Ahmed Barhoum
email: ahmed.barhoum@science.helwan.edu.eg

Heba H El-Maghrabi

Amr A Nada

Syreina Sayegh

Stéphanie Roualdès

Aurélien Renard

Igor Iatsunskyi

Emerson Coy

Mikhael Bechelany
email: mikhael.bechelany@umontpellier.fr

Stéphanie Roualdes

Simultaneous Hydrogen and Oxygen Evolution Reactions using Free-Standing Nitrogen-Doped-Carbon-Co/CoOx Nanofiber Electrodes Decorated with Palladium Nanoparticles

Keywords: Electrochemical Water Splitting, Heterostructure Electrocatalyst, Electrospinning, Atomic Layer Deposition, Nitrogen-doped Carbon Nanofibers, Cobalt-Cobalt Oxide Heterojunction, Graphitic Layer Structures

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Hydrogen is particularly interesting as a green fuel to meet the global energy demand. It can be produced from water splitting without emission of any pollution. Moreover, when it recombines with oxygen in fuel cells, it produces electrical energy and water only as a by-product. [START_REF] Peter | Photoelectrochemical Water Splitting. A Status Assessment[END_REF][START_REF] El-Maghrabi | Synthesis of Mesoporous Core-Shell CdS@TiO2 (0D and 1D) Photocatalysts for Solar-Driven Hydrogen Fuel Production[END_REF][START_REF] Nada | Highly Textured Boron/Nitrogen Co-Doped TiO2 with Honeycomb Structure Showing Enhanced Visible-Light Photoelectrocatalytic Activity[END_REF] These characteristics make hydrogen an attractive fuel for electricity generation applications (e.g. cars, trains, ships, aircraft, portable power, and houses). [START_REF] Pavlenko | Silicon/TiO2 Core-Shell Nanopillar Photoanodes for Enhanced Photoelectrochemical Water Oxidation[END_REF] Water splitting (HOH) to generate H2 and O2 can be performed via an electrochemical reaction using two electrodes and an aqueous electrolyte solution (acidic or alkaline). [START_REF] Roger | Earth-Abundant Catalysts for Electrochemical and Photoelectrochemical Water Splitting[END_REF][START_REF] Shalan | Nanofibers as Promising Materials for New Generations of Solar Cells[END_REF] For practical uses, electrochemical water splitting powered by photovoltage (PV) cells is also under development. Electrocatalysts accelerate electron transfer between electrode and reactants and also increase the speed of the intermediate chemical transformation during each half-reaction (i.e. hydrogen evolution reaction and oxygen evolution reaction, HER, and OER). [START_REF] Zhang | Metal Atom-Doped Co 3 O 4 Hierarchical Nanoplates for Electrocatalytic Oxygen Evolution[END_REF][START_REF] El-Maghrabi | Design of Ni/NiO-TiO2/RGO Nanocomposites on Carbon Cloth Conductors via PECVD for Electrocatalytic Water Splitting[END_REF] Conversely, the electrocatalyst overpotential limits the efficiency of electrochemical water splitting. These overpotentials can be significantly reduced using electrocatalysts that harbor active sites for HER/OER. To date, Pt and Pt/C (for HER) and IrO2 and RuO2 (for OER) are considered the most powerful electrodes with the lowest overpotential. [START_REF] Du | Interface Engineering of Palladium and Zinc Oxide Nanorods with Strong Metal-Support Interactions for Enhanced Hydrogen Production from Base-Free Formaldehyde Solution[END_REF] However, their industrial applications are limited because they are very expensive and rare. [START_REF] Zhou | Ultrathin Cobalt Oxide Layers as Electrocatalysts for High-Performance Flexible Zn-Air Batteries[END_REF][START_REF] El-Maghrabi | Coaxial Nanofibers of Nickel/Gadolinium Oxide/Nickel Oxide as Highly Effective Electrocatalysts for Hydrogen Evolution Reaction[END_REF][START_REF] Zhao | Electrospun Cobalt Embedded Porous Nitrogen Doped Carbon Nanofibers as an Efficient Catalyst for Water Splitting[END_REF] Indeed, electrode activity, cost, durability, and stability are of major importance. Therefore, it is crucial to develop efficient and cheap electrocatalysts for HER/OER to meet the increasing industrial demands. [START_REF] Ramanavicius | Insights in the Application of Stoichiometric and Non-Stoichiometric Titanium Oxides for the Design of Sensors for the Determination of Gases and Vocs (Tio2-x and Tino2n-1 vs. Tio2). Sensors[END_REF] Carbon nanofibers (CNFs) incorporating metal and metal oxide nanoparticles (NPs) are interesting materials for electrochemical water splitting because of their high dimensional stability, large specific surface, good electrical conductivity, and electrocatalytic activity, and also ability to minimize electrode surface fouling. [START_REF] Wang | Engineering the Composition and Structure of Bimetallic Au-Cu Alloy Nanoparticles in Carbon Nanofibers: Self-Supported Electrode Materials for Electrocatalytic Water Splitting[END_REF][START_REF] Wang | Small and Well-Dispersed Cu Nanoparticles on Carbon Nanofibers: Self-Supported Electrode Materials for Efficient Hydrogen Evolution Reaction[END_REF][START_REF] Li | Ni Strongly Coupled with Mo 2 C Encapsulated in Nitrogen-Doped Carbon Nanofibers as Robust Bifunctional Catalyst for Overall Water Splitting[END_REF] CNF-based electrodes are among the most interesting electrode materials for water treatment, [START_REF] Nnaji | Engineered Nanomaterials for Wastewater Treatment: Current and Future Trends[END_REF][START_REF] El-Maghrabi | One Pot Environmental Friendly Nanocomposite Synthesis of Novel TiO2 -Nanotubes on Graphene Sheets as Effective Photocatalyst[END_REF][START_REF] Chen | Magnetic Nanofibers: Unique Properties, Fabrication Techniques, and Emerging Applications[END_REF] electrochemical biosensing, [START_REF] Abdel-Haleem | Coated-Wire, and Carbon-Paste Ion-Selective Electrodes for Potentiometric Determination of Galantamine Hydrobromide in Physiological Fluids[END_REF][START_REF] Abdel-Haleem | T-Butyl Calixarene/Fe2O3@MWCNTs Composite-Based Potentiometric Sensor for Determination of Ivabradine Hydrochloride in Pharmaceutical Formulations[END_REF][START_REF] Abdel-Haleem | T-Butyl Calixarene/Fe2O3@MWCNTs Composite-Based Potentiometric Sensor for Determination of Ivabradine Hydrochloride in Pharmaceutical Formulations[END_REF][START_REF] Abdel-Haleem | Polyvinyl Chloride Modified Carbon Paste Electrodes for Sensitive Determination of Levofloxacin Drug in Serum, Urine, and Pharmaceutical Formulations[END_REF][START_REF] Abdel-Haleem | Molecularly Imprinted Electrochemical Sensor-Based Fe2O3@MWCNTs for Ivabradine Drug Determination in Pharmaceutical Formulation, Serum, and Urine Samples[END_REF][START_REF] Abdel-Haleem | Molecularly Imprinted Electrochemical Sensor-Based Fe2O3@MWCNTs for Ivabradine Drug Determination in Pharmaceutical Formulation, Serum, and Urine Samples[END_REF] batteries, [START_REF] Haichao | Engineering Nanofibers as Electrode and Membrane Materials for Batteries, Supercapacitors, and Fuel Cells[END_REF] supercapacitors, [START_REF] Haichao | Engineering Nanofibers as Electrode and Membrane Materials for Batteries, Supercapacitors, and Fuel Cells[END_REF][START_REF] Barhoum | Review of Recent Research on Flexible Multifunctional Nanopapers[END_REF] and drug delivery. [START_REF] Sawy | Insights of Doxorubicin Loaded Graphene Quantum Dots: Synthesis, DFT Drug Interactions, and Cytotoxicity[END_REF][START_REF] Rasouli | Nanofibers for Biomedical and Healthcare Applications[END_REF] The interpenetrating CNF network offers many active sites, good electrical conductivity, mechanical stability, and electrolyte diffusion. [START_REF] Ding | Electrospun Nickel-Decorated Carbon Nanofiber Membranes as Efficient Electrocatalysts for Hydrogen Evolution Reaction[END_REF] It has been shown that many transition metals (e.g. Cu, [START_REF] Wang | Engineering the Composition and Structure of Bimetallic Au-Cu Alloy Nanoparticles in Carbon Nanofibers: Self-Supported Electrode Materials for Electrocatalytic Water Splitting[END_REF] Mo, [START_REF] Wang | A 3D Nanoporous Ni-Mo Electrocatalyst with Negligible Overpotential for Alkaline Hydrogen Evolution[END_REF] W, [START_REF] Kawashima | Characterization of Sputter-Deposited Ni-Mo and Ni-W Alloy Electrocatalysts for Hydrogen Evolution in Alkaline Solution[END_REF] Co, [START_REF] Li | Co-Ni-Based Nanotubes/Nanosheets as Efficient Water Splitting Electrocatalysts[END_REF] Ni, [START_REF] Han | Transition-Metal (Co, Ni, and Fe)-Based Electrocatalysts for the Water Oxidation Reaction[END_REF] , and Fe [START_REF] Navarro-Flores | Characterization of Ni, NiMo, NiW and NiFe Electroactive Coatings as Electrocatalysts for Hydrogen Evolution in an Acidic Medium[END_REF] ) are efficient electrocatalysts for HER and OER. [START_REF] Wang | Engineering the Composition and Structure of Bimetallic Au-Cu Alloy Nanoparticles in Carbon Nanofibers: Self-Supported Electrode Materials for Electrocatalytic Water Splitting[END_REF][START_REF] Wang | Small and Well-Dispersed Cu Nanoparticles on Carbon Nanofibers: Self-Supported Electrode Materials for Efficient Hydrogen Evolution Reaction[END_REF][START_REF] Li | Ni Strongly Coupled with Mo 2 C Encapsulated in Nitrogen-Doped Carbon Nanofibers as Robust Bifunctional Catalyst for Overall Water Splitting[END_REF] Transition metal/metal oxide NPs incorporated in CNFs also show superior electrocatalytic activity, stability, and selectivity in HER/OER. [START_REF] Coy | High Electrocatalytic Response of a Mechanically Enhanced NbC Nanocomposite Electrode Toward Hydrogen Evolution Reaction[END_REF][START_REF] Valencia | High Electrocatalytic Response of Ultra-Refractory Ternary Alloys of Ta-Hf-C Carbide toward Hydrogen Evolution Reaction in Acidic Media[END_REF] Other cheap and effective electrode materials for HER/OER include transition metal carbides, [START_REF] Lu | Ultrafine Dual-Phased Carbide Nanocrystals Confined in Porous Nitrogen-Doped Carbon Dodecahedrons for Efficient Hydrogen Evolution Reaction[END_REF] perovskites (ABO3 in which A and/or B are transition metals), and oxide spinel (AB2O4 in which A and/or B are transition metals).

However, in HER/OER tests, they show poor performances due to their low electrical conductivity and corrosion. [START_REF] Zhao | Electrospun Cobalt Embedded Porous Nitrogen Doped Carbon Nanofibers as an Efficient Catalyst for Water Splitting[END_REF] Recently, atomic layer deposition (ALD) has been used as a cost-effective technique to deposit small amounts of expensive electrocatalyst NPs on the CNF electrode. [START_REF] Weber | Sub-Nanometer Dimensions Control of Core/Shell Nanoparticles Prepared by Atomic Layer Deposition[END_REF] The electrocatalytic activity of ALD-deposited catalyst NPs is strongly influenced by their amount, size, geometry, and their distribution within the electrode surface/porous structure. [START_REF] Topuz | Atomic Layer Deposition of Palladium Nanoparticles on a Functional Electrospun Poly-Cyclodextrin Nanoweb as a Flexible and Reusable Heterogeneous Nanocatalyst for the Reduction of Nitroaromatic Compounds[END_REF] ALD allows controlled deposition of noble metal NPs with controlled morphologies and compositions (monometallic, [START_REF] Celebioglu | Surface Decoration of Pt Nanoparticles via ALD with TiO2 Protective Layer on Polymeric Nanofibers as Flexible and Reusable Heterogeneous Nanocatalysts[END_REF] bimetallic, [START_REF] Lu | Toward Atomically-Precise Synthesis of Supported Bimetallic Nanoparticles Using Atomic Layer Deposition[END_REF] and core-shell [START_REF] Cao | Controlled Synthesis of Pd/Pt Core Shell Nanoparticles Using Area-Selective Atomic Layer Deposition[END_REF] ), depending on the application. [START_REF] Kertmen | Photoelectrochemically Active N-Adsorbing Ultrathin TiO 2 Layers for Water-Splitting Applications Prepared by Pyrolysis of Oleic Acid on Iron Oxide Nanoparticle Surfaces under Nitrogen Environment[END_REF] Comparing with Pt and Au NPs, ALD of the Pd NP on the CNF electrode surface can enhance its electrochemical activity, improve chemical stability, and reduce the overpotential of HER and the overall cost. [START_REF] Cheng | Electrocatalysts by Atomic Layer Deposition for Fuel Cell Applications[END_REF][START_REF] Liu | Atomic Layer Deposition of Electrocatalysts for Use in Fuel Cells and Electrolyzers[END_REF] As controlled ALD allows reducing the Pd NP size to atomic clusters or even single atoms, it could lead to a major reduction of the Pd NP mass on the electrode surface and enhance the electrode electrochemical activity towards HER/OER reaction. [START_REF] Barhoum | Review of Recent Research on Flexible Multifunctional Nanopapers[END_REF][START_REF] George | Atomic Layer Deposition: An Overview[END_REF][START_REF] Johnson | A Brief Review of Atomic Layer Deposition: From Fundamentals to Applications[END_REF] However, the impact of the deposited layer thickness, heterojunctions, and nanointerfaces on the CNF electrocatalytic activity has not been thoroughly investigated yet. [START_REF] Song | Origin of Achieving the Enhanced Activity and Stability of Pt Electrocatalysts with Strong Metal-Support Interactions via Atomic Layer Deposition[END_REF][START_REF] Anitha | Anodic TiO2 Nanotubes Decorated by Pt Nanoparticles Using ALD: An Efficient Electrocatalyst for Methanol Oxidation[END_REF][START_REF] Young | Enhanced Photocatalytic Water Oxidation Efficiency with Ni(OH) 2 Catalysts Deposited on α-Fe 2 O 3 via ALD[END_REF] Aside from ALD of electrocatalyst catalyst NPs

Recent progress in electrospinning technique led to the development of low-cost electrospun

CNFs electrodes incorporating transition metals NPs with improved electrocatalytic activity towards HER/OER reactions close to Pt, Pt/C, IrO2, and RuO2 based electrodes. [START_REF] Guterman | Activity and Stability of Pt/C and Pt-Cu/C Electrocatalysts[END_REF] 7664-93-9, 97%, Sigma-Aldrich) were used as an electrolyte solution for the HER and OER tests.

Experimental

Fabrication of Free-Standing Nanofiber Electrodes

Three N-CNF mats incorporating Co/CoOx NPs were produced according to the following three-step procedure: i) electrospinning; ii) thermal oxidation, and iii) thermal pyrolysis. [START_REF] Barhoum | Nanofiber Technology: History and Developments[END_REF] Briefly, the PAN/cobalt acetate/DMF solution was prepared by dissolving an appropriate amount of cobalt (II) acetate tetrahydrate in 20 mL DMF, and then by adding PAN powder to the mixture that was continuously stirred for 15h to obtain a homogenous solution. Electrospinning was performed with solutions that contained 0%, 10%, or 20% cobalt acetate and loaded in 20 mL plastic syringes and the following parameters: tip-to-collector distance = 15 cm, operating voltage = 25 kV, flow rate = 1 ml/min, and drum rotating speed = 400 rpm. Then, the nanofiber mats underwent thermal peroxidation for 2h (250 °C, air atmosphere) and pyrolysis for 1h (1000 °C, nitrogen atmosphere, constant heating and cooling rate of 1 °C.min -1 ).

Decoration with Pd NPs (Co-catalyst) by ALD

The N-CNF-Co/CoOx surface (electrode) was decorated with Pd NPs (co-catalyst) by performing 100 or 200 ALD cycles [START_REF] Johnson | A Brief Review of Atomic Layer Deposition: From Fundamentals to Applications[END_REF] using a home-made ALD reactor and Pd(hfac)2 (i.e. the Pd precursor, heated to approximately 70 °C to achieve the appropriate vapor pressure), formalin (i.e. the reacting species, kept at 25 °C) and argon (Ar, i.e. the purging gas). The ALD lines were at 100 °C and the deposition chamber at 220 °C. A typical ALD cycle included a 5 seconds pulse of Pd(hfac)2 with an exposure time of 15 seconds followed by Ar purge for 10s, and finally, 1 seconds pulse of formalin for 15 seconds and Ar purge for 60 seconds, as previously described. [START_REF] Merenda | Fabrication of Pd-TiO 2 Nanotube Photoactive Junctions via Atomic Layer Deposition for Persistent Pesticide Pollutants Degradation[END_REF][START_REF] Weber | Enhanced Catalytic Glycerol Oxidation Activity Enabled by Activated-Carbon-Supported Palladium Catalysts Prepared through Atomic Layer Deposition[END_REF]

Characterization of the Prepared Electrodes

The crystal structure and crystallite size of the fabricated electrodes were analyzed by X-ray diffraction (XRD) with a PANAlytical Xpert-PRO device with Ni-filtered Cu-radiation (wavelength of 0.154 nm) as a radiation source. Their fiber diameter and surface morphology, composition, and crystallinity were measured by scanning electron microscopy (SEM, Hitachi S4800 microscope, Japan Zeiss EVO HD15 microscope) and high-resolution transmission electron microscopy (HR-TEM, JEOL ARM 200F, Japan) with an energy dispersive X-ray (EDX) analyzer. Raman spectra were recorded with a SENTERRA Raman microscope spectrometer (Bruker, Germany) with the double-frequency Nd:YAG and 532 nm lasers. X-ray photoelectron spectroscopy (XPS) was performed using a Kratos Axis Ultra spectroscope (Kratos Analytical, UK) and a monochromatic Al Kα source (1486.6 eV). Co elemental concentration in the as-prepared N-CNFs was determined by atomic absorption spectroscopy (ASS, AAnalyst 400, PerkinElmer). The mean pore diameter, volume, and surface area were assessed from the N2 adsorption-desorption isotherms using the BET method (Brunauer Emmett and Teller, Micromeritics ASAP 2010 equipment).

Electrochemical Tests

For HER and OER, a CHI 660E workstation was used with: (i) free-standing N-CNF, N-CNF-Co/CoOx, and N-CNF-Co/CoOx-Pd electrodes as working electrodes (obtained by cutting the prepared electrodes into circles with diameter =1 cm and thickness = 0.25 mm); (ii) a silver/silver chloride electrode as reference electrode (Potentials were referenced to a reversible hydrogen electrode (RHE) following the equation: ERHE = EAg/AgCl + 0.059  pH + 0.197); and (iii) a graphitic electrode as counter electrode. For all tests, 1 M KOH was used as the electrolyte solution. In HER and OER tests, the fabricated electrodes were tested alongside commercial Pt and IrO2 electrodes (working electrodes), respectively. All spectra were acquired after 10 cycles at a 100 mV s -1 scan rate to trigger and stabilize the prepared electrodes. Cycle sweep voltammetry (CSV) and linear sweep voltammetry (LSV) data were collected at a 5 mV s -1 scan rate for all tests. Electrochemical impedance spectrometry (EIS) was performed from 10 -1 to10 5 Hz at -0.3 V vs RHE and at 1.6 V vs RHE, for the HER and OER tests, respectively. To assess the electrocatalytic stability during HER, the chronoamperometric response (i-t curves) was recorded in 1 M KOH solution and -0.3 V vs. RHE for 12h. Hydrogen evolution was monitored by gas chromatography (Clarus-400, PerkinElmer, TCD (2m x1mm), He carrier) of gas samples collected with a syringe every 30 min at 100 mV.

Faradaic efficiency (FE) was calculated using the following equation:

FE (%) = [ 2F * mol H2 (GC) * 100] / [Q],
where F is the Faraday constant, Q is the gas volume % based on the charge passed, and GC is the gas volume %. [START_REF] Liu | Controlled Synthesis of FeP Nanorod Arrays as Highly Efficient Hydrogen Evolution Cathode[END_REF]

Results and Discussion

Through HER and OER, water electrolysis leads to the production of hydrogen fuel and oxygen gas, respectively. Among the different types of electrode materials, N-CNFs represent an excellent scaffold to support electrocatalytic metal NPs. In this study, free-standing N-CNF electrodes incorporating Co/CoOx NPs and coated with Pd NPs were developed and tested. Due to its high mechanical strength, excellent electrical conductivity, and elevated carbon yield (>65% wt/wt), PAN is the most promising polymer for N-CNF electrode production. Nitrogen doping of CNFs and incorporation of Co/CoOx NPs were performed by electrospinning a cobalt acetate-PAN solution, followed by ALD of PD NPs (Scheme 1).

During the peroxidation step, PAN or PAN-cobalt acetate nanofibers undergoes different chemical process such as oxidation, cyclization, crosslinking, aromatization and dehydrogenation that make the polymer (PAN or cobalt acetate/PAN) fibers denser and more thermally stable during the subsequent thermal carbonation step. This step prevents the fusion of individual fibers before furthering the carbonization step, by inducing additional oxygen into the PAN structure. In the subsequent step (thermal carbonation), nanofibers (PAN or cobalt acetate/PAN) begin to pyrolysis with a considerable release of volatile by-products. The characteristics of the CNFs are typically influenced by the thermal treatment parameters, especially the nature of gas, heating rate, the final temperature, and the time of the isothermal treatment. [START_REF] Barhoum | Nanofibers as New-Generation Materials: From Spinning and Nano-Spinning Fabrication Techniques to Emerging Applications[END_REF] T. Wanjun and C. Donghua 58 studied the mechanism of thermal decomposition of cobalt acetate tetrahydrate in N2 and H2 atmosphere. The authors reported that the residual compositions in the above processes were similar regardless of the atmosphere used. The dehydration process was followed by a major step concerning the decomposition of the acetate group, leading to basic acetate as an intermediate, which then produced and migration can be efficiently reduced. This also limits the corrosion and oxidation of the fabricated electrodes caused by the electrolyte. Therefore, N-CNFs are often chosen as the main component to enhance electrochemical stability, active site density, and electron/electrolyte transfer on the electrode surface. Moreover, the nitrogen atoms (oxidized N, pyrrolic, pyridinic, graphitic) [START_REF] Luo | Cobalt Nanoparticles Confined in Carbon Matrix for Probing the Size Dependence in Fischer-Tropsch Synthesis[END_REF] used for doping can interact with three C atoms at different locations on the graphene layer, strongly improving the electrode conductivity and electrochemical properties. As water electrolysis is generally performed in the presence of alkaline electrolyte solutions, rather than acidic electrolyte solutions, the developed electrode materials should show satisfactory performance in basic media. In the basic electrolyte solution (1 M KOH) used in this study, the Co atoms on the Co/CoOx NP surface can be transformed into Co(OH)2, which is an active-active phase for OER. Conversely, in the presence of an acidic electrolyte solution (0.5 M H2SO4), Co/CoOx NPs tend to be corroded and passivated during HER and OER.

Analysis of the Composite Nanofiber Electrode Composition and Morphology

Morphology and elemental composition of the prepared N-CNF and N-CNF-Co/CoOx electrodes were investigated by field-emission SEM and HR-TEM. SEM analysis (Figure 1) showed that the cobalt acetate to PAN weight ratio significantly influenced the size and morphology of the N-CNF, in particular, the number of incorporated Co/CoOx NPs and their sizes and size distribution. Indeed, the electrodes formed a 3D porous structure with randomly oriented CNF with diameters of 300±44 nm for N-CNF (Figure 1a), 220±33 nm for N-C-Co10 (Figure 1b), and ~200±29 nm for N-C-Co20 (Figure 1c). Diameter of the nanofibers depends mainly on solution properties. The addition of cobalt acetate changes the solution properties i.e., it increases the solution's electrical conductivity.

The increase in solution electrical conductivity improve the polymer solution spinnability and as a result affects the jet formation and decreases fiber diameter. [START_REF] Topuz | Influence of Salt Addition on Polymer-Free Electrospinning of Cyclodextrin Nanofibers[END_REF] As the cobalt acetate to PAN weight ratio increased from N-C-Co10 to N-C-Co20, larger numbers of rhombohedral Co/CoOx NPs were formed inside the N-CNFs and they covered also the N-CNFs surface (Figure 1b,c). The particle size of Co/CoOx NP located at the N-CNF surface slightly increased from 20-50 nm to 30-80 nm with the increase of the cobalt acetate percentage from N-C-Co10 to N-C-Co20, respectively (Figure 1b,c).

The elemental composition analysis (representative SEM-EDX results in Figure 1d) confirmed that:

(i) the elements C, Co, N, and O were homogeneously distributed, (ii) the thermal pyrolysis of thermally treated PAN/cobalt acetate nanofibers under nitrogen atmosphere allows for nitrogendoping of CNFs which might contribute to the electrode's excellent electrochemical activity. The ASS analysis indicated that Co total content was approximately 14wt% for N-C-Co10 and 25wt% for N-C-Co20 (Table S1, Supplementary Materials). HR-TEM analysis (Figure 2) showed that Co and Pd NPs were firmly attached to the N-CNF surface (Fig. 2d). Unlike SEM, the TEM has a much higher resolution but allows less amount of sample to be analyzed at a time. TEM images in Figure 2a-b show that the N-C-Co20-Pd100 surface was covered by 40±10 nm Co/CoOx NPs and 7±2 Pd NPs. Higher magnification TEM images (Figure 2e) indicated that Co/CoOx NPs were coated by a graphitic layer and that Pd NPs were deposited conformably on the graphitic layers. TEM-EDX (Figure 2d) with elemental mapping confirmed C, Co, and Pd homogeneous distribution at the N-C-Co20-Pd100 surface. Moreover, the lattice spacing of 0.35 nm (inset in Figure 2e) corresponded to the 002 crystal planes of the graphitic layers, in agreement with the XRD results. The TEM and SEM results indicated that the electrodes were made of interpenetrated crystalline graphitic layers coated by Co NPs grown in situ on the entire framework. Furthermore, the N-CNF architecture inhibited the restacking of graphitic nanosheets. The electrocatalytic activity should progressively increase with Co/CoOx NP and Pd NP higher dispersion and smaller size and with the presence of graphene layer structures. Similar studies reported that higher catalytic activity is obtained with Pd NPs of a specific size (e.g. 10 nm, Figure 2c,d). [START_REF] Barhoum | Atomic Layer Deposition of Pd Nanoparticles on Self-Supported Carbon-Ni/NiO-Pd Nanofiber Electrodes for Electrochemical Hydrogen and Oxygen Evolution Reactions[END_REF] Interestingly, in our work, the Pd-graphene-Co/CoOx nanostructure was observed by HR-TEM (Figure 2d) and confirmed by XPS and Raman spectroscopy (Figure 3b), but was not reported in previous similar studies. [START_REF] Liu | Nonenzymatic Glucose Sensor Based on Renewable Electrospun Ni Nanoparticle-Loaded Carbon Nanofiber Paste Electrode[END_REF][START_REF] Maiyalagan | Performance of Carbon Nanofiber Supported Pd-Ni Catalysts for Electro-Oxidation of Ethanol in Alkaline Medium[END_REF][START_REF] Guo | Pd-Ni Alloy Nanoparticle/Carbon Nanofiber Composites: Preparation, Structure, and Superior Electrocatalytic Properties for Sugar Analysis[END_REF] XRD analysis was applied for the prepared electrodes (N-CNF, N-C-Co10, and N-C-Co20) to study the effect of cobalt acetate incorporation into the N-CNF crystallinity and crystal structure (Figure 3a,b). XRD analysis showed that the peak at 2θ = 25° was associated with the hexagonal graphite (002) lattice planes (JCPDS card no. 41-1487), with an inner-layer spacing d = 0.336 nm. This suggests that the pristine N-CNF sample has a low degree of graphitization. [START_REF] Zhu | Graphitized Porous Carbon Nanofibers Prepared by Electrospinning as Anode Materials for Lithium Ion Batteries[END_REF] Conversely, the higher and stronger intensity of the (002) peak intensity in the N-C-Co20 and N-C-Co10 electrodes suggests a higher degree of crystallinity of the carbon matrix and Co/CoOx NPs in these two samples. Moreover, the higher intensity for the graphitic peak at 2θ = 25° indicated that higher percentage of Co/CoOx NPs accelerate the formation of graphitic carbon. The N-C-Co10 and N-C-Co20 electrodes showed also two sharp peaks at around 44.3° and 54° that were ascribed to the (111) and (200) planes of the fcc crystal structures of metallic Co (JCPDS 15-0806), respectively. The average crystallite sizes of Co/CoOx NPs, calculated with the Scherrer equation, [START_REF] Barhoum | Physicochemical Characterization of Nanomaterials: Polymorph, Composition, Wettability, and Thermal Stability[END_REF][START_REF] Nada | Mesoporous ZnFe2O4@TiO2 Nanofibers Prepared by Electrospinning Coupled to PECVD as Highly Performing Photocatalytic Materials[END_REF][START_REF] El-Sheikh | Preparation of Superhydrophobic Nanocalcite Crystals Using Box-Behnken Design[END_REF] S1, Supplementary Materials). When we compare the average crystallite size of N-C-Co10 to the average particle size, the average crystallite sizes of these samples are similar or slightly smaller than the average particle size estimated by SEM and TEM while the difference is bigger for N-C-Co20. This indicates that N-C-Co20 contain higher percentage of polycrystalline Co/CoOx NPs, while crystals formed in N-C-Co10 is mainly single crystals; these findings where discussed elsewhere. [START_REF] Barhoum | Simple Chemical Route for the Synthesis of Pure Nanocalcite Crystals[END_REF] The production of reducing by-products during thermal carbonization under N2 atmosphere could explain the presence of metallic Co nanocrystals instead of CoOx nanocrystals. [START_REF] Ji | Electrospun Carbon Nanofibers Decorated with Various Amounts of Electrochemically-Inert Nickel Nanoparticles for Use as High-Performance Energy Storage Materials[END_REF] Therefore, the pre-oxidation step of PNA-cobalt acetate followed by calcination under nitrogen atmosphere is main reason for the formation of CoOx together with Co. Oxygen content (about 2% according to the SEM-EDX and TEM-EDX analyses with elemental mapping) could be due to minimal N-CNF oxidation or to the generation of amorphous CoOx impurities during thermal peroxidation after electrospinning. XRD diffraction could not detect other phases (i.e. cobalt oxides, and cobalt carbides) for N-C-Co10 and N-C-Co20 possibly due to their small crystallite size, low content, and/or amorphous nature. [START_REF] Ji | Electrospun Carbon Nanofibers Decorated with Various Amounts of Electrochemically-Inert Nickel Nanoparticles for Use as High-Performance Energy Storage Materials[END_REF] However, the two collapsed diffraction peaks observed at 2θ of 44° (Figure 3b) could be explained by the presence of CoOx, at very low concentration, in the catalyst, as confirmed by XPS and Raman spectrometry. Slight shift in the peak positions could attributed to macrostrain present in the unit cell of the obtained crystals with increasing the cobalt acetate concertation from for N-C-Co10 to N-C-Co20. Previous studies showed that metallic Co 0 and CoOx can synergistically increase the catalytic performance. [START_REF] Yuan | Effects of Cobalt Precursor on Pyrolyzed Carbon-Supported Cobalt-Polypyrrole as Electrocatalyst toward Oxygen Reduction Reaction[END_REF] Raman analysis was applied for the prepared electrodes (N-CNF, N-C-Co10, and N-C-Co20) to study the effect of cobalt acetate incorporation into the N-CNF graphitic architecture (Figure 3c). Generally, N-CNF show two main peaks: the broad D band (1300 cm -1 ; amorphous carbon) and the G band (1600 cm -1 ; ordered graphitic structure). [START_REF] Zhu | Chemical Characterization of Electrospun Nanofibers[END_REF] The relative intensities (R) of these two bands (ID/IG) are positively influenced by the number of amorphous carbon (disordered sp 2 ) and graphitic layers (ordered sp 2 bonding carbon atoms). [START_REF] Zhu | Graphitized Porous Carbon Nanofibers Prepared by Electrospinning as Anode Materials for Lithium Ion Batteries[END_REF] The D (∼1320 cm -1 ) and G (~1590 cm -1 ) bands were detected in all analyzed electrode samples (Figure 3c). The ID/IG value was 1.13 for pristine N-CNF, but it gradually decreased with the cobalt acetate content: 0.92 for N-C-Co10 and 1.01 for N-C-Co20. [START_REF] Bao | Synergistic Effect of the Pd-Ni Bimetal/Carbon Nanofiber Composite Catalyst in Suzuki Coupling Reaction[END_REF] With higher Co/CoOx NP content, the carbonaceous structures should more easily form graphitic structures. Raman spectrometry was used also to investigate the CoOx structures. The N-C-Co10 and N-C-Co20 spectra showed four typical Co3O4 peaks between 474 and 677 cm -1 (Figure 3c) that matched the reported CoOx spectrum. [START_REF] Larrude | Multiwalled carbon nanotubes decorated with cobalt oxide nanoparticles[END_REF] The five Raman peaks for Co3O4 at 192, 474, 513, and 667 cm -1 are associated with T2g, Eg, and A1g symmetries. The presence of Co3O4 could be explained by Co NP surface oxidation by oxygen during peroxidation in air atmosphere after electrospinning. 75 XPS can be used to analyze the surface chemistry of the free-standing electrodes before and after ALD on Pd NPs on the electrode surface (Figure 4a). XPS survey spectra (Figure 4a) indicated that N-C-Co20 and N-C-Co20-Pd100 exhibited peaks related to C(1s), O(1s), N(1s), Co(2p), Pd(3d) core levels. When quantified these five peaks, the N-C-Co20 are consisted of 95.1 % C, 2.7% O, 1.4% N, 0.8 % Co, and 0.0% for Pd while the N-C-Co20-Pd100 exhibit 87.7 % C, 2.0% O, 1.0% N, 0.9 % Co, and 8.4% (Table S2, Supplementary Materials). The C1s peak (Figure 4b) comprised four different peaks that might be attributed to sp 2 carbon (C=C) and three low-intensity components (C-C/C-H, 284.9 eV), (C-O, 285.8 eV), (C=O, 287.0 eV). [START_REF] Di Blasi | Synthesis and Characterization of Electrospun Nickel-Carbon Nanofibers as Electrodes for Vanadium Redox Flow Battery[END_REF] The O1s peak (Figure 4c) could not be decomposed due to charge effects and/or partial overlapping of the O 1s and Pd 3p3/2 peaks. The high-resolution XPS spectra of N 1s (Figure 4d) point out the existence of N-doping of different nitrogen species pyridinic, nitrile, and quaternary nitrogen, which has a significant effect on HER/OER reactions. The CoOx (oxidation state) could not be easily quantified because of the low signal-to-noise ratio. However, we can confirm that the cobalt oxide and a cobalt metal state coexisted. [START_REF] Larrude | Multiwalled carbon nanotubes decorated with cobalt oxide nanoparticles[END_REF] The maximum intensity of the two peaks, observed at 778eV and 780eV, suggested the contribution of metal Co (0) and Co oxide (III) to the spectra (Figure 4e). [START_REF] Ji | Electrospun Carbon Nanofibers Decorated with Various Amounts of Electrochemically-Inert Nickel Nanoparticles for Use as High-Performance Energy Storage Materials[END_REF] The high-resolution XPS spectra of Pd 3d (Figure 4f) further confirm Pd NP deposition on the N-CNF and Co/CoOx NP surfaces. Analysis of the Pd 3d spectra showed a strong increase in the intensity of the Pd 3d5/2 peak of N-C-Co20-Pd100 that included metal Pd (0) at 335.2 eV and oxide Pd (+II) at 336.3eV (Table S3, Supplementary Materials). Measurement of the N2 adsorption/desorption isotherms, surface areas (BET method), and pore size of the different electrodes (Table S4, Supplementary Materials) showed that Co/CoOx NP in situ synthesis inside the N-CNF significantly increased the surface area by 35-fold. This effect might be explained by the formation of degradation products during the thermal steps after electrospinning that promote pore formation. The N-C-Co20 specific surface area was considerably higher than that of N-CNF and N-C-Co10 (239.7 cm 2 /g versus 6.7 cm 2 /g, and 114.1 cm 2 /g, with pore diameters of 5.6 nm versus 13.8 nm, and 3.8 nm, respectively) (Table S4, Supplementary Materials).

A higher specific surface area might enhance electrolyte diffusion into the electrode, and consequently also the electrochemical activity. However, the ALD of Pd NPs on the N-CNF-Co/CoOx surface slightly reduced the electrode surface area to 150.4 cm 2 /g for N-C-Co20-Pd100

(pore diameter of 8.1 nm) and to 113.2 cm 2 /g for N-C-Co20-Pd100 (pore diameter of 4.4 nm). The surface area and pore diameter reduction confirmed that ALD efficiently decorated the N-C-Co/CoOx surface and tiny pores between the fibers. The effect of using 100 or 200 ALD cycles on the surface area and the electrode activity during HER/OER will be investigated in the next section.

Hydrogen Evolution Reactions

The HER polarization LSV curves of the prepared electrodes were illustrated in Figure 5a.

HER performance of N-C-Co20-Pd100 has displayed the best electrochemical activity, with a very low overpotential (100 mV) at a current density of 10 mA cm -2 , compared with other prepared electrodes of N-CNF (498 mV), N-C-Pd (296 mV), N-C-Co10-Pd200 (167 mV), N-C-Co20-Pd200

(155 mV), and N-C-Co10-Pd100 (148 mV).

Tafel plots were employed in Figure 5b to reveal the HER performance from the kinetic viewpoint. Tafel slopes (determined with the following equation: η=b log j+a, where j represents the current density, η the overpotential, and b the Tafel slope) [START_REF] Young | Enhanced Photocatalytic Water Oxidation Efficiency with Ni(OH) 2 Catalysts Deposited on α-Fe 2 O 3 via ALD[END_REF] were used to thoroughly investigate the electrocatalytic activity and HER kinetics of the prepared electrodes as listed in (Table S5, Supplementary Materials).

Tafel slope of N-C-Co20-Pd100 is as low as 33 mV dec -1 ; compared with 73 mV dec -1 for N-C-Pd100, , 63 mV dec -1 for N-C-Co10-Pd200, 55 mV dec -1 for N-C-Co20-Pd200) and 39 mV dec -1

for N-C-Co10-Pd100 similar to the reference catalyst (Pt: 36 mV dec -1 ). [START_REF] Chen | Carbon Nanofiber-Supported PdNi Alloy Nanoparticles as Highly Efficient Bifunctional Catalysts for Hydrogen and Oxygen Evolution Reactions[END_REF] As demonstrated in previous studies, a lower Tafel slope is more HER catalysis advantageous since current density can undergo a sharper raise with a low η, thus benefiting HER process (ref). The low b of N-C-Co20-Pd100 may arise from a facile reaction mechanism following Volmer-Tafel reactions, which can be attributed to the lower energy barrier from the thermodynamic respect. Whereby, the multi-step HER reaction comprises an electrochemical Volmer reaction (H3O + + e -→Hads +H2O, 120 mV dec -1 ), followed by a chemical Tafel reaction (H3O + + e -→Hads +H2O, 120 mV dec -1 ) or an electrochemical Heyrovsky reaction (Hads + H3O + + e -→ H2↑ +H2O, 40 mV dec -1 ).

The exchange current density (J0) was extrapolated from the Tafel plots (Figure 5b) to investigate the intrinsic HER activity of N-C-Co20-Pd100. The highest J0 value was obtained with N-C-Co20-Pd100 (~1.8 mA cm -2 , versus 1.13 mA cm -2 for C-Pd, 1.5 mA cm -2 for N-C-Co10-Pd100, and 1.43 mA cm -2 for N-C-Co20-Pd200) (Table S5, Supplementary Materials). Which may result in the high active site density and the excellent electron transport capacity of N-C-Co10-Pd100. To confirm this, the charge transfers resistance (Rct) values were extrapolated from EIS measurements (Table S5 and Figure 5c Recent findings indicated that three major parameters are identified control the hydrogen generation activity (HER) in an aqueous system the availability of active sites for the reaction socalled electrochemical active surface area (ECSA). [START_REF] Mahmood | Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes: Mechanisms, Challenges, and Prospective Solutions[END_REF] The large surface area and network structure of free-standing electrodes increase effectively the contact area of active surfaces to the electrolyte. In this work, estimation of the ECSA was performed by computing the electrochemical double-layer capacitance (Cdl) from CSV plots. The results showed larger ECSA values for N-C-Co20-Pd100

(1750 cm 2 ) than for N-C-Pd100 (375 cm 2 ), N-C-Co10-Pd100 (750 cm 2 ), N-C-Co10-Pd200 (775 cm 2 ), and N-C-Co20-Pd200 (600 cm 2 ) (Figure 5d and Table S5). The higher capacitance and ECSA values of the N-C-Co20-Pd100 electrode indicate that it improved HER performance could be explained by its specific porous structure and larger active site surface.

In order to study the electrochemical activity for hydrogen production in presence of the elaborated N-CNF-Co/CoOx nanofiber electrodes. The hydrogen generation using N-C-Co20-Pd100 as a working electrode was monitored for 2 hours (Figure 5e). Measurements of the hydrogen evolution rate and Faraday efficiency have been used to describe the efficiency with which charge (electrons) is transferred between the electrolyte and the electrodes facilitating an HER reaction [START_REF] Kawrani | Segregation of Copper Oxide on Calcium Copper Titanate Surface Induced by Graphene Oxide for Water Splitting Applications[END_REF][START_REF] Kawrani | Enhancement of Calcium Copper Titanium Oxide Photoelectrochemical Performance Using Boron Nitride Nanosheets[END_REF] .

The current-time curves obtained from chronoamperometry tests for FE measurements of N-C-Co20-Pd100 electrode were illustrated in Fig( S1). The best electrode (N-C-Co20-Pd100) exhibited a high hydrogen evolution rate (250 μmol/h) and excellent faradaic efficiency (~95%) over 2 hr. This high electrochemical performance can be explained by the higher current density (as indicated by LSV measurements) and low Rct value, respectively. The overpotential value was not changed during the 40h chronoamperometry test at 100 mV (Figure 6d). Furthermore, the XRD analysis after the test (Figure 5f) highlighted a slight decrease in the (002) peak intensity for graphitic structure as well as the peaks ascribed to the (111) and ( 200) planes of the Co/CoOx NPs. However, no major intensity changes for the typical peaks of Co/CoOx crystals. This demonstrated the superior durability and chemical stability of the prepared free-standing N-CNF-Co/CoOx-Pd electrodes.

Literature survey has shown that the N-C-Co20-Pd100 overpotential value was also lower than that of previously reported CNF-based electrodes (Table 5,6) and similar to that of the commercial Pt (100 mV @10 mA cm -2 ) and Pt/C catalysts (36 mV dec -1 ) [START_REF] Chen | Carbon Nanofiber-Supported PdNi Alloy Nanoparticles as Highly Efficient Bifunctional Catalysts for Hydrogen and Oxygen Evolution Reactions[END_REF] . The effect of Pd particle size on the catalyst activity during HER has not been much studied. As Pd NPs with a diameter >10 nm behave like bulk Pd, size affects only below this diameter, particularly in the range of 2-5 nm. [START_REF] Khalily | Atomic Layer Deposition of Pd Nanoparticles on N-Doped Electrospun Carbon Nanofibers: Optimization of ORR Activity of Pd-Based Nanocatalysts by Tuning Their Nanoparticle Size and Loading[END_REF] , [START_REF] Gao | Shape and Size Effects on Photocatalytic Hydrogen Production: Via Pd/C3N4photocatalysts under Visible Light[END_REF] This can be explained by a geometric effect, and by the induction of structural changes through an epitaxial strain of Pd NPs. Moreover, as Pd NPs display facets with different crystallographic orientations, the catalytic activity might be influenced by size-mediated differences in their relative abundance. [START_REF] Navlani-García | Investigation of Size Sensitivity in the Hydrogen Production from Formic Acid over Carbon-Supported Pd Nanoparticles[END_REF] Here, the decreased density and size of Pd NPs (from 200 ALD cycles/10 nm to 100 ALD cycles 5 nm) could explain N-C-Co20-Pd100 better electrochemical performance during HER because the electrode electrochemical activity is reduced by Pd NP high surface coverage. Moreover, Pd NP aggregation or Ostwald-ripening during the HER test should be less frequent during 100 ADL cycles due to the lower Pd NP density (i.e. higher distance between Pd NPs). 82 hydrogen evolution rate and Faradic Efficiency of the electrodes; (f) XRD patterns of the best electrode (N-C-Co20-Pd100) after the chronoamperometric response (j-t) test.

Oxygen Evolution Reaction

OER is an important process for energy conversion and storage, particularly during water electrolysis. As O⁎, HO⁎, and HOO⁎ are the main OER intermediates, the M-O bonding interaction during OER plays a major role in their stabilization at the surface, and can significantly influence the overall water splitting efficiency. [START_REF] Lee | Synthesis and Activities of Rutile IrO 2 and RuO 2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions[END_REF] Like for HER, a volcano-type relationship can be observed for OER using (∆GO* -∆GHO*) as a descriptor. Many studies focused on Co3O4 nanostructures as OER catalysts due to their good activity and stability [START_REF] Cai | Nanomaterials With Different Dimensions for Electrocatalysis[END_REF] , abundance, low cost, and environmental friendliness. The many active sites present on Co3O4 (111) explain the higher electrocatalytic activity of ultrathin Co3O4 nanosheets during water and ethanol oxidation in alkaline solutions. [START_REF] Cai | Nanomaterials With Different Dimensions for Electrocatalysis[END_REF] The N-C-Co20-Pd100 electrode exhibited outstanding OER activity with an overpotential of approximately 160 mV at 10 mA cm -2 compared with N-CNF (537 mV), N-C-Pd (380 mV), N-C-Co10-Pd100 (270 mV), N-C-Co10-Pd200 (366 mV), and N-C-Co20-Pd200 (290 mV) (Figure 6a).

As observed for HER, increasing the amount of Pd NPs deposited on the electrode surface (from 100 to 200 ALD cycles) negatively affected the electrode electrochemical activity. N-C-Co20-Pd100 performance was similar to that of the commercial IrO2 electrode (210 mV @10 mA cm -2 ).

Moreover, N-C-Co20-Pd100 displayed the smallest Tafel slope value (113 mV dec -1 compared with 412 mV dec -1 for N-C-Pd100, 158 mV dec -1 for N-C-Co10-Pd100, 399 mV dec -1 for N-C-Co10-Pd200, and 316 mV dec -1 for N-C-Co20-Pd200) that was similar to that of IrO2 (156 mV dec -1 ) (Figure 6b). N-C-Co20-Pd100 displayed a J0 (1.22 mA cm -2 ) considerably more elevated than N-C-Pd (0.53 mA cm -2 ), N-C-Co10-Pd100 (0.62 mA cm -2 ), and N-C-Co20-Pd200 (0.55 mA cm -2 ) (Table S5, Supplementary Materials). This high J0 value reflects the electrode's higher surface-active area.

Moreover, the smaller Rct value (EIS measurements) of N-C-Co20-Pd100 (12.5 Ω, compared with 55 Ω for N-C-Pd100, 24 Ω for N-C-Co10-Pd100, 39 Ω for N-C-Co10-Pd200, and 21 Ω for N-C-Co20-Pd200) was in agreement with its higher ECSA (1750 cm 2 ) (Figure 6c). The chemical stability test has been performed to finally conclude that the N-C-Co20-Pd100 is a promising electrode for HER reaction. The long-term durability of the best electrode (N-C-Co20-Pd100) was assessed by recording its overpotential at -10 mA/cm 2 without iR compensation (Figure 6d). Altogether, these results indicated that the best electrode (N-C-Co20-Pd100) displays excellent activity during HER and OER (lowest overpotential at 10 mA cm -2 and smallest Tafel slope) compared with previously reported electrocatalysts (Table S6, Supplementary Materials). [START_REF] Wu | Non-Noble-Metal-Based Electrocatalysts toward the Oxygen Evolution Reaction[END_REF][START_REF] Wu | Partially Oxidized Ni Nanoparticles Supported on Ni-N Co-Doped Carbon Nanofibers as Bifunctional Electrocatalysts for Overall Water Splitting[END_REF][START_REF] Chen | Carbon Nanofiber-Supported PdNi Alloy Nanoparticles as Highly Efficient Bifunctional Catalysts for Hydrogen and Oxygen Evolution Reactions[END_REF][START_REF] Ding | Electrospun Nickel-Decorated Carbon Nanofiber Membranes as Efficient Electrocatalysts for Hydrogen Evolution Reaction[END_REF][START_REF] Li | Fabrication of Pt Nanoparticles on Nitrogen-Doped Carbon/Ni Nanofibers for Improved Hydrogen Evolution Activity[END_REF] However, the Pd NP amount used in our experiments was much smaller than what was previously described. Other similar work, Guo et. al. [START_REF] Guo | Strategic Atomic Layer Deposition and Electrospinning of Cobalt Sulfide/Nitride Composite as Efficient Bifunctional Electrocatalysts for Overall Water Splitting[END_REF]

  However, there is no studies have been published on the potential applications of free-standing CNF electrodes that incorporate Co/CoOx NPs as electrocatalyst and are decorated with Pd NPs (by ALD) for simultaneous HER and OER. This study aims to study simultaneous hydrogen and oxygen evolution reactions using free-Standing nitrogen-doped-carbon-Co/CoOx nanofiber electrodes decorated with Pd NPs. The free-standing nitrogen-doped CNF (N-CNFs) and N-CNF-cobalt/cobalt oxide (Co/CoOx)-Pd electrodes were produced in a multistep process in which a polyacrylonitrile (PAN) and cobalt acetate solution were electrospun to fabricate free-standing and binder-free N-CNF mats with Co/CoOx NPs that were thermally stabilized under air and carbonated under nitrogen. Finally, the electrode surface was decorated with Pd NPs by ALD. The N-CNF-Co/CoOx-Pd heterostructure provided an excellent electrocatalytic activity for HER/OER in an aqueous alkaline electrolyte (1 M KOH) with high chemical stability. The formation of graphitic layers in the N-CNF matrix facilitated Co/CoOx dispersion into the N-CNF bulk and on the electrode surface. The N-CNF architecture also favored the electrolyte accessibility to active Co/CoOx and Pd NP sites and increased electron transfer between electrode and electrolyte. Furthermore, Co/CoOx embedding in the graphitic layers and Pd NP deposition by ALD protected the Co/CoOx NPs from oxidation and corrosion during HER/OER and significantly increased the electrode durability. 26 By controlling the electrode composition and electrolysis conditions, the fabricated N-CNF-Co/CoOx-Pd electrodes displayed very good electrochemical activity (low overpotential values and Tafel slope during HER/OER) and stability relative to the reference Pt electrode.

2. 1 .

 1 Materials Polyacrylonitrile (PAN, Mwt ~150000, CAS 25014-41-9, Sigma-Aldrich), cobalt (II) acetate tetrahydrate (Co(OCOCH3)2•4H2O, 99%, CAS No. 6147-53-1, Sigma-Aldrich), N,Ndimethylformamide (DMF, 98%, CAS No. 68-12-2, Sigma-Aldrich) and ethanol (C2H5OH, 99%, CAS No. 64-17-5, Sigma-Aldrich) were used for fabricating free-standing N-CNF-Co/CoOx electrodes by electrospinning. Palladium (II) hexafluoroacetylacetonate (Pd(C5HF6O2)2, CAS 64916-48-9, 99%, Sigma-Aldrich) was used as Pd precursor and formaldehyde (HCHO, CAS No. 50-00-0, 37%, Sigma-Aldrich) as co-reactant for ALD deposition of Pd NPs on N-CNF-Co/CoOx electrodes. Potassium hydroxide (KOH, CAS 1310-58-3, Sigma-Aldrich) and sulfuric acid (H2SO4, CAS No.

  Scheme 1. Four main steps of the production of Free-Standing Nitrogen-Doped-Carbon-Co/CoOx

Figure 1 .

 1 Figure 1. Field-emission SEM photographs of the fabricated free-standing electrodes: (a) N-CNF;

Figure 2 .

 2 Figure 2. TEM analysis of a free-standing N-C-Co20-Pd100 electrode: (a) Low magnification TEM

  were 32.0±0.4 (N-C-Co10) and 35.1±0.3 (N-C-Co20) (Table

Figure 3 .

 3 Figure 3. XRD analysis and Raman spectrometry of the indicated free-standing electrodes: (a) XRD

Figure 4 .

 4 Figure 4. XPS analysis of N-C-Co-Pd electrode samples: (a) representative XPS survey scan; (b) C

  in Nyquist plots). The N-C-Co20-Pd100 electrode displayed a smaller Rct value (1.2 Ohm) compared with N-C-Pd100 (7.2 Ohm), N-C-Co10-Pd100 (2.5 Ohm), N-C-Co10-Pd200 (4 Ohm), and N-C-Co20-Pd200 (2.4 Ohm), in agreement with its HER activity results and lower overpotential, due to the higher conductivity and electrical exchange of N-CNF, Co/CoOx, and Pd NPs. These results indicate that: (i) HER proceeds via a Volmer-Heyrovsky mechanism when using N-CNF electrodes; (ii) increasing the Co/CoOx content from N-C-Co10 to enhances the electrode activity for HER; (iii) increasing the amount of Pd NPs deposited on the electrode surface (from 100 to 200 ALD cycles) negatively affected the electrode activity.[START_REF] Patil | Atomic Layer Deposition of NiOOH/Ni(OH) 2 on PIM-1-Based N-Doped Carbon Nanofibers for Electrochemical Water Splitting in Alkaline Medium[END_REF]

Figure 5 .

 5 Figure 5. Comparison of the performance of the commercial Pt electrode and the free-standing N-C-

Figure 6 .

 6 Figure 6. Comparison of the electrocatalytic activity during OER carried out with the commercial

95 Conclusion

 95 developed strategic atomic layer deposition and electrospinning techniques of cobalt sulfide/nitride composite as efficient bifunctional electrocatalysts for overall water Splitting. The prepared electrodes exhibited overpotentials of 20 mV for HER and 180 mV for OER at a current density of 10 mA cm -2 in basic medium. However, the lowest Tafel slope of 54.4 mV dec -1 was observed in 0.5 M H2SO4 electrolyte.The equivalent circuit applied for the evaluation of both (HER and OER) EIS measurements is illustrated in Figure6e. A classic expression of this circuit in a Nyquist plot was an incomplete semicircle with a line, which straightens out with decreasing frequency.[START_REF] Ramanavicius | Electrochemical Impedance Spectroscopy Based Evaluation of 1,10-Phenanthroline-5,6-Dione and Glucose Oxidase Modified Graphite Electrode[END_REF] Such a model under right selected circuit element values to fit well with gathered measurement data as seen in Figures6c and 5c. The simplified Randles cell consists of a solution resistance, double layer capacitance and a charge transfer or a polarization resistance. The double layer capacitance (Cdl) is in parallel with the charge transfer resistance (RCT) and this parallel combination is in series with the ohmic resistance (RΩ) as shown in the Figure6e.The OER and HER activity of the prepared electrodes are in the following order N-CNF < N-C-Co10 < N-C-Co20 < N-C-Pd100 < N-C-Co10-Pd200 < N-C-Co20-Pd200 < N-C-Co10-Pd100 < N-C-Co20-Pd100.Where, N-C-Co20-Pd100 manifested excellent electrocatalytic activity in water splitting due to: (i) the optimal Co/CoOx NP loading on CNFs; (ii) Co/CoOx NP formation within and on the CNF surface; (iii) N-CNF and Co/CoOx NP surface decoration with ~5 Pd NPs that modify the electronic structures of N-C-Co/CoOx electrodes, increase their conductivity, accelerate charge transfer, and enhances their electrochemical activity 61 and stability, resistance to corrosion and possibly chemical stability; (iv) the limited N-C-Co/CoOx surface and structural decomposition because the Co/CoOx NP surface is protected by an inert graphitic carbon layer and Pd NPs; (v) the N-CNF architecture that favor gas and electrolyte diffusion during HER/OER; (vi) heterostructures made of various active materials with different HBE (i.e. N-C-Co/CoOx-Pd) that lead to a better HER activity compared with N-CNF-Co/CoOx and N-CNF-Pd nanofibers, possibly due to the presence of accessible active sites or to the improved interface electronic configuration. 92,93,94 The N-C-Co/CoOx-Pd electrode heterostructure allows the efficient catalysis of H + reduction and Hads desorption in HER, improving HER kinetics and activity. In this study, solution electrospinning, thermal treatment, and ALD were combined to fabricate N-doped CNF-Co/CoOx-Pd electrodes with high electrocatalytic activity towards HER/OER. The incorporation of Co/CoOx and Pd NPs in CNFs results in excellent electrocatalytic activity and very good chemical stability in alkaline electrolyte solutions. Our results show that combining CNF graphitic nanostructures with Co/CoOx and Pd NPs offers great potential for improving catalyst performance in HER/OER and their chemical stability at a lower cost than with already commercially available catalysts (Pt and IrO2). The best electrode contained up to 25% of http://www.lcpme.cnr-nancy.fr) for XPS analysis. TEM analysis was performed by I.I and E.C. were partially supported by H2020-MSCA-RISE-2017, 'Novel 1D photonic metal oxide nanostructures for early-stage cancer detection' (Project number: 778157), and by the Poland National Science Centre (NCN) through the OPUS grant 2019/35/B/ST5/00248.

292 290 288 286 284 282 280 278

	Intensity (a.u.)	(e)		Co 2p Baseline	Co 2p 3/2		Intensity (a.u.)	(f)	Pd 3d5/2	Pd 3d3/2 Pd 3d Baseline
		810 805 800 795 790 785 780 775 770 Binding Energy (eV)	332 334 336 338 340 342 344 346 Binding Energy (eV)
	Intensity (a.u.)	Pd 3p1/2 Co 2p O KLL (a)	Pd 4s O 2s + Pd 4p Pd 3d N 1s Pd 3p1/2 + O 1s C 1s	Intensity (a.u.)	(b)	C 1s Baseline
		1000	800	600	400	200	0
			Binding Energy (eV)			Binding Energy (eV)
	Intensity (a.u.)	(c)				O 1s Baseline		Intensity (a.u.)	(d)	N 1s Baseline
		540 538 536 534 532 530 528 526		406	404	402	400	398	396	394
			Binding Energy (eV)			Binding Energy (eV)

Acknowledgment

Special thanks to the Egypt-France Joint Driver (Imhotep, Project No. 43990SF, 2020-2022) for financially supporting Dr. Ahmed Barhoum (Project PI from the Egyptian Side) during his research stay at the Institut Européen des Membranes (IEM). Dr. Ahmed Barhoum (Group Leader of NanoStruc, Research Group, Helwan University) would also like to thank the Irish Research Council (hereinafter referred to as 'the Council') for financing his research stay at the School of Chemical Sciences, Dublin City University under grant number (Project ID: GOIPD/2020/340). Authors thank Martine Mallet from the SMI LCPME facility (Université de Lorraine-CNRS-

Co, CoOx in traces, and few Pd NPs (100 ALD cycles), and displayed excellent electrocatalyst activity in HER and OER tests. This could be explained by the synergistic interaction between N-CNFs, Co/CoOx, and Pd NPs at the nanoscale interfaces. Moreover, the N-CNF architecture might underlie the fast gas desorption kinetics. The superior HER activity is explained by the N-CNF electrode architecture with more Co/CoOx edge sites and fewer Pd NPs that can improve electrocatalytic activity and accelerate electron transfer between graphitic layers, Co/CoOx, and Pd NPs, ultimately increasing the electrode conductivity and consequently its efficacy during HER. This approach could be adapted for incorporating in CNFs other transition metals and metal alloys (as alternatives to Pt and IrO2) for HER and OER industrial applications using water. The described N-CNF-Co/CoOx-Pd NPs can be used as a benchmark for developing HER/OER electrodes for electrolysis, metal-air batteries, and wastewater treatment. ALD of a few Pd NPs with very small diameters could also be exploited for water electrolysis and wastewater electrochemical oxidation.

Conflict of interest

All authors have declared the absence of conflicts of interest relative to their contribution to the present publication.


Réda Bendraou

Benoît Combemale

Xavier Crégut

Marie-Pierre Gervais

Definition of an eXecutable SPEM 2.0

One major advantage of executable models is that once constructed, they can be run, checked, validated and improved in short incremental and iterative cycles. In the field of Software Process Modeling, process models have not yet reached the level of precision that would allow their execution. Recently the OMG issued a new revision of its standard for Software Process Modeling, namely SPEM2.0. However, even if executability was defined as a mandatory requirement in the RFP (Request For Proposal), the adopted specification does not fulfill it. This paper presents a critical analysis on the newly defined standard and addresses its lacks in terms of executability. An approach is proposed in order to extend the standard with a set of concepts and behavioural semantics that would allow SPEM2.0 process models to be checked through a mapping to Petri nets and monitored through a transformation into BPEL.

Introduction

Since the earliest projects developing large software systems, one main concern of organizations was to provide a conceptual scheme for rationally managing the complexity of software development activities. Several Software Development Life Cycle (SDLC) have been proposed like the "Waterfall", "Spiral" and the "Incremental Model". However, the software process modeling community was unsatisfied with using these life-cycle descriptions. The granularity of SDLC models is too coarse-grained and fails to describe elementary process building blocks [START_REF] Curtis | Process modeling[END_REF]. Rapidly, the need to describe in more details processes that companies are actually performing during software development or maintenance emerged. The idea was to decompose these SDLC descriptions into sufficient detail so that they can provide more explicit guidance for executing a software development project. This is how the notion of Process Models (PM) appeared.

Goals that motivated the introduction of process models are manifold: they span from informal support and facilitating human understanding to direct assistance in process assessment, management and enactment [START_REF] Riddle | Session summary: Opening session[END_REF]. To be most effective in supporting this variety of objectives, process models must go beyond representation. The understanding of process participants about the contents and sequencing of process steps depends strongly on the degree of details provided in the process model. Recently, the pressure for greater granularity (i.e., more details) in process models is driven by the need to ensure process precision, the degree to which a defined process specifies all process steps needed to produce accurate results [START_REF] Humphrey | Introduction to software process improvement[END_REF]. Another pressure comes from the increasing demand for process validation and automation, which requires precise process models at relatively deep levels of detail.

In this paper, we will focus on two main objectives. The first one deals with the ability to validate a software process model at any stage of its lifecycle, i.e., during its design, when it is tailored for a given project and while it is conducted. Validation may be achieved through the behavioral checking of process model properties. The second objective relates to the automation of the execution support. We mean by execution support, the ability to monitor and to control a real process according to its defined process model. We try to reach these two objectives i.e. Validation and Execution in the context of SPEM2.0 (Software Process Engineering Metamodel) [18], the recently adopted OMG's standard dedicated to software process modeling. Addionnally to describing a concrete software development process or a family of related software development processes, the last version of the standard claims its ability for partially supporting process enactment, a facility which was intentionally kept out of the scope in SPEM1.1 [START_REF][END_REF].

P r e l i m i n a r y V e r s i o n

In the next section of this paper, we will start by giving a critical analysis on the enactment approaches proposed in SPEM2.0 and we will address their limits in supporting process model executions. In order to tackle these limits, in section 3, we define an extension of SPEM2.0 that allows the specification of executable process models. This extension brings a new compliance level to the standard. It comes in form of a set of elements and features that merge the SPEM2.0 metamodel. We also associate a behavioral semantics to the proposed extension. For validating and ensuring that processes can be executed with respect to constraints defined in the process model, an approach is proposed in section 4. Examples of such constraints are schedulability constraints, temporal constraints and those related to resource availabilities. The approach uses modelchecking. At this aim, process models are translated into a formal language: Petri nets. This has the primary advantage of leveraging the myriad of already existing modelcheckers. In section 5, we explore the possibility to use workflow tools for monitoring purposes. Thus, we propose a mapping between SPEM elements and BPEL (Business Process Execution Language) concepts. Finally, section 6 concludes this work.

2 Process Models Enactment with SPEM2.0 SPEM2.0 is the OMG's standard dedicated to software process modeling. It aims at providing organizations with means to define a conceptual framework offering the necessary concepts for modeling, interchanging, documenting, managing and presenting their development methods and processes [18]. Besides providing a standard way for representing organization's processes and expertise, SPEM2.0 comes with a new attractive vision. That latter consists in separating all the aspects, contents and material related to a software development methodology from their possible instantiation in a particular process. Thus, to fully exploit this framework, the first step would be to define all the phases, activities, artifacts, roles, guidance, tools, and so on, that may compose a methodology and then, to pick, according to the situation or process context, the appropriate method contents to use within a process definition.

SPEM2.0 comes in form of a MOF-compliant metamodel [START_REF]Core[END_REF] that reuses UML2.0 Infrastructure [START_REF]Unified Modeling Language (UML) 2.1.1 Infrastructure[END_REF] and UML2.0 Diagram Interchange specifications [START_REF]Diagram Interchange 1.0 Specification[END_REF]. It reuses from the UML Infrastructure basic concepts such as Classifier or Package. No concept from the UML2.0 Superstructure [START_REF]Unified Modeling Language (UML)[END_REF] is reused. The Standard comes also in form of UML Profile where each element from the SPEM2.0 metamodel is defined as a stereotype in UML2.0 Superstructure. The metamodel is composed of seven packages linked with the "merge" mechanism (cf [START_REF]Unified Modeling Language (UML) 2.1.1 Infrastructure[END_REF], §11.9.3), each package dealing with a specific aspect (cf. Fig. 1). However, even if process enactment was among the principal requirements when the SPEM2.0 RFP was issued [START_REF]Object Management Group, Inc. Software Process Engineering Metamodel (SPEM) 2.0 RFP[END_REF], the recently adopted specification does not address the enactment issue. Nevertheless, it clearly suggests two possible ways of enacting SPEM2.0 process models. In the following, we introduce them, we present the concepts that are supposed to be used in order to enact SPEM2.0 models and, we give some remarks on the feasibility of each approach.

Mapping the SPEM2.0 Processes Models into Project Plans

In this first approach the standard proposes to map SPEM2.0 processes into project plans by means of project planning and enactment systems such as IBM Rational Portfolio Manager or Microsoft Project. Once SPEM2.0 pro-P r e l i m i n a r y V e r s i o n cesses are mapped to project plans, the plans can be instantiated by means of planning tools and concrete resources can then be affected. However, whether this approach may be very useful for project planning, it is not considered as process enactment. It is still necessary to affect duration to tasks, persons to roles in order to get, at the end, an estimation of the development process period and resources needed for its realization. These plans are used by a project manager in order to estimate if the process will be in schedule or not, whether more persons need to be affected to process tasks, etc. There is no support for process execution, no automatic task affectations to responsible roles, no automatic routing of artifacts, no automatic control of work product states after each activity, no means to support agent and team communication and so on. Besides the fact that this approach does not provide concrete enactment support, it presents a major lack which is its tight dependence to the project planning tool. Another aspect that has to be taken into account is the impact of modifying or adding information within the planning tool and how this modification will be reflected / traced-up to the SPEM2.0 process model. Finally, process modelers have to deal with the compatibility of the process definition file format of the planning tool.

Linking SPEM2.0 process elements with external behavior formalisms

The SPEM2.0 standard does not provide any concepts or formalism for modeling precise process behavior models or execution. Rather, claiming for more flexibility, SPEM2.0 provides, through the Process Behavior package, a way to link SPEM2.0 process elements with external behavior models. The goal behind is not to restrict or to impose a specific behavior model but to give the process modeler the option to choose the one that fits his needs best. Further more, a mapping towards BPEL can be carried out in order to reuse BPEL execution engines. In addition, a WorkProduct can for instance be linked to a UML state diagram in order to model possible WorkProduct's states and transitions that can make this WorkProduct move from one state into another. Here again, a state machine engine has to be integrated to the process execution engine. SPEM2.0 defines a kind of proxy classes (i.e., Activity_ext, Con-trolFlow_ext, Transition_ext and State_ext) in order to link between SPEM2.0 process elements (i.e., WorkProductUse, WorkDefinition, RoleUse, Activity, WorkSequence) and external behavior model elements. It is up to the process modeler to link each process element with its equivalent in the behavior model. Since a single behavior model may not be expressive enough to represent all the behavioral aspects of the process, several behavior models can be combined.

Even if this approach may provide flexibility in representing behavioral aspects of SPEM2.0 processes, it presents some lacks. The first one is that the standard is not very clear on how the linking of process elements with behavioral models is realized. It just provides proxy classes that make reference to other elements in an external behavioral model. We suppose that this task is tool implementer's responsibility. A tool implementers have to define a specific behavioral model that has to be automatically generated from the SPEM2.0 process model. This is already the case in the free EPF1 tool which is meant to be the implementation of SPEM2.0. In EPF, a kind of a proprietary activity diagram is partially generated from a process definition. The latter can be refined in order to provide more details on the process activities and their coordination (control flows). However no execution is provided. The second lack is that the mapping from SPEM2.0 process elements into a specific behavioral model can be done differently from one organization to another, depending on the process modeler's interpretation. Thus, a standardization effort may be required in order to harmonize mapping rules between SPEM2.0 concepts and a specific behavior model such as BPEL for instance. At this aim, in section 5, we propose such mapping rules between a subset of SPEM2.0 concepts and the BPEL language. The third lack, which tightly relates to the previous one, is that more often concepts in behavior models are richer than in SPEM2.0. This is due to the fact that behavior modeling and execution languages provide additional concepts related to the technical support and execution of processes while SPEM2.0 concentrates on the "business concerns" of the software development process or methodology (i.e., Roles, Activities, Guidance, etc.). Consequently, a full executable code generation from SPEM2.0 is not possible which may impose some refinement steps in behavior models before they can be enacted. This in its turn poses the problem of traceability and how these refinements (changes) can be reflected in the initial SPEM2.0 model.

In the rest of the paper, we propose an approach for validating and executing SPEM2.0 process models. For this purpose, we need first to extend the SPEM2.0 metamodel in order to take into account some process execution aspects.

XSPEM: SPEM2.0 Extension for Process Enactment

In this section, we propose to extend the SPEM2.0 specification in order to take into account the support of process enactments while remaining standard. We call our proposition, XSPEM which stands for eXecutable SPEM. For sake of clarity, we only present the minimal subset of SPEM2.0 concepts required for process execution. These concepts are regrouped in the XSPEM Core package (section 3.1). Additional features are required in the purpose of tailor-P r e l i m i n a r y V e r s i o n ing a process for a given project. This includes defining properties specific to activity scheduling and resource allocations. They are introduced in XSPEM ProjectCharacteristics package (section 3.2). During execution, the process will evolve from one state into another. It is then required to provide concepts for characterizing all process states during enactment time. This is the aim of the XSPEM Proces-sObservability package (section 3.3). We also define the behavioral semantics needed for process execution. Finally, we defined events that trigger state changes in the XSPEM EventDescriptions package (section 3.4).

XSPEM Core

xSPEM Core reuses concepts offered by the Core and the ProcessStructure packages (grey packages on fig. 1). Fig. 2 shows xSPEM Core's concepts. An Activity is a concrete WorkDefinition that represents a general unit of work assignable to specific performers represented by RoleUse.

It can rely on inputs and produces outputs represented by

WorkProductUses. RoleUse models a set of competences required to perform an activity (ProcessPerformerMap). An activity may be broken down into sub-activities. Activities are ordered thanks to the WorkSequence concept whose attribute linkKind indicates when an activity can be started or finished. We do not reuse the remaining packages as their content do not have any impact on process executability.

XSPEM Project Characteristics

In order to tailor a process model for a given project, additional features have to be defined. It is required to dimension activities, i.e., specify the number of used resources, expected duration, etc., and to identify the concrete resources allocated to the project. In this paper, we consider that RoleUse and WorkProductUse may be considered as resources required to perform an activity. The direction attributes defined in WorkDefinitionParameter could be used to complete sequencing constraints expressed through the WorkSequence concept. They are not detailed in this paper due to lack of space but they would be treated in the same way as activities described below.

We have thus defined the xSPEM_ProjectCharacteristics package as an extension of the xSPEM_Core package using the OMG merge operator. It redefines the concepts of Activity, RoleUse and ProcessPerformerMap by adding: 1) the time interval during which an activity must finish (min_time and max_time on Activity); 2) the number of role occurrences required to perform an activity (occurrencesNb on RoleUse); 3) the work load affected to a role for an activity (charge on ProcessPerformerMap).

XSPEM Process Observability

In order to enact a process model, its semantics has to be defined. We apply a property-driven approach that we have described in [START_REF] Combemale | Towards a Formal Verification of Process Model's Properties -SimplePDL and TOCL case study[END_REF]. It helps a Domain Specific Language (DSL) expert in defining the DSL semantics. It first consists in identifying relevant properties and specifying them using a temporal logic. Then, they are used to point out states of interest for the DSL expert and transitions that lead from one state to another.

The formal semantics associated to the system can be seen as the set of maximal finite traces whose elements are model states. If the metamodel has a well defined operational semantics, it can be easily expressed as a modification of instance's attributes or a modification of the topology (dynamically creating or deleting instances). On the contrary, if the associated semantics is not formally defined, the states characterised by properties allow to define an observable operational semantics. Following this idea, if state properties rely on notions that cannot be directly expressed in the model (classical OCL queries), then the metamodel must be enriched to express these notions. The dynamic operational semantics, i.e. the Kripke structure that allows to build trace semantics, must then be approximated by defining transitions between characterised states. It is the work of the domain expert to describe them.

P r e l i m i n a r y V e r s i o n

This approach has three major advantages: it gives a method to define a formal semantics, it is incremental (properties may be added one after another) and it allows to easily define an observable approximation of the trace semantics.

Based on expertise described in [START_REF] Bendraou | UML4SPM : A UML2.0-Based Metamodel for Software Process Modelling[END_REF], we now apply our property-driven approach to define the behavioral semantics of SPEM2.0 metamodel.

Characterising Properties

We identify the following properties that every XSPEM model must satisfy. We split them in two classes; universal properties that have to be satisfied by every execution (every activity must start, all started activities must finish, all suspended activities must resume, once an activity is finished, it has to stay in this state, an activity is able to start or finish depending on worksequences constraints. . . ) and existential properties that must be true in at least for one execution (each activity must be performed in more than min_time and less than max_time, the overall process can to finish when all activities are finished between min_time and max_time).

Similar properties are defined on products. For example, an activity can only be started if its input products have been started (at least one activity producing them is started) or completed (all the activities producing them are finished). It could be of interest to know whether it is possible to complete all the products of a process.

Extending the Metamodel to Represent Dynamic Informations

The second step consists in adding features to the metamodel to capture states implied by the aforementioned properties. For XSPEM, we can identify two orthogonal aspects for the Activity element. First, an activity can be not started, started, suspended and finally finished. Secondly, there is a notion of time and clock associated each activity; but this time is only relevant for transition-enabling conditions (in our case transitions that start and finish an activity) and is not explicit in state properties. Thus it can be represented into the finite set of states {tooEarly, ok , tooLate}. This second orthogonal aspect is only relevant when the activity is finished. Now we have to express these states by extending the Activity element in order to introduce attributes that reflect dynamic informations, i.e.

the state of the current activity. We choose to add two attributes (in the xSPEM_ProcessObservability package): state ∈ {notStarted, started, suspended, f inished} and time ∈ {tooEarly, ok, tooLate}. It is also necessary to take into account the concept of a clock (clock ∈ R + ), internal to an activity. It is not represented in the metamodel because only the abstraction is necessary, the clock being taken into account by the execution engine.

An observational abstraction of the operational semantics of our processes with respect to our properties can now be defined. The expert has again to formalise the initial state and the transition relation. In our case, it is quite natural: the initial state is {a → (notStarted, ok)|a ∈ A}. The transition relation is defined for Activity in Fig. 3.

XSPEM Events Description

Definition of the observational semantics introduces states and transitions. States have been captured through features added on the XSPEM metamodel. Transitions are triggered by events that make the process evolve. Events can be exogenous (produced by the environment of the process) or endogenous (produced within the process). For example the transition between the states not started and started corresponds to the event StartActivity applied on the corresponding activity. Other events on an Activity are finish, suspend and resume. They are modelled in the xSPEM_EventsDescription package as specializations of Ac-tivityEvent, an abstract event that involves a target activity.

Events may be recorded as a trace so as to keep track of what happened in the process (package DSL_Trace). They can also be used to build scenarios that can be used to simulate a process.

Process Models Validation

In this section, we propose to implement semantics defined in the previous section in order to check xSPEM process models. Like for programming languages, there are several approaches.

Operational semantics allows to precisely describe the dynamic behavior of the language's constructions. In MDE (Model-Driven Engineering), it aims to express the behavioral semantics of a metamodel and thus provides executable models. We explore in [START_REF] Combemale | Towards a Rigorous Metamodeling[END_REF] two ways to achieve this purpose. The first one is closer to the operational semantics in programming languages (Structural Operational Semantics [START_REF] Plotkin | A Structural Approach to Operational Semantics[END_REF], natural semantics [START_REF] Kahn | Natural semantics[END_REF]). It consists in defining transformations between two execution states of a model (for example, in the ATL [START_REF] Jouault | Transforming models with ATL[END_REF] transformation language). The whole set of transformations defines the behavior of models. The second way consists in describing the behavior of each concept of the metamodel in an imperative manner using metaprogramming languages such as Kermeta [START_REF] Muller | Weaving executability into object-oriented meta-languages[END_REF], xOCL [START_REF] Clark | Applied metamodelling -a foundation for language driven development[END_REF] or an action language.

Translational semantics relies on a well-defined formalism to express the semantics of a given language [START_REF] Gunter | Semantic domains. In Handbook of Theoretical Computer Science[END_REF]. A translation is carried out from all concepts of the source language towards this formalism. This translation defines In MDE, it consists in translating towards another technical space [START_REF] Clark | Applied metamodelling -a foundation for language driven development[END_REF]. This approach allows to take advantage of all tools available in the target domain. This last approach is explored in the section hereunder in order to re-use model-checkers available in the model-checking community.

Translational Semantics to Petri Nets

In this experimentation, we choose to use the technical space of timed Petri nets as the target representation for formally expressing XSPEM process models. We also choose to generate our properties as LTL (Linear Temporal Logic) formulae over the Petri net associated to a process model. Then we manipulate timed Petri nets and LTL formulae within the Tina 2 toolkit [START_REF] Berthomieu | The tool TINA -construction of abstract state spaces for Petri nets and time Petri nets[END_REF] which includes: 1) nd (Net-Draw), an editing tool for automata and timed networks, under a textual or graphical form. It integrates a "step by step" simulator (graphical or textual) for the timed networks. 2) Tina, which builds the state space of a Petri net, timed or not. Tina performs classical constructs (marking graphs, covering trees) and allows abstract state space construction, based on partial order techniques. 3) selt, a modelchecker for formulae of an extension of temporal logic seltl (State/Event LTL) [START_REF] Chaki | State/event-based software model checking[END_REF]. In case of non satisfiability, selt is able to build a readable counter-example sequence usable by the TINA simulator to execute it step by step.

The XSPEM semantics is defined as a mapping to Petri nets (XSPEM2PETRINET). A PetriNet is composed of Nodes that denote Places or Transitions. Nodes are linked by Arcs. Arcs can be normal ones or read-arcs. An Arc specifies the number of tokens consumed in the source place or produced in the target one. A read-arc only checks tokens availability without removing them. Petri nets marking is defined by the number of tokens contained in places. Finally, a time interval can be expressed on Transitions.

An example of transformation from a process model to a Petri net model is given in Fig. 4. Each activity is trans-2 TIme Petri Net Analyser, http://www.laas.fr/tina/ The state Started records that the activity has been started. A Work-Sequence becomes a read-arc from one place of the source activity to a transition of the target activity. We also add five places that define a local clock. The clock will be in state TooEarly when the activity ends before min_time and in the state TooLate when the activity ends after max_time. This transformation has been written in ATL within the context of an execution dedicated XSPEM subset 3 .

The Petri net model is then translated into the concrete syntax of Tina using an ATL query PETRINET2TINA. To reuse other Petri nets tools, only this last transformation would have do be adapted.

Model-Checking on Process Models

Now that the process model is translated into a Petri net model, we can check xSPEM properties by using Tina. Properties expressed on the xSPEM metamodel leads to an P r e l i m i n a r y V e r s i o n ATL transformation that produces the corresponding LTL properties instantiated from the xSPEM model. So, whether a process may be completed or not may becomes a termination problem that can be expressed on the Petri net model. In xSPEM, a process is finished when all its activities are finished on time. On the Petri net model, it means that for places of such activities there is one token in the place _finished and none in _tooEarly and _tooLate places. It is defined by the macro-definition finished generated from the process model by an ATL query.

We can distinguish the partial correction "all deadlock is in final process state" ( (dead ⇒ finished)) and the termination ( ♦ dead) which ensures that any execution finishes. At this stage, we have a "strong consistency": any execution finishes and any finished execution is in deadlock. In practice, the constraints expressed in the xSPEM model do not allow to finish systematically.

Thus we can be interested by the "weak consistency". It means that at least one execution fulfills the constraints. We evaluate the -♦ finished property which, literally, expresses that no execution finishes. If the process is weakly consistent, this property is evaluated with False and the counterexample produced by selt gives one correct execution of the process. If the property is evaluated with True then the process does not allow any solution.

Note that the consistency study is generic: it only depends on the finished criterium.

Project Monitoring

Once process models are validated, an execution support is required in order to ensure orchestrations of activities. At this aim, we can envisage two approaches. The first one consists in developing from scratch, a process engine that will take as input, XSPEM process models in order to execute them. The process engine has then to ensure all enactment facilities such as activity sequencing, resources management, events handling, exceptions processing, etc. The second approach consists in reusing the current state of the art in the Workflow and Business Process Management (BPM) domains. Indeed, these domains have reached a certain maturity level and recently, a consolidation has led to a single language for business process executions: the Business Process Execution Language for Web Services (WS-BPEL, BPEL for short) [START_REF] Oasis | Web Services Business Process Execution Language Version 2.0[END_REF]. Rapidly, BPEL gained importance and became the "Language" for business process orchestrations. Many tool vendors already provide training supports and process engines for this standard: Ac-tiveBPEL4 , ApacheAgila 5 . For XSPEM process model executions, we decided to explore the second possibility and we opted for BPEL as process execution language. In order to reuse BPEL process engines, a mapping between XSPEM and the process execution language is required. After studying the BPEL standard, in table 1, we propose mappings between XSPEM elements and BPEL concepts.

While establishing these rules we have noticed three major issues. The first one relates to the fact that all XSPEM elements that provide semantics proper to software process modeling have no equivalent in BPEL. All elements such as RoleUse, ProcessPerformerMap, WorkProductUse, etc., are converted into simple BPEL process variables. The second aspect is BPEL's lack of user interactions and support for Human-oriented tasks. Since software processes are mainly composed of human creative tasks, this issue has to be tackled. As a solution, a very interesting work done by industrials known as "BPEL4PEOPLE" [START_REF]WS-BPEL Extension for People BPEL4People[END_REF] can be reused. In BPEL4PEOPLE, a new BPEL activity called People activity is introduced. A People activity is a basic activity, which is not realized by a piece of software but an action performed by a human being. It can be associated with a group of people, a generic role, etc. The extended BPEL engine creates for each People activity a generic user interface in order to highlight inputs/outputs of the activity, deadlines, to add the possibility to attach other materials (e.g., guidelines) and to ease communication between agents. Regarding the implementation, BPEL4PEOPLE leaves the choice to the modeler between five possible configurations. These five configurations, that we will not detail here, fall roughly into two kinds: Inline Activities and Standalone Activities. Inline activities are defined as part of the BPEL process (they have access to the process context, variables, etc.) while standalone activities are defined outside the process. Standalone activities may be accessed through 1) implementation-specific invocation mechanisms P r e l i m i n a r y V e r s i o n (i.e., no WSDL), 2) a Web service interface defined with WSDL or 3) a BPEL Invoke activity that calls a Web service implemented by the People activity (WSDL + binding). We opted for the latter configuration. Main reasons are: 1) to promote reusability of standalone activities by other processes, 2) to use tasks in a distributed environment since they offer a WSDL interface, 3) to avoid BPEL engine extensions, since that solution is generic and does not need a support of the new People activity kind. However, process modeler can decide to use another configuration among the five that BPEL4PEOPLE proposes if needed. Finally, the last issue relates to the fact that the generated BPEL is not usable straightforward after the transformation. Some data and configuration details have to be set first. Additionally, the process modeler can decide to add new elements or variables for execution aims. This raises the issue of traceability between the XSPEM process model and the generated BPEL process, and how coherence between the two definitions can be preserved.

Conclusion

In this paper, we proposed an extension of SPEM2.0 to provide concepts required to enact a process model. A subset of SPEM2.0 is used as a foundation and constitutes a new compliance level. Features have been added to model characteristics of a project, including defining concrete resources allocated to the project and dimensioning activities. We also define features to store process's states during enactment time. Once both process model and the project model have been defined, we propose two complementary approaches. The first one consists in validating models with formal tools (e.g., model-checkers available in the area of Petri nets). It consists in evaluating properties on SPEM2.0 by translating them into LTL properties on the corresponding Petri net model. Properties include termination (i.e., will the process finish) and can also exhibit examples of process planning that fulfill process and project constraints.

The second approach consists in monitoring the project. We proposed a mapping into BPEL, a standard process execution language in the BPM domain. When doing this mapping we identified some drawbacks; the major ones are the loss of semantics proper to software process modeling while mapping SPEM2.0 process models into BPEL, and BPEL's lack of user interaction supports. For this issue, we proposed to reuse the BPEL4PEOPLE proposition. Another deficit of this approach relates to the fact that the code generated after the mapping may need to be completed or modified for execution purposes. However, these modifications are not reflected (traced-up) to the SPEM process model. These approaches are currently evaluated in the context of the IST MODELPLEX and TOPCASED projects. More elaborated prototypes are under construction.

Figure 2 .

 2 Figure 2. XSPEM: an eXecutable SPEM2.0

Figure 3 .

 3 Figure 3. Event-based Transition Relation for Activitiesthe semantics of the source language. In MDE, it consists in translating towards another technical space[START_REF] Clark | Applied metamodelling -a foundation for language driven development[END_REF]. This approach allows to take advantage of all tools available in the target domain. This last approach is explored in the section hereunder in order to re-use model-checkers available in the model-checking community.

Figure 4 .

 4 Figure 4. An XSPEM activity as a Petri net lated into five places characterising its state (NotStarted, Started, InProgress, Suspended or Finished).The state Started records that the activity has been started. A Work-Sequence becomes a read-arc from one place of the source activity to a transition of the target activity. We also add five places that define a local clock. The clock will be in state TooEarly when the activity ends before min_time and in the state TooLate when the activity ends after max_time. This transformation has been written in ATL within the context of an execution dedicated XSPEM subset3 .The Petri net model is then translated into the concrete syntax of Tina using an ATL query PETRINET2TINA. To reuse other Petri nets tools, only this last transformation would have do be adapted.

  The Core

	MethodPlugin	<<merge>>
		<<merge>>
	ProcessBehavior	<<merge>>	Method ProcessWith	<<merge>>	MethodContent
	<<merge>>	<<merge>>		<<merge>>
		ProcessStructure <<merge>>	ManagedContent <<merge>>
			Core
	Figure 1. Structure of SPEM2.0 [18]
	package introduces classes and abstractions that build the
	foundation for all other metamodel packages. The build-
	ing block of this package is the WorkDefinition class, which
	generalizes any work within SPEM2.0. The Process Struc-
	ture package defines elements for representing basic pro-
	cess models in terms of a flow of Activities with their
	WorkProduct Uses and Roles Uses. However, the possibil-
	ity to textually document these elements (i.e., add properties
	describing the element) is not provided in this package but
	in the Managed Content package, which provides concepts
	for managing the textual description of process elements.
	Examples of such concepts are the Content Description
	class and the Guidance class. The Method Content package
	defines core concepts for specifying basic method contents
	such as Roles, Tasks and WorkProducts. The Process with
	Method package defines the set of elements required for in-
	tegrating processes defined by means of Process Structure
	package concepts with instances of Method Content pack-
	age concepts. The Method Plugin package provides mech-
	anisms for managing and reusing libraries of method con-
	tents and processes. This is ensured thanks to the Method
	Plugin and Method Library concepts. Finally, Process Be-
	havior package provides a way to link SPEM2.0 process
	elements with external behavior models such as UML2.0
	Activity Diagrams or BPMN (Business Process Modeling
	Notation) models.

Table 1 . Mappings between XSPEM and BPEL

 1

	xSPEM		BPEL
	Activity (Outermost)	BPEL Process (next, a BPEL Sequence Activity
			is required to incorporate nested activities)
	Activity (Nested)	BPEL Invoke Activity with a Receive Activity
	Activity's	proper-	BPEL Variable
	ties (min-time and
	max_time, state, time)
	WorkProductUse	BPEL Variable
	RoleUse		BPEL Variable
	WorkSequence (Fin-	BPEL Sequence Activity
	ishToStart)
	WorkSequence (Start-	BPEL Flow Activity combined with the Link el-
	ToStart, StartToFin-	ement for Synchronization
	ish, FinishToFinish)
	ProcessParameter	BPEL Variable with attribute MessageType
			equals to the WorkProductUse used as an Ac-
			tivity ProcessParameter. If the Activity has
			more than one ProcessParameter than one Part
			(name=processParameterName) and its type is
			to be defined for each ProcessParameter whithin
			the MessageType.
	ProcessPerformerMap BPEL Variable

This work is supported by the IST European project MODELPLEX (contract n • IST-3408) and the TOPCASED project.

Eclipse Process Framework Project, http://www.eclipse.org/epf

http://eclipse.org/m2m/atl/usecases/SimplePDL2Tina

http://www.active-endpoints.com/active-bpel-engine-overview.htm

http://wiki.apache.org/agila/