Damien Voiry

Mengfei Qiao

Ying Wang

Xamxikamar Mamat

Anran Chen

Guoan Zou

Lei Li

Guangzhi Hu

Shusheng Zhang

Xun Hu

Rational Design of Hierarchical Porous Cobalt, Nitrogen-doped Carbon Architectures as Efficient Oxygen Reduction Reaction Electrocatalysts

Developing highly active non-precious metal catalysts for the oxygen reduction reaction (ORR) is of great significance to reduce the cost of fuel cells. Three-dimensional ordered porous structures could substantially improve the performance of the catalysts because of its excellent mass diffusion properties and high speci fic surface area. Herein, we prepared ordered porous ZIF-67 by forced molding of a polystyrene template and obtained cobaltsupported, nitrogen-doped three-dimensional ordered porous carbon (Co-NOPC) after further carbonation. The resultant Co-NOPC exhibits excellent performance for the ORR in an alkaline medium with a half-wave potential (E1/ 2) of 0.86 V vs. RHE, w hich is greater than that of the state-of-the-art Pt/C (0.85V vs. RHE). Moreover, the substantially improved catalytic performance of Co-NOPC compared to cobalt-supported, nitrogen-doped carbon (Co-NC) reveals the key role of its hierarchical porosity for boosting ORR. Co-NOPC also performs a close-to-ideal four-electron transfer path, long-term durability, and methanol penetration resistance, making it promising for large-scale application.

Introduction

In recent decades, issues such as energy shortage and environment al pollution due to the extens ive exploitation and utilization of fossil fuels have stimulated the development of new energy materials and energy conversion technologies. [1] With the advantages of high energy density, environmental friendliness, easy implementation, and unlimited sources of reactants, proton exchange membrane fuel cells ( PEMFCs) are expected to play an important role in next-generation energy technologies. [2] One major bottlenec k to the commercial application of PEMFCs lies in the insufficient activity and durability of the electrocatalysts to accelerate the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode. [3] For high-perfor mance catalysts, most theoretical and exper imental studies believe that the active sites should have an inter mediate adsorption strength for the reactant and its inter mediates to reduce reaction barrier, and the substrate should have a high specific surface area and rich pore structure, thus achieving rapid mass transfer on electrode surface. [4] Meeting all these conditions is a major bottlenec k in the search for ideal ORR catalysts. Notably, the control of the porosity in order to optimize both the surface area and the efficient diffusion of the O2 to the active s ites request the preparation of catalytic materials w ith hierarchical porosity. [5] State-of-the-art catalysts Pt/C, w herein Pt nanoparticles are supported on high-surface-area carbon, exhibit a relatively low overpotential and ideal four-electron trans mission path for the ORR. [6 ] How ever, the high cost, resource constraints, poor stability and nonresistance to methanol penetration, hinder its large-scale application, necessitating the search for a better alternative. [7] Based on current research, metal-nitrogen-car bon (M-N-C) catalysts, w here M denotes a transition metal (typically w ith M = Fe or Co), ar e the most promising low -cost substitutes for Pt-based catalysts. [8]

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activity among all M-N-C mater ials, but the Fenton reactions (Fe 2+ + H2 O2), w here dissolved Fe ions combine w ith H2 O2, w ill generate significant amounts of active oxygen-containing hydroxyl and hydroper oxyl radicals, thereby lead ing to the irreversible erosion and degradation of the electrodes and diaphragm. [9] Therefore, Co-N-C based catalysts may be a promis ing substitute for Pt/C w hen considering both, a high catalytic activity and operational stability over reasonable service life. [10] In order to achieve a breakthrough in perfor mance, many inspir ing w orks have focused on obtaining a perfect combination of Co-N-C structure and excellent substrate materials. [11] Recently, metal-organic framew orks (MOFs), w hich possess structural diversity, easy functionalization and high specific surface area, have been selected as a self -sacrificing precursor/template for preparing nanoporous carbons. [12] A mong the MOFs, Z IF-67, w hich consists of Co 2+ and 2-medetomidine, has received w ide attention ow ing to easy synthesis, high porosity of the zeolite-like topology, and particularly it could be an ideal source of carbon and nitrogen. The coordination structure of Co 2+ and 2-methylimidazole can be in situtransformed into a Co-Nx structure, and the pore structure can be w ell inher ited during the pyrolysis of ZIF-67, w hich endow s it w ith high intr insic activity and active site stability. [13] How ever, their catalytic activity is limited because of deep burial of the active sites and low mass transfer of inherit microporous structure of ZIF-67, w hich stimulates the researcher to resolve this problem by combining w ith excellent conductive and high surface area carbon materials such as graphene, car bon nanofibers etc.. [14] Shen et al. reported a kind of ZIF-67 and reduced graphene oxide derived carbon compos ite named Z IF-C/r GO, w here the r GO integration led to better charge transfer channels and mor e micropores to load active sites. [15] Yin et al.

synthesized Z IF-67/PA N by the in-situ gr ow th of ZIF-67 nanocrystals on PA N fibers, and further carbonized it at 800 °C to obtain Z IF-67/PA N-800, w hich also show ed significantly improved performance. [16] Although these attempts have improved the cata lytic performance of oxygen reduction to a certain extent, they have failed to achieved benchmarks from practical applications. Introduc ing macropores into Z IF-67 should be a good idea to achieve efficient utilization of active sites by exposing more surfaces, meanw hile, macro/mesopores w ill also improve the mass transfer of macromolecules near the double layer. [17] Her ein, w e demonstrate the successful synthesis of porous ZIF-67 ( OP-Z IF-67) w ith the accurate contr ol of the macro and microporos ity by developing an original approach that combines hard template method and ultrasonic nucleation. [18 ] The ordered MOF superstructure translates, after carbonization, into optimized hierarchical order macro-microporous carbon skeleton for electrocatalytic mater ials, enabling efficient diffusion of the reactant to the actives w hile maximizing the density of active sites. The resultant car bonized product, cobalt-supported, nitrogen-doped three-dimens ional ordered porous carbon ( Co-NOPC), exhibits enhanced catalytic activity and long-ter m stability tow ards ORR under both ac idic and alkaline conditions.

The half-w ave potential of the obtained Co-NOPC w as 0.86 V vs.

RHE

Results and Discussion

The method for the synthesis of Co-NOPC is illustrated in Figure Additionally, the Co-NOPC, obtained by the further carbonation of OP-Z IF-67 at 500 °C, w as characterized by SEM ( Figure 1b, f-h). Although there is a certain degree of collapses, the pore structure and morphology are vastly consistent w ith those of the precursor. The inter nal structure of OP-Z IF-67 character ized by HRTEM ( Figure 2a-c) further revealed that the macropores are regularly distributed throughout the single-crystal particles, which w ere engraved reversely by the PS template (Figure S2).

After thermal treatment at 500 °C, the crystal surface (Figure 2df) becomes no longer s mooth as before, and the hierarchical ordered macro-micropore tr ansform into inter-connected car bon skeleton. The relatively disordered carbon phase (Figure 2f) implies that abundant defects, such as vacancies, dislocations and shear defects in favor of ORR catalysis exist in the car bon structure. [19] The EDS mapping in Figure 2g demonstrate that no dramatic migration and aggregation of metallic Co occurs after pyrolysis, w hich is attr ibuted to the in-situ conversion of coordination of Co and N under mild conditions. [20] For compar ison, the morphology of conventional Z IF-67 and carbonation derivative ( Co-NC) w ere also investigated in Figure S3. 100) planes of the graphite structure. [22] Moreover, there are no significant metal peaks in XRD, w hich indicates that no obvious metal nanoparticles w ere formed after annealing at 500 °C. pyrolysis. These results show that the introduction of macropores has no significant effect on the formation of micropores and mesopores. We believe that the mesoporosity originates from the structure of the Z IF-67 and for ms dur ing the graphitization process. How ever, a higher specific surface area of Co-NOPC (418 m 2 g -1 ) compar ed w ith that of Co-NC (214 m 2 g -1 ) confir ms that ordered macropores are responsible for the increased specific surface area. Raman spectra w ere analyzed to character ize the change in the carbon phase w ith the synthesis temperature ( Figure S6). The higher ID/IG ratio of Co-NOPC-500 reveals that an over-high temperature w ill reduce the defect positions w hich are typically active site centers. [25] This result agrees w ell w ith that of XRD, in that Co 2+ w ill be quickly reduced to Co 0 w ith increasing temperatur e, and that the recombination of chemical bonds w ill also reduce the number of active sites. [26] The catalysts obtained at different temperature w ere characterized and compared by X-ray photoelectron spectroscopy (XPS). [27] The survey of Co-NOPC ( Figur e S7, Table S1 and Table S2) (779.1 eV). [28] Co 2+ is know n to be more beneficial to the catalytic activity w hen in the form of coordinated Co-Nx obtained through the in-situ transfor mation of the coordination structure of Co 2+ and 2-methylimidazole. On the other hand, Co 0 , obtained through the reduction product of Co 2+ , tends to agglomeration and lose its ORR activity. [29] The relative proportion of Co 2+ over Co( 0) reduces from 91.5% to 68.6% w hen the annealing temperature increases from 500 °C to 700 °C, verifying the advantage of mild-temperature graphitization ( Figure 4c). To

reveals

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abilities. [31] In addition, part of pyridinic N and pyrrolic N atoms can serve as coordination sites for Co in the form of Ncoordinated Co structures: Co-Nx (x = 2 and 4) as prev iously identified using X-ray absorption spectroscopy (XAS) and

Mössbauer spectroscopy[. [19,32] Density functional theory ( DFT)

in

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leading to more exposed active s ites and accelerated mass transfer. [33] The influence of the hierarchical por osity is c learly Co 0 , implying a significant reduction of active sites Co-Nx ( Figure 4d) w hen the annealing temperature exceeded 600 °C, w hich matches w ell w ith the XPS ( Figure 4) and Raman spectra observations (Figure S6), w here the defect sites in the carbon phase decrease w ith increasing temperature. [34] In addition, the reductions of specific surface area and average pore size w ith enhanced temperature (Figure S5) suggest that higher temperature is responsible for the partial shrinkage of the mater ial structural frame resulting on the r educed mass transfer.

Besides the annealing temperature, the mass ratio of

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To verify the practical applications of the catalys ts, chronoamperometric measurements w ere used to evaluate their selectivity and long-ter m durability in O2-saturated 0.1 M KOH solution at the half -w ave potential of 0.86 V. [35] For long-ter m durability, Co-NOPC exhibits minimal loss of activity w ith a retention of 85% after 24 h, w hich is significantly larger than commercial Pt/C that exhibits an 69% retention, corresponding to 2-fold larger loss of activity ( Figure 5e). [36] To assess the consequences of methanol crossover on the performance of Co-NOPC, 10 mL of methanol w as instantaneously added to 100 mL of 0.1 M KOH solution to investigate methanol tolerance of catalysts during the chronoamperometric measurements (Figure 5f). [37] Upon the addition of methanol, the current of commercial The electron transfer number and the hydr ogen peroxide yield calculated based on the corresponding RRDE data.

Conclusions

In summary, w e demonstrated the synthetic method of ordered porous Z IF-67 using hard template and ultrasonic

Experimental Section

Materials. All the chemicals used in the experiment were analytical purity and no further purification was made. was u sed to evaluate catalytic activity as benchmark for ORR. Deionized water (≥18.2 MΩ) was prepared using a Simplicity®UV system and used for the preparation of all aqueous solution.

Synthesis of 3D ordered PS template. The 3D ordered polystyrene sphere (PS) template with a particle diameter of 110 nm were synthesized according to the following procedure. Firstly, 50 mL of washed styrene, 400 mL of deionized water and 1.5 g of PVP were added into a 1 L flask. Then the mixture was bubbled with nitrogen for 10 min and stirred at 92 °C for 30 min. Subsequently, 50 mL of 1 wt.% K2S2O8 was added into the flask to initiate the polymerization reaction of styrene. After keeping stirring at a rotation rate of 400 rpm for 24 h at this temperature, the mixture was cooled down and the obtained milk-like product was the monodispersed colloidal polystyrene spheres. The monodispersed polystyrene spheres were further filtered to form filter cakes. Finally, the 3D ordered PS templates were obtained after further washing with deionized water and drying at 60 °C for overnight.

Synthesis of OP-ZIF-67 and

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  PAPER the Co-Nx active sites obtained by the in-situ conversion of Co 2+ and 2-methylimidazole complexes confer to the catalyst high intrinsic activity and long-term stability. To get information on the morphology and structur e, the pristine OP-Z IF-67 and the carbonized Co-NOPC w ere investigated by field-emission scanning electron microscopy (FESEM) and high-resolution trans mission electron microscopy (HRTEM).

  Figure 1c-e show s the typical SEM images of OP-Z IF-67 at various magnifications. OP-Z IF-67 is found to adopt a w elldefined rhombic dodecahedron morphology w ith a particle size of 1 μm, w here macropores of approximately 110 nm are regularly distributed throughout the single-crystal particles.

Figure 3a show s

  Figure 3a show s the X-ray diffraction (XRD) patterns of Z IF-67, OP-Z IF-67 and simulated ZIF-67. The cons istency of peak types among the three samples confirms that ZIF-67 and OP-Z IF-67 both formed a complete crystal structure w ith no impurity phase in the materials. [21] The XRD patterns of the final samples of Co-NOPC and Co-NC are show n in Figure 3b. The diffraction peaks appear ing at approx imately 23.3° and 43.5° can be assigned to the (002) and (100) planes of the

Further

  Figure 1. (a) Schematic of the Co-NOPC synthes is. (b) SEM images of the Co-NOPC. (c-e) SEM images of OP-ZIF-67. (f-h) SEM images of Co-NOPC.

Figure 2 .Figure 3 .

 23 Figure 2. (a-c) HRTEM images of OP-Z IF-67. ( d-f) HRTEM images of Co-NOPC. (g) EDS mapping of Co-NOPC.

  the presence of C 1s, N 1s, O 1s, and Co 2p elements, w ithout any impurities. The content of N 1s, O 1s, and Co 2p decrease w hile the content of C 1s increases w ith the increasing pyrolysis temperature. The highresolution XPS spectrum of Co 2p 3/2 (Figure 4a) can be fitted into tw o components, corresponding to Co 0 (776.9 eV) and Co 2+

  precisely deter mine the close relationship betw een active site centers and chemical environ ment of the doped nitrogen, the N 1s spectra of all the samples w ith different pyrolysis temperatures w ere analyzed. Figure4bshow s that the highresolution N 1s spectrum split into four subpeaks at 397.3 eV, 398.1 eV, 399.2 eV and 401.6 eV, corresponding to pyridinic (Py-N), pyrrolic ( Pyr-N), graphitic like (Gra-N), and oxidized type (Oxi-N) nitrogen, respectively. According to the liter ature, Py -N and Gra-N can effectively boost oxygen adsorption to r educe the ORR overpotential because of their excellent electron-accepting abilities. 30] According to the literature, Py-N and Gra-N can effectively boost oxygen adsorption to reduce the ORR overpotential because of their excellent electron-accepting 10.1002/cssc.201903053 Accepted Manuscript ChemSusChem This article is protected by copyright. All rights reserved.

  agreement w ith exper imental studies has suggested that Co-Nx has a low er energy barrier for oxygen dissociation and thus acts as active s ite in the catalytic ORR process from Co-N-C based catalysts[10b, 1 1, 29] As show n in the Figure4d, the ratios of Py -N and Pyr-N in Co-NOPC r apidly decrease w ith the increase of annealing temperature. On the other hand, the rapid grow th of Gra-N signals at increasing annealing temperature ver ifies the results of Raman spectra. In order to optimize catalytic performance, w e adjusted the ratio of precursors for the synthesis of Co-NOPC. A series of OP-Z IF-67 typed mater ials was synthesized w ith different ratios of Co 2+ and 2methylimidazole precursor and named as OP-Z IF-67-n:m, w here n:m represents the feeding mass ratios of Co( NO3)2•6H2 O:2methylimidazole. We found that the r egular single crystal structure of ZIF-67 w ith ordered porosity w as only formed at the mass ratios of 3:4 of Co( NO3)2•6H2 O:2-methylimidazole. Our analyses also reveal that a low cobalt mass ratio does not lead to the grow th of single crystal MOF particles w hile a high cobalt mass ratio may lead to the uncontrolled shapes of crystal particles (Figure S8). Further more, w e evaluated the effect of feed amount of Co( NO3)2•6H2 O to active s ites using XPS ( Figure S9). The proportion of Co-Nx also tends to increase w ith increasing Co content, and reaches the maximum w hen the ratio of Co( NO3)2•6H2 O and 2-methy limidazole is 3:4. In addition, compared to contr olled sample Co-NC, Co-NOPC-3:4 had a higher fitting Co 2+ /Co 0 ratio, w hich might be related to a better coordination structures. All these ev idences suggest that Co-NOPC-3:4 is the most suitable candidate for the oxygen evolution reaction. The catalytic performance of Co-NOPC and Co-NC samples for ORR w ere firstly evaluated in O2-saturated 0.1 M KOH on a rotating disk electrode ( RDE) at a sw eep rate of 10 mV s -1 and a rotation rate of 1600 rpm. The cyclic voltammetry (CV) measurements w ere employed to evaluate the ORR catalytic activity of the as-prepared catalysts and the benchmar k Pt/C (20 wt.%) catalyst (Figur e S10). The CV curves of all the tested catalysts show ed obvious oxygen reduction peaks in O2saturated electrolytes, w hereas featureless CV curves w ere obtained in N2-saturated electrolytes. Notably, the more pos itive reduction peak potential of Co-NOPC (0.83 V), compared to 0.80 V and 0.83 V (vs. RHE) of Co-NC and Pt/C, respectively, suggests higher ORR catalytic activ ity. To further assess the ORR activ ities of the tested catalysts, w e recorded the linear sw eep voltammetry (LSV) polarization curves (Figure 5a). The onset potentials (Eonset) of Co-NOPC, Co-NC, and Pt/C w ere 0.95 V, 0.92 V, and 0.98 V (vs. RHE) respectively. The potential at w hich the reduction current sharply decreases on the LSV curves is know n as Eonset. Experimentally, w e used cross-point potential of tangent lines from the ris ing current and baseline current to deter mine the onset potential of the reaction. The halfwave potential ( E1/2) of Co-NOPC (0.86 V) is obv iously improved compared to that of Co-NC (0.82 V) and even compares favorably to commercial Pt/C catalyst (0.85 V). Additi onally, the Tafel slopecalculated from the LSV curvesof Co-NOPC (72 mV) is significantly low er than that of Pt/C (91 mV) confir ming Co-NOPC is an efficient ORR catalyst w ith improved reaction kinetics compared to Pt/C ( Figure5b). The enhanced steadystate diffusion current dens ity (jL = 5.2 mA cm -2 ) of Co-NOPC is comparable to that of commercial Pt/C (5.3 mA c m -2 ). The low er overpotential and lar ger limit current density from Co-NOPC is attributed to its ordered hierarchical macro/meso/microporosity

Figure 4 .

 4 Figure 4. The fine spectra of (a) Co 2p and (b) N 1s of Co-NOPC synthesized at different temperatures. (c) Corresponding fitting mass ratio of Co 2+ and Co 0 . (d) Corresponding fitting mass ratio of N w ith different chemical structures.

  noticeable w hen comparing the steady state current from Co-NOPC and Co-NC. On the other hand, the s mall Tafel slope and the positive Eonset may originate from the high intrinsic activity of Co-Nx (the coordinated structur e of Co w ith the pyrrolic/pyridinic N), w here N-coordinated Co sites serve as the ORR catalytic active sites. To further improve the catalytic activity, w e explored the influence of the annealing temperature and of the ratio of precursors: Co( NO3)2•6H2 O and 2-methylimidazole. The control of annealing temperature is indeed likely to modify the for mation of active sites and carbon phase of the substrate material.

Figure

  Figure S11 show s that both the onset potential and the halfwave potential ar e reduced w ith increasing annealing

  Co( NO3)2•6H2O:2-methylimidazole also influences the density of active sites because the coordination structure of ZIF-67 w ill translates, at the end, into tunable catalytic activity. Our analyses of the relationship betw een pyrrolic/pyridinic N content (Figure S9c) and the half -w ave potential ( Figure S12) support the fact that Co-Nx active sites play a central role in deliver ing high ORR catalytic performance. Next, w e investigated the reaction mechanis m from Co-NOPC by deter mining the number of electrons involved in the reaction of oxygen. According to the Damjanov ic model, a direct 4e - pathw ay, involving reduction of O2 to H2 O, is more efficient than a 2e -pathw ay, in w hich O2 is reduced to H2 O2 and further to H2 O sequentially. Hence LSV curves tested at var ious rotating speeds w ere used to evaluate the ORR kinetics of Co-NOPC and Pt/C (Figur e 5c). The fitted Koutecky-Levich (K-L) plots of Co-NOPC r anged from 0.78 to 0.58 V (vs. RHE) indicates that the reaction on Co-NOPC follow s the desirable 4e -reaction pathw ay and negligible for mation of H2O2 during the ORR process (Figure 5d). The electron transfer mechanis m and the formation of peroxide inter mediates on Co-NOPC during ORR were also monitored by rotating ring-disk electrode ( RRDE) measurements. Based on the recorded ring and disk and r ing currents (Figur e S13a, b), the average tr ansferred electron number (n) w as 3.93 and the yield of H2 O2 is limited to less than 5% (Figure S13c, d), proving that the four-electron reaction pathw ay is proceeding under alkaline conditions.

Figure 5 .

 5 Figure 5. (a) ORR polar ization curves of samples in O2 saturated 0.1 M KOH solution w ith a scan rate of 10 mV s -1 and (b) Tafel plot of corresponding polarization curves. (c) LSV curves of Co-NOPC at different rotating speeds w ith a scan rate of 10 mV s -1 and (d) the corresponding Koutecky-Levich plots. (e) Chronoamperometric response and (f) methanol crossover tests chronoamperometric response of Co-NOPC and Pt/C at half-w ave potential in 0.1 M KOH.

Figure 6 .

 6 Figure 6. (a) ORR polar ization curves of samples in O2 saturated 0.5 M H2 SO4 solution w ith a scan rate of 10 mV s -1 and (b) the Tafel plot of corresponding polarization curves . (c) The dis k current and the ring current of Co-NOPC and Pt/C. (d)

  nucleation, follow ed by carbonization to obtain cobalt-supported nitrogen-doped three-dimensional ordered porous carbon w ith controlled hierarchical poros ity. Compared to Co-NC derived from the or iginal Z IF-67, the three-dimensional ordered porous structure of Co-NOPC display ordered hierarchical macromicroporous structure w ith higher specific surface area to maximize the density of exposed active sites w hile allow ing excellent mass transfer. ORR performance tests show ed that 10.1002/cssc.201903053 Accepted Manuscript ChemSusChem This article is protected by copyright. All rights reserved.

  w ave potential of the final catalyst Co-NOPC (0.86 V) surpasses that of the state-of-the-art Pt/C catalyst (0.85 V) in alkaline solutions. Remarkably, the ORR activity in acidic have demonstrated a substantial improvement of the ORR for Co-NOPC compared to Co-NC. The super ior ORR activity or iginates from the optimization of the density of Co-Nx active sites in the mater ials and the controlled hierarchical porosity of the structure that induce both improved half -w ave potential and diffusion limited current density. Long-ter m tolerance and stability against methanol crossover in acidic and basic media make Co-NOPC an affordable alternative of Pt/C. Overall our r esults demonstrate a novel practical strategy for modifying MOF-derived mater ials for ORR. We expect such strategy can be extended to other electrocatalytic reactions for w hich the perfor mances are limited by poor diffusion of the reactants to the active sites.

  Co-NOPC.4.5 g of Co(NO3)2•6H2O and 6 g of 2-methylimidazole were added to 60 mL of methanol and fully mixed by stirring. Then, the PS template was soaked in the above solution and treated by ultrasound for 30 s to nucleate. After 24 h of static growth at room temperature, the bulk composite of ZIF-67@PS was formed after further dried at 40 °C. The obtained ZIF-67@PS was then dissolved in THF for 12 h to removed PS. After multiple centrifuging, washing and drying at 110 °C, the ordered porous ZIF-67 (OP-ZIF-67) was collected.Under nitrogen, the blue precursor was carbonized at 500 °C for 2 h to obtain cobalt-supported, nitrogen-doped 3D ordered porous carbon (Co-NOPC). For comparison, control samples, named as Co-NOPC-1:4, Co-NOPC-2:4, Co-NOPC-3:4, Co-NOPC-4:4 were prepared by adding Co(NO3)2•6H2O (n) and 2-methylimidazole (m) with different mass ratios: n:m. Co-NOPC-500, Co-NOPC-600 and Co-NOPC-700 was synthesized with the same mass ratio at 3:4 of Co(NO3)2•6H2O and 2-methylimidazole but different temperatures of 500 °C, 600 °C and 700 °C, respectively.Synthesis of ZIF-67 and Co-NC. For comparison, conventional ZIF-67and carbonation derivative (Co-NC) were also synthesized according to the following procedure. Firstly, 4.5 g of cobalt nitrate hexahydrate and 6 g of 2-methylimidazole were dissolved in 250 mL of methanol. After full mixing, the mixed solution was treated under ultrasonic for 20 min to nucleate and then stirred at room temperature and pressure for 12 h. Finally, ZIF-67 was obtained by several times washing with methanol, centrifugal collection and drying at 110 °C. The precursor of conventional ZIF-67 was annealed at 500 °C for 2 h with nitrogen protection to obtain Co-NC. Structural characterization. Morphology and the microstructure of the prepared samples were characterized on a field emission scanning electron microscope (SEM; Zeiss Supra55 VP, Germany) and a highresolution transmission electron microscope (HRTEM; Bruker Nano GmbH Berlin, Germany), which were both equipped with an energy dispersive X-ray spectroscope (EDS). Wide-angle X-ray diffraction (XRD) was performed by a Bruker D8 Advance X-ray diffractometer (Germany) with Cu Kα (λ = 0.15418 nm). Nitrogen adsorption -desorption measurements were performed on an Autosrob-1 (Quanta chrome INSTRUMENTS). The pore size distributions and specific surface areas were calculated by BJH method based on desorption branch , and the Brunauer-Emmett-Teller (BET) method, respectively. Raman spectroscopy measurements were characterized with a high -resolution laser confocal microscopy Raman spectrometer (Horiba evolution) with a 532 nm excitation laser. X-ray photoelectron spectroscopy (XPS) was recorded on a Thermo SCIENTIFIC ESCALAB 250 using 150 W Al Kα radiation. Electrochemical characterization. All electrochemical measurements were carried on PGSTAT 302N (Metrohm Auto Lab) and a standard electrochemical cell (Pine Instrument) in an O2 saturated electrolyte at room temperature (18-25 °C). The counter and reference electrodes were a platinum wire and Ag/AgCl/3M KCl, respectively. RHE calibration was performed in the high purity hydrogen saturated electrolyte with a Pt foil as the working electrode and a Pt wire as the counter electro de. Cyclic voltammetry (CV) was ran at a scan rate of 1 mV s -1 , and the average of two potentials at which the current cross zero was ta ken as 10.1002/cssc.201903053 Accepted Manuscript ChemSusChem This article is protected by copyright. All rights reserved.

  the hydrogen electrode reactions (FigureS1). The potential measured against an Ag/AgCl electrode was converted to the potential versus the reversible hydrogen electrode (RHE) according to E(RHE) = E(Ag/AgCl) + 0.976 V in 0.1 M KOH, and E(RHE) = E(Ag/AgCl) + 0.240 V in 0.5 M H2SO4. All data were presented without IR compensated. To prepare the working electrode, 10 µL catalysts inks (6 mg mL -1 ), prepared by dispersing 6 mg of catalysts (including Pt/C) in 1 m L of mixture solution (water: isopropanol = 1:2) with 0.1 wt% Nafion as binder, was deposited on the polished Φ5 mm glass carbon electrode (RDE, Pine Instrument). Linear sweep voltammetry (LSV) was used to characterize catalytic activity with sweep rates of 10 mV s -1 at a rotation rate of 1600 rpm in the O2-saturated solution. Then the ORR polarization curves were corrected by subtracting the background current measured under N2 atmosphere. The durability of catalysts was evaluated by performing chronoamperometric measurements at half-wave potential for 24 h. To evaluate the methanol tolerance of catalysts, 10 mL of methanol was added into 100 mL of electrolyte during the chronoamperometric measurement. For the Tafel plot, the kinetic current was calculated from the mass-transport correction of RDE by: Calculation of electron t ransfe rred nu mbe r fo r O RR. The electron tran sfe r numbers pe r oxygen molecule for oxygen reduction in 0.1M KOH and 0.5M H2SO4 a re calculated by Koute chy -Levich (K -L) j where j is the measured current density; jL is the diffusion limited current; jK is the kinetic current density; ω is the electrode rotation rate; B is the slope of the K-L plots; n represents the transferred electron number per oxygen molecules; F is the Faraday constant (F = 96485 C mol -1 ); D is the diffusion coefficient of O2 in 0.1 M KOH solution (D = 1υ is the kinetic viscosity of the electrolyte (υ = 0.01 cm 2 s -1). C0 is the bulk concentration of O2 (C0 = 1.2×10 -6 mol cm -3 ). The total electrontransfer number (n) and hydrogen peroxide yield (%H2O2) were determined by the RRDE approach using: and jR are the voltammetric currents at the disk and ring of electrode, respectively. N determined to be 0.37 is the RRDE collection efficiency determined.

  Fe-N-C catalysts show the highest [a] M.F.Qiao, Y .Wang, X. Mamat, Prof . G.A.Zou, Prof . G.Z.hu Key Laboratory of Chemistry of Plant Resources in Arid Regions State Key Laboratory Basis of Xinjiang Indige nous Medicinal Pla nts Resource Utilization, Xin jiang Technical,Institute of Phy sics and

			[d]	Prof . S.S.Zhang
				College of Chemistry and Molecular Engi neerin g, Zhengzhou
				Univ ersity Zhengzhou, 450000, China.
			[e]	Prof . X.Hu
		Chemistry ,Chinese Academy of Science		School of Material Science and Engineeri ng,Univ ersity of Jinan, Jinan
		Urumqi 830011, China.		250022, China.
		Emali : guangzhihu@y nu.edu.cn	[f ]	Prof . D. Voiry
	[b]	Dr. A.R.Chen, Prof . G.Z.hu		Institut Européen des Membranes, IEM, UMR 5635, Univ ersité
	[c]	School of Chemical Science and Technology , School of Energy , Y unnan Univ ersity ,Kunming 650091, China Prof . L.li, Prof . G.Z.hu College of Biological, Chemical Sciences and Engineerin g, Jiaxing	[g]	Montpellier, ENSCM, CNRS, 34095 Montpellier Cedex5, France Emali: damien.v oiry @umontpellier.f r M.F.Qiao
		Univ ersity
		Emali : leili@mail.zjxu.edu.cn

Univ ersity of Chinese Academy of Sciences, Beijing 100049, China.

Acknowledgements

This w ork w as supported by the National Natural Science Foundation of China (21677171, 21978111).

Keyw ords: Cobalt • Electron transfer • Fuel cell • Oxygen reduction reaction • ZIF-67

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A hierarchical porous cobalt, nitrogen-doped carbon were obtained by pyrolysis of ordered porous ZIF-67, that is prepared by forced molding of a polystyrene template. The hierarchical porosity result in a significant improvement in the performance of ZIF-67 derived carbon to oxygen reduction reaction in both alkaline and acidic conditions.

xt for Table of Contents


R F Orthous-Daunay

C Wolters

V Vuitton

J Isa

H Naraoka

ORBITRAP-MS AND CHROMATOGRAPHY IN PREPARATION FOR HAYABUSA2 MOLECULAR COMPLEXITY ANALYSES

Introduction: High-resolution mass spectometry (HRMS) is a powerful tool for unraveling the molecular content of mixtures [3]. Chromatography on its side is unmatched for molecular identification. Both have helped with the diversity and the polymeric structure of soluble organic matter in meteorites and laboratory analogs [1,4]. The HRMS indeed is able to detect each redundant mass differences and group masses by polymeric families. The chemical separation based of the retention in chromatographic columns helps identifying isomers that can have completely different molecular structure one another. Therefore, the combination of the two allows to tell if the synthesis patterns and the final composition are of one single process or if there is a complex history at the origin of a given mixture.

For instance, the evolution on the average aliphatic chains length can be related to growing of destructing processes. Given homologuous families (one common molecular structure with one or several aliphatic parts of variable length) have already been described in carbonaceous meteorites, the variability of the chains length is a test for the co-synthesis of the whole mixture [2].

The use of this techniques for priceless samples like regolith from the surface of Ryugu is high risk for high reward, with definitive answer on the distribution of organic molecules in the solar system. Here we present preliminary results concerning the rehearsal measurements done on Murchison with a LC-HRMS instrument.

Methods: The goal is to compare data from macrosample, here grams of Murchison to micro-sample. Few milligrams of Murchison were crushed manually and macerated in different solvents sequentially from apolar (dichloromethane) to polar (methanol). The extracts were then infused in a C-18 reversed phase column with a classical acetonitrile to water elution gradient. Both for macro and micro sample, a minimal consumption Nano-LC pump was used in line with an Exactive-Orbitrap mass spectrometer. High resolution mass spectra were acquired every ~1 seconds for 40 minutes at Kyushu University. This type of column and gradient are to separate mixtures by the polarity of their components. This strategy suits the Murchison mixture in the sense the dominant variability is the chain length which will control the interaction with the column and determine the retention time of each compound.

Data analysis were done using the ATTRIBUTOR house made software to represent and evaluate the mass distributions, ions formulas and their retention time. It provides a quick overview of the chromatographic behavior of the mixture through the so-called ion map plotting the abundance (color coded) of each peak in the retention time/mass space (fig. 1). For any domain of this ion map, the classical tools for HRMS are available such the Mass Defect vs Mass analysis and the stoichiometric network. New development on the automatic peak classification will be shown and used to ease the interpretation.

Results: For the macro sample, dichloromethane (DCM), methanol and water extracts are available for Murchison meteorite. DCM and methanol extracts have different yet comparable chromatograms. As expected, the column separates molecules by polarity. Two major groups are observed and can attributed to O-rich and O-poor distributions. The water extract has a very different chromatogram were the column separation is moderate. In the case of extreme polarity, the hydrophobic column may not be the best choice to investigate chemical diversity. Another type of column is considered to go further in highly polar extracts.

For the microsample of Murchison, only the methanol extract is available for comparison and more details will be discussed as the data acquisition is complete.

The figure 1 shows the ion map associated with the methanol extract and clearly shows several groups of compounds. The one with the lowest retention times (below 1000 seconds) are related to the compounds poorly retained by the hydrophobic column and therefore interpreted as the most polar molecules. They indeed exhibit typical negative mass defect patterns identified in direct infusion spectra. This is consistent with higher amount of oxygen bearing functions. They are of lower abundance compared to the other group with higher retention times (above 1000 seconds). Indeed, the group corresponds to the less polar compounds and exhibit a correlation between the retention time and the mass/charge. This is interpreted as the chain length variability identified in direct infusion spectra. This shows that the different families span a common mass and retention time (polarity) range.

The CH 2 distributions found in the bulk, direct infusion, spectrum are present and can be compared from one retention time to another. This tends to prove that retention time is controlled by the aliphatic chains variability, more than the heteroatomic functions attached to. Further investigations on the fine distribution of the CH 2 patterns will tell if the molecular growth process is the same from one homologous family to another.

Figure 1 :

 1 Figure 1: Ion map of the micro-sample of soluble in methanol Murchison extract. The denser part with retention time above 1000 seconds shows families of variable chain length, the latter being responsible of the correlation between polarity (increase of retention time) and mass.


Riccardo Cantoro

Nikolaos I Deligiannis

Matteo Sonza

Marcello Traiola

Emanuele Valea

Evaluating the Code Encryption Effects on Memory Fault Resilience

In most safety-critical systems, the robustness and the confidentiality of the application code are crucial. Such code is generally stored into Non-Volatile Memories (NVMs) that are prone to faults (e.g., due to radiation effects). Unfortunately, faults affecting the instruction code result very often into Silent Data Corruption (SDC). This condition lets faults remain undetected and it can lead to undesiderable errors that may compromise the system functionality. Thus, it is desirable that the system is able to detect faults affecting the code memory. To overcome this issue, designers often resort to expensive error detection/correction mechanisms. Furthermore, they also adopt memory encryption techniques to prevent unauthorized, hence malicious, access to the code or to protect it from any unauthorized copy. In this paper, we show that the presence of memory encryption alone is able to strongly reduce the probability of SDC, without the need of implementing expensive error detection.

We have performed some experiments on the OpenRISC1200 microprocessor in order to evaluate the impact on reliability stemming from different encryption methods.

I. INTRODUCTION

Designers of Embedded Systems are used to face many challenges: among them, it is increasingly common to find both safety and security. In fact, in many domains where embedded systems are used (e.g., automotive, biomedical, aerospace) the effects of possible failures due to faults affecting the hardware must be considered, and suitable safetyoriented techniques must be adopted in order to minimize the chances of faults (e.g., acting on the adopted semiconductor technology) or to minimize the impact of their effects (e.g., by introducing redundancy in the design). On the other side, in the last years there has been an exponentially growing interest (and concern) for the security-oriented techniques suited to prevent possible attacks that can deliberately affect the system behavior to change it (in the extreme case, blocking the system, or forcing it to perform illegal operations), or to extract private and/or precious information.

Focusing on security, a common issue which is often faced when designing embedded systems is the one of protecting the application code from possible attacks aimed at stealing it (e.g., to better understand the adopted algorithms for copying purposes, or to allow more intrusive attacks in the following).

For this purpose, in these cases the application code is often encrypted when stored in the memory (often a Non Volatile Memory, or NVM) and then decrypted before execution, or when the code is transferred to a RAM [START_REF] Yang | Improving memory encryption performance in secure processors[END_REF]. In this way the stored version of the code can hardly be used and understood, even if downloaded from the NVM. A suitable hardware block to execute the decryption is thus included within the circuit and activated when required. Several issues related to efficiently manage an encrypted NVM storing the application code are discussed and solved in [START_REF] Ye | Towards low-cost mechanisms to enable restoration of encrypted non-volatile memories[END_REF].

On the other side, since NVMs are prone to errors (e.g., caused by radiation effects), designers adopt redundancy solutions able to detect the occurrence of single-and multiplebit errors and to possibly correct some of them. The most common solution is based on Error Correction Codes (ECCs) able to detect and possibly correct errors up to a given multiplicity [START_REF] Xiao-Bo | Novel ecc structure and evaluation method for nand flash memory[END_REF]. The adoption of ECCs involves both hardware and performance overhead, because they require computing some extra bits each time the memory is written, and recomputing/checking them each time the memory is read. Moreover, the error detection/correction capabilities of ECCs depend on the number of errors they can manage, and the overhead quickly becomes unacceptable when multiple faults, other than double ones, are considered. Alternative solutions to ECCs have also been explored, based on suitably selecting the instruction opcodes, so that the chances that the processor can trigger an exception when a faulty instruction is fetched are maximized [START_REF] Martínez | A scheme to improve the intrinsic error detection of the instruction set architecture[END_REF]. However, this solution requires acting on the design of the CPU core, which is often impossible for system companies using for their applications processors developed by other companies.

Our work started from the observation that when a fault affects the code memory, it is likely that the decryption mechanism produces a code which triggers some fault detection mechanism in the processor (e.g., an illegal instruction exception). Hence, encrypting the code may turn into a mechanism that detects possible errors in the code memory content. The attractive consequence of this observation is that, if confirmed, it allows an existing mechanism to be used for both security and safety, thus reducing costs. In order to quantify this phenomenon, we performed a number of experiments on the OR1200 processor [START_REF] Community | OpenRISC[END_REF] with different encryption mechanisms and different applications. Results show that by suitably se-lecting the encryption mechanism, a significant percentage of single-and multiple-bit faults can be detected. Hence, a high error detection capability can be achieved without resorting to any Error Correction/Detection mechanism for the memory.

As a result, a major contribution of this paper lies in practically demonstrating that security-oriented mechanisms can be used for detecting at no extra cost hardware faults affecting the code memory.

The rest of the paper is organized as follows. Section II briefly describes the underlying theory of symmetric encryption and why it is applied to embedded system memories. Section III introduces the reference scenario of this work and details the proposed contribution. Section IV illustrates the experimental setup and the related results obtained. Finally, Section V draws the conclusions.

II. BACKGROUND

In this Section, we provide the background on memory encryption which is required to let the reader appreciate the evaluation proposed in the following of the paper. At first, we summarize the theory of symmetric encryption, introducing the two main categories of encryption mechanisms. Secondly, we explain how these principles are applied to the practical case of securing embedded system memories.

A. Symmetric Encryption

In cryptography, encryption is used to grant confidentiality of exchanged data between a sender and a receiver [START_REF] Menezes | Handbook of Applied Cryptography[END_REF]. Let us imagine a communication scheme where a sender wants to send a private message to a receiver. The incumbent security threat is that an attacker could eavesdrop the communication channel and potentially steal the content of the data. In order to protect the message, the sender and the receiver can rely on encryption, whose advantage is making the transmitted message unintelligible for the attacker. In particular, symmetric encryption schemes are based on the fact that the sender and the receiver share the same secret key.

The plaintext message m is encrypted by the sender using the secret key k and the encryption function E, in order to produce the ciphertext c = E(k, m). The ciphertext c is sent on the communication channel, but no attacker is able to deduce m from c without knowing the secret key. The receiver can easily retrieve the plaintext m using the secret key and the decryption function D. Indeed, m = D(k, c).

The couple (E, D) is called cipher. A cipher is defined as semantically secure, if it is not possible for an attacker to retrieve the plaintext merely observing the ciphertex. Most common ciphers can be classified into two categories: block ciphers and stream ciphers.

1) Stream Ciphers: stream ciphers are based on a theoretical cipher, called One Time Pad (OTP). In the OTP, the secret key must be as long as the message m. The encryption function is defined as E(k, m) = m ⊕ k, and the decryption function as D(k, c) = c ⊕ k. Normally, the plaintext and the ciphertext are processed as bitstreams. The key k is called keystream and it is combined one bit at a time with the plaintext to produce the ciphertext stream.

If the keystream is perfectly random (i.e., according to the uniform distribution), the OTP has perfect secrecy. This means that the produced ciphertext is indistinguishable from a random sequence (this is due to the properties of the XOR operation). In this case, it is impossible for an attacker intercepting the ciphertext to derive any information neither on the message nor on the key. However, from a practical point of view, the OTP is not implementable because the sender and the receiver should share a secret key having the same size as the message.

Stream ciphers are the implementation of the OTP. In stream ciphers the keystream is produced by a Pseudo-Random Generator (PRG). The PRG takes as input a value k, called seed of the stream cipher, and outputs the keystream S(k). The encryption and decryption functions are thus defined as

E(k, m) = m ⊕ S(k) and D(k, c) = c ⊕ S(k).
Therefore, the secret key that is exchanged between the sender and the receiver is the seed of the PRG (which is much shorter than the resulting keystream). As long as the PRG produces a keystream that is unpredictable, the resulting stream cipher is considered to be secure. In Fig. 1(a) the high-level structure of a typical stream cipher is represented.

Stream

For the purpose of this paper, it is worth pointing out that in stream cipher encryption, we always find a bit-tobit correspondence between plaintext and ciphertext. In fact, a one-bit difference on the plaintext leads to the same bit difference on the ciphertext.

2) Block Ciphers: block ciphers are based on mathematical objects called Pseudo Random Permutations (PRPs). They are invertible functions that take as input an n-bit value m and a secret key k, and output an n-bit value c. A PRP is considered secure if it is indistinguishable from a random bijective function on n-bit values. Block ciphers implement a secure PRP. They are made of (i) an encryption function that is able to encrypt a plaintext block into a ciphertext block using a secret key; (ii) a decryption function that performs the inverse operation and retrieves the plaintext block from the ciphertext.

Block cipher functions perform confusion and diffusion of the plaintext on the ciphertext. This means that two very similar plaintexts result in completely different ciphertexts after the encryption.

In Fig. 1(b) the high-level structure of a typical block cipher is represented.

In block cipher based encryption the plaintext and the ciphertext have a block per block correspondence. This means that the modification of one bit on the plaintext leads to the perturbation of an entire block of ciphertext.

B. Memory Encryption

In embedded systems, encryption is widely implemented in order to protect a large variety of assets. All the intellectual properties that are present inside a system could potentially be stolen by an attacker having physical access to the hardware. For this reason, it is important to provide confidentiality of the memory content [START_REF] Valea | Providing confidentiality and integrity in ultra low power iot devices[END_REF], as well as of the data that is exchanged with the other integrated circuits in the system [START_REF] Valea | A survey on security threats and countermeasures in ieee test standards[END_REF].

NVMs are sensitive elements in all embedded systems. For instance, flash memories can be opened and read out by any attacker having access to the device [START_REF] Kannan | Security vulnerabilities of emerging nonvolatile main memories and countermeasures[END_REF]. Since the application code is typically stored inside external flash memories, the encryption of the memory content is a functionality that is more and more needed by designers. Even when the NVM is part of a single chip also including the CPU, its content could be encrypted to make it more difficult to steal it via some peripheral interface.

The memory encryption model adopted in this paper relies on the symmetric encryption scheme. The sender is the code developer that stores the encrypted code into the NVM. The receiver is the microprocessor that reads out the code from the NVM and loads it into its internal memory (e.g., internal RAM or instruction cache) for being executed. The secret key is known by the code developer and configured inside the microprocessor in a secure way. At system boot, the application code is transferred from the external NVM to the internal RAM and processed on-the-fly by a decryption module using the secret key.

III. REFERENCE SCENARIO AND PROPOSED EVALUATION

When a single fault affects the non-encrypted NVM, where the application code is stored, this normally results in the flipping of one instruction bit. If the fault hits the part of the instruction containing the opcode, it may happen that the instruction is transformed into an illegal instruction, causing a failure in the code execution. On the other hand, if the fault affects the part of the instruction containing the operands, there is a higher probability that the resulting instruction is executed by the processor without causing a failure. Thus, the computation carries on with corrupted data, resulting in Silent Data Corruption (SDC).

In this paper, we show that when memory encryption is implemented, faults affecting the application code could produce an effect that may be propagated and amplified by the decryption mechanism. This increases the probability of an illegal instruction condition, thus it decreases the SDC probability. The experiments that we have conducted (discussed in Section IV) show that the SDC reduction heavily depends on the cipher that is used.

For the purpose of this paper we have considered two different ciphers to implement memory encryption:

• RC4: a very lightweight stream cipher which is widely used in hardware implementations; • Advanced Encryption Standard (AES): the most widely used block cipher. It encrypts the plaintext per blocks of 128 bits. Block ciphers can be implemented according to different modes of operation. A mode of operation is an encryption scheme that repeatedly applies a single-block operation in order to securely transform a plaintext that is longer that one encryption block.

In the proposed experiments, we have considered memory encryption with AES configured according to the following modes of operation:

• Electronic Codebook (ECB): blocks are encrypted independently, such that one block of plaintext is mapped to one block of the ciphertext. Keystream blocks are computed in parallel encrypting blocks produced by a counter. The resulting keystream blocks are XORed with the plaintext blocks in order to compute the ciphertext blocks. Depending on the error propagation properties of the aforementioned encryption techniques, we can classify them into the following categories:

• Non-Propagating configurations: if a fault affects one bit of the encrypted memory, after decryption the fault will not propagate to other bits. Thus, in this case, the resulting code is as much corrupted as it would be without memory encryption. This category includes all the stream ciphers, as well as OFB and CTR modes of operations of block ciphers. For the sake of clarity, in the following of the paper we will refer to these encryption methods with the generic name of stream ciphers. • Propagating configurations: if a fault affects one bit of the encrypted memory, during the decryption the fault propagates to at least one encryption block. If the processor runs 32-bit instructions and the encryption Fig. 2: Experiments' Setup method is based on 128-bit blocks, one bit flip on the memory results in at least four consecutive instructions that are entirely corrupted. ECB, CBC and CFB modes of operations of block ciphers belong to this category. For the sake of clarity, in the following of the paper we will refer to these encryption methods with the generic name of block ciphers. For these reasons, implementing memory encryption based on a propagating configuration may lead to an increased reliability of the system, substantially reducing the risk of SDC. In the next Section, we will verify these hypotheses through some experiments.

IV. EXPERIMENTAL RESULTS

A. Experimental setup and environment

For the purposes of our experiments, we selected a set of programs representative of typical application codes. Matrix multiplication, the Dijkstra algorithm and the Bubble-sort algorithm were considered. These programs were written using ANSI C, then compiled for the OR1200 processor, providing us with the .or32 binary files. The OR1200 is a 32-bit scalar RISC with Harvard micro-architecture and 5 stage integer pipeline. The OR1200 core is mainly intended for embedded, portable and networking applications. The RTL description of the core was taken from [START_REF] Community | OpenRISC[END_REF] and it was synthesized using the Silvaco 45nm Open Cell library [START_REF] Silvaco | 45nm Open Cell Library[END_REF].

To analyze the effects of faults affecting the code memory content, we corrupted the binary files with a Python script. Given the binary file, the encryption method to be used and the number of faults to inject, the script (i) performs the encryption, (ii) injects the specified amount of faults, and (iii) performs the decryption. As a result, the corrupted version of the binary file is obtained. For the exhaustive 1bit injection experiments we used the OpenSSL library [START_REF]OpenSSL Software Library[END_REF] for encrypting/decrypting. Finally, the corrupted binary format was transformed into a hex-file for the purpose of the Logic Simulation with ModelSim, which follows.

To know when to set a timeout for every test program's simulation, we simulated the OR1200 with the original noncorrupted binary files and so we defined a timeout margin as:

T timeout = T i + ∆ (1)
• i {M atrixM ultiplication, Dijkstra, BubbleSort}.

• ∆ is a time constant: ∆ = 10% × T i .

After finding the timeout values and furthermore acquiring the golden values for every program, we run the Logic Simulation with the corrupted binary files. After the simulation end, each fault is classified as:

• Silent: the signatures are the same (effect-less fault).

• Silent Data Corruption: the signatures differ.

• Detected: the application crashed or the timeout condition was triggered. The above described process (depicted in Figure 2) was fully automated using two bash scripts. The first script was responsible for corrupting the .or32 file, running the simulation and comparing the results with the golden values. The second script was used to run multiple experiments while changing the experiment's variables (e.g., the number of faults to inject).

B. Results and discussion

A set of experiments was devised and performed on the OR1200 microprocessor resorting to the described environment and procedure. To have a point of reference, we started by doing single-bit fault injections without using encryption. for (l = 1; l ≤ 1000; l + +) do 5:

Simulate the injection of k faults using j on i; end for 10: end for Fig. 3: Random Fault Injection Procedure The next step was to understand the ability of encryption to detect multiple-bit faults. For this purpose we computed, for each encryption mechanism, how many bit should be flipped before the number of SDC faults is reduced to 0. We started by injecting a small number of bit-flips so that we could distinguish the difference between the detection rates of stream ciphers and block ciphers. Then we moved to an increasingly large number of bit-flips. Lastly, we considered as many samples as possible from every method and every type of injection. Considering also the computational time, the complexity of the simulations and also based on some experiments, we selected a number of iterations for each experiment equal to 1,000. The procedure is described by the pseudo-code in Figure 3.

The graphs in Figures 4 to 6 show that using block ciphers over stream ciphers provides significantly higher detection rates for multiple-bit fault injection as well. These graphs also show that encryption alone is able to guarantee the detection of faults starting from a given multiplicity k. The value of k changes depending on the encryption method. In some cases, encryption is able to detect any fault with multiplicity greater than 4 or 5 bits. This is a remarkable result, since error correction codes are typically unable to guarantee similar detection capabilities.

V. CONCLUSIONS

Embedded systems are increasingly employed in safetycritical scenarios, both in industrial and in consumer environments. As a consequence, robustness and confidentiality of the application code running on those systems are key requirements. This generally results in implementing protection mechanisms for the memory containing the application code, in order to prevent both random error effect propagation (e.g., due to radiations) and unauthorized accesses. Encryption mechanisms are widely adopted to protect all the intellectual properties within the system. At the same time, designers often resort to expensive error detection/correction mechanisms to prevent random error effects that would cause unexpected results. In general, it is crucial to avoid the silent propagation of random fault effects, either by correcting the error or by preventing it from silently propagate.

In this work, we investigate the possibility to exploit the features of some encryption mechanisms to detect errors in the system: when a random fault strikes and impacts one or more bits of the encrypted code in the memory, the error will propagate to other bits of the code thanks to the decryption process. This will likely cause instruction corruptions, thus it will prevent the error from silently propagate.

Experimental results showed that, by using block cipher encryption mechanisms, the detection rate of single-bit faults increases on average by 58.2%, compared to non-encrypted code, without resorting to any ECC. In absolute terms, single bit-flip fault detection values, achieved with block cipher These results support the claim that it is possible to avoid the implementation of costly detection/correction mechanisms by reusing encryption mechanisms already implemented to protect the system from illegal accesses. Moreover, some encryption/decryption mechanisms proved to have very good detection properties with respect to multiple-bit faults, thus complementing the limited capabilities of ECCs in this direction.

Work is currently being done to extend the performed work considering more application programs and different types of encryption mechanisms.

Fig. 1 :

 1 Fig. 1: High-level architecture of: (a) stream cipher; (b) block cipher.

Fig. 4 :

 4 Fig. 4: Detection Rates on Matrix Multiplication

TABLE I :

 I Detection Rates on 1-bit Injections Average absolute improvements compared to unencrypted code For the Dijkstra program, we injected 200 one-bit random faults, since we could not adopt the exhaustive injection due to the large application code size, which resulted in excessively long simulation times. Hence, we reduced the number of iterations to 200 since they were enough to provide accurate samples. For all the test programs we also performed onebit fault injections for all the encryption methods in our set. The results of these experiments are shown in TableI. The table reports the detection rates for single-bit fault injections obtained by applying the considered proposed approach. The obtained results show that, even for single-bit error scenarios, block cipher encryption mechanisms provide a drastic detection increment (58.2% on average) compared to unencrypted code. Values spanning from 81.5% to 93.5% single bit-flip detection were achieved. The single-bit error event is the most difficult to detect via encryption/decryption. Indeed, this condition quite often leads to corrupted data (SDC), silently propagating the error to the rest of the computation.

							Encryption Methods
		Programs		Stream Ciphers	Block Ciphers
			NO ENC	CTR		OFB	RC4	CBC	ECB	CFB
		Matrix Multiplication	20.2%	22%	22.2%	21.8%	90 %	89.9%	84.1%
		Bubble Sort	35.1%	34.6%	34.4%	34.2%	85.1%	85 %	81.5%
		Dijkstra	34 %	33%		35%	35,5%	93.5%	93.5%	89.5%
		Average	29.8% 29.9% 30.5% 30.5% 89.5% 89.5% 85.0%
				NO ENC	Stream Ciphers Block Ciphers
		Average detection	29.8%		30.3%	88.0%
		Average Improvements*	0% (ref.)		0.5%	58.2%
	2:	for j = {CT R, OF B, CBC, ECB, CF B, RC4} do
	3:	for k = {10, 20, 50, 100, 200, 500} do
	4:

*1: for i = {M atrixM ult, BubbleSort, Dijkstra} do

VI. ACKNOWLEDGMENTS

Part of this work has been supported by the Center for Automotive Research and Sustainable mobility@PoliTO (CARS).


F.-R Orthous-Daunay

C Wolters

L Flandinet

V Vuitton

P Beck

L Bonal

J Isa

F Moynier

D Voisin

S Moran

Comparison of Molecular Complexity Between Chondrites, Martian Meteorite and Lunar Soils

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

Comparison of Molecular Complexity Between Chondrites, Martian Meteorite and Lunar Soils.

F.-R. Orthous-Daunay 1 , C.Wolters 1 , L.Flandinet 1 , V. Vuitton 1 , P.Beck 1 , L.Bonal 1 , J.Isa 1 , F.Moynier 2 , D.Voisin 3 , S.Moran 4 , S.Horst 4 , G.Danger 5 , V.Vinogradoff 5 , L.Piani 6 , D.Bekaert 6 , L. Tissandier 6 , Y.Isono 7 , S.Tachibana 8 , H.Naraoka 9 and R.Thissen 10 1 IPAG (CS 40700 38058 Grenoble Cedex 9, France, frod@univ-grenoble-alpes.fr) 2 IPGP, (1 Rue Jussieu, 75005 Paris, France) 3 IGE,(CS 40700 38 058 Grenoble Cedex 9, France) 4 JHU,( 3400 N. Charles St.Baltimore, MD 21218) 5 PIIM,( Avenue Escadrille Normandie-Niémen 13397 Marseille cedex 20, France) 6 CRPG,( Université de Lorraine, 15 rue Notre Dame des Pauvres, BP 20, 54501 Vandoeuvre-lès-Nancy, France) 7 Hokkaido University, (N10 W8, Sapporo 060-0810, Japan) 8 University of Tokyo(7-3-1 Hongo, Tokyo 113-0033, Japan) 9 Kyushu University, (744 Motooka, Nishi-ku, Fukuoka 819-0395 Japan) 10 LCP, (310 Rue Michel Magat, 91400 Orsay)

Introduction: Meteorites and comets bear organic molecules ranging in size from one single to an arbitrarily large number of carbon atoms. This contrasts with the limited size of free molecules detected in space environments [1]. Understanding the organic matter cycle in the solar system requires to identify the time and place where the molecule are formed and destroyed.

The High Resolution Mass Spectrometry technique has been used for almost a decade to characterize extraterrestrial material [START_REF] Schmitt-Kopplin | Proc. Natl. Acad. Sci[END_REF]. The technique only accesses the soluble part of the samples and is used both with and without liquid chromatography for molecular identification [START_REF] Yamashita | [END_REF]. Several authors have proposed an origin for the sample mixtures. For instance, aqueous alteration has been invoked to explain the growth of N-bearing molecules [4] in Murchison.

During the last years, the IPAG group has developed a tool dedicated to the interpretation of the polymerization degree of any sample. We highlighted a peculiar pattern in meteorites extract that can be only reproduced with highly reducing gas phase polymerization experiments [5]. This is interpreted as a protoplanetary disk origin.

The work presented here is a comparison between the typical protosolar pattern and the features found or not in Martian meteorites and lunar soils.

Method: We extracted the organic mixtures from 4 lunar soils and from the NWA7533 "black beauty" Martian meteorite [6] by maceration in Toluene and Methanol for 1 week at room temperature. We did the same for several carbonaceous chondrites including Orgueil, Murchison, 4 CR class samples. Laboratory experiment residues produced from ionized gaz [7], [START_REF] Kuga | Proc. Natl. Acad. Sci[END_REF], photon irradiated ices [START_REF] Danger | [END_REF] or photon irradiated liquids [10] were also analyzed to provide comparison to well constrained synthesis environments. Mass spectra were acquired with a Thermo LTQ Orbitrap XL at its highest resolving power, coupled with Electrospay ionization (ESI) source.

Results: The Orbitrap mass spectrometry provides the mass distribution of mixtures with resolution and precision high enough to undoubtfully identify polymeric patterns. In every sample, CH 2 patterns are detected. A CH 2 family has only molecules with R-(CH 2 ) n formula. From each CH 2 family, the free parameters of the Wesslau model [11] for polymerization can be adjusted to match the distribution. Each sample has from 3 to 15 CH 2 families with up to 30 members. In the synthetic samples, the parameters depend mainly on the precursors mixture. The meteorites exhibit larger variations of the polymerization parameters than synthetic samples. We discuss the relevance of the various candidates for the emergence of molecular diversity of asteroids.

Polymeric patterns in the NWA7533 involve C, H and O. Only CH 2 , C 2 H 2 and C 2 H 4 O patterns seem to be responsible of the molecular complexity. Heteroatomic (O-bearing) pattern is the major difference between chondritic and Martian organics. Another major difference is the absence of nitrogen in any cations observed whereas it was a key feature in Murchison.

The patterns in lunar soils are highly variable. Some soils exhibit very little polymerization patterns, if any. One has a pattern comparable to the asteroidal one and one has a peculiar mass distribution that doesn't match any other extraterrestrial sample. The latter is the most exposed to solar wind and the mass range of its mixture is the most extended. We will discuss the interpretation of such feature in terms of origin and possible evolution of the organic matter delivered to the moon.


C He

S M Horst

N K Lewis

J I Moses

Exploring R Kempton

M. S. Marley P A Mcguiggan

C V Morley

J A Valenti

Véronique Vuitton

Exploring Exoplanetary Atmospheres from Laboratory Simulations

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

1

Exploring Exoplanetary Atmospheres from Laboratory Simulations

Chao He 1 , Sarah M. Hörst 1,2 , Nikole K. Lewis 3 , Julianne I. Moses 4 , Eliza Miller-Ricci Kempton 5 , Patricia A. McGuiggan 1 , Mark S. Marley 6 , Caroline V. Morley 7 , Jeff A. Valenti 2 , Véronique Vuitton 8 , and Xinting Yu 1,9 1 Johns Hopkins University, Baltimore, MD, USA (che13@jhu.edu), 2 Space Telescope Science Institute, Baltimore, MD, USA, 3 Cornell University, Ithaca, NY, USA, 4 Space Science Institute, Boulder, CO, USA, 5 University of Maryland, College Park, MD, USA, 6 NASA Ames Research Center, Mountain View, CA, USA, 7 University of Texas at Austin, Austin, TX, USA, 8 Université Grenoble Alpes, Grenoble, France, 9 University of California Santa Cruz, Santa Cruz, CA, USA.

Introduction: The majority of discovered exoplanets (over 4,000 by November, 2019) are super-Earths and mini-Neptunes (with size or mass between Earth's and Neptune's), and their atmospheres are expected to exhibit a wide variety of atmospheric compositions. Clouds and/or hazes are likely to be present in these atmospheres as they exist in every solar system planetary atmosphere. However, the photochemical processes for haze formation in these exoplanet atmospheres remain largely unknown as the atmospheric phase space has not been explored previously. To understand haze formation in these atmospheres, we have conducted a series of laboratory experiments simulating a range of atmospheric compositions at four different temperatures (300, 400, 600, and 800 K) [1,2,3,4]. Experimental Setup: We carried out the experiments using the PHAZER setup (Figure 1) at Johns Hopkins University [5], which allows us to conduct simulation experiments over a broad range of atmospheric parameters with two different energy sources (AC plasma or FUV photons). Figure 2 shows the initial gas mixtures for our experiments, which is prepared from high-purity gases. The gas mixture flows through a heating coil that heats the gas mixture to the required experimental temperature, and then flows into the chamber where the heated gas mixture is exposed to AC plasma, or UV photons. The gas flowing out the chamber is monitored with a Residual Gas Analyzer (RGA, a quadrupole mass spectrometer). We run the experiment for 72 hr, and collect the solid samples in a dry (<0.1 ppm H2O), oxygen free (<0.1 ppm O2) N2 glove box.

Results and Conclusions:

The mass spectra of the gas phase show the compositional changes during the experiments, suggesting that distinct chemical processes happen in the experiments as a function of different initial gas mixture and different energy sources (plasma or UV photons). We identified new gas products that could be indicative to photochemistry and haze formation in these atmospheres [1]. All simulated atmospheres resulted in haze formation with both energy sources, but the production rates varied substantially with different conditions [2,3,4]. The resulting haze particles display different properties, such as color, size distribution, particle density, and composi-

Figure 1 :

 1 Figure 1: Schematic of the PHAZER setup.

Figure 2 :

 2 Figure 2: Initial gas mixtures used in the experiments, which span a range of temperatures (300-800 K) and initial gas mixtures (100 to 10,000x solar metallicity).

2 tion. Our laboratory results indicate that complex atmospheric photochemistry can happen and produce new gas products and haze particles in diverse exoplanet atmospheres, including compounds (O2 and organics) that could be falsely identified as biosignatures.