Kristine Tolod
email: kristine.tolod@polito.it

Tapish Saboo

Simelys Hernandez
email: simelys.hernandez@polito.it

Hilmar Guzman

Micaela Castellino

Rowshanak Irani

Peter Bogdanoff

Elsje Fatwa F Abdi

Alessandra Quadrelli

Nunzio Russo

Kristine Rodulfo Tolod

Simelys Hernández

Hilmar Guzmán

Elsje Alessandra Quadrelli

Insights on the surface chemistry of BiVO4 photoelectrodes and the role of Al overlayers on its water oxidation activity

Keywords: BiVO 4, Al overlayer, passivation layer, photoanode, surface chemistry, water splitting, artificial photosynthesis, surface organometallic chemistry

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Introduction

Photoelectrochemical water splitting is a sustainable means to produce H 2 , which can serve as a clean source of fuel. In this process, liquid water is split into its gaseous H 2 and O 2 components (H 2 O + photons  H 2 + ½ O 2 ), via the use of photo-activated semiconductor catalysts, by utilizing the photons coming from the sun and using them to directly convert the solar energy into chemical energy. To be able to split water into hydrogen and oxygen efficiently and sustainably, the photo-voltage must be sufficient, there should be an appropriate band alignment in the semiconductor, a substantial absorption of the solar spectrum and an efficient and fast transport of charges between the semiconductor and the electrolyte [START_REF] Chen | Photoelectrochemical Water Splitting Standards, Experimental Methods, and Protocols[END_REF][START_REF] Walter | [END_REF]. In addition, the semiconductor must be stable and cost-effective. The search for suitable materials that satisfy these requirements is the focus of the current on-going studies in photoelectrochemistry and photocatalysis because no single material can fulfil all these requirements; trade-offs exist among the light-harvesting ability, the charge transport and charge transfer processes, and the catalyst's influence in the kinetics of the reaction.

The bottleneck in the photoelectrochemical water splitting process is the water oxidation reaction in the photoanode [3,4], which is the focus of this study. Metal oxides [5,6] are preferred as photoanodes, mainly because of their resistance to oxidizing conditions [START_REF] Walter | [END_REF]. Compared to other metal oxides like hematite (α-Fe 2 O 3 ) and WO 3 , bismuth vanadate (BiVO 4 ) is preferred because its conduction band edge position nearly coincides with the thermodynamic hydrogen evolution potential [7,8], which is instrumental in promoting an earlier photocurrent onset and enabling the generation of higher photocurrents in the low bias region [9]. It also has a relatively low band gap energy (2.4-2.5 eV), which makes it readily absorb visible light [10,11]. The theoretical maximum photocurrent density of BiVO 4 is 7.5 mA/cm 2 , which translates to a maximum of 9% of solar-to-hydrogen (STH) efficiency [12,13] assuming all of the sunlight photons with energies greater than 2.4 eV are absorbed by the semiconductor and have contributed to the photocurrent. However, solid semiconductors such as BiVO 4 have surface states that are associated with the abrupt termination and discontinuity of the solid crystal at the crystal boundary. As a result, the electronic band structure is quite different from the bulk, as characterized by dangling bonds [14]. These surface states have a modified band structure that may give rise to defect energy levels, which may lie within the previously forbidden band gap region away from the band edges. These can mediate electron-hole recombination via the Shockley-Read-Hall mechanism wherein the electron from the conduction band can move towards the defect level, release the energy as photon or phonons, and eventually relax into the valence band, annihilating a hole in the process.

In photoelectrochemical systems, passivation layers have been used as an effective strategy to improve the charge separation and transfer processes across semiconductor-liquid interfaces. These thin layers can reduce the charge recombination at surface states, increase the water oxidation reaction kinetics, and protect the semiconductor from chemical corrosion. These layers are <100 nm thick but quite often, only 1-2 nm are used to allow the charge transfer by means of tunneling. Passivation layers can be fabricated through the use of several relatively scalable techniques such as atomic layer deposition (ALD), spin-coating, electrochemical deposition, sputtering, electron beam evaporation, floating transfer and dip-casting [15][START_REF] Van De Krol | Photoelectrochemical Hydrogen Production[END_REF][START_REF] Tolod | [END_REF].

Al 2 O 3 has been used as a passivation layer for both p-and n-type silicon solar cells. Al 2 O 3 is a wide band gap material (5-9 eV) which exists in different crystalline forms, exhibits low leakage current, and has a modest value of dielectric constant [18]. It finds its use as a gate dielectric in metal oxide semiconductor (MOS) transistors and is a suitable insulator for various electronic applications. In the photovoltaic field, the importance and crucial impact of surface recombination to the resulting photoactivity of conventional semiconductors has been very much highlighted. Al 2 O 3 as a passivation layer is unique because it combines the effect of both chemical and field effect passivation by providing hydrogen to the Si interface during the post-deposition thermal treatments and also for having a very high density of negative charges located near the interface [19]. However, the use of Al 2 O 3 as a passivation layer for BiVO 4 photoanodes has not been largely investigated. To date, only one paper reported the use of Al 2 O 3 as a passivation layer for BiVO 4 [20]. It was shown that adding an Al overlayer with an optimal thickness can double the photocurrent density, however, the nature of the specific surface interactions between the Al overlayer and the BiVO 4 remains to be studied. This study presents novel findings about the use of ultrathin overlayers of Al to enhance the photoactivity of BiVO 4 . Moreover, an organometallic chemistry approach was used to study this very important surface as well as to introduce surface modifications that could help improve the photoactivity and the stability of this material.

Experimental

Materials and Methods

Synthesis Procedures

A full description of materials and methods used for the synthesis of both BiVO 4 powders and films is reported in the Supporting Information (SI, sections S1 and S2). Briefly, the BiVO 4 powder was synthesised by hydrothermal method employing a solution of bismuth nitrate and ammonium metavanadate as precursors, under acidic media (pH=0), in an autoclave at 180 o C for 12h, according to a previous published procedure [21]. The crystalline BiVO 4 powder was obtained after calcination under air atmosphere at 400 o C for 2h. The BiVO 4 films were synthesised based on a previously optimized electrodeposition method [22]. In brief, BiOI was electrodeposited on F-doped Tin oxide (FTO)/glass substrate by using a solution of bismuth nitrate, potassium iodide, p-benzoquinone and ethanol. The BiOI film was subsequently transformed in BiVO 4 by addition of a solution of vanadyl acetylacetonate in dimethyl sulfoxide and calcination under air atmosphere at 425 o C for 2h. The photoelectrode area is of 1 cm 2 . The BiVO 4 powders and thin films were modified by the addition of Al 2 O 3 with the procedure described in the SI (sections S1.4 and S2.3, respectively). In particular, the BiVO 4 films with different amounts (or cycles) of deposited Al 2 O 3 were investigated.

Materials Characterization

A Merlin Zeiss Field Emission -Scanning Electron Microscope (FESEM) equipped with an Energy Dispersive X-ray Spectroscopy System (EDS) was used to study the morphology of the samples. The XRD spectra were obtained by using an X'Pert Phillips diffractometer under Cu Kα radiation (λ =1.5418 Å) set at 40 kV and 40 mA. N 2 adsorption and desorption isotherms were obtained at 77K on a degassed sample using Belsorp-Max from BEL-JAPAN. The DRIFTS spectra were obtained using a Nicolet 6700-FT spectrometer. Gas chromatography (GC) measurements were determined using a HP 5890 gas chromatograph equipped with a flame ionization detector (FID) and a KCl/Al 2 O 3 on fused silica column (50 m x 0.32 mm). UV-Vis absorption spectra of BiVO 4 films were measured using a Perkin-Elmer Lambda 950 double-beam spectrometer equipped with an integrating sphere. X-ray photoelectron spectroscopy (XPS) measurements were performed) with a monochromatic Al K radiation (h = 1486.74 eV), in order to determine the surface composition of the BiVO 4 electrodes and their stability after the photoelectrochemical tests. Bare BiVO 4 samples were first analysed by using a PHI 5000 Versa Probe (Physical Electronics) system. Subsequently, a SPECS FOCUS 500 monochromator and a hemispherical electron analyzer (SPECS PHOIBOS 100) were employed to further analyse the bare and Al-modified BiVO4 samples before and after testing. The binding energies for all spectra were calibrated with respect to the adventitious carbon C 1s peak at 284.5 eV. Peak fitting was done using XPSPEAK software with a Shirley background subtraction.

Photoelectrochemical (PEC) Characterization

A Pt coil counter electrode (CE), an Ag/AgCl (3M KCl) reference electrode (RE), and the BiVO 4 photoanode as the working electrode (WE), were assembled in a single-compartment quartz cell containing a 0.1 M NaPi solution (pH 7). The simulated sunlight irradiation was from a Newport 450 W Xe lamp source that was equipped with an AM 1.5G filter (intensity of 100 mW/cm 2 ). Illumination was performed under back illumination on a geometric area of 1 cm 2 , which corresponds to the entire deposited area of the electrode.

The PEC tests were performed using a BioLogic VSP 300 potentiostat. Linear sweep voltammetry (LSV) in continuous and chopped light modes and chronoamperometry (CA) were performed to characterize the photoactivity of the electrodes. Linear sweep voltammetry was performed at the potential range of -0.3 V to 1 V vs. Ag/AgCl, using a scan rate of 10 mV/s. Chronoamperometry was done at 0.61V vs. Ag/AgCl. The following Nernst equation was used to convert the potentials versus the Ag/AgCl (3M KCl) reference electrode to RHE (NHE at pH=0):

(1) MS-SPEIS (Mott Schottky-Staircase Potentio Electrochemical Impedance Spectroscopy) analysis was done under an applied potential of -0.8 V to 1 V (vs. Ag/AgCl), and a frequency of 7.5 kHz. Mott-Schottky plots were used to extrapolate the flat band potentials (E fb ) and donor densities (N D ) of the photoanodes, according to the equation: [START_REF] Walter | [END_REF] where C is the interfacial capacitance, is the dielectric constant of the semiconductor, o is the permittivity of free space, A is the area of the interfacial capacitance, N D is the donor density, E is the applied potential, E fb is the flat-band potential, k B is the Boltzmann's constant, T is the temperature and e is the electronic charge. A plot of versus the potential yields a linear region.

Charge transport and transfer properties of the photoanodes were studied using electrochemical impedance spectroscopy (EIS), at frequencies from 0.1 Hz to 0.5 MHz, a potential of 0.61 V vs. Ag/AgCl, under simulated sunlight illumination.

Monochromatic photocurrents (i ph ) of BiVO 4 films were measured under the illumination of a Xe lamp source (Oriel, 300 W) coupled with a grating monochromator (Acton SpectraPro 150i). A long-pass filter (Schott, 3 mm thick) was placed between the monochromator and the sample in order to remove any secondorder diffracted light. The optical power of lights reaching the films (P) was measured using a calibrated photodiode (Ophir, PD300R-UV with Nova II controller). An EG&G Princeton Applied Research 283 potentiostat was used to apply the bias potential. An Ag/AgCl (saturated KCl, XR300 Radiometer Analytical) and a platinum wire were used as the reference and counter electrode, respectively. Incident photon-to-current conversion efficiency (IPCE) values for each wavelength () were calculated using the following formula:

(3)

For O 2 detection, differential electrochemical mass spectrometry measurements were performed using a setup described in details elsewhere [23,24]. Signal calibration was performed by measuring the O 2 signal vs. current for a Pt sheet. The Faradaic efficiency (FE) was then calculated using the following equation, under the assumption that the Pt sheet has 100% Faradaic efficiency for the O 2 evolution reaction:

(4)
Finally, rapid scan voltammetry was performed. A constant potential of 1.5 V vs Ag/AgCl (2.1 V vs RHE) was applied for 2 minutes as a conditioning step, followed by a quick potential sweep to -0.2 V vs Ag/AgCl (0.4 V vs RHE) with a scan rate of 1 V/s. Comparison of the curves with and without conditioning yields the surface capacitance (C) through the equation C=(J with cond -J w/out cond )/scan rate.

Results and Discussions

Morphology and Structure

BiVO 4 has three main crystal forms: monoclinic scheelite, tetragonal zircon-type and tetragonal scheelite. There is an irreversible transition from the tetragonal zircon-type to the monoclinic scheelite structure that occurs at 400-500 °C. At 255°C, another reversible transition also occurs between the monoclinic-scheelite and the tetragonal scheelite structures. Furthermore, mechanical grinding at room temperature can also irreversibly transform the tetragonal structure into a monoclinic structure [25]. Among these three crystal structures, the monoclinic scheelite structure is the most commonly used for photocatalysis because of its higher photoactivity compared to the other structures [26]. We prepared two different form of samples: BiVO 4 powders that were synthesized hydrothermally and BiVO 4 thin-film electrodes prepared via electrodeposition. The reader is referred to the Supporting Information (S1 and S2) for the detailed synthesis procedures. The XRD spectra in Erreur ! Source du renvoi introuvable. showed that both the BiVO 4 electrode and the BiVO 4 powder manifested the monoclinic scheelite structure (JCPDS No. 14-0688, space group: I2/a, a=5.195, b=11.701, c=5.092, =90.38 o ), which is evident from the peak splitting at the 2 values of 18.5 o and 35 o . No traces of other phases were observed in these diffraction patterns, which points out the suitability of the chosen synthesis methods towards the monoclinic phase of BiVO 4 . For an easy comparison, the XRD reference spectra of BiVO 4 with monoclinic scheelite and tetragonal zircon-type structures are also shown in the Figure 1. From the FESEM images in Figure 1, it is evident that there were very distinct differences between the resulting morphologies of the BiVO 4 powder and the thin-film electrode. The BiVO 4 powder morphology consists of a more agglomerated and compact structure that is made up of big grain sizes, while the thin film electrode showed a more porous structure that is not agglomerated, and with smaller grains. In agreement with this, the crystallite sizes for the powder and the thin film electrode, which were determined from the Xray diffraction data, are found to be 320 and 112 nm, respectively. More intense peaks were observed from the powder samples because of the higher amount of material present than in the thin-film electrode. From FESEM-EDS, the ratio of Bi/V for each of the samples was both found to be 1, which means that the 1:1 ratio of Bi: V is well-maintained in the bulk for both samples. Nitrogen adsorption-desorption isotherms were used for the BET surface area measurements for the BiVO 4 powder, which revealed a rather low surface area of 2-4 m 2 /g.

BiVO4 powder

The nature of the BiVO4 surface was studied by the chemical titration of the BiVO4 powder with nbutyllithium (n-BuLi), a carbanionic reagent, typically used in surface organometallic chemistry for the titration of surface acid protons (see SI, section S1.3, for more details). The n-BuLi is known to react with the surface hydroxyls and chemisorbed water of inorganic oxides [27] to yield butane (C4H10) that can be quantified using a gas chromatograph (GC).

From the n-BuLi reaction, 8.8 μmol of butane gas (C4H10) was evolved when contacted with the thermally-treated, dehydroxylated BiVO4. If one extrapolated such chemical reaction to exclusive reaction with surface reactive sites, the result translates to 1.5 surface reactive sites/nm 2 on the BiVO4 surface. The reader is referred to the SI (section S3) for the gas chromatography calculations. In a study by Starr et. al. [28], it was shown that single crystal BiVO 4 exhibited the presence of surface hydroxyl groups, however, there is no evidence in literature that confirms their presence in polycrystalline structures such as BiVO 4 powders.

The IR spectra of the pristine BiVO4 powder (black line) and the titrated BiVO4 powder (red line) in the 500 to 2000 cm -1 region is shown in Figure 2. The BiVO4 powder titrated with n-BuLi showed two distinct IR bands in the 1098.5 cm -1 and 1259 cm -1 regions. This preliminary data necessitates further studies to characterize the surface chemical species and to see if hydroxyl to O-Li + titration with concomitant alkane stoichiometric release is the dominant surface reactivity. As of now, the 1.5 OH/nm 2 can be considered as an upper boundary estimate of the hydroxyl population on the BiVO4 surface. To the best of our knowledge, this quantification of the OH groups in BiVO4 surface has not been previously reported in literature.

The addition of an Al overlayer was also performed on BiVO 4 powder in order to model the BiVO 4 -Al interface formed on the photoanode. Such modeling on the powder's surface, which has a 10 4 -fold larger surface area than that of the thin film electrode (ca. 2 m²/g for powder vs. few cm²/g for the electrode), allows to use millimolar range routine characterization techniques (such as DRIFT IR, elemental analysis by combustion, GC analysis of the gas phase), thus, giving molecular-level insight to the surface processes, which would be more difficult to harness directly from the electrode surface [29].

The accepted simplified ALD-mechanism for Al 2 O 3 growth on OH-terminated surfaces is shown below, with * denoting the surface species [30].

When the reaction is performed in a sequential manner, i.e. A-B-A-B-A-B…, a controlled layer-bylayer growth of Al 2 O 3 is achieved. However, to the best of our knowledge, the mechanism of the growth of an Al overlayer on BiVO 4 has not been investigated to date. We have earlier established and quantified the presence of reactive sites on the BiVO 4 surface, and we have hypothesized that some OH groups can serve as initial binding sites for the Al atoms as indicated in sequence A. Consequently, the growth of the Al overlayer in an A-B-A-B-A-B sequential manner should also be possible for BiVO 4 .

pulse of AlMe 3 and 1 cycle AlMe 3 -H 2 O at (A) 2000-500 cm -1 wavenumbers, (B) 3000-2000 cm -1 wavenumber, (C) 4000-3000 cm -1 wavenumber

Figure 3 shows the IR spectra of (i) vacuum pre-treated pristine BiVO 4 , (ii) BiVO 4 powder with 1 pulse of AlMe 3 , and (iii) after reaction with H 2 O. The starting bare BiVO 4 , i.e., before the 1 st pulse of AlMe 3 , is characterized by peaks at 720 cm -1 , 826 cm -1 , and 977 cm -1 which correspond to V-O waging, symmetric, and asymmetric vibrations, respectively [31]. Peaks associated with O-H stretching vibrations are expected in 3374 cm -1 and 3700 cm -1 wavenumbers, although for the case of BiVO 4 it is not readily seen in the IR spectra.

Upon addition of the first pulse of AlMe 3 , the presence of the AlCH 3 * surface species was confirmed by the presence of the diagnostic (C-H) stretching vibrations seen at 2822-2948 cm -1 . The 3015, 1207, 1015, 1264 and 1302 cm -1 peaks are yet unassigned. The subsequent H 2 O pulse resulted to a broad O-H band at 3400 cm -1 in agreement with expected AlO-H stretching vibrations from AlOH* surface species in the 3600 cm -1 region. The contribution to this band from OH vibration mode of physiosorbed water molecules should be limited, due to the vacuum treatment step undergone by the sample between the water addition and the IR analysis.

Upon water addition on the Al-Me terminated surface (see reaction B above), the volatiles were condensed, analyzed and quantified by gas chromatography. In agreement with reaction B, methane was detected in the gas phase. The quantification of methane translated to 1.2 CH 4 evolved per reactive group titrated on the initial surface. This result is in the correct order of magnitude for the simplified growth mechanism proposed for ALD cycles to alumina and is also compatible with the possible formation of dipodal surface species [(-O) 2 AlMe].

BiVO4 thin-film electrode

The chemical composition of the BiVO 4 thin film electrodes was determined by XRD, XPS, FESEM-EDS and Raman analyses.

The high resolution XPS spectra for Bi4f, V2p, and O1s binding energy regions are shown in Figure 4Erreur ! Source du renvoi introuvable.. In agreement with the XRD data (see section 3.1), the thin-film photoelectrodes manifest the spin-orbit splitting for both the Bi4f 7/2 and Bi4f 5/2 , and the V2p 3/2 and V2p 1/2 , that are in good agreement with those reported in literature for monoclinic-scheelite structure of BiVO 4 [32]. Also, it is noteworthy that the Bi4f 7/2 and Bi4f 5/2 peaks appear in the binding energy region (158.5 eV and 163.8 eV, respectively) that is much higher than that of metallic Bi, i.e. 156.8 eV and 162.2 eV [32], respectively. These Bi4f peaks indicate the presence of Bi occurring mainly as Bi 3+ species in the surface of the electrodes. Meanwhile, the V2p 3/2 and V2p 1/2 peaks at 516.1 eV and 523.7eV, respectively, are characteristic of V occurring mainly as V 4+ species on the surface of the photoanode. This is different from the bulk V species, which have a +5 oxidation state, as implied by EDS measurements and by earlier studies by us with depth profile XPS measurements [22]. The reduction of V 5+ to V 4+ on the surface of the BiVO 4 electrode induces oxygen vacancies that directly impact the electronic structure of the monoclinic scheelite BiVO 4 . In fact, these oxygen vacancies has been observed from the O 1s spectra, the deconvolution of which yielded lattice oxygen at 529.2 eV, non-lattice oxygen at 530.3 eV, and chemisorbed or dissociated oxygen from water molecules at 532.0 eV [32][33][34][35][36]. The non-lattice oxygen refers to oxygen-deficient regions which might be populated with OH groups in order to correct the charge balance. Furthermore, surface Bi/V atomic ratios revealed a Bi-rich surface for the BiVO 4 electrode before PEC tests (Bi/V: 2.3), while EDS measurements show a Bi/V ratio of 1 in the bulk. The Bi-rich surface of the BiVO 4 is a consequence of the segregation phenomena that has been observed for Bi-containing oxides, wherein the oxygen segregates to the surface, depleting the subsurface of oxygen, and leading to Bi-O surface terminations [37].

Upon Al-overlayer deposition, XPS investigation was also performed, and the spectra are shown in Figure S6. As expected, Al peak is clearly seen for the BiVO 4 sample that is modified with Al-overlayer deposition, whereas none can be observed for the bare BiVO 4 . No change is observed for the V 2p peaks upon Al-overlayer deposition, but additional peaks at higher binding energies can be observed in the Bi 4f spectra. These are assigned to Bi 5+ , as also reported in the literature [38][39]. This shows that the Aloverlayer deposition slightly oxidize the surface of the BiVO 4 . The Raman spectra of bare and Al-modified BiVO 4 thin film electrodes are shown in Figure 6A. The most intense band at 822 cm -1 is assigned to the V-O stretching mode and the weak shoulder at 709 cm -1 is assigned to antisymmetric V-O stretching. The bands at 375 cm -1 and 335 cm -1 correspond to the symmetric and antisymmetric bending of VO 4 tetrahedra. There was a slight difference observed at the 201-214 cm -1 bands, but these are ascribed to the external mode of BiVO 4 , which is not exactly structural in nature [40]. Therefore, based on the Raman spectra, it can be also confirmed that the Al overlayer did not induce structural modifications on the BiVO 4 . This result is also supported by the UV-Vis absorption analysis shown in Figure S3 in the SI. Only minor differences are observed in the absorption spectra of all the bare BiVO 4 and Al-modified BiVO 4 samples (Figure S3A), which is within the sample-to-sample variation. From these absorption spectra, Tauc analyses were performed in order to determine the bandgaps. Tauc plots for direct and indirect bandgap analyses are shown in Figure S3B andC, respectively, where only very little changes of bandgaps are observed (2,48 and 2,41 ± 0.01 eV, respectively). Overall, as expected, the Al overlayer does not affect the bulk structural and optical properties of BiVO 4 . The effect of varying the number of cycles in the ALD-like deposition method on the photoactivity of the BiVO 4 thin film electrodes was investigated. Figure 6B andC shows the linear sweep voltammetry and chronoamperometry curves under chopped light conditions for all the samples modified with Al, as compared with that of the bare BiVO 4 thin film electrode. Indeed, for very few pulses of AlMe 3 -H 2 O cycles, the formation of alumina oxide cannot be inferred. All curves manifested the initial current spikes appearing readily when the light source was turned on, which eventually dampened, which suggests the occurrence of fast electron-hole recombination. For the bare BiVO 4 sample, a photocurrent density of 0.35 mA/cm 2 at 1.23 V vs RHE was achieved. It can be clearly seen from the same figure that an improvement in photocurrent density values was achieved with 2 cycles and 5 cycles of Al deposition, yielding 0.54 mA/cm 2 and 0.49 mA/cm 2 at 1.23 V vs RHE, respectively, in the LSV curves. Under CA tests at 1.23 V vs RHE, the photocurrents of these two samples stabilized at a bit lower and similar value of 0.38 mA/cm 2 . However, lower photocurrent density was observed when the number of Al-deposition cycles was increased to 8. The influence of Al-deposition cycles to the incident photon-to-current conversion efficiency (IPCE) was also investigated and the resulting curves for the different samples under back-side illumination are shown in Figure 6D. The IPCE increases from the bare BiVO 4 sample to that with 2 cycles Al deposition, and it decreases again for higher number of Al deposition cycles. The differences between samples are rather larger than those observed from the LSV curves. The reason for this may be related to the instability of the films, which will be discussed later in the manuscript. Nevertheless, the overall trend qualitatively agrees with the LSV and CA measurements.

It is noteworthy that the reaction temperature used in the ALD-like deposition of the Al overlayer on the BiVO 4 surface, which yielded the better performance is low enough at 60 o C, which is not commonly demonstrated in typical ALD processes involving Al 2 O 3 , commonly performed at 125-500 o C [30]. To confirm that the higher photocurrent values of the Al-modified BiVO 4 electrodes are mainly due to water oxidation, we measured the O 2 generated by the highest performing Al-modified samples (i.e. with 2 cycles) using differential electrochemical mass spectrometry (DEMS) (see experimental section for measurement details). Figure 7A shows a linear sweep voltammetry curve, directly compared with the O 2 signal simultaneously measured with DEMS. Both curves correspond well with each other, suggesting that the O 2 generation can be correlated with the photocurrent. Indeed, by using the same system but under chronoamperometry condition, the Faradaic efficiency for O 2 evolution was determined to be 100.0 ± 0.6 % at 1.0 V vs. RHE. All photocurrents can therefore be attributed to the water oxidation reaction.

In order to verify if the Al overlayer is indeed passivating the surface states in the BiVO 4 electrode surface, rapid scan voltammetry was performed to determine the surface capacitance of the electrodes. As shown in Figure 7B, the surface capacitance decreases by orders of magnitude with increasing number of cycles of Al, in agreement with passivating surface states that serve as charge recombination sites. This is important because the photocurrent of BiVO 4 has been reported to be limited more by surface recombination than the charge transfer [41].

Meanwhile, the Nyquist impedance plot is shown in Figure 7C. The charge transfer resistance, which was indicated by the diameter of the arc, is seen to be highest for the 8 cycles Al electrode (green arc), and greater than that of bare BiVO 4 (black arc). This is consistent with the trend observed for the photocurrent density, where it was seen that the bare BiVO 4 performed better than the 8 cycles Al overlayer. Therefore, the thickest layer of Al overlayer contributed in slowing down the transfer of holes on the BiVO 4 -electrolyte interface. Also, it can be seen from Figure 7C, that the 2 cycles Al overlayer (red arc) and the 5 cycles Al overlayer (blue arc) have lower charge transfer resistances than the bare BiVO 4 , thus, both having higher photocurrent density relative to the bare BiVO 4 . However, between these 2 electrodes, it turned out that the 5 cycles Al has a lower charge transfer resistance than the 2 cycles Al, which was the opposite behaviour of the observed photocurrent density. It appears that the higher photocurrent density of the 2 cycles Al electrode is explained by the donor densities as derived from the Mott-Schottky plots (shown in Figure S4 in the SI), wherein the 2 cycles Al electrode manifested a donor density of 1.49 x 10 20 cm -3 , while the 5 cycles Al electrode has a lower donor density of 1.16 x 10 20 cm -3 . A higher donor density can improve the PEC response by raising the Fermi level, which creates more band bending that enhances electric field in the space charge layer, thus, lowering the electron-hole recombination. Second, it enhances the electrical conductivity of the photoanode, thus, improving the charge transport properties within the material. Moreover, the onset potentials of these electrodes were almost equal at 0.39 V, 0.37 V, 0.38 V, and 0.37 V vs RHE, for bare BiVO 4 , 2 cycles Al, 5 cycles Al, and 8 cycles Al, respectively. The 2 cycles Al electrode, which exhibited the best photocurrent density, was subjected to a stability test along with the bare BiVO 4 electrode. The result of the stability test is shown in Figure 8. The bare BiVO 4 (black line) showed an 85% decay in photocurrent density after 7.5 hours, while the 2 cycles Al (blue line) electrode showed 59.6% decay. To understand the reason of such decay in the performance, post-reaction FESEM-EDS and XPS analyses were performed. Figure 5D andE show the FESEM images of the 2 cycles Al modified sample after testing. An increase of the BiVO 4 particles size and rearrangement of the porous structure of the film after the PEC test are observed. The same behavior was also noticed in the bare BiVO 4 film (see Figure S5, SI); however, uncoated regions of the FTO substrate clearly appeared after the 7.5 h of operation in the bare BiVO 4 film (see Figure S5E, SI), while the Al-modified film was already preserved from this point of view. From EDS analyses it was observed a Bi-enrichment of the films: the Bi/V ratio increased from 1.0 to 1.2 and 1.1, in the bare BiVO 4 and the 2 cycles Al-modified electrode, respectively, which can be explained by the dissolution of V atoms from the BiVO 4 surface during the PEC tests [42]. Indeed, the Bi/V ratio from XPS analysis for the bare BiVO 4 slightly increases from 2.3 (vide supra) to 2.8 after PEC measurement. In addition, Bi atoms on the surface of the bare BiVO 4 sample become partially oxidized to Bi 5+ ([Bi 5+ ] increased from 0 to ~20%, see Figure S7A), indicating that the photocurrent decay may be related to an oxidative photo-corrosion process. In contrast, although the Bi atoms are already partially oxidized to begin with in the 2 cycles Al-modified sample, PEC measurement only slightly oxidize the surface further ([Bi 5+ ] slightly increased from 23 to 29%, see Figure S7B). Instead, the instability observed in the Al-modified BiVO 4 may be related to the detachment of Al, as observed from disappearance of peak in the Al 2p core level spectra (see Figure S7C). Further investigation on better adhesion of Al on the BiVO 4 surface is therefore needed in order to extend the stability.

Overall, although the stability of the BiVO 4 photoanode clearly remains an issue, it was demonstrated here that the presence of an Al overlayer on the BiVO 4 surface arrested some of the photocorrosion and reduced the photocurrent decay by at least 15%.

Conclusion

In order to minimize the inefficiencies due to electron-hole recombination and passivate the surface states, ultrathin Al oxo/hydroxyls overlayers were deposited onto the BiVO 4 thin film electrodes in an ALDlike manner (alternating pulses of trimethyl aluminum and water). This was also performed in order to protect the BiVO 4 surface from photocorrosion and increase its stability. A photocurrent density of 0.54 mA/cm 2 at 1.23 V vs RHE was obtained for the 2 cycles Al overlayer -BiVO 4 electrode, which was a 54% improvement from the bare BiVO 4 that demonstrated a photocurrent density of 0.35 mA/cm 2 at 1.23 V vs RHE. A 15% increase in stability of the 2 cycles Al overlayer-BiVO 4 electrode was also observed over 7.5 hours of continuous irradiation. Finally, through surface capacitance measurements, it was shown that the Al overlayer was indeed passivating the surface states of the BiVO 4 electrodes.

Chemical insight into the nature of the Al-modified BiVO 4 surface was also gained by studying the larger surface area BiVO 4 powder, used as a model of the photoanode. Chemical titration of the BiVO 4 powder with n-BuLi led to the evolution of butane (1.5 molecule/nm² of BiVO 4 surface), thus highlighting the existence of surface reactive groups on BiVO 4 , whose chemical nature has not yet been defined. If the presence of surface hydroxyls, postulated elsewhere in the literature, is confirmed, this work suggests a maximum of 1.5 of such reactive site/nm 2 on BiVO 4 . Such surface site reacts with AlMe 3 to yield surface alkyls, which can be hydrolyzed by a further pulse of water and concomitant methane release (1.2 CH 4 molecules released by the reactive group of the initial surface). This work on model powder BiVO 4 confirms that chemically reasonably well-behaved ALD-like half-cycles of AlMe 3 and H 2 O can be alternated on BiVO 4 surfaces, thus providing a molecular model for the useful chemical modification reported herein for the thin film BiVO 4 photoanodes.

Figure 1 .

 1 Figure 1. XRD (left) and FESEM (right) images of bare BiVO 4 powder and thin-film electrode. The reference XRD spectra of monoclinic scheelite and tetragonal zircon-type structures of BiVO 4 are shown for comparison in the bottom of the left image.

4 +Figure 2 .Figure 3 .

 423 Figure 2. FTIR spectra of bare BiVO 4 and BiVO 4 titrated with n-BuLi

Figure 4 .

 4 Figure 4. O1s, Bi 4f and V2p XPS spectra for bare BiVO 4 electrode

Figure 5 .

 5 Figure 5. FESEM-EDS analyses of Al-modified BiVO 4 electrode made by 2 cycles: Low magnification FESEM image and EDS maps of Al, Bi, V and O elements on the fresh electrode (A); FESEM images before (B, C) and after (D, E) photo-electrochemical water oxidation tests in 0.1M NaPi buffer.

Figure 6 .

 6 Figure 6. (A) Raman spectra of bare and Al-modified BiVO 4 electrodes. PEC water oxidation tests of bare and Almodified BiVO 4 : (B) LSV plots under chopped light (scan rate: 10 mV/s), (C) chronoamperometries under chopped light, (D) IPCE spectra and (D) Raman spectra. Tests were made with 0.1 M Na-Pi buffer pH 7.

Figure 7 .

 7 Figure 7. (A) Measurement of O 2 signal detected simultaneously during linear sweep voltammetry on 2 cycles Almodified electrode. (B) Surface capacitance and (C) Nyquist impedance plots for bare and Al-modified BiVO 4 .

Figure 8 .

 8 Figure 8. Stability test (chronoamperometry) plot for 2 cycles Al and bare BiVO 4

Acknowledgments

The authors thank the EACEA Erasmus+ SINCHEM Grant FPA 2013-0037 for the financial support. Part of the work described in this paper has been conducted within the project SUNCOCHEM. This project has received funding from the European Union's Horizon 2020 Innovation Action programme under the Grant Agreement No 862192. From Politecnico di Torino, we acknowledge Mauro Raimondo for the FESEM measurements and Enrico Sartoretti for the assistance in Raman spectra interpretation. The support of CNRS, Université de Lyon 1 and CPE Lyon Sustainable chair is also gratefully acknowledged.


Bhanu Chandra Marepally

Claudio Ampelli

Chiara Genovese

Elsje Francesco Tavella

Alessandra Quadrelli

Siglinda Perathoner

Gabriele Centi

B C Marepally

T Saboo

E A Quadrelli

Electrocatalytic reduction of CO2 over dendritic-type Cu-and Fe-based electrodes prepared by electrodeposition

Keywords: nanofoam (NF), electro-catalytic (EC), electro-deposition, dendrites, liquid phase cell

Introduction

The combat against the rising world energy needs [START_REF]Key World Energy Statistics[END_REF][START_REF]World energy outlook 2016[END_REF][START_REF]IRENA -Renewable energy and jobs 2015[END_REF][START_REF]Global Energy Trends -2016 report[END_REF], the global climate change due to rising CO 2 and other green-house gases in the atmosphere, temperature rise, and other concerns [START_REF]Greenhouse Gas Inventory Data Explorer -2014[END_REF][START_REF] Canadell | [END_REF][START_REF] Le Quere | Trends in the sources and sinks of carbon dioxide[END_REF][START_REF]CO 2 and temperature rise prediction[END_REF][START_REF] Wigley | [END_REF][START_REF] Hansen | Global surface temperature change[END_REF][START_REF] Houghton | the Science of Climate Change[END_REF] made the world research to race towards understanding and creating new paths to efficient and sustainable renewable energy technologies [START_REF]IRENA -Renewable energy and jobs 2015[END_REF][START_REF]Global Energy Trends -2016 report[END_REF][START_REF] Canadell | [END_REF][START_REF]World Energy Assesment Report: Energy and the Challenge of Sustainability[END_REF]. One key path which addresses both these problems is artificial photosynthesis which utilizes renewable energy sources to convert CO 2 to fuels [START_REF] Centi | [END_REF][14][15][16][17][18][19] using photo-electro catalytic (PEC) cells or artificial leafs [20][21][22][23]. This system is sub-divided into photo-catalytic [24,25] and electro-catalytic [26][27][28] to ease the study and understanding of the fundamental aspects in these processes.

Nano-porous materials having high surface area, very low densities, large number of active sites are attractive materials for use in catalysis. Many studies towards CO 2 reduction with different metal/metal-oxide nanofoams (Zn, Sn, Bi, Pb, Ag, Au, Cu) already exist in literature [29][30][31][32][33][34][35][36][37][38] but, with Cu metal there are very few [36][37][38] and with Fe nanofoams are untouched yet. Studies on tuning the Cu nano-foams physical and structural properties, already reported in the past [39] and also, Fe dendrites were synthesized via hydrothermal method and used for electro-magnetic applications [40][41].

In this work, we modelled Cu and Fe dendritic nanofoams based catalytic electrodes on various substrates of Cu, Fe, Ti, Al, FeCrNi, Al grid, GDL using electro-deposition techniques. We report, Fe dendrites and nanofoams, first-of-its-kind to be synthesized via electro-deposition and tested for CO 2 reduction to fuels. Also, we propose the deposition of the nanofoams on GDL, which would enable us to work in both liquid phase and well as gas phase conditions for CO 2 reduction to fuels. We provide a detailed comparison on the structural changes of nanofoams on different base metallic substrates and their respective CO 2 reduction activity. Moreover, we would like to communicate the high faradaic efficiencies (F.E. -42 % to 92%) achieved and also, high F.E. selectivity (> 40 %) towards higher alkyl C 2 -product (acetic acid) with low-cost, highly abundant Fe, nanofoam or dendrite based catalysts. ], hydrochloric acid (HCl, 37%), sulphuric acid (H 2 SO 4 , 95%), Sodium Hydroxide (NaOH), potassium bicarbonate (KHCO 3 , 99.7%), Nafion perflourinated resin were obtained from Sigma-Aldrich and used without any further purification. Gas Diffusion layer (GDL 25 BC) was obtained from Sigracet®. Nafion membrane (Nafion 117 ) was obtained from Ion power and was pre-treated before use. Cu Foil, Al Foil, Titanium Foil were obtained from Sigma-Aldrich and were pre-treated before use. Fe Foil, Al grid were obtained from Good-Fellow and were also, pre-treated before use. Gas Diffusion Layer (GDL 25-BC) was obtained from Sigracet ® .

Experimental

Materials and Reagents

Synthesis of the Nanofoams of Copper (Cu NF)

Nanofoams of Cu are synthesized using electro-deposition technique. Various kinds of nanofoams are achieved by altering conditions such as electrolyte, electrolytic P H , electric current, time of deposition, substrate metal, and pre-treatment of the substrates. A schematic of this process is shown in the Fig. 1.

Synthesis Method-I: (Cu and Fe Nanofoams)

Prior to the synthesis, the substrate metal foils (Cu foil, Fe foil, Ti Foil, Al foil, Al grid, Fe-Cr-Ni alloy) are mechanically polished using a P280 abrasive sheet and then, cleaned with distilled water and dried. Later, the foils are etched using dilute 0.5 M HCl aqueous solution for 5 minutes, and again, cleaned using distilled (DI) water and dried. This forms the cathode of the electro-deposition setup. The anode metal (Cu) foil also, undergoes above polishing and etching procedure. The anode and cathode are fixed at a separation distance of 1 cm. The electrolyte used is a mixture of 0.4 M CuSO 4 or Cu(CH 3 COO) 2 , 1.5 M H 2 SO 4 and 50 mM HCl considered from the literature work [39]. The electro-chemical deposition of Cu NF is performed with a constant current of 1.2 A for time periods of 1 min, 2 min and 4 min. Also, in a specific case, with Fe as substrate, an under-potential deposition is performed for 30 min of deposition time. After the deposition, the NFs are cleaned using DI water carefully not to detach the NFs. The clean NF samples were dried at 80 °C overnight, and then, calcined at 200 °C (5 °C rate) for 1 hr, thus forming the final dark brown-black working electrodes. On similar lines, Fe nanofoams were prepared on Fe, FeCrNi, Al substrates.

Synthesis Method-II: (Cu Nanofoams)

The pre-treatments of the substrate metal foils (Cu foil, Fe foil, Ti Foil, Al foil, Al grid, Fe-Cr-Ni alloy), the arrangement of the electro-deposition setup and final treatments are maintained as described in the synthesis 1. For GDL, it is directly used without any treatments. The only change made is the P H of the electrolyte by addition of NaOH. The electrolyte used is 0.4 M CuSO 4 , and added 1 M NaOH till the P H reaches value of 12-13. The electro-deposition of Cu NF is performed with a constant current of 1.2 A for time periods of 0.5 min, 1 min, 2 min and 4 min. The smaller time depositions were performed to understand the initial formation stage flowery crystal growth of the foams as seen in SEM data.

Synthesis Method-II: (Fe Nanoofoams)

The pre-treatments of the substrate metal foils (Fe foil, Ti Foil, Al foil, Fe-Cr-Ni alloy), the arrangement of the electro-deposition setup and the final treatments are maintained remain same as in the synthesis 1. Except for GDL, which is directly used and for Ti instead of HCl, HF was used for a better etching. The electrolyte used is 0.4 M FeSO 4 or Fe 2 (SO 4 ) 3 . We cannot add NaOH as it forms a strong Na salt on top of the substrate which blocks the foam formation. The electro-chemical deposition of Fe NF is performed with a constant current of 1.2 A for time periods of 15 sec, 30 sec, 45 sec, and 90 sec. As observed from the SEM data, the best times were -Fe(II) on Ti base (15 sec) ; Fe(II) on GDL or Fe base (90 sec) ; Fe(III) on GDL base (45 sec) ; Fe(III) on Ti or Fe or Al base (90 sec).

Liquid Phase Electrochemical (EC) Cell

The liquid phase cell prototype designed in the past in our laboratory [42] was used for performing the catalytic tests for different nanofoam based electrodes. The cell works on a three-electrode configuration, as it is graphically represented in Fig. 2. The cathode side houses the working electrode (circular, diameter 2.7 cm). A Pt ring makes up for the electric contact and closing of the external circuit between the working electrode and the counter electrode. A commercial Pt ball-edged rod (Amel) acts as the counter electrode, in the anodic compartment. And the reference electrode (Ag/AgCl) is placed close to the working electrode in the cathodic compartment of the cell. Constant voltage conditions are applied to the electrodes using a potentiostat-galvanostat (Amel mod. 2049A).

Fig. 2 Schematic of the experimental setup of liquid phase cell for CO 2 reduction to fuels.

The electro-catalytic cell is divided into two compartments: 1) anodic and 2) cathodic with the protonexchange membrane (Nafion® 117) acting as the partition. A 0.5 M aqueous KHCO 3 electrolytic solution is used in the anodic compartment to generate H + ions necessary for performing the reaction, while in the cathodic compartment a CO 2 (10 ml/min, pure) saturated electrolyte is flowed using an external tank and peristaltic pump. The fuels are analysed using Gas Chromatography-Mass Spectrometer (GC-MS) and Ion Chromatography (IC).

Results and Discussions

Characterization and Measurements

The nanofoams were characterized using techniques such as SEM, EDX, XRD, XPS and current profiles to determine the physical and structural properties. From the SEM surface and cross-sectional views, for the NFs via method-I, which formed leady dendrite-like structures with an actual weight of about 25-30 mg, measured by peeling of the NF from the metal base, and the respective average porosity of about 50 % (for average thickness of 15 µm).

For the NFs via method-II lead to the formation of truncated hollow-conical foamy structures with additional intra-porous structures. The NFs weight was measured to be about 10-15 mg by peeling and has an average porosity of 84 % with a Surface Area of about 38 cm 2 (per 1 cm 2 geometrical area).

SEM Morphological Measurements

Scanning Electron Microscopy (SEM) was performed by Phenom ProX Scanning Electron Microscope (SEM) equipped with EDX analyser to understand the structural and morphological surface characteristics of the nano-foams. Fig. 3 (a) &(b), shows the surface with Cu foamy structures formed on Cu foil with deposition time = 1 min and 4 min, respectively (synthesis method-I). In the former figure, it can be observed that dendrite structures are yet to be formed, while in the later a flake-like structure formed bulk up into flakes due to increased time of deposition of the dendrites. b), shows the thick leaf-like Cu foams formed on Fe metal foil as the base substrate. It can be observed that these dendrites are more compact and closely packed as compared to that of the Cu foil based foams. Moreover, the lengths of these systems ranged from 100 nm to 100 μm. Since the thickness and length are very high, they collapse under their own shear weight and form a dense intricate structure. Fig. 4 (c) &(d), shows the Cu foam formed on an Al grid as base substrate. The foam is compact and thick. Not many dendrites are seen for Al as the base. In addition, from the cross-sectional view, we can calculate the thickness of the foam, which is about 4-5 μm. Increasing the deposition time would make the foam peel-off since the base is cylindrical, thereby it has additional bending forces.

Fig. 5 SEMmethod-II (a) Cu NF on GDL (2 min), (b) Cu NF on GDL cross-section, (c) Cu crystal flowers on GDL (30 sec), (d) Fe NF on GDL (45 sec), Fe NF on Ti -(e) Fe 2+ (15 sec) & (f) Fe 3+ (90 sec). Surface Analysis

A quantitative surface analysis was performed by X-Ray photoelectron spectroscopy (XPS) using PHI 5000 VersaProbe II spectrometer with a monochromatic Al-Kα X-ray source operated at 100 W, 20 kV.. The survey gives an insight into the oxidation state of the Cu foam formed. The C-1s binding energy of adventitious carbon (284.9) was used as the reference. Fig. 6 (a), gives the general survey spectra of the Cu nano-foam, while (b) shows a high-resolution spectrum evidencing the peaks at 932.3 eV and 952.1 eV. They represent the characteristic binding energies for the CuO species, which are further confirmed by satellite peaks at 942.5 eV and 962.6 eV.

Fig. 6 (a) XPS wide scan survey of Cu NF on Cu (b) High-resolution spectra of the Cu 2p doublets.

Energy Dispersive X-ray spectroscopy (EDX)

EDX was carried out on different nano-foam samples, to get an idea of the composition of the different elements present. Fig. 7 shows the EDX spectra for Cu NF on Cu foil, and the respective atomic concentrations are Cu (76.4 %), O (22.6 %), C (1%). Other samples EDX are discussed in the supplementary section.

Fig. 7 EDX spectra of the Cu nano-foam with base substrate as Cu foil.

Structural Analysis

X-Ray Diffraction (XRD) was performed to determine the crystal structure of the nano-foam using D2 Phaser Bruker diffractometer equipped with a Cu-Kα radiation (1.5406 Å) source operating at 30 kV and 10 mA. A scan rate of 0.025°/s with a 2θ range in 12° to 100°. Fig. 8 shows the XRD patterns for Cu NF on Cu foil. From the diffraction pattern, the 3 peaks at 2θ = 43.54°, 50.61°, and 74.25°, corresponding to the planes (111), (200) (220), respectively of the FCC of Cu metal. Also, 2 peaks are observed at 2θ = 36.76° and 58.64°, which may correspond to CuO formed.

Fig. 8 XRD pattern of the Cu nano-foam with base substrate as Cu foil.

Current profile

The current profiles (in amperes) for the CO 2 reduction reactions, for the different Cu NFs on different base substrates and Cu Foil as a reference are shown below in Fig. 9. (i) A constant potential of -1.5 V is applied to the cell with the goal to compare these catalysts. It can be observed that the currents at the beginning of the reaction were quite high and then stabilize as an equilibrium P H was achieved in the electrolytes in the anodic and cathodic compartments. Although the initial currents vary from 0.1 to 0.5 A, the equilibrium current lies in the range of 5 mA to 20 mA. (ii) Different potentials were applied, taken from cyclic voltammetry-reduction potentials to improve and achieve high faradaic efficiencies.

Fig. 9 Current profiles -(i) Constant Potential of -1.5V -(a) Cu NF on Cu, (b) Cu foil, (c) Cu NF on FeCrNi, (d) CuNF on (Al grid+Cu foil) ; (ii) Different potentials -(a) Cu NF on (Al grid+FeCrNi), (b) Cu NF on GDL, (c) Fe NF on GDL, (d) Fe NF on Ti (Fe 2+ ), (e) Fe NF on Ti (Fe 3+ ). Cyclic Voltammetry (CV)

To understand the reduction potentials and achieve higher fuel efficiencies, CV was performed on individual catalytic electrodes from which the high Faradaic efficiencies (F.E.) shown in Table 2 were accomplished. The CVs are shown in the Fig. 10.

Fuel Productivity

The fuels produced during the electro-chemical reduction of CO 2 were analysed using various techniques, which include IC, GC-Mass, and GC-TCD. The major liquid products detected in the liquid electrolyte on the cathode side were formic acid, methanol and acetic acid. Minor quantities of acetone, isopropanol and ethanol were also found. The gaseous products detected were H 2 , CH 4 and CO. The results are expressed in micromoles of the products obtained for a given time of the reaction and the experiments were performed under same conditions, with potentiostatic (-1.5 V), and constant CO 2 flow rate of 10 ml/min and the catalytic surface area ~ 5.7 cm 2 , so that the catalytic activity of different materials can be well-compared under these fixed conditions as shown in Table 1. Higher faradaic efficiencies were achieved with specific catalysts (chosen from the above fixed condition experimental results) with the catalytic reduction tests performed at different electric potential conditions acquired from the cyclic voltammetry curves as seen in Table 2.

The fuel values for the NF electrodes synthesized via electro-chemical deposition using method-I and the respective plain metal electrodes (which acted as the base substrate for the foams) are shown in Table 1. It is clearly evident that for the nano-foams the net C-products increased by several factors for different combinations of metal foams on metal substrates as compared to that of plain metal foil electrodes, thereby confirming that the activity is mainly due to the NFs. The plain metals showed near inert activity expect for the Cu metal. In addition, there is a huge amount of H 2 production for the NFs (several orders higher than plain metals). The highest productivity obtained was for Cu NF on FeCrNi foil, which was about 44.6 μmol/hr C-products (i.e. 313 μmol/hr.cm 2 per gm of catalyst) and closely followed by a bi-electrode system (CuNF on Al grid + CuNF on Cu foil) with a value of 44.8 μmol/hr (240 μmol/hr.cm 2 per gm, since dual electrode). Here, the priority was not faradaic efficiencies, but to make a good comparison for various catalysts under constant conditions. Also, for the NF based electrodes a better productivity of oxygenates and hydrocarbons with higher chain (C2-C3) was observed as compared to plain metal electrodes. Higher amounts of Cproducts along with higher H 2 values indicate that the NF electrodes utilize the energy supplied more efficiently. Moreover, there were gaseous products (CH 4 , CO) observed for NFs, which were not seen for pure metal foil electrodes. The foams were synthesized via a simple technique and for some cases without application of electricity using only the reduction potential differences of metals, thereby enabling this method to be easily scalable for mass production of foams.

Table 1 Productivity for the CO 2 electro-catalytic reduction in liquid phase at fixed potential of -1.5 V for nano-foam electrodes (Synthesis type -I) (Units: µmoles per hr) A time dependent study conducted on the products formed is shown in Fig. 11 (a) & (b), for two samples -Cu NF on FeCrNi foil and (Cu NF on Al grid + Cu NF on Cu foil). For these samples, the formation of H 2 and C-products continue to rise steadily with time. Also, reaction run-time was increased from 4 hr to 20 hr and also, the working electrodes were reused, to see, if there is a significant loss in the catalytic activity. The fuel productivity values were found to be steadily increasing with time and also, the re-use of the electrodes (2 times) showed a slight loss of about 5-10 % in products indicating the stability and long-term usage of the NF based electrodes. Table 2 shows the NF electrodes synthesized via method-II and their respective fuel productivity in terms of faradaic efficiencies. High F.E.s were achieved by applying specific potentials chosen from cyclic voltammetry. For Cu NF on different base substrates (Cu, FeCrNi, GDL) the net-F.E. ranged from 42 % to 83 %, with highest F.E. for Cu NF on GDL. In-addition to having 75 % higher total F.E, it also has about 1.6 times higher formic acid efficiency (F.E., 58 %) compared to Cu NF cited in literature [36] and much higher when compared with state-of-the-art Cu crystalline electrodes. A comparison of the results is provided in the Table 2 for convenience.

For the Fe NFs, the achieved F.E. was about 92-95 % with the best F.E. for Fe NF on Ti (Fe 3+ precursor). Moreover, it can be noticed that the there is significant improvement and shit towards the higher alkyl product (acetic acid) with highest value of 49 %. Although, Fe NF on GDL (F.E. 92 %) is only slightly lower, it has an added advantage to the electrode which can be used in gas phase catalytic systems (electrolyte-less cells), in-addition to conventional liquid phase cells.

Conclusion

Here, we presented two main types of catalysts and their respective electro-catalytic CO 2 reduction study. The first type is Cu based nanofoams or dendrite-like structures, which are formed via electrodeposition on different substrates (Cu, Fe, FeCrNi, Al, Al grid, GDL) and tested for CO 2 reduction. With this, we showed that the NFs structure and catalytic activity are both altered greatly with varying base substrates, (the best productivity of 313 μmol/hr.cm 2 per gm of catalyst Cu NF on FeCrNi). We tried to improve the electro-deposition conditions used to synthesize the Cu NF, thereby created new set of Cu NF electrodes with much uniform and higher porous foams, which in-turn proved to be better catalysts for CO 2 reduction, with the a substantial improvements in productivity, with the highest faradaic efficiency (83 %) seen for Cu NF on GDL at -0.3V/Ag/AgCl. This is also, nearly twice the efficiency than the best Cu NFs cited in literature [36] working in simple, non-ionic electrolytes.

CO 2 reduction test were performed using just the base substrates alone to see their contribution to the NF performance, and found that the activity is mostly dependent on the NF alone. Also, time dependant study was performed to see the loss in catalytic activity as the reaction progresses.

The second work, was successful development of precise conditions of electro-deposition of uniform, highly porous Fe nanofoams and dendritic structures. The novel Fe NFs, modelled are firstof-its-kind, to be prepared via a cost effective and simple electro-deposition technique. Also, the use of Fe NFs in CO 2 electro-catalytic reduction is the first ever to be reported. This opened doors towards the highly efficient, very cheap, abundant Fe NF based catalytic electrodes in CO 2 reduction to fuels with faradaic efficiencies ranging from 92-95% for Fe NFs on GDL, and Ti substrates. Inaddition, Fe NF showed to be more effective towards reducing CO 2 to higher alkyl products (best Faradaic efficiency for acetic acid, 49%). Another, important outcome was to be able to develop nanofoam on GDLs, which put-forward possible usage of the NF based catalysts in both electrolyteless (gas phase) and liquid phase conditions moving us a step closer towards artificial leafs.

Fig. 1

 1 Fig. 1 Schematic of Electro-deposition of Nanofoam on polished metal substrates.

Fig. 3

 3 Fig. 3 SEM image (top view) of the Cu NF on Cu foil with deposition time (a) 1 min, (b) 4 min.

Fig. 3

 3 Fig. 3 SEM images of Cu NF on Cu (2 min) -dendrite structures (c) top view, (d) cross-section view.

Fig. 3

 3 Fig. 3 (c), shows the top view of the Cu dendrite structures formed on Cu foil with deposition time = 2 min. The dendrites resemble a leaf like model, with each leaf edge branching out into more leaves with smallest edge measuring about 100 nm. The leaf branches measure about 3, 5, 8 μm and the maximum about 13 μm before it collapses due to the increasing weight. The leaf branching approximately follows a Fibonacci sequence. Moreover, holey areas can be observed, which are formed due to the H 2 bubbles pushing out of the foamy structures formed during the electrodeposition. Fig. 3 (d), shows the cross-sectional view of the NFs, evidencing a thickness of the Cu NF ranging between about 15-20 μm.

Fig. 4

 4 Fig. 4 SEM images of Cu NF on Fe foil (2 min) (a) top view, (b) dendrite close-up ; Cu NF on Al grid (2 min) (c) top view, (d) cross-section view.

Fig. 10

 10 Fig. 10 CV curves -(a) CuNF on GDL, (b) FeNF on GDL, (c) FeNF on Ti (Fe 2+ ), (d) FeNF on Ti (Fe 3+ ).

Fig. 11

 11 Fig. 11 Time vs productivity of formic acid, acetic acid, CO and H 2 for (a) Cu NF on Inconel, (b) Cu NF on Al grid + Cu NF on Cu foil.

  Chemicals used in our synthesis route are Iron (III) nitrate nonahydrate [Fe(NO 3 ) 3 .9H 2 O], Iron (III) acetyl acetonate [Fe(C 2 H 7 O 2 ) 3 or Fe(acac) 3 ], Iron (II) sulphate heptahydrate [FeSO 4 .7H 2 O], Iron (III) sulphate hydrate [Fe 2 (SO 4 ) 3 .H 2 O], Copper (II) sulphate pentahydrate [CuSO 4 .5H 2 O], Copper (II) acetate monohydrate [Cu(CH 3 COO) 2 .H 2

CATALYSTS (Synthesis type I) FUELS ( µmol•hr -1 )

	Cu NF on Cu (4 min)		6.16	0.14	0.03	0.00	0.00	0.02	-	-	340.90	6.35
	Cu NF on FeCrNi (2 min)	23.60	0.26	0.84	0.01	0.00	0.01	-	19.90	777.86	44.62
	Cu NF on Al (2 min)		11.03	6.43	0.29	0.01	0.00	0.01	-	3.30	226.58	21.07
	Cu NF on Fe *		0.16	0.42	0.37	0.00	0.00	0.00	2.25	2.13	61.58	5.33
	Fe	NF	on
	FeCrNi.		5.31	0.63	0.62	0.00	0.01	0.02	-	-	1538.04	6.59
	(2 min)
	Fe NF on Al (2 min)		0.36	0.82	0.07	0.00	0.00	0.00	-	-	480.27	1.25
	Cu NF on (Al grid + Cu Foil) **	37.19	2.52	1.45	0.01	0.00	0.02	-	3.62	62.17	44.81
		* Cu NF on Fe electrode prepared using under potential ; ** Combo-electrode Cu NF on (Al grid + Cu foil)
				Formic Acid	Acetic Acid	Methanol	Acetone	Iso-Propanol	Ethanol	CH 4	CO	H 2	C-Products (NET)
	Metal
	Cu Foil		12.48	0.06	0.37	0.01	0.01	0.02	2.07	2.80	264.52	17.81
	FeCrNi Foil		0.12	0.00	-	-	-	-	-	-	22.83	0.12
	Al Foil			0.58	0.00	-	-	-	-	-	-	89.28	0.58
	Fe Foil			0.16	0.00	-	-	-	-	-	-	57.20	0.16
	Nanofoam (NF)
	Cu NF on Cu (1 min)		1.96	0.07	0.00	0.00	0.00	0.00	-	-	97.82	2.03
	Cu NF on Cu (2 min)		7.87	0.28	0.03	0.00	0.06	0.05	2.24	11.52	1069.55	22.05

Table 2

 2 Productivity for the CO 2 electro-chemical reduction in liquid phase cell for nano-foam electrodes (Synthesis type -II) in terms of Faradaic Efficiencies.

CATALYSTS (

Synthesis type II) NanoFoams Potential (V) E vs Ag/AgCl Major C-Products FUELS (Faradaic efficiency %), F.E.

	Cu NF on Cu		-1.50	HCOO -, CO, EtOH	37.24	1.67	7.30	48.02	49.0
	Cu NF on FeCrNi		-0.95	HCOO -, CO, CH3COO -	34.01	6.69	17.99	39.75	58.7
	Cu NF on GDL		-0.30	HCOO -, CO, CH3COO -	58.06	13.21	11.57	15.82	82.8
	Fe NF on GDL		-0.35	HCOO -, CH3COO -, CO	44.57	27.16	20.27	7.63	92.0
	Fe NF on Ti (Fe 2+ )		-0.85	CH3COO -, HCOO -, CO	24.48	48.81	18.69	7.11	91.9
	Fe NF on Ti (Fe 3+ )		-0.40	CH3COO -, CO, HCOO -	17.52	43.61	33.36	4.94	94.5
	REFERENCES
	Cu NF on Cu	[36]	-1.5	HCOO -, CO	37	-	7	55	47
	Cu Poly crystalline [43]	-0.8 vs RHE	HCOO -, CO	26	-	18	50	48.5
	Cu [44]	Nanoparticles	-1.8	CH4, C2H4	3.5	-	0.1	49.7	35.7
	Cu NF hollow fibre [37]	-0.5 vs RHE	CO, HCOO -	10	-	72	14	85
						Formic Acid	Acetic Acid	CO	H 2	C-Products (NET)

Acknowledgements

The author thanks the European Joint Doctoral Programme on Sustainable Industrial Chemistry (SINCHEM -Erasmus+ Action 1 Programme (FPA 2013-0037)) for funding and supporting his PhD at the University of Messina in collaboration with C2P2 (UMR 5265 CNRS-UCBL -CPE Lyon). We thank Dr. Salvatore Abate, Dipartimento di Ingegneria, Università di Messina for the XPS analysis. References