Razik Djara

Dr Nathalie Masquelez

Dr Marie-Agnès Lacour

Abdelhafid Merzouki

Prof. D Julien Cambedouzou

David Cornu
email: david.cornu@enscm.fr

Sophie Tingry
email: sophie.tingry@umontpellier.fr

Yaovi Holade
email: yaovi.holade@enscm.fr

Prof ] R A Djara

Merzouki

Dr S Cornu

Dr Y Tingry

Holade

Self-Supported Electrocatalysts Derived from Nickel-Cobalt Modified Polyaniline Polymer for H2-Evolution and O2-Evolution Reactions

Keywords: electrocatalysis, hydrogen evolution reaction, noble metal-free, oxygen evolution reaction, polyaniline

The water splitting aims to produce H2 for fuel cells and a number of industrial processes such as the ammonia synthesis and the metals refining. There is actually a major limitation about the development of cost-effective and efficient electrocatalysts to lower the cell voltage that must be overcome to perform sequentially the overall reaction that is 2H2O → 2H2 + O2. In the present study, we combined Ni and Co as the promising alternative catalytic metals to Pt-group metals as well as N and S as the doping elements to fabricate self-supported electrocatalysts for hydrogen evolution (HER) and oxygen evolution (OER) in alkaline media. To these ends, the polymerization of the aniline into polyaniline (PANI) in the presence of Ni(II) and Co(II) followed by calcination in air resulted in the formation of heterogeneous catalytic materials self-supported onto a matrix made of C, N, S, Ni and Co. Enhanced activity and durability were recorded for the hybrid and bimetallic PANI-NiCo composite.

Introduction

The electrochemical approach emerges as a sustainable route for the production of hydrogen, which is a high-energy density fuel for clean electricity production and an important feedstock for the synthesis of platform chemicals. [1] Yet, finding electrocatalytic materials that combine high activity, long-term durability and low cost is a challenging task as it would require foregoing precious metals, i.e. those of the platinum group. So among the conducting polymers, polyaniline (PANI) has received the most important interest in heterogeneous electrocatalysis during the last years. [2] This interest is due to the fact that PANI can either be used directly as a support/electron relay because of its electrical conductivity, [2d,3] or as a source of nitrogen doping after a well-controlled calcination phase. [2c,4] One of the logical solution to augment the electrocatalytic performance is to incorporate Ni and/or Co species. Song et al. [5] showed that the electro-polymerization of aniline onto a nickel foam significantly decreases the overpotential towards hydrogen evolution reaction (HER) of 0.1 V at a metric current density of 10 mA cm -2 in comparison to the bar nickel electrode. Feng et al. [3f] reported that the modification of the CoP by PANI leads to a PANI-CoP material with high electrochemically active surface area and superior electrocatalytic activity towards HER in an acid medium. It was argued that PANI captures H + species from hydronium ions to form protonated amine groups that have higher positive charge density than those of hydronium ions from the solution, thus accelerating the kinetics of HER at the PANI-CoP surface. [3f] One can be summed up that the chemistry behind these strategies requires the co-material (e.g. CoP, nickel foam) to be prepared in advance and cannot be elegantly be generalized to metallic cations such Ni 2+ or Co 2+ . Indeed, those ionic species could be readily lost during the electrocatalysis and decrease the catalytic activity and durability. Also, the above conclusion cannot meaningfully be implemented in alkaline media since the adsorption of H2O molecules instead of H + and their splitting is necessary (2H2O + 2e -→ H2 + 2HO -, E° = 0 V vs reversible hydrogen electrode (RHE)). Other limitations are the coverage of some active sites of the catalytic material by the PANI when the latter is deposited after the formation of metallic particles as well as the obvious absence of the creation of a strong metal-support interaction.

The emerged scenario to tackle the above limitations is the one-pot chemical polymerization in the presence of metallic cations followed by a proper thermal treatment to yield electrocatalysts with high activity. [2c,4b,4e] We recently reported that the use of the ammonium persulfate (APS, (NH4)2S2O8) as the oxidizing agent and the nickel nitrate resulted in the fabrication of Ni3S2 species after thermal treatment at 700-1000 °C; and demonstrated an outstanding efficiency towards HER and oxygen evolution reaction (OER) in alkaline media. [4b] Furthermore, nickel cobaltite (NiCo2O4) was studied for a long time as a promising electrode for O2 electrocatalysis. [6] The most utilized synthesis approach is the hydrothermal process combined with heat treatment (300-500 °C), which can lead to spinel nickel cobaltite with high water splitting activity (i.e. HER and OER). [7] The origin of the high performance of NiyCo3-yX4 (X = O or S, 0 < y < 2) results from to synergetic effect arising from the mixed transition metal cations, [6a,8] which enable controlling the adsorption of the reactants and the removal of the products.

N and S are known to allow the manipulation of the catalytic performance, [9] however, precisely how both elements could be incorporated into the same material to target high efficiencies remains a puzzle.

Herein, we report the combination of the chemical polymerization of the aniline (C6H5-NH2, C and N source) by the ammonium persulfate ((NH4)2S2O8, N and S source) in the presence of Ni(II)-Co(II) salts and the thermal treatment to fabricate a self-supported heterogeneous PANI-NiCo material as a multifunctional and efficient electrocatalyst for both HER and OER in alkaline media. The performed comprehensive structural characterization showed that the transformation at 550 °C under air allow converting the polymerization reaction mixture into a nanostructured carbon-sulfur-nitrogen network, which supports various Ni-Co-X (X = O, S) phases among which the bipyramid nickel cobaltite. This heterogeneous PANI-NiCo material demonstrated outstanding electrocatalytic activity and stability towards HER and OER in a basic electrolyte.

Results and Discussion

In order to synthesize a nanostructured matrix made of C, N, S, Ni and Co species, we implemented the chemical polymerization of the aniline in the presence of metallic precursors followed by the thermal treatment. Figure 1a resumes the adapted strategy for the synthesis. While the present study focuses on NiCl2•6H2O and CoCl2•6H2O precursors, the nitrate salts were however used in control synthesis. The mixture of the polymerization reaction underwent a solvent removal in a rotary evaporator to efficiently trap metal cations in the solid that will dry out afterwards at 80 °C. We previously found that this rotovap step is important because a simple filtration over Buchner results in the loss of Ni(II) and low yield of the synthesis. [4b] The second step was the thermal treatment to allow the assembling of Ni and Co elements into Ni-Co-X (X = O, S) particles that are self-supported on a matrix made of C, N, S, Ni and Co. We hypothesized that the use of air as the environment for the thermal treatment instead of an inert gas such as nitrogen or argon will allow reaching an optimal compromise between the formation of a certain number of bipyramid nickel cobaltite particles onto a carbon-nitrogen-sulfuroxygen-nickel-cobalt network and the removal of excess carbon, chloride, and sulfur species. Indeed, not only this thermal transformation defines the bonds in carbon-heteroatom but also the type of Ni-or Co-heteroatom junction, which all above will regulate the electrocatalytic performance. Seminal reports of Rasiyah et al. [6c,8] showed that the NiCo2O4 spinel phase can be obtained by a freeze drying of a solution of nickel and cobalt nitrate followed by the vacuum decomposition and thermally treatment in air at 400 °C. Recent reports utilized the calcination under different conditions, 375 °C in air, [10] 350 °C in Ar, [7a] 350 °C in N2, [7b] 400 °C (no indication of the type of gas that was used), [7c] 350 °C in air, [7d] and at 300 °C in air. [11] It should be emphasized that the implementation of those temperatures of 350-400 °C in the present study could probably failed because the polyaniline material has complex behavior in this temperature range. [12] In order to clarify the above points, we conducted thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis to probe the effect of the addition of the metal precursor and the nature of the gas that should be utilized during the thermal treatment. Figures 1b andS1-S3 show the obtained results. The minor processes occurring below 150 °C are attributed to the evaporation of the water molecules that are adsorbed and/or trapped between the hydroscopic chains of the PANI polymer [12][13] and confirm that the materials were well dried. Then, the elimination of the residual doping agent and the decomposition of smaller fragments of PANI occur at 200-350 °C with an endothermic peak at 241 °C while the degradation of the main chains takes place above 350 °C. [12b,c] As hypothesized above, the introduction of nickel and cobalt salts triggers a positive shift towards 282 °C and 386 °C for the first peak and second peak, respectively. When the gas is nitrogen, these two peaks are located at 285 and 387 °C, respectively. Those findings could indicate that those Ni(II) and Co(II) species have an effect on the phenomena occurring there. Since this range of temperature regulates the rearrangement of the different atoms leading the bonds (carbon-carbon, carbonnitrogen, nickel-sulfur, cobalt-sulfur, nickel-oxygen, and cobaltoxygen), it became obvious that a temperature below 400 °C is not suitable for obtaining NiyCo3-yX4 (X = O, S). We conducted several control experiments, which demonstrated that the calcination under air at 550 °C was the optimal temperature. Unless otherwise specified, the next discussion focuses of the deep study of the physicochemical, electrochemical and electrocatalytic characteristics of the as-synthesized PANI-Ni, PANI-Co, and PANI-NiCo after calcination under air at 550 °C. We first utilized N2 as probing molecule to access the textural properties of the as-synthesized PANI-Ni, PANI-Co, and PANI-NiCo materials. More specifically, we evaluated the specific surface area (the so-called Brunauer-Emmett-Teller, SBET(m 2 g -1 )) as well as the pore volume and established the pore size distribution (Barrett, Joyner and Halenda (BJH)). Figure 1c shows the adsorption-desorption isotherms of N2 on the different materials. The mesoporous character is evidenced by the slightly hysteresis loop. The extracted quantitative data are reported in Table S1. The determined surface area is SBET = 10, 5, and 9 m 2 g -1 for PANI-Ni, PANI-Co, and PANI-NiCo, respectively. The pore diameter (or pore size = 2×RBJH) decreases as follows: PANI-Ni (47 nm) < PANI-Co (44 nm) < PANI-NiCo (29 nm). Those results suggest that the materials exhibit a mesoporous character. Furthermore, the pore volume is 0.049, 0.029, and 0.041 cm 3 g -1 for PANI-Ni, PANI-Co, and PANI-NiCo, respectively. It can be observed that the introduction of Co species in the PANI-Ni material enables decreasing the pore size from 44 to 29 nm while keeping the same pore volume and BET surface. The shape of the isotherms at lower pressures implies that the above volume mainly results from mesopores.

We then used powder X-ray diffraction (PXRD) to determine whether or not we succeeded in the preparation of a heterogeneous material comprising various structures including nickel cobaltite. Indeed, by using a relatively low calcination temperature of 550 °C, we were targeting the formation of NiyCo3-yX4 (X = O, S) to coexist with other species from C, N, S, O, Ni, and Co. Our goal was to release a material with multiple structures and capable of providing synergic effect during the catalysis. Figure 1d (♥, JCPDS 48-0083) and Ni(OH)2 (♦, JCPDS 14-0117). Note that the main characteristic peaks (denoted by ↓ signs in Figure 1d) of the NiCo2O4 spinel structure (JCPDS 20-0781) could hardly be resolved in the PANI-NiCo sample. This would mean that the signal from the nickel cobaltite would be blurred into that of the other structures as explained above. Figure S4a shows the peaks at 18.9°, 30.9°, 36.7°, 38.3°, and 44.4°, correspond to the (111), ( 220), ( 311), ( 222), (400) planes, respectively, of NiCo2O4. The gain a deep knowledge on the morphology of those PANI-based materials, the scanning electron microscopy (SEM) was used. Figure 2 shows the backscattered SEM images in order to make a better contrast between metallic and other species. The morphology of the material is in agreement with the data from the N2 adsorption-desorption isotherms, especially the low BET surface of the PANI-Co. The first finding is that PANI-Ni and PANI-Co do not show any significant presence of bipyramid particles in comparison to PANI-NiCo. Additional backscattered SEM images reported in Figures S4a-S4d substantiate those observations. In other words, the simultaneous presence of Ni and Co is a prerequisite to obtain this structure. Similar shape was reported and attributed to NiCo2O4 octahedron exposing <111> crystal planes after complementary analysis by XRD, and high-resolution transmission microscopy. [11] The control synthesis of PANI-NiCo materials performed at 350 °C (Figure S5, no obvious presence of bipyramid particles), 450 °C (Figure S6, no obvious presence of bipyramid particles), and 650 °C (Figure S7, presence of bipyramid particles), definitely underpins our postulated hypothesis that a temperature below 400 °C is not sufficient to generate the targeted particles. Finally, the backscattered SEM images of the control synthesis of PANI-NiCo material by the calcination at 550 °C under N2 (Figure S8) reveal the presence truncated octahedron particles, which were previously reported to be NiCo2O4 exposing <111> and <100> crystal planes. [11] The above SEM observations only provide a qualitative analysis of the materials. So, we next utilized the energy dispersive X-ray spectroscopy (EDX) to question whether a NiyCo3-yO4-xSx heterogeneous could be obtained or not. Indeed, the simultaneous self-assembly of Ni and Co with O or S to form NiyCo3-yX4 (X = O, S) is well-known to be more favorable than individual NiXz or CoXz. [9a,11,14] As both S and O were present in the synthesis conditions, we specifically utilized the micro-scale EDX mapping to clarify the situation. Figures 3a-c depict the backscattered SEM images, the corresponding mapping and quantification of the Ni, S, C, O and Co for the three type of materials obtained under air at 550 °C, for instance, PANI-Ni, PANI-Co, and PANI-NiCo. By tracking the positions of S and Co, it can be clearly observed from Figures 3b and3c that their mappings do not overlap. The result means that Co and S are not simultaneously present in the same structure. On the other hand, it can be seen in Figure 3c that the positions of the Co and Ni elements overlap perfectly with a significantly higher oxygen concentration than that of PANI-NiCo material obtained under N2. Those observations qualitatively rule out the formation of NiyCo3-yO4-xSx species, but support the obtaining of NiyCo3-yO4 that coexists with other nanostructured species.

The EDX spectra are reported in Figures S10-S14 and the main quantitative data are shown in Figure 3 (extended data are gathered in Table S2). The nitrogen content was N(at%) = 0.83 ± 0.23, 0.76 ± 0.44, and 0.60 ± 0.28 for PANI-Ni, PANI-Co, and PANI-NiCo, respectively. The sulfur amount was found to be S(at%)= 10.93 ± 0.60 (PANI-Ni), 15.01 ± 0.95 (PANI-Co), and 13.29 ± 1.31 (PANI-NiCo). The found atomic ratio of Co/Ni = 1.88 ± 0.09, and 2.37 ± 0.12 in PANI-NiCo obtained by thermal treatment under air and nitrogen confirm the theoretical expectation. It can however be observed that the O/(Ni+Co) atomic ratio of 2.52 ± 0.18 (PANI-Ni), 4.41 ± 0.13 (PANI-Co), and 4.54 ± 0.13 (PANI-NiCo) do not support the obtaining of a NiyCo3-yO4. For the monometallic PANI-Ni and PANI-Co materials, the EDX analysis suggests that metallic Ni and Co coexist with oxidized species. These outcomes are in agreement with the PXRD results that previously suggested the presence of multiple structures. The EDX profiles at the different places of the PANI-NiCo material and reported in Figure S13 confirm this hypothesis. Based on those physicochemical characterizations, it can be concluded that the thermal treatment of the polyanilinebased nickel-cobalt composites results in the formation of a heterogeneous materials. This heterogeneity is often very interesting from an electrocatalytic point of view when at least one reaction is targeted for the same material. This is the purpose of the present study where HER and ORR are targeted. In practice, this avoids having to change the electrode composition once the reaction is changed. So the next section aims to examine the electrochemical properties of those PANI-Ni, PANI-Co and PANI-NiCo materials.

The preceding discussion demonstrated that the optimal combination of the chemical polymerization and the thermal treatment allows synthesizing composite materials with different structures. Hence, we sought to study carefully the impact of this heterogeneity on the electrocatalytic ability towards two processes, HER and OER in alkaline media. Before investigating those electrocatalytic properties, we preliminary utilized the cyclic voltammetry (CV) as a powerful tool to determine their electrochemical characteristics. Figure S15 displays the recorded voltammograms at 50 mV s -1 scan rate in 1 M KOH electrolyte (25 °C). The electrode based on PANI-Co material does not show any significant redox peaks in comparison to the PANI-Ni, but there is a continuous increase of the current from 1 V vs RHE. For the PANI-Ni electrode, the pair of oxidation and reduction peaks between 1.2 and 1.5 V vs RHE are historically attributed to the reversible electrochemistry of Ni(OH2) hydroxide and NiOOH oxyhydroxide because the metallic nickel is irreversibility electrooxidized during the first cycle. [15] For the bimetallic PANI-NiCo, the faradaic process associated to this oxyhydroxide/hydroxide significantly augments and the reduction peak shifts negatively of about 100 mV. This can be translated to as a synergic effect between nickel and cobalt species. Seminal studies of NiCo2O4 electrode attributed those redox processes to the simultaneous Co 2+ /Co 3+ and Ni 2+ /Ni 3+ transitions and even the Co 3+ /Co 4+ . [8,16] It is known that the presence of two valence states on the sub-lattices and the possibility of excess oxygen in a NiCo2O4 spinel (Co 2+ 1-xCo 3+ x [Ni 2+ y N 3+ 1-yCo 3+ 1.0]O 2-4 (x=y)) create a synergetic effect in this mixed transition metals, which enables regulating the adsorption of the reactants for a better electrocatalysis. [6a,8,16] Based on the large electrical charge associated with the redox peaks of the PANI-NiCo material and thus a larger number of active sites, it is anticipated that this electrode will result in a better electrocatalytic performance. This especially applied to the case of OER whose electrocatalytic kinetics depends on what actually happens in the (oxyhydr)oxides region. To access the electrocatalytic activity towards HER and OER of the as-synthesized materials, the catalytic inks were firstly loaded onto a gas diffusion carbon electrode (carbon paper). [17] Figures 4a-f shows the obtained results, which highlight the superior performance of the bimetallic PANI-NiCo as compared to the monometallic PANI-Ni and PANI-Co. The polarizations of HER in Figure 4a show that at E = -0.3 V vs RHE, j = -8.2, -1.4, and -51.0 mA cm -2 for PANI-Ni, PANI-Co, and PANI-NiCo, respectively. These data show that the achieved current with PANI-NiCo is 6.2 and 36.4 times higher in comparison to PANI-Ni and PANI-Co, respectively. Figure S16 reports the comparison with the state-of-the-art Pt/C catalyst. In Figures 4a,b and at the metric current density of j = -10 mA cm -2 that is employed in the literature to compared the efficiency in HER and OER, [18] E = -0.32, -0.40, -0.22, and -0.09 V vs RHE for PANI-Ni, PANI-Co, PANI-NiCo, and Pt/C, respectively. The determined Tafel slopes of 85-123 mV dec -1 (Figure 4c) suggest that the Volmer step is the rate determining step. [18b] This can explain the low overpotential of the bimetallic electrode where the presence of both metals at different valence states fosters the adsorption of H2O molecules. For comparison, an overpotential of 250 mV at j = -10 mA cm -2 and a Tafel slope of 94 mV dec -1 were reported for NiCo2O4 octahedron. [11] The summarized comparative performance of relevant HER on Nibased catalysts from literature in alkaline solutions is reported in Table S3. The data indicate that the overpotential at j = -10 mA cm -2 ranges from 40 to 500 mV, which means that the present PANI-NiCo exhibits a promising efficiency. The polarization curves for OER in Figures 4d andS16b and the histograms of the overpotential at the metric current density of j = 10 mA cm -2 in Figure 4e show that E = 1.72, 1.62, and 1.60, 1.67 V vs RHE for PANI-Ni, PANI-Co, PANI-NiCo, and Pt/C, respectively. Those results also point out the good performance of the PANI-NiCo material, which is also supported by the Tafel slope of 45 mV dec -1 ; Figure 4f. The comparison with relevant Ni-based catalysts from literature (Table S4, overpotentials of 215-400 mV and Tafel slopes of 36-87 mV dec -1 ) shows that PANI-NiCo has an outstanding performance. Figure S17 reports the control analysis of the HER performance for PANI-NiCo materials obtained by thermal treatment under different atmospheres (air and nitrogen). The data highlight the superior activity of the material obtained by calcination in air, thus confirming the difference in the morphology as previously observed by SEM (Figure 2c1-c3 versus Figure S8a). Similar trend was reported for NiCo2O4 octahedron (enclosed by <111> faces) that has a higher activity than NiCo2O4 truncated octahedron (exposing <111> and <100> faces). [11] Taken together, those set of results demonstrate that the as-synthesized heterogeneous bimetallic PANI-NiCo material can be used as a multifunctional electrocatalyst for both HER and OER.

The previous electrochemical measurements were based on a carbon paper substrate (gas diffusion electrode). This support works better for cathode reactions that do not required high electrode potentials, which could cause the oxidation of the carbon. [17] To avoid/limit this deterioration of the support, the bulk nickel is widely used as supporting material, particularly for the water splitting reactions in alkaline electrolytes. So to fairly evaluate the durability of the PANI-NiCo that has shown the best performance in both HER and OER, the catalytic ink was loaded onto nickel foam (NF). Figures 5a-f show the obtained results. The used program for the accelerated durability test (ADT) consisted of a chronopotentiometry at |j| = 10 mA cm -2 for both HER (Figure 5b) and OER (Figure 5e). Figure 5a shows the recorded voltammograms of the fresh and aged electrodes. After the HER's ADT, the increase in the current density can be attributed to the reduction of the remaining oxidized surface species, the conditioning of the nickel-based electrode in a cathodic potential is routine in electrochemistry. [15c] The presence of two anodic oxidation peaks after the OER's ADT can be attributed to the reorganization of the surface and is similar to the seminal observation at a NiCo2O4-based electrode where the first anodic peak was assigned to a cobalt transition and the second oxidation peak was attributed to either cobalt or nickel transition given the different valence states. [6a,8,16] Figure 5c shows that at j = -10 mA cm -2 , E = -0.26, -0.24, -0.24, and -0.18 V vs RHE for NF_before-ADT, NF_after-ADT, NF-PANI-NiCo_before-ADT, and NF-PANI-NiCo_after-ADT, respectively. In other words, the overpotential decreases of about 60 mV after the HER's ADT for NF-PANI-NiCo, which is 3 times larger than the blank NF (20 mV). This decrease can also be observed at j = -50 mA cm -2 and where E = -0.4, -0.35, -0.32, and -0.25 V vs RHE for NF_before-ADT, NF_after-ADT, NF-PANI-NiCo_before-ADT, and NF-PANI-NiCo_after-ADT, respectively. At a much higher current density of j = -100 mA cm -2 , E = -0.42, and -0.28 V vs RHE for NF_after-ADT and NF-PANI-NiCo_after-ADT, respectively. Those results indicated that, after the electrode conditioning; a small overpotential of 280 mV is necessary to achieve a current density of 0.1 A cm -2 for a Tafel slope of 79 mV dec -1 (Figure 5d). For OER, the current density of j = 10 mA cm -2 was achieved at E = 1.50, 1.66, 1.65, and 1.62 V vs RHE for NF_before-ADT, NF_after-ADT, NF-PANI-NiCo_before-ADT, and NF-PANI-NiCo_after-ADT, respectively (Figure 7f). The NF-PANI-NiCo maintains its Tafel slope of ca. 89 mV dec -1 after the ADT (Figure S18). Those results definitely confirm the excellent activity and stability of the as-synthesized heterogeneous PANI-NiCo as a multifunctional electrocatalyst for both HER and OER.

It was observed in this work that bipyramid NiCo2O4 particles contribute to a higher performance in comparison to the corresponding truncated octahedron particles. It is also wellknown that nanostructured networks composed of carbon, nitrogen, and sulfur (in addition to nickel, and cobalt elements herein) provide good electrocatalytic performance towards both HER and OER. So, a unique structure-activity trend cannot be claimed for the as-prepared material PANI-NiCo given its heterogeneous nature.

Conclusion

We report a two-step sacrificial method to thermally convert the product of the polymerization of aniline in the presence of Ni(II) and Co(II) into an heterogeneous self-supported PANI-NiCo electrocatalyst for both HER and OER in alkaline media. The SEM observations showed that the conditions of the thermal treatment (temperature and gas) are the determining parameters for obtaining a bipyramid phase of nickel cobaltite NiCo2O4 selfsupported onto a matrix made of C, N, S, Ni and Co. The optimized condition that makes a better comprise between the morphology of the particles and the electrocatalytic properties was the calcination at 550 °C under air. For HER, the potential needed to achieved a metric current density of j = 10 mA cm -2 were found to be E = -0.32, -0.40, -0.22, and -0.09 V vs RHE for PANI-Ni, PANI-Co, PANI-NiCo, and Pt/C (catalytic ink loaded onto a carbon paper substrate). For OER, the current density of j = 10 mA cm -2 was reached at E = 1.72, 1.62, and 1.60, 1.67 V vs RHE for PANI-Ni, PANI-Co, PANI-NiCo, and Pt/C. Those results clearly indicated that the performance of the bimetallic PANI-NiCo surpasses those of the monometallic PANI-Ni and PANI-Co, thus evidencing the synergic effect when nickel and cobalt are put together. Furthermore, the electrochemical measurements highlighted that after 20 h of accelerated durability test at j = -10 mA cm -2 , the PANI-NiCo material (catalytic ink loaded onto a nickel foam) still exhibits high performance towards both HER and OER. Specifically, small overpotentials of 180 and 280 mV for current densities of 10 and 100 mA cm -2 were recorded for HER (Tafel slope of 79 mV dec -1 ). This enhanced electrocatalytic characteristics of the PANI-NiCo material was attributed to its heterogeneous composition, which opens new directions towards the design of advanced self-supported electrocatalysts for the electrochemical hydrogen production.

Figure 1 .

 1 Figure 1. (a) Sketch of the used strategy for the synthesis of nickel cobaltite self-supported onto a matrix made of C, N, S, Ni and Co. (b) TGA (left y-axis, solid lines) and DSC (right y-axis, dotted lines) curves in an aluminum crucible in the range of 25-500 °C (5 °C min -1 , 100 mL min -1 ). (c) N2 adsorptiondesorption isotherms and the corresponding pore size (diameter) distribution. (d) XRD patterns, see text for details.

  displays the PXRD patterns. Many peaks attributable to multiple crystalline structures with large lattice parameters appear in all patterns. Metallic fcc Ni (JCPDS 04-850) and hcp Ni (JCPDS 45-1027) are not clearly observable in the PXRD patterns of PANI-Ni and PANI-NiCo, as their main peak should show up at 44.8° (corresponding to (111) peak for the fcc phase and (011) peak for the hcp phase). Also, metallic fcc Co (JCPDS 15-0806) and hcp Co (JCPDS 05-727) are also not clearly visible in the PANI-Co and PANI-NiCo patterns. Therefore pure Co or Ni particles are not dominant, which was expected because the materials underwent calcination at 550 °C under air. The main crystal structures are rather composed of a complex mixture of sulfates, carbonates, hydroxides and carbohydroxides. A tentative indexation of the main peaks in the PANI-NiCo sample involves CoSO4 (♣, JCPDS 46-0004), Ni2CO3(OH)2•H2O (otwayite, ♠, JCPDS 29-0868), Co2CO3(OH)2

Figure 2 .

 2 Figure 2. Backscattered SEM images of the as-synthesized materials. (a1-a3) PANI-Ni, (b1-b3) PANI-Co, (c1-c3) PANI-NiCo.

Figure 3 .

 3 Figure 3. Backscattered SEM images; the corresponding micro-scale EDX mapping and quantitative composition of the materials: (a) PANI-Ni, (b) PANI-Co, and (c) PANI-NiCo. Scale bar = 10 μm.

Figure 4 .

 4 Figure 4. Catalytic ink deposited onto a carbon paper electrode. (a) HER polarization curves recorded at 5 mV s -1 . (b) Overpotential of HER at j = -10 mA cm -2 :  = E-Eeq, Eeq = 0 V vs RHE. (c) Tafel plots of HER. (d) OER polarization curves recorded at 5 mV s -1 . (e) Overpotential of OER at j = 10 mA cm -2 :  = E-Eeq, Eeq = 1.23 V vs RHE. (f) Tafel plots of OER. The electrolyte was 1 M KOH and the temperature was 25 °C.

Figure 5 .

 5 Figure 5. Catalytic ink deposited onto a nickel foam electrode. (a) iRuncorrected CVs recorded at 50 mV s -1 . (b) Chronopotentiometry of HER at j = -10mA cm -2 during the ADT. (c) HER polarization curves recorded at 5 mV s -1 . (d) Tafel plots of HER. (e) Chronopotentiometry of HER at j = -10mA cm -2 during the ADT. (f) OER polarization curves recorded at 5 mV s -1 . The electrolyte was 1 M KOH and the temperature was 25 °C.

Acknowledgements

This work at IEM Montpellier was supported by the CNRS Energy unit through the project PEPS19-ELECTROFUEL. D. Razik gratefully acknowledges the financial support from Campus France for his stay at IEM Montpellier through the program PROFAS B+ of "algéro-français cooperation". We thank Didier Cot (IEM Montpellier), Bertrand Rebiere (IEM Montpellier) Bernard Fraisse (Réseau des Rayons X et Gamma, Université de Montpellier) for assistance during SEM, EDX and XRD measurements.

Conflict of Interest

The authors declare no conflict of interest.

Entry for the Table of Contents

PANiNi & Co. Chemical polymerization of aniline into polyaniline in the presence of nickel-cobalt salts and calcination are combined to engineer self-supported electrocatalysts. The simultaneous presence of aniline and ammonium persulfate permits us to thermally convert, by a two-step sacrificial method, the reaction mixture into heterogeneous materials. The as-constructed framework composed of nickel cobaltite and a network of carbon, nitrogen, sulfur, nickel, and cobalt allows reaching high electrocatalytic performance towards water splitting reactions of H2-evolution and O2-evolution in alkaline media.


Paul Morandi

Nazym Tuleushova

Sophie Tingry

Julien Cambedouzou

Shelley D Minteer

David Cornu

Yaovi Holade

Bromide-Regulated Anisotropic Growth of Desert Rose-like Nanostructured Gold onto Carbon Fiber Electrodes as Free-Standing Electrocatalysts

Keywords: anisotropic growth, catalyst, gold, gas-diffusion electrodes, glycerol oxidation

Three-dimensional (3D) gas diffusion electrodes (GDEs) as support for catalytic nanoparticles are important to the electrolysis and fuel cells fields to facilitate reactant transport and distribution, and reach higher current densities. Hence, the assembly of nanostructured catalytic particles directly onto GDEs during the synthesis could be a powerful one-step strategy to target high performance and long-term durability thanks to the induced strong nanoparticlesupport interaction. We report herein the use of bromide anions to shape the anisotropic growth of nanostructured desert-rose-like gold particles directly onto the GDE, as a freestanding GDE-Au catalyst for straightforward use in electrolysis without an additional step. We showed that the bromide ions determine the anisotropy of the particles, resulting in a growth of gold into nanoplates enclosed by (111) facets. The formation of hierarchical nanostructured 3D Au at the GDE surface was explained by the nanoparticle-mediated aggregation mechanism and the preferential adsorption of Br -on the (111) facet leaving behind dominant planar structured nuclei, which then self-assemble. Electrocatalytic tests towards the glycerol electrooxidation demonstrated that the as-synthesized freestanding surfactant-and binder-free GDE-Au material (100 µgAu cm -2 ) delivered a high specific peak current density of jp = 33 mA cmAu -2 , which is 3 and 7 higher than the tested commercial Au/C nanocatalyst at loadings of 76 and 152 µgAu cm -2 , respectively. This study provides future directions for the synthesis and application of nanostructured catalysts for alcohol oxidation as an alternative to anodic oxygen evolution, which consumes the majority of the electricity input during the electrolysis.

INTRODUCTION

Gas diffusion electrodes (GDEs) are utilized in electrolysis and fuel cells to overcome mass transport limitations, improve the catalyst utilization efficiency, and increase the current density. [START_REF] Del Castillo | Continuous electroreduction of CO2 to formate using Sn gas diffusion electrodes[END_REF][START_REF] Jhong | The Effects of Catalyst Layer Deposition Methodology on Electrode Performance[END_REF][START_REF] Higgins | Gas-Diffusion Electrodes for Carbon Dioxide Reduction: A New Paradigm[END_REF] A GDE is typically a three-dimensional (3D) material of tens to hundreds of micrometer thickness and made of carbon micro-or nano-fibers, thus constituting a porous electrode with tunable hydrophobicity. The issue is that the most frequently reported strategies for the synthesis of metal nanoparticles as catalysts are based on the reduction of the corresponding metal salts. The result is the production of nanocatalysts in powder or liquid form.

Hence, before being used in electrolysis or fuel cells, the collected nanoparticles need to be physically attached to an electrically conductive support, which is the GDE before the integration into a membrane-electrode-assembly type mounting in order to obtain the most compact arrangement. It was reported that the activity generally measured at small electrodes (e.g., glassy carbon electrode) using a typical three-electrode cell is a poor indicator of performance in the solid-state polymer electrolyte membrane system. [START_REF] Xu | Earth-Abundant Oxygen Electrocatalysts for Alkaline Anion-Exchange-Membrane Water Electrolysis: Effects of Catalyst Conductivity and Comparison with Performance in Three-Electrode Cells[END_REF] The challenge is that a liquid mixture has to be prepared by re-dispersing the collected nanoparticles in the presence of a binder to form a homogeneous catalytic ink. A binder such as Nafion ionomer is required during this immobilizing step to guarantee the stability, which does not simplify the electrode assembly and leads to loss of performance during operation. It should be mentioned that the ionomer also play an important role as an ion conductor within the electrode. Those approaches also lead to the need for a relatively high loading of metals of several hundreds or even thousands of micrograms of metal per square centimeter of the electrode. Given the high price and scarcity of platinum group metals, a significant reduction of the loading is desired. [START_REF] Shao | Recent Advances in Electrocatalysts for Oxygen Reduction Reaction[END_REF][START_REF] Vesborg | Addressing the terawatt challenge: scalability in the supply of chemical elements for renewable energy[END_REF] So in order to avoid collecting and immobilizing high amounts of the nanoparticles after the synthesis, the obvious and convenient solution is the preparation of the active sites directly on the GDE. During the last years, researchers have reported attempts via atomic layer deposition (ALD), [START_REF] Weber | Enhanced electrocatalytic performance triggered by atomically bridged boron nitride between palladium nanoparticles and carbon fibers in gas-diffusion electrodes[END_REF][START_REF] Barr | Enhancement of Pd Catalytic Activity toward Ethanol Electrooxidation by Atomic Layer Deposition of SnO2 onto TiO2 Nanotubes[END_REF] electrodeposition, [START_REF] Yu | Pd Nanocrystals with Continuously Tunable High-Index Facets as a Model Nanocatalyst[END_REF][START_REF] Wei | Concentration-Mediated Shape Evolution of Palladium Nanocrystals and Their Structure-Electrocatalytic Functionality[END_REF][START_REF] Song | Interfacial Sites between Cobalt Nitride and Cobalt Act as Bifunctional Catalysts for Hydrogen Electrochemistry[END_REF] electrospinning, [START_REF] Li | RuNi Nanoparticles Embedded in N-Doped Carbon Nanofibers as a Robust Bifunctional Catalyst for Efficient Overall Water Splitting[END_REF][START_REF] Engel | One-Pot Route to Gold Nanoparticles Embedded in Electrospun Carbon Fibers as an Efficient Catalyst Material for Hybrid Alkaline Glucose Biofuel Cells[END_REF] reactive spray deposition, [START_REF] Yu | Palladium-ceria nanocatalyst for hydrogen oxidation in alkaline media: Optimization of the Pd-CeO2 interface[END_REF][START_REF] Ayers | Pathways to ultra-low platinum group metal catalyst loading in proton exchange membrane electrolyzers[END_REF] and chemical/hydrothermal [START_REF] Zeng | Nano-engineering of a 3D-ordered membrane electrode assembly with ultrathin Pt skin on open-walled PdCo nanotube arrays for fuel cells[END_REF][START_REF] Zhao | Vertically Aligned MoS2/Mo2C hybrid Nanosheets Grown on Carbon Paper for Efficient Electrocatalytic Hydrogen Evolution[END_REF] processes.

Relatively small organic compounds such as methanol, ethanol, glycerol, and glucose are oxidized at an oxidation potential below 1 V vs. reversible hydrogen electrode (RHE) at platinum-palladium-gold surfaces. This is significantly lower than that of the oxygen evolution reaction (OER, higher than 1.4 V vs RHE), so that the oxidation of those organic compounds instead of water at the anode in electrolyzers is expected to significantly lower the input energy. [START_REF] Verma | Co-electrolysis of CO2 and glycerol as a pathway to carbon chemicals with improved technoeconomics due to low electricity consumption[END_REF][START_REF] Boukil | Enhanced electrocatalytic activity and selectivity of glycerol oxidation triggered by nanoalloyed silver-gold nanocages directly grown onto gas diffusion electrodes[END_REF][START_REF] González-Cobos | Development of Bismuth-Modified PtPd Nanocatalysts for the Electrochemical Reforming of Polyols into Hydrogen and Value-Added Chemicals[END_REF][START_REF] Holade | Recent advances in the electrooxidation of biomass-based organic molecules for energy, chemicals and hydrogen production[END_REF] Also, because the anode reaction only occurs in a potential range well below 1 V vs RHE, carbon fiber papers as support in anode were used in organic-fuelled reactors without concern for their oxidation. [START_REF] Verma | Co-electrolysis of CO2 and glycerol as a pathway to carbon chemicals with improved technoeconomics due to low electricity consumption[END_REF][START_REF] Glüsen | Dealloyed PtNi-Core-Shell Nanocatalysts Enable Significant Lowering of Pt Electrode Content in Direct Methanol Fuel Cells[END_REF] This emerging research direction constitutes a meaningful option to advance the development of an efficient anode to be associated with the electricity-driven half-cell processes of hydrogen evolution reaction (HER), CO2 reduction reaction (CO2RR), and N2 reduction reaction (N2RR) for the production of valuable fuels and chemicals. Verma et al. [START_REF] Verma | Co-electrolysis of CO2 and glycerol as a pathway to carbon chemicals with improved technoeconomics due to low electricity consumption[END_REF] reported that the anodic electrooxidation of glycerol during CO2RR into ethylene and ethanol can reduce the consumed electricity by up to 53%, thus decreasing the operating costs and the environmental footprint. It should be emphasized that glycerol is a co-product of the industrial manufacturing of biodiesel and soap, where biodiesel produces 10% wt./wt. of crude glycerol.

Indeed, the main motivation of the research during the last years was to find a method that promotes the selective oxidization of glycerol into valuable chemicals instead of the total oxidation into CO2 that will contribute to pollution. So the coupling of a selective oxidation at the anode of an electrolyzer to an electroreduction reaction such as CO2RR at the cathode is expected to significantly reduce the energy consumption and the release in the atmosphere of polluting carbon species, thus the decrease in the costs and the environmental impact. It is even envisaged that if CO2 were the reaction product, it would be sent to the cathode to produce higher value-added chemical intermediates such as ethylene and ethanol. [START_REF] Verma | Co-electrolysis of CO2 and glycerol as a pathway to carbon chemicals with improved technoeconomics due to low electricity consumption[END_REF][START_REF] Bardow | Converting two wastes to value[END_REF] Furthermore, gold is the only element that provides a good balance between catalytic activity, selectivity, and stability for the glycerol oxidation. [START_REF] Kwon | Mechanism of the Catalytic Oxidation of Glycerol on Polycrystalline Gold and Platinum Electrodes[END_REF][START_REF] Beden | Electrocatalytic oxidation of saturated oxygenated compounds on gold electrodes[END_REF][START_REF] Gomes | Spectroscopic Studies of the Glycerol Electro-Oxidation on Polycrystalline Au and Pt Surfaces in Acidic and Alkaline Media[END_REF] Its nanostructuration is desired to fabricate cost-effective catalysts by increasing the number of active sites. It is worth mentioning that the reported precious metal-free materials to electro-catalyze the oxidation of glycerol need a potential range of 1.4-1.9 V vs RHE in order to get high current densities, as recently reviewed by the group of Baranova. [START_REF] Houache | Study catalyst selection for electrochemical valorization of glycerol[END_REF] Given the higher potential, the idea of using an organic compound to lower the potential of the anode apparently violates the initial need of reducing the proportion of energy consumed by the anode, of the order of 90% in conventional water electrolyzer. [START_REF] Verma | Co-electrolysis of CO2 and glycerol as a pathway to carbon chemicals with improved technoeconomics due to low electricity consumption[END_REF][START_REF] Bardow | Converting two wastes to value[END_REF] Those facts explain why the presence of the three metals Au, Pt or Pd, which are the only species giving good activity in low potentials, is practically unavoidable for the development of reliable anodes for the organics oxidation. Thus, the smart solution should be their nano-structuring and/or the combination with other elements (Bi, Ni, Ag, etc.) to significantly reduce their quantity while providing a synergistic effect.

To date, various methods were reported to engineer catalytically active Au, Pd, and Pt nanoparticles of different shapes, morphologies and sizes for application in heterogeneous surface reactions. [START_REF] Yu | Pd Nanocrystals with Continuously Tunable High-Index Facets as a Model Nanocatalyst[END_REF][START_REF] Wei | Concentration-Mediated Shape Evolution of Palladium Nanocrystals and Their Structure-Electrocatalytic Functionality[END_REF][START_REF] Xu | Hollow AuxAg/Au core/shell nanospheres as efficient catalysts for electrooxidation of liquid fuels[END_REF][START_REF] Holade | Halide-regulated growth of electrocatalytic metal nanoparticles directly onto a carbon paper electrode[END_REF][START_REF] Li | Residual Silver Remarkably Enhances Electrocatalytic Activity and Durability of Dealloyed Gold Nanosponge Particles[END_REF] This was mainly facilitated by the use of capping agents such as organic molecules that adsorb on the surface of a crystal to control its size and selectively stabilize certain crystallographic planes by regulating growth in aqueous or organic solutions.

This advantage finally turns into a major obstacle when these particles have to be used in electrocatalysis, since it becomes almost impossible to remove completely these organic molecules from the metal surface. The remaining molecules of the surfactant thereby impede access to all catalytically active sites and have the potential to undesirably alter the selectivity of the reaction under investigation. [START_REF] Rizo | Shape-Controlled Nanoparticles as Anodic Catalysts in Low-Temperature Fuel Cells[END_REF][START_REF] Arán-Ais | Electrochemical Characterization of Clean Shape-Controlled Pt Nanoparticles Prepared in Presence of Oleylamine/Oleic Acid[END_REF][START_REF] Chen | Carbon monoxide-controlled synthesis of surface-clean Pt nanocubes with high electrocatalytic activity[END_REF] To benefit from the strong nanoparticle-substrate interaction, avoid the use of organic molecule-based organic surfactants or capping agents, and target high electrocatalytic performance, we reported the direct growth of anisotropic gold nanostructures directly onto a GDE support, wherein the key role of halides (I -, Br -, and Cl -) was emphasized. [START_REF] Holade | Halide-regulated growth of electrocatalytic metal nanoparticles directly onto a carbon paper electrode[END_REF] Enhanced electrocatalytic activity and stability towards the electrooxidation of glucose in a basic solution was observed in a half-cell as well as a record performance in a direct glucose fuel cell with a maximum power density of 3 mW cm -2 in unstirred solution.

However, in this previous study, the particles were deposited on both sides of the GDE. At the end, our initially developed strategy [START_REF] Holade | Halide-regulated growth of electrocatalytic metal nanoparticles directly onto a carbon paper electrode[END_REF] is only applicable for simple use where the electrode is supposed to be dipped in the solution. This is not necessarily optimal for the membraneelectrode-assembly setup, whereby the membrane is sandwiched between an anode and a cathode, both of which being based on GDEs. Therefore, only the side facing the membrane should constitute the active phase, i.e. contain the catalytic nanoparticles. Furthermore, for the electrocatalytic glycerol oxidation, much less attention was given to the strategy of direct synthesis of nanoparticles exactly at one face of the GDE. It is thus important to develop a strategy that allows nanostructured particles to grow directly on only one side, which is the purpose of the present study. So, we revisited this method in order to directly put Au particles onto GDE (GDE-Au) without using an organic molecule as a capping agent or surfactant during the synthesis. Hence, a binder such as Nafion ionomer is consequently not needed during the electrochemical measurement. Our results showed that bromide halide triggers the anisotropic growth of desert rose-like nanostructured gold particles on the carbon fibers surface, which leads to the recovery of 5×5 cm 2 electrodes as freestanding electrocatalysts. The comparative electrocatalytic study towards the glycerol oxidation in alkaline media with the commercial Vulcan-supported gold nanoparticles showed that GDE-Au exhibits a higher performance than the reference nanocatalyst.

RESULTS AND DISCUSSION

Growth of Anisotropic Gold Particles onto GDE Support.

Suitability of the GDE as a support for the synthesis requires deep physical scrutinizing to assess whether or not the surface of its constituting fibers would be suitable for the growth of gold particles. Specifically, the hydrophobic/hydrophilic character (wettability) is a prerequisite for purposes of bottom-up synthesis and electrocatalysis in aqueous media. In fact, in the case of a repulsing force between the support surface and metallic particles being deposited, it becomes essential to carry out an activation phase that enables obtaining high loading and significant electrocatalysis. Recent analysis by X-ray photoelectron spectroscopy (XPS) revealed the presence of a substantial amount of oxygen species supported by water contact angle measurements, [START_REF] Weber | Enhanced electrocatalytic performance triggered by atomically bridged boron nitride between palladium nanoparticles and carbon fibers in gas-diffusion electrodes[END_REF] which means that the GDE is suitable for direct use in aqueous solutions without further pre-treatment stages. Figure 1a presents a representative scanning electron microscopy (SEM) image of a bare GDE-material. For all SEM analysis, a thin layer (1-5 nm) of platinum was deposited to enhance the imaging capability. Additional low and high-resolution images of the GDE are reported in Figure S1. The images show that the GDE is composed of ∼7 μm microfibers leading to a 3D network made of a stack of 190/7 = 27 layers. revisited "halide-regulated method" using L-ascorbic acid as a reducing agent. [START_REF] Holade | Halide-regulated growth of electrocatalytic metal nanoparticles directly onto a carbon paper electrode[END_REF] The preliminary results from this method in terms of SEM and HRSEM images of nanostructured gold grown on GDE are reported in Figures S2; using different halides Cl -, Br -, and I -. [START_REF] Holade | Halide-regulated growth of electrocatalytic metal nanoparticles directly onto a carbon paper electrode[END_REF] It can be observed that the gold particles are preferentially and mainly deposited on the surface of the first 2 or 3 layers of microfibers of the GDE out of approximately 27. These results are similar to earlier findings with Au, Pt, and Pd nanostructured metals through chemical reduction on a GDE and for which the augmentation of precursors concentration does not change the conclusion. [START_REF] Holade | Halide-regulated growth of electrocatalytic metal nanoparticles directly onto a carbon paper electrode[END_REF] This would mean that the first step of the reduction of metal salts by the reducing agent occurs on the outer fibers.

Then, this first seeding layer of resulting particles serves as an active site where further reduction takes place and promotes the diffusion of metal salts from the interior towards the surface of the GDE. The crystals' size range is 5-7 µm for individual particles (Figure 1g) and 10-15 µm for clusters (Figure 1e). Each individual nanostructured particle or cluster of particles is composed of 3D self-assembly of sheets of about ten nanometers thick. Early-stage SEM observations (Figure S3) for the reaction time of 2 and 5 min showed that this desert rose-like morphology was present at the beginning. Figure S4 reports the energy-dispersive X-ray (EDX) analyses, which demonstrate that those particles are actually metallic gold since no presence of bromide or chloride was observed. The present nanostructured particles are slightly different from those initially obtained by the halide-regulated growth, HRSEM showed sponge-like 3D structures with gold particles of 600 ± 200 nm and consisting of intertwined nanoplates of 50 ± 10 nm. [START_REF] Holade | Halide-regulated growth of electrocatalytic metal nanoparticles directly onto a carbon paper electrode[END_REF] The present results could, therefore, be explained by the fact that the synthesis was realized under inert atmosphere and in a much more acidic medium (2.4 mM HAuCl4 in 1.8 M HNO3, pH ~0) whereas the initial methodology was performed in 2.4 mM HAuCl4•3H2O.

Scheme 1 shows the illustration of the different steps in the present study. The introduction of the ionic ligand Br -in the reaction mixture results in the conversion of [AuCl4] -ion-complexes (pKd = 25) into the more stable [AuBr4] -ones (pKd = 32). [START_REF] Usher | A spectrophotometric study of aqueous Au(III) halide-hydroxide complexes at 25-80 °C[END_REF] The ionic radii of bromide is larger compared to chloride (r(Br -) = 195 pm, r(Cl -) = 181 pm). Based on that and the specific adsorption of Br -on the crystallographic facet (111) of gold, [START_REF] Grzelczak | Shape control in gold nanoparticle synthesis[END_REF][START_REF] Xin | One-Step Synthesis of Tunable-Size Gold Nanoplates on Graphene Multilayers[END_REF][START_REF] Zhang | Investigation of Halide-Induced Aggregation of Au Nanoparticles into Spongelike Gold[END_REF] the complete replacement of chloride with bromide should increase the steric hindrance around the metallic cation and slow down the Au(III) to Au(0) reduction rate. Indeed, E°([AuBr4] -/Au) = 0.85 V vs. SHE, which is smaller than E°([AuCl4] -/Au) = 1.00 V vs. SHE. This will further control the seed growth towards an anisotropic mechanism. Specifically, the difference between the standard redox potential E°(V) between the reducing agent couple (ascorbic acid) and that of Au(III)/Au(0) defines that equilibrium rate constant ( n

E K 06 . 0 log   
, n = exchange number of electrons, which is here equal to six). Furthermore, the growth anisotropy describes the condition in which the growth rates are not equal in all crystallographic (hkl) directions. The surface energies and thus the growth rates of the crystal faces dictate the final shape of a nanoparticle. For a face-centered cubic symmetry metal like gold, the (111) plane has the smallest of the surface energies compared to the (100) or (110) planes, so that nanoparticles enclosed by the (111) facets are often obtained. Experimentally, the modulation of the growth along a certain orientation rather than others is triggered by the specific adsorption of capping agent molecules on targeted (hkl) facets, which results in the formation of 1D, 2D, and 3D materials. In the present study, bromide ion was used as an ionic capping agent so that the ion could be easily washed out at the completion of synthesis.

Scheme 1. Sketch of the desert rose-like gold crystal growth on the fiber of a carbon paper.

Similar types of structures have been reported for Ag [START_REF] You | Surface-enhanced Raman scattering-active desertrose-like Ag mesoparticles prepared using cyclic voltammetric methods[END_REF] and Au, [START_REF] Fang | Gold Mesostructures with Tailored Surface Topography and Their Self-Assembly Arrays for Surface-Enhanced Raman Spectroscopy[END_REF]40 which are referred to as "mesoparticles or mesocrystals". It is reported that, those crystals are three-dimensional wellordered superstructures of particles resulting from the mechanism of the particle-mediated growth, which involves an organized assembly of separate sub-units of nanoparticles. [START_REF] You | Surface-enhanced Raman scattering-active desertrose-like Ag mesoparticles prepared using cyclic voltammetric methods[END_REF][START_REF] Fang | Gold Mesostructures with Tailored Surface Topography and Their Self-Assembly Arrays for Surface-Enhanced Raman Spectroscopy[END_REF][START_REF] Watzky | Transition Metal Nanocluster Formation Kinetic and Mechanistic Studies. A New Mechanism When Hydrogen Is the Reductant: Slow, Continuous Nucleation and Fast Autocatalytic Surface Growth[END_REF][START_REF] Niederberger | Oriented attachment and mesocrystals: Non-classical crystallization mechanisms based on nanoparticle assembly[END_REF][START_REF] Kulak | Continuous Structural Evolution of Calcium Carbonate Particles: A Unifying Model of Copolymer-Mediated Crystallization[END_REF] The mechanism that was identified for the formation of the present nanostructured gold particles on GDE by nanoparticle-mediated growth, which is not a simple aggregation of individual particles with each other, has been described as follows. In the starting, [AuBr4] -ion-complexes are chemically reduced to Au atoms by the L-ascorbic acid (L-dehydroascorbic acid as the byproduct), which leads to an increase of the concentration of Au atoms on the GDE surface with time, while the precursor ([AuBr4] -) concentration diminishes. When the point of supersaturation is reached, Au atoms start to aggregate into small clusters on the GDE surface known as nuclei or seeds. Those seeds rapidly grow into nanocrystals, and the amount of the primary and individual Au atoms decreases when nanocrystals increase in size, until the growth stops due to the concentration of the building block units, which falls below the supersaturation threshold that defines an equilibrium state. In these particular conditions, a phenomenon called "oriented attachment or oriented aggregation" occurs, i.e., the already produced gold nanoparticles are aligned in a similar crystallographic orientation to ensure that the interface energy is minimized. [START_REF] You | Surface-enhanced Raman scattering-active desertrose-like Ag mesoparticles prepared using cyclic voltammetric methods[END_REF][START_REF] Niederberger | Oriented attachment and mesocrystals: Non-classical crystallization mechanisms based on nanoparticle assembly[END_REF][START_REF] Kulak | Continuous Structural Evolution of Calcium Carbonate Particles: A Unifying Model of Copolymer-Mediated Crystallization[END_REF][START_REF] You | Synthesis of colloidal metal and metal alloy nanoparticles for electrochemical energy applications[END_REF] The driving force of this stage is traditionally provided by the elimination of solvents or moieties from the surface of nanoparticles and the establishment of new chemical bonds via epitaxial connections. [START_REF] Niederberger | Oriented attachment and mesocrystals: Non-classical crystallization mechanisms based on nanoparticle assembly[END_REF][START_REF] Kulak | Continuous Structural Evolution of Calcium Carbonate Particles: A Unifying Model of Copolymer-Mediated Crystallization[END_REF] The final stage is the reduction of the remaining [AuBr4] -ion-complexes by the L-ascorbic acid directly on the oriented Au nanoscale crystals to form micro-crystals. Herein, the most likely scenario involves bromide ions that regulate the growth rate along (hkl) directions to yield desert rose-like Au crystals. Indeed, Xin et al. [START_REF] Xin | One-Step Synthesis of Tunable-Size Gold Nanoplates on Graphene Multilayers[END_REF] showed that bromide determines the anisotropic growth of gold into nanoplates enclosed by (111) facets on graphene multilayers. In the present study, the 3D structure of GDE is different from the graphene sheets, which indicates that (111) could be one of the preferential directions.

In the next sections, the use of additional characterization techniques of PbUPD and HRTEM will enable access to the full information.

To interrogate a possible influence of the temperature of the synthesis reactor on the morphology of the particles, we prepared additional GDE-Au materials at 5, 10, 20 and 40 °C.

For a temperature-dependent process, it is expected that the augmentation or decrease of the temperature affect the reaction rates through the Arrhenius's relationship. The results of the GDE-Au material obtained at a temperature of 35 °C were discussed above, SEM images of HRSEM images clearly demonstrate that all materials follow the same anisotropic gold particle growth on GDE, leading to the desert rose-like structure. The impact of the temperature is only 10 visible for the density of the particles on the microfibers, which increases with the temperature.

This finding would, therefore, mean that only the first stages in the reduction of [AuBr4] -ioncomplexes by the L-ascorbic acid and the subsequent nucleation are impacted. The results are the production of a significant amount of gold nuclei, resulting in a high degree of microfiber coverage. Consequently, a higher amount of gold nuclei will lead to a larger number of desert rose-like gold crystals. The preceding discussion shows that not only were we able to tightly regulate the formation of desert rose-like gold mesocrystals directly at one face of a GDE support, but also, the reaction temperature does not impact the morphology significantly. We then questioned whether the nonvisible effects of temperature on the morphology of the particles could be a major factor in the electrocatalytic activity. This is due to the simple reason that morphology is not the only parameter that dictates electrocatalytic performance. To elucidate that point, we thus studied the

XRD, PbUPD, and HRTEM Investigations.

To gain further insights into the as-fabricated GDE-Au electrode materials, we next performed a number of physicochemical characterizations and electrochemical probing. We specifically utilized X-ray diffraction (XRD) to determine the crystallographic structure of the as-synthesized materials. Figure 4a and (111) facets of gold. [START_REF] Wang | Engineering the internal surfaces of three-dimensional nanoporous catalysts by surfactantmodified dealloying[END_REF][START_REF] Hernández | Gold nanoparticles synthesized in a water-inoil microemulsion: electrochemical characterization and effect of the surface structure on the oxygen reduction reaction[END_REF][START_REF] Hebié | Advanced Electrocatalysts on the Basis of Bare Au Nanomaterials for Biofuel Cell Applications[END_REF] Due to the reversibility of those adsorption-desorption processes, the reversible desorption of the lead on (111) and (110) corresponds to the peaks at 0.45 and 0.58 V vs. RHE during the backward scan, respectively. As we previously showed, [START_REF] Holade | Halide-regulated growth of electrocatalytic metal nanoparticles directly onto a carbon paper electrode[END_REF][START_REF] Wang | Engineering the internal surfaces of three-dimensional nanoporous catalysts by surfactantmodified dealloying[END_REF] a careful look at the region between those peaks reveals the presence of a shoulder around 0.50 V vs RHE that corresponds to (100) facet. Hence, the presence of low-coordination number (111) facet is indeed confirmed by both XRD and PbUPD. The complementary observation (100) and ( 110) by PbUPD can be correlated with the presence of the diffraction peaks associated to the high-index facets of ( 200) and (220) as previously evidenced by XRD. The comparison of the profiles of the obtained peaks of the PbUPD for each facet with the existing literature of single and nanoscale crystals of gold should indicate that the (111) facet is predominant. [START_REF] Wang | Engineering the internal surfaces of three-dimensional nanoporous catalysts by surfactantmodified dealloying[END_REF][START_REF] Hernández | Gold nanoparticles synthesized in a water-inoil microemulsion: electrochemical characterization and effect of the surface structure on the oxygen reduction reaction[END_REF][START_REF] Hamelin | Underpotential deposition of lead on gold single crystal faces: Part II. General discussion[END_REF][START_REF] Hamelin | Underpotential deposition of lead on single crystal faces of gold: Part I. The influence of crystallographic orientation of the substrate[END_REF][START_REF] Hebié | Advanced Electrocatalysts on the Basis of Bare Au Nanomaterials for Biofuel Cell Applications[END_REF][START_REF] Hebié | Insight on the Surface Structure Effect of Free Gold Nanorods on Glucose Electrooxidation[END_REF] By considering the intensities and areas of the peaks, the PbUPD characteristics in Figure 4c clearly reveal that the bromide ion predominantly favors the crystallographic direction (111). This set of data greatly supports the hypothesis of the growth of gold particles in different orientations resulting in desert rose-like nanostructured gold particles containing various facets. To date, Miller's low index facets (100), ( 110) and ( 111) are thought to increase the electrocatalytic activities of nanostructured gold materials. [START_REF] Wang | Engineering the internal surfaces of three-dimensional nanoporous catalysts by surfactantmodified dealloying[END_REF][START_REF] Hebié | Advanced Electrocatalysts on the Basis of Bare Au Nanomaterials for Biofuel Cell Applications[END_REF][START_REF] Wang | Shapedependent electrocatalytic activity of monodispersed gold nanocrystals toward glucose oxidation[END_REF] To further elucidate the structural and textural characteristics of the as-synthesized nanostructured gold materials, a typical sample was prepared using a double beam focused ion beam (dbFIB) microscope. Figure 4d 111) and (200), respectively. In our preliminary study, [START_REF] Holade | Halide-regulated growth of electrocatalytic metal nanoparticles directly onto a carbon paper electrode[END_REF] other interplanar spaces of 0.14, 0.19, and 0.29 nm were observed from HRTEM pictures acquired at different locations. Hence, taking into account all the results obtained here from XRD and PbUPD, we can argue that the absence of those lattice fringes results

Performance Comparison with the Commercial Vulcan-Au Catalyst.

To assess the potential for electrocatalytic applications of the as-fabricated freestanding GDE-Au electrode materials obtained by Au growth on one face of GDE (referred herein to as CP-Au, about 100 µgAu cm -2 ), we carried out a set of control experiments for the glycerol oxidation reaction. The electrocatalytic activity was studied in 1.0 M NaOH + 0.1 M glycerol. To make a fair comparison with the state-of-the-art Vulcan-Au (20 wt%) as well as the relevant metallic electrocatalysts in alkaline media from literature, we utilized different metrics for the current density normalization. Figure S5 illustrates the method used for the ECSA by CV (Table S1).

Among our investigated methods, the current was normalized by using the electrode area (Figure 5a), ECSA (Figure 5b), gold weight (Figure S6a), and simultaneously by the mass and ECSA of the gold (Figure S6b). The different loading of Au for the commercial Au/C (Vulcan-Au-38, Vulcan-Au-76, Vulcan-Au-152, the numbers indicate the mass of the gold in micrograms per square centimeter) was used to make sure that the comparison can be neither overestimated nor underestimated. For the CVs that are based on the electrode surface, the peak current density is, jp = 78, 47, 55, and 52 mA cm -2 for CP-Au, Vulcan-Au-38, Vulcan-Au-76, and Vulcan-Au-152, respectively. Those peak current densities of those Vulcan-Au electrodes are 1.6, 1.4, and 1.5 times lower in comparison with CP-Au. The specific current density based on ECSA is shown in Figure 5b, which indicates that CP-Au still outperformed the commercial catalyst. In terms of the mass-activity, jp was evaluated to be 0.8, 1.3, 0.7, and 0.07 A mgAu -1 for CP-Au, Vulcan-Au-38, Vulcan-Au-76, and Vulcan-Au-152, respectively. Based on the metal loading of the as-fabricated CP-Au (100 µgAu cm -2 ), it cannot be concluded that the present desert rose-like gold crystals exhibit an enhanced electrocatalytic activity towards glycerol electrooxidation. By taking into account both ECSA and gold content, jp was determined to be 1.33 cmAu -2 mgAu -1 (CP-Au), 0.30 cmAu -2 mgAu -1 (Vulcan-Au-76), and 0.01 A cmAu -2 mgAu -1 (Vulcan-Au-152). This corresponds to an increase of nearly one order of magnitude in catalytic activity for an equivalent gold content. Table S2 summarizes the performance of nanostructured mono-and bi-metallic catalysts for the electrooxidation of glycerol in basic solutions, which have a peak current density of jp = 20-290 mA cm -2 based on the area of the electrode or jp = 0.2-18 A mgAu -1 , and where metal content is fairly high of 16-500 µgAu cm -2 . [START_REF] González-Cobos | Development of Bismuth-Modified PtPd Nanocatalysts for the Electrochemical Reforming of Polyols into Hydrogen and Value-Added Chemicals[END_REF][START_REF] Xu | Hollow AuxAg/Au core/shell nanospheres as efficient catalysts for electrooxidation of liquid fuels[END_REF][START_REF] Chen | Promoted Glycerol Oxidation Reaction in an Interface-Confined Hierarchically Structured Catalyst[END_REF][START_REF] Li | Designed multimetallic Pd nanosponges with enhanced electrocatalytic activity for ethylene glycol and glycerol oxidation[END_REF][START_REF] Hong | Bimetallic PdPt nanowire networks with enhanced electrocatalytic activity for ethylene glycol and glycerol oxidation[END_REF][START_REF] Zhou | Seed-mediated synthesis of PtxAuy@Ag electrocatalysts for the selective oxidation of glycerol[END_REF][START_REF] Xu | Solvent-mediated length tuning of ultrathin platinum-cobalt nanowires for efficient electrocatalysis[END_REF][START_REF] Zalineeva | Self-Supported PdxBi Catalysts for the Electrooxidation of Glycerol in Alkaline Media[END_REF] Except for few examples of nanoalloynanoporous AuAg [START_REF] Boukil | Enhanced electrocatalytic activity and selectivity of glycerol oxidation triggered by nanoalloyed silver-gold nanocages directly grown onto gas diffusion electrodes[END_REF] and Pt-PtCo, [START_REF] Chen | Promoted Glycerol Oxidation Reaction in an Interface-Confined Hierarchically Structured Catalyst[END_REF][START_REF] Xu | Solvent-mediated length tuning of ultrathin platinum-cobalt nanowires for efficient electrocatalysis[END_REF] it can, therefore, be deduced that the performance of the asfabricated GDE-Au freestanding material is superior to the relevant activities reported in the literature for gold-, palladium-, and platinum-based electrocatalysts. [START_REF] González-Cobos | Development of Bismuth-Modified PtPd Nanocatalysts for the Electrochemical Reforming of Polyols into Hydrogen and Value-Added Chemicals[END_REF][START_REF] Li | Designed multimetallic Pd nanosponges with enhanced electrocatalytic activity for ethylene glycol and glycerol oxidation[END_REF][START_REF] Hong | Bimetallic PdPt nanowire networks with enhanced electrocatalytic activity for ethylene glycol and glycerol oxidation[END_REF][START_REF] Zhou | Seed-mediated synthesis of PtxAuy@Ag electrocatalysts for the selective oxidation of glycerol[END_REF][START_REF] Zalineeva | Self-Supported PdxBi Catalysts for the Electrooxidation of Glycerol in Alkaline Media[END_REF][START_REF] Zhou | Selective Electro-Oxidation of Glycerol to Dihydroxyacetone by PtAg Skeletons[END_REF][START_REF] Palma | Effect of Co-catalyst on the Selective Electrooxidation of Glycerol over Ruthenium-based Nanomaterials[END_REF][START_REF] Xu | Novel surfactant-directed synthesis of ultra-thin palladium nanosheets as efficient electrocatalysts for glycerol oxidation[END_REF][START_REF] Holade | Toward the Electrochemical Valorization of Glycerol: Fourier Transform Infrared Spectroscopic and Chromatographic Studies[END_REF] To our knowledge, this is the first time a free-standing electrocatalyst made of desert rose-like nanostructured gold has been obtained directly onto carbon fiber electrode and demonstrate high performance towards the glycerol electrooxidation. The enhanced electrocatalytic performance of the present GDE-Au results from the absence of organic impurities or binding substances on the surface of the nanostructured particles, the particular morphology of the latter, and the strong particle-support interaction. The electrolyte was 1.0 M NaOH, the glycerol was at 0.1 M, the temperature was 25 °C and the scan rate for CVs was 50 mV s -1 .

Figures S6c-f displays the CVs collected at various sweep rates (v). Figure 5c shows the plots of log(jp) vs log(v). The obtained slope of 0.2-0.3, which falls in the case where the adsorption, diffusion, and electron transfer limit the overall reaction. Indeed, the seminal study on the effect of the scan rate on the peak current density (log(jp) vs log(v), the relationship is also valid for the peak current without normalization) of the electrooxidation of organic molecules resumed that:

(i) a slope of 1 is the pure adsorption, (ii) a slope of 0.5 is the pure diffusion, (iii) a slope of 0.5-1 is a mixed regime of adsorption and diffusion, (iv) a slope below 0.5 is a complicated kinetics involving parallel or successive reactions (can be referred to as cascade reactions). [START_REF] Parpot | Electrocatalytic oxidation of saccharose in alkaline medium[END_REF] It is thus obvious that the case (iv) is a combination of the case (iii) and the electron transfer. This conclusion was expected, since the glycerol electrooxidation reaction is often considered to be a process involving the transfer of at least two electrons and two protons to oxidize the secondary or primary alcohol group into a stable product. [START_REF] Boukil | Enhanced electrocatalytic activity and selectivity of glycerol oxidation triggered by nanoalloyed silver-gold nanocages directly grown onto gas diffusion electrodes[END_REF][START_REF] González-Cobos | Development of Bismuth-Modified PtPd Nanocatalysts for the Electrochemical Reforming of Polyols into Hydrogen and Value-Added Chemicals[END_REF][START_REF] Zalineeva | Self-Supported PdxBi Catalysts for the Electrooxidation of Glycerol in Alkaline Media[END_REF][START_REF] Holade | Toward the Electrochemical Valorization of Glycerol: Fourier Transform Infrared Spectroscopic and Chromatographic Studies[END_REF] Therefore, the reactant/product diffusion/adsorption and the electron transfer can simultaneously limit the overall reaction kinetics.

The electrochemical impedance spectroscopy (EIS) was then utilized to better understand the origin of the high catalytic efficiency of the GDE-Au electrode material. EIS allows modeling the electrocatalytic interface. [START_REF] Orazem | Electrochemical Impedance Spectroscopy[END_REF][START_REF] Lasia | Electrochemical Impedance Spectroscopy and its Applications[END_REF] Figures 5d-e show the plots of the complex-plane Nyquist impedance at two potentials, Eappl = 0.84 V vs. RHE (Figure 5d) and 0.94 V vs RHE (Figure 5e).

The combination R+QCPE//Rct is the representative equivalent electrical circuit that best describes the electrocatalytic interface. The ohmic resistance also referred to as the uncompensated resistance was modeled by R, the constant phase element that represents an imperfect capacitance of the double-layer was symbolized by QCPE and the charge transfer resistance was schematized by Rct. [START_REF] Orazem | Electrochemical Impedance Spectroscopy[END_REF][START_REF] Lasia | Electrochemical Impedance Spectroscopy and its Applications[END_REF] Basically, Rct is inversely dependent to the standard rate constant of the reaction, which is proportional to the exchange current density (function of the Tafel slope and equilibrium redox potential). [START_REF] Bard | Electrochemical Methods: Fundamentals and Applications[END_REF][START_REF] Voiry | Best Practices for Reporting Electrocatalytic Performance of Nanomaterials[END_REF] Therefore, it is expected to have the lowest value of Rct for a faster electron transfer capacity for a given overpotential (higher value of the rate constant). The ohmic resistance is R = 1  cm 2 for CP-Au and 2  cm 2 for Vulcan-Au.

Those values correspond to ~4  by taking into account the electrode area, which is the typical value of conductive electrocatalytic materials in 1 M NaOH at room temperature. R depends on the electrolyte conductivity, the position of the reference electrode relative to the working electrode, and the geometry of the working electrode, which were kept unchanged during our measurements. Furthermore the charge transfer resistances at 0.94 V vs. RHE are Rct = 5  cm 2 (CP-Au), 19  cm 2 (Vulcan-Au-38), 7  cm 2 (Vulcan-Au-76), and 9  cm 2 (Vulcan-Au-152).

The lowest value of Rct for the CP-Au electrode confirms its high ability to transfer electrons and deliver high current. We finally evaluated the Tafel slope expressed as mV dec -1 , a semiquantitative measure of the kinetics and/or mechanisms of a charge transfer process. [START_REF] Bard | Electrochemical Methods: Fundamentals and Applications[END_REF] Indeed, the slope of the Tafel indicates the electron transfer ability between the surface of an electrode and the substrate under study. In practice, a catalytic material having a small Tafel slope is better because a lower value implies that increasing the activity by an order of magnitude (dec -1 ) low overpotential (mV), which implies faster charge transfer kinetics. Tafel plots by current density were reported in Figure 5f. The evaluated values are 122, 144, 130, and 187 mV dec -1 for CP-Au, Vulcan-Au-38, Vulcan-Au-76, and Vulcan-Au-152, respectively. To confirm this trend, we also determined the Tafel slope by the alternative method of E-iR vs. log[Rct -1 ], 68- 70 as reported in Figure S7. The determined values by this method are 130, 193, 174, and 218 mV dec -1 for CP-Au, Vulcan-Au-38, Vulcan-Au-76, and Vulcan-Au-152, respectively. Hence, both methods demonstrate that the CP-Au electrode has the lowest Tafel slope, which is consistent with the high electrochemical performance by CV mentioned above.

It is important to mention that the present electrocatalytic performance was obtained in halfcell experiments, i.e. in a three-electrode configuration. Hence, we emphasis that it is unknown how these free-standing GDE-Au electrodes would perform in an MEA when implemented in an electrolyzer and where the mass transport is critical. This dedicated investigation including a fair optimization is a future direction for the type of materials reported herein.

CONCLUSION

Metallic nanoparticles as electrocatalysts are traditionally produced as solution or powder before being re-dispersed in the presence of a binder to form a homogeneous catalytic ink for incorporating on the support. This strategy of nanostructured materials does not benefit from the metal-support interaction and sometimes suffers from the remaining surfactants on particles, which do not permit maximizing the catalytic performance in terms of available active sites, activity, and durability. We demonstrated herein a facile and simplified method for the direct growth of three-dimensional desert rose-like nanostructured gold particles directly at one face of a model carbon paper substrate, which is currently used in fuel cells and electrolyzers as a gasdiffusion electrode (GDE). The preparation of a membrane-electrode-assembly includes the GDE for better management of the reactant distribution and the achievement of high current densities. Our developed one-step method involved the chemical reduction of Au(III) ions using L-ascorbic acid as the reducing agent in tandem with bromide used as the ionic capping agent for easy removal after the synthesis. This bromide-regulated method allows synthesizing desert roselike nanostructured gold particles that were strongly anchored on the surface of the GDE's microfibers. The nanoparticle-mediated aggregation mechanism and the preferential adsorption of Br -on the (111) facet leaving behind dominant planar structured nuclei was used to explain the formation of those hierarchical 3D Au structures. The nanosheet self-assembly to yield desert rose-like gold particles resulted in a high electrocatalytic activity towards glycerol oxidation in alkaline media, which was larger than the commercial Vulcan-supported gold nanoparticles. We also point out that a future study in a complete electrolysis cell with a MEA is necessary to confirm the performance obtained herein by a half-cell setup. The present study outlines further directions for the fabrication of organic surfactant-free metallic nanostructured crystals as freestanding electrocatalysts for application in heterogeneous electrocatalysis.

Notes

The authors declare no competing financial interest.

EXPERIMENTAL SECTION

Chemicals and Materials.

GDE-based carbon paper electrode (AvCarb MGL190, 190 μm thickness) was obtained from Fuel Cell Earth LL (USA). Nitric acid (HNO3, 65%, Sigma-Aldrich), sodium hydroxide (NaOH, 99.4%, Fisher Scientific), trihydrate tetrachloroauric acid (HAuCl4•3H2O, 99.9%, Sigma-Aldrich), isopropanol (iPrOH, ≥99.5%, Sigma-Aldrich), potassium bromide (KBr, 99%, Alfer Aesar), lead nitrate (Pb(NO3)2, ≥99.0%, Sigma-Aldrich), L-(+) ascorbic acid (AA, >99%, Alfer Aesar), Nafion ® suspension (5 wt%, Sigma-Aldrich), commercial catalyst Au/C (20 wt%, ≈4 nm particles size, Premetek Co., USA) were used as received. All ultrapure water (MQ) was 18.2 M cm at 20 °C, obtained from a Milli-Q Millipore source.

Chemical growth of Au onto 3D GDE: GDE-Au.

The synthesis was performed as previously with few modifications. [START_REF] Holade | Halide-regulated growth of electrocatalytic metal nanoparticles directly onto a carbon paper electrode[END_REF] The as-received GDE was cut in a square to yield 5 cm high and 5 cm width. It was washed 3× by iPrOH under middle-shacking (5 min per round) using an orbital shaker (RSLAB-7PRO, RS Lab) followed by drying in an oven at 50 °C (1 h), and referred to as CP. For Au particle deposition onto one face of CP, an electroplating tape (3M Company) was used so that the working geometric area is 25 cm 2 ; however, given its 3D morphology, it is obvious that the real geometric surface area is larger than that value. The synthesis was carried out in an engineered double-jacket reactor (cleaned with "aqua regia" from one synthesis to another) that operated at different temperatures. Typically, 18.8 mg of HAuCl4•3H2O were dissolved with 20 mL of 1.8 M HNO3 and gently shaken for five minutes using the previous shaker. Then, 546 mg KBr (229 mM) was introduced and the solution was shaken for an additional five minutes. Next, the as-cleaned CP piece was introduced for further shaking while outgassing for ten minutes. Then, 1.912 mL of 0.1 M of ascorbic acid was added at 0.3 L h -1 through a syringe pump (KD Scientific). The reaction proceeded under shaking and the color of the electrode changed from greyish to brown/red, which is typical of gold. After 1 h (except control experiments of 2 and 5 min), the reaction was quenched by removing the electrode rapidly. Finally, the sample was thoroughly rinsed with ultrapure water and dried in the oven at 50 °C overnight. Once dried, the tape was easily peeled off. GDE-Au was similarly prepared as previously described [START_REF] Holade | Halide-regulated growth of electrocatalytic metal nanoparticles directly onto a carbon paper electrode[END_REF] for morphological comparison.

Material Characterizations.

A Hitachi S-4800 microscope was used for SEM analysis. A ZEISS EVOHD 15 microscope was used for EDX study. XRD patterns were collected in Bragg-Brentano mode from 20 to 80° (in 2θ) using a PANalytical Xpert-PRO diffractometer operating at 40 kV and 20 mA ((CuK1,2) = 1.541 Å). Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis was carried out on a spectrometer Optima 2000 DV (PerkinElmer). A representative sample for HRTEM (JEOL JEM-2800, 200 kV) analysis was prepared by dbFIB microscope (FEI Helios NanoLab 650, 2-30 kV).

Electrochemical Measurements.

Electrochemical Setup and Characterization. All electrochemical measurements were carried out in a three-electrode cell (25 ± 1 °C) with an AUTOLAB PGSTAT 204 potentiostat (Metrohm, Netherlands). For the synthesized GDE-Au catalysts, the working electrode was a GDE cut into an L-shape of 0.5×0.5 cm 2 (not taking into account the 3D structure of the GDE) and leaving adequate space on top for the electrical connection with a gold wire. The counter and reference electrodes were a slab of glassy carbon plate (12.4 cm 2 ) and a mercury-mercury oxide electrode (Hg|HgO|NaOH, MOE, Origalys), respectively. The MOE is isolated from the solution via a Haber-Luggin bridge. However, the potentials were scaled with respect to RHE according to the scaling relation E(V vs. RHE) = E(V vs. MOE) + ΔE with ΔE = 0.920, and 0.946 V for 0.1 and 1.0 M NaOH, respectively. This was achieved by a steady-state CV recorded in the electrolyte saturated with H2 and employing Pt plate, Pt mesh, and MOE as the working, counter, and reference electrodes, respectively. Catalytic ink preparation for the commercial Vulcan-Au material was done by ultrasonically mixing MQ water (260 µL), iPrOH (100 μL), and Nafion ®

Figure 1 .

 1 Figure 1. Morphological characterization of blank GDE and gold decorating GDE (synthesized at 35 °C). (a) SEM micrograph of the bare GDE electrode. (b-i) (HR)SEM with different magnifications of nanostructured gold particle growth on GDE: (b-d) Overview SEM, (e-g) near view, and (h, i) close view that shows HRSEM micrographs.

Figure 1 .

 1 Figure 1. Figures 2a-d shows SEM/HRSEM micrographs of GDE-Au materials with different magnifications for the temperatures of 5, 10, 20, and 40 °C (from top to down for the sequence).

Figure 2 .

 2 Figure 2. Morphological characterization of the nanostructured gold decorating GDE. (a-c) SEM and HRSEM images for the nanostructured gold particle growth on GDE at the reaction temperature of: (a) 5 °C, (b) 10 °C, (c) 20 °C, and (c) 40 °C.

  Figure3athat the onset potential of the five types of electrodes are the same (0.65 ± 0.02 V vs. RHE). Furthermore, the iR-uncorrected cyclic voltammograms (CVs) normalized by the electrode area (Figure3b) confirmed the previous results. Indeed, the onset potential (Eonset) is Eonset ≈ 0.7 V vs. RHE. During the forward scan, a peak current density was reached at Epeak = 1.4 V vs. RHE. Then, the current density decreases because of the formation of inactive gold oxides. During the backward scan, the reduction of those oxides into metallic species allows new reactant molecules to adsorb and get oxidized again (as demonstrated by electroanalytic studies[START_REF] Holade | Electrocatalytic and Electroanalytic Investigation of Carbohydrates Oxidation on Gold-Based Nanocatalysts in Alkaline and Neutral pHs[END_REF] ), thus leading to a new oxidation peak at 1.1-1.4 V vs. RHE. For the forward scan, the peak current density was jp = 21, 23, 20, 27, and 31 mA cm -2 for GDE-Au obtained at 5, 10, 20, 35, and 40 °C, respectively. Since the materials have similar morphology and the onset potential is the same for the different electrodes, the observed differences might be attributed to the number of active sites. Specifically, the electrochemically active surface area is ECSA = 0.12, 0.22, 0.18, 0.26 and 0.35 cm 2 for 5, 10, 20, 35, and 40 °C, respectively. Those significant changes of the electrocatalytic activity and ECSA are consistent with the increase in the density of gold particles on the outer fibers of the GDE with the increase in temperature of the synthesis reactor, as previously observed. Of course, taking into account the active surfaces, the trend would be reversed for the specific current density. However, from an application point of view, it is more convenient to look at the result in terms of geometric surface area. So, it is expected to obtain the highest current density per geometrical surface. Based on this strategy, the outcomes

Figure 3 .

 3 Figure 3. Electrochemical performance of the as-synthesized GDE-Au material at different temperatures. (a) Recorded OCP upon stabilization in 0.1 M NaOH + 0.1 M glycerol. (b) iRuncorrected CVs recorded in 0.1 M NaOH + 0.1 M glycerol at 50 mV s -1 . Experiments were performed at 25 °C.

  shows the obtained patterns of the representative samples of GDE-Au obtained at 35 °C and GDE as the blank material. For both GDE and GDE-Au materials, the peak localized at 53.9° corresponds to the (004) reflection peak of graphite coming from the carbon support (as well as a strong (002) peak of graphite at 26.4°, not shown for clarity). The presence of gold in GDE-Au is shown by the peaks localized at 38.2°, 44.4°, 64.5°, and 77.6°, which correspond to the crystallographic orientations (111), (200), (220), and (311) of facecentered cubic of gold, respectively. We next utilized the CV recording in the electrolyte and the model of underpotential deposition of lead (PbUPD), which are efficient tools to characterize the surface of different types of gold and the evaluation of their crystallographic structures.[START_REF] Wang | Engineering the internal surfaces of three-dimensional nanoporous catalysts by surfactantmodified dealloying[END_REF][START_REF] Hernández | Gold nanoparticles synthesized in a water-inoil microemulsion: electrochemical characterization and effect of the surface structure on the oxygen reduction reaction[END_REF][START_REF] Hamelin | Underpotential deposition of lead on gold single crystal faces: Part II. General discussion[END_REF][START_REF] Hamelin | Underpotential deposition of lead on single crystal faces of gold: Part I. The influence of crystallographic orientation of the substrate[END_REF][START_REF] Hebié | Advanced Electrocatalysts on the Basis of Bare Au Nanomaterials for Biofuel Cell Applications[END_REF][START_REF] Hebié | Insight on the Surface Structure Effect of Free Gold Nanorods on Glucose Electrooxidation[END_REF] To these ends, two types of electrodes were considered, GDE-Au electrode by Au growth on one face of GDE (n(KBr)/n(HAuCl4) = 100) and GDE-Au electrodes by Au growth on both faces of GDE. For the latter, the synthesis was carried out either in the presence of KBr (n(KBr)/n(HAuCl4) = 100, GDE-Au_KBr-100) or in the absence of KBr (n(KBr)/n(HAuCl4) = 0, GDE-Au_KBr-0) in order to interrogate the effect of the halide, i.e. bromide. Figures4b and 4c

  depict the recorded steady-state voltammograms. The voltammograms in the electrolyte show well-defined electrochemical features of gold. The first region (0.1-0.7 V vs. RHE) represents the capacitive current and the second the chemisorption of OH -resulting in the formation of Au(OH)x as well as their oxidation to form AuOx above 1.2 V vs. RHE. Then, the formed oxides during the positive scan of the potential are reduced during the reverse scan with a peak centered at a potential of 1.1 V vs. RHE. Figure4cclearly shows that the introduction of potassium bromide during the synthesis enables fabricating an electrode material with an augmented electrochemically active surface area since the current associated to the reduction of the oxides of GDE-Au_KBr-100 is greatly larger than that of GDE-Au_KBr-0. Even though the PbUPD for different electrodes exhibits the same overall profile of an adsorption-desorption reversible process, the peak currents have much higher intensity in the case of GDE-Au_KBr-100 in comparison with GDE-Au_KBr-0. During the negative potential sweep, the strong peaks appearing at 0.55 and 0.41 V vs. RHE correspond, respectively, to the lead deposition on (110)

  displays the high-solution transmission electron microscopy (HRTEM) image and its corresponding picture of the Inverse Fast Fourier Transition (IFFT) is shown in Figure4e. Both configurations allow identifying the crystal structures unambiguously. For the analyzed part of the sample, the measured interplanar space of 0.24 nm is consistent with the (111) crystalline plane of the face-centered cubic symmetry of the gold. This finding supports the above hypothesis of the promotion of (111) face of Au by the bromide halide. The present analysis at the atomic level corroborates the earlier physical characterization through XRD and electrochemical probing via PbUPD. Indeed, for a lattice parameter a = 4.08 Å of gold (face-centered cubic structure), the values of the interplanar space

  408, 0.288, 0.236, and 0.204 nm for (100), (110), (

Figure 4 .

 4 Figure 4. Crystallographic orientation characterization. (a) XRD patterns of the nanostructured gold particle growth on GDE (GDE-Au) and the control material (GDE). (b) CVs of the GDE-Au electrode at 50 mV s -1 scan rate of the blank in 0.1 M NaOH and UPD of lead in 0.1 M NaOH containing Pb(NO3)2 at 1 mM: Au growth on one face of GDE (are = 0.25 cm 2 ). (c) CVs of the blank and UPD of lead obtained at 50 mV s -1 (0.1 M NaOH + 1 mM Pb(NO3)2): GDE-Au electrode synthesized at n(KBr)/n(HAuCl4) = 100 (GDE-Au_KBr-100) and at n(KBr)/n(HAuCl4) = 0 (GDE-Au_KBr-0): Au growth on both faces of GDE (area = 2×0.25 = 0.5 cm 2 ). Transform an HRTEM micrograph (d) to IFFT image with lattice fringes (e).

Figure 5 .

 5 Figure 5. Comparison of the performance of the commercial material Au/C referred to as Vulcan-Au and the as-fabricated GDE-Au material referred to as CP-Au. (a, b) iR-uncorrected CVs related to: (a) the electrode area, and (b) the specific active surface of gold. (c) Plot of the peak current density (jpeak) vs. the scan rate (v) in log-log scale. (d, e) Complex-plane Nyquist impedance (normalized by the electrode area and inset the equivalent electrical circuit) at: (d) Eappl = 0.84 V vs. RHE, and (e) Eappl = 0.94 V vs. RHE. (f) iR-free Tafel plots from the panel (b).

  The following files are available free of charge. Additional SEM/HRSEM images of GDE; SEM images of GDE-Au materials obtained in the presence of Cl -, Br -and I -; Early stage (2 and 5 min) SEM/HRSEM of GDE-Au; EDX of GDE-Au; additional CVs recorded 1 M KOH; additional CVs recorded in 1 M KOH + 0.1 glycerol; Tafel plots by Rct; Table of the determination of the ECSA; and comparison of the performance of relevant metallic catalysts for the glycerol electrooxidation reaction from literature. *D.C.: E-mail: david.cornu@enscm.fr *Y.H.: E-mail: yaovi.holade@enscm.fr

We next investigated the glycerol electrooxidation by changing the sweep rate of the potential during cyclic voltammetry in an attempt to interrogate the reaction process electrochemically.

ACKNOWLEDGMENT

This work was supported by the European Institute of Membranes of Montpellier (Exploratory Project COGENFC, PAT-Energy-Axis, 2018-2019) and the Army Research Office MURI (W911NF-14-1-0263). This work made use of the University of Utah USTAR shared facilities supported, in part, by the MRSEC Program of NSF under Award No. DMR-1121252 for the characterization by S/TEM and dbFIB. We thank Didier Cot, Bertrand Rebiere, and Arie van der Lee of IEM for assistance during SEM, EDX XRD characterizations.

AUTHOR INFORMATION

Corresponding Author