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Room Temperature Wet Chemical Synthesis of Au NPs/TiH2/nanocarved Ti Self-supported Electrocatalysts for Highly Efficient H2 Generation Mohammed A. Amin, Sahar A Fadlallah, Ghaida S Alosaimi, Emad M Ahmed, Nasser Y Mostafa, Pascal Roussel, Sabine Szunerits, and Rabah Boukherroub ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07611 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017

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ACS Applied Materials & Interfaces

Room Temperature Wet Chemical Synthesis of Au NPs/TiH2/nanocarved Ti Selfsupported Electrocatalysts for Highly Efficient H2 Generation

Mohammed A. Amin1,3*, Sahar A. Fadlallah4, Ghaida S. Alosaimi1, Emad M. Ahmed2,5, Nasser Y. Mostafa1,6, Pascal Roussel7, Sabine Szunerits8, Rabah Boukherroub8 1

Materials science and engineering group, Department of Chemistry, Faculty of Science, Taif University, 888 Hawiya, Saudi Arabia

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Materials science and engineering group, Department of Physics, Faculty of Science, Taif University, 888 Hawiya, Saudi Arabia 3

Department of Chemistry, Faculty of Science, Ain Shams University, 11566 Abbassia, Cairo, Egypt 4

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Chemistry Department, Faculty of Science, Cairo University, 12613 Giza, Egypt

Solid State Physics Department, National Research Center, Dokki, Giza 12311, Egypt

Chemistry Department, Faculty of Science, Suez Canal University, Ismailia 41522, Egypt

Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité de Catalyse et Chimie du Solide, F-59000 Lille, France 8

Univ. Lille, CNRS, Centrale Lille, ISEN, Univ. Valenciennes, UMR 8520 - IEMN, F-59000 Lille, France

*The Corresponding Author: E-mail: [email protected] Tel: +966560480239

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Abstract Self-supported electrocatalysts are a new class of materials exhibiting high catalytic performance for various electrochemical processes and can be directly equipped in energy conversion devices. We present here, for the first time, sparse Au NPs self-supported on etched Ti (nanocarved Ti substrate selfsupported with TiH2) as promising catalysts for the electrochemical generation of hydrogen (H2) in KOH solutions. Cleaned, as-polished Ti substrates were etched in highly concentrated sulphuric acid solutions without and with 0.1 M NH4F at room temperature for 15 min. These two etching processes yielded a thin layer of TiH2 (the corrosion product of the etching process) self-supported on nanocarved Ti substrates with different morphologies. While F--free etching process led to formation of parallel channels (average width: 200 nm), where each channel consists of an array of rounded cavities (average width: 150 nm), etching in presence of F- yielded Ti surface carved with nano-grooves (average width: 100 nm) in parallel orientation. Au NPs were then grown in situ (self-supported) on such etched surfaces via immersion in a standard gold solution at room temperature without using stabilizers or reducing agents, producing Au NPs/TiH2/nanostructured Ti catalysts. These materials were characterized by scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), Grazing Incidence X-Ray Diffraction (GIXRD), and X-ray photoelectron spectroscopy (XPS). GIXRD confirmed the formation of Au2Ti phase, thus referring to strong chemical interaction between the supported Au NPs and the substrate surface (also evidenced from XPS), and a titanium hydride phase of chemical composition TiH2. Electrochemical measurements in 0.1 M KOH solution revealed outstanding hydrogen evolution reaction (HER) electrocatalytic activity for our synthesized catalysts, with Au NPs/TiH2/nanogrooved Ti catalyst being the best one among them. It exhibited fast kinetics for the HER with onset potentials as low as -22 mV vs. RHE, high exchange current density of 0.7 mA cm-2, and a Tafel slope of 113 mV dec-1. These HER electrochemical kinetic parameters are very close to those measured here for a commercial Pt/C catalyst (onset potential: -20 mV, Tafel slope: 110 mV dec-1, and exchange current density: 0.75 mA cm-2). The high catalytic activity of these materials was attributed to the catalytic impacts of both TiH2 phase and self-supported Au NPs (active sites for the catalytic reduction of water to H2), in addition to their nanostructured features which provide a large-surface area for the HER. Keywords: Self-supported electrocatalysts; Hydrogen evolution reaction; Nanostructured titanium; Titanium hydride; Supported gold nanoparticles. 2


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1.


Introduction Hydrogen is a versatile energy carrier with favorable characteristics, and a high-energy-density

fuel capable of addressing global and environmental challenges.1,2 Its production has become substantial due to enhanced global energy demand and also essential to diminish our dependence on fossil fuels. In industry, steam methane reformer method is widely used to generate hydrogen, carbon monoxide, and natural gas. However, the main disadvantage of this method, besides its requirements of high-energy (heat) input, is the production of CO2 gas as a by-product.2-4 CO2-free hydrogen can be produced at room temperature via water electrolysis. For water electrolysis process to be efficient, an electrocatalyst capable of reducing the overpotential of the HER, and accelerating its kinetics should be used. Materials based on platinum are the most stable and promising electrocatalysts for the HER. However, these materials are highly expensive and therefore increase the cost of water electrolyzers. Consequently, the fabrication of new, non-precious active HER electrocatalysts is essential for the development of large scale hydrogen generation technology.5,6 Numerous reports focused on Mo-, Niand Co-based alloys as potential replacement of the Pt-based electrocatalysts for the HER.7-9 Also, CoP,10 Co-S,11 Co-B,12 and Co-chalcogenides13,14 were reported as active, non-precious HER electrocatalysts. Recently, materials based on molybdenum disulfide (MoS2) were also applied as highly active, inexpensive electrocatalysts for photochemical and electrochemical HER.15-17 Carbides of tungsten and molybdenum,18,19 and nickel phosphides20 acted as effective HER electrocatalysts. Also, carbon-based materials exhibited superior activity for the HER, and found to be good alternatives to the Pt electrocatalysts.21 Unfortunately, most of these non-precious electrocatalysts suffered from poor stability and durability in alkaline media.

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The Ti surface is a poor hydrogen evolution catalyst, and therefore has to be modified for improved HER catalytic performance for energy conversion purposes. However, literature revealed very little reports on the modification of Ti surface for enhanced HER. In this respect, Liu et al.22 coated Ti with a uniform layer of phytic acid through a simple drop-drying process, for improved HER electrocatalytic activity. Lu et al.23 combined Ti with copper (Cu) to produce a hierarchical nanoporous highly efficient copper-titanium bimetallic electrocatalyst for the HER in alkaline solutions. In our lab, the surface of Ti was decorated with highly dispersed Ag24 or Au25 nanoparticles for enhanced electrochemical generation of H2. Self-supported electrocatalysts, directly grown on conductive substrates or as free-standing films, have recently attracted the attentiveness of many researchers,26 because of their outstanding performance for various electrochemical processes with many substantial advantages in comparison with their powder counterparts. In this work, for the first time, TiH2 (one of the most substantial class of hydrogen storage materials27) is self-supported on nanostructured Ti substrates (Ti surface carved with nanoscale pores and parallel grooves) via wet chemical synthesis at room temperature for efficient electrochemical generation of H2 in KOH solutions. Dispersed Au NPs were also supported on the TiH2/nanostructured Ti catalysts for further improvement of the HER catalytic performance. HER electrocatalytic studies were performed in KOH solutions employing polarization and impedance measurements. The accelerating influence of substrate roughness, morphology and composition on the kinetics of the HER was reported and discussed. Outstanding HER activity (very close to that of the Pt/C catalyst) and stability were reported for our synthesized Au NPs/TiH2/nanostructured Ti catalysts. The synthetic approach described here is not limited to Au NPs, but can be extended to a diverse array of metal nanoparticles (M NPs) such as Co NPs, Ni NPs, and Cu NPs, providing a general approach to M NPs on self-supported TiH2/nanostructured Ti as a new class of highly efficient cathode catalysts for water 4


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splitting and hydrogen generation. In addition, the TiH2/nanostructured Ti catalyst has an advantage of being self-supported, thus excluding the overpotential due to the catalyst/support interface.

2. Experimental 2.1. Preparation of catalysts 2.1.1. Catalysts 1 and 2 (self-supported TiH2/nanostructured Ti) The Ti foils (99.7% metal basis and 0.25 mm thick) were first cut into small sheets of 1.0 cm × 1.0 cm and polished as described elsewhere,25,26 then dipped into 10 mL of two etching baths, namely 98% H2SO4 solution without (bath 1, for the preparation of catalyst 1) and with 0.1 M NH4F (bath 2, for catalyst 2 synthesis) at room temperature. After 15 min of soaking, the etched Ti sheets (i.e., catalysts 1 and 2) were taken away from the etching bath, washed several times with water and ethanol before drying their surfaces with the N2 gas, to be ready for surface analysis and electrochemical characterization.

2.1.2. Catalysts 3-6 (sparse Au NPs of different sizes and densities loaded on catalysts 1 and 2) A standard gold solution (Gold(III) trichloride acid 1000 ± 3 µg/mL in 2% HCl) with high purity was used to elaborate sparse Au NPs of different sizes and densities on catalysts 1 and 2 to yield catalysts 36. This was achieved via immersing catalysts 1 and 2 in the gold precursor solution for 5 and 20 min at room temperature without using reducing agents or stabilizers. Bath chemical composition and the operating conditions used to synthesize catalysts 1-6 are shown in Table 1. All wet chemical syntheses were carried out in absence of light, to exclude its influence on the Au NPs' deposition process.

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2.2. Physical characterizations Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), High-resolution TEM (HRTEM), Grazing Incidence X-Ray Diffraction (GIXRD), and X-ray photoelectron spectroscopy (XPS) examinations were used to fully characterize the synthesized catalysts (Supporting Information, Section 1).

2.3. Electrochemical tests Electrochemical characterizations were carried out in a standard, 200 mL volume, three-electrode cell equipped with a saturated calomel reference electrode (SCE, Princeton Applied Research) and a graphite rod (Sigma-Aldrich, 99.999%) as an auxiliary electrode. The reference electrode was dipped in a Luggin capillary full of with KNO3 solution (2.0 M). This capillary functions as the salt bridge to reduce the junction potentials, and prevent contamination of the test solution (the alkaline environment) with Cl- in the SCE. The Luggin capillary can also limit significantly diffusion of OH- from the test solution into the reference electrode, thus hindering the change of the SCE to Hg/HgO. This in turn will maintain the potential of the SCE stable throughout the run.28 Stability test of the SCE was reported in the Supporting Information file (section 2, Figure S1). The potential of the working electrode was measured here versus SCE, but reported versus the scale of reversible hydrogen electrode as reported in section 3 (Supporting Information). The test solution was 0.1 M KOH (Sigma-Aldrich, 99.99%), prepared using Milli-Q water (18.2 MΩ), and was continuously purged with Ar. Electrolyte temperature was maintained constant (25 oC ± 0.2 oC) throughout the run using a temperature-controlled water bath. Electrochemical responses were recorded via connecting the cell to an Autolab Potentiostat/Galvanostat (PGSTAT30). 6


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Electrocatalytic activity of our synthesized catalysts for the HER was assessed in 0.1 M KOH aqueous solutions based on linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) techniques. LSV measurements were carried out by sweeping the potential of the working electrode cathodically starting from the corrosion potential (Ecorr) up to -2 V vs. SCE at sweep rate 5.0 mV s-1. Impedance spectra were recorded from 100 kHz to 0.01 Hz at various cathodic potentials. A suitable equivalent circuit was proposed to fit impedance data. The HER catalytic activity of our catalysts was evaluated versus a commercial 20wt% Pt/C catalyst, which was used as received, and prepared for electrochemical measurements as described elsewhere.29 Cyclic voltammetry and chronoamperometry techniques were used to test stability and durability of the best catalyst. In cyclic voltammetry measurements, the potential of the working electrode is first cathodically scanned (sweep rate 100 mV s-1) starting from the corrosion potential (Ecorr) till an end potential of -2 V (SCE), and then turned toward Ecorr with the same scan rate, forming one cycle. This process was reiterated 10,000 times (repetitive cycling till the end of the 10,000th cycle). For chronoamperometry (current vs. time) measurements, the cathode was held at a fixed cathodic potential, and the resultant current was monitored up to 24 h. Stability tests were performed in 0.1 M KOH aqueous solution at room temperature. Each run was repeated at least three times to ensure data reproducibility, which was found to be excellent. The various parameters derived from the experimental findings are the average values from at least three independently repeated experiments. The HER Faradaic efficiencies of the tested catalysts were estimated by measuring the amount of H2 produced during a controlled chronoamperometry experiment, in which the catalyst is held at -0.8 V vs RHE for 1 h in 0.1 M KOH solution at 25 oC, using gas chromatography (GC). This amount of 7



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liberated H2 is then divided by theoretical volume of H2 (calculated, assuming 100% Faradaic efficiency, from the charge passed through the working electrode (WE) during the chronoamperometry run. Section 4 in the Supporting Information file describes in details the GC employed in this study.

3. Results and discussion

3.1. Morphologies and characterizations of the synthesized catalysts 3.1.1. Catalysts 1 and 2 (etched Ti substrates: self-supported TiH2/nanostructured Ti) The surface of dental implants can be perfectly textured by acid etching.30 Figure 1 shows SEM images at different magnifications captured on the Ti surface after 15 min of chemical etching in 98% H2SO4 (images a1-a4), designated here as catalyst 1, and 98% H2SO4 + 0.1 M NH4F (images b1-b6), catalyst 2, at 25 oC. It can be seen that etching grooved the surface of Ti with channels, which are parallel in many regions. The edges of these channels are in the nanoscale, protruding white corrosion products. The morphology and dimension of these channels vary depending on the composition of the etching bath. Etching in 98% H2SO4 (images a1-a4) led to formation of wide channels (average width: 200 nm). Each channel consists of an array of cavities (almost rounded) with an average width of 150 nm, images a3 and a4. On the other hand, etching in (98% H2SO4 + 0.1 M NH4F) created grains with obvious and distinguishable boundaries (image b1). The surface of these grains is characterized mainly by nanogrooves (average width: 100 nm) in parallel orientation; quite clear in images b3 and b4. Images b5 and b6 present two different grains whose surfaces exhibited other shapes of parallel grooves, narrower than those in image b4. In image b5, the surface consists of individual Ti nano-grooves in many regions. Most of them are parallel to each other, and some of them are branched. The nanowires tend to stick together (possibly due to capillary effects), leading to the aspect of a wet hair, or of a corn field after a storm-like 8


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structure. Typical parallel nano-grooves are shown in image b6. It seems that the presence of F- (highly aggressive to Ti passivity) in the etching bath destroyed the boundaries and the passive regions among the rounded cavities (images a), making them coalesce and interfere. This, in turn, formed the topography of the parallel grooves (images b). The rounded cavities can be considered as circular objects, characterized by an aspect ratio (i.e., length to width ratio) close to unity. On the other hand, the shape of the parallel grooves resembles that of elongated objects with higher aspect ratios.

3.1.2. Au NPs loaded on etched Ti surfaces (catalysts 3-6) Catalysts 1 and 2 were then decorated by sparse Au NPs of different sizes and densities via electroless deposition (dipping in the SGS at room temperature) at different time intervals (5 and 20 min), yielding catalysts 3-6. The morphologies of such catalysts are displayed in Fig. 2. Our previous study25 revealed that the formation of metallic Au on Ti surface was not due to a simple galvanic displacement between metallic Ti and Au3+ in the precursor solution. The actual reducing agent by which Au3+ ions are reduced to the corresponding metallic form was proved to be the atomic hydrogen (H) produced as a result of the interaction between water molecules and the base metal (Ti). It follows from Fig. 2 that the extent of Au NPs' deposition varies according to duration of the deposition process and substrate morphology. In all cases, it can be seen that deposition took place preferentially on the edges rather than inside of the grooves. After 5 min of immersion, dispersed spherical Au NPs with different sizes (15-38 nm) were observed on the etched substrates. Some aggregations of such nanoparticles can be seen. Obviously, population of supported Au NPs increases with the time of immersion, permitting further NPs’ aggregation. The shape of these aggregates depends on substrate morphology. While Au NPs’ aggregation on catalysts 3 and 4 appears as chain-like structures, (images a1-a3), they form large spheres on catalysts 5 and 6 (images c1-c3). 9



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The particle size distribution curves, constructed after counting at least 100 Au NPs, of such supported Au NPs are exhibited in Figure S2, Supporting Information. Catalyst 5 exhibited the lowest average particle size (15 nm) while catalyst 4 displayed the highest average particle size (38 nm). The high average particle size of the latter can be attributed to the higher agglomeration degree of Au NPs on its surface. The average Au NPs' size on catalyst 3 and catalyst 6 is found to be 28 and 21 nm, respectively. It seems therefore that supported Au NPs grow with time. Au NPs also form in solution (unsupported Au NPs). This is confirmed and fully supported employing UV-Vis spectroscopy, GIXRD, and HRTEM, as shown in Figs. 3 and 4. UV-Vis spectra showed that the surface plasmon band appears at 530 nm, confirming the presence of Au NPs. The high resolution transmission electron microscopy image depicts Au NPs with an average diameter of 17 nm (Fig. 4d). The high magnification HRTEM images show lattice fringes representing atomic layers with measured lattice d-spacing of 0.235 nm consistent with (111) diffraction plans and d-spacing of 0.202 nm consistent with (200) diffraction plans (Fig. 4b). The selected area electron diffraction (SAED) pattern displays the concentric spots corresponding to 111, 200, 220, 311 and 222 reflections plans of face-centered cubic (fcc) gold structure (Fig. 4c).

3.2. Characterization of the synthesized catalysts EDX analysis (Figure S3, Supporting Information,) revealed, as expected, that the etched Ti surface is mainly Ti (4.508 keV/Kα, mass% 96.08), with very little contribution from oxygen. A little contribution from S (2.309 keV/Kα, mass% 0.58) is most likely due to the adsorption of SO42(evidenced from XPS, see later). A relatively detectable percentage of N was found when NH4F is added to the etching bath. Little oxygen contribution is most probably attributed to thinning (destabilization) of the passive oxide film induced by the aggressive F- anions. EDX analysis also confirmed that the 10


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nanoparticles supported on catalysts 3-6 are metallic Au. Inspect, as a representative example, EDX and elemental mapping studies performed on catalyst 4 (Figure S4, Supporting Information). It has been reported that Ti etching dissolves its protective passive film, and subsequently small native hydrogen ions can diffuse into the base metal (Ti) surface, enriching it with hydrogen. This in turn helps form the titanium hydride (TiHx) phase. Grazing Incidence X-ray diffraction study is used here to confirm the presence of such hydride phase due to the conditions of acid etching employed here. The effect of loading Au NPs on the etched surface on the XRD intensity of that phase is also studied. In this respect, Fig. 5 is constructed, which shows GIXRD patterns of pure, as-polished Ti and catalysts 1, 2, and 6. For the untreated Ti sample (Fig. 5a), diffraction peaks due to α-Ti were observed at 2θ of 35.0°, 38.4°, 40.2°, and 52.9° diffracted from the (100), (002), (101), and (102) planes (Reference code: 01089-4893, crystal system: Hexagonal structure, space group P63/mmc). On the other hand, for the two etched Ti samples (Fig. 5b and c), two additional peaks were observed at 35.9○ and 41.0○ attributed to the (101) and (110) ppeaks of the titanium hydride phase (TiH2) (Reference code: 00-009-0371, crystal system: Tetragonal, space group: 14/mmm). Ban et al.31, who prepared a TiH2 layer on Ti by etching in 48% H2SO4 solutions at 60 oC for 0.25-8 h, obtained similar results. Ren and coworkers also obtained almost similar results for the Ti/TiH2 phase prepared by an acid-hydrothermal method.32 It is seen that the two TiH2 diffraction peaks of catalyst 2 (Fig. 5c) are smaller in intensity than those recorded for catalyst 1, Fig. 5b. Since no experimental conditions were changed between the two measurements, this can be attributed to the presence of F- anions in the etching bath used to prepare catalyst 2 (re-inspect Table 1), which may dissolve part of the self-supported TiH2 phase.33 For the etched Ti sample decorated with Au NPs (catalyst 6, Fig. 5d), the XRD pattern revealed, in addition to the peaks of α-Ti, an additional broad peak at 2θ 44.4○ assigned to the (200) plane of 11



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metallic gold (Reference code: 01-089-3697, crystal system: Cubic, space group: Fm-3m). The main peak (100% intensity) of gold is located at 38.1○, which is superimposed with that of the α-Ti(002). The inset of Fig. 5, which shows the simulated XRD pattern of gold (red line) overlaid on that of Ti (the black one), is constructed to clarify the main peak of gold. Moreover, an additional XRD peak is observed at 41.1○ which is matching to the 100% intensity peak coming from the (111) plane of the Au2Ti phase (Reference code: 00-029-0651, crystal system: Tetragonal, space group: 14/mmm). It seems that some of the loaded Au NPs combined chemically with atoms of the substrate (Ti) to form the Au2Ti phase. In addition, the high density of Au NPs supported on catalyst 6 (Fig. 2, images d) may cover the TiH2 layer partially; the reason why the two peaks corresponding to TiH2 appeared weaker. The oxidation state of the substrate (here catalyst 2) and the supported Au NPs of catalyst 6 was determine precisely by means of X-ray photoelectron spectroscopy, Fig. 6. The peaks due to Au4f, S2p, N1s, and Ti2p can be seen. The possibility of residual gold precursor on the surface can be excluded, as we did not observe any Cls peak. The core level Au4f XPS signal showed the Au4f7/2 and Au4f5/2 bands located at binding energies of 87.6 and 83.8 eV, respectively (Fig. 6(a)). These values are consistent with the literature values of Au4f5/2 (87.7 eV) and Au4f7/2 (84.0 eV) peaks, respectively, confirming the formation of metallic Au as reported by others.34,35 Additional peaks of lower intensity were also recorded in the XPS spectrum of the Au4f region. Such minor peaks are positioned 0.7 eV higher in binding energy than the main peaks, referring to the oxidation of some Au atoms during binding to the substrate.36 The slightly negative shift from 84.0 to 83.8 eV in the Au4f7/2 peak is a sign for charge transfer between Au and Ti, thus referring to metal–metal bonding at Au/Ti interface.33 This strong interaction between Au and Ti may refer to and support the formation of the Au2Ti phase on catalyst 6, as evidenced from XRD examinations (Fig. 5). 12


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The XPS spectra were also used to investigate the etched Ti surface before (Fig. 6(b), curve (1)) and after (Fig. 6(b), curve (2)) the deposition. The Ti2p XPS spectrum of the etched surface, curve (1), shows the Ti2p1/2 band at 464.2 eV and the Ti2p3/2 at 458.5 eV. The splitting value varies with the chemical state of Ti. The 5.7 eV difference is clearly characteristic for TiO2.37-39 The high resolution XPS spectrum of the S2p region is depicted in Fig. 6(c), characterized by the S2p1/2 and S2p3/2 doublet positioned at 170.7 eV and 169.5 eV with an energy separation of 1.2 eV, consistent with the sulphate species.40,41 This result is a good indication for the adsorption of SO42anions due to sulphuric acid. Adsorbed N due to NH4F that is present in bath 2 for the preparation of catalyst 2 (Table 1) is also evident from XPS, Fig. 6(d), where two bands at 400.2 eV and 398.3 eV were observed. The lower value corresponds most likely to Ti-N and the higher one is assigned to Ti-ON.42 The Ti2p XPS spectra of etched Ti-Au NPs, curve (2), exhibited the Ti2p1/2 and Ti2p3/2 bands located at binding energy values very close to those recorded for the etched Ti-free Au NPs, curve (1), but with larger contributions (Fig. 6b). This larger contribution of TiO2, which has occurred when the nanoparticles of Au are loaded on the surface of etched Ti, refers to increased thickness of TiO2, and hence enhanced passivation rate. It seems therefore that the loaded Au NPs improve the surface passivation, thus contributing to catalyst stability (see later).

3.2. Electrocatalysis The as-prepared TiH2/etched Ti catalysts (unsupported and supported with Au NPs), namely catalysts 16 were washed and transferred into 0.1 M KOH solution for cathodic polarization measurements, to evaluate their electrocatalytic activity for the HER, Fig. 7(a). The obtained results were compared with as-polished (unmodified) Ti to shed more light on the catalytic influence of substrate morphology and 13



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Au NPs on the HER kinetics, and also to clarify the role of etched Ti (i.e., TiH2 self-supported on a nanostructured Ti substrate) as a support for Au NPs. The Pt/C catalyst was also considered as a reference point. The corresponding overpotential vs. log j plots of such linear cathodic polarization plots is shown in Fig. 7(b). The linear parts in the Tafel plots were fitted to Tafel equation, as shown in the inset of Fig. 7(b), to determine the various electrochemical kinetic parameters of the HER (Table 2). The current density was referred to the electrochemical active surface area (EASA) estimated from the EIS data via the calculation of the roughness factor (Rf), as described elsewhere.29 The obtained Rf, and hence EASA, values are depicted in Table S1 (Supporting Information). Cathodic polarization curves of catalysts 3-6 (curves 4-7) exhibit increased cathodic currents at low overpotential values, designated here as onset potential for the HER (EHER). Beyond EHER, a sudden increase in the current density has occurred, denoting accelerated HER kinetics. The HER catalytic activity of catalysts 3-6 went far from the etched ones (catalysts 1 and 2, curves 2 and 3, respectively), approaching that of Pt/C. The best one among them (catalyst 6, curve 7) even exhibited a Pt-like activity, curve 8. This is clear from Table 2, where catalyst 6 (the best one here) recorded an EHER value of -22 mV vs. RHE, very close to Pt/C (-20 mV vs. RHE), more anodic than those measured for catalysts 3-5 (-40, -140, and -160 mV vs. RHE, respectively), and much more anodic than those obtained for catalysts 1 and 2 (-300 and -230 mV vs. RHE, respectively). The low values of EHER are translated into increased exchange current densities (jo). The best catalyst recorded, as shown in Table 2, a jo value of 0.72 mA cm-2, extremely near to Pt/C catalyst (0.75 mA cm-2) and the highest among the others (0.09, 0.19, 0.2, 0.29, and 0.58 mA cm-2 for catalysts 1-5, respectively). The catalytic performance of the synthesized catalysts was also evaluated from the overpotential (η10) at which 10 mA cm-2 cathodic current is obtained. Here again catalyst 6 (the best catalyst) achieved η10 of 127 mV, very near to that measured for the Pt/C (125 mV) and the least among the tested catalysts 14


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(400, 340, 260, 208, and 165 mV for catalysts 1-5, respectively). Further inspection of Fig. 7 and Table 2 reveals that the catalytic activity of any of the etched Ti substrates (catalysts 1 and 2) is much greater than that exhibited by as-polished Ti. For instance, catalyst 2 exhibited jo value of (0.19 mA cm-2) which is 32 times larger than that recorded for as-polished Ti (6 × 10-3 mA cm-2). These findings refer to the important role of substrate morphology and roughness (revisit morphologies in Fig. 1) in accelerating HER kinetics (see later). The catalytic activity is always greater in presence of supported Au NPs. This is clear when the results obtained for catalysts 1 and 2 (self-supported TiH2/nanostructured Ti) were compared with those measured for catalysts 3-6 (Au NPs/self-supported TiH2/nanostructured Ti). The results, as clearly seen in Table 2, are always in the side of catalysts 3-6. These findings reflect the catalytic impact of supported Au NPs, which enhances with their density on the substrate surface. This is obvious from Fig. 7 and Table 2, where catalytic activity of catalyst 4 (its morphology is depicted in Fig. 2, images b), Fig. 7(a) curve 5, is greater than that of catalyst 3 (Fig. 2, images a), Fig. 7(a) curve 4, as the former supports higher density of Au NPs. Also, catalyst 6 (Fig. 2, images d) is more efficient than catalyst 5 (Fig. 2, images c) for the same reason. Tafel slope is also a good indicator to assess and compare electrocatalytic performance, and also to clarify the HER mechanism.43,44 It is well documented in the literature that the HER proceeds in the alkaline solutions following either Volmer-Heyrosky or Volmer-Tafel mechanisms:24,45 Volmer step: H2O + e- + S* ↔ Hads + OH-

(1)

Heyrosky step: Hads + H2O + e- ↔ H2 + OH- + S*

(2)

Tafel step: 2Hads ↔ H2 + S* + S*

(3)

where S* is the hydrogen active adsorption site.

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It follows from Table 2 that catalyst 6 (the best catalyst here) recorded a Tafel slope value of 113 mV dec-1, very close to that measured on Pt/C (110 mV dec-1). This result infers that the kinetics of the HER proceeds on catalyst 6 in a way similar to that occurring on the Pt/C catalyst, where the Volmer step, with a Tafel slope near 120 mV dec-1, is the rate-limiting step for the HER.24,45 The obvious decrease in bc from catalyst 1 (191 mV dec-1) to catalyst 6 (113 mV dec-1) is a sign for increased HER kinetics, most probably due to the substrate roughness and morphology, and supported Au NPs, as lower Tafel slopes refer to plenty of attainable catalytic sites.11,46

3.3. Impedance Impedance measurements were also carried out at different cathodic potentials (Ec) to gain more information onto HER kinetics. Figure 8 illustrates impedance plots, in the complex-plane format, measured for the studied catalysts in comparison with Pt/C. Measurements were performed in 0.1 M KOH solutions at Ec = -0.3 V vs. RHE at 25 oC. Similar results were obtained at higher cathodic potentials, namely -0.5 and -0.7 V vs. RHE (Figure S5, Supporting Information). In all cases the Nyquist plots exhibited two depressed semicircles with different diameters. The first semicircle, observed at high frequencies, is small and corresponds to a charge-transfer resistance R1. The second one is large, with diameter R2, and covers a wide frequency range (from medium to low frequency values). Each semicircle can be assigned to an Rct-Cdl network for the HER; a combination of the charge-transfer resistance (Rct) and double-layer capacitance (Cdl) for water reduction at the catalyst/solution interface.47 Figure S6 (Supporting Information) depicts the equivalent circuit used to simulate the experimental impedance data. The fitting data are listed in Table 3. Obviously, for any studied catalyst, the values of R1 and R2 decrease as the applied potential goes to more cathodic values (Table 3), indicating that the total charge-transfer resistance (Rct = R1 + R2) of 16


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studied interfaces is related to the kinetics of the HER. At any potential, the Rct values measured for catalyst 6 and Pt/C are very close to each other, and the smallest among the other tested catalysts. For instance, at Ec = -0.7 V vs. RHE, an Rct value of 69.4 Ω cm2 was measured for catalyst 6, which is near to that recorded for the Pt/C (60 Ω cm2), and much smaller than those obtained for the other studied catalysts (1265, 1056, 610, 403, and 213 Ω cm2 for catalysts 1-5, respectively) at the same potential. Lower Rct values allow fast electron transfer during the HER, reflecting high catalytic activity. Table 3 also reveals that, at any applied potential, the largest capacitance values (C and Q) were measured for catalyst 6. For example, considering the values of C (C = C1 + C2) measured at Ec = -0.7 V vs. RHE, catalyst 6 recorded a capacitance value of 1884 µF cm-2, which is very close to that obtained for the Pt/C catalyst (1891 µF cm-2) and 19, 13, 9, 6 and 5 times larger than those measured for catalysts 1-5, respectively. This large capacitance value of catalyst 6 reflects its large accessible active surface area, as evidenced from EASA measurements (Table S1, Supporting Information), which in turn helps promote HER kinetics. These findings corroborate the results of polarization measurements, confirming superior catalytic activity of catalyst 6. The Faradaic efficiency (FE) value associated with the HER is an important parameter to evaluate materials' catalytic performance. The moles of H2 liberated under experimental conditions is first measured (designated here as nmeasured) using gas chromatography (as described in the experimental part). The charge passed through the working electrode (WE) during the experimental H2 evolution, see Fig. 9, was used to calculate theoretical moles of H2 (ncalculated), assuming 100% Faradaic efficiency. ncalculated is determined from the relation: ncalculated = (Coulombs)/(2F). The value of FE is then estimated for each studied catalyst from the relation: FE = {nmeasured / ncalculated} × 100%. The obtained data are 17



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shown in Table 4 in a comparison with Pt/C. Table 4 reveals that the value of nmeasured for catalyst 6 (20 µmol h-1, with a corresponding FE of 99%) is very close to that of the Pt/C catalyst (20.5 µmol h-1, FE 99.5%), and is the highest among the other tested catalysts. The above findings confirm each other, pointing to superior HER activity of catalyst 6, which is very close to that of the Pt/C.

3.5. Origin of catalytic activity 3.5.1. Catalysis due to substrate roughness and morphology Surface roughness of the Ti substrate induced by etching, revisit substrates’ morphology in Fig. 1, resulted in improved catalytic performance for the HER. This is clear in Fig. 7(a), compare between curve 1 for as-polished Ti and curves 2 and 3 for the two etched Ti substrates (catalysts 1 and 2). The remarkable increase in Rf of the etched substrates, as compared with that of the as-polished Ti substrate (Table S1, Supporting Information), may be one of the reasons responsible for their enhanced catalytic activity. For instance, as shown in Table S1 (Supporting Information), the Rf value at a cathodic potential of -0.5 V vs. RHE has significantly increased (thus referring to large accessible surface area) from 1.84 for as-polished Ti to 112.6 for the first etched Ti substrate (catalyst 1). The large accessible surface area of the etched substrates favors hydrogen generation at lower overpotentials, denoting accelerated HER kinetics. Substrate morphology may also affect catalytic activity. For example, the catalytic performance varies from catalyst 1 (Fig. 7(a), curve 2) to catalyst 2 (Fig. 7(a), curve 3), as they exhibited different morphologies (Fig. 1, images (a) and (b)). The higher Rf value of catalyst 2 (149 @ 0.7 V vs. RHE) as compared with that of catalyst 1 (128 @ -0.7 V vs. RHE), Table S1 (Supporting Information), may account for the higher catalytic activity of the former.

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3.5.2. Catalysis due to substrate composition (catalytic impact of the hydride phase) Chemical composition of substrate surface is another important factor controlling catalytic activity. TiH2 phase is formed during etching (as evidenced from XRD, Fig. 5) and its role in enhancing HER kinetics cannot be ignored. This phase may help create new active adsorption sites for water reduction. To confirm the catalytic impact of such a hydride phase, a TiH2-free nanoporous Ti substrate (Figure S7, Supporting Information,) were synthesized via room temperature chemical etching in a concentrated lactic acid solution containing 1.0 M NH4F. Such porous Ti substrate, which lacks TiH2 phase (as evidenced from XRD analysis, Figure S7), exhibited lower catalytic activity (Figure S8, Supporting Information) than catalysts 1 and 2 (the two self-supported TiH2/etched Ti catalysts), despite its higher Rf value (estimated from impedance measurements performed in 0.1 M KOH solutions as a function of potential, namely -0.3, -0.5, and -0.7 V vs. RHE) as compared with those of catalysts 1 and 2 (Table S1, Supporting Information). These findings are strong evidence for the catalytic impact of the TiH2 phase.

3.5.3. Catalysis due to supported Au NPs Electrocatalytic studies revealed that the catalytic performance of the studied TiH2/etched Ti catalysts (catalysts 1 and 2) greatly enhanced upon loading on their surfaces dispersed Au NPs, referring to Au NPs' catalytic contribution. Supported Au NPs not only helped increase Rf (i.e., EASA), Table S1, Supporting Information, due to their small size and high dispersion, but also provided the surface with active catalytic sites for water reduction. Each supported Au NP is assumed to serve as a nanocathode for efficient electrochemical H2 production. Also, the important role of the etched Ti substrate in catalyzing HER cannot be ignored. The high EASA values (Table S1, Supporting Information) of the etched Ti substrates help sparse the nanoparticles of Au and limit their size, thus favoring the exposure of more active catalytic sites for the HER. Strong chemical interaction between the supported Au NPs 19



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and the substrate, as evidenced from the formation of the Au2Ti phase (XRD, Fig. 5) and metal–metal bonding at Au/Ti interface (XPS, Fig. 6), is another important role the substrate plays in accelerating the HER. This strong chemical interaction guarantees speedy electron transfer from the substrate to supported Au NPs thus, accelerating the kinetics of the HER.

3.6. Catalyst stability and durability Cyclic voltammetry (CV) and chronoamperometry (CA) tests were applied to assess durability and stability of catalyst 6 (the best catalyst). Measurements were performed in 0.1 M KOH solutions at the room temperature, and the obtained results are displayed in Fig. 10. Figure 10(a) reveals an obvious increase in the cathodic current after 10,000 of repetitive cycling, denoting catalyst activation. The best catalyst Tafel analysis of the 10,000th cycle (inset of Fig. 10(a)) confirmed that activation, where bc and jo values of 105 mV dec-1 and 0.87 mA cm-2 were recorded. These findings indicate that the HER catalytic performance of the activated catalyst surpasses that of the Pt/C itself (110 mV dec-1 and 0.75 mA cm-2). Chronoamperometry measurements (Fig. 10(b)) led to the same conclusion, revealing catalyst activation during the operation. In Fig. 10(b), during the first minute of the operation, the cathodic current density decreased suddenly from -0.4 A cm-2 to ~ -0.03 A cm-2, referring to catalyst deactivation. This deactivation is most probably due to accumulation of H2 bubbles. The catalyst is then re-activated and H2 is released, clear from the current decay over the 24 h of continuous operation till steady-state, ~ -0.4 A cm-2. The above findings referred to catalyst activation during stability tests. This increased catalytic activity could be attributed to the evolved hydrogen, which can incorporate into the catalyst lattice in the near-surface region.48,49 This, in turn, results in roughening of the catalyst surface and as a consequence results in the catalyst’s specific surface area increase. To confirm increased specific surface area due to 20


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hydrogen incorporation, an impedance experiment was performed on the best catalyst at the end of the stability test, namely CV measurements (i.e., after 10,000 of repetitive cycling), and the roughness factor was estimated, as shown in Table S1 (Supporting Information). According to Table S1, after 10,000 of repetitive cycling, the Rf value of the best catalyst @ -0.7 V vs. RHE has increased from 1150 prior to the stability test to 1425 at the same potential, further evidence for the catalytic impact of the evolved hydrogen.

21



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4. Conclusions In this work, two nanostructured Ti substrates self-supported with a hydrogen storage material, namely TiH2 were synthesized via chemical etching in 98% H2SO4 solutions without and with 0.1 M NH4F at room temperature for 15 min. The two etching baths carved the surface of Ti with parallel channels, and nanoedges protruding white corrosion products. The morphology of these channels varied with the chemical composition of the etching bath. Etching in 98% H2SO4 yielded relatively wide channels (average width: 200 nm). Each channel consisted of an array of pores with an average width of 150 nm. On the other hand, etching in presence of F- anions created narrower channels or nanogrooves (average width: 100 nm) with no pores. Dispersed Au NPs, with different sizes and densities, were then in situ grown (self-supported) on such etched Ti substrates via soaking in a gold precursor at room temperature without using stabilizers or reducing agents. The above wet chemical syntheses produced Au NPs/TiH2/nanostructured Ti catalysts (fully characterized by SEM/EDX, low angle XRD, and XPS) of various morphologies and compositions for efficient electrochemical generation of H2 in deaerated 0.1 M KOH aqueous solutions. The formation of the TiH2 and Au2Ti phases on catalysts' surfaces was confirmed from GIXRD. Au2Ti phase formation was a sign for a strong chemical interaction between the supported Au NPs and the substrate surface, an important key factor in catalysis. Au NPs/TiH2/nanogrooved Ti catalyst recorded the highest catalytic performance (onset potential: -22 mV vs. RHE, exchange current density: 0.7 mA cm-2, and Tafel slope: 113 mV dec-1), which is very close to the Pt/C catalyst (-20 mV vs. RHE, 110 mV dec-1, and 0.75 mA cm-2), among the tested catalysts. The TiH2 phase and self-supported Au NPs exhibited considerable catalytic contribution towards the HER, besides the catalytic impact of the etched Ti substrates themselves due to their high roughness factor (i.e., large accessible electrochemical surface area) arised from their nanostructured features.

22


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5. Acknowledgments All authors are gratefully acknowledge financial support from the Centre National de la Recherche Scientifique (CNRS), the University of Lille, and the Hauts-de-France region.

6. Brief Description of the Supporting Information Physical characterizations, SCE stability, Conversion of the working electrode's potential from the SCE scale to the RHE scale, Faradaic efficiency measurements, Particle size distribution of the supported Au NPs of catalysts 3-6, SEM/EDX of catalysts 1 and 2, SEM/EDX and elemental mapping of catalyst 4, Values of roughness factor estimated from impedance measurements, Complex-plane impedance plots measured for the studied catalysts at -0.5 and -0.7 V vs. RHE, The equivalent circuit used to model and analyze the experimental impedance data, Low angle (2o) XRD and SEM examinations for a TiH2-free

nanoporous Ti catalyst, Cathodic polarization curve and Tafel analysis for a TiH2-free nanoporous Ti catalyst

23



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25



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Figure 1 - SEM micrographs at different magnifications recorded for cp-Ti after 15 min of chemical etching in 98% H2SO4 without (catalyst 1, images a1-a4) and with 0.1 M NH4F (catalyst 2, images b1-b6) at 25 oC.

26


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Figure 2 - SEM micrographs at different magnifications recorded for Au NPs on catalysts 1 and 2 as a function of immersion time, yielding catalysts 3-6. Catalyst 1 and 2 (revisit their morphology in Fig. 1) are dipped in the SGS for 5 min (images a1-a3 and c1-c3) and 20 min (images b1-b3 and d1-d3) at 25 oC.

27



0.3

530 nm

Absorbance (%)

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0.2

0.1

0.0 300

400

500

600

700

800

Wavelength (nm) Figure 3 - UV-Vis absorbance of Au NPs in solution (unsupported Au NPs), obtained during the preparation of catalyst 6; 20 min of catalyst 2 immersion in the SGS at 25 oC.

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Figure 4 - (a, b) HRTEM image of Au NPs separated in the electrolysis solution; (c) selected area electron diffraction (SAED) pattern; (d) particle size distribution of Au NPs.

29



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Figure 5 – Grazing Incidence X-ray Diffraction (GIXRD) patterns recorded for (a) cp-Ti (as-polished), (b) catalyst 1, (c) catalyst 2, and (d) catalyst 6.

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Figure 6 – High resolution XPS spectra of (a) Au4f (due to the supported Au NPs of catalyst 6), (b) Ti2p {due to etched Ti surface before (catalyst 2, curve 1) and after (catalyst 6, curve 2) the deposition}, and (c) S2p and (d) N1s due to catalyst 6.

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0.00

1 (a)

-2

-0.01

j / A cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.02

5 3

-0.03

6

2 -0.04 -0.8

-0.7

-0.6

4

-0.5

-0.4

7 8 -0.3

E / V(RHE)

33


-0.2

-0.1

0.0


0.4

(b)

Overpotential / V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2 5

1

0.3

3 4 6

0.2

7

8

0.1

0.0 -5

-4

-3

-2

-1

-2

log ( j / A cm ) Figure 7 – (a) Polarization curves and (b) corresponding Tafel plots of the studied catalysts in comparison with bare (unmodified) Ti and Pt/C catalysts. Measurements were conducted in 0.1 M KOH solutions at a scan rate of 5.0 mV s-1 at 25 oC. (1) bare Ti; (2) catalyst 1; (3) catalyst 2; (4) catalyst 3; (5) catalyst 4; (6) catalyst 5; (7) catalyst 6; (8) Pt/C.

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2000

1

200

2 4

-Zim / Ω cm

2

3

2

1500

-Zim / Ω cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150 5 100

50

6 8

0 0

50

100

1000

150

7 200

250

300 2

350

400

Zre / Ω cm

500

1 6 5

0 0

500

3

4 1000

1500

2000

2 2500

Zre / Ω cm

3000

3500

4000

2

Figure 8 - Complex-plane impedance plots recorded for the studied catalysts in comparison with bare (unmodified) Ti and Pt/C catalysts. Measurements were conducted in 0.1 M KOH solutions at -0.3 V vs. RHE. (1) bare Ti; (2) catalyst 1; (3) catalyst 2; (4) catalyst 3; (5) catalyst 4; (6) catalyst 5; (7) catalyst 6; (8) Pt/C.

35



4

8 7 6 5 4 3

3

Charge / C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2 2

1

1

0 0

10

20

30

40

50

60

Time / min

Figure 9 - Charge - time plots of the studied catalysts during 1 h of a controlled potential electrolysis run at -0.8 V vs. RHE in 0.1 M KOH aqueous solution. (1) Ti (as-polished); (2) catalyst 1; (3) catalyst 2; (4) catalyst 3; (5) catalyst 4; (6) catalyst 5; (7) catalyst 6 (the best one); (8) Pt/C.

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0.0

(a)

-2

-0.1

0.5

-0.2

-0.3 st

1

e cl y c th

-0.4

00 0 , 10

O verpotential / V

j / A cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


e cl y c

0.4 0.3 0.2 j o = 0.87 mA cm

0.1

-2

105 m V dec

-1

0.0 -4

-3

-2

-1

0

-2

log ( j / A cm )

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

E / V(RHE)

37


-0.2

-0.1

0.0


0.0

(b)

-2

-0.1

j / A cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.2

-0.3

-0.4 0

20000

40000

60000

80000

Time / s

Figure 10 – Stability tests of catalyst 6 (the best catalyst): (a) effect of repetitive cycling (10,000 cycles) between Ecorr and -0.75 V vs. RHE at a scan rate of 50 mV s-1 - Inset: fitting to the Tafel equation after the 10,000th cycle has been completed. (b) Chronoamperometry measurements (j vs. t) measured at -0.8 V vs. RHE. Measurements were performed in 0.1 M KOH aqueous solutions at 25 oC.

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Table 1 – Designation of our synthesized catalysts based on the composition of the etching bath and duration in the gold precursor. All wet chemical syntheses were performed in the dark at room temperature. Bath composition

Duration time

Yielded catalyst

98% H2SO4

15 min

Catalyst 1

98% H2SO4 + 0.1 M NH4F

15 min

Catalyst 2

Catalyst 1 in the SGS*

5 min

Catalyst 3


5 min

Catalyst 4


20 min

Catalyst 5


20 min

Catalyst 6 (the best)

*Standard Gold Solution (SGS): 1000 ± 3 µg Au/mL in 2% HCl. Table 2 - Mean value (standard deviation) of the electrochemical kinetic parameters for the HER on the surfaces of the synthesized catalysts in 0.1 M KOH solution at 25 oC, in a comparison with those recorded for as-polished Ti and Pt/C electrodes. EHER /

-βc /

jo /

mV (RHE)

mV dec-1

mA cm-2

as-polished Ti

-490(9.6)

177(3)

6(0.17) × 10-3

570(12)

Catalyst 1

-300(5)

191(3.4)

9(0.2) × 10-2

400(7.8)

Catalyst 2

-230(4.2)

176(2.8)

18.8(0.6) × 10-2

340(6.6)

Catalyst 3

-160(3.8)

146(3.1)

0.2(0.02)

260(4.8)

Catalyst 4

-140(3.1)

140(2.6)

0.29(0.025)

208(4)

Catalyst 5

-40(1.2)

131(2.7)

0.58(0.03)

165(3.6)

Catalyst 6 (the best)

-22(0.8)

113(2.2)

0.7(0.04)

126(3)

Pt/C

-20(0.6)

110(2)

0.75(0.02)

121(2)

Tested cathode

39


η10 / mV


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Table 3: Mean value (standard deviation) of the impedance parameters recorded for the studied catalysts in comparison with Ti (as-polished) and Pt/C. Measurements were conducted in 0.1 M KOH solutions at cathodic potentials -0.3, -0.5, and -0.7 V vs RHE at 25 oC. Catalyst

E

Q1

Q2

13.2(0.3)

3369(26)

23(0.4)

1600(15)

0.91

16.6(0.35)

2198(22)

951(9.5)

878(14.2)

1670(16)

0.95

896(9.2)

2224(22)

0.93

1144(12)

1059(19)

1383(14)

0.94

1085(11.3)

1863(21)

320(5)

0.90

1262(12.6)

1183 (25)

945(10)

0.92

1194(12.2)

1265(15)

1208(21.2)

448(5.2)

0.93

1153(10.8)

1071(20.6)

1302(14)

0.94

1094(10.4)

1750(19.2)

-0.5

1429(26.7)

330(3.6)

0.92

1339(13.4)

1220(25.8)

1135(12)

0.93

1250(12.9)

1465(15.6)

-0.7

1650(29.8)

206(2.4)

0.89

1444(12.7)

1332(27.2)

850(9.2)

0.91

1348(13.7)

1056(11.6)

-0.3

3520(34.2)

246(3.2)

0.93

3482(17.2)

3130(31.6)

630(7.4)

0.93

3294(16.6)

876(10.6)

-0.5

3926(36.8)

180(2.6)

0.91

3793(19)

3406(34.8)

560(6.3)

0.92

3603(18.4)

740(8.9)

-0.7

4411(38)

120(2.2)

0.88

4044(23.4)

3644(36.7)

490(5.2)

0.90

3887(22.8)

610(7.4)

-0.3

4475(45.6)

168(2.3)

0.91

4350(24.6)

4022(37.8)

420(4.9)

0.92

4210(23)

588(7.2)

-0.5

5038(47)

112(1.8)

0.89

4694(23.8)

4293(39)

370(4.2)

0.91

4494(24.7)

482(6.2)

-0.7

5548(51)

88(1.2)

0.87

4984(27.6)

4474(41.2)

315(3.7)

0.89

4668(25.4)

403(4.9)

-0.3

6229(53.8)

110(1.7)

0.91

6000(25.7)

5335(42.7)

280(3.9)

0.92

5525(24.8)

390(5.6)

-0.5

7929(56.4)

72(1.1)

0.88

7345(33)

6574(46.2)

250(3.6)

0.89

6990(31)

322(4.7)

-0.7

8966(59.8)

58(0.98)

0.89

8270(31.6)

7795(48)

155(3.2)

0.90

7960(33.8)

213(4.18)

-0.3

8825(57.6)

49(1.5)

0.90

8040(33.2)

7856(51)

145(3.2)

0.92

7945(34.7)

194(4.7)

Catalyst 6

-0.5

10666(61)

34.6(1.3)

0.89

9430(34)

9177(55.8)

91(2.1)

0.91

9015(35.6)

125.6(3.4)

(the best)

-0.7

12273(67.3)

20.6(1.1)

0.88

10175(39)

10783(59)

48.8(1.4)

0.89

9960(37.5)

69.4(2.5)

-0.3

10895(59)

40(1.1)

0.90

9935(36)

9384(56.6)

125(1.6)

0.92

9515(33.8)

165(2.7)

-0.5

12848(68.2)

22(0.8)

0.89

10990(37.6)

10740(58)

82.3(1.2)

0.91

10610(35)

104.3(2)

-0.7

15452(71)

12.7(0.6)

0.88

12375(41)

13055(62)

38.2(0.8)

0.89

11980(38)

50.9(1.4)

-0.3

10804(48)

44(0.9)

0.89

9855(26.8)

9267(46.2)

138(2.9)

0.91

9495(28.9)

172(3.8)

-0.5

12440(53)

28.8(0.7)

0.88

10815(35)

10686(47.8)

87(1.8)

0.89

10590(31)

115.8(2.5)

-0.7

15328(56)

16.8(0.6)

0.86

12290(37)

12922(51)

43.2(1.2)

0.88

11935(33)

60(1.8)

Catalyst 4

Catalyst 5

Catalyst 6*

Pt/C

-0.3

16.1(0.25)

1050(12)

-0.5

20.6(0.4)

-0.7

-2

2

Rct

0.94

Ω cm

-1

C2

2601(17)

S (ω cm )

n

n2

4301(34)

V (RHE)

-2

R2

11.7(0.22)

Catalyst 3

2

C1

0.96

Catalyst 2

-2

n1

Ω cm2

Catalyst 1

-1

R1

µF cm

Ti

n

µF cm

S (ω cm )

Ω cm

0.97

14.2(0.23)

13.3(0.3)

3251(22)

768(9)

0.95

16.6(0.4)

16.2(0.4)

27.6(0.5)

598(7)

0.93

20.3(0.44)

-0.3

988.3(15)

554(6)

0.94

-0.5

1193(22)

480(7)

-0.7

1382(28)

-0.3

-2

*Catalyst 6 after 10,000 of repetitive cycling in 0.1 M KOH aqueous solutions at 25 oC. The cathodic cyclic polarization curves were swept between Ecorr and -0.75 V vs. RHE at a scan rate of 50 mV s-1. 40


Page 41 of 49

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Table 4 - Mean value (standard deviation) of the amounts of hydrogen (measured and calculated) per hour by a controlled potential electrolysis process* and the Faradaic Efficiency values, FE (%), for the studied catalysts in comparison with Ti (as-polished) and Pt/C.

Catalyst Ti Catalyst 1 Catalyst 2 Catalyst 3 Catalyst 4 Catalyst 5 Catalyst 6 (the best) Pt/C

H2 measured by gas chromatography (H2 / µmol h-1) 2.7(0.06) 11.6(0.3) 14.2(0.33) 15.2(0.28) 16.3(0.32) 18(0.34) 20.1(0.3) 20.5(0.2)

Calculated H2 based on the charge passed during electrolysis Charge passed / C H2 / µmol h-1

1.1 (0.02) 2.6(0.08) 3.05(0.1) 3.2(0.04) 3.4(0.06) 3.6(0.05) 3.9(0.08) 3.98(0.06)

*

5.7(0.1) 13.5(0.4) 15.8(0.5) 16.6(0.2) 17.6(0.31) 18.7(0.26) 20.3(0.4) 20.6(0.31)

FE (%)

47.4(0.2) 85.9(0.3) 89.9(0.8) 91.4(0.5) 92.6(0.2) 96.3(0.4) 99(0.4) 99.5(0.5)

A chronoamperometry run where the catalyst is held at -0.8 V vs RHE for 1 h in 0.1 M KOH solution at 25 oC.

41



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TOC Graphic

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239x194mm (96 x 96 DPI)


TiH2

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