High Electrocatalytic Response of Ultra-refractory Ternary Alloys of Ta

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

High Electrocatalytic Response of Ultra-Refractory Ternary Alloys of Ta–Hf-C Carbide towards Hydrogen Evolution Reaction In Acidic Media Drochss P Valencia, Luis Yate, Willian Aperador, Yanguang Li, and Emerson Coy J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08123 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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The Journal of Physical Chemistry

High Electrocatalytic Response of Ultra-Refractory Ternary Alloys of Ta– Hf-C Carbide towards Hydrogen Evolution Reaction In Acidic Media Drochss P. Valencia1, Luis Yate2, Willian Aperador3, Yanguang Li4, Emerson Coy5* 1. 2. 3. 4. 5.

Pontificia Universidad Javeriana de Cali calle 18 No 118-250 Cali, Colombia, Teléfono: (+57-2) 321-82-00 CIC biomaGUNE, Paseo Miramón 182, 20009 Donostia-San Sebastian, España. Departamento de Ingeniería, Universidad Militar Nueva Granada, Carrera 1, 101-80, 49300 Bogotá, Colombia Institute of Functional Nano and Soft Materials, Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, China NanoBioMedical Centre, Adam Mickiewicz University, 85 Umultowska str., 61614, Poznan, Poland

* Corresponding author: [email protected]

Abstract Transition metal carbide alloys are promising materials to replace platinum as electrodes in electrocatalysts towards the hydrogen evolution reaction (HER). Although Tantalum Hafnium carbides (Ta-Hf-C) have shown outstanding refractory and structural properties, there is no clear role of their electrochemical efficiency. Here we report on the electrochemical activity of such thin films, (Ta2C)(100x)%·(Hf2C)(x)%,

x=100, 70, 30, and 0, deposited by magnetron sputtering. Grazing incidence X-ray

diffraction (Gi-XRD) showed no evidence of phase segregation and XPS confirms the well-controlled stoichiometry of the electrodes. The HER kinetics was studied in strong acidic conditions and it was found that the (Ta2C)

70%·(Hf2C)30%

was the most active material towards HER in this acid media and

displayed an onset overpotential of −198 mV vs. NHE and a Tafel slope of 129 mV. dec−1. Our results suggest that the strong affinity of Hf-C towards oxygen reduction reaction (ORR) could be responsible for the high catalytic response and strong oxidation resistance of the ternary carbide alloys. Finally, we show that, in fact, the Ta-Hf-C alloys can be competitive materials towards HER. 1. Introduction The goal of clean and self-sustaining energy has led to a focus on highly efficient electrochemical and catalytic materials for splitting of water, a field which has attracted great scientific interest in recent years1,2 because of their potential applications in ultra-high purity hydrogen production without the emission of undesired waste1–3. Materials for the electrochemical catalytic reduction of water to molecular hydrogen via the hydrogen evolution reaction (HER) are expected to increase the efficiency of this process by reducing the overpotential and drive the kinetically rate-limiting steps involved1,4–6. 1 ACS Paragon Plus Environment

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The most effective HER materials for electrocatalysts incorporate noble metals, such as Pt, because of their low overpotential and fast kinetics for driving the H2 generation2,3. However, Pt is a precious, scarce, and soft metal that cannot withstand continuous mechanical stresses, rendering it highly expensive and in continuous demand in the industry. In order to explore low-cost alternatives to metal noble catalysts, different materials have been proposed for application towards HER

1,3,7.

Most of the new materials

include transition metals combined with non-metals1,5,8–13. These combinations have turned out to be highly efficient catalysts for HER reaction in different environments and transition metal carbides are usually the most often proposed substitutes of the Pt cathode catalyst for the HER5. Transition metal carbides (TMC) have received great attention in the past few years due to the combination and tunability of their attractive physical and chemical properties, such as: low wear, high hardness, high electrical conductivity, good corrosion resistance, and good chemical stability

14–19.

In

addition to this, transition metal carbides are the most promising substitute of Pt group metals to date, being the most successful ones WC and MoC7,17,18 nevertheless, the whole family of transition metal carbides has shown activity towards HER17,18. Moreover, recent studies have shown the competitiveness of niobium carbide as printable, hard, and flexible electrodes towards HER20, paving the way for other unexplored metal carbides17 . Among binary metal carbides, tantalum and hafnium carbides are of particular interest, due to the extremely high melting points, and relatively high hardness 20–23. More importantly, recent studies have shown the high catalytic response of Ta-C nanocomposites in acidic media24. Although, a general drawback of this system is the large difference between the catalytic efficiency of Pt/C and that of Ta-C, which has been mainly associated to the uptake of oxygen by the Ta-C surface25, effect that observed even after tribological and mechanical testing26. On the other hand, Hf has shown some modest catalytic use, especially when combined with sulfur27 or doping titanium oxides for photocatalysis28–30. However what is more interesting, is the experimental observation by C. Rodenbücher et al, in which Hf shows strong affinity towards the formation of carbide structures, even over oxides, at relatively high temperatures31. Additionally, recent studies suggest that Hf-C can allow to reduce the catalytic barrier for the oxygen reduction reaction (ORR)32, which, although more similar to the oxygen evolution reaction (OER), could avoid the oxidation of the active HER catalyst, by adsorbing oxygen and directing it towards H2O2, even in acidic media33. Moreover, first principle studies have suggested that the strong adhesion of Hf atoms to Ta-C could be the main reason behind the strong oxidative corrosion resistance previously shown by Ta-Hf-C34. These facts, combined with recent studies on the Ta-Hf-C ternary alloy,

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an intermediate solution between Ta-C and Hf-C binary carbides, in which it was suggested that this material possess the highest melting point temperature for any solid35, motivate this study. In this work we investigate the catalytic properties of binary (Ta-C and Hf-C) and ternary (Ta-Hf-C) polycrystalline alloys towards HER. We aim to investigate the effect of partial substitution of chemical elements (Hf/Ta)-C on the general catalytic performance of the materials. The carbide alloys were deposited by magnetron sputtering in thin film form and characterized by different structural and chemical techniques. Samples of different weight content (Ta2C)(100-x)%·(Hf2C)(x)% ( x = 100, 70, 30, and 0) were deposited in order to determine the effect of Hf on the alloy and electrocatalytic response. We show that the catalytic activity of the alloys does not follow a simple mixture law assisted by Linear Voltametry and Impedance Spectroscopy. Finally, we show that the maximum electrocatalytic activity lies between the responses of Hf and Ta carbides, providing an experimental study of the promising role of Hf-C incorporation in Ta-C alloys and the applicability of the presented system. 2. Material and Methods 2.1 Electrodes deposition The binary and ternary Ta-Hf-C alloy films were deposited at room temperature on silicon (100) wafers and AISI 316LVM stainless steel discs by means of non-reactive magnetron co-sputtering. Deposits were performed in a single run on both substrates in an AJA-ATC 1800 system with a pressure of 1 x 10 7 Pa according to our previously published paper34. The deposition of the films was done with three separate elemental Ta, Hf, and C targets (purity > 99.95%, Demaco-Holland), in a confocal configuration at a pressure of 4 x 10 5 Bar of Ar 99.99%. Ta-Hf-C films with different compositions were obtained by varying the Ta and Hf target power (total power applied (Ta + Hf) = 100 W), The percentage of Ta and Hf contained in the alloys were shown to be proportional to the power applied to the targets and hereafter referred to as Ta-C (100W), 70Ta-C-30Hf-C (70, 30 W), 30Ta-C-70Hf-C (30, 70 W) and HfC (100W), respectively while keeping the carbon target power constant at 380 W34. 2.2 Characterization Grazing incidence X-ray diffraction (Gi-XRD) measurements were carried out in a PANalytical Empyream diffractometer with Cu-Kα1 (1.540598 Å) and Cu-Kα2 (1.5444 Å) x-ray sources. Data analysis was performed with HighScore Plus software in combination with the ICSD PANalytical database and the free version of Crystallography Open Database (COD). Gi-XRD measurements were 3 ACS Paragon Plus Environment


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performed on samples deposited on Si (100) substrates, in order to observe the phase composition of the binary and ternary Ta-Hf-C alloy films, without interference of stainless steel substrates. X-ray photoelectron spectroscopy measurements were carried out using a SAGE HR100 with a nonmonochromatic source (Mg Kα 1283.6 eV). The selected resolution was 30 and 7 eV of Pass Energy and 0.5 and 0.15 eV/step, for the general survey and the high resolution spectra, respectively. Sputter cleaning was done with Ar+ ions at 3.0 kV energy in order to remove adventitious contamination. The peak fitting was performed with asymmetric and Gaussian-Lorentzian functions after a Shirley background correction where the FWHM of all the peaks were constrained while the peak positions and areas were set free. The AFM experiments were performed on a Nanoscope V Multimode Atomic Force Microscope Bruker using Tapping Mode TM. Finally, nanomechanical and nanowear studies were performed on a Hysitron Ti-950 Triboindenter equipped with a Berkovich tip, mechanical values were obtained by Oliver and Pharr method36, detailed procedures are described elsewhere37. Advancing contact angle was measured with a DSA100 Contact Angle Measuring System (Germany) with a DSA100 control from the Kruss Company. Measurements were conducted at room temperature approaching a 3µl droplet at a volume flux of 500 µl and an error of ±1º, average values were obtained from 4 independent measurements. 2.3 Electrochemical measurements An aqueous 0.10 mol L─1 H2SO4 solution, prepared from commercial H2SO4 (Panreac Quimica SLU), 98% purity and ultra-pure water (18.1 M-cm resistivity) obtained by a MilliQ system (MILLIPORE), was utilized as the electrolyte in all electrochemical measurements which were performed at 25 oC with a potentiostat-galvanostat system, a Gamry unit, model PCI 4 equipped with electrochemical impedance spectroscopy (EIS) employing a Gamry controlled software. The experiments were carried out in a threeelectrode cell configuration using a Ag/AgCl(NaClsat) as a reference electrode, a Pt wire as the counter electrode and the AISI 316LVM stainless steel disc surface modified with the Ta-Hf-C alloy as the working electrode. The geometrical area of the working electrodes used was 0.237 cm2. The obtained current for the Ta-Hf-C electrodes was normalized to the geometric surface area of AISI 316LVM disc surface electrode. Nyquist plots were obtained with frequency sweeps between 10.0 mHz and 100 kHz using an amplitude of 10.0 mV for the sinusoidal signal. 3. Experimental Results 3.1 Structural and Chemical 4 ACS Paragon Plus Environment

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Figure 1a) shows the Gi-XRD patterns of the binary and ternary Ta-Hf-C alloy films deposited on Si(100) substrates. The Ta-C sample shows the cubic Fm3-m crystalline structure, according to the JCPDS-ICDD 98-008-5868 card, with the Ta-C (111), (002), (022), and (113) crystalline planes at 2 = 34.9, 40.4, 58.7, and 70.2º, respectively. The ternary 70Ta-C-30Hf-C alloy retains the structure of the Ta-C with no evidence of Hf-C. However, the diffraction peaks are observed to be slightly shifted to lower angles compared to the binary Ta-C sample indicating some compressive stress within the crystalline structure. On the other hand, the Hf-C sample also presents the cubic Fm3-m crystalline structure of hafnium carbide, according to the JCPDS-ICDD 98-008-5719 card, with the Hf-C (111), (002), (022), and (113) crystalline planes at 2 = 33.4, 38.7, 55.7, and 66.8º, respectively. In the case of the 30Ta-C70Hf-C

sample, only the Hf-C (111) peak was recorded, suggesting that this sample is highly oriented.

Moreover, the Hf-C (111) peak in the 30Ta-C-70Hf-C is shifted to slightly higher angles compared to the Hf-C sample indicating some possible tensile stress.

Figure 1. a) Gi-XRD patterns of the binary and ternary Ta-Hf-C alloy films. inset show the typical reflectivity and smooth surface of the samples. b) XPS spectra of the binary and ternary Ta-Hf-C alloy films focusing on the C 1s and Ta and Hf 4f regions. Insets show a representative image of the contact angle measured for the samples. It is worth noting that none of the ternary alloys show evidence of mixed Ta-C and Hf-C phases. This can be attributed to the fact that both carbides share the Fm3-m cubic structure and have similar 5 ACS Paragon Plus Environment


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lattice parameters with only a difference of 4%, where the Ta and Hf)atoms can be replaced by Hf and Ta atoms respectively, in the Ta-C (and Hf-C) structures. In addition to this, nanomechanical studies, compiled in Table 1, show the superior mechanical and tribological properties of the ternary alloys over their binary constituents, the hardness, elastic modulus and nanowear properties are clearly increased, proving further enhancement of their functional properties and applicability. The chemical quantification of Ta, Hf, and C was performed by means of XPS, and is shown in Table 1. The samples have sub-stoichiometric amounts, i.e., they lack carbon. However, this kind of behavior has been previously described for tantalum-hafnium-carbide materials38. The high resolution spectra of the different samples deposited on the silicon surfaces can be observed in Figure 1b), while the general survey spectra are shown in the Figure S1. The region of the C1s, Ta, and Hf 4f peaks for the binary and ternary alloy films shows the evolution from Pure Hf-C to pure Ta-C. In the spectra, the main component of the carbon C1s peak is shifted from 282.9 to 282.0 eV from the Ta-C to the Hf-C sample, which is in agreement with the binding energies of the Ta-C and Hf-C bonds at around 283.039 and 282.2 eV40, respectively. A minor amount of C-C bonds at around 284.8 eV41 is also observed. Table 1. Composition obtained by XPS; RMS roughness obtained by AFM; Hardness, Elastic Modulus obtained by nanoindentation; thickness obtained by SEM cross section; Nanowear. Sample

Ta (at.%)

Hf (at.%)

C (at.%)

O (at.%)

Ta-C

58.8

0.0

33.4

7.8

70Ta-C-30Hf-C

41.6

14.1

37.8

6.5

30Ta-C-70Hf-C

12.2

46.1

35.1

6.5

Hf-C

0.0

63.0

29.1

8.0

RMS (nm)

H (GPa)

E (GPa)

t (nm)

2.9±0.3 13.2±0.5 160±20 200±2 1.3±0.1 23.3±0.8 230±40 285±4 0.5±0.1 25.5±0.3 240±20 295±3 2.9±0.2

19±0.2

180±20 300±2

Nanowear Contact Angle (µm3) (º) 0.44±0.05 86.3 +/2.9 0.21±0.02 89.0 +/0.6 0.32±0.03 94.6 +/2.0 1.19±0.1 93.6 +/0.9

The tantalum Ta 4f and hafnium Hf 4f spectra consist of two doublet peaks or spin-orbit splitting (4f7/2 and 4f5/2)42. The main Ta 4f7/2 peak position in the Ta-C binary alloy film is around 23.0 eV and is attributed to Ta-C bonds34. In the Hf-C binary alloy, the main Hf 4f7/2 peak is around 14.6 eV and is attributed to Hf-C bonds34. In contrast, the Ta and Hf 4f7/2 peak positions for pure metals are around 21.6 and 14.3 eV43, respectively. Furthermore, in the case of the 70Ta-C-30Hf-C and 30Ta-C-70Hf-C ternary alloy films, the fitting of the region of the Ta and Hf 4d peaks reveals the co-existence of both the Ta-C and Hf-C bonds in different ratios, confirming thus the presence of a ternary carbide as suggested by the Gi-XRD analysis. 6 ACS Paragon Plus Environment

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Surface morphology of the binary and ternary Ta-Hf-C alloy films are shown in Figure S2 in supplementary information. The average roughness measurements (Ra) were taken from 3 different areas of the sample. The values were calculated using the Gwyddion software and are shown in Table 1. In general, the binary alloys showed larger grain sizes and higher roughness than the ternary alloys. The lower roughness values in the ternary alloys can be attributed to the enhanced ion bombardment of the simultaneous deposition from the Ta and Hf targets. Finally, differences in roughness and chemical composition showed negligible differences in contact angle of the surfaces Table 1, thus, discarding that any differences on electrochemical behavior are due to wettability of the surfaces.

Figure 2. Electrocatalytic performance of the binary and ternary Ta-Hf-C alloys and of Pt in H2SO4 0.10 mol L−1. a) Linear sweep voltammetric plots, scan rate: 50 mV s−1. Dashed square shows the slope change typical of ORR b) Tafel plots at scan rate 1 mV s−1. c) Initial and post-potential (100 sweeps) linear sweep voltammetry of Pt and 70Ta-C-30Hf-C surfaces at 50 mV s−1. Potential sweeps were between 0.1 and -0.8 V vs NHE. d) Nyquist plots of the binary and ternary Ta-Hf-C alloys in 0.10 mol L-1 H2SO4 at -0.015 V vs Ag/AgCl.



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3.2 Electrochemical tests 3.3.1 Linear sweep voltammetry Electrochemical tests are presented in Figure 2. Linear sweep voltammograms (Figure 2a) curves for the binary and ternary Ta-Hf-C alloy films immersed in 0.1 mol L-1 H2SO4 solution at a scan rate of 50 mV s−1. It can be seen that the Ta-C binary sample showed a small onset potential, about −228 mV vs. normal hydrogen electrode (NHE)) for the HER, beyond that, the current decreased at more negative potentials, down the point in which the mechanism clearly changes, Figure 2a, this is the classical behavior of Hf-C towards ORR32,44. It is interesting that the 70Ta-C-30Hf-C ternary alloy showed an even more positive peak potential at -0.198 mV than the Ta-C with no ORR signatures. In sharp contrast, the 30Ta-C-70Hf-C

ternary alloy exhibited a more negative and indistinct potential of onset (-327 mV)

towards HER and a small current density. This onset potential value is lower than that of the pure materials (Ta-C = −228 and Hf-C = −282 mV, respectively).

Figure 3. Electrocatalytic stability of 70Ta-C-30Hf-C surfaces, from 10 to 1000 scans. Inset shows the % of degradation, which remains below ~6% after 1000 cycles.

In order to assess the suitability of the investigated materials for the HER, steady-state polarization measurements in 0.1 mol L−1 H2SO4 solution were performed. The registered HER overpotentials (η) for the binary and ternary Ta-Hf-C alloys and bare Pt are shown as a function of the of the logarithm of the current density to give a plot known as Tafel curve, and are displayed in Figure 2b. From the linear part 8 ACS Paragon Plus Environment

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of the Tafel plot, the characteristic parameters providing information about the mechanism and the rate of the process, i.e. Tafel slope (b) and exchange current density (j0), could be determined. Tafel slope refers to the overpotential required to raise the current density by one order of magnitude2. The calculated Tafel slopes were 124, 124, 129, 118, and 32 mV dec─1 for Ta-C, 70Ta-C-30Hf-C, 30Ta-C-70Hf-C, Hf-C, and Pt, respectively. The calculated Tafel slope of 32 mV dec─1 for Pt is consistent with the known mechanism of HER on Pt45–48. The Tafel curves of the binary and ternary alloys clearly demonstrated that the HER occurs through a Volmer-Heyrovský mechanism, that is, the slow step is the electrochemical desorption of Hads and H3O+ to form hydrogen1,46,48. To determine the long-term durability of the samples, potential sweeps were conducted from 0.0 to −1.0 V vs NHE for 100 cycles. After cycling, the sample retained a linear sweep voltammetric curve similar to that before testing (see Figure 2c and S3), which suggests the high stability of the different surfaces over a long time in an acidic environment for HER. Additionally, further tests focusing only on the long-term stability of 70Ta-C-30Hf-C are shown in Figure 3, the variation of potential is presented in percentage for logarithmically increasing sweep quantities (inset Figure 3), the test variation remains below ~6% after 1000 cycles, showing long term stability of the sample. The change of the minus logarithm of exchange current density, -log (j0), with respect to the percentage of the Ta-C, (not shown), where the variation of j0 can be observed for Ta-C with and without Hf-C, irregular changes are observed since there is not a direct relation between j0 and the atomic percentage of Ta-C, j0 is a measure of the rate of the charge transfer at the interface in equilibrium conditions and can effectively be used as a measure of the electrocatalytic properties of an interface towards HER, depending on the material. As can be seen, in Figure 2 the material with highest affinity towards the HER reaction is the 30Ta-C-70HfC with j0 = 10−7.90 A cm−2. For Ta-C, 70Ta-C- 30Hf-C, and Hf-C system, the j0 were 10−7.78, 10−7.20, and 10−6.56 A cm−2, respectively. 3.3.2 Electrochemical impedance spectroscopy Further characterization of the investigated materials toward the HER was carried out by electrochemical impedance spectroscopy (EIS). Recorded impedance plots obtained at the overpotential value η = −10 mV are presented in Figure 2d. Initially, at higher frequencies, the Zimag = f (Zreal) dependence is close to linear and then goes smoothly into one or two deformed semicircles at lower frequencies. The as-measured on the binary and ternary alloys reaction currents do not directly reflect the intrinsic sample behavior due to the effects of Ohmic resistance, however the low frequency frustrated 9 ACS Paragon Plus Environment


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semicircle can be represented by the non-ideal capacitive element, the constant phase element (CPE), thus the impedance of this contribution can be model as a R-CPE circuit as follows: Z*R-CPE=R/(1+RQ(iw)α)

(1)

where Q denotes the amplitude and α deals with the phase of the CPE. The typical values range from α ≤0.6-1, α = 1 being the value for an ideal capacitor, and in this case C=Q. Thus, the charge transfer resistance (Rct) for each sample was determined as Ta-C = 108.10 kΩ cm2, 70Ta-C-30Hf-C = 205.21 kΩ cm2, 30Ta-C-70Hf-C = 516.12 kΩ cm2, and Hf-C = 79.83 k Ω cm2. It is worth noting that 70Ta-C-30Hf-C has a smaller resistance than

30Ta-C-70Hf-C

by almost double, which is consistent with a better HER

performance5,9,44. However, when comparing the performance of Ta-C and Hf-C in both Figure 2a and 2d, with the mixtures, the small resistance of both Hf-C and Ta-C shows a discrepancy with the onset potential values, where Hf-C shows lower performance than Ta-C and almost identical resistance. 4. Discussion At this point it is important to remark that although some studies have shown that Hf-C has poor, but visible activity towards HER its applicability on ORR is much more well established18. Conversely, Ta-C has shown poor performance towards ORR and high response towards HER applications 44. In the case of Ta-C, this behavior is accompanied, and also explained, by its well-known high resistivity towards hot corrosion

34,49,50

and an important increment of the melting temperature of Ta-C based

composites51. In addition to this, the ternary alloy of Ta-Hf-C is theorized since the 1960s, to be one of the highest melting point materials52, only surpassed relatively recently by estimations on Hf-C-N. Furthermore, early studies by D. L. Deadmore 53 showed that a composition close to 80Ta-C-20Hf-C had the highest melting point and lowest oxidation rate for the entire group of partially substituted compounds. Thus, it is clear at this point that the improved catalytic activity and resilience of our 70TaC-30Hf-C has to be related to its improved oxidation resistance. O2 + 4H+ + 4𝑒 ― → 2H2O

(2)

O2 + 2H+ + 2𝑒 ― → H2O2

(3)

H2O2 + 2H+ + 2𝑒 ― → 2H2O

(4)

+ + 𝑒 ― →Hads Haq

(5)

+ + 𝑒 ― → H2 Hads + Haq

(6)


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Figure 4. Electrocatalytic reactions pathways a) for ORR on Hf-C (top) and HER on Ta-C (bottom). b) Diagram of the cooperative effect between ORR and HER in 70Ta-C-30Hf-C. From the catalytic point of view, Hf-C should present one of the known mechanism for ORR in Pt 54–56 , Figure 4a) and Equation 2), the so called direct four-electron pathway, in which O2 is reduced to H2O without the formation of hydrogen peroxide (H2O2) and the indirect two step reaction in which O2 is converted into H2O, with an intermediate step of H2O2, Equation 3 and 4. On the other hand, since Ta-C should follows the Volmer-Herovsky in acidic solutions, as shown in Figure 2b, the general pathway dictates that electrolyte species (H+aq) are adsorbed on the Ta-C surface (Hads) and the further generation of H2, Figure 4a, Equation 5 and 6. In general, although the reactions deal with different mechanism, both HER and ORR comprehends the adsorption of H and O species on the catalytic surfaces. It is important remarking, that the typical overportential region for the ORR pathway in Hf-C is different from the one reported here44. However, as mentioned before in the results section, the second current drop observed for Hf-C, Figure 2a, is a strong indication of ORR taking place, which irrespective of the overpotential region, is found as a second drop at negative current densities as in our case. Moreover, since the typical set-up for ORR studies includes spinning electrodes and saturated oxygen solutions33,44, when compared with our experiments, the overpotential region for the ORR can be shifted to a region in which the oxygen adsorption, and thus its availability, overcomes the HER dynamics. 11 ACS Paragon Plus Environment


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Further theoretical studies by S. Yang et al 32 on Hf-C and Y Meng et al 57 on Ta-C, suggest that the catalytic activity of Hf-C towards ORR can be enhanced by using a 4:1 ratio of co-catalyst, while a ratio of 1:1 was suggested for Ta-C, which incidentally increased its oxidation resistance. In fact, 70TaC-30Hf-C is close to such ratio if one considers a homogenous distribution of Hf-C, surrounded by Ta-C unit cells. Conversely, Ta-C is the most active towards HER, from the here studied compounds, as shown in Figure 2a. Moreover, studies by S.T. hunt et al

58

show that small alloying quantities of Ta-C can

improve the oxidation resistance of platinum like carbides (WC) due to the fast creation of small and easily overcome passivation layers. Additionally, the theoretical studies of D. Liu et al59 suggested that the formation of C-O bounds, crucial for the oxidation of the material, is much higher for Hf-C than for Ta-C. Therefore, it is sensible to assume that a cooperative catalytic process between ORR and HER is taking place on the 70Ta-C-30Hf-C solid solution. Figure 4b, illustrates how both catalytic pathways are allowed in the solid solution, while the oxidation of HER active Ta-C is strongly prevented by the highly Hf-C centers, which in time, will release oxidative species in H2O molecules. Finally, our results show that the performance of the 70Ta-C-30Hf-C is comparable or superior to that of other state of the art electrodes, such as Mo2N60, bulk-WS261, 2D-WS262, MoS263, NbC20, BGraphene6,64, and MoSe265. Nevertheless, further research and modeling is needed in order to understand the role of Hf-C(%) in this compound and other carbide based electrodes towards HER and ORR, additionally, a more detailed study of the electron transfer dynamics, in which different pH and cell current are applied, is needed in order to understand the discrepancies observed between the Rct and potential response, as mentioned in section 3.3.2, between the binary and ternary carbides. 5. Conclusions Binary and ternary thin film alloys from the Ta-Hf-C system have been successfully synthesized by the magnetron co-sputtering method. The electrochemical investigation showed that Ta-Hf-C working electrodes exhibited a good electrocatalytic activity towards the HER reaction in acid media, and exhibited a superior electrocatalytic behavior in the ternary films. Particularly, the 70Ta-C-30Hf-C ternary alloy exhibited an excellent HER performance, with a low onset potential of -198 mV, a Tafel slope of 129 mV dec ─1 and good electrochemical stability. A model in which the well-known high oxidation resistance of the solid solution (70Ta-C-30Hf-C) is explained by the cooperative role of ORR and HER catalyst centers, opens the possibility for further research regarding chemical optimization and catalytic modeling of TM-carbides. Finally, our results


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clearly demonstrate the potential use of Ta-Hf-C systems for H2 production, due to their strong oxidation resistance and competitive electrochemical performance. Acknowledgements This research was supported by “Vicerrectoría de Investigaciones de la Universidad Militar Nueva Granada” under contract Ing-2630- 2018 and Dirección General de Investigaciones USC Project DGICOCEIN-No. 531-6211116-A33. Authors acknowledge the partial financial support from the National Science Centre of Poland by the SONATA project number UMO-2016/23/D/ST3/02121 Supporting Information It includes: XPS general survey (Figure S1), AFM micrographs (Figure S2) and Electrochemical stability of the electrodes (Figure S3). Authors Contributions: D.P.V: Electrochemical experiments and wrote the first draft, L.Y: Electrode Deposition, XPS analysis and Contact Angle measurements W. A: Supervision and Analysis of Electrochemical experiments and Discussion. Y.L. Preparation and critical reading of the manuscript. E.C: Manuscript preparation, experiments and analysis of:

Nanoindentation, Nanowear, Gi-XRD, Atomic Force

Microscopy, Analysis of Electrochemical results, General conception of the experiments, and Interpretation of Results. References (1) (2) (3) (4) (5)

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High Electrocatalytic Response of Ultra-refractory Ternary Alloys of Ta

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