Trends and descriptors of metal-modified transition metal carbides for

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Trends and descriptors of metal-modified transition metal carbides for hydrogen evolution in alkaline electrolyte Qian Zhang, Zhao Jiang, Brian M. Tackett, Steven R. Denny, Boyuan Tian, Xi Chen, Bo Wang, and Jingguang G Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03990 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 10, 2019

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Trends and descriptors of metal-modified transition metal carbides for hydrogen evolution in alkaline electrolyte

Qian Zhanga, Zhao Jiangb,Brian M. Tacketta, Steven R. Dennya, Boyuan Tianc, Xi chend, Bo Wangd and Jingguang G. Chen*a aDepartment

of Chemical Engineering, Columbia University, New York, NY 10027, USA of Chemical Engineering, Xi’an Jiaotong University, Xi’an, 710049, China cState Key Laboratory of Advanced Transmission Technology (Global Energy Interconnection Research Institute Co., Ltd.)，Beijing 102209，China dGlobal Energy Interconnection Research Institute North America, San Jose, California 95134, USA bDepartment

Corresponding Author: *[email protected]

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Abstract In this study, the experimentally measured HER exchange current densities for metal monolayermodified transition metal carbides (TMCs) are correlated with density functional theory (DFT) calculations of adsorbed hydrogen and hydroxyl binding energies. The correlation reveals a volcano relationship in alkaline electrolytes, while the hydroxyl binding energy does not appear to show a strong correlation. These results should provide guidance for further improving the electrocatalytic activity of metal-modified TMCs in alkaline environment. Key words: hydrogen evolution reaction (HER); transition metal carbides (TMC); density

functional theory (DFT); hydroxyl binding energy (OHBE); hydrogen binding energy (HBE).


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1. Introduction Water electrolysis can provide CO2-free hydrogen for energy storage and chemical synthesis that is crucial for enabling increased solar energy capacity1 and CO2 utilization technologies2. As such, the hydrogen evolution reaction (HER) has been extensively studied over the past decade3–5, with emphasis on conditions relating to the compact and versatile polymer electrolyte membrane (PEM) cells6. While acidic (proton transport membrane) HER is catalyzed by acceptably small loadings of Pt, water electrolysis at low pH is critically hindered by the requirement for the scarce and expensive IrOx catalysts for the oxygen evolution reaction (OER). To circumvent this problem, alkaline (hydroxide transport membrane) OER is readily catalyzed by many non-precious metal oxides, notably Fe and Ni compounds.7,8 However, alkaline HER kinetics are slower than in acid, increasing the Pt loading requirement. Thus, recent research has been devoted to both discovering an inexpensive replacement for the Pt HER catalyst and understanding the cause of slower HER kinetics at high pH. Understanding descriptors and trends for HER can provide insight into reaction mechanisms and help predict new active catalysts.9,10 For example, Norskov et al.11 used the well-established experimental correlation between the M-H bond energy on a metal surface and its HER activity12 to develop a database of density functional theory (DFT)-calculated hydrogen binding energies (HBEs) for monometallic catalysts. This resulted in a volcano curve when plotted with experimentally measured exchange current density. Their work provided new thermodynamic data about the adsorbed hydrogen intermediate being involved in the acidic Volmer-Heyrovsky-Tafel HER mechanism (Eq. 1- 3), which consists of the Volmer step followed by either the Heyrovsky or the Tafel step. This approach also allowed the DFTcalculated HBE to be used as a screening tool for other acid HER catalysts. This was especially useful for developing low-cost electrocatalysts with single atom-thick layers (monolayers) of Pt supported on transition metal carbides (TMCs).13,14 Later, Sheng et al. showed that a similar volcano correlation existed between DFT-calculated HBEs and experimental alkaline HER activity on monometallic catalysts.15 A subsequent study revealed that HBE on Pt was dependent on the pH value of the electrolyte, which correlated HBE with HER activity over a wide pH range.16 However, it is unclear whether such DFT calculations will be useful for Pt-modified TMCs, which are important for reducing Pt loading in alkaline HER. 3 ACS Paragon Plus Environment

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In this work, we first aim to extend the DFT-calculated HBE correlation to metalmodified TMCs to determine the utility of using HBE for this class of materials in alkaline HER. We then explore the relationship between DFT-calculated hydroxyl binding energy (OHBE) and alkaline HER activity on metal-modified TMCs. The hydroxyl group appears in the alkaline Volmer step (Eq. 4) and Heyrovsky step (Eq. 5), so it likely impacts HER kinetics more than in the acidic case. As stated above, alkaline HER is not completely described by HBE alone, and recent studies have aimed to describe the exact role of the hydroxyl for alkaline HER (mostly on Pt surfaces). Some conclude that adsorbed OH directly participates in the Volmer step,17 while others state that hydroxyl only indirectly impacts HER kinetics through competitive adsorption18 or hydrogen stabilization.16,19 Whatever the case, clearly hydroxyl binding energy should be understood to describe alkaline HER. This is especially true for TMC surfaces, which are typically more oxophilic, and should bind hydroxyl more strongly, than Pt. 𝐻 + + ∗ + 𝑒 ― ↔𝐻 ∗

Eq. 1

𝐻 ∗ + 𝐻 + + 𝑒 ― ↔𝐻2 + ∗

Eq. 2

𝐻 ∗ + 𝐻 ∗ ↔𝐻2 + 2 ∗

Eq. 3

Alkaline Volmer Step

𝐻2𝑂 + ∗ + 𝑒 ― ↔𝐻 ∗ + 𝑂𝐻 ―

Eq. 4

Alkaline Heyrovsky Step

𝐻 ∗ + H2O + 𝑒 ― ↔𝐻2 + 𝑂𝐻 ―

Eq. 5

Volmer Step Heyrovsky Step Tafel Step

Thin films of molybdenum (Mo), tungsten (W), niobium (Nb), tantalum (Ta) and vanadium (V) carbides were synthesized, and they were also modified with monolayers (MLs) of Pt, Au, Ag, or Cu. HER electrochemical testing was performed to obtain HER activity, specified by their exchange current density (i0). DFT calculations were preformed to determine the free energy change of adsorbed hydrogen (∆GH) and hydroxyl group (∆GOH) for each surface, directly correlating to HBE and hydroxyl binding energy (OHBE). To ensure the validity of the obtained HER activity, sample stability was examined by electrochemical stability testing coupled with XPS characterization before and after electrochemical testing. The correlation was then established between the calculated HBEs and OHBEs and their acid and alkaline HER activity (i0).


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2. Experimental and theoretical methods 2.1 DFT calculations Density functional theory (DFT) calculations were carried out using the Vienna Ab initio Simulation Package (VASP)20. The Kohn-Sham one-electron wave functions were expanded by using a plane wave basis set with a kinetic energy cutoff of 400 eV 21. The Perdew-Wang-91 (PW91) functional with the generalized gradient approximation (GGA) was used to deal with the electronic exchange and correlation energies22. For all metal surfaces, a 3×3 (111) unit cell with four layers was used. The transition metal terminated TMC surfaces were modeled using a 3×3 surface slab cell with four bilayers (a bilayer contains a unit of one TM layer and one C layer) of atoms, using closed-packed (111) surfaces for NbC, TaC, TiC, and VC and (0001) surfaces for WC, W2C and Mo2C. For the Pt-, Pd-, Au-, Ag-, Cu-modified TMC surfaces, an admetal monolayer was placed on the TMC surfaces. These calculations were done with a 3×3×1 k-point mesh. The bottom two layers of the slab were fixed, and the top two layers were allowed to relax. The Gibbs free energy change of adsorbed hydrogen (∆GH) and adsorbed hydroxyl group (∆GOH) were calculated using the equations shown below: ∆G=∆E+∆ZPE-T∆S

Eq. 6

where E is the total energy of a species from DFT calculations and defined in the equation below; ZPE and S are the zero point energy and entropy of a species, respectively; and T is the temperature at 298.15 K. ∆E = E(H-slab) ‒ E(slab) ‒ 0.5 × E(H2)

Eq. 7

∆E = E(OH-slab) ‒ E(slab) ‒ E(OH)

Eq. 8

where E(H–slab) is the total energy of the slab with 1/9 monolayer (ML) hydrogen adsorbed, E (slab) is the total energy of the slab in a vacuum, E(H2) and E(OH) are the total energy of hydrogen and hydroxyl in the gas phase. 2.2 Synthesis of TMC thin films Metal foils of tungsten (W), molybdenum (Mo), tantalum (Ta), niobium (Nb), and vanadium (V) (Alfa Aesar, 99.95%) were first cut to sizes of 1 cm x 1.5 cm. The foils were then preconditioned to remove surface grease and oxides23 by first sonicating in acetone for fifteen minutes followed by soaking in 0.3 M NaOH for thirty minutes . The foils were then inserted 5 ACS Paragon Plus Environment

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into a quartz tube in a furnace equipped with hydrogen (H2), methane (CH4) and passivation gas (1% O2, 99% N2) flow. For samples of W, Ta, Nb, and V, the carbonization process was carried out at 1273 K for one hour in a gas flowing environment of 100 mL min-1 H2 and 20 mL min-1 CH424,25. The temperature was ramped down and held at 1123 K for thirty minutes with a flow of 100 mL mins-1 H2 to remove excess surface carbon.26 The temperature was then ramped down gradually to room temperature (RT) in a reduced H2 flow of 70 mL min-1 followed by passivation for one hour in a 30 mL min-1 1% O2/99% N2 gas environment at RT. The temperature ramping procedure and steps for reducing surface carbon have been previously described in detail25. Regarding the carburization procedure for Mo, the procedure was followed as described in previous literature27, by carburizing at 1123 K for one hour in a gas environment of 42 mL min-1 CH4 and 85 mL min-1 H2. CH4 was then turned off with reduced H2 flow of 70 mL min-1 during the cooling process followed by the same passivation as described above. These procedures were identical to previously published studies on carbide thin films, and the XRD characterization of all carbide thin films was presented in those results25. 2.3 Physical vapor deposition (PVD) of Pt and Au on TMC surfaces The deposition of Pt or Au on the prepared TMC thin films were conducted by a PVD method28 in ultra-high vacuum (UHV) conditions by resistively heating tungsten filaments wrapped with 99.99% pure metal wires of Pt or Au. Before conducting PVD of Pt or Au, TMC foils were dipped in 0.3 M NaOH for fifteen minutes to remove passivated oxide layers. After unmodified TMC thin film samples were rinsed with DI water and air dried, they were mounted on a sample holder and loaded in an ultra-high vacuum (UHV) chamber with a base pressure of 5*10-9 Torr equipped with PVD metal sources. X-ray photoelectron spectroscopy (XPS) was measured with an Al X-ray source. The desired coverage of metal was achieved by controlling the deposition time, and coverage was quantified using XPS, as described previously29. The fitting procedure for determining these peak areas was described in our previous work performed on ML Pt-modified WC30. 2.4 Electrochemical Measurement Electrochemical tests were performed at RT both in acid (0.5M H2SO4) and alkaline (0.1MKOH) electrolytes in a three-electrode electrochemical cell set-up with graphite (SigmaAldrich, 99.995 % trace metal basis) as the counter electrode and a reversible hydrogen electrode 6 ACS Paragon Plus Environment

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(RHE, Hydroflex) as the reference electrode. Princeton Applied Research Versa STAT 4 was used as a potentiostat to conduct linear scanning voltammetry (LSV), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) testing.31 Thin film samples, as working electrodes, were covered with electroplating tape to ensure that only a 1 cm2 geometric surface area was exposed to the electrolyte. Prior to measurements，the electrolyte was purged with Argon (Ar) for fifteen minutes. All electrodes were first cleaned in Ar-saturated acid or alkaline electrolyte with 10 CV cycles between 0.05 and ~ 0.5 V vs. RHE using a scan rate of 100 mV s-1. For Pt and Pd polycrystalline thin films, the CV scan range was set from 0.05V to 1.4 V vs. RHE. Next, the electrolyte was purged with ultra-high purity H2 gas for twenty minutes. Cathodic LSV testing was conducted starting from 0.15V vs RHE at a scan rate of 20 mV s-1 while H2 continued to purge the electrolyte. Cell resistance was measured at the beginning of each LSV measurement using electrochemical impedance spectroscopy (EIS). All reported potentials were obtained after cell resistance correction. The LSV tests were usually performed three times to ensure the reproducibility of the LSV results, and the last LSV curve was used for data analysis. After testing, the samples were rinsed in DI water and then air dried for post-HER XPS characterization. The log10(i0) values of unmodified and metal-modified TMCs were obtained from extrapolation of Tafel plots within the range of current densities between -1 mA cm-2 to -4 mA cm-2, as described in literature15,32. 3. Results and Discussion 3.1 HER activity for Pt- and Au-modified TMCs in acid Several metal-modified carbide thin films have been evaluated experimentally in previous studies for acid HER and have shown good correlation with DFT-calculated HBE values14. Metal-modified TaC and VC thin film HER experiments have not been previously reported and were used here to validate the experimental procedure. Fig. 1 illustrates HER current density for various coverage of Pt on TaC (a) and Au on TaC (b) in 0.5 M H2SO4 electrolyte. Unmodified TaC was the least active for HER and required more than 650 mV vs. RHE applied potential to reach 5 mA cm-2. As the Pt and Au coverage increased, the overpotentials required to achieve the same current density decreased. The LSV curve for 1.8


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ML Pt/TaC overlapped with that of the Pt foil, and 2.8 ML Au/TaC LSV matched well with the Au foil LSV.

Fig. 1 Acid HER LSV curves for various MLs Pt-modified TaC (a) and Au-modified TaC (b) in H2-saturated 0.5 M H2SO4; Acid HER activity for Pt and Au foils samples incorporated; LSVs after cell resistance correction at RT and collected at a scan rate of 20 mV s-1; The acid HER activity indicated by log10(i0) as a function of the different number of Pt or Au MLs coverage on the TaC substrates shown in (c) and (d), respectively; The current density was normalized by the geometric surface area of the each tested sample.

The log10(i0) values were obtained from Tafel plots of the LSV data and plotted as a function of Pt coverage (Fig. 1c) and Au coverage (Fig. 1d) on TaC. These plots show an increase in log10(i0) with increasing Pt and Au coverage. There was a plateau at the Pt coverage of 1.8 ML and Au coverage of 2.8 ML, where io was similar to Pt and Au foils, respectively. This result is expected, because the DFT-calculated HBEs for Pt/TaC and Au/TaC are similar to that of bulk Pt and bulk Au, respectively. This behavior is similar to previous reports for various 8 ACS Paragon Plus Environment

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metal-modified TMCs30,32–34. Acid HER log10(i0) for the tested samples are summarized in Table 1 along with a complete list of acid HER log10(i0) for metals, carbides, and metal-modified carbides reported in literature14. Bold-face values are from literature, and normal font values are experimental observations from this work. Table. 1 The summary of log10(i0) in 0.5 M H2SO4;The values in italics were from published results summarized in literature14. catalyst

Log10(io [A cm-2]) in 0.5 M H2SO4

Log10(io [A cm-2]) in 0.1 M KOH

Ag

-6.3

-7.66

AgML/NbC

-6.3

-5.74

AgML/Mo2C

-6.57

-5.76

Au

-5.75

-5

AuML/Mo2C

-5.6

-5.47

AuML/NbC

-6.73

AuML/TaC

-6.1

AuML/VC

-6.15

AuML/WC

-5.1

-5.81

Cu

-5.24

CuML/Mo2C

-6.14

-5.60

Mo2C

-6.1

-5.66

NbC

-6.8

-6.2

Pd

-3.2

-4.12

PdML/Mo2C

-4

PdML/NbC

-4.4

PdML/WC

-3.7

Pt

-3.1

-3.72

PtML/Mo2C

-3.2

-4.5

PtML/NbC

-3.5

-4

PtML/TaC

-3.41

-4.52

PtML/TiC

-3.15

PtML/VC

-3.65

PtML/W2C

-3.5

PtML/WC

-3.3

-4.1

TaC

-7.55

-7.03

TiC

-5.8

VC

-7.11

W2C

-5.75

-4.6

-6.09


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WC

-5.75

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-5.63

3.2 HER activity and stability for metal-modified carbides in alkaline In alkaline electrolyte, the HER activity and stability were examined for five different TMCs, TaC, VC, Mo2C, WC and NbC with these surfaces modified with Au and Pt overlayers. Pt/TaC and Au/TaC are first examined to determine if the procedures demonstrated for acid HER are applicable to alkaline HER. The alkaline HER curves for Pt-modified TaC (Fig. 2a) reached closer to the Pt foil with increasing Pt coverage. The trend was similar for Au-modified TaC in alkaline HER, as shown in Fig. 2b. Figures 2c and 2d show acquired log10(i0) values for different coverage of Pt and Au on TaC, respectively. They are consistent with the trend indicated by the LSV curves. It should also be noted that, as compared to acid, the Pt foil in alkaline illustrated slower HER kinetics, as indicated by the LSV curves and log10(i0) values. To achieve alkaline HER current density of 5mA cm-2, Pt foil required an overpotential of approximately 130 mV (Fig. 2a) with log10(i0) of -3.8 (Fig. 2b), while it only required 25 mV (Fig. 1a) with log10(i0) of 3.1 (Fig. 1c) in acid. These results are consistent to the literature32. The HER trends of metalmodified TaC in alkaline are similar to those in acid.


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Fig. 2 alkaline HER LSV curves for various MLs Pt-modified TaC (a) and Au-modified TaC (b) in H2-saturated 0.1 M KOH; Alkaline HER activity for Pt and Au foils samples incorporated; LSVs after cell resistance correction at RT and collected at a scan rate of 20 mV s-1; The alkaline HER activity indicated by log10(i0) as a function of the different number of Pt or Au MLs coverage on the TaC substrates shown in (c) and (d), respectively; The current density was normalized by the geometric surface area of the each tested sample.

Catalyst stability during alkaline HER testing was examined by XPS measurements before and after electrochemical testing. All Pt-modified TMCs and Au-modified TMCs appeared to be stable catalysts during alkaline HER tests. The Pt-modified TaC are shown as an example in Fig. 3a. The Pt and Au overlayers decreased by approximately 20% to 30% after onehour HER measurement in alkaline, shown in Fig. 3a and 3c, respectively. The signal decrease could be either attributed to Pt overlayer dissolution or agglomeration32. To determine the metal coverage reduction mechanism, an additional hour of HER testing containing CV cleaning with three repeating LSV measurements was further performed on the spent 1 ML Pt/TaC sample. If the Pt dissolution occurred, continuing decrease in the ratio from one-hour testing to two-hour testing should be detected. However, both Fig. 3a and 3c revealed that the decrease in the metal overlayers after an additional hour of stability testing were insignificant, indicating that the 11 ACS Paragon Plus Environment

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decrease in the XPS ratios were most likely due to Pt agglomerations during the initial electrochemical measurements.

Fig. 3 Metal overlayer coverages after one-hour and two-hours alkaline HER testing for different Pt overlayer-modified TaC shown in (a) and different Au overlayer-modified TaC (c); Pt/Ta and Au/Ta XPS signal ratios for one-hour and two-hours alkaline HER testing for 1ML Pt-modified TaC (b) and 2 ML Au-modified TaC (d), respectively.

3.3 Correlation between HER activity and possible descriptors in acid and alkaline Table 2. DFT calculations of ∆GH on Pt, Pd, Au, Ag-modified carbides, Pt(111), Pd(111), Au(111), Ag(111) and unmodified carbides surfaces

Category

Surfaces

∆GH (eV)

∆GOH (eV)

Pt/carbides

PtML/Mo2C(0001) PtML/NbC(111) PtML/TaC(111) PtML/TiC(111) PtML/VC(111) PtML/W2C(0001)

0.01 −0.28 −0.07 −0.03 0.02 0.09

-2.39 -2.41 -2.51


-2.26

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Pd/carbides

Au/carbides

Ag/carbides Cu/carbides Bulk metals

Unmodified-carbides

PtML/WC(0001) PdML/Mo2C(0001) PdML/NbC(111) PdML/WC(0001) AuML/Mo2C(0001) AuML/NbC(111) AuML/TaC(111) AuML/VC(111) AuML/WC(0001) AgML/NbC(111) AgML/Mo2C(0001) CuML/Mo2C(0001) Ag(111) Au(111) Cu(111) Pd(111) Pt(111) Mo2C(0001) NbC(111) TaC(111) TiC(111) VC(111) W2C(0001) WC(0001)

−0.13 −0.03 −0.19 −0.19 0.48 0.57 0.47 0.51 0.14 0.44 0.40 0.12 0.37 0.36 0.36 -0.36 -0.26 −0.75 −0.90 −0.99 −0.96 −0.84 −0.60 −0.81

-2.81

-2.39 -2.31 -2.51

-2.81 -2.88 -3.52 -2.44 -1.61 -2.34 -2.21 -4.6 -4.85 -4.96 -4.91 -4.19

The Gibbs free energy change of adsorbed hydrogen (∆GH) was calculated by DFT over ML metal-modified TMC surfaces, as shown in Table 2. Based on previously published results14 , HBE had been reported as an effective descriptor for HER activity in acid. A volcano correlation was established between the HER activity for examined catalysts with the respective calculated HBE. Figure 4 shows a volcano relationship between acid HER activity (summarized in Table 1) on the vertical axis and ∆GH (Table 2) on the horizontal axis. Catalysts not previously evaluated for HER activity, including unmodified TaC, VC Pt/TaC, Pt/VC, Au/TaC, Au/VC, Ag/Mo2C, and Cu/Mo2C, fit well on the volcano plot, as shown in Fig. 4. These results are expected, because the acid HER trend is well-established for metal-modified TMCs.


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Fig. 4 log10 (i0) for unmodified, metals-modified TMCs and monometallic metals as a function of corresponding ∆GH in 0.5 M H2SO4; Arrow pointed data were measured for this manuscript and other log10 (i0) results were from published results summarized in literature14.

The descriptor for alkaline HER has been under debate due to the differences in proposed mechanisms in acid and alkaline electrolytes, as discussed in the introduction. Therefore, the Gibbs free energy change of the adsorbed hydroxyl group (∆GOH) was also calculated and summarized in Table 2. Pt, Au, Ag, and Cu-modified TMCs were used to examine possible correlations between alkaline HER activity (log10(i0)) and either ∆GH (Fig. 5a) or ∆GOH (Fig. 5b). The alkaline HER activity for metal-modified TMCs forms a volcano-shaped relationship with ∆GH, as shown in Fig. 5a. This indicates that ∆GH is, indeed, an important descriptor for alkaline HER activity on metal-modified carbides. According to Fig. 5a, the best HER activity in alkaline, obtained on the Pt surface, should correspond to the most appropriate ∆GH located at the apex. Based on the Sabatier principle35, when hydrogen atoms were bound to surfaces more strongly than Pt (∆GHPt), the hydrogen surface coverage was relatively lower. As a result, higher energy (higher overpotential) was required to 14 ACS Paragon Plus Environment

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reach the same alkaline HER activity. The trend was formed by the electrocatalysts of Au, Ag, and Cu-modified carbides. Unlike the volcano-shaped relationship formed with ∆GH, ∆GOH shows a much weaker correlation with alkaline HER activity (Fig. 5b). Au-modified TMCs have similar OHBEs as Pt and Pt-modified TMCs, but the Au/TMCs alkaline exchange current densities are 2 or 3 orders of magnitude lower than their Pt counterparts. Additionally, Ag, Cu, and carbides modified with Ag and Cu have a wide range of OHBEs, but show no discernible HER trend across that range. This indicates that OHBE is not a good descriptor for alkaline HER on TMCs and metal-modified TMCs. It also suggests that the adsorbed hydroxyl group does not play a direct role in the rate determining step of alkaline HER kinetics on these surfaces. Therefore, as compared to ∆GH, ∆GOH should be considered as a less effective descriptor for alkaline HER of examined TMCbased catalysts. Thus, the decreased HER activity for metal-modified TMCs in alkaline compared to acid cannot be attributed to HBE and OHBE alone. Instead it is likely that the alkaline environment influences the water structure or hydrogen stability, as suggested by some other studies on Pt surfaces.16,18,19


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Fig. 5 log10 (i0) for metals, TMCs, and metal-modified TMCs, as a function of ∆GH (a) or ∆GOH (b) in 0.1 M KOH.

4. Conclusions Metal-modified TMC films were evaluated under alkaline HER conditions, and the HER activity correlates strongly with DFT-calculated HBE values. A volcano trend on metalmodified TMCs indicates that HBE should be a good descriptor for this class of materials under both alkaline and acid conditions. The hydroxyl binding energies do not show the expected volcano relationship with measured alkaline HER activity, suggesting that the adsorbed hydroxyl group does not directly participate in the rate determining step of the alkaline HER.

Acknowledgements This work was supported by the U.S. Department of Energy, Office of Science, Catalysis Program (DE-FG02-13ER16381). We also acknowledge partial support by GEIRI North America, San Jose, California, under State Grid fund SGRIDLKJ (2016) 794 (code: SGRI-DL-71-16-015). The DFT

calculations were performed using computational resources at the Center for Functional 16 ACS Paragon Plus Environment

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Nanomaterials, a user facility at Brookhaven National Laboratory supported by the U.S. DOE under Contract No. DE-AC02-05CH11231.


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Abstract graphic


Trends and descriptors of metal-modified transition metal carbides for

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