Adsorbed Hydroxide Does Not Participate in the Volmer Step of

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Adsorbed Hydroxide Does Not Participate in the Volmer Step of Alkaline Hydrogen Electrocatalysis Saad Intikhab, Joshua D. Snyder, and Maureen Tang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02787 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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Adsorbed Hydroxide Does Not Participate in the Volmer Step of Alkaline Hydrogen Electrocatalysis Saad Intikhab, Joshua D. Snyder, Maureen H. Tang* Chemical and Biological Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States

ABSTRACT: The sluggish kinetics of the alkaline hydrogen electrode have been attributed to the need to adsorb both H and OH optimally. In this work, single-crystal voltammetry and microkinetic modeling show that an OH-mediated mechanism is not viable on Pt (110). Only a direct Volmer step can explain observed kinetic trends with OH adsorption strength in KOH and LiOH electrolytes. OH instead behaves as a rapidly equilibrated spectator species that decreases available surface sites and slows hydrogen kinetics. These results identify kinetic barriers from interfacial water structure, not adsorption energies, as key to explaining changes in hydrogen kinetics with pH. (99 words)

KEYWORDS: electrocatalysis, Volmer step, microkinetic modeling.

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It is well accepted that the hydrogen oxidation and hydrogen evolution reactions (HOR/HER) are slower in basic than acidic electrolytes1–3. Understanding the root of this observation is critical for electrocatalyst design, not only for HER/HOR, but also for CO2 reduction, nitrogen fixation, and other engineering challenges of the 21st century. Several explanations for the pH dependence of HER/HOR kinetics have been proposed. One possibility is that OH- stabilizes the Pt-H bond for stronger binding and slower catalysis4,5. Central to this hypothesis is an experimentally measured shift with pH in the peak potential for hydrogen underpotential deposition (H-UPD) on the (100) and (110) oriented sites on polycrystalline Pt. However, Pt (111) exhibits almost no shift in H-UPD, but similar losses in HER/HOR activity when shifting from acid to base2,6,7, and weak-binding catalysts such as gold and copper do not improve activity at high pH8,9, as this mechanism predicts. The pH-dependent shift in H-UPD peak is instead caused by changes to competitive OH adsorption10,11, as shown in Scheme 1. Scheme 1: Direct Volmer step ∗ + + ↔ + [I] + ↔∗ +

[II]

Janik et al showed that on Pt(110) and Pt(100) at high pH, water preferentially solvates adsorbed alkali cations instead of OHad, weakening OHad and pushing the peak to higher potentials12. In recognition of this competition, we refer to this voltammetric peak as H/OH exchange (HOH-X). Weaker OH adsorption at high pH has also been suggested as the cause of pH dependence of HER/HOR kinetics, based on observed synergies between Pt (111) and either Pt or Ni(OH)2 nanoclusters13–16. Improved HER kinetics were attributed to facile water


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dissociation on oxophilic sites, generating a bifunctional mechanism for efficient electrocatalysis (Scheme 2). Scheme 2: Indirect or OH-mediated Volmer step + ↔∗ +

[II]

2 ∗ + ↔ +

[III]

A third explanation points to a possible pH dependence of surface water orientation, independent of OH adsorption10,17. Recent experiments suggest that Ni(OH) 2 nanoclusters shift the potential of zero charge on Pt (111) by approximately -25 mV, closer to the reversible hydrogen potential7. It was hypothesized that at the potential of zero charge, a more fluid and dynamic water structure lowers barriers to solvent reorganization. In this work, we investigate specifically the hypothesis that adsorbed OH actively participates in the alkaline Volmer step on Pt (110). We focus on the so-called “H-UPD” region (0 to 0.5 V vs. RHE) to isolate the Volmer step of H adsorption and eliminate complications from bulk H2 transport or surface-H recombination. Microkinetic modeling of the indirect (OH-mediated) and direct (OH-as-spectator) Volmer step predicts that changes to OH adsorption affect the rates of these mechanisms differently. Experimentally varying the OH binding strength via the electrolyte cation can therefore determine which mechanism dominates. Voltammograms of Pt (110) in 0.1 M KOH, 0.1 M LiOH, and 0.1M HClO4 are shown in Fig. 1(a). The observed difference of HOH-X peaks in the different electrolytes is consistent with literature12,18 and attributed to the interaction of metal cations with oxygenated species like OHad and surface water. As shown in Fig. 1(a), the (110) peak shifts by approximately -15 mV from


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KOH to LiOH, indicating stronger OH binding in LiOH. This effect is expected due to noncovalent interactions between hydrated alkali cations and OHad13,19 and because the lower redox potential of Li is expected to decrease the surface cation coverage and subsequent OHad desolvation12. In HClO4, the (110) peak shifts to even lower potentials, indicating even stronger binding of OHad in acid, consistent with theoretical predictions12. Evidence in literature and Fig. 1b suggests that metal cations do not interact with adsorbed hydrogen12,18,20. On Pt (111), hydrogen and hydroxide adsorption are separated by about 0.8V, and while the hydrogen region remains unaffected from KOH to LiOH, there is a change in the adsorption potential of hydroxide species as shown in Fig. 1(b). The unaffected hydrogen region demonstrates that metal cations do not affect the H adsorption energy. Relating voltage scan rate to peak potential splitting Δ (the difference between the potential of peak anodic and cathodic currents) is a common technique for extracting heterogeneous rate constants for soluble redox couples21 and can also be applied to surface adsorption reactions22–25. Fig. 2 plots Δ vs. peak current density (the combined anodic and cathodic peak heights minus double-layer charging) for the HOH-X region of Pt (110). For simplicity, in all the figures (except Figure 1), the currents are normalized by the electrochemical active surface area (ECSA) based on the traditional H-UPD method with the constant 147 µC/cm26. Complete CVs and scan rate dependence are shown in Figs. S1-S5. In all electrolytes, Δ is minimal at slow scan rates. At pH=1, ΔE remains small at faster scan rates, demonstrating reversible kinetics. At pH=13,

increasing ΔE with scan rate shows slower HOH-X kinetics, with greater ΔE in LiOH indicating slower adsorption than in KOH.


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Previous models of the Pt CV have neglected competition between H and OH22–25 or considered only thermodynamic effects, not kinetics12,26. Time-dependent expressions for current and voltage as functions of H and OH adsorption energy for Schemes 1 and 2 are derived fully in the Supporting Information and summarized in Tables 1 and 2. Key parameters include the free ∗ energy of H and OH adsorption at 0V (Δ∗ and Δ ), and the kinetic parameters , , , and

, . As described here, , includes an attempt frequency and an exponential barrier

dependence27–29. Numeric solutions to the models of Tables 1-2 are compared to experimental HOH-X peaks at 20 mV/s in Fig. 3. Any mechanism can describe peak positions and height at slow scan rate regardless of the values of , because the reaction proceeds close to the

equilibrium potential, i.e. Δ ≈ 0. The direct mechanism matches the (110) peak position in

∗ ∗ KOH with Δ∗ = -0.37 eV and Δ = 0.185 eV. Strengthening Δ to 0.158 eV describes the

15 mV shift in LiOH. All three values compare reasonably well to theoretical predictions12. For ∗ the indirect mechanism, DFT-predicted values12,30,31 for Δ∗ , Δ , and result in ≈

10!", while water’s self-ionization constant in the absence of surface adsorption is well known

as 10!#. Matching experiment requires arbitrary adjustment of = 0.001 eV, yielding = 1412 (KOH) or 4041 (LiOH). These unrealistic values shed considerable doubt on the indirect

mechanism. In both mechanisms, simulating the experimental peak height requires reducing the maximum surface concentration from the theoretical value of 1.52 × 10' mol/cm2 (147

µC/cm26) to 7.6 × 10!* mol/cm2, indicating that the surface likely reaches saturation at sub-

monolayer coverage6,12. At 1200 mV/s, the increased ΔE requires that at least one of the kinetic parameters , ,

, or , is slow. To consider the limiting cases, one step was assumed to be equilibrated


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while the kinetic parameter of the other step was reduced until the simulated Δ matched

experiment. As shown in Fig 4, the experimentally measured ΔE of 122 mV (a) in KOH can be described by any of the four different mechanisms: direct/indirect, OH-limiting (b), direct, H!

limiting (c), or indirect, dissociation-limiting (d). Setting , > 10, for equilibrated steps and -

, ≈ 10# for rate-limiting steps describes experiment well. Barriers of 0.15 – 0.44 eV and frequency factors of 1013 have been applied to previous models of the Volmer step on Pt(111) in !

acid27,28, corresponding to of 2 × 10!* to 9.4 × 100 , in reasonable agreement with the -

model. The unrealistic values required of to simulate the H/OH peak positions at 20 mV/s already favor the direct (Scheme 1) over the indirect (Scheme 2) Volmer step, but the fast scan (1200 mV/s) rate data provides more definitive evidence for the direct mechanism. After estimating the ∗ kinetic parameter from the peak splitting in KOH, the effect of stronger Δ in LiOH (27 meV

in our simulation) can be shown to affect each mechanisms differently. If OH adsorption is ratelimiting in either Scheme 1 or 2, increasing OH binding strength makes the overall kinetics faster and decreases ΔE (b). If H adsorption is the rate-limiting step in Scheme 1, increasing OH

binding strength slows kinetics and increases ΔE (c). If water dissociation/recombination is

rate-limiting in Scheme 2, a 27 meV change affects so drastically that the reaction does not

reach completion (d). Experimentally (a), stronger OH binding increases ΔE (145 mV), a trend seen only in Fig. 4c, a direct mechanism limited by H adsorption. We emphasize that the primary objective of this work is not to quantitatively simulate experimental CVs, but to identify mechanisms and rate-limiting steps through qualitative effects.


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∗ ∗ Although multiple combinations of Δ∗ and Δ (direct) or Δ and (indirect) can

∗ describe the peak positions at 20 mV/s, only strengthening Δ can describe the peak shift

∗ is stronger, only the direct Volmer step with Hbetween KOH and LiOH (Fig. S1). When Δ

limiting can qualitatively describe slower HOH-X kinetics. Fig. 4c also uniquely replicates the experimental asymmetry, in which the anodic peak is much less sensitive to scan rate than the cathodic peak. Further support for fast OH adsorption is shown on Pt (111) (Figs. S6-S7). When OH and H do not compete for surface sites, ΔE remains small in both electrolytes, showing that stronger OH binding in LiOH does not affect the rapid kinetics. ∗ Detailed examination of the model explains the opposite effects of Δ on ΔE in OH- and

H-limited schemes. When OH adsorption is limiting, strengthening the relatively weak Pt-OH bond increases the rate via the exponential prefactor in Eqs. 4 and 10. When H adsorption is limiting, stronger OH adsorption increases OHad coverage at lower potentials. More overlap between OH and H limits the number of empty sites at any given potential (Fig. 5a), thereby decreasing the exchange current density (b) and driving up overpotential. The poisoning effect of OHad in LiOH therefore sheds light onto the difference between acid and base. The lower HOH∗ X peak in 0.1M HClO4 corresponds in our simulations to Δ = -0.09 eV (Fig. S8). The rapid

HOH-X kinetics in acid (Fig. 2) are too fast for meaningful microkinetic analysis, showing that, in acid, increased OH binding strength does not have the poisoning effect predicted by the model. Therefore, the kinetic parameter , must be so fast in acid that H adsorption is equilibrated despite an even lower availability of surface sites. This study therefore provides conclusive evidence that the pH dependence of H adsorption kinetics on Pt (110) is caused not by thermodynamic differences to adsorbate energies or reaction pathways, but differences in the kinetic frequency factors of elementary steps7,17,32. Because the enthalpy of H-adsorption is


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independent of pH, the difference must originate from differences to the transition state energy, or to the frequency factor. Low frequency factors in electrocatalytic reactions have previously been attributed to relatively slow dynamics of interfacial solvent reorganization29,33,34. Our study therefore lends further support to water reorganization as a descriptor for hydrogen kinetics7. In conclusion, we have developed microkinetic models for the direct and OH-mediated Volmer steps and shown by their comparison with single-crystal voltammetry that the OH-mediated mechanism is extremely unlikely to govern alkaline hydrogen electrocatalysis on Pt (110). The OH-mediated mechanism requires unrealistic values for Gw and Keq and is furthermore inconsistent with experimentally observed reduction in HOH-X kinetics with stronger OH binding. The model indicates that the alkaline Volmer step proceeds through a direct mechanism with rapid OH and slow H adsorption. Our results indicate that, contrary to published results13-16, adsorbed OH is not an active participant in the Volmer step on Pt, but leads to a decrease in the number of active sites for H adsorption. However, because H and OH do not overlap on Pt (111), these effects will be observed only on (110) and (100)-like sites. This work emphasizes that thermodynamic adsorption energies are insufficient descriptors for hydrogen adsorption kinetics. The great challenge to designing next-generation electrocatalysts is controlling not only the adsorption strength of intermediates, but also interfacial water structure in order to facilitate low transition state barriers and rapid solvent reorganization. (main text: 1970 words)


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FIGURES.

(a)

(b)

Figure 1: CVs of Pt (110) (a) and Pt (111) (b) in 0.1M KOH and 0.1M LiOH at 50 mV/s

Figure 2: Peak splitting Δ vs. peak current density for HOH-X on (110) sites.


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Figure 3: Comparison of experiment and direct/indirect HOH-X model at slow (20 mV/s) scan rate. Dashed lines: experimental CV (Hads/des region) on Pt (110) in 0.1 M KOH (blue) or 0.1M ∗ LiOH (red). Solid lines: direct/indirect model for Pt (110) with Δ = 0.185 eV (blue) or 0.158

eV (red).


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(a)

(b)

(c)

(d)

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∗ = 0.185 eV (KOH, Figure 4: Comparison of direct/indirect models at fast scan rate with Δ

blue) or 0.158 eV (LiOH, red). a) Experimental CV on Pt (110) in 0.1 M KOH (blue) and 0.1M


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!

!

!

LiOH (red). b) Direct/indirect, OH-limiting (, 1 10, - , , 1 1100 or , 1 6000 , !

!

!

, 1 10, - ) c) Direct, H-limiting (, 1 45,000 - , , 1 10, - ) d) Indirect, dissociation!

!

limiting (, 1 10, - , , 1 75,000 - ).

(a)

(b)

Fig. 5: Effect of competitive adsorption on equilibrium surface coverage (a) and exchange ∗ ∗ current density (b) with weaker (Δ = 0.185 eV, blue) or stronger (Δ = 0.158 eV, red) OH

binding.


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TABLES. Table 1: Microkinetic model of direct Volmer step (Scheme 1) 23 4 23 4 8 1 , 23 4 exp =

56 1 8 57

[1]

56 1 −8 57

[2]

C1 − >DF >Δ∗ >F B CG − H D − expC CG − H DI A 6 C1 − 6 − 6 D!B EexpC − ?@ ?@ ?@

8 1 , 23 4 exp =−

∗ >Δ >F C1 − >DF !B CG − H D − expC A 6 C1 − 6 − 6 DB [expC − CG − H D] ?@ ?@ ?@

H 1 − H 1

Δ∗ ?@ 1 − 6 − 6 + ln F F 6

∗ Δ ?@ 6 + ln F F 1 − 6 − 6

8 + 8 1 −

NO F

[3]

[4]

[5]

[6]

[7]

Table 2: Microkinetic model of indirect Volmer step (Scheme 2) 23 4 23 4 8 1 , 23 4 exp =−

56 1 8 57

[8]

56 1 8 − 8 57

[9]

∗ >Δ >F C1 − >DF !B CG − H D − expC A 6 C1 − 6 − 6 DB [expC − CG − H D] ?@ ?@ ?@

8 1 , 23 4 [C1 − 6 − 6 D − H 1

6 6 ]

∗ Δ ?@ 6 + ln F F 1 − 6 − 6

1 exp =−

∗ P∗ + P − A ?@

8 1 −

NO F

[10]

[11]

[12]

[13] [14]


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AUTHOR INFORMATION Corresponding Author *Maureen Tang, [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National Science Foundation through the Catalysis Program in the Division of Chemical, Biological, Environmental and Transport Systems, under Award #1602886. . ABBREVIATIONS CV, cyclic voltammetry; HER, hydrogen evolution reaction; HOR, hydrogen oxidation reaction; HOH-X, hydrogen/hydroxide exchange; H-UPD, hydrogen underpotential deposition; RHE, reversible hydrogen electrode. REFERENCES (1)

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TOC FIGURE:


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Adsorbed Hydroxide Does Not Participate in the Volmer Step of

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