(16e) Kinetics of Hydrogen Evolution and Oxidation Reactions on Pt Contacting a Vapor-Equilibrated PFSA Thin Film

The membrane electrode assembly (MEA) is a highly scalable electrochemical device architecture which uses a water-swollen ion-conducting polymer (ionomer) as both the electrolyte and separator. In MEA devices, electrochemical reactions occur in catalyst layers. These layers are complex porous networks of catalyst particles coated and bound by inhomogenous thin films of ionomer facilitating ions access to/from catalyst sites in the electrode. Facile electrochemical kinetics in the catalyst layer are critical to device performance, however, characterizing these kinetic parameters is challenging.

Two primary obstacles make characterization of kinetic parameters for MEA devices difficult: 1) electrode geometries in real devices are nonuniform and complex precluding straightforward isolation of kinetic parameters, 2) reaction conditions in traditional well-defined systems such as rotating disk electrodes are unrepresentative of those in MEA devices making the extracted kinetic parameters inapplicable to MEAs.

Reaction conditions in MEA electrodes are nonuniform in both the micron and nanoscale. Reactant/product concentrations and ohmic resistances vary through the thickness of the electrodes (μm-scale) causing nonuniform current distributions.¹ At the nanoscale, inhomogenous ionomer coverage results in regions of catalyst coated by thick ionomer layers while other regions are left bare, without ionomer coating.² To further complicate matters, liquid water often condenses in smaller pores of the electrode affecting both transport and, likely, kinetics in these regions. As such, extracting intrinsic kinetic parameters from these devices is extremely challenging. Instead, MEA-level kinetic studies often fit overall cell kinetics to Tafel kinetic expressions—anodic and cathodic kinetics are difficult to isolate as reference electrodes cannot easily be integrated into MEA devices.³ Often, kinetic parameters fitted from MEAs are specific to a set of operating conditions (e.g. relative humidity) and are not transferable to different MEA systems even when the same catalyst material is used.

Rotating-disk electrode, liquid-phase microelectrode, and other more well-controlled systems have been used to characterize intrinsic kinetic parameters, but generally, kinetic activities observed in these systems are much larger than those expected in MEA devices.⁴ The primary discrepancy between MEAs and more well-controlled systems is thought to be the local reaction environment at the catalyst-electrolyte interface. Several studies show that changing this local environment (e.g. by changing cation identity, water structure, or adding interfacial additives) significantly affects the kinetics for a variety of reactions including hydrogen and oxygen oxidation/reduction and CO₂ reduction. In more well-controlled systems, typically a liquid electrolyte is used. In contrast, MEAs use solid electrolytes (i.e. ionomers) which have drastically different species (e.g. sulfonic-acid moieties) and solvent structures at the catalyst-ionomer interface than liquid electrolytes.

Here we develop a solid-state cell which is well-controlled and more representative of reaction conditions in MEA devices. We apply this system to interrogate the kinetics of hydrogen oxidation (HOR) and evolution (HER) reactions on a polycrystalline Pt surface coated by a film of water-swollen perflourosulfonic acid (PFSA) ionomer, the prototypical ionomer used in PEM systems. The HOR/HER reactions are relevant for fuel cell/water electrolyzer MEA devices and are more amenable to a detailed kinetic study as the mechanism is well-defined in comparison to other reactions (e.g. oxygen evolution/reduction). The water content of the PFSA ionomer is varied through the relative humidity (RH) of gas fed to the electrode chamber and polarization curves are collected at each RH. To isolate intrinsic kinetic parameters, we develop a thermodynamic framework to calculate the H⁺ activity, which varies with ionomer water content. Given that acidic HOR/HER kinetics are notoriously fast, we carefully correct for gas transport in our system. After accounting for H⁺ activity and gas transport, we fit polarization curves to a Volmer-Heyrovsky model to extract intrinsic kinetics at each RH. We find the intrinsic HOR/HER kinetics are faster at higher humidity, suggesting that the water structure, which changes with humidity, plays an important role in the electrochemical kinetics.

References

[1] Yoon, W., & Weber, A. Z. (2011). Modeling Low-Platinum-Loading Effects in Fuel-Cell Catalyst Layers. Journal of The Electrochemical Society, 158(8), B1007. https://doi.org/10.1149/1.3597644

[2] Girod, R., Lazaridis, T., Gasteiger, H. A., & Tileli, V. (2023). Three-dimensional nanoimaging of fuel cell catalyst layers. Nature Catalysis, 6(5), 383–391. https://doi.org/10.1038/s41929-023-00947-y

[3] Neyerlin, K. C., Gu, W., Jorne, J., & Gasteiger, H. A. (2006). Determination of Catalyst Unique Parameters for the Oxygen Reduction Reaction in a PEMFC. Journal of The Electrochemical Society, 153(10), A1955. https://doi.org/10.1149/1.2266294

[4] Durst, J., Simon, C., Hasché, F., & Gasteiger, H. A. (2015). Hydrogen Oxidation and Evolution Reaction Kinetics on Carbon Supported Pt, Ir, Rh, and Pd Electrocatalysts in Acidic Media. Journal of The Electrochemical Society, 162(1), F190–F203. https://doi.org/10.1149/2.0981501jes