Reaction Condition Dependence of Different Overpotential Components in Electrochemical Hydrogen Catalysis
Author(s)
Tang, Bryan Y.
Advisor(s)
Surendranath, Yogesh
Date Issued
September 2023
Publisher
Massachusetts Institute of Technology
Abstract
The ability to drive redox reactivity at electrode interfaces necessitates polarization away from equilibrium. This manifests as an applied or measured overpotential. However, there are many ways to generate non-equilibrium conditions, and correspondingly overpotential can arise from multiple different sources. Thus, the applied overpotential in an electrocatalytic system is always the aggregate of different overpotential components, each of which can be uniquely affected by different reaction conditions. This thesis seeks to not only isolate, but understand how different overpotential components are affected by changes in reaction condition.
Chapter 2 explores how the applied overpotential for the hydrogen evolution reaction (HER) can be partitioned into a charge transfer overpotential, which drives proton-coupled electron transfer, and a chemical overpotential arising from increasing surface H activity. Typical experiments provide no information about the relative contributions from these two components in hydrogen evolution catalysis. We employ a Pd membrane double cell to spatially isolate charge transfer and chemical reaction steps in HER catalysis, deconvoluting their overpotential contribution under different reaction conditions. We analyze how pH, and the introduction of poisons and promoters affect each component, and find that for a given H2 release rate, only charge transfer overpotential is affected by reaction conditions. These findings suggest that reaction condition dependent-HER efficiencies are driven predominantly by changes to the charge transfer kinetics rather than the chemical reactivity of surface H.
Chapter 3 explores how proton consumption necessary for the HER can lead to non-equilibrium interfacial pH environments that differ substantially from the bulk. This pH swing subsequently manifests as a concentration overpotential which superimposes onto the aggregate overpotential in the HER. Using open circuit potential decay transients, we develop and validate a general methodology for temporal isolation of the proton concentration overpotential on Pt. This then allows us to experimentally quantify polarization-induced interfacial pH swings. Using this method, we quantify the impact of buffer strength, supporting electrolyte composition, and the presence of cation exchange polymer overlayers on the polarization-induced pH swings. We find that (1) modest current densities of −30 mA cm⁻² are sufficient to sustain pH swings of > 2 pH units, even for strongly buffered solutions; (2) addition of alkali supporting electrolyte to unbuffered, acidic electrolyte can induce pH swings so large that the polarized electrode environment becomes strongly alkaline and (3) the presence of a Nafion polymer overlayer containing fixed anionic charges serves to further augment the interfacial pH swing, resulting in a similar pH swing at half the applied current density. The transport characteristics of these systems were analytically modelled, enabling direct calculation of boundary layer thickness and quantitative prediction of the OCP decay transient.
MIT Department
Massachusetts Institute of Technology. Department of Chemistry
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