Imaging Water-Splitting Electrocatalysts with pH-Sensing Confocal Fluorescence Microscopy

Abstract

Hydrogen generation by water electrolysis is promising for energy storage, and imaging the reaction dynamics near a H 2 -evolving electrode can provide valuable insights. We utilized laser-scanning confocal fluorescence microscopy to map the product/reactant concentration at a H 2 -evolving electrode at micron-scale resolution which identifies areas with fast reaction kinetics. Small concen-trations of a pH indicator dye added to the aqueous electrolyte enables ratiometric fluorescence sensing for quantitative pH detection over the range pH 5.3-7.5 and minimally perturbs the local environment. To overcome diffusion limitations, a miniature flow cell was utilized in the microscope to achieve micron-scale resolution in steady state while applying galvanostatic current. We demonstrated the technique using F:SnO 2 -coated glass with varied metal catalyst patterns and compositions resulting in clear images of increased pH near areas of high water-reducing activity. Simulations of the pH profiles near the electrolyte-patterned catalyst interface were also performed using the COMSOL finite-element software package to solve the convection/diffusion equations, and the calculation results agreed well with the experimentally-observed fluorescence profiles. Flow cell fluoresecence microscopy shows promise in imaging comparative catalyst activity as well as three-dimensional product/reactant profiles in complex electrode architectures. There is a critical worldwide need for renewable energy sources, but many promising options such as solar or wind energy are intermit-tent by nature, with limited ability to produce dispatchable power on demand, thus limiting integration of renewables into the power grid. Storing energy in chemical bonds by using renewably-generated elec-tricity to split water could provide a clean and scalable fuel in hydrogen gas or in liquid hydrocarbons after further processing. 1 While current water electrolyzers can provide high efficiencies, they often use ex-pensive catalysts and corrosive electrolytes, 2 motivating research on electrocatalysts that can operate with less overpotential and in neu-tral aqueous electrolyte. However, traditional catalyst characterization techniques rely on simple current-voltage curves which average the reaction rate over all exposed surface features of the electrode. In contrast, imaging the spatially-resolved reactant or product concen-tration near the electrode reveals specific areas of high activity and reaction rate resulting from e.g., use of heterogeneous catalysts or high surface area electrode topology. While the evolved gas bubble growth rate can provide a good measure of the local reaction rate, 3