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

Hydrogen generation by water electrolysis is promising for energy storage, and imaging the reaction dynamics near a H2-evolving electrode can provide valuable insights. We utilized laser-scanning confocal fluorescence microscopy to map the product/reactant concentration at a H2-evolving electrode at micron-scale resolution which identifies areas with fast reaction kinetics. Small concentrations 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:SnO2-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 intermittent 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 electricitytosplitwatercouldprovideacleanandscalablefuelinhydrogen gas or in liquid hydrocarbons after further processing. 1 While current water electrolyzers can provide high efficiencies, they often use expensive catalysts and corrosive electrolytes, 2 motivating research on electrocatalysts that can operate with less overpotential and in neutralaqueouselectrolyte.However,traditionalcatalystcharacterization 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 concentration 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 bubbles do not necessarily nucleate near the surface features of interest and can block the flow of reactants. Scanning electrochemical microscopy (SECM) provides submicron resolution and many analytical capabilities, but the presence of the probe significantly perturbs the local environment. 4 Detecting the local pH at a water-splitting cathode indicates the consumed H + and evolved OH − concentrations, indirectly measuring the evolved H2. pH changes near large platinum electrodes have been quantified by amperometric SECM, identifying both steady-state H2 evolution as well as transient adsorption and surface oxidation features. 5 By using the potentiometric mode, threedimensional pH profiles near micron-scale platinum electrodes have also been mapped with SECM with less perturbation 6,7 but require the