Probing Bubble Properties during Hydrogen Evolution Reaction on Platinum Micropatterns Using Scanning Electrochemical Microscopy
Authors: Shao Xiaohan , Zhu Qianhong , Wang Ting , Jarin Mourin , Xie Xing , Tarpeh William Abraham 
Published: February 2024

Many electrochemical reactions involve gas bubble consumption and generation at liquid-solid interfaces, creating a three-phase interfacial zone (e.g., solid electrode, liquid electrolyte, gaseous products). For example, hydrogen and oxygen evolution reactions (HER, OER) generate gas, the nitrate reduction reaction (NO3RR) makes gaseous and aqueous products, and the carbon dioxide reduction reaction (CO2RR) consumes gas while making gaseous and aqueous products. These reactions correlate gas bubbles to important energy and sustainability applications such as aqueous electrolysis, photoelectrochemical energy storage, environmental catalysis, and water treatment. In electrochemical water treatment, gas bubbles have been shown to catalyze chemical reactions and enhance the efficiency of water treatment by degrading organic pollutants and reducing fouling of water treatment components (e.g., membranes). However, gas bubbles grow into macroscopic sizes after nucleating on solid surfaces, which interferes with chemical processes by blocking the reactants from reaching the liquid-solid interface and impedes reaction rate and energy efficiency. The liquid-solid interface is a critical zone that dictates the performance of electrochemical processes. Bubble formation at the liquid-solid interface also presents a major challenge for establishing molecular understanding of reaction mechanisms, kinetics, stability, and selectivity toward desired products, and therefore complicates process designs that leverage gas bubble properties to facilitate electrochemical reactions. Questions such as how surface bubbles form and grow on surfaces, how they impact chemical reactivity at surfaces, and how electrode geometries (e.g., the size, shape, and arrangement of the catalysts for reactions) guide bubble formation and transport should be answered to inform rational design of electrochemical processes to efficiently utilize the bubbles.
In this study, we use HER as a model reaction to probe bubble properties using scanning electrochemical microscopy (SECM) and optical microscopy to gain understandings of the fate and dynamics of bubbles (i.e., nucleation, aggregation, transport, and dissolution). Specifically, we developed and fabricated micro-scale platinum patterned substrates that vary in Pt pattern size (20-200 µm diameter), gap spacing (1-10x diameter apart), and angle (60° and 90°between Pt patterns) to facilitate bubble nucleation. The patterns (120 nm in height) were deposited on n-type silicon wafer (525 µm thickness) using photolithography and electron beam evaporation. We then identified the locations of bubble nucleation on the patterned substrates with optical microscopy. We correlated the locations of bubble formation with the distribution of Pt patterns on the electrode by deconvoluting the measured currents due to bubble aggregation from conductivity of Pt patterns via SECM, a powerful tool to acquire spatial and electrochemical activities for various use cases (e.g., corrosion, catalysis, gas evolution). In addition, we investigated the effects of current densities and reaction time on the nucleation and stability of the bubbles on the fabricated patterned substrates. Furthermore, the lifetime of the patterned substrates was examined, which will provide insights into future engineering design of robust and effective substrates that leverage bubble properties to advance electrochemical water treatment. The results of this study are generalizable to other reactions involving gas bubble generation to enhance optimization and sustainability of other industrial electrochemical processes.