Development of a New Rapid Screening Method for High Throughput Electrochemical Characterization of NiMo Hydrogen Evolution Catalysts

Abstract

In order to effectively study new electrocatalysts for hydrogen evolution (using water as a renewable and clean feedstock) new advancements must be made with the analytic techniques used to characterize these electrocatalysts. In this thesis, two new methods are presented for measuring heterogeneous electrocatalyst electron transfer kinetics. The first of these methods is intended to advance the capabilities of high throughput rapid screening methods for electrocatalysts. By using piezoelectric printing, precursors for metal and metal oxide catalysts are printed onto a substrate, yielding arrays of 64 catalyst samples to be analyzed by linear sweep voltammetry (LSV). Combining piezoelectric printing technologies with automated process controls (used to control the electrochemical reaction systems) and automated high throughput COMSOL Multiphysics simulations allows for the number of catalyst samples that can be measured in one set are increased exponentially compared to other methods capable of returning kinetic parameter data (k0eff) without sample crosstalk. To verify the capabilities of this method, 2% incremental composition NiMo bimetallic hydrogen evolution catalyst arrays (from 100% Ni to 100% Mo) were studied in both acidic and neutral pH electrolyte solutions, yielding catalytic reactivity maps of the composition arrays. For the acidic reaction conditions, peak activity of the NiMo catalyst occurred in the composition range of 78-90% Ni, whereas for the neutral reaction conditions, peak activity regions occurred at 12-18% Ni and 46-62% Ni. The second method of measuring kinetics of electron transfer reactions is an improved controls system for Scanning Electrochemical Microscopy (SECM) motion control. To improve the efficiency of the SECM approach for electrodes, a new variable approach speed technique was developed to employ custom fuzzy logic control algorithms for automatically adjusting the speed of approach for the electrode based on tip size, detection method, enhancement factor, and distance of the electrode tip from the set-point. This algorithm is able to automatically switch from a coarse to a fine motion controller when the tip electrode is sufficiently close to the substrate. We validated that approach curves obtained using the fuzzy logic algorithm matched well with simulated approach curves for both large microelectrodes (d = 175 μm) and for conventional SECM ultramicroelectrodes (d = 6.2 μm and d = 4.2 μm). Using positive feedback approach curves, we obtained a tip/substrate gap distance of 6.5 μm for the 175 μm electrode, 800 nm for the 6.2 μm electrode, and 580 nm for the 4.2 μm electrode. The gap distances were obtained using the fuzzy logic control algorithm in roughly one-third of the time compared to the conventional constant-speed approach method. Also, the gap distances were closer using the fuzzy logic algorithm, with the 4.2 μm electrode gap being 120 nm smaller than the constant-speed approach method. In addition, at the 580 nm gap distance obtained with the fuzzy logic algorithm, linear sweep voltammetry was performed allowing us to quantify the kinetic rate constant (k0) for the oxidation reaction of ferrocenemethanol to be 0.20 ± 0.05 cm s-1.