Operando Electrochemical UV-Vis Absorption Spectroscopy with Microsecond Time Resolution

Abstract

Operando spectroscopic characterization is effective for examining electrocatalytic reaction mechanisms. However, most operando characterization techniques currently used are based on (quasi-)steady-state spectroscopy, which often cannot directly measure transient changes occurring on the micro-millisecond time scale. Herein, an operando electrochemical UV-Vis absorption spectroscopy with 3 μs time resolution was realized by introducing bias pulses and synchronizing the bias pulse and spectral signals. Comparing the time-dependence curves of the bias pulse, collected spectral curve, and controlling voltage, a good time consistence for the three signals was observed, demonstrating the time-resolved ability of the novel apparatus. More importantly, two oxidation reactions, water oxidation reaction and hole sacrifice reagent oxidation reaction, showed distinct dynamics, verifying the reliability of the time-resolved kinetics. The water oxidation kinetics on a ferrihydrite (Fh) electrocatalyst were studied by this novel operando spectroscopic system. Different water oxidation steps were decoupled by analyzing the accumulation and decay dynamics of the operando time-resolved UV-Vis absorption data with various pulse widths and magnitudes of applied bias. A long bias pulse with width above 1s enabled the continuous accumulation of reaction intermediates in Fh electrocatalyst to reach a quasi-equilibrium state with electron extraction into the external circuit. In addition, a fast decay for water oxidation was observed after the applied bias was turned off. Importantly, when a short bias pulse with tens of ms width was applied, an abnormal intermediate accumulation process was observed after the applied bias was shut off, revealing a spontaneous species transformation process. These results confirm the validity of this novel method for examining species transformation kinetics at a fast timescale. The formation, transformation, and reaction kinetics of water oxidation reaction intermediates were directly studied on a µs to s time scale. Therefore, operando electrochemical UV-Vis absorption spectroscopy with µs time resolution can promote the understanding of various electrocatalytic reaction mechanisms and be used to guide the design and synthesis of novel high-efficiency electrocatalysts.