OperandoMethods in Electrocatalysis

Electrocatalysis has been the cornerstone for enhancing energy efficiency, minimizing environmental impacts and carbon emissions, and enabling a more sustainable way of meeting global energy needs. Elucidating the structure and reaction mechanisms of electrocatalysts at electrode–electrolyte interfaces is fundamental for advancing renewable energy technologies, including fuel cells, water electrolyzers, CO2 reduction, and batteries, among others. One of the fundamental challenges in electrocatalysis is understanding how to activate and sustain electrocatalytic activity, under operating conditions, for extended time periods and with optimal activity and selectivity. Although traditional ex situ methods have provided a baseline understanding of heterogeneous (electro)catalysts, they cannot provide real-time interfacial structural and compositional changes under reaction conditions, which calls for the use of in situ/operando methods. Herein, we provide a selective review of in situ and operando characterizations, in particular, the use of operando synchrotron-based X-ray techniques and in situ atomic-scale scanning transmission electron microscopy (STEM) in liquid/gas phases to advance our understanding of electrode–electrolyte interfaces at macro- and microscopic levels, which dictate the charge transfer kinetics and overall reaction mechanisms. The use of scanning electrochemical microscopy (SECM) enables direct probing of the local activity of electrocatalysts at the nanometer scale. In addition, differential electrochemical mass spectrometry (DEMS) and the electrochemical quartz crystal balance (EQCM) enable the simultaneous identification of multiple reaction intermediates and products for mechanistic studies of electrocatalyst selectivity and durability. We anticipate that continuous advances of in situ/operando techniques and probes will continue to make significant contributions to establishing structure/composition-reactivity correlations of electrocatalysts at unprecedented atomic-scale and molecular levels under realistic, real-time reaction conditions.