In situ Electrochemical Characterization Techniques for Active Hydrogen in Electrocatalytic Nitrate Reduction to Ammonia

Electrocatalytic nitrate reduction to ammonia (NitRR) represents a sustainable pathway that integrates environmental remediation with the production of high-value ammonia. However, the activity and selectivity of this process are limited by the generation, consumption, and dynamic equilibrium of the key transient intermediate—active hydrogen (*H). Precise modulation of *H necessitates a comprehensive understanding of its behavior, which relies fundamentally on advanced in situ electrochemical characterization techniques. However, a systematic review dedicated to the rational selection and application of in situ electrochemical techniques for the specific characterization of *H in NitRR, with clear definitions of each technique's "detectable targets, quantitative accuracy, and application boundaries" remains absent. This review aims to fill this methodological gap by systematically summarizing in situ electrochemical techniques for probing *H behavior in NitRR, including in situ spectroscopic techniques, in situ electrochemical methods, and scanning electrochemical microscopy. We construct a problem-oriented hierarchical methodological framework tailored for *H characterization in NitRR. By detailing the principles, representative applications, and limitations of these techniques, this review provides a reliable guide for selecting appropriate characterization methods in future *H research, thereby deepening the mechanistic understanding of NitRR. Additionally, future research directions and development trends are discussed, offering insights to advance efficient and selective green ammonia synthesis techologies.