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 technologies.

The electrocatalytic nitrate reduction reaction (NitRR) represents a promising approach toward achieving economically and environmentally sustainable ammonia. However, it remains a challenge to regulate the size effect of electrocatalysts to optimize the catalytic activity and ammonia selectivity. Herein, the Cu-based catalysts were tailored at the atomic level to exhibit a size gradient ranging from single-atom catalysts (SACs, 0.15–0.35 nm) to single-cluster catalysts (SCCs, 1.0–2.8 nm) and nanoparticles (NPs, 20–30 nm), with the aim of studying the size effect for the NO₃⁻-to-NH₃ reduction reaction. Especially, the Cu SCCs exhibit enhanced metal–substrate and metal–metal interactions by taking advantageous features of Cu SACs and Cu NPs. Thus, Cu SCCs achieve exceptional electrocatalytic performance for the NitRR with a maximum Faradaic efficiency of ca. 96% and the largest yield rate of ca. 1.99 mg ${}_{{\mathrm{N}\mathrm{H}}_3}$·h⁻¹·cm⁻² at −0.5 V vs. reversible hydrogen electrode (RHE). The theoretical calculation further reveals the size effect and coordination environment on the high catalytic activity and selectivity for the NitRR. This work provides a promising various size-controlled design strategy for aerogel-based catalysts effectively applied in various electrocatalytic reactions.