Sn Catalysts with Build-in [NCN]^2- as Proton Relay for Industrial-Grade CO₂ Reduction at Low Overpotential

Electrochemical CO₂ reduction (ECO₂R) involves multiproton electron transfer, characterized by sluggish reaction kinetics and challenges including high overpotential and subsequently poor stability. Facilitating proton transfer through rational catalyst design to reduce the applied potential enables effective and durable CO₂ reduction. Herein, a proton migration promotion strategy to accelerate the CO₂ protonation and the overall ECO₂R process was explored by Sn-based cyanamide catalysts. Catalysts with varying [NCN]^2- content, from SnO to Sn₂ONCN and SnNCN, were investigated to elucidate the proton transfer mechanism of build-in [NCN]^2-. The reversible structural transformation between [N═C═N]^2- and [N≡C─N]^2- from [NCN]^2- group achieves an efficient proton relay effect. More efficient CO₂ conversion to HCOOH is achieved with a higher content of build-in [NCN]^2- groups in structures. Among these catalysts, SnNCN showed a current density of 450 mA cm^-2 at a low overpotential of 360 mV, with a maximum Faradaic efficiency (FE) of 97.4% for HCOO^- production and a peak cathodic energy conversion efficiency (ECE) of 79.6%. Furthermore, it produced formate (HCOO^-) continuously for over 110 h at 200 mA cm^-2 and pure formic acid (HCOOH) solution for 192 h at 135.8 mA cm^-2. This study provides a novel material system and a fresh perspective for the design of efficient ECO₂R catalysts.