The surface gradient doping of Sn enables CuO high selectivity for electro-reducing CO2 to HCOOH

As an effective strategy to turn waste into treasure, electrochemical CO 2 reduction reaction (eCO 2 RR) producing value-added chemicals such as HCOOH attracts unremitting interest. However, the practical applications are limited by poor activity and selectivity, the key is to develop economical-efficient catalysts. Herein, gradient-doped Sn on the CuO surface (Sn-CuO) was developed for eCO 2 RR in an H-cell. Typically, the main reaction pathway of *OCHO → HCOOH was obtained via in situ experiments and further verified by theoretical calculations. Furthermore, the lowest Δ G *OCHO and Δ G *OCHO − Δ G *H as well as the highest Δ G *COOH enlist Sn-CuO with sufficient proton to produce HCOOH while inhibiting CO and H 2 by-products. Finally, the synthesized Sn-CuO delivers a maximum partial current density (23.6 mA cm −2 ) at −1.3 V REH and Faradic efficiency (91.5 %) at −1.2 V RHE for HCOOH, indicating excellent activity and selectivity.