Thermodynamic phase control of Cu-Sn alloy electrocatalysts for selective CO2 reduction.

In the electrochemical CO2 reduction reaction (CO2RR), Cu alloy electrocatalysts can control the CO2RR selectivity by modulating the intermediate binding energy. Here, we report the thermodynamic-based Cu-Sn bimetallic phase control in heterogeneous catalysts for selective CO2 conversion. Starting from the thermodynamic understanding about Cu-Sn bimetallic compounds, we established the specific processing window for Cu-Sn bimetallic phase control. To modulate the Cu-Sn bimetallic phases, we controlled the oxygen partial pressure (pO2) during the calcination of electrospun Cu and Sn ions-incorporated nanofibers (NFs). This resulted in the formation of CuO-SnO2 NFs (full oxidation), Cu-SnO2 NFs (selective reduction), Cu3Sn/CNFs, Cu41Sn11/CNFs, and Cu6Sn5/CNFs (full reduction). In the CO2RR, CuO-SnO2 NFs exhibited formate (HCOO-) production and Cu-SnO2 NFs showed carbon monoxide (CO) production with the faradaic efficiency (FE) of 65.3% at -0.99 V (vs. RHE) and 59.1% at -0.89 V (vs. RHE) respectively. Cu-rich Cu41Sn11/CNFs and Cu3Sn/CNFs enhanced the methane (CH4) production with the FE of 39.1% at -1.36 V (vs. RHE) and 34.7% at -1.50 V (vs. RHE). However, Sn-rich Cu6Sn5/CNFs produced HCOO- with the FE of 58.6% at -2.31 V (vs. RHE). This study suggests the methodology for bimetallic catalyst design and steering the CO2RR pathway by controlling the active sites of Cu-Sn alloys.

Vitamin C-induced CO2 capture enables high-rate ethylene production in CO2 electroreduction.

High-rate production of multicarbon chemicals via the electrochemical CO2 reduction can be achieved by efficient CO2 mass transport. A key challenge for C-C coupling in high-current-density CO2 reduction is how to promote *CO formation and dimerization. Here, we report molecularly enhanced CO2-to-*CO conversion and *CO dimerization for high-rate ethylene production. Nanoconfinement of ascorbic acid by graphene quantum dots enables immobilization and redox reversibility of ascorbic acid in heterogeneous electrocatalysts. Cu nanowire with ascorbic acid nanoconfined by graphene quantum dots (cAA-CuNW) demonstrates high-rate ethylene production with a Faradaic efficiency of 60.7% and a partial current density of 539 mA/cm2, a 2.9-fold improvement over that of pristine CuNW. Furthermore, under low CO2 ratio of 33%, cAA-CuNW still exhibits efficient ethylene production with a Faradaic efficiency of 41.8%. We find that cAA-CuNW increases *CO coverage and optimizes the *CO binding mode ensemble between atop and bridge for efficient C-C coupling. A mechanistic study reveals that ascorbic acid can facilitate *CO formation and dimerization by favorable electron and proton transfer with strong hydrogen bonding.