Solar-Driven Reduction of 1 atm CO ₂ to Formate at 10% Energy-Conversion Efficiency by Use of a TiO ₂ -Protected III-V Tandem Photoanode in Conjunction with a Bipolar Membrane and a Pd/C Cathode Electrocatalyst

Abstract

A solar-driven CO2-reduction (CO2R) cell, consisting of a tandem GaAs/InGaP/TiO2/Ni photoanode in 1.0 M KOH(aq) (pH=13.7) to facilitate the oxygen-evolution reaction (OER), a Pd/C nanoparticle-coated Ti mesh cathode in 2.8 M KHCO3(aq) (pH=8.0) to perform the CO2R reaction, and a bipolar membrane to allow for steady-state operation of the catholyte and anolyte at different bulk pH values, was constructed. At the operational current density of 8.5 mA cm -2 , in 2.8 M KHCO3(aq), the cathode exhibited <100 mV overpotential and >94% Faradaic efficiency for the reduction of 1 atm of CO2(g) to formate. The anode exhibited 320 ± 7 mV overpotential for the OER in 1.0 M KOH(aq), and the bipolar membrane exhibited ~480 mV voltage loss with minimal product crossover as well as >90% and >95% selectivity for protons and hydroxide ions, respectively. The solardriven CO2R cell converted sunlight to fuels at an energyconversion efficiency of ~10%.

Production of Liquid Solar Fuels and Their Use in Fuel Cells

“…147 A high solar-to-fuel energy conversion efficiency of 10% for CO 2 reduction to formate was achieved by using a solar-driven photovoltaic-assisted electrosynthetic cell composed of a tandem GaAs/InGaP/TiO 2 /NiO x photoanode in 1.0 M KOH(aq), a Pd/C nanoparticle-coated Ti mesh cathode in 2.8 M KHCO 3 (aq), and a bipolar membrane (BPM) with a purified feed stream of 1 atm of CO 2 (g). 148 The BPM effectively coupled two electrolytes for CO 2 reduction and water oxidation, resulting in lower total overpotentials and higher efficiency than those obtained in a single-electrolyte CO 2 reduction cell. 148 As far as the solar-to-energy conversion efficiency for CO 2 reduction to formate is concerned, the 10% value seems to be quite promising.…”

Performance Limits of Photoelectrochemical CO2 Reduction Based on Known Electrocatalysts and the Case for Two-Electron Reduction Products

“…This drop in photocurrent is compensated by the increasing energy content (per electron) of the formate or CO products so the overall result is a similar STF efficiency. In fact, if a CO 2 reduction catalyst existed, which could make either CO or formate at overpotentials similar to HER on Pt (in 2015 such a catalyst was reported for formate 41 and very recently it showed great performance in PEC application 43 ), CO 2 reduction could reach higher STF efficiency than water splitting. This is illustrated in Figure 3, which shows the ideal performance under conditions of zero electrochemical losses, but otherwise standard conditions (Table 1).…”

“…This effect underscores the disadvantage of moderate pH electrolytes and the need for clever device designs, e.g. using bipolar membranes 44,45 to enable use of high ionic strength electrolytes 43 and minimize ionic resistance.…”

Pushing the activity of CO2 electroreduction by system engineering

“…5c). Xiang and co-workers [74] developed an artificial photosynthesis system to convert CO 2 into formate, with a 10% solar-to-formate energy efficiency. A tandem GaAs/InGaP/TiO 2 /Ni photoanode was used for light absorption and OER, while the CO 2 RR was catalyzed by a Pd/C-coated Ti mesh cathode ( Fig.…”