Solar-Driven Reduction of 1 atm of CO₂ to Formate at 10% Energy-Conversion Efficiency by Use of a TiO₂-Protected III–V Tandem Photoanode in Conjunction with a Bipolar Membrane and a Pd/C Cathode

Abstract

A solar-driven CO2 reduction (CO2R) cell was constructed, 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. 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 a 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 crossovers and >90 and >95% selectivity for protons and hydroxide ions, respectively. The bipolar membrane facilitated coupling between two electrodes and electrolytes, one for the CO2R reaction and one for the OER, that typically operate at mutually different pH values and produced a lower total cell overvoltage than known single-electrolyte CO2R systems while exhibiting ∼10% solar-to-fuels energy-conversion efficiency.

Recent Progress on Photo‐Electrocatalytic Reduction of Carbon Dioxide

“…TiO 2 can also function as a protective layer for other photoanodes. For example, a novel solar‐driven CO 2 reduction cell consisting of a tandem GaAs/InGaP/TiO 2 /Ni photoanode in 1.0 m aqueous KOH for oxygen‐evolution reaction and a Pd/C nanoparticle‐coated Ti mesh cathode in 2.8 m aqueous KHCO 3 for CO 2 reduction reaction separated by a bipolar membrane was developed . The overpotentials of the cathode and anode are 100 and 320 mV, respectively.…”

Semiconductor‐Based Photoelectrochemical Conversion of Carbon Dioxide: Stepping Towards Artificial Photosynthesis

“…Recently, instead of using a Nafion membrane, a bipolar membrane (BPM) between the cathode and anode chambers has been used to sustain steady pH values in each compartment (Figure ); this allows for the individual optimization of each electrolyte at a suitable pH and a combined cell overvoltage . BMP is composed of an AEL and CEL that allow protons to transfer to the cathode and hydroxide ions to be transported to the anode; this balances consumed protons and hydroxide ions, respectively, in the electrode reaction.…”

“…Wired to a tandem GaAs/InGaP/TiO 2 /NiO x photoanode in 1.0 m KOH (aq) (pH 13.7) and a Pd/C nanoparticle coated Ti mesh cathode in 2.8 m KHCO 3(aq) (pH 8.0), the cathode exhibited an overpotential less than 100 mV and FE>94 % for CO 2 reduction to formate, and the anode exhibited an overpotential of (320±7) mV for the OER. The total solar‐to‐fuel energy conversion efficiency reached up to 10 % . Taking into consideration visible‐light absorption, metal oxynitrides are more suitable because many of them have an absorption edge at λ >600 nm.…”

Photocatalytic fuel production

“…Reducing CO 2 by H 2 O in efficient photovoltaic-assisted systems accompanying chemical bias [15] . The photoanode is a tandem-junction GaAs/InGaP two-photon absorber coated by an amorphous TiO 2 -Ni nanolayer for water oxidation catalysis, as previously demonstrated for water splitting systems [16] .…”

“…Carbon dioxide is reduced at a Pd/C-coated Ti mesh cathode in close to neutral solution (2.8 M HCO 3 − , pH = 8.0) exposed to 1 atm of CO 2 gas. The bipolar membrane [17][18][19][20] allows to maintain the large pH difference of the cell under continuous operation at a light-limited photocurrent density of 8.5 mA cm −2 and > 94% Faradaic efficiency for CO 2 reduction to formate [15] . Modest voltage loss of 480 mV at the bipolar membrane and low overvoltages for CO 2 reduction ( < 100 mV) and O 2 evolution (320 mV) enable a remarkable 10% energy conversion efficiency.…”