Photo‐Electrochemical Conversion of CO₂ Under Concentrated Sunlight Enables Combination of High Reaction Rate and Efficiency

Abstract

Photo‐electrochemical production of solar fuels from carbon dioxide, water, and sunlight is an appealing approach. Nevertheless, it remains challenging to scale despite encouraging demonstrations at low power input. Higher current densities require notable voltage input as ohmic losses and activation overpotentials become more significant, resulting in lower solar‐to‐CO conversion efficiencies. A concentrated photovoltaic cell is integrated into a custom‐made heat managed photo‐electrochemical device. The heat is transferred from the photovoltaic module to the zero‐gap electrolyzer cell by the stream of anodic reactant and produce synergetic effects on both sides. With solar concentrations up to 450 suns (i.e., 450 kW m−2) applied for the first time to photo‐electrochemical reduction of CO2, a partial current for CO production of 4 A is achieved. At optimal conditions, the solar‐to‐CO conversion efficiency reaches 17% while maintaining a current density of 150 mA cm−2 in the electrolyzer and a CO selectivity above 90%, representing an overall 19% solar‐to‐fuel conversion efficiency. This study represents a first demonstration of photo‐electrochemical CO2 reduction under highly concentrated light, paving the way for resource efficient solar fuel production at high power input.

Photovoltaic-Powered Electrochemical CO2 Reduction: Benchmarking against the Theoretical Limit

“…Another approach being explored is the individual CO 2 reduction to CO and water splitting to produce hydrogen, using a downstream Fischer–Tropsch process to make the desired hydrocarbons. , Since solar CO is the intermediate of these two approaches, photovoltaic-powered electrochemical (PEC ,, ) CO 2 reduction to CO is the central technology for both of them. Despite efforts toward the development of efficient and selective catalysts, − the most recently reported solar-to-chemical (STC) energy conversion efficiency was still below 20% . We believe it is the right time to address the fundamental issue, i.e., how close PEC CO 2 reduction can approach the theoretical limit.…”

“…For PEC CO 2 reduction, both dual-junction and triple-junction photoabsorbers are considered. Most of the relevant devices were driven by triple-junction photoabsorbers (or solar cells), ,,,− ,, because E rx,CO > 2.2 V is usually needed for selective CO 2 reduction. Figure a shows the efficiency limit variations with E rx,CO .…”

Recent advancement in heterogeneous CO2 reduction processes in aqueous electrolyte

“…However, with increased catalyst loading >3.33 mg cm −2 , an astonishingly high current density of 1.32 A cm −2 was observed with a C 2+ partial current density of 1.21 A cm −2 along with a significant cathodic energy efficiency (EE) of nearly 45%. In 2022, Boutin et al 179 made a GDE by spray coating Ag NPs and sustainion solution on carbon paper with IrO x as an anode. The zero-gap GDE flow cell (basically an MEA) is operated with ultrapure water as anolyte.…”

“…They also tested it in outdoor sunlight (<100 mW cm −2 ) conditions and got a result close to the laboratory experiment, with ∼18.7% of STF efficiency at 2.2 V cell voltage. In 2022, Boutin et al 179 utilized a 0.92 cm 2 GaInP/GaInAs/Ge triple-junction concentrated PV cell (CPV) which could work up to 450 sun. They also found that the open circuit voltage increased from 2.33 V to 2.58 V when the light intensity increased from 132 sun to 450 sun.…”

“…There is only one way of increasing the STF efficiency for a selective electrode: increasing the solar current density. However, using concentrated light for PV cells can be a solution for industrial viability, as shown by Boutin et al 179 Many solar cells with low open circuit voltages can be used in series to obtain the required cell potentials. In that case, current density decreases, consequently reducing the STF efficiency.…”