Efficient solar-driven electrocatalytic CO2 reduction in a redox-medium-assisted system

Abstract

Solar-driven electrochemical carbon dioxide (CO₂) reduction is capable of producing value-added chemicals and represents a potential route to alleviate carbon footprint in the global environment. However, the ever-changing sunlight illumination presents a substantial impediment of maintaining high electrocatalytic efficiency and stability for practical applications. Inspired by green plant photosynthesis with separate light reaction and (dark) carbon fixation steps, herein, we developed a redox-medium-assisted system that proceeds water oxidation with a nickel-iron hydroxide electrode under light illumination and stores the reduction energy using a zinc/zincate redox, which can be controllably released to spontaneously reduce CO₂ into carbon monoxide (CO) with a gold nanocatalyst in dark condition. This redox-medium-assisted system enables a record-high solar-to-CO photoconversion efficiency of 15.6% under 1-sun intensity, and an outstanding electric energy efficiency of 63%. Furthermore, it allows a unique tuning capability of the solar-to-CO efficiency and selectivity by the current density applied during the carbon fixation.

Fig. 1

Schematic illustration of the redox-medium-assisted CO₂ electroreduction system consisting of both light reaction and (dark) carbon fixation. a Reaction pathway of natural photosynthesis. b Energy diagram of each part in the redox-medium-assisted system

Fig. 2

CO₂ electroreduction on nano-Au catalysts. a SEM image of nano-Au catalysts on carbon papers. b HRTEM image and c SAED pattern of a representative nano-Au at the zone axis of <002 > direction. d Linear sweep voltammetry curve of nano-Au electrocatalysts in CO₂-saturated 0.5 M KHCO₃ solution at a sweep rate of 5 mV s⁻¹. e Faradaic efficiency for CO (red bars) and H₂ (blue bars) production on nano-Au at various potentials ranging from –0.27 to –0.77 V vs. RHE. f The current-voltage (j–V) curve of CO production partial current density versus potential on nano-Au and kinetics analysis of the CO₂ electroreduction on nano-Au. g Stability of CO₂ electroreduction activity of nano-Au at –0.47 V vs. RHE, including the total current density (left y-axis) and FE_CO (right y-axis). The scale bars are 10 μm in a, 2 nm in b and 5 1/nm in c

Fig. 3

Oxygen evolution and CO₂ reduction. a The j–V curve of electrochemical oxygen evolving in the two-electrode Zn//NiFe hydroxide system from linear sweep voltammetry. The scan rate was 5 mV∙s⁻¹. b, c Photographs of hydrogen evolution on a Zn plate at a voltage of 2.0 and 2.1 V, respectively. d The chronovoltammetry profile, FE_CO and concentration of CO production in the two-electrode Zn//nano-Au system at a current density of 5 mA∙cm⁻². e FE_CO (red bars) and concentration of CO production (blue bars) at different current densities of 2, 5 and 8 mA∙cm⁻²

Fig. 4

The redox-medium assisted CO₂ electroreduction system. a Schematic and b photograph of the redox-medium-assisted system. c Photovoltaic and electrocatalytic current-voltage curves of the GaAs solar cell (blue curve) and two-electrode O₂ evolution (red curve). The red dot indicates the maximum power conversion efficiency (PCE) point of the designed artificial light reaction, based on photoelectrochemical O₂ evolution and Zn deposition. d Voltage-versus-time and FE_CO profiles of the overall photosynthesis under chopped simulated AM 1.5 G illumination. The current density for CO₂ reduction was 5 mA∙cm⁻². e The voltage-versus-time profile, FE_CO, and solar-to-CO PCE of the redox-medium-assisted system at a current density of 10 mA∙cm⁻²