Improvements in Long-Term Durability of CO2 Electrolyzer Producing Formate Designed for Integration with Solar Cells for Artificial Photosynthesis
Authors: Kato Naohiko , Kawai Yasuaki , Nojiri Natsumi , Shiozawa Masahito , Kikuzawa Yoshihiro , Suzuki Nobuaki , Kosaka Satoru , Hamaguchi Tsuyoshi , Takeda Yasuhiko 
Published: August 2024

Artificial photosynthetic cells have been extensively studied for carbon neutrality, yet in lab-scale. We have previously constructed practically large-sized cells consisting of CO2 electrolyzers and crystalline silicon solar cells.1,2 Our CO2 electrolyzers adopt a facing cathode-anode configuration, using a Ru complex polymer (RuCP) catalyst realizing a low overpotential for CO2 reduction and an IrO
x
-based catalyst with a high catalytic activity for water oxidation.3 1000 cm2-sized cells achieved a solar-to-chemical energy-conversion efficiency (η
STC) of 7.2%,1 and a further scaled-up 1 m2-cell demonstrated a higher η
STC of 10.5%, yielding a large formate production rate of 1.2 mol/h.2

In the present study, we improved the long-term durability of the CO2 electrolyzer. We prepared the 75 cm2-sized CO2 electrolyzer using the improved cathode and the anode, and achieved high formate Faradaic efficiencies (FE) exceeding 80% and electricity-to-chemical energy-conversion efficiencies (η
ETC) of around 60% even after 3000 h operation under practical conditions, as shown in Fig. 1.

In the previous large-sized cells, the operating current gradually decreased during continuous operation within dozen of hours. We investigated the causes of the degradation and identified three key factors. Two of these factors were associated with the cathode electrodes: detachment of the RuCP catalyst from the carbon supports and peeling off of the carbon supports from the conducting Ti substrates. An introduction of a pyrrole derivative containing amino group in the RuCP, coupled with an application of a UV-ozone treatment to create carboxyl groups on the carbon supports, effectively reduced the detachment of the RuCP catalyst by forming strong chemical bonds. A newly-developed chemically-resistant graphite adhesive composed of graphite particles and a polyvinylidene fluoride (PVDF) binder prevented the carbon supports from peeling off from the Ti-plates. The post-loading process of the RuCP, i.e., RuCP-loading on the carbon supports already-bonded on the Ti-plates using the new adhesive eliminated the detrimental impact of the heat treatment for curing the adhesive. The third factor involved detachment of the anode catalyst of IrO
x
particles from the Ti-plates.4 Therefore, we replaced the IrO
x
anodes with highly durable counterparts comprising IrO
x
-TaO
x
/Pt-Metal oxide/Ti-plates.

Another critical issue we addressed was the validity of laboratory-based evaluations for reflecting outdoor operations. In our previous study, we vertically installed the integrated CO2 electrolyzers and solar cells on the ground and evaluated their performance under continuous simulated sunlight (ca. 1 sun).2,3 This vertical setup was chosen to prevent a decrease in η
STC caused by crossover reactions that would occur when the electrolyzers are inclined. Indeed, it has been reported that oxygen reduction reaction (ORR) of the O2 bubbles generated at the anode and transferred to the cathode can be substantial in a single-chamber water-splitting reactor.5 By contrast, although the present CO2 electrolyzer was installed at an inclined angle of 30 °, which is approximately the optimal value for receiving more solar energy, the crossover reactions were well-suppressed because the porous separator film inserted between the facing cathode and anode impeded the transfer of O2 bubbles from the anode to the cathode. However, the use of a separator film raised another issue: the O2 bubbles trapped on the film narrowed the effective film area. Intermittent operation corresponding to changes in solar energy during the day mitigates the detrimental impact of the second issue.

Thus, we established the groundwork for the widespread use of integrated CO2 electrolyzers and solar cells for artificial photosynthesis, demonstrating high durability, exceptional FE, and η
ETC.

References

N. Kato, et al., A large-Sized Cell for Solar-Driven CO2 Conversion with a Solar-to-Formate Conversion Efficiency of 7.2%, Joule, (2021) 5, 687.

N. Kato, et al., Solar Fuel Production from CO2 using 1 m-square-sized Reactor with a Solar-to-Formate Conversion Efficiency of 10.5%, ACS Sustain. Chem. Eng.,(2021) 9, 16031.

T. Arai, S. Sato, T. Morikawa, A Monolithic Device for CO2 Photoreduction to Generate Liquid Organic Substances in a Single-Compartment Reactor, Energy Environ. Sci., (2015) 8, 1998.

M. Shiozawa, et al., Improved Durability of Highly Active IrO
x
Electrodes for Electrocatalytic Oxygen Evolution Reaction, Electrocatalysis, (2022) 13, 830.

K. Obata, et al., Multiphase Fluid Dynamics Simulations of Product Crossover in Solar-driven, Membrane-less Water Splitting, Cell Rep. Phys. Sci, (2021) 2, 100358.

Figure 1