Integrating hydrogen utilization in CO₂ electrolysis with reduced energy loss

Electrocatalytic CO₂ reduction reaction (CO₂RR) offers a promising pathway for sustainably producing fuels and chemicals. Despite significant progress in materials development and technological advancements enabling high-rate and selective product formation, high energy input and low efficiency remain major hurdles. The anode's oxygen evolution reaction (OER) is both energetically demanding and kinetically slow, producing low-value diatomic oxygen. Furthermore, oxygen production often leads to significant carbon loss, further diminishing energy efficiency. Carbonate or bicarbonate ions formed at the cathode can migrate to the anode, where lower pH causes protonation and CO₂ release alongside O₂. In AEM-based CO₂ electrolyzers, carbon loss from CO₂ crossover can reach ~70%, with CO₂ recovery energy penalties significantly exceeding the electrolysis energy. "Paired electrolysis," coupling CO₂RR with a more favorable half-reaction, offers a potential solution. While electro-oxidation of various organics has been explored, this strategy faces limitations due to market size mismatches between cathode and anode chemicals and challenges in product separation and purification. Pairing CO₂RR with hydrogen oxidation (HOR) is theoretically advantageous, addressing the aforementioned challenges while reducing energy input. Green hydrogen generation via water electrolysis involves OER under more favorable thermodynamic and kinetic conditions (neutral or weaker alkaline electrolytes, lower temperatures) than in CO₂RR reactors. The anodic overpotential in advanced water electrolysis cells is significantly lower than in CO₂RR reactors. This study proposes "transferring" the OER from CO₂RR to a water electrolyzer to enhance energy efficiency.

The authors review existing literature on CO2RR, highlighting the challenges of high energy consumption and low efficiency due to the oxygen evolution reaction (OER) at the anode. They discuss the concept of paired electrolysis as a solution, citing examples of coupling CO2RR with the oxidation of various organic compounds. However, they point out the limitations of this approach due to market size mismatches between anode and cathode products and the difficulty of product separation. The review then focuses on the potential of using hydrogen oxidation reaction (HOR) paired with CO2RR, emphasizing the thermodynamic and kinetic advantages of performing OER in a separate water electrolyzer. Previous works on paired electrolysis, electro-oxidation of organic compounds and hydrogen generation via water electrolysis are referenced to support the proposed approach.

The researchers designed a single electrochemical cell directly coupling CO₂ electrolysis with HOR at the anode. CO2-to-CO and CO2-to-formate conversions served as model reactions representing gaseous and soluble product formation. A flow cell with a Ni(OH)₂/NiOOH mediator was implemented to prevent carbon loss and HOR catalyst poisoning by migrated CO₂RR products. A gas-diffusion electrode (GDE) with a gradient functional layer was developed to minimize cathodic overpotential loss. For CO₂-to-CO, Zn nanosheets electrodeposited on Cu foam were used as the catalyst. The electrodeposition time was varied to optimize catalyst performance. Materials characterization techniques including XRD, SEM, BET, and electrochemical methods (LSV, EIS) were employed to analyze catalyst structure and activity. CO₂RR product analysis was conducted via gas chromatography and NMR. For CO₂-to-formate, a Bi₂O₃ porous nanosphere catalyst was synthesized using a hydrothermal method with a carbon template. Characterization methods were similar to those used for the CO₂-to-CO catalyst. The reaction kinetics of the Ni(OH)₂/NiOOH mediator were examined using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), comparing its performance to OER catalysts in weaker alkaline electrolytes. The stability of the mediator was assessed using chronopotentiometry. A gradient functional layer was developed for the Zn-based GDE to improve mass transport and enhance CO2RR performance. The full cell performance was evaluated by measuring Faradaic efficiency (FE), cell voltage, and overpotential at various current densities. Online gas chromatography (GC) and differential electrochemical mass spectrometry (DEMS) were employed to monitor product formation and to confirm the absence of oxygen evolution at the anode during NIOR. For HOR, a Pt/C GDE was used. The cell's long-term stability was tested for over 100 hours. Finally, the energy efficiency of the integrated system was compared with conventional CO₂RR, considering both cell voltage and energy consumption per tonne of product, considering also upstream hydrogen generation via alkaline water electrolysis (AWE) and solid oxide electrolysis cells (SOEC). Techno-economic analysis was performed on a pilot plant producing 100 tonnes of CO per day.

The study demonstrated that integrating hydrogen oxidation at the anode significantly enhances CO2 electrolysis efficiency. * **High selectivity and stability:** The H₂-integrated CO₂RR cell achieved high selectivity for CO (up to 81.9% at 150 mA cm⁻²) and formate (up to 95.3% at 150 mA cm⁻²) with excellent long-term stability (>100 h). The Zn-Cu-500 catalyst showed the highest FE toward CO formation (85.8% at -1.0 V vs RHE) in the H-cell configuration, and a Bi2O3 catalyst showed high formate selectivity. * **Reduced energy consumption:** The cell voltage was significantly lower (0.15-0.23 V) compared to conventional CO₂RR at the same current densities, attributable to the replacement of OER with the kinetically faster NIOR. Including hydrogen generation, total energy consumption was reduced by up to 42% in comparison to conventional CO2RR, even when considering the energy needed for anodic CO2 recovery. The integration with a SOEC further decreased energy costs. The energy savings were significant, ranging from 23% to 42%, across different operating conditions. * **Suppression of carbon loss and catalyst poisoning:** The Ni(OH)₂/NiOOH mediator effectively prevented carbon loss and protected the HOR catalyst from poisoning by migrated CO₂RR products; no oxygen evolution was observed at the anode during NIOR, based on GC and DEMS measurements. * **Flexible cell design:** The cell design was flexible, accommodating both membrane-free and membrane-based configurations. * **Techno-economic analysis:** A preliminary techno-economic analysis suggested that while initial capital expenditure is higher due to the use of a mediator and advanced water electrolyzer, the increased versatility and energy savings could compensate for this, particularly when the plant operates as a battery energy storage system and can produce hydrogen.

The findings directly address the research question of improving the energy efficiency of CO₂RR. The successful integration of hydrogen oxidation, facilitated by the Ni(OH)₂/NiOOH mediator, significantly reduced the overall energy consumption of the process. The shift of the OER to a separate water electrolyzer under more favorable conditions was key to achieving this improvement. The high selectivity and stability observed for both CO and formate production demonstrate the practical viability of the proposed approach. The results highlight the synergistic potential of combining CO₂RR with established energy storage technologies, paving the way towards more sustainable and cost-effective chemical production. The study suggests that a system without the redox mediator would offer further energy advantages, calling for future research in this area to find robust solutions to address CO2 crossover and to develop poisoning-resistant HOR catalysts.

This research successfully integrated CO₂RR with hydrogen oxidation in a single electrochemical cell, significantly reducing the operating voltage and energy consumption. The use of a Ni(OH)₂/NiOOH mediator proved crucial in mitigating catalyst poisoning and preventing carbon loss. The integration with existing water electrolysis technologies further improved energy efficiency. This work demonstrates a promising pathway for integrating various energy conversion and storage approaches to enhance the sustainability and economic feasibility of CO₂RR, while paving the way for future studies that eliminate the need for a redox mediator and develop robust HOR catalysts.

While the study demonstrates significant improvements in energy efficiency, limitations remain. The study primarily focused on CO and formate production. Further research is needed to evaluate the system's performance for other multicarbon products. The techno-economic analysis is preliminary and needs further refinement to fully account for all factors involved in scaling-up the technology. Future research could explore more efficient and cost-effective HOR catalysts to reduce overpotential loss. The long-term stability needs to be evaluated under even more demanding real-world conditions.