Integrating hydrogen utilization in CO2 electrolysis with reduced energy loss

Abstract

Electrochemical carbon dioxide reduction reaction using sustainable energy is a promising approach of synthesizing chemicals and fuels, yet is highly energy intensive. The oxygen evolution reaction is particularly problematic, which is kinetically sluggish and causes anodic carbon loss. In this context, we couple CO₂ electrolysis with hydrogen oxidation reaction in a single electrochemical cell. A Ni(OH)₂/NiOOH mediator is used to fully suppress the anodic carbon loss and hydrogen oxidation catalyst poisoning by migrated reaction products. This cell is highly flexible in producing either gaseous (CO) or soluble (formate) products with high selectivity (up to 95.3%) and stability (>100 h) at voltages below 0.9 V (50 mA cm^-2). Importantly, thanks to the "transferred" oxygen evolution reaction to a water electrolyzer with thermodynamically and kinetically favored reaction conditions, the total polarization loss and energy consumption of our H₂-integrated CO₂ reduction reaction, including those for hydrogen generation, are reduced up to 22% and 42%, respectively. This work demonstrates the opportunity of combining CO₂ electrolysis with the hydrogen economy, paving the way to the possible integration of various emerging energy conversion and storage approaches for improved energy/cost effectiveness.

Fig. 1. H₂-integrated CO₂RR and the cell configuration.

a Illustrative comparison of H₂-integrated and conventional CO₂RR; b Comparison of Nernst potentials (E₀) between conventional and H₂-integrated CO₂RR; c Potential carbon loss and product crossover at the anode side in a typical CO₂ flow electrolyzer; d Cell configuration and detailed working principles of the H₂-integrated CO₂RR cell.

Fig. 2. Characterizations and performance of Zn and Ni(OH)₂/NiOOH electrodes.

a XRD patterns; SEM images of b Cu foam and c Zn-Cu-500, the inset compares the optical images of Cu foam before and after Zn deposition; d Potential-dependent Faradaic efficiency for CO generation (the error bar represents standard deviation from three independent measurements); e Plots of j_CO as a function of potential bias during CO₂RR; f LSV curves of Ni(OH)₂ and Co₃O₄ electrodes (the solution resistance was 1.25 ~ 1.35 Ω); g Tafel plots of NiOR and OER; h Nyquist plots of Ni(OH)₂ electrode acquired at 1.40, 1.50, 1.65, and 1.80 V vs. RHE; i Nyquist plots of Ni(OH)₂/NiOOH electrode acquired at different DoC at 1.55 V vs. RHE, charging is defined as the Ni(OH)₂-to-NiOOH conversion; j Nyquist plots measured at 1.55 V vs. RHE of Ni(OH)₂/NiOOH electrode (DoC = 0% and =100%) and Co₃O₄. All solid black lines in Nyquist plots represent the fitting results based on the equivalent circuit, where R_s is the solution resistance, CPE is the constant phase element, R_ct is the charge-transfer resistance and R_W is the Warburg impedance. Source data are provided as a Source Data file.

Fig. 3. Characterizations of GDE based on the Zn-coated Cu foam.

a The schematic illustration of GDE; cross-sectional SEM images of b merged foam with carbon-PTFE layer after drop-casting the first layer (inset shows the surface image) and c final gradient functional layer; d SEM image of the top layer surface.

Fig. 4. Cell performance in CO₂RR.

Faradaic efficiencies of CO production in a CO₂RR + NiOR and b CO₂RR + OER; c Polarization curve comparison of CO₂RR + NiOR and CO₂RR + OER for CO production; d Anode overpotentials of NiOR and OER during CO₂RR at different current densities; e Faradaic efficiencies of formate production in CO₂RR + NiOR; f Polarization curve comparison of CO₂RR + NiOR and CO₂RR + OER for formate production; g Chronopotentiometry curves of Step 1 (CO₂RR+NiOR) for CO production at 20 and 50 mA cm⁻²; h O₂ production at the anode in Step 1 at different current densities. The error bar represents standard deviation from three independent measurements. Source data are provided as a Source Data file.

Fig. 5. Cell performance in hydrogen conversion and the overall performance assessment.

a Polarization and power density curves of Step 2; b Polarization curves of CO₂RR directly coupling anodic HOR, the inset shows the FTIR spectroscopy of Pt/C GDE during the HOR process; c Chronopotentiometry curves of Step 2 at 20 and 50 mA cm⁻²; d Swap between Step 1 and Step 2 for CO generation; e Voltage efficiency of Step 1 and Step 2; f Multi-swap test between Step 1 and Step 2 at 50 mA cm⁻² for CO generation; g Comparison of operating voltages of H₂-integrated CO₂RR and state-of-the-art CO₂RR in the literature with (green) and without (black) paired electrooxidations from refs. ^–,,,; h Contributions of polarization losses in H₂-integrated CO₂RR coupled with water electrolyzers and in conventional CO₂RR at 50 mA cm⁻²; i Comparison of energy consumptions between H₂-integrated CO₂RR coupled with water electrolyzer and conventional CO₂RR at 50 mA cm⁻², assuming CO₂ recovery costs 4 GJ per tonne of CO₂ ^[20]. Source data are provided as a Source Data file.