Highly active oxygen evolution integrating with highly selective CO2‑to‑CO reduction

As global carbon emissions continue to rise, the development of efficient and sustainable artificial photosynthesis systems has become a critical challenge. A promising strategy involves coupling sunlight-driven water oxidation with CO₂ reduction to produce valuable fuels. However, the sluggish kinetics of the oxygen evolution reaction (OER) and poor selectivity of CO₂ reduction have long limited the overall efficiency of such systems. Now, researchers from the Lanzhou Institute of Chemical Physics, led by Prof. Yingpu Bi, have developed a breakthrough photoelectrochemical (PEC) system that achieves record solar-to-fuel conversion efficiency by rationally engineering the coordination environments of both photoanode and cathode catalysts.

Why This Integration Matters

• Efficient Water Oxidation: A highly active BiVO₄ photoanode decorated with low-coordinated FeNi catalysts delivers an exceptional photocurrent density of 6.51 mA cm^-2 at 1.23 V<sub> under AM 1.5G illumination.
• Selective CO₂ Reduction: A single-atom cobalt phthalocyanine (CoPc) cathode anchored on nitrogen-rich carbon substrates enables >90% Faradaic efficiency for CO production.
• Record Solar-to-Fuel Efficiency: The integrated system achieves a 5.41% solar-to-fuel conversion efficiency under bias-free conditions, setting a new benchmark for PEC CO₂ reduction systems.

Innovative Design and Features

• Photoanode Engineering:
  • FeNi oxyhydroxide catalysts with oxygen vacancies (FeNi-Ov) were uniformly deposited on nanoporous BiVO₄.
  • Ar plasma treatment reduces metal coordination, enriching surface electron density and enhancing hole transfer for OER.
  • The resulting photoanode exhibits 93.5% IPCE at 360 nm and maintains stable performance over 6 hours.
• Cathode Optimization:
  • Single-atom Co–N₅ sites were constructed by anchoring CoPc onto conductive N-rich carbon (NC) derived from ZIF-8.
  • XANES and EXAFS confirm the atomic dispersion of Co and the formation of Co–N₅ coordination.
  • CO₂-TPD and in situ FTIR reveal enhanced CO₂ adsorption and activation on CoPc-NC compared to pristine CoPc.
• System Integration:
  • A bias-free PV-PEC device was constructed by combining the BiVO₄/NiFe-Ov photoanode with a polycrystalline Si solar cell to harvest sunlight >500 nm.
  • The system produces 62.2 μmol cm^-2 CO and 35.3 μmol cm^-2 O₂ in 1 hour, with a stoichiometric H₂:CO:O₂ ratio close to 2:1:1.

Mechanism and Performance Highlights

• Water Oxidation: Photo-generated holes migrate to FeNi-Ov sites, facilitating O–O bond formation and releasing electrons/protons for CO₂ reduction.
• CO₂ Reduction: CO₂ molecules adsorb on Co–N₅ sites, forming *COOH intermediates that are rapidly reduced to CO with high selectivity.
• Stability: The system maintains >85% Faradaic efficiency for CO over 10 hours and across 5 cycles, with no detectable performance degradation.

Challenges and Future Outlook

The study highlights the importance of coordination environment tuning in boosting PEC performance. Future work will focus on:

• Scaling up the synthesis of single-atom catalysts for practical applications.
• Extending light absorption of photoanodes into the near-infrared region for higher solar utilization.
• Integrating with gas diffusion electrodes for continuous-flow CO₂ conversion.

This work provides a blueprint for rational catalyst design in solar fuel systems, demonstrating that simultaneous optimization of both half-reactions can unlock unprecedented efficiencies. Stay tuned for more innovations from Prof. Yingpu Bi’s team at LICP-CAS!