(Invited) P-Type Transparent Conducting Oxide Protection Layers for Sustainable Photoelectrochemical Water Oxidation
NiCo2O4 was deposited on n-Si (100) and np-Si (100) by reactive sputtering at a substrate temperature of 70-100 °C. The film structure was amorphous, as shown by the lack of XRD and Raman features. P-type conductivity was confirmed by measurement of a positive Seebeck coefficient. Light transparency of >70% (l > 400 nm) was achieved with a NiCo2O4 thickness of 40 nm. The hole conductivity was 50-60 S/cm and, as expected, p-NiCo2O4 forms a rectifying contact to n-Si.
Photoelectrochemical (PEC) evaluation was performed in aqueous 1M NaOH (pH 14) with simulated AM1.5 illumination; these conditions would rapidly corrode the n-Si photoanodes in the absence of a protection layer. PEC performance of the np-Si/p-NiCo2O4 structures is excellent, particularly when a thin NiFe oxygen evolving catalyst is applied. An onset potential of 0.95 V vs. RHE is observed, which is one of the lowest reported for a Si-based photoanode. The current density at the reversible potential for water oxidation (+1.23 V vs. RHE) is >25 mA cm-2, and the current rises to a limiting value of 30 mA cm-2 at more anodic potentials, which demonstrates the attractive combination of transparency and low-resistance hole conductivity in the NiCo2O4. Long-term testing indicates multi-day stability with minimal decrease in performance or observable corrosion of the Si photoanode. In depth characterization of both the solid-solid interfaces between NiCo2O4 and light absorber/catalyst and of the solid/electrolyte interface will be discussed.
This work demonstrates that p-TCOs are promising as corrosion protection layers for stable water oxidation photoanodes.
This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.