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Charge Transfer Kinetics and Thermodynamics Control the Energy Conversion Efficiency of a Gallium Phosphide Solar
Kathleen Becker1, Li Wang1, Frank E Osterloh1
1Department of Chemistry, University of California, Davis, California 95616, United States.
The Journal of Physical Chemistry. C, Nanomaterials and Interfaces
|October 17, 2024
Summary
Gallium phosphide photocathodes show improved hydrogen evolution efficiency with a cadmium sulfide overlayer. This study reveals dynamic junction properties crucial for optimizing solar fuel production.
Area of Science:
- Materials Science
- Electrochemistry
- Photocatalysis
Background:
- P-type gallium phosphide (GaP) photocathodes have theoretical potential for hydrogen evolution but exhibit lower actual performance.
- Understanding kinetic and thermodynamic factors is key to improving GaP photocathode efficiency for the hydrogen evolution reaction (HER).
Purpose of the Study:
- To evaluate factors limiting energy conversion in GaP photocathodes for HER.
- To investigate the effect of a cadmium sulfide (CdS) overlayer and platinum (Pt) cocatalyst on photocathode performance.
- To analyze the dynamic junction properties and their influence on photovoltage.
Main Methods:
- Utilized vibrating Kelvin probe surface photovoltage (VKP-SPV), open circuit potential (OCP) measurements, and photoelectrochemical (PEC) experiments.
- Investigated GaP/water junctions and optimized GaP/CdS/Pt photocathodes.
- Analyzed the impact of gas atmospheres (hydrogen, oxygen) on photovoltage.
Main Results:
- The bare GaP-H2O junction photovoltage is limited by surface state recombination.
- A CdS overlayer forms an n-p junction, enhancing both photovoltage and photocurrent.
- Optimized GaP/CdS/Pt photocathodes achieved 62% quantum efficiency at 400 nm and 0.43 V open circuit photovoltage.
- Pt improved photocurrent via HER kinetics but reduced photovoltage by promoting recombination.
- Gas atmospheres dynamically altered band bending, indicating Schottky-like junction behavior.
Conclusions:
- The performance of GaP photocathodes is significantly influenced by surface states, junction properties, and cocatalyst effects.
- Dynamic junction behavior, controlled by the electrolyte's electrochemical potential, must be considered for optimizing photoelectrodes.
- VKP-SPV offers a direct measurement of open circuit photovoltage, complementing OCP data for a comprehensive understanding of illuminated photoelectrodes and solar fuel production.
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