NREL shows graded catalytic-protective layer boosts longevity of high-efficiency photocathodes for renewable hydrogen
Researchers at the US Department of Energy’s National Renewable Energy Laboratory (NREL) have developed a method which boosts the longevity of high-efficiency photocathodes in photoelectrochemical water-splitting devices. Their works demonstrates the potential of utilizing a hybridized, heterogeneous surface layer as a cost-effective catalytic and protective interface for solar hydrogen production.
In a paper published in the journal Nature Energy, they show that annealing a bilayer of amorphous titanium dioxide (TiOx) and molybdenum sulfide (MoSx) deposited onto GaInP2 results in a photocathode with high catalytic activity and stability for the hydrogen evolution reaction. The study showed that the annealing results in a graded MoSx/MoOx/TiO2 layer that retains much of the high catalytic activity of amorphous MoSx but with stability similar to crystalline MoS2.
Using a photoelectrochemical (PEC) device is a promising way to produce hydrogen. A PEC cell absorbs sunlight and converts that energy into hydrogen and oxygen by splitting water molecules. Unfortunately, high efficiency devices developed to date quickly degrade in the acidic solution to which the cell is exposed. The challenge of making a more durable cell must be overcome before renewable hydrogen from PEC devices can become commercially viable.
Oxide semiconductor materials, such as Fe2O3, WO3, SrTiO3 and TiO2, have been studied for many years for PEC water splitting. However, the slow charge transport kinetics and/or large bandgaps that typically define these oxide semiconductors result in very low energy conversion efficiencies. In contrast, conventional photovoltaic semiconductors, such as GaInP2, offer excellent transport properties and smaller bandgaps than oxides, but are susceptible to corrosion during the water-splitting process. To realize and commercialize future solar hydrogen concepts based on PEC devices, durability of tens of thousands of hours and a device cost of hundreds of dollars per square meter must be achieved.
Here, we show that annealing a bilayer of a-MoSx/TiOx, applied to a GaInP2 photoelectrode, results in a graded g-MoSx /MoOySz/MoOx/c-TiO2 interfacial layer, endowing the photocathode with PEC properties superior to those of a GaInP2 electrode coupled with a PtRu alloy HER catalyst.—Gu et al.
During a 20-hour durability test, the photocathode retained 80% of the initial electricity generated. The TiOx and MoSx produced a catalyst protection layer and served to protect the GaInP2 from the acidic solution.
The concept of using an integrated tandem cell based on the NREL high-efficiency tandem solar cell to split water and produce hydrogen was developed 18 years ago by research fellow John Turner, who has been with the laboratory since 1979. He designed a tandem solar cell containing layers of gallium indium phosphide (GaInP2) and gallium arsenide (GaAs) semiconductors to absorb the sunlight and produce the power necessary for the photoelectrochemical water-splitting reaction. Turner’s device held the record for the highest solar-to-hydrogen efficiency, until it was finally eclipsed in 2015.
The research was funded by the Energy Department’s Office of Science, Office of Basic Energy Sciences, Solar Photochemistry Program. This effort benefits from work on photoelectrochemical water splitting being funded out of the department’s Fuel Cell Technologies Office.
Jing Gu, Jeffery A. Aguiar, Suzanne Ferrere, K. Xerxes Steirer, Yong Yan, Chuanxiao Xiao, James L. Young, Mowafak Al-Jassim, Nathan R. Neale & John A. Turner (2017) “A graded catalytic–protective layer for an efficient and stable water-splitting photocathode” Nature Energy 2, Article number: 16192 doi: 10.1038/nenergy.2016.192