(Keynote) Interfacial Layers and Catalyst Approaches for Stabilizing a p-GaInP2 Photoelectrode for Hydrogen Production
Authors: Turner John A 
Published: February 2020

The use of sunlight to drive the conversion of water into H2 and O2 via a photoelectrochemical (PEC) reactor provides a strategy for storing solar energy, providing an energy carrier for transportation and a feedstock for ammonia production. To collect enough sunlight to produce the amounts of hydrogen necessary will require covering large land areas. Utilizing precious metal catalysts over such large areas could prove to be difficult, thus alternative catalyst systems need to be considered. Alternative catalyst schemes include non-precious metal hydrogen evolution catalysts based on materials such as MoS2 and homogeneous catalysts. Homogeneous catalysts provide a rich area for both hydrogen and oxygen catalysts with a variety of redox potentials and catalytic activity. Combining the high photoconversion efficiency of a III-V semiconductor electrode with these catalysts could prove to be a winning combination making PEC hydrogen production viable.

Our work has shown that by employing a hybrid molecular/semiconductor interface with atomic layer deposited (ALD) TiO2 as an intermediate layer, a robust and corrosion resistant GaInP2-TiO2-cobaltoxime-TiO2 photocathode can be operated in alkaline media (pH =13). 

An earth-abundant first row transition Cobalt catalyst Co(oxm)(pyCOOH) [oxm=bis-glyoxime; pyCOOH=4-carboxyl pyridine] as shown in figure 1 was attached onto a TiO2 modified GaInP2 surface through a carboxylic linkage group. A 35 nm amorphous TiO2 layer was deposited onto bare GaInP2 as protection and linkage layer. To further protect the linkage stability, an additional 10 cycle (∿0.4 nm) TiO2 ALD layer was deposited on top of the catalyst layer.

The photoelectrochemical photocurrent density (J) versus potential (V) plots of the cobaltoxime modified electrode showed that its activity and stability were comparable to the similarly modified Pt electrodes.

Note that for homogeneous catalysts used in a PEC system, the stability is less important, due to the fact that the system will be inoperative every night allowing for the possibility of rejuvenating the catalyst system by some means.

In an alternative scheme, we developed a robust and catalytically active interfacial layer for stabilizing a p-GaInP2 photocathode based on MoSx/MoOx/TiO2. We find that annealing a bilayer of amorphous titanium dioxide (TiOx) and amorphous molybdenum sulfide (MoSx) deposited onto GaInP2 results in the MoSx/MoOx/TiO2 coating that provides the GaInP2 photocathode with good efficiency (current density of 11 mA/cm2 at a potential of 0 V vs. RHE under 1 sun illumination) and stability (retains 80% of its initial photocurrent density over a 20 h period) for the hydrogen evolution reaction in strong acid solution compared with traditional surface-deposited platinum-ruthenium cocatalysts on a GaInP2 photocathode. We explore the composition and phase of the interfacial layer using transmission electron microscopy , scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, electron energy loss spectroscopy, and x-ray photoelectron spectroscopy before and after annealing. These studies reveal that 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.