Ultrathin coatings for enhanced durability in (photo)electrochemical hydrogen production
Keywords: Chemical Engineering.

With society transitioning from fossil fuel energy to renewable energy, there is an increasing need for robust means of storing the energy captured from these more variable resources. One concept for storing the vast, effectively limitless solar energy resource is called solar water splitting—a photoelectrochemical (PEC) process that produces hydrogen fuel from one of earth's most abundant chemical feedstocks. This work focuses on developing ultrathin coatings that enhance the durability of these PEC hydrogen-producing systems in the acidic aqueous electrolytes commonly employed for this reaction. We investigated the canonical PEC protective coating, titania (TiO2), to understand the reason that heat treatments in oxygen-containing environments improve the durability of (photo)electrochemical devices assembled with these ultrathin coatings. In electrocatalytic hydrogen production experiments, conductive silicon substrates coated with a 5-nm thick TiO2 coating and a layer of Pt showed dramatic improvements in both their activity (in terms of voltage required to achieve a set rate of hydrogen production) and durability when the TiO2 had been heat treated at temperatures of 200-500°C, with 500°C engendering the best performance by both metrics. In situ grazing incidence x-ray diffraction experiments under heating conditions in air revealed two crystallization regimes for these ultrathin TiO2. From temperatures about 250-400°C, the crystallization from amorphous to the anatase phase of TiO2 resembles the nucleation of anatase crystallites. From temperatures about 420-450°C, there appears to be growth of these crystallites to encompass increasing fraction of the film in the anatase phase. With ultrathin TiO2 requiring this advanced processing to meet durability requirements, we sought to explore another transition metal oxide that we predicted to have better intrinsic stability—tungsten oxide (WO3). WO3 coatings were synthesized by atomic layer deposition (ALD), a process known to yield highly conformal films with a high degree of thickness control. When utilized as coating on conductive silicon substrates with a Pt catalyst overlayer, a 4 nm WO3 film imparted the best durability by sustaining efficient hydrogen evolution over 15 h of durability testing. A photoactive device prepared on semiconducting silicon with a 4 nm WO3 film and Pt catalyst layer produced a light-driven voltage of 0.31 V and sustained a photocurrent density (which is proportional the rate of hydrogen production) of -20 mA cm-2 for 22 h of PEC durability testing. Adapting this ALD-type WO3 deposition to a copper gallium selenide (CuGa3Se5) semiconducting light absorbing film led to a CuGa3Se5 [vertical line] WO3 [vertical line] Pt devices that sustained continuously illuminated PEC hydrogen production over six weeks of durability testing. The most-durable device established a new durability milestone for any non-silicon device by passing 21,490 C cm-2 of charge, which is proportional to the total amount of hydrogen produced. Tungsten sulfide (WSx) films were synthesized from ALD-prepared WO3 films and investigated for their electrocatalytic activity and durability. Through x-ray photoelectron spectroscopy (XPS), the films synthesized at temperatures of 250-450°C were shown to be composed of WS2, with the 350-450°C-treated samples exhibiting some WS3 character that increased with increasing temperature. The WSx coating synthesized at 250°C demonstrated some electrocatalytic activity for hydrogen production, which was largely absent with the 350-450°C synthesized films, but degraded dramatically after 18 h of durability testing. The 350°C-synthesized WSx film was adapted to a photoactive silicon device with a catalytic Pt overlayer, and showed reasonable PEC hydrogen production activity but poor durability