Due to its high energy density, hydrogen could play an important role to store chemical energy in GW scale. To produce it in large quantities, “artificial leaf”-type structures can be used to convert solar light into hydrogen by photoelectrochemical splitting of water. Since this process is most efficiently working under acidic conditions, acid-stable semiconducting materials are required to efficiently absorb sunlight and generate electron-hole pairs, the energy of which must be high enough to split water. In addition, cheap and abundant electrocatalysts are needed to minimize the overvoltages at cathode and anode.

To replace costly platinum as well as RuO₂ as efficient electrocatalysts for hydrogen and oxygen evolution, resp., alternative catalysts such as MoCo, (Mo,Co)S_x and (NH₄)₂Mo₃S₁₃ for pH <7 [1] and amorphous CoO_xOH_y, Mn₂O₃ for pH >7 [2] have been tested as hydrogen (HE) and oxygen evolution (OE) catalysts, respectively. The materials were first deposited on conductive glass by reactive magnetron sputtering, spin coating or electrochemical deposition techniques and investigated electrochemically. Highest activity as HE catalysts was found by depositing (NH₄)₂Mo₃S₁₃ on amorphous MoS_x/FTO and alloys of MoCo, while porous layers of Mn₂O₃ deposited on FTO glass showed high activity as OE catalyst. Due to their remarkable behavior as dark catalysts (η_(Mn2O3) = 340 mV and η_(MoSx) = ‑170 mV at j = 10 mA/cm²), the materials were afterwards deposited on p- and n-type photosensitive transition metal chalcogenide layers to investigate their behavior under illumination.

Surprisingly, (NH₄)₂Mo₃S₁₃, deposited on highly 001-textured polycrystalline p-type WSe₂ film, behaved as a photosensitive hydrogen evolving electrode. Here the deposited catalyst film operates as an electrocatalyst for hydrogen evolution, but also as a semiconductor at the catalyst-semiconductor interface forming a buried heterojunction.

References:

[1] Jesse D. Benck, Sang Chul Lee, Kara D. Fong, Jakob Kibsgaard, Robert Sinclair, Thomas F. Jaramillo, Adv. Energy Mater. 4 (2014) 1400739-1400739.

[2] Bogdanoff P., Stellmach D., Gabriel O., Stannowski B., Schlatmann R., van de Krol R., Fiechter S, Energy Technology 4, (2016) 230–241, DOI:number:10.1002/ente.201500317

[3] Stellmach D., Bogdanoff P., Gabriel O., Stannowski B., Schlatmann R., van de Krol R, Fiechter S., Nanostructured MoS₂ particles as a novel hydrogen evolving catalyst integrated in a PV-hybrid electrolyzer, Materials and Processes for Energy: Communicating, Current Research and Technological Developments, Vol. 1 (A. Méndez-Vilas, Ed.), FORMATEX 2013, 880-886.

[4] Moritz Kölbach, Sebastian Fiechter, Roel van de Krol, Peter Bogdanoff,

Catalysis Today, 290 (2017) 2–9, https://doi.org/10.1016/j.cattod.2017.03.030

[5] Ramírez A., Hillebrand, P., Stellmach D., May M.M., Bogdanoff P., Fiechter S.,Phys Chem. C, 118 (2014) 14073-14081 and SI.