Development of hybrid photocathodes for solar hydrogen production

Thiphaine Bourgeteau 1
1 LICSEN - Laboratoire Innovation en Chimie des Surfaces et NanoSciences
NIMBE UMR 3685 - Nanosciences et Innovation pour les Matériaux, la Biomédecine et l'Energie (ex SIS2M)
Abstract : One of the challenges of the 21st century is to produce clean and inexpensive energy at the TW scale to face the increasing energy demand and the global climate change. Because renewable energies are intermittent, they must be converted and stored in order to use them at the same scale of fossil energies. Hydrogen appears to be an ideal energy carrier when it is produced from water and sunlight. This fuel can be stored, transported and use on-demand by its combination with oxygen, for example in a fuel cell. Photo-electrochemical (PEC) cells able to carry out the photo-electrolysis of water are not yet cost-effective, because most of the materials used for their fabrication are rare or expensive (platinum, crystalline semiconductors). Producing hydrogen in a PEC cell at industrial scale depends on the finding of readily-available and easily-processed materials. In this thesis, the development of a noble-metal free hydrogen-evolving photocathode was undertaken, to reduce protons from light and acidic water. The photo-converting unit was based organic semiconductors organized in a polymer-fullerene bulk-heterojunction layer (P3HT:PCBM) coupled to amorphous molybdenum sulfide (MoS3) as a catalyst. In the device, the P3HT:PCBM layer absorbs the photons and the photogenerated electrons are then transported to the interface with the catalyst, which uses the electrons to produce hydrogen. After studying each material (catalyst and solar cell) separately and checking the alignment of their energy levels, the first assemblies were made by solution processes. The deposition methods were adapted depending on the nature of the materials. Spin-coating and spray were used for the deposition of the light-harvesting unit and the catalyst, respectively. With the photo-electrochemical characterization setup, a photocurrent of up to 100 μA cm–2 was obtained, corresponding to production of hydrogen, as analyzed by gas chromatography. These first results proved the viability of the concept of this hybrid noble-metal free photocathode. In order to improve the photocathode performance, new configurations were designed. Firstly, interfacial materials placed between P3HT:PCBM and MoS3 (electron-extracting layer, EEL) were studied to improve charge collection by the catalyst. Among studied materials, photocathodes with titanium protected aluminum reached up to 10 mA cm–2 of photocurrent. The presence of aluminum induced instability in aqueous media, so that oxides (TiOx) and organic materials (C60 fullerene and graphene) were considered. TiOx brought only a slight improvement compared to photocathodes without EELs, while C60 allowed to reach 5 mA cm–2 but with a lower stability compared to metallic EELs. The origin of the increased performances with EELs was attributed to the burying of the photovoltaic junction, removing the influence of the electrolyte. Secondly, the material between the transparent electrode and the photovoltaic part, i.e. the holeextracting layer (HEL), was replaced by amorphous oxides (graphene oxide (GO), MoOx, NiOx). It led to the fabrication of performant photocathodes, stables for several hours, by process temperatures below 150 °C in the case of MoOx and GO. The increase of the performance seemed to be related to the increase of the HEL work function, leading to the suggestion that the Fermi level difference between the HEL and the electrolyte has an impact on the capacity of the photocathode to separate the charges and use them for photocatalysis. The most performant photocathodes (several mA cm–2 and 0.6 V of photovoltage) were the one with MoOx, i.e. the material with the largest work function, and had a much better stability than the photocathodes with metallic EELs.
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Thiphaine Bourgeteau. Development of hybrid photocathodes for solar hydrogen production. Material chemistry. Ecole polytechnique X, 2015. English. ⟨tel-01870902⟩

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