Titanium-Based Nanoparticles from the Gas-Phase as Catalyst Carrier on Anodes for PEM Water Electrolysis—Synthesis, Characterization and Processing

In near future, hydrogen will play an important role as an energy carrier. Produced from water by electrolysis with renewable energy, hydrogen has the potential to act as a climate-friendly and CO2-free alternative for energy applications and chemical industry. Since renewable sources like photovoltaic or wind turbines generate fluctuating energy, dynamic operation conditions are required for electrolyzers of the future. Proton exchange membrane water electrolyzers (PEMWE) can handle these dynamics. However, efficiency losses from high anodic overpotential and the use of expensive critical raw materials (CRM) like Iridium with limited availability hinders the commercial use on large scale. The long-term stability of low-cost components is under investigation and seems limited, hindering their use. Corrosion issues from the acidic cell environment and high electrode potentials still necessitate CRMs for the anodic electrode to prevent passivation of the porous transport layer (PTL). Recent studies show strategies to reduce the amount of iridium. Here, electrically conductive nanostructured ceramic materials with implemented oxygen deficiencies and high specific surface area act as carrier material for the catalytic iridium nanoparticles.
In this work, a novel approach to obtain advanced PEMWE electrodes based on conductive titanium oxides and carbides as catalyst support is described. Using an optimized electrode structure with graduated porosity and electrochemical catalyst deposition, a significant decrease of the required amount of iridium is achieved, which results into reduced material costs in comparison to state-of-the-art PEMWE electrodes.
Particle synthesis takes place in gas-phase processes by thermal decomposition of the precursor titanium tetraisopropoxide (TTIP). Two different approaches are being pursued here: sub-stoichiometric titanium oxides are produced by means of spray flame synthesis and composite materials of titanium oxide and titanium carbide are produced in a microwave plasma process. The advantages of these gas-phase synthesis processes are continuous operation, reproducibility and high product purity. The products are characterized by BET (specific surface area), XRD (crystal structure) and electron microscopy (shape and size distribution).
A commercially available microstructured titanium sintered material serves as basis for the porous transport layer (PTL) of the manufactured electrode. Dispersions consisting of either TiOx or TiC in a carrier liquid are applied to this structure using a spraying process. A following laser sintering process of the nanomaterial coated PTL results in a mechanical and electrical bonding of the nanoparticles- on the microstructure, which exhibits a significantly higher specific surface area compared to uncoated PTL. To stabilize and generate oxygen deficiencies or electrically conductive crystallite structures the laser-sintering is performed under inert- or reducing gas atmosphere. To deposit Iridium catalysts on these surfaces galvanic deposition is performed. The electrical conductivity and bonding are verified by measuring the contact resistance and performing impedance spectroscopy of the prepared electrodes. The achievement of a finely structured and mechanically bonded surface is examined using electron microscopy. The evaluation compares the effects of different powders on the electrode properties. With all samples it is possible to maintain the fine structure and at the same time produce a mechanically stable and electrically conductive layer. Successful catalyst deposition and suitability as an anode in the PEMWE with reduced catalyst loading is exemplified.