Catalyst Carrier for PEM Water Electrolysis—Synthesis, Characterization and Processing

A necessary precondition for proton exchange membrane (PEM) water electrolysis to become a game-changer in hydrogen industry is the significant reduction of noble metal loading as well as robust electrode manufacturing techniques. Highly specific conductive nanomaterials with high specific surface area offer a huge potential to be used as a supporting material for catalysts in electrode structures. The combination of advanced catalyst deposition technologies for targeted deposition only on the electrically conductive electrode surface and the large surface area of the nanostructured carrier material enables higher proportions of active catalyst surface and thus leads to a reduction in precious metal consumption.
Three different nanostructured support materials for anodes (Titania, doped Silicon) and cathodes (Graphene) have been generated in gas-phase reactors in the pilot plant scale.
We demonstrate the synthesis of sub-stoichiometric Titania nanoparticles and their use as iridium carrier material in PEM technology. For this purpose, blue-coloured Titania particles with oxygen deficiencies are produced in combustion processes, deposited on electrical connection geometries and subsequently laser sintered to nanostructured surfaces under reducing atmospheres. SEM analyses confirm process engineering successes both in laser sintering and in the subsequent galvanic deposition of iridium particles, which are clearly visible on the surface and are confirmed by EDX analyses. Cyclic voltammetry on TiO_2-X/Ir - systems clearly proofs an activity of oxygen generation (OER) at 1.25 V and lower slopes of the data in Tafel representation indicate an increased catalytic activity compared to bulk Ir.
Another approach for the generation of nanostructured support materials is represented by the use of highly boron doped silicon particles as support material. Here, Monosilane (SiH₄) and Diborane (B₂H₆) have been used as precursor material to produce degenerated silicon semiconductors. As catalysts laser-based synthesized iridium nanoparticles from a laser ablation in liquids (LAL) and laser fragmentation of microparticles in liquids (MP-LFL) are used. Here, particles sizes of 7 nm and 4 nm are applied. XRD analysis reveals a distribution of Si, Ir and IrO₂ phases in the analyzed samples, both of the last two being well-known for being highly active catalytically. The fact that no compound phases between any of Si, B and/or Ir were observed, shows that no alloying takes place. Cyclic voltammograms recorded with Ir decorated Si:B nanoparticles followed the well-studied Ir oxidation path. The laser fabricated Ir nanoparticles exhibit a smaller size than the commercial ones, creating a higher electrochemical surface area reaching lower overpotential and onset overpotential values while presenting similar Tafel slopes.
Additionally, due to high electrical conductivity graphene is a potential candidate to be used as a support material, too. To address this, we performed eco-friendly synthesis using easily accessible ethanol as precursor material. For this purpose, ethanol is evaporated and subsequently fed through an argon-hydrogen plasma generated by microwave radiation. To investigate the influence of precursor concentration on the material properties, the feeding rate of EtOH is stepwise adjusted from 200 g/h to 800 g/h. TEM, SEM and Raman analysis proof that typical graphene sheets with lowest carbon particle presence are formed almost exclusively at lowest feeding rates, which can be explained by reduced carbon saturation during material formation. In good agreement with material morphology highest specific conductivity σ = 3,19 S/cm with ρ= 0.23 g/cm³ at 100 N/cm² is obtained for material with high graphene/particle ratio in a powder conductivity test cell at increased pressures. Measurements on commercial reference materials show lower conductivities by a factor of 8.6.