Water electrolysis is expected to play a crucial role to the transition towards a hydrogen-based, low carbon economy. Among the various technologies for water electrolysis technologies, the one using proton exchange membranes (PEM) holds promise due to its high efficiency, load flexibility and compact design. However, the acidic environment in the PEM creates a harsh operating environment which entails the use of scarce and expensive metals (Pt and Ir) which account for 38% of the cost of a catalyst coated membrane (CCM). Moreover, the high scarcity of Ir is considered as the grand challenge of this technology.¹

To relieve high raw material costs and to mitigate the Ir-dependence of PEM electrolysis, the development of advanced CCMs with improved Ir utilization is essential. Most of studies so far have focused on the development of high-structured catalysts, either by maximizing the Ir dispersion with using high surface area supports or by using alternative catalyst nanostructures.² In all these conventional routes, the CCM manufacturing process is multi-step and primarily ink-based. Steps include ink preparation (from metallic nanoparticles), ink application onto the PEM to yield the catalyst-coated product, and several intermediate drying stages.

The use of improved catalyst layer manufacturing techniques has been proposed as an alternative route to produce catalyst-coated membranes (CCMs) with low Ir loadings, without compromising in activity or durability. However, such concepts are less represented in literature. Despite this, the use of vapor-based processes for the manufacturing of nanomaterials for PEM electrolysers has key benefits over ink-based processes including the simplification of the production process, the reduction of catalyst loading, and the deposition of more uniform thin layers of material.³

In this work, we prepared CCMs using a solvent free, gas-phase method, comprising spark ablation and impaction,⁴ to produce iridium-based nanoparticles (IrNPs) for PEM water electrolysers. IrNPs were produced at ambient temperature and pressure conditions in the gas phase using Ir rods as the nanomaterial source. The nanoparticle aerosol was then directed towards a nozzle and deposited onto a Nafion 115 membrane via inertial impaction. By securing the membrane on an XY-stage, a 4 cm² catalyst coating was printed. The Ir content on the Nafion membrane was controlled by varying the printing speed.

The prepared CCMs were characterized using RBS, ICP, SEM, HR-TEM, XRD, and XPS. The CCMs prepared via spark ablation comprised of a homogeneous, dense, porous layer of amorphous IrO₂ nanoparticles (Figure 1a) with an average particle size of 2 nm. The Ir-based CCMs were employed in a single-cell PEM water electrolyser and complete performance assessment was performed at 60 °C, focusing both on activity and durability.

Our results show that the CCMs prepared with spark ablation outperform the benchmark (commercial) CCM (Figure 1b) despite using considerably less Ir (Figure 1c). Specifically, up to 5-fold improvement in Ir utilization (i.e. 5-fold decrease in Ir-specific power density) was obtained.

To assess durability, both transient and dynamic service conditions were simulated by using different time-profiles for the power input. The overall profile of the constant load durability test indicated that irreversible changes occurred at the CCM during the first 30 h, while afterwards a stable degradation rate of 0.05 mV h^-1 was sustained for >60 h. A similar degradation rate was also obtained with the benchmark CCM. Based on this rate and assuming a cut-off voltage of 2.2 V, the expected lifetime of our CCM is 10,000 h, which is the minimum requirement for industrial operation.

Overall, our results showcase the feasibility of the spark ablation technology as a scalable and efficient method to achieve reduced Ir loading and better performing CCMs for PEM water electrolysis. Taking into account the simplicity of the process, this technology has also the potential to relieve the high CCM manufacturing costs of conventional approaches, which currently account for 42% of the total CCM cost.¹

1. IRENA, Green Hydrogen Cost Reduction – Scaling up electrolysers to meet the 1.5 °C climate goal, International Renewable Energy Agency (IRENA), Abu Dhabi, (2020).
2. F. M. Sapountzi, J. M. Gracia, C. J. (Kees-J. Weststrate, H. O. A. Fredriksson, and J. W. (Hans) Niemantsverdriet, Prog. Energy Combust. Sci., 58, 1–35 (2017).
3. R. J. Ouimet et al., Energy & Fuels, 35, 1933–1956 (2021).
4. M. F. J. Boeije et al., in Spark Ablation: Building Blocks for Nanotechnology,, p. 49–108, CRC Press (2019).