Topologically Optimized Anode Catalyst Layers of Proton Exchange Membrane Water Electrolyzers
Authors: Charoen-amornkitt Patcharawat , Alizadeh Mehrzad , Suzuki Takahiro , Tsushima Shohji 
Published: September 2023

Climate change and global warming are ongoing global problems that will severely impact the environment if they are not addressed seriously. Replacing traditional fossil fuels with clean and sustainable energy from renewable energy sources, such as wind and solar, is necessary to reduce carbon emissions. However, these renewable energy sources are intermittent and thus require a system to help stabilize power generation. In this regard, hydrogen, the most sustainable form of clean energy, is proposed as a potential energy carrier. In such a system, water electrolyzers are used together with fuel cells. This allows hydrogen to be produced and stored in times of excess power generation. When there is demand and insufficient power generation, the produced hydrogen will be converted into electricity using fuel cells, resulting in a stable power grid. Although fuel cells have been improving significantly, water electrolyzers are an essential bottleneck for the widespread commercialization of the hydrogen economy.
Due to the intermittency of renewable energy, the water electrolysis system applied to renewable energy should be able to turn on and off depending on renewable energy sources. As a result, low-temperature water electrolysis has gained widespread attention. Due to its potentially high-performance and high-purity hydrogen gas, the proton exchange membrane water electrolyzer (PEMWE) is one of the most promising technology for producing clean hydrogen. However, concerning widespread industrial use, PEMWE is affected by high costs due to its high loadings of expensive iridium-based catalysts. Therefore, it is necessary to improve its performance. As PEMWE technology grows, the working current density is inevitably higher. When the current density is higher, the amount of oxygen and hydrogen that must be removed from the cell must be more significant to deliver water to the reaction site efficiently. To meet the demands of the application, it is crucial to design an efficient electrode, especially at the anode, where the water must be supplied to the catalyst layer, and, at the same time, it is necessary to transfer oxygen gas from the reaction area to the flow channel.
Topology optimization, an emerging tool to optimize physical systems, has recently been applied to systems with multi-physics. However, attempts to implement the approach in the system with reactions still lacked in the literature. Only a few studies were interested in utilizing such an approach in electrochemical energy device applications. Roy et al. [1] used topology optimization to develop the porous electrode. According to the findings, the developed electrode outperformed the traditional electrode. However, topology optimization is just a numerical approach. Our research group showed how a topologically optimized structure of porous reactors connected to the situation of the most uniform and minimum entropy generation [2]. To utilize topology optimization with PEMWE, a model for the anode catalyst layer is necessary. Although there are existing models to simulate the phenomena of PEMWE [3,4], to the best of our knowledge, studies have yet to look into how to model the phenomena inside the anode catalyst layer of the PEMWE.
Therefore, the main goal of this study is to develop a model to simulate the physics of the anode and utilize it in topology optimization to search for the optimal structure of the anode catalyst layer. The optimization problem was formulated to maximize the cell performance by controlling the volume fractions of ionomer and catalyst materials. Figure 1a-c displays the heterogeneous structure obtained by topology optimization. The electrochemical reaction increased by approximately 40% compared to a case with the homogeneously distributed electrode. The topologically optimized structure was similar to the structure proposed by Dong et al. [5], where their ordered structure with Ir loading of 0.2 mg cm−2 can significantly decrease the overpotentials compared with conventional MEA with an Ir loading of 2 mg cm−2. Therefore, such a structure might be a way forward for high-performance PEMWE system.

Acknowledgment

The authors would also like to express their gratitude to Office of the Permanent Secretary, Ministry of Higher Education, Science, Research and Innovation (OPS MHESI) (Grant No. RGNS 65-084), Thailand Science Research and Innovation (TSRI) and King Mongkut’s University of Technology Thonburi. This work was partially supported by Grant-in-Aid for JSPS Fellows number 22J20603 and JSPS KAKENHI Grant number 21H04540

References

[1] Roy, T., et al., Struct. Multidiscipl. Optim. 65 (2022) 171.
[2] Charoen-amornkitt, P., et al., Int. J. Heat Mass Transf. 202 (2023) 123725
[3] Lopata, J., et al., J. Electrochem. Soc., 168 (2021) 054518.
[4] Lopata, J.S., et al., Electrochim. Acta, 424 (2022) 140625
[5] Dong, S., et al., Nano Lett. (2022) (In Press)

Figure 1