High current density operation of water electrolysis enables mass production of hydrogen in a small system. With respect to high current density operation, alkaline water electrolysis, which are currently widely used for hydrogen production, has material-related limitations like ohmic resistance. However, polymer electrolyte membrane water electrolysis (PEMWE) has less limitations for higher current density operation, so that high current density operation of PEMWE is an advantageous feature compared with other water electrolysis technologies. High current density operation also leads to lower capital cost by reducing electrode area. A large amount of heat is however generated in high current density operation¹, which could lead to nonuniform temperature distribution in the cell and therefore understanding the internal cell temperature distribution is important. Some researchers studied internal cell temperature distribution by numerical simulation^{2, 3}, but, to the best of our knowledge, no study has been done so far in a current density range of up to 10 A cm^-2. The aim of this study is, by numerical analysis, to clarify internal cell temperature distribution around 10 A cm^-2 which is the operating range of future PEM water electrolysis.⁴

Experimental

A two-dimensional computational fluid dynamics (CFD) model for a 1 cm×1 cm PEM electrolysis cell consisting of MEA, GDL and flow channel was developed for numerical analysis of I-V characteristics and cell temperature distribution. CFD software COMSOL Multiphysics ver. 6.0 was used for the simulation. The cell temperature was fixed at 80 ^oC and the flow rate of water was set to be 5 to 15 ml min^-1. Water was supplied from left hand side to the anode, and from right hand side to the cathode, as shown in Fig 1(b). The thickness of electrolyte membrane was set to be 51, 127, and 183 μm simulating Nafion 212, 115, and 117. I-V characteristics were analyzed by simulating the potential scan from 1.3 V to 2.5 V. The cell temperature distribution was analyzed in a steady state. The essential material property parameters for the calculations were procured from literature survey. Mass transport resistance, not yet understood sufficiently,⁵ was not considered in this calculation. The relation between current density and internal cell temperature distribution was investigated.

Results and discussion

Figure 1 shows I-V characteristics of a PEMWE cell simulated by following operating conditions: electrolyte membrane thickness of 51 μm; the flow rate of the supply water of 5 ml min^-1, and the cell voltage of 2.5 V (current density of 11.8 A cm^-2) in a steady state. Figure 1(a) shows linear I-V characteristics consisting predominantly ohmic overpotential in high current density region due to ignored mass transport resistance in this calculation. The cell temperature distribution in Fig. 1(b) shows that the cell temperature increased towards the outlet of water supply on the anode side and that temperature difference of approximately 20 K was generated between the inlet and outlet of water supply on the anode side. Temperature difference was suppressed by increasing the flow rate of water supplied. This result suggests the importance of appropriate rate of water supply for uniform cell temperature distribution especially in high current density operation.

References

1. M. Suermann, T. J. Schmidt, and F. N. Büchi, Electrochim. Acta, 211, 989–997 (2016).
2. S. Toghyani, E. Afshari, E. Baniasadi, S. A. Atyabi, and G. F. Naterer, Energy, 152, 237–246 (2018).
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