Modelling of the Water Mass Transport through the Membrane of Direct Toluene Electro-Hydrogenation Electrolyzers
Authors: Atienza-Márquez Antonio , Reyna-Peña Fatima Isabella , Shiono Ryuhei , Araki Takuto , Nagasawa Kensaku , Mitsushima Shigenori 
Published: February 2024

The organic hydride toluene/methylcyclohexane (MCH) is an attractive carrier for the bulk and long-distance transport of green hydrogen. On the other hand, the direct toluene electro-hydrogenation in a proton exchange membrane electrolyzer exhibits advantages concerning the conventional two-step toluene hydrogenation process, such as the lower theoretical voltage by avoiding thermal losses [1]. Figure 1 (a) shows the internal structure and illustrates the basic working principle of this electrochemical device.

However, the effective toluene supply to the cathode catalyst is a central issue. It is assumed that the accumulation of water dragged from the anode side inhibits the toluene mass transport to the reaction site [2]. Therefore, in practice, a fraction of the toluene stream entering the cathode fails to convert to MCH; instead, hydrogen bubbles are generated. According to the theoretical approach illustrated in Fig. 1 (a), protons transported across the membrane and can be the carrier of associated water. Nevertheless, the water transfer mechanism inside a cell is complex, diverse, and unclear (e.g., as mist, small droplet) [3].

According to most conventional modelling approaches, electro-osmosis and back diffusion mainly govern the total water flux through electrolytic membranes. The open literature dealing with PEM fuel cells agrees that the water content of the membrane (correlated with the water activity by earlier studies) is a central factor concerning not only these mass transport mechanisms inside the membrane, but also concerning the mass transfer resistance on its surface. Indeed, to calculate both the electro-osmotic drag and the diffusion coefficients, authors have developed numerous correlations based on a relationship with the membrane water content. However, the experimental data available in the open literature dealing with PEM fuel cells devices largely disagree. Complex factors such as the Schroeder’s Paradox in Nafion® membranes [4] can explain those discrepancies. This leads into a huge uncertainty in the parametrization of those parameters involved in the numerical modelling and simulation of the water transport in electrochemical devices based on PEM technology [5].

The objective of this work is to qualitatively study, through numerical simulation, the influence of the main parameters involved in the transport of water through the electrolytic membrane of a direct toluene electro-hydrogenation electrolyzer. To accomplish this aim, a sensitivity analysis was conducted, and the probability distributions set for each modelling parameter (i.e., potential variation range) are consistent with the state-of-the-art. The anode reactants considered were pure water and also diluted solutions of sulfuric acid with concentrations between 0.1-1.5 L/mol. Figure 1 (b) shows the preliminary results obtained when supplying pure water as anode reactant and under the assumption of null water mass transport by back diffusion mechanism. The experimental data gathered from the monitoring of an in-house toluene electro-hydrogenation cell are used as reference to discuss the results obtained from the simulations and to tighten the modelling parameters.

Acknowledgements

Antonio Atienza-Márquez acknowledges the Ministerio de Universidades of Spain and the Recovery, Transformation and Resilience Plan for financially supporting the postdoctoral contract 2021URV-MS-01.

References

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Figure 1