(Invited) Suppression of Electronic Leakage in Proton-Conducting Cells
Authors: Matsumoto Hiroshige , Vediyappan Veeramani , Kwati Leonard 
Published: December 2024

Solid oxide cells (SOC) utilizing proton-conducting metal oxides as electrolytes offer the advantage of reduced operating temperatures compared to conventional SOCs. When used as fuel cells, they exhibit high fuel utilization rates, and when applied to steam electrolysis, they offer the benefit of producing hydrogen without steam. However, there are still several challenges to address, and the electronic leakage is most crucial. Proton-conducting oxides generally exhibit characteristics of hole conduction in oxidative atmospheres. Hole conduction affects the entire cell, even if one side of the cell is in a reducing atmosphere, resulting in decreased Faraday efficiency during steam electrolysis for hydrogen production and extra fuel consumption without contributing to power generation in fuel cell applications. Therefore, electronic leakage phenomena lead to a decrease in energy efficiency in both types of devices.

In this presentation, it is demonstrated how electronic leakage phenomena in proton-conducting solid oxide cells appear experimentally in both steam electrolysis and fuel cell modes, and discussed on the possible methods to suppress this phenomenon.

In steam electrolysis, electronic leakage is observed as a decrease in Faraday efficiency. Ideally, with the electrolyte being fully ionically conductive, the amount of hydrogen generated by a charge of electrical quantity Q [C] should be Q/2F [mol], where F is the Faraday constant. However, actual hydrogen generation is reduced due to electronic leakage. Depending on the electrolyte, Faradaic efficiency can be from 20% to 30%, resulting in a significant portion of input power dissipating as Joule heat. Even achieving 80% to 90% Faraday efficiency leaves a considerable loss of 10% to 20%. In the fuel cell mode, electronic leakage is observed as excessive fuel consumption. Even in an open-circuit state with hydrogen and air introduced into the cell, water formation is observed. This is due to internal short circuits caused by electronic leakage in the electrolyte.

One proposed method to suppress this phenomenon is to find materials that do not exhibit hole conduction (or exhibit it to a lesser extent). Is this feasible? The mechanism by which proton conduction occurs in metal oxides is explained by the hydration of oxygen vacancies. When the temperature is sufficiently low, around 600°C or below, for chemical equilibrium hydration to progress in a humid atmosphere, oxygen vacancies are hydrated, and protons (hydrogen ions) appear as conducting species within the lattice. On the other hand, oxygen vacancies can also accept oxygen from the atmosphere, resulting in hole conduction that causes the aforementioned leakage phenomenon. In other words, the equilibrium reactions that produce protons and holes are very similar, both involving the filling of oxygen vacancies with oxygen. Therefore, as long as materials are designed to achieve proton conduction through this mechanism, eliminating only hole conduction seems theoretically difficult.

As an alternative approach to suppress electronic leakage in proton-conducting cells, the authors are investigating the control of transport phenomena involved in electronic leakage reactions. The figure depicts reactions around the anode (oxygen-evolving electrode) in steam electrolysis, where a mixed conductor of oxide ions and electrons is used as the electrode. The correct reaction involves water splitting into protons and oxide ions at the electrolyte-electrode interface, but leakage current flows when holes and electrons are generated at the same interface. To mitigate this, an improvement in Faraday efficiency was observed by inserting gadolinium-doped ceria (GDC) as an electron-blocking layer in part of the electrode. Thus, it seems possible to suppress electronic leakage by controlling the flow of specific substances through the implementation of layers with specific functions in the electrolyte or electrodes and utilizing this control effectively.

Acknowledgement

This paper is partially based on results obtained from a project, (JPNP20003), commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Figure 1