To Carbon or Not to Carbon: Rethinking Electrode Design in Unitized Reversible Fuel Cells

Abstract

The development of efficient and scalable energy storage systems remains a major challenge in the transition to renewable energy. Unitized reversible fuel cells (URFCs), capable of operating in both electrolysis and fuel cell modes, offer a promising solution. In this context, integrating the chlor-alkali process into URFCs enables not only cost-effective energy storage but also environmental benefits such as CO₂ capture via alkaline absorption. While chlor-alkali electrolysis is well established, the reversible operation is not well known. This study addresses a key design question: the role of carbon-based materials in electrode architecture, specifically in the use of a carbon-based microporous layer. Titanium felt electrodes were modified with microporous layers (MPLs) containing 1, 2, and 3 mgC/cm² and coated with a RuO₂-Pt catalyst using a Pechini-type polymeric precursor method. The results showed that increasing the carbon content, the electrode resistance was reduced and surface hydrophobicity was enhanced, achieving the best results with 2 mgC/cm² in the MPL. Moreover, in electrolysis mode, the hydrogen production efficiency improved with temperature, reaching 15 mgH₂/Wh at 60 °C (surpassing industrial benchmarks). The system also achieved high Faradaic efficiency for hydrogen production (>98%) and enabled simultaneous CO₂ capture via cathodic alkaline absorption. In fuel cell mode, the optimized electrode reached a peak power density of ∼30 mW/cm² at 60 °C, an order of magnitude higher than previously reported in the literature for similar systems. The results are very promising and position chlor-alkali-based reversible electrochemical cells as a promising platform for efficient, scalable, and multifunctional energy storage and conversion technologies.