The Development of Zn-Ce Hybrid Redox Flow Batteries for Energy Storage and Their Continuing Challenges

The Zn-Ce flow battery is a recently introduced hybrid redox flow battery (RFB) but has been extensively studied in the laboratory and at the industrial pilot scale since its introduction in 2005. The cell has the highest open-circuit cell potentials amongst aqueous RFBs, which can exceed 2.4 V at full charge. Although original patents were filed in 2004 and 2005, the history of the half-cell reactions stretches back many decades, with the Ce-IV/Ce-III redox couple being well studied in sulfuric acid as a redox mediator in organic electrosynthesis or in nitric acid for specialist cleaning in the chemical and nuclear industries; in addition, zinc deposition and stripping in acid media are well known in hydrometallurgy and electroplating of metals as well as in other secondary batteries using zinc negative electrodes. Methanesulfonic acid electrolytes were introduced in surface finishing several decades ago but their use in flow batteries is only 20 years old. This Review considers the thermodynamics and kinetics of the electrode reactions (desired and secondary) in each half-cell, operational variables, materials for cell components, cell design and performance of the zinc-cerium flow battery. Continuing challenges are highlighted and critical research needs for the science and technological development are considered.