Zinc–cerium batteries are a type of redox flow battery first developed by Plurion Inc.(UK) during the 2000s.[1][2] Negative zinc electrolyte and positive cerium electrolyte are stored in two separated reservoirs and are circulated during the operation. Negative and positive electrolyte compartments are separated by a Nafion (DuPont) cation-exchange membrane.
Due to the high standard electrode potentials of both zinc and cerium redox reactions in aqueous media, the open-circuit cell voltage is as high as 2.43 V.[3] Among the other proposed flow battery systems, this system has the largest cell voltage and power density per electrode area. Methanesulfonic acid was used as the supporting electrolyte, as it allows both zinc (2.16 M)[4] and cerium electroactive species to dissolve at concentration larger than 1 M.[5]
Since zinc is electroplated during charge at the negative electrode and redox reactions of Ce(III)/ Ce(IV) take places at the positive electrode, this system is often classified as a hybrid flow battery. Unlike the chemistry used in zinc–bromine and zinc–chlorine redox flow batteries, no condensation device is needed to dissolve the halogen gases.
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At the negative electrode (anode), zinc is electroplated and stripped on the carbon polymer electrodes during charge and discharge, respectively.[6]
Zn2+ + 2e− ↔ Zn (−0.76 V vs. NHE)
At the positive electrode (cathode) (titanium based materials or carbon felt electrode), Ce(III) oxidation and Ce(IV) reduction take place during charge and discharge, respectively.
2Ce3+ − 2e− ↔ 2Ce4+ (between +1.28 and +1.72 V vs. NHE)
Because of the large cell voltage, hydrogen (0 V vs. NHE) and oxygen (+1.23 V vs. NHE) could evolve theoretically as side reactions during battery operation (especially on charging).
The zinc–cerium redox flow battery was first proposed by Clarke and co-workers in 2004,[7][8] which has been the core technology of Plurion Inc. (UK). In 2008, Plurion Inc. suffered a liquidity crisis and was under liquidation in 2010. However, the information of the experimental conditions and charge-discharge performance described in the early patents of Plurion Inc. are limited. Since the 2010s, the electrochemical properties and the characterisation of a zinc–cerium redox flow battery have been identified by the researchers of Southampton and Strathclyde Universities. During charge/discharge cycles at 50 mA cm−2, the coulombic and voltage efficiencies of the zinc–cerium redox flow battery were reported to be 92 and 68 %, respectively.[9] In 2011, a membraneless (undivided) zinc–cerium system based on low acid concentration electrolyte using compressed pieces of carbon felt positive electrode was proposed. Discharge cell voltage and energy efficiency were reported to be approximately 2.1 V and 75 %, respectively. With such undivided configuration (single electrolyte compartment), self-discharge was relatively slow at low concentrations of cerium and acid.[10] Major installation of the zinc–cerium redox flow battery was the > 2 kW testing facility in Glenrothes, Scotland, installed by Plurion Inc.