Energy storage is a key enabling technology that can significantly reduce the whole energy system cost of deploying large amounts of renewables. Electrochemical energy storage devices are one of the most efficient ways of converting and storing electrical energy. Out of many options, redox flow batteries (RFB) or regenerative fuel cells are one of the most promising for large scale energy storage, especially for high energy applications. Although a number of different chemistries have been explored, a major breakthrough is still required in order to fully deliver on their promise. In this work a novel chemistry, based upon hydrogen and cerium, is reported with the potential to deliver high energy density, low cost and high performance. The novel chemistry benefits from multi-electron transfer reactions and the use of alternative material to the typical carbon based electrodes. A global target has been set to reduce greenhouse gas emissions by 40% to 70% by 2050 compared to 2010 levels. 1 However energy generation from renewable sources suffers from their intermittent and unreliable nature. Hence large-scale energy storage has become a vital technology in order to meet the set targets through the development of smart grids. Redox flow batteries (RFBs), like other electrochemical storage systems, have the ability to convert chemical energy into electrical energy; often referred to as reversible or regenerative fuel cells, they typically utilise two soluble redox couples contained in separate electrolyte tanks external to the battery system. Each electrolyte is pumped through their respective half-cell such that energy is stored/released as the RFB is charged/discharged.3 These two halfcells are separated by an ion exchange membrane that allows selective ions to pass from one electrolyte to another, with the electrons passing through an external circuit.4 The crucial requirements for grid level energy storage applications are all met by RFBs, namely fast response time (~milliseconds), site independence, low environmental footprint, high depth of discharge, high reliability, high round trip efficiency (ca. 85%) and long life cycle (>>13 000 cycles).5-7 Additionally, they are highly modular as the electrolyte volume and concentration determine the energy of the system, while the power depends on the stack size and the active surface area of each cell 8 , hence RFBs are used for a wide range of energy storage applications. A large number of redox couples has been investigated in RFB research such as Fe/Cr, Poly-Sulphide/Br 2 , Zn/Cl 2 , Zn/Ce, allvanadium redox battery (VRB), V/Ce and Zn/Br 2 .5 However, only the VRB and the Zn/Br 2 systems have been developed to a commercial level, and are currently exploited in applications such as load levelling, power quality control, and solar and wind deployment. 9Recent developments in RFB research has shown increasing interesting redox couple combinations that deliver high cell voltage and hence high energy density systems.10 Among them the cerium couple Ce(III)/Ce(IV) has bee...

In this study, a time dependent model for a regenerative hydrogen-vanadium fuel cell is introduced. This lumped isothermal model is based on mass conservation and electrochemical kinetics, and it simulates the cell working potential considering the major ohmic resistances, a complete Butler-Volmer kinetics for the cathode overpotential and a Tafel-Volmer kinetics near mass-transport free conditions for the anode overpotential. Comparison of model simulations against experimental data was performed by using a 25 cm 2 lab scale prototype operated in galvanostatic mode at different current density values (50 − 600 A m −2 ). A complete Nernst equation derived from thermodynamic principles was fitted to open circuit potential data, enabling a global activity coefficient to be estimated. The model prediction of the cell potential of one single charge-discharge cycle at a current density of 400 A m −2 was used to calibrate the model and a model validation was carried out against six additional data sets, which showed a reasonably good agreement between the model simulation of the cell potential and the experimental data with a Root Mean Square Error (RMSE) in the range of 0.3-6.1% and 1.3-8.8% for charge and discharge, respectively. The results for the evolution of species concentrations in the cathode and anode are presented for one data set. The proposed model permits study of the key factors that limit the performance of the system and is capable of converging to a meaningful solution relatively fast (s-min). Redox flow batteries are considered to be an exceptional candidate for grid-scale energy storage. One attractive feature is their capability to decouple power and energy.1-4 All-Vanadium Redox Flow Batteries (VRFBs) have been considered a promising system due to the limited impact of cross-contamination. However, they have faced challenges related to cost, scale-up and optimization. Current research is also focused on improvement of electrolyte stability for use over a wider temperature window and concentrations, development of electrode materials resistant to overcharge, and mitigation of membrane degradation.1,2 Cost dependency with regarding to vanadium can be mitigated through utilization of new systems that employ only half of the vanadium.1 Recently, a Regenerative Hydrogen-Vanadium Fuel Cell (RHVFC) based on an aqueous vanadium electrolyte V(V) and V(IV) and hydrogen has been introduced 5 and is illustrated schematically in Figure 1. This system contains a porous carbon layer for the positive electrode reaction, membrane and catalyzed porous carbon layer for the negative electrode reaction. Hydrogen evolution, which is an adverse reaction in VRFBs, is here the main anodic process. During discharge, V(V) is reduced to V(IV) and H 2 is oxidized, while the reverse process occurs during charge and H 2 is stored. The vanadium reaction takes place in the positive electrode (cathode), while the hydrogen reaction occurs in the catalyst layer (CL) of the negative electrode (anode). The redox reactions that occur ...

The positioning of reference electrodes in redox flow batteries without disturbing the cell operation represents a great challenge. However decoupling anode and cathode processes is crucial in order to fully understand the losses in the system so it can be further optimized. The feasibility of a regenerative fuel cell based on an V(IV)/V(V) electrolyte and hydrogen gas has previously been demonstrated. In this investigation, using electrochemical impedance spectroscopy, the various losses of the cathode, anode and whole cell were established using an alternative reference electrode set-up. The findings showed that the largest irreversible losses under the conditions tested arose from diffusion limitations in the cathode and the effect of vanadium crossover and therefore adsorption onto the platinum layer of the hydrogen electrode leading to higher losses on the anode. These results highlight the potential for further improvement and optimization of cell design and materials for both electrodes in the Redox flow batteries (RFBs) also called regenerative fuel cells are one of the promising candidates for large scale energy storage. They offer the ability to convert electrical energy into chemical energy which is stored in external tanks containing two redox couples. The anolyte and catholyte are respectively pumped through the anode and cathode of the electrochemical cell where at discharge the chemical energy is converted back to electrical energy.1 One of the attractive features of these batteries is their flexibility in decoupling power and energy as the power is determined by the stack size and active surface area of each cell while the energy available depends on the electrolyte volume and concentration.2 Furthermore, RFBs also present additional advantages such as fast response time (∼milliseconds), site independence, low environmental footprint, high depth of discharge, high reliability, high energy efficiency (ca. 85%) and long life cycle (>13 000 cycles). 3-5The all-vanadium redox battery (VRB), initially developed by Skyllas-Kazacos, is regarded as one of the most promising RFBs and is already available commercially where it is utilized for load levelling, power quality control and renewable energy deployment. 6,7 Expertise gained on proton exchange membrane fuel cells (PEMFCs) can be applied to RFB research due to their similarities. Accordingly, an increase in hydrogen based hybrid RFBs has been observed in recent years for example with systems such as H 2 /Fe, H 2 /Br 2 , H 2 /Ce, as well as the H 2 /V discussed here. [8][9][10][11][12][13] Those systems benefit from the fast kinetics of the hydrogen reaction, and absence of crosscontamination through mixing of liquid anolyte and catholyte. While the crossover of the catholyte is still possible, it can be collected on the anode side and pumped back.In order to decouple the anode and cathode processes, reference electrodes (REs) are usually incorporated into RFB rigs. Similarly to fuel cells, the positioning of a reference electrode in an RFB is not a ...