(103g) High Voltage Vanadium-Metal Hydride Hybrid Flow Battery As Electrochemical Energy

By coupling the advantages of the vanadium redox flow battery and the acid/alkaline hybrid battery concepts, a novel Vanadium-metal hydride (V-MH) hybrid rechargeable flow battery is developed. The V-MH flow battery consists of graphite felt positive electrode in flowing VOSO₄-H₂SO₄ electrolyte and metal hydride negative electrode in flowing KOH, the two flowing streams are separated by a bipolar membrane. Hybridizing the V⁴⁺/V⁵⁺ coupled with metal hydride eliminates the problem of V²⁺ oxidation as in VRF battery. In the meantime, the higher voltage and higher capacity achieved is a result of the extra energy store/released in acid-alkaline neutralization corresponding to =-79.85 kJ/mol.

The charge and discharge electrode reactions in this acid/alkaline Vanadium-Metal Hydride flow system are as follows:

Graphite positive electrode:

VO₂⁺(aq.) + 2H⁺(aq.) + e^-  ↔ VO²⁺(aq.) + H₂O  (E^o=1.0 V)

Metal Hydride negative electrode:

MH_x + OH^-(aq.) ↔ MH_x-1 + H₂O + e^- (E^o= -0.8 V)

These two electrodes are separated by a bipolar membrane, which provides ionic contacts and acts as barrier for proton and hydroxide ions transport in the pH differential cells.

Overall Discharge(®) and Charge(¬) reactions are

VO₂⁺(aq.) + 2H⁺(aq.) + MH_x + OH^-(aq.) ↔ VO²⁺(aq.) + MH_x-1 + 2H₂O  (E^o=1.8 V)

In principle, this hybrid battery system of V⁴⁺/V⁵⁺ and metal hydride can deliver an overall cell capacity of 110 mAh/g, cell voltage of 1.8 V, and a much higher energy density of 200 Wh/kg than that of the conventional all vanadium redox flow (VRF) battery (60.5 Wh/kg). The high voltage (1.8 V) reported is among the highest in the Vanadium-based hybrid flow systems, about 40% higher than conventional VRF battery (1.4 V) and nickel metal hydride battery (1.2 V). The electrochemical properties which includes the charge/discharge characteristics, cycle test and discharge polarization curves, of this new hybrid flow battery at different flow rates and electrolyte concentrations will be demonstrated and optimized. This V-MH flow battery has a huge potential to implement as effective electrochemical energy storage system from intermittent energy sources like wind and solar power.

The next stage of this V-MH battery would be to replace the metal-hydride electrode by hydrogen, resulting in a Vanadium-Hydrogen (V-H₂) flow battery. Since alkaline hydrogen electrode has an extremely high capacity and favorable reversible electrochemical kinetics, further increase the energy and power density of the system. In theory, this V-H₂ battery system can yield a cell voltage of 1.83 V, and reach a higher energy density of about 300 Wh/kg (which is five times that of VRF battery).

Acknowledgements

This research is supported by Innovation Technology Fund of Hong Kong (ITF ITS/290/12), Research Grants Council of Hong Kong (GRF HKU 700209P and GRF HKU 700210P), University of Hong Kong SRT, University of Hong Kong Small Project Funding, The University Development Fund on the Initiative of Clean Energy and Environment, and Zentric Inc.

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