Polyoxometalates for energy storage applications

Abstract

In this thesis, the use of polyoxometalates (POMs) in fuel cells and redox flow batteries (RFBs) is explored. Regarding their use in fuel cells, the primary topic is the development of a method for characterising the kinetic parameters of homogeneous biomass oxidation by POMs using electrochemical techniques. The method involves allowing biomass oxidation to proceed for a fixed period of time followed by applying an anodic current to the solution which results in oxidation of the POM back to its initial state. Phosphomolybdate (P Mo12) and glucose are used as example species and the kinetic parameters (rate constants, Arrhenius factor, activation energy) are determined using this technique. It is also shown that as the glucose becomes oxidised the reaction rate slows appreciably to the point where it effectively stalls, leaving a mixture of partially oxidised glucose and POMs which is difficult to separate. Consequently, the immobilisation of POMs directly onto an electrode surface was inves- tigated. The aim was to develop a heterogeneous catalyst so that the separation of POMs and biomass was less problematic. It was shown that TBA3PMo12O40 could be deposited on a carbon felt electrode by soaking the electrode in an acetonitrile solution containing a small quantity of the POM followed by evaporating the solution. Analysis of the cyclic voltam- mograms showed that, at room temperature, the POM was indeed confined to the electrode surface and remained there over a long period of time and over repeated electrochemical oxi- dation and reduction. The mass of POM deposited on the electrode surface was also estimated using voltammetry. Methods for increasing the accessible capacity of polyoxometalate redox flow batteries are also investigated using 3-electrode bulk electrolysis cells and 2-electrode flow batteries. The methods for increasing accessible capacity mainly involved altering the supporting elec- trolyte properties. The use of phosphomolybdic acid as a redox flow battery electrolyte is also explored. It is shown that P Mo12 electrolytes have a high capacity retention and coulombic efficiency when used as a redox flow battery anolyte. A flow battery utilising 1 mM solu- tion of P Mo12 as the anolyte underwent 142 charge/discharge cycles without capacity loss. On testing a solution with a higher concentration of P Mo12, the capacity diminished more rapidly and the accessible capacity was reduced but the results are still relatively promising. The use of silicotungstic acid (SiW12) as a redox flow battery anolyte is also investigated, building on the work reported previously by Stimming group. [1, 2] It is shown here that the accessible capacity can be increased by a factor of 6 by adjusting the pH of the electrolyte. Cyclic voltammetry and bulk electrolysis experiments of SiW12 indicate that the POM can be reduced and oxidised by at least 12 electrons per molecule reversibly. Many of the redox waves associated with aqueous solutions of SiW12 are in the -1.2–0.5 V vs SHE range, which allowed aqueous RFBs with a potential difference of ≥ up to 1.5 V to be demonstrated. The use of polyamide based nanofiltration membranes as redox flow battery separators is briefly explored. The rationale for this was that the pores in the nanofiltration membrane are small enough to allow charge carriers such as (H+, Li+, Na+ etc. through, but not POMs, which are substantially larger. The results showed that POMs which exist in equilibrium with smaller ions (specifically [HxPV14O42]9–x (P V14) are not compatible with this type of membrane because the smaller species are able to cross over. Furthermore, upon exposure to aqueous solutions the polyamide membrane appeared to rot after a period of weeks. A calculator for redox flow batteries is presented which takes electrochemical parameters of the electrolytes, electrodes, and membranes, as well as the costs of critical components and returns the capital cost of a system with a breakdown of each component cost for any given power, energy, and efficiency combination. The capital cost of various different systems including iron-chrome, all vanadium, various POM batteries, and the AQDS-HBr batteries are compared and contrasted. A theory of the thermodynamic properties of species with multiple redox couples is pre- sented and verified. The well known Nernst equation allows the reduction potential of a single redox couple/reaction to be calculated. The theory described in chapter 3 builds on the Nernst equation to arrive at a series of equations which can describe an electrochemical system with any number of redox couples, meaning that the reduction potential of species such as POMs (which have many possible oxidation states) can be calculated precisely. More- over, if the reduction potential and the standard reduction potentials are known, then the concentration of species in each oxidation state can also be precisely calculated. The method presented in chapter 3 removes a lot of the guess work in typical Tafel analysis. The theoretical predictions of this model are verified experimentally. Using this theory in conjunction with rotating disk electrode experiments we show that it is possible to calculate the rate constants, transfer coefficients, and mass transport limiting current of the redox reactions of silicotungstic acid on a glassy carbon electrode. Finally, an electrochemical model which describes the behaviour of POM redox flow bat- teries is presented and briefly verified against literature data. The model makes use of the equations presented in chapter 3 of this thesis, a Butler-Volmer description of the overpo- tential according to the kinetic and mass transport characteristics of the electrolyte, as well as diffusion, migration, and convection of species within the electrodes and through the mem- brane. It is capable of generating a variety of data including a prediction of the voltage vs capac- ity, concentration of various species with respect to their position in the electrode, expected power output, and amount of crossover of various species. The primary contributions of this thesis are the development of a variety of electrochem- ical techniques and theory which can be used to investigate and explain the properties of polyoxomletalates in energy applications. In summary, this thesis builds upon the work pre- viously carried out by Stimming group on polyoxometalate redox flow batteries, as well as contributing towards the improvement of biomass fuel cells, and the understanding of fun- damental electrochemical principles that apply to polyoxometalates and various other elec- trochemically active species.