Electrogenerated hydrogen peroxide and fuel cells

2017-03-02T03:18:58Z (GMT) by McDonnell-Worth, Ciaran James
This thesis focuses on the electrochemical production of H₂O₂ via water oxidation and its subsequent use as a fuel in direct H₂O₂ fuel cells. H₂O₂ is a useful chemical both in industrial and scientific applications as it is not only a strong oxidant but also produces no harmful by-products when it is used as such. Shortly prior to the commencement of this thesis it was found that H₂O₂ may be produced electrochemically via water oxidation. This process only occurred in a specific electrolyte, namely one in which the salt contained an ammonium-based cation and free amine was present in solution. It was hypothesised that the presence of free amine helped to stabilise the H₂O₂. To further understand this phenomenon this research focused on changing the parameters involved in the water oxidation mechanism. This included using different ammonium-based salts in the electrolyte, measuring H₂O₂ production rates at different pHs and different oxidative potentials and observing the rate of H₂O₂ production over different oxidation times. It was found that H₂O₂ was only generated efficiently in a narrow range of oxidation potentials and pHs. It was proposed that the role of the free amine in solution was two-fold, one to help stabilise the H₂O₂ during water oxidation and prevent its further oxidation to O₂, and the other to act as a pH buffer at the surface of the water oxidation catalyst. The research went on to attempt to utilise this electrogenerated H₂O₂ in direct H₂O₂ fuel cells (DHPFCs). DHPFCs are a fascinating alternative to H₂ gas or methanol fuel cells as H₂O₂ can be used as both the fuel and the oxidant. This is because H₂O₂ can be reduced and oxidised relatively easily compared to other fuels. As a consequence of this DHPFCs may be designed and utilised in a number of different ways which has certain advantages in cost, efficiency and safety. As part of this thesis novel catalysts for H₂O₂ reduction and oxidation were tested as anodes and cathodes in DHPFCs and were found to perform well. Specifically, using a mixed cobalt oxide-carbon black film as a cathode in combination with a heat treated nickel foam anode was found to give quite high open circuit potentials compared to other catalysts that have been used previously in similar conditions. Ultimately, this thesis was able to demonstrate a fully reversible system in which H₂O₂ was generated electrochemically and then used to produce power in a single-electrolyte fuel cell without any processing of the fuel or electrolyte. This reversibility is important if such a system were to be used in real world applications and shows great promise for this technology.