Energy and the Physical Sciences: Hydrogen Production using a Proton Electron Buffer

We propose to develop 'proton-electron-buffers' (PEBs) using redox-active polyoxometalate (POM) clusters that will be able, for the first time, to address the problem of simultaneous oxygen and hydrogen production during the electrolysis of water. It is anticipated that the use of a PEB in the water-splitting reaction will allow new catalysts, electrodes, and device architectures to be employed in electrolysers, and we will investigate both these new designs and the use of PEBs with exisiting electrolyser technology. Using a PEB in an electrolyser could also bring significant advantages with regards to intermittent power supplies (such as renewables) by reducing the instantaneous voltages required for electrolysis to occur. There could also be significant advantages In addition to exploring water splitting through the paradigm of the proton-electron-buffer, we will also explore the use of reduced polyoxometalate clusters as an intermediate "fuel source", by reacting the reduced PEBs with reducible chemical substrates to produce storable fuels. Thus this work could pave the way to a totally new route to 'clean' low-carbon H2 production temporally separated from the production of oxygen, as well as reducing energy consumption through technological advances informed by a whole system understanding as highlighted by the RCUK Energy Programme.

In 2011, total worldwide hydrogen production exceeded 31 million metric tons, making hydrogen the world's most produced chemical in terms of number of moles made (> 15.5 trillion moles). Over 80% of this was used in the Haber-Bosch process to make ammonia (and thus the fertilisers upon which the earth's population rely) and the majority of the balance was used in the petro-chemicals industry. However, these already large hydrogen production figures will be dwarfed if and when the "hydrogen economy" becomes a reality, or if the production of fuel-stuffs such as methanol from hydrogen and CO2 becomes economical. A hydrogen and/or methanol economy would make sense from the point of view of both energy security and the environmental impact of continued fossil fuel use. However, both of these arguments are only true if the hydrogen can be sourced in disparate locations and in a renewable fashion. Currently, well over 90% of the world's supply of hydrogen actually comes from fossil fuels in the first place, and so can be considered neither environmentally friendly nor "secure". One route that can deliver hydrogen renewably and almost anywhere is the electrolysis of water. This produces hydrogen and oxygen, and when the hydrogen is subsequently burnt in air or in a fuel cell the only products are water and energy. Hence there is no chemical pollution in this energy storage cycle, provided that the power input used to initially electrolyse the water came from a renewable source. Proton-electrolyte-membrane electrolysers (PEMEs) are favoured for this renewable-driven water electrolysis, because they respond well to the variable and intermittent power delivered by renewable power sources (wind, solar, etc.). During the electrolysis, water is oxidised to give oxygen in one half of the cell and hydrogen in the other half. PEMEs tend to use precious metal electrodes and, in order to offset the costs involved, they tend to operate in fairly compact formats with both gases made at high current densities and high pressures in their respective sides of the cell. Although the two cell compartments are separated by an ostensibly gas-impermeable membrane (typically Nafion), at the high pressures reached (200 bar), hydrogen can permeate through the membrane, with attendant membrane degradation, efficiency losses due to parasitic hydrogen oxidation at the anode and ultimately requiring the electrolyser to be shut down to prevent explosive oxygen/hydrogen mixtures forming in the cell head-space. Despite these drawbacks, the US DOE believes high-pressure electrolysis to be amongst the most promising candidates for low-cost and sustainable hydrogen production for the hydrogen economy, if the issues surrounding gas-mixing and concomitant electrolyser degradation can be surmounted. In this research we propose to address the problems described above by developing a 'proton-electron-buffers' (PEBs) using redox-active polyoxometalate (POM) clusters that will be able to address the problem of simultaneous oxygen and hydrogen production during the electrolysis. It is anticipated that the use of a PEB in the water-splitting reaction will allow new catalysts, electrodes, and device architectures to be employed in electrolysers, and we will investigate both these new designs and the use of PEBs with exisiting electrolyser technology. In addition to exploring water splitting through the paradigm of the proton-electron-buffer, we will also explore the use of reduced polyoxometalate clusters as an intermediate "fuel source", by reacting the reduced PEBs with reducible chemical substrates to produce storable fuels. We therefore think the impact of this research could be large in both academia, industry, and the development of new electrolyzer technologies and we describe the routes to maximise impact in our pathways to impact document including our collaboration with ACAL energy, who are already using Polyoxometalates in next generation fuel cells.