Ultra-Reduced Polyoxometalates as Electron-Coupled-Proton-Systems for Energy Storage

As our reliance on renewable energy sources grows, so too does our need to store this energy in order to store excess energy, & also respond when demand exceeds the generating capacity in the system. Amongst the numerous solutions that have been proposed for this challenge, two stand out in terms of their flexibility and scalability: storage of energy as electrical charge in batteries, and storage of energy via conversion to chemical fuels. Both of these approaches bring their own unique set of advantages and drawbacks, and it is often not obvious as to which would make the better choice in any particular circumstance. Against this background, energy storage solutions that can act as both batteries and fuel generation devices (depending on the user's requirements) could have a transformative effect on how renewable energy is utilised. For renewable fuel generation, the electrolysis of water to give hydrogen fuel is attractive. However, renewables tend to be intermittent giving serious problems when operating conventional electrolysers using such stop/start inputs, such as unacceptably high levels of mixing of the product gases and accelerated degradation of expensive cell components. Previously, we showed how low-power energy inputs (characteristic of renewables) could be used to electrolyse water to produce pure hydrogen and oxygen regardless of the electrolytic current density by employing a polyoxometalate cluster as soluble redox mediator (an "Electron-Coupled-Proton Buffer", ECPB) in a new type of electrolyser device. This also enabled a new approach to be taken to on-demand hydrogen production via electrolysis: the hydrogen can now be produced remotely from the electrochemical cell over a fixed catalyst bed, increasing the rate of H2 production by a factor of over 30 compared to state-of-the-art proton exchange membrane electrolysers at equivalent catalyst loadings.

However, our previously-reported systems all suffer from rather low electron storage densities: normally only two electrons can be stored reversibly per mediator molecule, which means that large volumes of solution are required for decoupled electrolytic hydrogen production. The large volumes of solution involved also preclude the use of the reduced electrolyte as an energy storage medium in its own right: as so much liquid is needed to store a few electrons it is not practical to use this as a long-term energy carrier (e.g. in a redox flow battery). If the number of electrons stored per mediator molecule could be increased by an order of magnitude, then one would have a viable electrolyte system which could be reduced in an electrochemical device using renewable power inputs, and then directed either to decoupled hydrogen (fuel) production or used as a high energy-density electrolyte in a redox flow battery (direct energy storage), see Figure 1. Such a system would have the potential to completely revolutionise the storage of renewable energy.
Here, we aim to investigate a new range of polyoxometalates as redox mediators that can be reduced by at least 18 electrons per molecule. Preliminary results indicate that the some POMs can be reversibly reduced and re-oxidised by at least this number of electrons in aqueous solution, provided that the concentration is high and the pH is kept below a certain value. With this as our starting point, we will use our expertise in the construction of polyoxometalate-based electrochemical devices to develop systems that can hold an ever-greater number of electrons per volume of electrolyte. At a fundamental level, we will apply a battery of cutting-edge techniques to unravel the underlying causes of the remarkable stability of these ultra-reduced species in aqueous solution, and develop models that explain the nature of these species. We will explore the use of new POM-based materials and device architectures in order to produce energy storage systems with the maximum flexibility and energy density.

Energy storage is a vital component of any energy supply system where renewable energy is a significant component. However, in many situations it is not obvious whether it is more effective to store this energy directly (in batteries for example) or to store it by conversion to a chemical fuel (such as hydrogen). This proposal describes a new concept in energy storage by developing the concept of the electron-coupled-proton buffer (ECPB) which we previously discovered with EPSRC support (EP/K023004/1; see Science, 2014, 345, 1326-1330 and Nature Chem. 2013, 5, 403-409). In the current proposal, we will explore the ability of ECPBs to act as both as mediators for electrolytic hydrogen production and as energy storage vectors in their own right (as the electrolyte in a redox flow battery), based on fundamental studies into the nature and stability of the reduced states of the polyoxometalates.

The need for better energy storage systems is a priority for the UK as described in Government's Industrial Strategy. Much of this stems from the potential of hydrogen as a clean-burning fuel. Likewise, interest in redox flow batteries as a means to iron-out peaks and troughs in electricity supply ("grid balancing") for both large-scale and more distributed power generation systems is at an all-time high. The UK is committed to a target of reducing carbon dioxide emissions by 80% by 2050, with an increased uptake of renewably-generated energy being a cornerstone of its strategy. It is, therefore, in the UK's national interest to support research into more flexible and efficient means of storing this renewable energy, which includes the development of new energy storage and conversion systems. Thus this research has the potential to directly benefit society in the UK (and worldwide), by enabling new methods for the storage of renewably-generated power, thus making increased reliance on this renewable power a more realistic proposition.

We will collaborate with a number of companies including BAE-systems, and also seek to transfer technology to spin-outs. The IPGroup, a recent investor in our lab has already helped us spin out a company with £5M of investment and we will aim to replicate this success with this project.