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

随着我们对可再生能源的依赖日益增长，我们也需要储存这些能源，以储存多余的能源，并在需求超过系统的发电能力时做出反应。在针对这一挑战提出的众多解决方案中，有两种解决方案在灵活性和可扩展性方面脱颖而出：将能量存储为电池中的电荷，以及通过转换为化学燃料来存储能量。这两种方法都有其独特的优点和缺点，而且在任何特定情况下，哪一种方法是更好的选择通常并不明显。在这种背景下，既可以作为电池又可以作为燃料发电设备的储能解决方案（取决于用户的要求）可能会对如何利用可再生能源产生变革性的影响。对于可再生燃料发电，电解水产生氢燃料是有吸引力的。然而，当使用这种停止/启动输入操作传统电解槽时，可再生能源往往是间歇性的，这会带来严重的问题，比如产品气体的混合水平高得令人无法接受，以及昂贵的电池组件的加速降解。之前，我们展示了如何使用低功率能量输入（可再生能源的特征）来电解水，以产生纯氢和氧，而不考虑电解电流密度，通过在新型电解槽装置中使用多金属氧酸盐簇作为可溶性氧化还原介质（“电子耦合质子缓冲液”，ECPB）。这也为通过电解按需制氢提供了一种新方法：氢气现在可以在固定的催化剂床上从电化学电池远程生产，与同等催化剂负载的最先进的质子交换膜电解槽相比，氢气的生产速度提高了30倍以上。然而，我们之前报道的系统都有相当低的电子存储密度：通常每个介质分子只能可逆地存储两个电子，这意味着解耦电解制氢需要大量的溶液。所涉及的大量溶液也排除了将还原电解质作为其自身权利的能量存储介质的使用：由于需要如此多的液体来存储少量电子，因此将其用作长期能量载体（例如在氧化还原液流电池中）是不切实际的。如果每个介质分子存储的电子数量可以增加一个数量级，那么就会有一个可行的电解质系统，可以在使用可再生能源输入的电化学装置中减少，然后直接用于解耦氢（燃料）生产或用作氧化还原液流电池（直接储能）中的高能量密度电解质，见图1。这样一个系统将有可能彻底改变可再生能源的储存方式。在这里，我们的目标是研究一种新的多金属氧酸盐作为氧化还原介质，每个分子可以被至少18个电子还原。初步结果表明，在水溶液中，只要浓度高且pH值低于一定值，某些POMs可以被至少这个数目的电子可逆还原和再氧化。以此为出发点，我们将利用我们在构建多金属氧酸盐电化学装置方面的专业知识，开发出每体积电解质可以容纳更多电子的系统。在基础层面上，我们将应用一系列尖端技术来揭示这些超还原物种在水溶液中显着稳定性的潜在原因，并开发解释这些物种性质的模型。我们将探索使用新的基于pom的材料和器件架构，以生产具有最大灵活性和能量密度的储能系统。

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.