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

随着我们对可再生能源的依赖增加，我们也需要储存这些能源来储存多余的能量，当需求超过系统的发电能力时也会做出反应。在针对这一挑战提出的众多解决方案中，有两个在灵活性和可伸缩性方面表现突出：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。这两种方法都有各自独特的优点和缺点，在任何特定情况下，哪种方法更好的选择往往并不明显。在这种背景下，既可用作电池又可用作燃料发电装置的储能解决方案(取决于用户的要求)可能会对可再生能源的利用方式产生革命性的影响。对于可再生燃料的生产，电解水得到氢燃料是很有吸引力的。然而，可再生能源往往是间歇性的，在使用这种停止/启动输入操作传统电解槽时会产生严重的问题，例如产品气体混合水平高得令人无法接受，以及昂贵的电池组件加速降解。以前，我们展示了如何通过在新型电解槽设备中使用多金属氧酸盐簇合物作为可溶性氧化还原介体(电子耦合质子缓冲器，ECPB)，在不考虑电解电流密度的情况下，如何利用低功率能量输入(可再生能源的特性)来电解水来生产纯氢和纯氧。这也使得一种通过电解按需制氢的新方法得以采用：现在可以通过固定催化剂床从电化学电池远程生产氢气，与同等催化剂负载的最先进的质子交换膜电解槽相比，氢气的产生速度提高了30倍以上。然而，我们之前报道的系统都存在电子存储密度较低的问题：通常每个介体分子只能可逆存储两个电子，这意味着解耦电解氢生产需要大量的溶液。所涉及的大量溶液也排除了将还原电解液本身用作能量存储介质的可能性：由于需要如此多的液体来存储几个电子，因此将其用作长期能量载体是不切实际的(例如，在氧化还原液流电池中)。如果每个介体分子存储的电子数能增加一个数量级，那么人们将拥有一个可行的电解液系统，它可以在使用可再生电力输入的电化学设备中被减少，然后被用于去耦合的氢(燃料)生产或用作氧化还原流动电池(直接储能)中的高能量密度电解液，见图1。这样的系统将有可能彻底改变可再生能源的存储。在这里，我们的目标是研究一系列新的多金属氧酸盐作为氧化还原介体，每个分子至少可以减少18个电子。初步结果表明，在水溶液中，只要浓度较高，pH值控制在一定值以下，某些多酚类化合物可以被至少这个数目的电子可逆地还原和再氧化。以此为起点，我们将利用我们在构建基于多金属氧酸盐的电化学设备方面的专业知识来开发能够在每体积电解液中容纳越来越多的电子的系统。在根本层面上，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具有非凡稳定性的根本原因，并开发解释这些物种性质的模型。我们将探索使用基于POM的新材料和设备架构，以生产具有最大灵活性和能量密度的能量存储系统。

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.