Department of Chemistry and Macromolecules Innovation Institute, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA.
Chem Commun (Camb). 2018 Jun 21;54(51):6965-6974. doi: 10.1039/c8cc01664j.
The dominant charge transfer mechanism in a vast number of metal-organic frameworks (MOFs) is that of redox hopping, a process best explained through the motion of electrons via self-exchange reactions between redox centers coupled to the motion of counter-balancing ions. Mechanistic studies of redox hopping transport in MOFs reveal characteristics that recall pioneering studies in linear redox polymers. When MOFs are employed as electrocatalysts, consideration must be given to both the catalytic properties - turn-over frequency (TOF) and energetic requirements (overpotential, TON) - and the charge transport properties - rate of charge hopping, measured via an apparent diffusion coefficient (Dapp). Herein, we provide a mathematical framework to provide constraints to MOF catalyst development by relating Dapp, TOF, and film thickness in the context of providing 10 mA cm-2 of catalytic current. Lastly with the mechanistic studies discussed as a foundation, design rules for future MOF electrocatalysts are provided and the challenges to the community to optimize MOF charge transport are laid out.
在大量金属-有机骨架（MOFs）中，占主导地位的电荷转移机制是氧化还原跳跃，通过氧化还原中心之间的自交换反应来解释电子的运动，这种反应与抗衡离子的运动相耦合。MOFs 中氧化还原跳跃输运的机理研究揭示了一些与线性氧化还原聚合物的开创性研究相呼应的特征。当 MOFs 被用作电催化剂时，必须考虑到催化性能—— turnover frequency（TOF）和能量需求（过电势、TON）——以及电荷输运性能——通过表观扩散系数（Dapp）测量的电荷跳跃速率。在此，我们提供了一个数学框架，通过将 Dapp、TOF 和薄膜厚度联系起来，在提供 10 mA cm-2 的催化电流的情况下，为 MOF 催化剂的发展提供了限制。最后，在讨论了机理研究的基础上，为未来的 MOF 电催化剂提供了设计规则，并为该领域提出了优化 MOF 电荷输运的挑战。
