# Metal-Organic Frameworks Toward Electrocatalytic Applications
## 2 Charge Transport in Metal-Organic Frameworks (MOFs)
### Redox Hopping  
Redox-hopping mechanism occurring in polymeric thin films has been known for more than two decades [32; 33]. At an appropriate applied potential, redox hopping can happen in the polymer with anchored redox-active sites that are spatially close to each other and possess a similar redox potential; the transport of electrons can oxidize or reduce all the redox-active sites and render the entire polymeric thin film electrochemically addressable. The diffusion of counter ions from the bulk electrolyte also plays an important role in the redox-hopping process. Counter ions need to diffuse through the polymeric thin film and adsorb on or desorb from the redox-active sites in order to maintain the local electroneutrality and enable the redox hopping to take place smoothly. Similarly, the redox-hopping mechanism applies to MOFs. When redox-active linkers or metal-base clusters are presented as repeating units within the framework, the transport of electrons via redox hopping may take place to render the MOF thin film electrochemically addressable at a reasonable applied potential. As the physical movement of the redox-active sites within most rigid MOFs is theoretically impossible, the overall rate of the electrochemical process presented in such redox-active MOF thin films is controlled by either the rate of charge hopping rate, the rate of diffusion for counter ions, or the total amount of redox-active sites presented within the MOF thin film. The redox-hopping behavior in MOF thin films was first observed by [PERSON] et al. in 2012 [34]; the redox-active linkers presented in a zinc-based MOF can enable charge transport and render the MOF thin film electrochemically active. Similar behaviors were also observed by [PERSON] et al. [35]. In the study, the redox-active naphthalene dimide (NDI)-based linkers were utilized to electrochemically address most linkers presented in the zinc-based MOF thin film, and such MOF thin films were found to exhibit electrochromic behavior. In 2013, [PERSON] et al. reported the similar findings in the thin film of a cobalt-based MOF constructed from redox-active porphyrinic linkers, and investigated the charge hopping mechanism in the MOF thin film via electrochemical and spectroelectrochemical approaches [36]. By utilizing the redox-hopping mechanism, even though the framework is not electrically conductive, the MOF thin film may still show a remarkable electrochemical performance in the presence of counter ions and an applied potential. For example, in 2013, [PERSON] et al. observed the redox-hopping behavior in the thin films of a zirconium-based MOF (Zr-MOF), NU-901 (Figure 2) [37]. Although NU-901 is electrically insulating [38], by electrochemically oxidizing the pyrene-based linkers in the presence of counter ions in the electrolyte, more than 90% of the pyrene units within the micrometer-thick MOF thin film can be electrochemically addressable; the resulting MOF thin film exhibits remarkable electrochromism with reversibility. Such redox-hopping strategies have been widely utilized in a range of MOF thin films for electrochemical purposes and their mechanistic insights have been investigated during recent years [39; 40; 41; 31]. Very recently, [PERSON] et al. found that the thin films of a Zr-MOF constructed from NDI-based linkers can perform stable redox-hopping behavior in aqueous electrolytes [42]. It should be noted that since the charge transport in these MOFs relies on the redox reaction of the repeating units within the framework, the redox hopping may not be stable during electrochemical operations if the redox reaction is not reversible in certain electrolytes; the charge transport may be ceased to work at this situation even though the framework is still structurally intact. In some examples, the redox hopping in MOF thin films is not efficient enough, which strongly limits the overall reaction rate of the electrocatalytic process and hinders the corresponding electrochemical performance [43; 44].