## 2 MOFs for Fuel-Cell Applications
### MOFs as Electrolyte Application  
MOFs with high proton conductivities have recently attracted a lot of attention due to their outstanding properties. The coordination skeleton can deliver protons directly or through carrier particles [85,86]. MOFs, however, are restricted in their migration of proton conductors as a result of their grain-boundary structure, which leads to an insufficient conductivity. Furthermore, because of the unique and diverse crystal structures of MOFs, it is extremely hard to directly process them for direct use in fuel cells [87]. A practical solution to this problem is to combine MOFs with materials of a polymeric nature and their advancement as composite membranes [85,88]. It is generally possible to divide the PEMs modified by MOFs into two basic types. Phytic@MIL, PIL@MIL, and acids@MIL are some of the proton carriers that have been immersed in the pores of MOFs that can be used in this method. Moreover, an alternative approach is to augment the hydrophilic and acidic properties of MOFs by adding useful chemical components (-SO\({}_{3}\)H, -NH\({}_{2}\), -2 COOH) to the organic ligands [89, 90, 91]. Despite the excitement surrounding these findings, reports in this direction are scarce with much to discover about various MOFs. There will be a brief summary of the performance enhancements of PEMs by using different MOFs in this section, including the UIO-series, MIL-series, ZIF-series and other MOFs. A MOF has the advantage of being able to be customized to meet the specific requirements of any application. Because of their specific properties, MOFs might possess the ability to enhance nafion functionality, which is a state-of-the-art electrolyte, and thus improve water containment in structures and the proton conductivity of nafion-based layers. Polymers and MOF composites can normally be improved in their proton conductivity in two separate ways: either by saturating their pores with protons or by modifying their organic groups with functional moieties to provide them with increased hydrophobicity and acidity [92,93]. In this study, [PERSON] et al. developed a high-performance composite membrane made from nafion and MOFs. They were interested in illustrating that graphene oxide (GO) can be grown on a porous metal-organic framework (ZIF-8) and subsequently be introduced into the nafion matrix. It was also found that ZIF-8 and GO had a synergetic effect that improved the proton conductivity by a significant amount [92]. In order to improve PEMFC performance, UIO-66 with high stability has been extensively studied. Recent research by [PERSON] et al. revealed the use of nafion membranes for obtaining graphene nanosheets covalently bonded to UIO-66 (Figure 6a-c) [94]. The aim of this study was to prepare different sizes of NH\({}_{2}\)-UiO-66 for use in this experiment (40, 90, and 140 nm), which were then attached to polydopamine-coated GO. Interestingly, the conductivity varied with the filler content, which reached its maximum value when it was at 0.6 wt.%. In addition, it also depended on the MOF size, which was ideally 40 nm. MOFs have not been studied in detail for their mechanical properties or stability. In contrast to nafion membranes with smaller NH\({}_{2}\)-UiO-66 (90 nm) and 140 nm, the proton permeability of nafion membranes with larger NH\({}_{2}\)-UiO-66 (90 nm) was lower, most likely related to the large interconnection of NH\({}_{2}\)-UiO-66 and the GO substrate, which may lead to an interruption in proton-transfer channels. As a result of UiO-66's remarkably high stability, the material has been extensively studied for potential applications in PEMCs. Recently, an organic material designated porous organic cages has been shown to be a potential filler for proton-exchange membranes. The in situ crystallization of nafion matrixes with Cage 3 attached has been reported as shown in Figure 7a,b [95]. Cage 3 is known to possess intrinsic 3D channels that may assist proton conduction.