## 2 MOFs for Fuel-Cell Applications
### MOFs as Electrolyte Application
A composite membrane can be greatly enhanced in terms of water-retention capacity by integrating Cage 3 into the nafion matrix, which is typically degraded at 320 \({}^{\circ}\)C (Figure 7c). Even though Cage 3 is potentially useful as a PEM, no mechanical studies have been conducted to determine the impact of incorporating it into a polymeric matrix. There was a significant improvement in proton conductivity when Cage 3 was added, but there was no substantial improvement in the methanol permeability. Under the same operating conditions, a nafion membrane containing Cage 3 achieved 0.27 S cm\({}^{-1}\) with a mass fraction of 5% when heated to 90 \({}^{\circ}\)C and used at 95% relative humidity. This is a substantial improvement over recast nafion (0.08 S cm\({}^{-1}\)) under identical conditions (Figure 7d,e). Figure 6: (**a**) Conductivity measurement plot of protons (at 95% RH) of nafion and nafion/NH\({}_{2}\)-UiO-66@GO-fused membranes under variable loadings of NH\({}_{2}\)-UiO-66@GO. (**b**) Proton conductivity measurements (at 95% RH) of nafion and different nafion membrane compositions with 0.6 wt.% filler content. (**c**) Proton conductivity measurements (at 95% RH) of nafion and nafion/NH\({}_{2}\)-UiO-66@GO composite membranes with different NH2-UiO-66@GO sizes [94]. In addition, reports have been published about the use of non-classical MOFs, including nafion-based composites. Specifically, [PERSON] et al. have identified MOF-W\({}_{1}\)/nafion and MOF-W\({}_{2}\)/nafion as two composite membranes, as shown in Figure 8[96], which contain the same type of acid: 5,5\({}^{\prime}\)-(butane-1,4-diylbis(oxy)) di-isophthalic acid (H\({}_{4}\)BDD). It appears that the molecular system MOF-W\({}_{1}\) has binuclear molecules that are connected through hydrogen atoms, primarily hydrogen bonds between uncoordinated carboxylic groups and water molecules. The MOF-W\({}_{2}\) is a supramolecular two-dimensional chain formed from one-dimensional hydrogen bonds between the uncoordinated and coordinated carboxylate groups. MOF-W\({}_{1}\)/nafion and MOF-W\({}_{2}\)/nafion have proton conductivity values of 4.04 \(\times\) 10\({}^{-7}\) and 4.94 \(\times\) 10\({}^{-7}\), respectively, at 30 \({}^{\circ}\)C. Recently, [PERSON] et al. studied the  
Figure 7: (**a**) Schematic illustration of preparation of nafion-Cage 3 composite membrane; (**b,c**) water-uptake capacity [WU (wt.%)] and thermogravimetric analysis (TGA) data of recast nafion and nafion-Cage 3 composite membrane; (**d**) dependence of temperature (40% RH); (**e**) dependence of humidity (90 \({}^{\circ}\)C) proton conductivity of recast nafion and nafion-Cage 3 composite membrane [95]. impact of doping NH\({}_{3}\)-modified Zn-MOF inside a nafion matrix. In order to improve the electrochemical performance of nafion hybrid membranes, NH\({}_{3}\)-modified Zn-MOFs were embedded as guest molecules inside nafion as a host (Figure 9a) [97]. The proton conduction of the designed hybrid membranes was greatly enhanced by the diverse and abundant hydrogen bonds. Upon doping Zn-MOF in the nafion membrane, the conductivity enhancement was 1.87-fold purer than nafion, whereas by doping NH\({}_{3}\)-Zn-MOF inside the nafion membrane, the conductivity enhancement was 5.47-times purer than nafion. In terms of performance, we see that 212-mW cm\({}^{-2}\) of maximum power density and 660 mA cm\({}^{-2}\) of maximum current density are both quite good for a single cell (Figure 9b). A comparison of proton conductivity of nafion/MOF-based hybrid electrolyte is shown in Table 2.  
Figure 8: Proton conductivity and water uptake of nafion/W\({}_{1}\), nafion/W\({}_{2}\) and nafion membranes [96]. Figure 9: (a) Schematic diagram showing the synthesis of MOF engaging in host-guest associations specific to hydrogen bonding in the Zn-MOF structural unit, and (b) discrete H\({}_{2}\)/O\({}_{2}\) Fuel-cell current-density-voltage plot of the hybrid membrane at 60 \({}^{\circ}\)C [97].