## 2 MOFs for Fuel-Cell Applications
### MOFs as Electrode Application
#### 2.1.1 H\({}_{2}\)/O\({}_{2}\) Fuel Cells
In order to achieve optimal performance, the proportion of Mn and Zn was adjusted to yield 200 Mn-NC SACs having a life of 1000 h at 0.7 V. Cobalt is another multivalent metal that shows excellent electrocatalytic participation, so the Co-N-C type SACs have also been investigated as catalysts devoid of Fe for H\({}_{2}\)/O\({}_{2}\) fuel cells [78]. By a similar calcination procedure, they were also able to synthesize the Co-N-C catalyst, which behaved similar to the Fe-N-C catalysts and displayed a similar performance to that of the Fe-N-C catalysts owing to the well-distributed CoN\({}_{4}\) operating  
Figure 4: (**a**) The schematic showing the FeN\({}_{x}\)/HOPC-c-1000 synthesis steps; (**b**) model of the OMS-Fe-ZIF-8 seen from different crystallographic orientations; (**c,d**) SEM images of the OMS-Fe-ZIF-8 showing growth along multiple crystallographic planes; (**e**) SEM image and details about the inner geometry of a broken crystal of OMS-Fe-Z(F-8); (**f**) the SEM image shows the internal structure of one of the FeN\({}_{4}\)/HOPC-c-1000 particles that has broken in two; (**g**) illustration of the PEMFC Fuel-cell assembly; (**h**) LSV curves of FeN\({}_{4}\)/HOPC-c-1000 and FeN\({}_{4}\)/C in 0.5 M H\({}_{2}\)SO\({}_{4}\), with a scan rate of 10 mVs\({}^{-1}\) in 0.5 M H\({}_{2}\)SO\({}_{4}\) under unsaturated O\({}_{2}\); (**i,j**) polarization and power\({}^{-}\) density measurements carried out using a cathode catalyst of H\({}_{2}\)-O\({}_{2}\) and a catalyst of FeN\({}_{4}\)/HOPC-c-1000 at 1 bar and (**k**) a constant-potential test to determine Fuel-cell durability [60]. sites, which were afforded by the MOF template [43; 62]. In spite of the encouraging catalytic activity of catalysts containing Co-N-C, the presence of agglomerated regions of Co might reduce the intrinsic capabilities of Co and N, which are atomically dispersed within the ZIF-8 precursor, forming a Co-N-C@F127 catalyst after a composition of Pluronic F127 block copolymer is utilized as a surface-active agent to confine the Co aggregates and collapse the porous structure. As predicted by density functional theory calculations, CoN\({}_{2+2}\) active sites, with their relatively low energy of activation 0.69 eV as compared with CoN\({}_{4}\), were capable of catalysing the 4e\({}^{-}\) ORR process [64; 79]. The problem of functional deficiency from high voltage exposure must, however, be addressed in more detailed studies. Due to the success of separating sites singularly as applied to M-N-C electrocatalysts in H\({}_{2}\)/O\({}_{2}\) fuel cells, great efforts have been made to combine different active metal centres for fuel cells. An N-doped carbon matrix was synthesized by [PERSON] et al. [80] to be a Pt-free catalyst that consists of Fe and Co automatically dispersed within the matrix (Figure 5a). There is encouraging evidence to suggest that the existence of double metal sites are necessary for the initiation of the O-O bonds from passive to active forms and the engagement of the 4e\({}^{-}\) ORR processes, as the hydrogen-oxygen Fuel-cell experiments and hydrogen-air Fuel-cell experiments clearly demonstrate. It was determined that the proposed (Fe,Co)/N-C catalyst performs better in both the H\({}_{2}\)/O\({}_{2}\) and H\({}_{2}\)/air conditions than most reported Pt-free catalysts. This shows that the dual metal sites have a high stability in the Fuel-cell test, suggesting that they have the potential to be produced commercially (Figure 5b,c) [80]. Figure 5: (**a**) Schematic illustration for the preparation of (Fe,Co)/N-C; (**b**) H\({}_{2}\)/O\({}_{2}\) Fuel-cell polarization plots. Cathode: \(\sim\)0.77 mg cm\({}^{-2}\) of (Fe,Co)/N-C; 100% RH; O\({}_{2}\), 0.1 and 0.2 MPa partial pressures. Anode: 0.1 mg Pt cm\({}^{-2}\) Pt/C; 100% RH; H\({}_{2}\), 0.1 MPa partial pressure; cell 353 K; 25 cm\({}^{2}\) electrode area and (**c**) polarization plots for hydrogen/air fuel cells. The cathode has a partial pressure of 0.2 MPa; the catalyst is (Fe,Co)/N-C; 100 percent relative humidity is used.