Metal-organic framework membranes for proton exchange membrane fuel cells: A mini-review

As eco-friendly energy, the proton exchange membrane fuel cells (PEMFCs) utilize H2 and O2 as energy sources wherein the water is the only by-product. As a result, they are the suitable candidates for replacement of combustion engines. Various materials such as hydrocarbon polymers, nafions, CNTs, graphene ionic liquids, etc., have been used as conducting membranes to efficiently transport and transfer charges such as protons across the electrode interfaces. However, many of these materials show mechanical instability at high temperatures and exhibit conductance dependent on relative humidity. They also foul quickly. All these drawbacks reduce the conductivity of the PEMs-constituted layer and lead to the poor output voltage of the fuel cells. The most promising materials to be used as part of the electrode interfaces for fuel cells are Metal Organic Frameworks (MOFs). They have good structural diversity and variability in dimensions/coordination, mechanical and thermal stability and have high surface area, adsorption, catalytic property, luminescence, magnetic properties, etc. MOFs have a high proton conductivity. Due to their structural diversity and variable coordination modes, many MOFs have been identified for their proton conductivity. In this current mini-review, we give a summary report on proton conductivities of metal carboxylates, phosphonates, sulphonates and other related MOFs as mem-branes for polymer membrane exchange fuel cells (PEMFCs). Charge transport and transfer in these MOF membranes have been established to occur via the proton-mediated conduction as proposed in the Grotthuss or the Vehicle mechanism.