1.IntroductionThe rising global energy demand and the environmental impact of energy use from traditional sources pose serious challenges to human health, energy security, environmental protection and the sustainability of natural resources. It is widely understood that increased energy use presents the easiest route to achieving prosperity and enhancing quality of life for large segments of the human population.1Over the last century, this goal has been accomplished by burning fossil fuels with adverse environmental consequences. The continued growth in global population, steadily increasing per-capita energy consumption in developing countries, and dependence on fossil fuel resources located in politically volatile regions make the fossil fuel-based economy unsustainable. Recently, a hydrogen-based economy has been proposed2as an alternative to the current fossil-fuel economy, but it is far from being widely realized. Considerable progress in the fundamental science of sustainable hydrogen production, storage and efficient use in fuel cells is needed to realize the vision of a hydrogen economy.Proton exchange membrane (also known as polymer electrolyte membrane) fuel cells (PEMFCs) represent a critical step in the proposed hydrogen economy. PEMFCs convert chemical energy of the fuel (hydrogen, methanol or formic acid) into electrical energy with high efficiency (∼60%) and minimal environmental pollution. Since PEMFCs are modular and have a simple design, they can be scaled up in size to suit the demands of a variety of applications. They have the potential to revolutionize transportation, which consumes 60% of the worldwide production of petroleum,3improve stationary power generation thereby reducing dependence on the electrical grid and provide reliable portable power for our ever-increasing array of personal electronic devices. Fuel cell technology has been used to power automobiles, scooters, buses, backup power generators, boats, underwater vehicles, locomotives and laptop computers.4Widespread commercialization of PEMFC technology has been stymied by high cost and the need for significant advances in hydrogen production and storage, and the performance, durability and service life of fuel cells under operating conditions.The heart of the PEMFC is a polymer electrolyte membrane (PEM) that separates the reactant gases and conducts protons as shown inFig. 1. The hydrogen fuel (on the anode side) and oxygen from air (on the cathode side) flow separately through grooves in the bipolar plate, then through a porous gas diffusion layer and in to a catalyst layer loaded with precious metal catalysts on carbon support. At the anode catalyst layer, hydrogen dissociates into protons that are transported through the membrane and electrons that flow through an external circuit. At the cathode catalyst layer, oxygen combines with the protons and electrons to form water and heat as by-products. The desired membrane properties for PEMFC include excellent proton conductivity and poor electron conductivity, low permeability for reactant gases, minimal crossover of water and fuel especially for direct methanol and formic acid fuel cells (DMFCs and DFAFCs), compatibility with electrode materials, dimensional stability during fuel cell operation (minimal swelling and shrinking), good mechanical strength, chemical and thermal stability, durability under prolonged operation (∼5000 h for transportation applications5) at elevated temperatures and during freeze–thaw cycles, and low cost. None of the available membranes meets all of these requirements.Schematic diagram of a PEM fuel cell. BP, GD, CL and Mem represent bipolar plates, gas diffusion layers, catalyst layers and membrane, respectively.A 2005 cost analysis6of an 80 kW PEMFC system for transportation, assuming high volume production of 500 000 units, projected a cost of $108 kW−1, which is below the United States Department of Energy's (DOE) 2005 target of $125 kW−1but considerably higher than the 2015 DOE target of $30 kW−1. The cost of the stack was projected to be $68 kW−1and the rest ($40 kW−1) arises from assembly and balance-of-plant components for water, heat and fuel management. The membrane–electrode assembly (MEA) accounts for 83% of the cost of the stack ($56 kW−1). The pressing need to overcome MEA cost and membrane degradation issues has driven an exponentially growing interest in this field as shown inFig. 2by the number of journal articles found in the scientific database Scopus.7The article search was performed using all of the following keywords: polymer, electrolyte, membrane, fuel and cell.Number of journal articles on polymer electrolyte membrane fuel cell based on a search using Scopus.7The most widely used PEM, Nafion®, was developed 40 years ago by DuPont Inc.8In Nafion, a hydrophobic perfluorinated polyethylene backbone and a highly hydrophilic sulfonic acid-terminated perfluoro vinyl ether pendant are known to form nanoscale domains within which ionic transport occurs.9,10The chemical structure of Nafion is shown inFig. 3. Typical values ofxandyare 7 and 1, respectively. Nafion has endured as the prototypical membrane for PEMFCs, because it offers high proton conductivity11(0.13 S cm−1at 75 °C and 100% relative humidity [RH]), chemical stability, and longevity (> 60 000 h) in a fuel cell environment.12However, it has several deficiencies that have stimulated a vigorous search for better membrane materials. Nafion is expensive, allows methanol crossover easily in DMFCs with adverse effect on performance, cannot function well at high temperatures (above 80 °C) or low humidity (below ∼80% RH), and requires external humidification and therefore management of water.12Chemical structure of Nafion.Proton dissociation from the SO3H groups and subsequent proton transport in Nafion depends on the presence of liquid water, because water molecules facilitate proton transfer and serve as shuttles for proton transport.13The need for liquid water in the membrane constrains the operating temperature below 100 °C in principle and 80 °C in practice. Moreover, protons traversing the hydrated membrane drag water molecules along from the anode to the cathode (electro-osmotic drag) as illustrated inFig. 1. There is also a flux of water molecules (back-diffusion) from the cathode to the anode driven by the water concentration gradient. Imbalance between these two fluxes can cause severe performance degradation due to drying of the anode catalyst layer and flooding of the cathode catalyst layer, mechanical stresses in the membrane, delamination of the catalyst layer, and pinhole formation in the membrane leading to gas crossover, catalyst sintering and failure.12,14The need to externally manage water distribution in the membrane adds to the system volume, weight, complexity and cost.Operation of PEMFCs above 120 °C is desirable for a number of reasons. Elevated temperature operation enhances the kinetics of electrode reactions and improves CO tolerance. At 80 °C, CO adsorbs on the Pt catalyst and diminishes the fuel cell performance. To avoid this CO poisoning, the CO content has to be maintained as low as 20 ppm in the fuel stream at 80 °C.15CO tolerance increases to 1000 ppm at 130 °C and 30 000 ppm at 200 °C due to CO desorption.15At temperatures above 120 °C, flooding of the catalyst layers can be avoided due to the elimination of liquid water, the cooling system will be simplified as a result of a larger temperature difference with the ambient, waste heat may be recovered, and it may be feasible to use non-precious metal catalysts.16In view of these considerations, DOE has set the following 2010 targets for PEM: conductivity: 0.1 S cm−1during operation at 120 °C and 1.5 kPa inlet water vapour partial pressure; unassisted start from −40 °C; O2and H2crossover: 2 mA cm−2; elevated temperature durability: 2000 h with cycling; and cost: $20 m−2.17In order to realize the potential of fuel cells, there is a need for rational design of novel membrane materials based on a fundamental understanding of membrane chemistry, proton transfer, nanophase segregation, membrane morphology, proton and molecular transport, and mechanical properties. Current membrane research is focused on developing membranes that retain water at elevated temperatures or that provide good conductivity in the absence of water. Several articles have reviewed various aspects of membrane development for PEMFCs.9,12,15,16,18–30The strategies being explored include modifying the pendant groups of existing membranes, incorporating hygroscopic oxides and heteropoly acids in the membrane to retain water, using aromatic backbone polymers, and replacing water with a less volatile proton solvent.22The aim of this review is to highlight developments over the past five years in membranes for PEMFCs. The references cited here are not meant to be comprehensive but serve as starting points for further exploration of this fascinating subject. The review is organized as follows: section 2 reports on the current experimental and theoretical understanding of Nafion membranes; section 3 discusses other perfluorinated membranes; section 4 presents recent developments in aromatic backbone membranes; section 5 discusses anhydrous membranes; section 6 presents the summary and outlook for membrane development.