Fan the flame with water: Current ignition, front propagation and multiple steady states in polymer electrolyte membrane fuel cells

F uel cells have tantalized engineers for more than a century as efficient devices to directly convert chemical energy to electrical energy. However, fuel cell use has been limited to niche applications, like space vehicles and emergency backup power systems, due to their expensive components and specialized fuel requirements. During the past two decades polymer electrolyte membrane (PEM) fuel cells have attracted the attention of the transportation industry, because they operate at modest temperatures (0–1008C), and do not involve highly corrosive liquids as electrolytes. Although PEM fuel cells are a promising technology they present new design challenges to engineers. This Perspective illustrates how classical chemical reactor engineering can help solve one of the most vexing problems of PEM fuel cells — how to manage water. PEM fuel cells are coupled chemical reactors where oxidation and reduction reactions are physically separated and coupled by transport of ions and electrons. Hydrogen is adsorbed onto a catalyst surface and is oxidized to protons and electrons at the anode. A chemical potential gradient drives the protons to migrate across the electrolyte. Electrons move through a parallel path in an electronic conductor, where they do useful work. The protons and electrons meet and react with oxygen on a catalyst surface at the cathode. PEM fuel cells have complex structures that facilitate chemical reaction and heat and mass transport. At the heart of the PEM fuel cell is the membrane-electrode assembly (MEA), a polymer electrolyte membrane coated on both sides with a thin layer of carbon supported Pt catalyst. Nafion is the most common PEM; it is a copolymer of tetrafluoroethylene and a perfluoro alkyl ether terminated with a covalently bonded sulfonic acid. Water absorbs into Nafion solvating the sulfonic acid groups, swelling the membrane and permitting mobility of the protons. The catalyst-coated membrane is sandwiched between a porous carbon layer approximately 200–400 mm thick, called the gas diffusion layer (GDL). The GDL carries the electronic current while permitting the reactant gases to diffuse to the membrane/catalyst interface. The MEA is placed between metal or graphite plates with flow channels (bipolar plates). The flow channels deliver the reactant gases across the MEA, collect and remove the product water and conduct the electronic current. Water management is essential to efficient PEM fuel cell operation. Proton conductivity in Nafion increases from 10 7 to 10 1 S-cm as water activity increases from 0 to 1 (relative humidity increasing from 0 to 100%). To achieve the highest proton conductivity and minimize energy losses in the polymer electrolyte, PEM fuel cells have been operated with humidified feeds. However, with humidified feeds, the product water condenses and liquid water can hinder reactant gas transport from the gas-flow channel through the GDL to the membrane/catalyst interface. The brute force engineering solution for PEM fuel cells is to humidify the feeds and operate with high reactant gas flow rates to blow the liquid drops out of the fuel cell; the high-gas-flow results in low-fuel conversion per pass, necessitating recovery and recycle systems. The flow patterns for the gas-flow channels are generally complex, most often consisting of long serpentine type channels 10–1,000 cm long with cross sections , 1 mm. Liquid water accumulates at various locations in the GDL and the gas-flow channels, resulting in a condition known as flooding. Liquid water can hinder mass transport, locally starving the fuel cell, resulting in variable local current densities and voltages. Understanding the spatiotemporal dynamics of water formation and motion in PEM fuel cells remains a great challenge for fuel-cell engineering, especially when the water production and flow rates change in time as occurs with startup, shutdown and with variable load. Recently several groups have developed probes for local current, composition and temperature measurements in Perspective