The Prospects of Developing Ultrahigh Energy Efficiency Hydrogen–Oxygen Fuel Cells

Abstract

A recent trend in science has been focused on improving the energy efficiency of electrochemical and other energy conversion devices. This work continues the theme by reviewing the prospects for developing viable low-temperature fuel cells (LTFCs) that utilize hydrogen and oxygen and operate with exceedingly high energy efficiencies of 75–85%. The best currently available commercial LTFCs, for example, the polymer electrolyte membrane fuel cells (PEMFCs) used in hydrogen hybrid vehicles, typically operate at energy efficiencies of ≤60%, generating ≤20.0 kWh of electricity per 1 kg of hydrogen consumed. When combined with state-of-the-art commercial water electrolyzers, which require ∼50 kWh to produce 1 kg of hydrogen, these systems offer a round-trip efficiency of only ∼40%. This makes them uncompetitive for large-scale energy storage. By contrast, future LTFCs operating at 75–85% energy efficiency would yield 25.0–28.3 kWh per 1 kg of hydrogen. When combined with recently developed water electrolyzers that require only ∼38.0–41.5 kWh to produce 1 kg of hydrogen, such systems could achieve round-trip efficiencies of up to 75%. This would be competitive with large-scale energy storage systems like pumped hydro, which provide a necessary alternative to batteries. Although Li-ion batteries can achieve round-trip efficiencies of ∼90%, their high cost and limited storage capacity hinder their use in grid-scale applications. Developing ultrahigh energy efficiency fuel cells therefore offers substantial promise as an alternative energy storage system. Achieving 75–85% efficiency at 80 °C requires fuel cells to operate at voltages of 0.9–1.0 V. This Review explores recent advances in electrocatalysts, interelectrode membranes, and other developments that have enabled PEMFCs, anion exchange membrane fuel cells (AEMFCs), and alkaline fuel cells (AFCs) that produce notable current densities at cell voltages of ≥0.9 V. The prospects of extending such technical achievements to the creation of fuel cells capable of viably operating at 75–85% energy efficiency are discussed. The most promising pathway to such ultrahigh energy efficiency is found to involve pressurizing the fuel cell gases to pressures notably higher than those used in commercial fuel cells at present (e.g., 4–10 bar).