**Preface to "Proton Exchange Membrane Fuel Cells (PEMFCs)"**

A proton exchange membrane fuel cell (PEMFC) spontaneously produces power from a fuel and oxygen with heat and water byproducts. Hydrogen is preferred as the fuel because it is renewable if generated by water electrolysis using electricity derived from renewable energy. PEMFCs are more efficient than an internal combustion engine because reactants are directly converted into energy through a one-step electrochemical reaction. Fuel cells coupled with hydrogen production can improve resiliency by fulfilling the energy storage needs of electric grids constrained by intermittent renewable energy generation (e.g., solar, wind). More specifically, discharge (fuel cell) and recharge (water electrolysis) durations exceeding a few days for power plant ratings below the megawatt level are not presently accessible to other energy storage technologies. Furthermore, fuel cells combined with water electrolyzers powered by renewable energy mitigate global warming concerns with reduced carbon dioxide emissions. PEMFCs are commercially available for a few applications including vehicles, buildings, and power backup systems. Improvements in cost, performance, and durability are needed to assist commercialization efforts because the technology is not yet mature.

Papers published in the *Molecules* Special Issue on PEMFCs are collected in this book. The Special Issue was initially intended to attract contributions focusing on all PEMFC scientific and technological aspects that decrease cost and increase performance and durability, including novel characterization methods, mathematical models, and accelerated stress tests to gain additional insight, as well as degradation mechanisms, innovative materials, and original designs for components, cells, stacks, and systems.

The collected papers comprise recent advancements in PEMFC technology aimed at reducing cost, improving performance, and extending durability. Almost all key materials, as well as their integration into a cell, are discussed: the bus plates that collect the electrical current, the gas diffusion medium that distributes the reactants over catalysts promoting faster reactions, and the membrane separating oxygen and hydrogen gases and closing the electrical circuit by transporting protons. Fuel cell operation below the freezing point of water and with impure reactant streams, which impacts durability, is also discussed. Papers focusing on materials and their integration appear at the beginning and follow the sequence: membrane, catalyst layer, gas diffusion layer, membrane electrode assembly, and bus plate. Subsequent papers center on operational aspects. All papers include at least one author with significant fuel cell experience. Authors originate from all organization types, universities, national laboratories, and companies, and are distributed over Asia, Europe, and North America. The varied origins of the contributors indicate a continued and widespread interest in fuel cell technology to address energy and climate issues.

> **Jean St-Pierre, Shangfeng Du** *Editors*

*Review*
