▪ Abstract The past 10 years have witnessed a tremendous acceleration in research devoted to non-fluorinated polymer membranes, both as competitive alternatives to commercial perfluorosulfonic acid membranes operating in the same temperature range and with the objective of extending the range of operation of polymer fuel cells toward those more generally occupied by phosphoric acid fuel cells. Important requirements are adequate membrane mechanical strength at levels of functionalization (generally sulfonation) and hydration allowing high proton conductivity, and stability in the aggressive environment of a working fuel cell, in particular thermohydrolytic and chemical stability. This review provides an overview of progress made in the development of proton-conducting hydrocarbon and heterocyclic-based polymers for proton exchange and direct methanol fuel cells and describes the various approaches made to polymer modification/synthesis and salient properties of the materials formed, including those relating to proton transport and proton conductivity, e.g., water diffusion and electro-osmotic drag. The microstructure, deduced from small angle X-ray and neutron diffraction measurements of representative non-fluorinated polymers is compared with that of perfluorosulfonic acid membranes. Different degradation mechanisms and aging processes that can result in chemical and morphological alteration are considered, and recent characterization of membrane-electrode assemblies (MEAs) in direct methanol and hydrogen-air (oxygen) fuel cells completes this review of the state of the art. While several types of non-fluorinated polymer membrane have demonstrated lifetimes of 500–4000 h, only a limited number of systems exist that hold promise for long-term operation above 100°C. ¹

Electrospinning is attracting close interest as a versatile fabrication method for one dimensional mesostructured organic, inorganic and hybrid nanomaterials of controlled dimensions prepared as random or oriented continuous nanofibres with possibilities of ordered internal morphologies such as core–sheath, hollow or porous fibre, or even multichannelled microtube arrangements. The dimensionality, directionality and compositional flexibility of electrospun nanofibres and mats are increasingly being investigated for the targeted development of electrode and electrolyte materials, where the specific properties associated with nanoscale features such as high surface area and aspect ratios, low density and high pore volume allow performance improvements in energy conversion and storage devices. We present here a review on the application of electrospinning for the design and fabrication of architectured, nanofibrous materials for dye sensitised solar cells, fuel cells, lithium ion batteries and supercapacitors, with particular emphasis on improved energy and power density imparted by performance improvement to, inter alia, ionic conductivity, cyclability, reversibility, interfacial resistance and electrochemical stability, as well as mechanical strength, of electrospun electrode and electrolyte components.

This article provides a comprehensive perspective of perfluorosulfonic acid fuel cell membrane degradation phenomena, reviews and appraises the effectiveness of key concepts for the mitigation strategies and identifies future research priorities.

We report a comprehensive analysis of the catalytic oxygen reduction reaction (ORR) reactivity of four of today's most active benchmark platinum group metal-free (PGM-free) iron/nitrogen doped carbon electrocatalysts (Fe–N–Cs). Our analysis reaches far beyond previous such attempts in linking kinetic performance metrics, such as electrocatalytic mass-based and surface area-based catalytic activity with previously elusive kinetic metrics such as the active metal site density (SD) and the catalytic turnover frequency (TOF). Kinetic ORR activities, SD and TOF values were evaluated using in situ electrochemical NO₂ ⁻ reduction as well as an ex situ gaseous CO cryo chemisorption. Experimental ex situ and in situ Fe surface site densities displayed remarkable quantitative congruence. Plots of SD versus TOF (“reactivity maps”) are utilized as new analytical tools to deconvolute ORR reactivities and thus enabling rational catalyst developments. A microporous catalyst showed large SD values paired with low TOF, while mesoporous catalysts displayed the opposite. Trends in Fe surface site density were linked to molecular nitrogen and Fe moieties (D1 and D2 from ⁵⁷Fe Mössbauer spectroscopy), from which pore locations of catalytically active D1 and D2 sites were established. This cross-laboratory analysis, its employed experimental practices and analytical methodologies are expected to serve as a widely accepted reference for future, knowledge-based research into improved PGM-free fuel cell cathode catalysts.

When complexed with a strong inorganic acid such as sulfuric or phosphoric acid, films of polybenzimidazole (PBI) show two types of behaviour, depending on the time of immersion in the corresponding acid bath. The first type, prepared at shorter doping times, has conductivity in the range 10^–5–10^–4 S cm^–1, whilst the second, of conductivity >10^–3 S cm^–1 is formed after more prolonged immersion. The ‘switch-over' from one state to the other is at 10–11 h in H₃PO₄, and 2–3 h in H₂SO₄. PBI has a remarkable capacity to concentrate H₃PO₄ and, even in an acid bath of concentration 3 mol dm^–3, the acid concentration within a PBI membrane is ca. 14.5 mol dm^–3. IR spectroscopy performed on hydrated PBI membranes as a function of temperature, and on acid-complexed membranes as a function of the amount of sorbed acid confirms proton transfer from H₃PO₄ to the imino groups of PBI and, at high doping levels, the presence of undissociated H₃PO₄.