Nafion Matrix and Ionic Domain Tuning for High-Performance Composite Proton Exchange Membranes

Although proton exchange membranes (PEMs) are widely deployed in an array of commercial applications, limitations linked to their proton conductivity, water retention, and gas permeability still limit ultimate device performance. While ex situ studies have shown additives can enhance membrane stability and mass transport, to date few have demonstrated that these performance enhancements are maintained when tested in commercially relevant electrochemical technologies, such as fuel cells or electrolyzers. Herein, a new multifunctional additive, 2D poly(triazine imide) (PTI), is demonstrated for composite PEMs, which is shown to boost proton conductivity by 37% under optimal high relative humidity (RH) conditions and 67% at low RHs. PTI also enables major improvements (over 55%) in both current and power densities in industrially relevant PEM fuel cells (PEMFCs). Most importantly, in situ and ex situ characterization suggests that the enhanced performance is due to polymer aggregate-PTI clusters that form with increasing 2D character and improved long-range connectivity, while acid-base interactions with pyridinic nitrogen facilitate the critical proton hopping mechanism at all RHs. Hence, this work offers both a new membrane concept with proven benefits for important electrochemical technologies, as well as design principles for future optimization of proton transport and water management within PEMs

Operando Electrochemical Atomic Force Microscopy of Solid–Electrolyte Interphase Formation on Graphite Anodes: The Evolution of SEI Morphology and Mechanical Properties

Understanding and ultimately controlling the properties of the solid–electrolyte interphase (SEI) layer at the graphite anode/liquid electrolyte boundary are of great significance for maximizing the performance and lifetime of lithium-ion batteries (LIBs). However, comprehensive in situ monitoring of SEI formation and evolution, alongside measurement of the corresponding mechanical properties, is challenging due to the limitations of the characterization techniques commonly used. This work provides a new insight into SEI formation during the first lithiation and delithiation of graphite battery anodes using operando electrochemical atomic force microscopy (EC-AFM). Highly oriented pyrolytic graphite (HOPG) is investigated first as a model system, exhibiting unique morphological and nanomechanical behavior dependent on the various electrolytes and commercially relevant additives used. Then, to validate these findings with respect to real-world battery electrodes, operando EC-AFM of individual graphite particles like those in commercial systems are studied. Vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are shown to be effective additives to enhance SEI layer stability in 1 M LiPF6/ethylene carbonate/ethyl methyl carbonate (EC/EMC) electrolytes, attributed to their role in improving its structure, density, and mechanical strength. This work therefore presents an unambiguous picture of SEI formation in a real battery environment, contributes a comprehensive insight into SEI formation of electrode materials, and provides a visible understanding of the influence of electrolyte additives on SEI formation

Mass transport in PEM water electrolysers: A review

While hydrogen generation by alkaline water electrolysis is a well-established, mature technology and currently the lowest capital cost electrolyser option; polymer electrolyte membrane water electrolysers (PEMWEs) have made major advances in terms of cost, efficiency, and durability, and the installed capacity is growing rapidly. This makes the technology a promising candidate for large-scale hydrogen production, and especially for energy storage in conjunction with renewable energy sources – an application for which PEMWEs offer inherent advantages over alkaline electrolysis. Improvements in PEMWE technology have led to increasingly high operational current densities, which requires adequate mass transport strategies to ensure sufficient supply of reactant and removal of products. This review discusses the current knowledge related to mass transport and its characterisation/diagnosis for PEMWEs, considering the flow channels, liquid-gas diffusion layer, and polymer electrolyte membrane in particular

Determining the electrochemical transport parameters of sodium-ions in hard carbon composite electrodes

Sodium-ion batteries offer advantages over conventional Li-ion batteries, including cost and safety. However, much less is known about their operation and performance properties, particularly at the anode. The electron and ion transport in the active materials and composite electrode significantly impact battery performance. Understanding the changes in transport properties as a function of state-of-charge and state-of-health is essential for effective electrode design and performance assessment. In this work, the resistivity and diffusivity of sodium transport in hard carbon composite electrodes are studied at different states-of-health, using Galvanostatic Intermittent Titration Technique (GITT), Electrochemical Impedance Spectroscopy (EIS), and Electrochemical Potential Spectroscopy (EPS) in a stable 3-electrode test cell configuration. The reference electrode eliminated some voltage errors arising from the overpotentials on the counter electrode. The resistance contributions from the surface electrolyte interface, electrolyte transport in the electrode pores, and the charge transfer resistance are extrapolated from the impedance measurements and the diffusion coefficient from the GITT and EPS. The different techniques indicate similar trends in the diffusion coefficient during sodiation, desodiation, and ageing, although different orders of magnitude were observed between the EPS and GITT data. The accuracy of the parameters calculated using the different electrochemical techniques is discussed in detail

Characterizing Batteries by In Situ Electrochemical Atomic Force Microscopy: A Critical Review

Although lithium, and other alkali ion, batteries are widely utilized and studied, many of the chemical and mechanical processes that underpin the materials within, and drive their degradation/failure, are not fully understood. Hence, to enhance the understanding of these processes various ex situ, in situ and operando characterization methods are being explored. Recently, electrochemical atomic force microscopy (EC-AFM), and related techniques, have emerged as crucial platforms for the versatile characterization of battery material surfaces. They have revealed insights into the morphological, mechanical, chemical, and physical properties of battery materials when they evolve under electrochemical control. This critical review will appraise the progress made in the understanding batteries using EC-AFM, covering both traditional and new electrode–electrolyte material junctions. This progress will be juxtaposed against the ability, or inability, of the system adopted to embody a truly representative battery environment. By contrasting key EC-AFM literature with conclusions drawn from alternative characterization tools, the unique power of EC-AFM to elucidate processes at battery interfaces is highlighted. Simultaneously opportunities for complementing EC-AFM data with a range of spectroscopic, microscopic, and diffraction techniques to overcome its limitations are described, thus facilitating improved battery performance

Engineering Catalyst Layers for Next-Generation Polymer Electrolyte Fuel Cells: A Review of Design, Materials, and Methods

Polymer electrolyte fuel cells (PEFCs) are a promising replacement for the fossil fuel–dependent automotive and energy sectors. They have become increasingly commercialized in the last decade; however, significant limitations on durability and performance limit their commercial uptake. Catalyst layer (CL) design is commonly reported to impact device power density and durability; although, a consensus is rarely reached due to differences in testing conditions, experimental design, and types of data reported. This is further exacerbated by aspects of CL design such as catalyst support, proton conduction, catalyst, fabrication, and morphology, being significantly interdependent; hence, a wider appreciation is required in order to optimize performance, improve durability, and reduce costs. Here, the cutting-edge research within the field of PEFCs is reviewed, investigating the effect of different manufacturing techniques, electrolyte distribution, support materials, surface chemistries, and total porosity on power density and durability. These are critically appraised from an applied perspective to inform the most relevant and promising pathways to make and test commercially viable cells. This holistic view of the competing aspects of CL design and preparation will facilitate the development of optimized CLs, especially the incorporation of novel catalyst support materials

Neutron studies of Na-ion battery materials

The relative vast abundance and more equitable global distribution of terrestrial sodium makes sodium-ion batteries (NIBs) potentially cheaper and more sustainable alternatives to commercial lithium-ion batteries (LIBs). However, the practical capacities and cycle lives of NIBs at present do not match those of LIBs and have therefore hindered their progress to commercialisation. The present drawback of NIB technology stems largely from the electrode materials and their associated Na+ion storage mechanisms. Increased understanding of the electrochemical storage mechanisms and kinetics is therefore vital for the development of current and novel materials to realise the commercial NIB. In contrast to x-ray techniques, the non-dependency of neutron scattering on the atomic number of elements (Z) can substantially increase the scattering contrast of small elements such as sodium and carbon, making neutron techniques powerful for the investigation of NIB electrode materials. Moreover, neutrons are far more penetrating which enables more complex sample environments including in situ and operando studies. Here, we introduce the theory of, and review the use of, neutron diffraction and quasi-elastic neutron scattering, to investigate the structural and dynamic properties of electrode and electrolyte materials for NIBs. To improve our understanding of the actual sodium storage mechanisms and identify intermediate stages during charge/discharge, ex situ, in situ, and operando neutron experiments are required. However, to date there are few studies where operando experiments are conducted during electrochemical cycling. This highlights an opportunity for research to elucidate the operating mechanisms within NIB materials that are under much debate at present

Neutron studies of Na-ion battery materials

The relative vast abundance and more equitable global distribution of terrestrial sodium makes sodium-ion batteries (NIBs) potentially cheaper and more sustainable alternatives to commercial lithium-ion batteries (LIBs). However, the practical capacities and cycle lives of NIBs at present do not match those of LIBs and have therefore hindered their progress to commercialisation. The present drawback of NIB technology stems largely from the electrode materials and their associated Na+ion storage mechanisms. Increased understanding of the electrochemical storage mechanisms and kinetics is therefore vital for the development of current and novel materials to realise the commercial NIB. In contrast to x-ray techniques, the non-dependency of neutron scattering on the atomic number of elements (Z) can substantially increase the scattering contrast of small elements such as sodium and carbon, making neutron techniques powerful for the investigation of NIB electrode materials. Moreover, neutrons are far more penetrating which enables more complex sample environments including in situ and operando studies. Here, we introduce the theory of, and review the use of, neutron diffraction and quasi-elastic neutron scattering, to investigate the structural and dynamic properties of electrode and electrolyte materials for NIBs. To improve our understanding of the actual sodium storage mechanisms and identify intermediate stages during charge/discharge, ex situ, in situ, and operando neutron experiments are required. However, to date there are few studies where operando experiments are conducted during electrochemical cycling. This highlights an opportunity for research to elucidate the operating mechanisms within NIB materials that are under much debate at present

Spatial quantification of dynamic inter and intra particle crystallographic heterogeneities within lithium ion electrodes

The performance of lithium ion electrodes is hindered by unfavorable chemical heterogeneities that pre-exist or develop during operation. Time-resolved spatial descriptions are needed to understand the link between such heterogeneities and a cell’s performance. Here, operando high-resolution X-ray diffraction-computed tomography is used to spatially and temporally quantify crystallographic heterogeneities within and between particles throughout both fresh and degraded Li_{x}Mn_{2)O_{4} electrodes. This imaging technique facilitates identification of stoichiometric differences between particles and stoichiometric gradients and phase heterogeneities within particles. Through radial quantification of phase fractions, the response of distinct particles to lithiation is found to vary; most particles contain localized regions that transition to rock salt LiMnO_{2} within the first cycle. Other particles contain monoclinic Li_{2}MnO_{3}near the surface and almost pure spinel Li_{x}Mn_{2}O_{4} near the core. Following 150 cycles, concentrations of LiMnO_{2} and Li_{2}MnO_{3} significantly increase and widely vary between particles

New use of global warming potentials to compare cumulative and short-lived climate pollutants

Parties to the United Nations Framework Convention on Climate Change (UNFCCC) have requested guidance on common greenhouse gas metrics in accounting for Nationally determined contributions (NDCs) to emission reductions1. Metric choice can affect the relative emphasis placed on reductions of ‘cumulative climate pollutants’ such as carbon dioxide versus ‘short-lived climate pollutants’ (SLCPs), including methane and black carbon2, 3, 4, 5, 6. Here we show that the widely used 100-year global warming potential (GWP100) effectively measures the relative impact of both cumulative pollutants and SLCPs on realized warming 20–40 years after the time of emission. If the overall goal of climate policy is to limit peak warming, GWP100 therefore overstates the importance of current SLCP emissions unless stringent and immediate reductions of all climate pollutants result in temperatures nearing their peak soon after mid-century7, 8, 9, 10, which may be necessary to limit warming to “well below 2 °C” (ref. 1). The GWP100 can be used to approximately equate a one-off pulse emission of a cumulative pollutant and an indefinitely sustained change in the rate of emission of an SLCP11, 12, 13. The climate implications of traditional CO2-equivalent targets are ambiguous unless contributions from cumulative pollutants and SLCPs are specified separately