Alkaline Stability of Low Oxophilicity Metallopolymer Anion-Exchange Membranes

Polymer Backbone Chemistry Shapes the Alkaline Stability of Metallopolymer Anion-Exchange Membranes

Anion-exchange membrane fuel cells and water electrolyzers have garnered significant attention in past years due to their potential role in sustainable and affordable energy conversion and storage. However, the chemical stability of the polymeric anion-exchange membranes (AEMs), the key component in these devices, currently limits their lifespan. Recently, metallopolymers have been proposed as chemically stable alternatives to organic cations, using metal centers as ion transporters. In metallopolymer AEMs, various properties such as alkaline stability, water uptake, flexibility, and performance, are determined by both the metal complex and polymer backbone. Herein we present a systematic study investigating the influence of the polymer backbone chemistry on some of these properties, focusing on the alkaline stability of low-oxophilicity gold metallopolymers. Despite the use of a common N-heterocyclic carbene ligand, upon gold metalation using the same reaction conditions, different polymer backbones end up forming different gold complexes. These findings suggest that polymer chemistry affects the metalation reaction in addition to the other properties relevant to AEM performance.

Quantifying the Resistive Losses of the Catalytic Layers in Anion-Exchange Membrane Fuel Cells

The existing gap in the ability to quantify the impacts of resistive losses on the performance of anion-exchange membrane fuel cells (AEMFCs) during the lifetime of their operation is a serious concern for the technology. In this paper, we analyzed the ohmic region of an operating AEMFC fed with pure oxygen followed by CO2 -free air at various operating currents, using a combination of electrochemical impedance spectroscopy (EIS) and a novel technique called impedance spectroscopy genetic programming (ISGP). Presented here for the first time in this work, we isolated and quantified the individual effective resistance (Reff ) values occurring in the AEMFC and their influence on performance as operating conditions change. We believe that this first work is vital to help distinguish the influence of the individual catalytic and mass-transfer processes in this technology thereby providing valuable data to the AEMFC community, with potentially wider applicability to other electrochemical devices where individual physical processes occur simultaneously and need to be sequestered for deeper understanding.

Alkaline Stability of Anion-Exchange Membranes

Recently, the development of durable anion-exchange membrane fuel cells (AEMFCs) has increased in intensity due to their potential to use low-cost, sustainable components. However, the decomposition of the quaternary ammonium (QA) cationic groups in the anion-exchange membranes (AEMs) during cell operation is still a major challenge. Many different QA types and functionalized polymers have been proposed that achieve high AEM stabilities in strongly alkaline aqueous solutions. We previously developed an ex situ technique to measure AEM alkaline stabilities in an environment that simulates the low-hydration conditions in an operating AEMFC. However, this method required the AEMs to be soluble in DMSO solvent, so decomposition could be monitored using ¹H nuclear magnetic resonance (NMR). We now report the extension of this ex situ protocol to spectroscopically measure the alkaline stability of insoluble AEMs. The stability ofradiation-grafted (RG) poly(ethylene-co-tetrafluoroethylene)-(ETFE)-based poly(vinylbenzyltrimethylammonium) (ETFE-TMA) and poly(vinylbenzyltriethylammonium) (ETFE-TEA) AEMs were studied using Raman spectroscopy alongside changes in their true OH^- conductivities and ion-exchange capacities (IEC). A crosslinked polymer made from poly(styrene-co-vinylbenzyl chloride) random copolymer and N,N,N',N'-tetraethyl-1,3-propanediamine (TEPDA) was also studied. The results are consistent with our previous studies based on QA-type model small molecules and soluble poly(2,6-dimethylphenylene oxide) (PPO) polymers. Our work presents a reliable ex situ technique to measure the true alkaline stability of AEMs for fuel cells and water electrolyzers.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.