Comparing Ammonium and Tetraaminophosphonium Anion-Exchange Membranes Derived from Vinyl-Addition Polynorbornene Copolymers.

Herein, we systematically examined how composition influenced the properties of vinyl addition polynorbornene anion exchange membranes (AEMs) prepared from 5-n-hexyl-2-norbornene and 5-(4-bromobutyl)-2-norbornene. Copolymerization kinetics revealed that 5-n-hexyl-2-norbornene is consumed faster than 5-(4-bromobutyl)-2-norbornene, leading to a portion of the chain being richer in bromoalkyl groups. The alkyl halide pendants can then be converted to either trimethylammonium or tetrakis(dialkylamino)phosphonium cations through straightforward substitution with trimethylamine or a tris(dialkylamino)phosphazene. A series of cationic ammonium polymers were synthesized first, where conductivity and water uptake increased as a function of increasing ionic content in the polymer. The optimized copolymer had a hydroxide conductivity of 95 ± 6 mS/cm at 80 °C. The living polymerization of the two monomers catalyzed by a cationic tert-butylphosphine palladium catalyst also enabled precise changes in the molecular weight while keeping the functional group concentration constant. Molecular weight did not have a significant impact on hydroxide conductivity over the range of ∼60-190 kg/mol (Mn). The optimized tetraaminophosphonium AEM had the highest conductivity for any tetraaminophosphonium polymer to date (70 ± 3 mS/cm at 80 °C). Clear phase separation and larger domains were observed for the phosphonium-based AEM compared to the ammonium at an identical composition, which is attributed to the larger occupied volume of the phosphorus cation. Fuel cell studies with the two membranes resulted in peak power densities of 1.59 and 0.79 W/cm2 for the ammonium and tetraaminophosphonium membrane electrode assemblies, respectively. The ammonium-based membrane was more water permeable as evidenced by water limiting current studies, which likely contributed to the improved performance.

Discovering Reactant Supply Pathways at Electrode/PEM Reaction Interfaces Via a Tailored Interface-Visible Characterization Cell.

In situ and micro-scale visualization of electrochemical reactions and multiphase transports on the interface of porous transport electrode (PTE) materials and solid polymer electrolyte (SPE) has been one of the greatest challenges for electrochemical energy conversion devices, such as proton exchange membrane electrolyzer cells (PEMECs), CO2 reduction electrolyzers, PEM fuel cells, etc. Here, an interface-visible characterization cell (IV-CC) is developed to in situ visualize micro-scaled and rapid electrochemical reactions and transports in PTE/SPE interfaces. Taking the PEMEC of a green hydrogen generator as a study case, the unanticipated local gas blockage, micro water droplets, and their evolution processes are successfully visualized on PTE/PEM interfaces in a practical PEMEC device, indicating the existence of unconventional reactant supply pathways in PEMs. Further comprehensive results reveal that PEM water supplies to reaction interfaces are significantly impacted with current densities. These results provide critical insights about the reaction interface optimization and mass transport enhancement in various electrochemical energy conversion devices.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies.

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Discovering and Demonstrating a Novel High-Performing 2D-Patterned Electrode for Proton-Exchange Membrane Water Electrolysis Devices.

Proton-exchange membrane water electrolysis (PEMWE) produces hydrogen with high efficiency and purity but uses high-loading platinum-group metal (PGM) catalysts. Such concerns call for the development of novel electrode architectures to improve catalyst utilization and mass activity, thus promoting PEMWE cost competitiveness for large-scale implementation. In this study, we demonstrated, for the first time, a novel two-dimensional (2D)-patterned electrode with edge effects to address these challenges. The edge effect was induced by membrane properties, potential distribution, and counter electrode coverage and could be optimized by tuning the catalyst layer dimensions. To achieve identical PEMWE performance, the optimal pattern saved the 21% anode PGM catalyst compared with the conventional catalyst fully covered electrode. The PGM catalyst could be further reduced by 61% to boost mass activity with no significant performance loss. The results also indicated that the electrode uniformity in PEMWE cells might not be as critical as that in PEM fuel cells. The novel 2D-patterned electrode could effectively reduce PGM catalyst loading, accelerating affordable and large-scale production of hydrogen and other value-added chemicals via electrolysis.

Role of H3O· Radical in the Degradation of Fuel Cell Proton-Exchange Membranes.

Membrane durability in proton-exchange membrane fuel cells (PEMFCs) is one of the major obstacles limiting its applications, especially in heavy-duty vehicles. Membrane degradation reactions are thought to be attacks by radicals such as hydroxyl (HO•) or hydrogen atom (H•) generated during fuel cell operation. For the H• case, computational modeling results have suggested that the reaction between H• and the sulfonic group should be the dominant degradation pathway. However, experimental work implies that the tertiary fluorine (t-F) attack is the dominant H• reaction pathway, apparently contradicting the theoretical prediction. Based on previous experimental evidence on isotopic substitution, we postulate that the hydronium radical (H3O•) might be present in PEMFCs. Our ab initio modeling indicates that this radical can be stabilized by the sulfonic anion on the polymer side chain. With the assistance of explicit water, the polymer side chain can undergo a conformational change, leading to a greatly reduced barrier for the t-F degradation reaction. Thus, our H3O• hypothesis is able to explain not only the previous isotopic substitution experiment but also why the t-F degradation reaction is a highly plausible H• degradation mechanism for proton-exchange membranes. To our knowledge, this is the first suggestion that H3O• radicals could be present in electrochemical devices with both experimental and theoretical support.

The alkali degradation of LDPE-based radiation-grafted anion-exchange membranes studied using different ex situ methods.

Radiation-grafted anion-exchange membranes (RG-AEM) in alkaline membrane fuel cells (AEMFC) exhibit promising performances (low in situ resistances, high power outputs and reasonably high alkali stabilities). Much research is focused on developing AEMs with enhanced chemical stabilities in the OH--forms at temperatures >60 °C. This study contributes towards this effort by providing a comparison of three different ex situ methods of screening alkali stabilities (where different laboratories conducted experiments on exactly the same batches of RG-AEM). Vinylbenzyl chloride monomer was radiation-grafted onto 25 µm thick low-density polyethylene (LDPE) precursor film in a single batch. This batch of grafted membrane was then split into three sub-batches, which were converted into RG-AEMs via amination with either: trimethylamine (TMA), N-methylpyrrolidine (MPY), or N-methylpiperidine (MPIP). Samples of each RG-AEM (l-AEM-TMA, l-AEM-MPY, and l-AEM-MPIP) were then distributed between the three collaborating institutes for evaluation using each institutes' test protocols. Out of the three head-group chemistries, the l-AEM-TMA generally exhibits the best balance of conductivity and ex situ alkali degradation, especially in lower humidity environments. The l-AEM-TMA also exhibited interestingly high Cl- ion conductivities (ca. 100 mS cm-1) when heated to 80 °C in a relative humidity RH = 95% atmosphere, a measurement frequently overlooked in favour of determining conductivities of RG-AEMs submerged in water (conductivities of submerged RG-AEMs can be suppressed due to excessive water contents and swelling).

Perspectives on Low-Temperature Electrolysis and Potential for Renewable Hydrogen at Scale.

Hydrogen is an important part of any discussion on sustainability and reduction in emissions across major energy sectors. In addition to being a feedstock and process gas for many industrial processes, hydrogen is emerging as a fuel alternative for transportation applications. Renewable sources of hydrogen are therefore required to increase in capacity. Low-temperature electrolysis of water is currently the most mature method for carbon-free hydrogen generation and is reaching relevant scales to impact the energy landscape. However, costs still need to be reduced to be economical with traditional hydrogen sources. Operating cost reductions are enabled by the recent availability of low-cost sources of renewable energy, and the potential exists for a large reduction in capital cost withmaterial and manufacturing optimization. This article focuses on the current status and development needs by component for the low-temperature electrolysis options.

A Roadmap to Low-Cost Hydrogen with Hydroxide Exchange Membrane Electrolyzers.

Hydrogen is an ideal alternative energy carrier to generate power for all of society's energy demands including grid, industrial, and transportation sectors. Among the hydrogen production methods, water electrolysis is a promising method because of its zero greenhouse gas emission and its compatibility with all types of electricity sources. Alkaline electrolyzers (AELs) and proton exchange membrane electrolyzers (PEMELs) are currently used to produce hydrogen. AELs are commercially mature and are used in a variety of industrial applications, while PEMELs are still being developed and find limited application. In comparison with AELs, PEMELs have more compact structure and can achieve higher current densities. Recently, however, an alternative technology to PEMELs, hydroxide exchange membrane electrolyzers (HEMELs), has gained considerable attention due to the possibility to use platinum group metal (PGM)-free electrocatalysts and cheaper membranes, ionomers, and construction materials and its potential to achieve performance parity with PEMELs. Here, the state-of-the-art AELs and PEMELs along with the current status of HEMELs are discussed in terms of their positive and negative aspects. Additionally discussed are electrocatalyst, membrane, and ionomer development needs for HEMELs and benchmark electrocatalysts in terms of the cost-performance tradeoff.

Synthesis of Platinum-nickel Nanowires and Optimization for Oxygen Reduction Performance.

Platinum-nickel (Pt-Ni) nanowires were developed as fuel cell electrocatalysts, and were optimized for the performance and durability in the oxygen reduction reaction. Spontaneous galvanic displacement was used to deposit Pt layers onto Ni nanowire substrates. The synthesis approach produced catalysts with high specific activities and high Pt surface areas. Hydrogen annealing improved Pt and Ni mixing and specific activity. Acid leaching was used to preferentially remove Ni near the nanowire surface, and oxygen annealing was used to stabilize near-surface Ni, improving durability and minimizing Ni dissolution. These protocols detail the optimization of each post-synthesis processing step, including hydrogen annealing to 250 °C, exposure to 0.1 M nitric acid, and oxygen annealing to 175 °C. Through these steps, Pt-Ni nanowires produced increased activities more than an order of magnitude than Pt nanoparticles, while offering significant durability improvements. The presented protocols are based on Pt-Ni systems in the development of fuel cell catalysts. These techniques have also been used for a variety of metal combinations, and can be applied to develop catalysts for a number of electrochemical processes.

Relating alkaline stability to the structure of quaternary phosphonium cations.

Alkali-stable quaternary phosphonium (QP) is a type of cationic group for hydroxide exchange membranes (HEMs). To elucidate the relationship between structure and alkaline stability, we investigated the kinetics and degradation mechanism of a series of QP cations by both experiment and computation, and established a semi-empirical formula based on the Taft equation to directly estimate alkaline stability of QP cations from the 31P NMR chemical shift δ and the steric substituent constant E s, facilitating the search for QP cations with improved alkaline stability.

Exceptional Oxygen Reduction Reaction Activity and Durability of Platinum-Nickel Nanowires through Synthesis and Post-Treatment Optimization.

For the first time, extended nanostructured catalysts are demonstrated with both high specific activity (>6000 µA cmPt -2 at 0.9 V) and high surface areas (>90 m2 gPt -1). Platinum-nickel (Pt-Ni) nanowires, synthesized by galvanic displacement, have previously produced surface areas in excess of 90 m2 gPt -1, a significant breakthrough in and of itself for extended surface catalysts. Unfortunately, these materials were limited in terms of their specific activity and durability upon exposure to relevant electrochemical test conditions. Through a series of optimized postsynthesis steps, significant improvements were made to the activity (3-fold increase in specific activity), durability (21% mass activity loss reduced to 3%), and Ni leaching (reduced from 7 to 0.3%) of the Pt-Ni nanowires. These materials show more than a 10-fold improvement in mass activity compared to that of traditional carbon-supported Pt nanoparticle catalysts and offer significant promise as a new class of electrocatalysts in fuel cell applications.

Single-Molecule Nanocatalysis Shows In†Situ Deactivation of Pt/C Electrocatalysts during the Hydrogen-Oxidation Reaction.

By coupling a Pt-catalyzed fluorogenic reaction with the Pt-electrocatalyzed hydrogen-oxidation reaction (HOR), we combine single-molecule fluorescence microscopy with traditional electrochemical methods to study the real-time deactivation kinetics of a Pt/C electrocatalyst at single-particle level during electrocatalytic hydrogen-oxidation reaction. The decay of the catalytic performance of Pt/C could be mainly attributed to the electrocatalysis-induced etching or dissolution of Pt nanoparticles. Spontaneous regeneration of activity and incubation period of the Pt electrocatalyst were also observed at single-particle level. All these new insights are practically useful for the understanding and rational design of highly efficient electrocatalysts for application in fuel cells.

Shape-directional growth of Pt and Pd nanoparticles.

The design and synthesis of shape-directed nanoscale noble metal particles have attracted much attention due to their enhanced catalytic properties and the opportunities to study fundamental aspects of nanoscale systems. As such, numerous methods have been developed to synthesize crystals with tunable shapes, sizes, and facets by adding foreign species that promote or restrict growth on specific sites. Many hypotheses regarding how and why certain species direct growth have been put forward, however there has been no consensus on a unifying mechanism of nanocrystal growth. Herein, we develop and demonstrate the capabilities of a mathematical growth model for predicting metal nanoparticle shapes by studying a well known procedure that employs AgNO3 to produce {111} faceted Pt nanocrystals. The insight gained about the role of auxiliary species is then utilized to predict the shape of Pd nanocrystals and to corroborate other shape-directing syntheses reported in literature. The fundamental understanding obtained herein by combining modeling with experimentation is a step toward computationally guided syntheses and, in principle, applicable to predictive design of the growth of crystalline solids at all length scales (nano to bulk).

Platinum-coated copper nanowires with high activity for hydrogen oxidation reaction in base.

Platinum (Pt)-coated copper (Cu) nanowires (Pt/CuNWs) are synthesized by the partial galvanic displacement of CuNWs and have a 100 nm diameter and are 25-40 µm length. Pt/CuNWs are studied as a hydrogen oxidation reaction (HOR) catalyst in base along with Cu templated Pt nanotubes (PtNT (Cu)), a 5% Cu monolayer on a bulk polycrystalline Pt electrode (5% ML Cu/BPPt), BPPt, and carbon supported Pt (Pt/C). Comparison of these catalysts demonstrates that the inclusion of Cu benefited the HOR activity of Pt/CuNWs likely by providing compressive strain on Pt; surface Cu further aids in hydroxyl adsorption, thereby improving the HOR activity of Pt/CuNWs. Pt/CuNWs exceed the area and mass exchange current densities of carbon supported Pt by 3.5 times and 1.9 times.

Moving beyond mass-based parameters for conductivity analysis of sulfonated polymers.

The proton conductivity of polymer electrolytes is critical for fuel cells and has therefore been studied in significant detail. The conductivity of sulfonated polymers has been linked to material characteristics to elucidate trends. Mass-based measurements based on water uptake and ion exchange capacity are two of the most common material characteristics used to make comparisons between polymer electrolytes, but they have significant limitations when correlated to proton conductivity. These limitations arise in part because different polymers can have significantly different densities and because conduction occurs over length scales more appropriately represented by volume measurements rather than mass. Herein we establish and review volume-related parameters that can be used to compare the proton conductivity of different polymer electrolytes. Morphological effects on proton conductivity are also considered. Finally, the impact of these phenomena on designing next-generation sulfonated polymers for polymer electrolyte membrane fuel cells is discussed.

Dynamic behavior of water within a polymer electrolyte fuel cell membrane at low hydration levels.

Protonic conduction across the membrane of a polymer electrolyte fuel cell is intimately related to the dynamic behavior of water present within the membrane. To further the understanding of water dynamics in these materials, quasielastic neutron scattering (QENS) has been used to investigate the picosecond dynamic behavior of water within a perfluorosulfonated ionomer (PFSI) membrane under increasing hydration levels from dry to saturation. Evaluation of the elastic incoherent structure factor (EISF) reveals an increase in the characteristic length-scale of confinement as the number of water molecules in the membrane increases, tending to an asymptotic value at saturation. The fraction of elastic incoherent scattering observed at high Q over all hydration levels is well fit by a simple model that assumes a single, nondiffusing hydronium ion per membrane sulfonic acid site. The quasielastic component of the fitted data indicates confined dynamic behavior for scattering vectors less than 0.7 A(-1). As such, the dynamic behavior was interpreted using continuous diffusion confined within a sphere at Q < 0.7 A(-1) and random unconstrained jump diffusion at Q > 0.7 A(-1). As the number of water molecules in the membrane increases, the characteristic residence times obtained from both models is reduced. The increased dynamical frequency is further reflected in the diffusion coefficients predicted by both models. Between low hydration (2 H2O/SO3H) and saturation (16 H2O/SO3H), the continuous spherical diffusion coefficient changes from 0.46 +/- 0.12 to 1.04 +/- 0.12 (10(-5) cm2/s) and jump diffusion indicates an increase from 1.21 +/- 0.03 to 2.14 +/- 0.08 (10(-5) cm2/s). Overall, the dynamic behavior of water has been quantified over different length scale regimes, the results of which may be rationalized on the basis of the formation of water clusters in the hydrophilic domain that expand toward an asymptotic upper limit with increased hydration.