Recent Insights on Catalyst Layers for Anion Exchange Membrane Fuel Cells

Crystal-Phase- and B-Content-Dependent Electrochemical Behavior of Pd─B Nanocrystals toward Oxygen Reduction Reaction

The manipulation of crystal phases in metal-nonmetal interstitial alloy nanostructures has attracted considerable attention due to the formation of unique electronic structures and surface atomic arrangements, resulting in unprecedented catalytic performances. However, achieving simultaneous control over crystal phase and nonmetal elements in metal-nonmetal interstitial alloy nanostructures has remained a formidable challenge. Here, a novel synthesis approach is presented for Pd─B interstitial alloy nanocrystals (NCs) that allows investigation of the crystal-phase- and B-content-dependent catalytic performance. Through comparison of the oxygen reduction reaction (ORR) properties of Pd─BX interstitial alloy NCs with different crystal phases and B contents, achieved by precise control of reaction temperature and time, the influences of crystal phase and B contents in the Pd─BX interstitial alloy NCs on ORR are precisely investigated. The hexagonal closed packed (hcp) PdB0.5 NCs exhibit superior catalytic activity, with mass activities reaching 2.58 A mg-1, surpassing Pd/C by 10.3 times, attributed to synergistic effects by the hcp crystal phase and relatively high B contents. This study not only provides a novel approach to fabricate interstitial alloy nanostructures with unconventional crystal phases and finely controlled nonmetal elements but also elucidates the importance of crystal phase and nonmetal element content in optimizing electrocatalytic efficiency.

Catalytically Active Carbon for Oxygen Reduction Reaction in Energy Conversion: Recent Advances and Future Perspectives

The shortage and unevenness of fossil energy sources are affecting the development and progress of human civilization. The technology of efficiently converting material resources into energy for utilization and storage is attracting the attention of researchers. Environmentally friendly biomass materials are a treasure to drive the development of new-generation energy sources. Electrochemical theory is used to efficiently convert the chemical energy of chemical substances into electrical energy. In recent years, significant progress has been made in the development of green and economical electrocatalysts for oxygen reduction reaction (ORR). Although many reviews have been reported around the application of biomass-derived catalytically active carbon (CAC) catalysts in ORR, these reviews have only selected a single/partial topic (including synthesis and preparation of catalysts from different sources, structural optimization, or performance enhancement methods based on CAC catalysts, and application of biomass-derived CACs) for discussion. There is no review that systematically addresses the latest progress in the synthesis, performance enhancement, and applications related to biomass-derived CAC-based oxygen reduction electrocatalysts synchronously. This review fills the gap by providing a timely and comprehensive review and summary from the following sections: the exposition of the basic catalytic principles of ORR, the summary of the chemical composition and structural properties of various types of biomass, the analysis of traditional and the latest popular biomass-derived CAC synthesis methods and optimization strategies, and the summary of the practical applications of biomass-derived CAC-based oxidative reduction electrocatalysts. This review provides a comprehensive summary of the latest advances to provide research directions and design ideas for the development of catalyst synthesis/optimization and contributes to the industrialization of biomass-derived CAC electrocatalysis and electric energy storage.

Nanostructure Engineering of Cathode Layers in Proton Exchange Membrane Fuel Cells: From Catalysts to Membrane Electrode Assembly

The membrane electrode assembly (MEA) is the core component of proton exchange membrane fuel cells (PEMFCs), which is the place where the reaction occurrence, the multiphase material transfer and the energy conversion, and the development of MEA with high activity and long stability are crucial for the practical application of PEMFCs. Currently, efforts are devoted to developing the regulation of MEA nanostructure engineering, which is believed to have advantages in improving catalyst utilization, maximizing three-phase boundaries, enhancing mass transport, and improving operational stability. This work reviews recent research progress on platinum group metal (PGM) and PGM-free catalysts with multidimensional nanostructures, catalyst layers (CLs), and nano-MEAs for PEMFCs, emphasizing the importance of structure-function relationships, aiming to guide the further development of the performance for PEMFCs. Then the design strategy of the MEA interface is summarized systematically. In addition, the application of in situ and operational characterization techniques to adequately identify current density distributions, hot spots, and water management visualization of MEAs is also discussed. Finally, the limitations of nanostructured MEA research are discussed and future promising research directions are proposed. This paper aims to provide valuable insights into the fundamental science and technical engineering of efficient MEA interfaces for PEMFCs.

Activating and Identifying the Active Site of RuS2 for Alkaline Hydrogen Oxidation Electrocatalysis

Searching for highly efficient and economical electrocatalysts for alkaline hydrogen oxidation reaction (HOR) is crucial for the development of alkaline polymer membrane fuel cells. Here, we report a valid strategy to active pyrite-type RuS2 for alkaline HOR electrocatalysis by introducing sulfur vacancies. The obtained S-vacancies modified RuS2-x exhibits outperformed HOR activity with a current density of 0.676 mA cm-2 and mass activity of 1.43 mA μg-1, which are 15-fold and 40-fold improvement than those of Ru catalyst. In situ Raman spectra demonstrate the formation of S-H bond during the HOR process, identifying the S atom of RuS2-x is the real active site for HOR catalysis. Density functional theory calculations and experimental results including in situ surface-enhanced infrared absorption spectroscopy suggest the introduction of S vacancies can rationally modify the p orbital of S atoms, leading to enhanced binding strength between the S sites and H atoms on the surface of RuS2-x, together with the promoted connectivity of hydrogen-bonding network and lowered water formation energy, contributes to the enhanced HOR performance.

CO2 Electrolyzers

CO2 electrolyzers have progressed rapidly in energy efficiency and catalyst selectivity toward valuable chemical feedstocks and fuels, such as syngas, ethylene, ethanol, and methane. However, each component within these complex systems influences the overall performance, and the further advances needed to realize commercialization will require an approach that considers the whole process, with the electrochemical cell at the center. Beyond the cell boundaries, the electrolyzer must integrate with upstream CO2 feeds and downstream separation processes in a way that minimizes overall product energy intensity and presents viable use cases. Here we begin by describing upstream CO2 sources, their energy intensities, and impurities. We then focus on the cell, the most common CO2 electrolyzer system architectures, and each component within these systems. We evaluate the energy savings and the feasibility of alternative approaches including integration with CO2 capture, direct conversion of flue gas and two-step conversion via carbon monoxide. We evaluate pathways that minimize downstream separations and produce concentrated streams compatible with existing sectors. Applying this comprehensive upstream-to-downstream approach, we highlight the most promising routes, and outlook, for electrochemical CO2 reduction.

Investigating Electrode-Ionomer Interface Phenomena for Electrochemical Energy Applications

The endeavor to develop high-performance electrochemical energy applications has underscored the growing importance of comprehending the intricate dynamics within an electrode's structure and their influence on overall performance. This review investigates the complexities of electrode-ionomer interactions, which play a critical role in optimizing electrochemical reactions. Our examination encompasses both microscopic and meso/macro scale functions of ionomers at the electrode-ionomer interface, providing a thorough analysis of how these interactions can either enhance or impede surface reactions. Furthermore, this review explores the broader-scale implications of ionomer distribution within porous electrodes, taking into account factors like ionomer types, electrode ink formulation, and carbon support interactions. We also present and evaluate state-of-the-art techniques for investigating ionomer distribution, including electrochemical methods, imaging, modeling, and analytical techniques. Finally, the performance implications of these phenomena are discussed in the context of energy conversion devices. Through this comprehensive exploration of intricate interactions, this review contributes to the ongoing advancements in the field of energy research, ultimately facilitating the design and development of more efficient and sustainable energy devices.

A trinary support of Ni/NiO/C to immobilize Ir nanoclusters for alkaline hydrogen oxidation

The hydrogen oxidation reaction is an important process in anion exchange membrane fuel cells with alkaline solutions. The pursuit of efficient catalysts for alkaline hydrogen oxidation has attracted considerable attention. In this study, we present a precursor route for the synthesis of a new Ir-based catalyst (Ir-Ni/NiO/C), in which Ir nanoclusters were immobilized on the generated Ni/NiO/C support derived from a metal-organic framework. The small size of Ir clusters facilitates the exposure of catalytically active sites. The electronic interplay between the Ir nanoclusters and the Ni/NiO/C support optimized the hydrogen binding energy (HBE) and hydroxide binding energy (OHBE) on the surface, which is unattainable on the contrasting Ir-C, Ir-Ni/C, and Ir-NiO/C products. The optimized catalyst shows excellent mass activity for alkaline hydrogen oxidation, which is 3.1 times that of the Pt/C catalyst. This study presents a promising pathway for the development of advanced HOR catalysts.

A lignin-derived N-doped carbon-supported iron-based nanocomposite as high-efficiency oxygen reduction reaction electrocatalyst

Fuel cells are a promising renewable energy technology that depend heavily on noble metal Pt-based catalysts, particularly for the oxygen reduction reaction (ORR). The discovery of new, efficient non-precious metal ORR catalysts is critical for the continued development of cost-effective, high-performance fuel cells. The synthesized carbon material showed excellent electrocatalytic activity for the ORR, with half-wave potential (E1/2) and limiting current density (JL) of 0.88 V and 5.10 mA·cm-2 in alkaline electrolyte, respectively. The material has a Tafel slope of (65 mV dec-1), which is close to commercial Pt/C catalysts (60 mV dec-1). Moreover, the prepared materials exhibited excellent performance when assembled as cathodes for zinc-air batteries. The power density reached 110.02 mW cm-2 and the theoretical specific capacity was 801.21 mAh g-1, which was higher than that of the Pt/C catalyst (751.19 mAh g-1). In this study, with the assistance of Mg5(CO3)4(OH)2·4H2O, we introduce an innovative approach to synthesize advanced carbon materials, achieving precise control over the material's structure and properties. This research bridges a crucial gap in material science, with potential applications in renewable energy technologies, particularly in enhancing catalysts for fuel cells.

Indispensable Nafion Ionomer for High-Efficiency and Stable Oxygen Evolution Reaction in Alkaline Media

Addressing the challenge of sluggish kinetics and limited stability in alkaline oxygen evolution reactions, recent exploration of novel electrochemical catalysts offers improved prospects. To expedite the assessment of these catalysts, a half-cell rotating disk electrode is often favored for its simplicity. However, the actual catalyst performance strongly depends on the fabricated catalyst layers, which encounter mass transport overpotentials. We systematically investigate the role and sequence of electrode drop-casting methods onto a glassy carbon electrode regarding the efficiency of the oxygen evolution reaction. The catalyst layer without Nafion experiences nearly 50% activity loss post stability test, while those with Nafion exhibit less than 5% activity loss. Additionally, the sequence of application of the catalyst and Nafion also shows a significant effect on catalyst stability. The catalyst activity increases by roughly 20% after the stability test when the catalyst layer is coated first with an ionomer layer, followed by drop-casting the catalysts. Based on the half-cell results, the Nafion ionomer not only acts as a binder in the catalyst layer but also enhances the interfacial interaction between the catalyst and electrolyte, promoting performance and stability. This study provides new insights into the efficient and accurate evaluation of electrocatalyst performance and stability as well as the role of Nafion ionomer in the catalyst layer.

Hydrocarbon Ionomeric Binders for Fuel Cells and Electrolyzers

Ionomeric binders in catalyst layers, abbreviated as ionomers, play an essential role in the performance of polymer-electrolyte membrane fuel cells and electrolyzers. Due to environmental issues associated with perfluoroalkyl substances, alternative hydrocarbon ionomers have drawn substantial attention over the past few years. This review surveys literature to discuss ionomer requirements for the electrodes of fuel cells and electrolyzers, highlighting design principles of hydrocarbon ionomers to guide the development of advanced hydrocarbon ionomers.

Intermetallic Nanocrystals for Fuel-Cells-Based Electrocatalysis

Electrocatalysis underpins the renewable electrochemical conversions for sustainability, which further replies on metallic nanocrystals as vital electrocatalysts. Intermetallic nanocrystals have been known to show distinct properties compared to their disordered counterparts, and been long explored for functional improvements. Tremendous progresses have been made in the past few years, with notable trend of more precise engineering down to an atomic level and the investigation transferring into more practical membrane electrode assembly (MEA), which motivates this timely review. After addressing the basic thermodynamic and kinetic fundamentals, we discuss classic and latest synthetic strategies that enable not only the formation of intermetallic phase but also the rational control of other catalysis-determinant structural parameters, such as size and morphology. We also demonstrate the emerging intermetallic nanomaterials for potentially further advancement in energy electrocatalysis. Then, we discuss the state-of-the-art characterizations and representative intermetallic electrocatalysts with emphasis on oxygen reduction reaction evaluated in a MEA setup. We summarize this review by laying out existing challenges and offering perspective on future research directions toward practicing intermetallic electrocatalysts for energy conversions.

Advanced Architectures of Air Electrodes in Zinc-Air Batteries and Hydrogen Fuel Cells

The air electrode is an essential component of air-demanding energy storage/conversion devices, such as zinc-air batteries (ZABs) and hydrogen fuel cells (HFCs), which determines the output power and stability of the devices. Despite atom-level modulation in catalyst design being recently achieved, the air electrodes have received much less attention, causing a stagnation in the development of air-demanding equipment. Herein, the evolution of air electrodes for ZABs and HFCs from the early stages to current requirements is reviewed. In addition, the operation mechanism and the corresponding electrocatalytic mechanisms of ZABs are summarized. In particular, by clarifying the air electrode interfaces of ZABs at different scales, several approaches to improve the air electrode in rechargeable ZABs are reviewed, including innovative electrode structures and bifunctional oxygen catalysts. Afterward, the operating mechanisms of proton-exchange-membrane fuel cells (PEMFCs) and anion-exchange-membrane fuel cells (AEMFCs) are explained. Subsequently, the strategies employed to enhance the efficiency of the membrane electrode assembly (MEA) in PEMFCs and AEMFCs, respectively, are highlighted and discussed in detail. Last, the prospects for air electrodes in ZABs and HFCs are considered by discussing the main challenges. The aim of this review is to facilitate the industrialization of ZABs and HFCs.

Defect-Engineered ZIF-Derived Non-Pt Cathode Catalyst at 1.5 mg cm-2 Loading for Proton Exchange Membrane Fuel Cells

Due to the sluggish kinetics of the oxygen reduction reaction (ORR) by non-Pt based catalyst, high loading of catalyst is required to achieve satisfactory fuel cell performance, which inevitably leads to the increase of the catalyst layer thickness with serious mass transport resistance. Herein, a defective zeolitic imidazolate framework (ZIF) derived Co/Fe-N-C catalyst with small mesopores (2-4 nm) and high density of CoFe atomic active sites are prepared by regulating the Fe dosage and pyrolysis temperature. Molecular dynamics simulation and electrochemical tests indicate that > 2 nm mesopores show insignificant influence on the diffusion process of O2 and H2 O molecules, leading to the high utilization of active sites and low mass transport resistance. The proton exchange membrane fuel cell (PEMFC) shows a high-power density of 755 mW cm-2 with only 1.5 mg cm-2 of non-Pt catalyst in the cathode. No apparent performance loss caused by concentration difference can be observed, in particular in the high current density region (1 A cm-2 ). This work emphasizes the importance of small mesopore design in the Co/Fe-N-C catalyst, which is anticipated to provide essential guidance for the application of non-Pt catalysts.

Development of Hydrogen-Oxygen Fuel Cells Based on Anion-Exchange Electrolytes and Catalysts with Reduced Platinum Content

Studies have been carried out to optimize the composition, formation technique and test conditions of membrane electrode assemblies (MEA) of hydrogen-oxygen anion-exchange membranes fuel cells (AEMFC), based on Fumatech anion-exchange membranes. A non-platinum catalytic system based on nitrogen-doped CNT (CNT_N) was used in the cathode. PtMo/CNT_N catalysts with a reduced content of platinum (10-12 wt.% Pt) were compared with 10 and 60 wt.% Pt/CNT_N at the anode. According to the results of studies under model conditions, it was found that the PtMo/CNT_N catalyst is significantly superior to the 10 and 60 wt.% Pt/CNT_N catalyst in terms of activity in the hydrogen oxidation reaction based on the mass of platinum. The addition of the Fumion ionomer results in minor changes in the electrochemically active surface area and activity in the hydrogen oxidation reaction for each of the catalysts. In this case, the introduction of ionomer-Fumion leads to a partial blocking of the outer surface and the micropore surface, which is most pronounced in the case of the 60Pt/CNT_N catalyst. This effect can cause a decrease in the characteristics of MEA AEMFC upon passing from 10PtMo/CNT_N to 60Pt/CNT_N in the anode active layer. The maximum power density of the optimized MEA based on 10PtMo/CNT_N was 62 mW cm^-2, which exceeds the literature data obtained under similar test conditions for MEA based on platinum cathode and anode catalysts and Fumatech membranes (41 mW cm^-2). A new result of this work is the study of the effect of the ionomer (Fumion) on the characteristics of catalysts. It is shown that the synthesized 10PtMo/CNT_N catalyst retains high activity in the presence of an ionomer under model conditions and in the MEA based on it.

Solid-Liquid-Gas Management for Low-Cost Hydrogen Gas Batteries

Aqueous nickel-hydrogen gas (Ni-H₂) batteries with excellent durability (>10,000 cycles) are important candidates for grid-scale energy storage but are hampered by the high-cost Pt electrode with limited performance. Herein, we report a low-cost nickel-molybdenum (NiMo) alloy as an efficient bifunctional hydrogen evolution and oxidation reaction (HER/HOR) catalyst for Ni-H₂ batteries in alkaline electrolytes. The NiMo alloy demonstrates a high HOR mass-specific kinetic current of 28.8 mA mg^-1 at 50 mV as well as a low HER overpotential of 45 mV at a current density of 10 mA cm^-2, which is better than most nonprecious metal catalysts. Furthermore, we apply a solid-liquid-gas management strategy to constitute a conductive, hydrophobic network of NiMo using multiwalled carbon nanotubes (NiMo-hydrophobic MWCNT) in the electrode to accelerate HER/HOR activities for much improved Ni-H₂ battery performance. As a result, Ni-H₂ cells based on the NiMo-hydrophobic MWCNT electrode show a high energy density of 118 Wh kg^-1 and a low cost of only 67.5 $ kWh^-1. With the low cost, high energy density, excellent durability, and improved energy efficiency, the Ni-H₂ cells show great potential for practical grid-scale energy storage.

Impact of Anion Exchange Ionomers on the Electrocatalytic Performance for the Oxygen Reduction Reaction of B-N Co-doped Carbon Quantum Dots on Activated Carbon

Composite electrocatalytic electrodes made from B-N co-doped carbon quantum dots (CQD) and various anion exchange ionomers (AEI) are studied for the oxygen reduction reaction (ORR) in alkaline solutions. The quantity and positions of dopants in CQD, prepared by hydrothermal synthesis, are analyzed by various spectroscopies, including ¹¹B NMR spectroscopy that evidenced boronic acid at edge sites. The AEI are synthesized with various backbones, including more hydrophilic polysulfone, hydrophobic poly(alkylene biphenyl), and poly(2,6-dimethyl-1,4-phenylene oxide) with intermediate hydrophilicity; the functional groups are trimethylammonium moieties grafted on long (LC) or short (SC) side chains. The CQD/AEI ink is drop-casted on activated carbon paper, and the samples are fixed on a rotating disk electrode and studied in three-electrode configuration in oxygen-saturated 0.1 M KOH. The onset potentials are among the best in the literature (E_onset ≈ 0.94 V vs RHE). The highest electrocatalytic activity is observed for electrodes containing AEI with long side chains; the sample containing PPO LC attains excellent ORR currents approaching that of benchmark Pt/C cloth. The electrocatalytic performances are discussed in view of the many relevant AEI parameters, including hydrophilicity, oxygen permeability, catalyst dispersivity, and ionic conductivity.

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.

Emerging Electrochemical Processes to Decarbonize the Chemical Industry

Electrification is a potential approach to decarbonizing the chemical industry. Electrochemical processes, when they are powered by renewable electricity, have lower carbon footprints in comparison to conventional thermochemical routes. In this Perspective, we discuss the potential electrochemical routes for chemical production and provide our views on how electrochemical processes can be matured in academic research laboratories for future industrial applications. We first analyze the CO₂ emission in the manufacturing industry and conduct a survey of state of the art electrosynthesis methods in the three most emission-intensive areas: petrochemical production, nitrogen compound production, and metal smelting. Then, we identify the technical bottlenecks in electrifying chemical productions from both chemistry and engineering perspectives and propose potential strategies to tackle these issues. Finally, we provide our views on how electrochemical manufacturing can reduce carbon emissions in the chemical industry with the hope to inspire more research efforts in electrifying chemical manufacturing.

Anion-exchange membrane water electrolyzers and fuel cells

The key components, working management, and operating techniques of anion-exchange membrane water electrolyzers and fuel cells are reviewed for the first time.

Application of Molecular Catalysts for the Oxygen Reduction Reaction in Alkaline Fuel Cells

The development of precious group metal-free (PGM-free) catalysts for the oxygen reduction reaction is considered as the main thrust for the cost reduction of fuel cell technologies and their mass production. Within the PGM-free category, molecular catalysts offer an advantage over other heat-treated PGM-free catalysts owing to their well-defined structure, which enables further design of more active, selective, and durable catalysts. Even though non-heat-treated molecular catalysts with exceptional performance have been reported in the past, they were rarely tested in a fuel cell. Herein, we report on a molecular catalyst under alkaline conditions: fluorinated iron phthalocyanine (FeFPc) supported on cheap and commercially available high-surface area carbon─BP2000 (FeFPc@BP2000). It exhibits the highest activity ever reported for molecular catalysts under alkaline conditions in half-cells and fuel cells.

Cathodic Electrodeposition of MOF Films Using Hydrogen Peroxide

Metal-organic framework (MOF) films can be made by cathodic electrodeposition, where a Brønsted base is formed electrochemically which deprotonates the MOF linkers that are present in solution as undissociated/partially dissociated weak acids. However, the co-deposition of metal and the narrow range of possible metal nodes limit the scope of this method. In this work, we propose the use of hydrogen peroxide (hydrogen peroxide assisted cathodic deposition or HPACD), to overcome these limitations. Electrochemical measurements indicate that in DMF, hydrogen peroxide is reduced to superoxide anions that deprotonate the carboxylic ligands. This single-electron reduction happens at much higher potentials than all previous reported methods. This prevents the co-deposition of metal and extends the range of possible metal nodes. Various pure MOF films (HKUST-1, MIL-53(Fe) and MOF-5) were prepared via this approach. HPACD was also used for the preparation of patterned MOF films and of flexible Cu-BTC coated paper membranes which reject 99.1 % of Rose Bengal from water with a permeance of 8.4 L m^-2  h^-1  bar^-1 .