Noncarbon Support Materials for Polymer Electrolyte Membrane Fuel Cell Electrocatalysts

Degradation and Photocatalytic Properties of Nanocomposites

The world is suffering from energy consumption and environmental pollution challenges for the next generation era [...].

Oxygen-deficient tungsten oxide inducing electron and proton transfer: Activating ruthenium sites for hydrogen evolution in wide pH and alkaline seawater

The design of electrocatalysts for the hydrogen evolution reaction (HER) that perform effectively across a broad pH spectrum is paramount. The efficiency of hydrogen evolution at ruthenium (Ru) active sites, often hindered by the kinetics of water dissociation in alkaline or neutral conditions, requires further enhancement. Metal oxides, due to superior electron dynamics facilitated by oxygen vacancies (OVS) and shifts in the Fermi level, surpass carbon-based materials. In particular, tungsten oxide (WO3) promotes the directed migration of electrons and protons which significantly activates the Ru sites. Ru/WO3-OV is prepared through a simple hydrothermal and low-temperature annealing process. The prepared catalyst achieves 10 mA cm-2 at overpotentials of 23 mV (1 M KOH), 36 mV (0.5 M H2SO4), 62 mV (1 M PBS), and 38 mV (1 M KOH + seawater). At an overpotential corresponding to 10 mA cm-2 in 1 M KOH and 1 M KOH + seawater, the mass activity of Ru/WO3-OV is about 7.7 and 7.86 times that of 20 wt% Pt/C. The improvement in activity and stability arises from electronic modifications attributed to metal-support interaction. This work offers novel insights for modulating the HER activity of Ru sites across a wide pH range.

Silica-Derived Nanostructured Electrode Materials for ORR, OER, HER, CO2RR Electrocatalysis, and Energy Storage Applications: A Review

Silica-derived nanostructured catalysts (SDNCs) are a class of materials synthesized using nanocasting and templating techniques, which involve the sacrificial removal of a silica template to generate highly porous nanostructured materials. The surface of these nanostructures is functionalized with a variety of electrocatalytically active metal and non-metal atoms. SDNCs have attracted considerable attention due to their unique physicochemical properties, tunable electronic configuration, and microstructure. These properties make them highly efficient catalysts and promising electrode materials for next generation electrocatalysis, energy conversion, and energy storage technologies. The continued development of SDNCs is likely to lead to new and improved electrocatalysts and electrode materials. This review article provides a comprehensive overview of the recent advances in the development of SDNCs for electrocatalysis and energy storage applications. It analyzes 337,061 research articles published in the Web of Science (WoS) database up to December 2022 using the keywords "silica", "electrocatalysts", "ORR", "OER", "HER", "HOR", "CO2RR", "batteries", and "supercapacitors". The review discusses the application of SDNCs for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO2RR), supercapacitors, lithium-ion batteries, and thermal energy storage applications. It concludes by discussing the advantages and limitations of SDNCs for energy applications.

Metal-Support Interaction between Titanium Oxynitride and Pt Nanoparticles Enables Efficient Low-Pt-Loaded High-Performance Electrodes at Relevant Oxygen Reduction Reaction Current Densities

In the present work, we report on a synergistic relationship between platinum nanoparticles and a titanium oxynitride support (TiOxNy/C) in the context of oxygen reduction reaction (ORR) catalysis. As demonstrated herein, this composite configuration results in significantly improved electrocatalytic activity toward the ORR relative to platinum dispersed on carbon support (Pt/C) at high overpotentials. Specifically, the ORR performance was assessed under an elevated mass transport regime using the modified floating electrode configuration, which enabled us to pursue the reaction closer to PEMFC-relevant current densities. A comprehensive investigation attributes the ORR performance increase to a strong interaction between platinum and the TiOxNy/C support. In particular, according to the generated strain maps obtained via scanning transmission electron microscopy (STEM), the Pt-TiOxNy/C analogue exhibits a more localized strain in Pt nanoparticles in comparison to that in the Pt/C sample. The altered Pt structure could explain the measured ORR activity trend via the d-band theory, which lowers the platinum surface coverage with ORR intermediates. In terms of the Pt particle size effect, our observation presents an anomaly as the Pt-TiOxNy/C analogue, despite having almost two times smaller nanoparticles (2.9 nm) compared to the Pt/C benchmark (4.8 nm), manifests higher specific activity. This provides a promising strategy to further lower the Pt loading and increase the ECSA without sacrificing the catalytic activity under fuel cell-relevant potentials. Apart from the ORR, the platinum-TiOxNy/C interaction is of a sufficient magnitude not to follow the typical particle size effect also in the context of other reactions such as CO stripping, hydrogen oxidation reaction, and water discharge. The trend for the latter is ascribed to the lower oxophilicity of Pt-based on electrochemical surface coverage analysis. Namely, a lower surface coverage with oxygenated species is found for the Pt-TiOxNy/C analogue. Further insights were provided by performing a detailed STEM characterization via the identical location mode (IL-STEM) in particular, via 4DSTEM acquisition. This disclosed that Pt particles are partially encapsulated within a thin layer of TiOxNy origin.

Nanoengineered, Pd-doped Co@C nanoparticles as an effective electrocatalyst for OER in alkaline seawater electrolysis

Water electrolysis is considered one of the major sources of green hydrogen as the fuel of the future. However, due to limited freshwater resources, more interest has been geared toward seawater electrolysis for hydrogen production. The development of effective and selective electrocatalysts from earth-abundant elements for oxygen evolution reaction (OER) as the bottleneck for seawater electrolysis is highly desirable. This work introduces novel Pd-doped Co nanoparticles encapsulated in graphite carbon shell electrode (Pd-doped CoNPs@C shell) as a highly active OER electrocatalyst towards alkaline seawater oxidation, which outperforms the state-of-the-art catalyst, RuO2. Significantly, Pd-doped CoNPs@C shell electrode exhibiting low OER overpotential of ≈213, ≈372, and ≈ 429 mV at 10, 50, and 100 mA/cm2, respectively together with a small Tafel slope of ≈ 120 mV/dec than pure Co@C and Pd@C electrode in alkaline seawater media. The high catalytic activity at the aforementioned current density reveals decent selectivity, thus obviating the evolution of chloride reaction (CER), i.e., ∼490 mV, as competitive to the OER. Results indicated that Pd-doped Co nanoparticles encapsulated in graphite carbon shell (Pd-doped CoNPs@C electrode) could be a very promising candidate for seawater electrolysis.

First-Row Transition Metals for Catalyzing Oxygen Redox

Rechargeable zinc-air batteries are widely recognized as a highly promising technology for energy conversion and storage, offering a cost-effective and viable alternative to commercial lithium-ion batteries due to their unique advantages. However, the practical application and commercialization of zinc-air batteries are hindered by the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Recently, extensive research has focused on the potential of first-row transition metals (Mn, Fe, Co, Ni, and Cu) as promising alternatives to noble metals in bifunctional ORR/OER electrocatalysts, leveraging their high-efficiency electrocatalytic activity and excellent durability. This review provides a comprehensive summary of the recent advancements in the mechanisms of ORR/OER, the performance of bifunctional electrocatalysts, and the preparation strategies employed for electrocatalysts based on first-row transition metals in alkaline media for zinc-air batteries. The paper concludes by proposing several challenges and highlighting emerging research trends for the future development of bifunctional electrocatalysts based on first-row transition metals.

Trimetallic non-noble NiCoSn alloy as an efficient electrocatalyst towards methanol oxidation and oxygen reduction reactions

The development of a non-noble, highly efficient bifunctional catalyst for methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) is the bottleneck in the alkaline direct methanol fuel cells (ADMFC). Ni-based bi/tri metallic alloys are the promising candidates next to the noble metals in the alkaline medium.Herein we present a facile hydrazine-assisted hydrothermal technique to synthesize a trimetallic nickel-cobalt-tin (NiCoSn) alloy as an efficient electrocatalyst for MOR and ORR reactions. The physiochemical analysis confirms the formation of trimetallic alloys with a high surface area. The as-synthesized trimetallic NiCoSn electrocatalyst exhibited superior MOR activity in terms of mass activity (509 mA mg^-1 at 1.55 V vs RHE) and stability than the bimetallic alloys in 1.0 M KOH electrolyte. Further, the trimetallic alloy delivered a lower onset and half-wave potential of 0.8 and 0.72 V vs RHE with the favorable four-electron transfer in the oxygen reduction reactions. This work highlights a facile approach for preparing Ni-based trimetallic alloys as a promising candidate for the alkaline direct methanol fuel cells/other catalytic applications.

Modification of multi-walled carbon nanotubes with platinum-osmium to develop stable catalysts for direct methanol fuel cells

In the study, a new bimetallic catalyst was synthesized for methanol oxidation using multi-walled carbon nanotube (MWCNT)-supported platinum-osmium (PtOs) nanoparticles (PtOs@MWCNT NPs). The morphological structures of the prepared NPs were examined using different techniques, such as scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrochemical characterization of the synthesized PtOs@MWCNT catalysts, such as chronoamperometry (CA), cyclic voltammetry (CV), scan rate (SR) analysis, cyclic catalytic test, and electrochemical surface area (ECSA) evaluation, were performed in an alkaline medium. From the results obtained, the size of the NPs was found to be 3.12 nm according to the Debye-Schrrer equation, and the MWCNTs were clearly observed by SEM imaging. After the characterization of the prepared nanomaterials, the PtOs@MWCNT catalysts were employed in the methanol oxidation reaction, and a high oxidation current value of 220.86 mA cm^-2 was observed. Besides, according to the CA results, the catalyst exhibited high stability for 4000 s, and it was seen that Os metal improved the catalytic activity of the main catalyst. These results show that the PtOs@MWCNT catalyst is highly stable and reusable, and provides high electrocatalytic activity in the methanol oxidation reaction. Moreover, the obtained catalyst gave ideal results in terms of CO tolerance and activity. These data show that the obtained catalyst will provide significant improvement and superior efficiency in fuel-cell applications.

Synergistic Mechanism of Sub-Nanometric Ru Clusters Anchored on Tungsten Oxide Nanowires for High-Efficient Bifunctional Hydrogen Electrocatalysis

The construction of strong interactions and synergistic effects between small metal clusters and supports offers a great opportunity to achieve high-performance and cost-effective heterogeneous catalysis, however, studies on its applications in electrocatalysis are still insufficient. Herein, it is reported that W18 O49 nanowires supported sub-nanometric Ru clusters (denoted as Ru SNC/W18 O49 NWs) constitute an efficient bifunctional electrocatalyst for hydrogen evolution/oxidation reactions (HER and HOR) under acidic condition. Microstructural analyses, X-ray absorption spectroscopy, and density functional theory (DFT) calculations reveal that the Ru SNCs with an average RuRu coordination number of 4.9 are anchored to the W18 O49 NWs via RuOW bonds at the interface. The strong metal-support interaction leads to the electron-deficient state of Ru SNCs, which enables a modulated RuH strength. Furthermore, the unique proton transport capability of the W18 O49 also provides a potential migration channel for the reaction intermediates. These components collectively enable the remarkable performance of Ru SNC/W18 O49 NWs for hydrogen electrocatalysis with 2.5 times of exchange current density than that of carbon-supported Ru nanoparticles, and even rival the state-of-the-art Pt catalyst. This work provides a new prospect for the development of supported sub-nanometric metal clusters for efficient electrocatalysis.

Controlled Synthesis of Carbon-Supported Pt-Based Electrocatalysts for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells are playing an increasing role in postpandemic economic recovery and climate action plans. However, their performance, cost, and durability are significantly related to Pt-based electrocatalysts, hampering their large-scale commercial application. Hence, considerable efforts have been devoted to improving the activity and durability of Pt-based electrocatalysts by controlled synthesis in recent years as an effective method for decreasing Pt use, and consequently, the cost. Therefore, this review article focuses on the synthesis processes of carbon-supported Pt-based electrocatalysts, which significantly affect the nanoparticle size, shape, and dispersion on supports and thus the activity and durability of the prepared electrocatalysts. The reviewed processes include (i) the functionalization of a commercial carbon support for enhanced catalyst–support interaction and additional catalytic effects, (ii) the methods for loading Pt-based electrocatalysts onto a carbon support that impact the manufacturing costs of electrocatalysts, (iii) the preparation of spherical and nonspherical Pt-based electrocatalysts (polyhedrons, nanocages, nanoframes, one- and two-dimensional nanostructures), and (iv) the postsynthesis treatments of supported electrocatalysts. The influences of the supports, key experimental parameters, and postsynthesis treatments on Pt-based electrocatalysts are scrutinized in detail. Future research directions are outlined, including (i) the full exploitation of the potential functionalization of commercial carbon supports, (ii) scaled-up one-pot synthesis of carbon-supported Pt-based electrocatalysts, and (iii) simplification of postsynthesis treatments. One-pot synthesis in aqueous instead of organic reaction systems and the minimal use of organic ligands are preferred to simplify the synthesis and postsynthesis treatment processes and to promote the mass production of commercial carbon-supported Pt-based electrocatalysts.

Graphical Abstract

This review focuses on the synthesis process of Pt-based electrocatalysts/C to develop aqueous one-pot synthesis at large-scale production for PEMFC stack application.

Corrosion Chemistry of Electrocatalysts

Electrocatalysts are the core components of many sustainable energy conversion technologies that are considered the most potential solution to the worldwide energy and environmental crises. The reliability of structure and composition pledges that electrocatalysts can achieve predictable and stable performance. However, during the electrochemical reaction, electrocatalysts are influenced directly by the applied potential, the electrolyte, and the adsorption/desorption of reactive species, triggering structural and compositional corrosion, which directly affects the catalytic behaviors of electrocatalysts (performance degradation or enhancement) and invalidates the established structure-activity relationship. Therefore, it is necessary to elucidate the corrosion behavior and mechanism of electrocatalysts to formulate targeted corrosion-resistant strategies or use corrosion reconstruction synthesis techniques to guide the preparation of efficient and stable electrocatalysts. Herein, the most recent developments in electrocatalyst corrosion chemistry are outlined, including corrosion mechanisms, mitigation strategies, and corrosion syntheses/reconstructions based on typical materials and important electrocatalytic reactions. Finally, potential opportunities and challenges are also proposed to foresee the possible development in this field. It is believed that this contribution will raise more awareness regarding nanomaterial corrosion chemistry in energy technologies and beyond.

Supported Iridium-based Oxygen Evolution Reaction Electrocatalysts - Recent Developments

The commercialization of acidic proton exchange membrane water electrolyzers (PEMWE) is heavily hindered by the price and scarcity of oxygen evolution reaction (OER) catalyst, i. e. iridium and its oxides. One of the solutions to enhance the utilization of this precious metal is to use a support to distribute well dispersed Ir nanoparticles. In addition, adequately chosen support can also impact the activity and stability of the catalyst. However, not many materials can sustain the oxidative and acidic conditions of OER in PEMWE. Hereby, we critically and extensively review the different materials proposed as possible supports for OER in acidic media and the effect they have on iridium performances.

Robust Oxygen Reduction Electrocatalysis Enabled by Platinum Rooted on Molybdenum Nitride Microrods

Robust oxygen reduction electrocatalysis is central to renewable fuel cells and metal-air batteries. Herein, Pt nanoparticles (NPs) rooted on porous molybdenum nitride microrods (Pt/Mo₂N MRs) are rationally constructed toward the oxygen reduction reaction (ORR). Owing to the desired composition with strong electronic metal-support interactions (EMSIs) and a porous one-dimensional structure supporting ultrafine NPs, the developed Pt/Mo₂N MRs possess much higher ORR mass and specific activities than commercial Pt/C. In situ Raman and density functional theory calculations reveal that the EMSI weakens the adsorption of intermediates over Pt/Mo₂N MRs via an associative mechanism. Moreover, the porous Mo₂N support stabilizes these high activities. Impressively, a homemade zinc-air battery driven by Pt/Mo₂N MRs delivers excellent performance including a peak power density of 167 mW cm^-2 and a high rate capability that ranged from 5 to 50 mA cm^-2. This work highlights the role of EMSI in promoting robust ORR electrocatalysis, thus providing a promising approach for efficient and robust cathode materials for advanced metal-air batteries.

Recent Advances in Anode Electrocatalysts for Direct Formic Acid Fuel Cells - Part I - Fundamentals and Pd Based Catalysts

Direct formic acid fuel cells (DFAFCs) have gained immense importance as a source of clean energy for portable electronic devices. It outperforms other fuel cells in several key operational and safety parameters. However, slow kinetics of the formic acid oxidation at the anode remains the main obstacle in achieving a high power output in DFAFCs. Noble metal-based electrocatalysts are effective, but are expensive and prone to CO poisoning. Recently, a substantial volume of research work have been dedicated to develop inexpensive, high activity and long lasting electrocatalysts. Herein, recent advances in the development of anode electrocatalysts for DFAFCs are presented focusing on understanding the relationship between activity and structure. This review covers the literature related to the electrocatalysts based on noble metals, non-noble metals, metal-oxides, synthesis route, support material, and fuel cell performance. The future prospects and bottlenecks in the field are also discussed at the end.

Magnéli TiO2 as a High Durability Support for the Proton Exchange Membrane (PEM) Fuel Cell Catalysts

Proton exchange membrane fuel cells (PEMFCs) cathode catalysts’ robustness is one of the primary factors determining its long-term performance and durability. This work presented a new class of corrosion-resistant catalyst, Magnél TiO2 supported Pt (Pt/Ti9O17) composite, synthesized. The durability of a Pt/Ti9O17 cathode under the PEMFC operating protocol was evaluated and compared with the state-of-the-art Pt/C catalyst. Like Pt/C, Pt/Ti9O17 exhibited exclusively 4e− oxygen reduction reaction (ORR) in the acidic solution. The accelerated stress tests (AST) were performed using Pt/Ti9O17 and Pt/C catalysts in an O2-saturated 0.5 M H2SO4 solution using the potential-steps cycling experiments from 0.95 V to 0.6 V for 12,000 cycles. The results indicated that the electrochemical surface area (ECSA) of the Pt/Ti9O17 is significantly more stable than that of the state-of-the-art Pt/C, and the ECSA loss after 12,000 potential cycles is only 10 ± 2% for Pt/Ti9O17 composite versus 50 ± 5% for Pt/C. Furthermore, the current density and onset potential at the ORR polarization curve at Pt/C were significantly affected by the AST test. In contrast, the same remained almost constant at the modified electrode, Pt/Ti9O17. This demonstrated the excellent stability of Pt nanoparticles supported on Ti9O17.

Study of the Relationship between Metal–Support Interactions and the Electrocatalytic Performance of Pt/Ti4O7 with Different Loadings

The application potential of Pt/Ti4O7 has been reported, but the lack of research on the relationship between Pt loading, MSI, and catalytic activity hinders further development. Micron-sized Ti4O7 powders synthesized by a thermal reduction method under an H2 atmosphere were used as a support material for Pt-based catalysts. Using a modified polyol method, Pt/Ti4O7-5, Pt/Ti4O7-10, and Pt/Ti4O7-20 with different mass ratios (Pt to Pt/Ti4O7 is 0.05, 0.1, 0.2) were successfully synthesized. Uniformly dispersed platinum nanoparticles exhibit disparate morphologies, rod-like for Pt/Ti4O7-5 and approximately spherical for Pt/Ti4O7-10 and Pt/Ti4O7-20. Small-angle deflections and lattice reconstruction induced by strong metal–support interactions were observed in Pt/Ti4O7-5, which indicated the formation of a new phase at the interface. However, lattice distortions and dislocations for higher loading samples imply the existence of weak metal–support interactions. A possible mechanism is proposed to explain the different morphologies and varying metal–support interactions (MSI). With X-ray photoelectron spectroscopy, spectrums of Pt and Ti display apparent shifts in binding energy compared with commercial Pt-C and non-platinized Ti4O7, which can properly explain the changes in absorption ability and oxygen reduction reaction activity, as described in the electrochemical results. The synthetic method, Pt loading, and surface coverage of the support play an important role in the adjustment of MSI, which gives significant guidance for better utilizing MSI to prepare the target catalyst.

Au(111)@Ti6O11 heterostructure composites with enhanced synergistic effects as efficient electrocatalysts for the hydrogen evolution reaction

Developing cost-effective electrocatalysts for the hydrogen evolution reaction (HER) is of great significance for the renewable energy field. The Magnéli phase Ti_nO_2n-1 (4 ≤ n ≤ 10) has attracted much attention as a promising carbon-free support for electrocatalysts due to its high electrical conductivity and favorable electrochemical stability. Herein, we report the synthesis of a specific crystal-plane coupling heterostructure between Au(111) nanoparticles (NPs) and Ti₆O₁₁ by photoreduction. Benefitting from the modification of the electronic structure and synergistic effects of the heterostructure, the electron density around Au atoms is enhanced, and the Gibbs free energy of hydrogen absorption (ΔG_H*) was dramatically optimized to facilitate the HER process. The best electrocatalyst Au(111)@Ti₆O₁₁-50 exhibits a lower overpotential of 49 mV at a current density of -10 mA cm^-2 and a Tafel slope of 39 mV dec^-1 in 0.5 M H₂SO₄, and shows long-term electrochemical stability over 30 h. Au(111)@Ti₆O₁₁-50 shows a mass activity of 9.25 A mg_Au^-1, which is about 18 times higher than that of commercial Pt/C (0.51 A mg_Pt^-1). Meanwhile, the density functional theory (DFT) calculations suggest that the ΔG_H* of Au(111)@Ti₆O₁₁ is -0.098 eV, which is comparable to that of Pt (-0.09 eV). This work would be a powerful guide for the realization of efficient utilization of noble metals in catalysis.

High Selectivity Electrocatalysts for Oxygen Evolution Reaction and Anti-Chlorine Corrosion Strategies in Seawater Splitting

Seawater is one of the most abundant and clean hydrogen atom resources on our planet, so hydrogen production from seawater splitting has notable advantages. Direct electrolysis of seawater would not be in competition with growing demands for pure water. Using green electricity generated from renewable sources (e.g., solar, tidal, and wind energies), the direct electrolytic splitting of seawater into hydrogen and oxygen is a potentially attractive technology under the framework of carbon-neutral energy production. High selectivity and efficiency, as well as stable electrocatalysts, are prerequisites to facilitate the practical applications of seawater splitting. Even though the oxygen evolution reaction (OER) is thermodynamically favorable, the most desirable reaction process, the four-electron reaction, exhibits a high energy barrier. Furthermore, due to the presence of a high concentration of chloride ions (Cl−) in seawater, chlorine evolution reactions involving two electrons are more competitive. Therefore, intensive research efforts have been devoted to optimizing the design and construction of highly efficient and anticorrosive OER electrocatalysts. Based on this, in this review, we summarize the progress of recent research in advanced electrocatalysts for seawater splitting, with an emphasis on their remarkable OER selectivity and distinguished anti-chlorine corrosion performance, including the recent progress in seawater OER electrocatalysts with their corresponding optimized strategies. The future perspectives for the development of seawater-splitting electrocatalysts are also demonstrated.

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.

Three-dimensional nano-framework CoP/Co2P/Co3O4 heterojunction as a trifunctional electrocatalyst for metal–air battery and water splitting

Three-dimensional nano-framework CoP/Co2P/Co3O4 heterojunctions exhibit superior trifunctional electrocatalyst performances toward metal–air batteries and water splitting.

Lanthanum-based double perovskite nanoscale motifs as support media for the methanol oxidation reaction

We have correlated the performance of double perovskite metal oxides as support media for the methanol oxidation reaction (MOR) with their intrinsic size, shape, and composition.

Molten salt synthesis of carbon-supported Pt–rare earth metal nanoalloy catalysts for oxygen reduction reaction

The synthesis mechanism of Pt–RE nanoalloy particles prepared by one-step molten salt synthesis as an advanced ORR catalyst is proposed.

Nanostructural synergism as MnNC channels in manganese (IV) oxide and fluffy g-C3N4 layered composite with exceptional catalytic capabilities

The avenues of catalysis and material science are always accepted and it is hoped that a state-of-the-art catalyst with exceptional intrinsic redox characteristics would be produced. This study focused on developing a multi-featured catalyst of high economical and commercial standards to meet the multi-directional applications of environmental and energy demands. Manganese (IV) oxide nanosheets made of fluffy-sheet-like g-C₃N₄ material were successfully synthesized by pyrolysis method. The electron-rich g-C₃N₄ network and semiconducting metallic oxides of MnO₂ nanosheets generated high electron density interfaces within the intra-composite structure. The input of active interfaces along with strong metal-to-support interactions achieved between two parallel nanosheets in MnO₂/g-C₃N₄ catalyst intrinsically boosted up its electrochemical and optical characteristics for it to be used in multi-catalytic fields. Successful trails of catalysts' performance have been made in three major catalytic fields with enhanced activities such as heterogeneous catalysis (reduction of nitrobenzene with rate constant of "K = 0.734 min^-1" and hydrogenation of styrene with "100% conversion" efficiency, including negligible change in five consecutive cycles), photocatalysis (degradation of methylene blue dye model within 20 min with negligible change in five consecutive cycles) and electrocatalysis (oxygen reduction reactions having comparable "diffusion-limited-current density" behaviour with that of the commercial Pt/C catalyst). The enhanced performance of catalysts in transforming chemicals, degrading organic pollutant species and producing sustainable energy resources from air oxygen can mitigate the challenges faced in environmental and energy crises, respectively.

Advanced Nanocarbons for Enhanced Performance and Durability of Platinum Catalysts in Proton Exchange Membrane Fuel Cells

Insufficient stability of current carbon supported Pt and Pt alloy catalysts is a significant barrier for proton-exchange membrane fuel cells (PEMFCs). As a primary degradation cause to trigger Pt nanoparticle migration, dissolution, and aggregation, carbon corrosion remains a significant challenge. Compared with enhancing Pt and PtM alloy particle stability, improving support stability is rather challenging due to carbon's thermodynamic instability under fuel cell operation. In recent years, significant efforts have been made to develop highly durable carbon-based supports concerning innovative nanostructure design and synthesis along with mechanistic understanding. This review critically discusses recent progress in developing carbon-based materials for Pt catalysts and provides synthesis-structure-performance correlations to elucidate underlying stability enhancement mechanisms. The mechanisms and impacts of carbon support degradation on Pt catalyst performance are first discussed. The general strategies are summarized to tailor the carbon structures and strengthen the metal-support interactions, followed by discussions on how these designs lead to enhanced support stability. Based on current experimental and theoretical studies, the critical features of carbon supports are analyzed concerning their impacts on the performance and durability of Pt catalysts in fuel cells. Finally, the perspectives are shared on future directions to develop advanced carbon materials with favorable morphologies and nanostructures to increase Pt utilization, strengthen metal-support interactions, facilitate mass/charge transfer, and enhance corrosion resistance.

Electronic and lattice strain dual tailoring for boosting Pd electrocatalysis in oxygen reduction reaction

Deliberately optimizing the d-band position of an active component via electronic and lattice strain tuning is an effective way to boost its catalytic performance. We herein demonstrate this concept by constructing core-shell Au@NiPd nanoparticles with NiPd alloy shells of only three atomic layers through combining an Au catalysis with the galvanic replacement reaction. The Au core with larger electronegativity modulates the Pd electronic configuration, while the Ni atoms alloyed in the ultrathin shells neutralize the lattice stretching in Pd shells exerted by Au cores, equipping the active Pd metal with a favorable d-band position for electrochemical oxygen reduction reaction in an alkaline medium, for which core-shell Au@NiPd nanoparticles with a Ni/Pd atomic ratio of 3/7 exhibit a half-wave potential of 0.92 V, specific activity of 3.7 mA cm^-2, and mass activity of 0.65 A mg^-1 at 0.9 V, much better than most of the recently reported Pd-even Pt-based electrocatalysts.

In Situ Growth of Pt-Co Nanocrystals on Different Types of Carbon Supports and Their Electrochemical Performance toward Oxygen Reduction

Carbon-supported Pt-M (M = Co, Ni, and Fe) alloy nanocrystals are widely used as catalysts toward oxygen reduction, a reaction key to the operation of proton-exchange membrane fuel cells. Here we report a colloidal method for the in situ growth of Pt-Co nanocrystals on various commercial carbon supports. The use of different carbon supports resulted in not only variations in size and composition for the nanocrystals but also their catalytic activity and durability toward oxygen reduction in acidic media. Among the nanocrystals, those grown on Vulcan XC72 and Ketjenblack EC300J showed the highest specific and mass activities in the 0.1 M HClO4 and 0.05 M H2SO4 electrolytes, respectively. Additionally, the catalysts also showed different durability depending on the strength of the interaction between the nanocrystals and the carbon support. Our analysis demonstrated that the difference in catalytic performance could be ascribed to the distinct effects of carbon support on both the synthetic and catalytic processes.

Mesoporous WCx Films with NiO-Protected Surface: Highly Active Electrocatalysts for the Alkaline Oxygen Evolution Reaction

Metal carbides are promising materials for electrocatalytic reactions such as water electrolysis. However, for application in catalysis for the oxygen evolution reaction (OER), protection against oxidative corrosion, a high surface area with facile electrolyte access, and control over the exposed active surface sites are highly desirable. This study concerns a new method for the synthesis of porous tungsten carbide films with template-controlled porosity that are surface-modified with thin layers of nickel oxide (NiO) to obtain active and stable OER catalysts. The method relies on the synthesis of soft-templated mesoporous tungsten oxide (mp. WOx ) films, a pseudomorphic transformation into mesoporous tungsten carbide (mp. WCx ), and a subsequent shape-conformal deposition of finely dispersed NiO species by atomic layer deposition (ALD). As theoretically predicted by density functional theory (DFT) calculations, the highly conductive carbide support promotes the conversion of Ni2+ into Ni3+ , leading to remarkably improved utilization of OER-active sites in alkaline medium. The obtained Ni mass-specific activity is about 280 times that of mesoporous NiOx (mp. NiOx ) films. The NiO-coated WCx catalyst achieves an outstanding mass-specific activity of 1989 A gNi -1 in a rotating-disc electrode (RDE) setup at 25 °C using 0.1 m KOH as the electrolyte.

Differences in the Electrochemical Performance of Pt-Based Catalysts Used for Polymer Electrolyte Membrane Fuel Cells in Liquid Half- and Full-Cells

A substantial amount of research effort has been directed toward the development of Pt-based catalysts with higher performance and durability than conventional polycrystalline Pt nanoparticles to achieve high-power and innovative energy conversion systems. Currently, attention has been paid toward expanding the electrochemically active surface area (ECSA) of catalysts and increase their intrinsic activity in the oxygen reduction reaction (ORR). However, despite innumerable efforts having been carried out to explore this possibility, most of these achievements have focused on the rotating disk electrode (RDE) in half-cells, and relatively few results have been adaptable to membrane electrode assemblies (MEAs) in full-cells, which is the actual operating condition of fuel cells. Thus, it is uncertain whether these advanced catalysts can be used as a substitute in practical fuel cell applications, and an improvement in the catalytic performance in real-life fuel cells is still necessary. Therefore, from a more practical and industrial point of view, the goal of this review is to compare the ORR catalyst performance and durability in half- and full-cells, providing a differentiated approach to the durability concerns in half- and full-cells, and share new perspectives for strategic designs used to induce additional performance in full-cell devices.

Enhancement of Activity and Development of Low Pt Content Electrocatalysts for Oxygen Reduction Reaction in Acid Media

Platinum is a main catalyst for the electroreduction of oxygen, a reaction of primary importance to the technology of low-temperature fuel cells. Due to the high cost of platinum, there is a need to significantly lower its loadings at interfaces. However, then O₂-reduction often proceeds at a less positive potential, and produces higher amounts of undesirable H₂O₂-intermediate. Hybrid supports, which utilize metal oxides (e.g., CeO₂, WO₃, Ta₂O₅, Nb₂O₅, and ZrO₂), stabilize Pt and carbon nanostructures and diminish their corrosion while exhibiting high activity toward the four-electron (most efficient) reduction in oxygen. Porosity of carbon supports facilitates dispersion and stability of Pt nanoparticles. Alternatively, the Pt-based bi- and multi-metallic catalysts, including PtM alloys or M-core/Pt-shell nanostructures, where M stands for certain transition metals (e.g., Au, Co, Cu, Ni, and Fe), can be considered. The catalytic efficiency depends on geometric (decrease in Pt-Pt bond distances) and electronic (increase in d-electron vacancy in Pt) factors, in addition to possible metal-support interactions and interfacial structural changes affecting adsorption and activation of O₂-molecules. Despite the stabilization of carbons, doping with heteroatoms, such as sulfur, nitrogen, phosphorus, and boron results in the formation of catalytically active centers. Thus, the useful catalysts are likely to be multi-component and multi-functional.

Solvent assistance induced surface N-modification of PtCu aerogels and their enhanced electrocatalytic properties

A facile method involving the nitrogen modification of PtCu aerogel surfaces with N-methyl pyrrolidone as the sole nitrogen source is reported. The half wave potential (E1/2) of the PtCu aerogels was 0.932 V and the electrochemical active surface area (ECSA) was 102.04 m2 g-1 for the oxygen reduction reaction (ORR), and the mass activity (MA) for the methanol electrooxidation reaction (MOR) was measured to be 4.08 A mg-1, values better than those of a commercial Pt/C catalyst and other reported Pt-based catalysts.

Recent advances in metal-free heteroatom-doped carbon heterogonous catalysts

The development of cost-effective, efficient, and novel catalytic systems is always an important topic for heterogeneous catalysis from academia and industrial points of view. Heteroatom-doped carbon materials have gained more and more attention as effective heterogeneous catalysts to replace metal-based catalysts, because of their excellent physicochemical properties, outstanding structure characteristics, environmental compatibility, low cost, inexhaustible resources, and low energy consumption. Doping of heteroatoms can tailor the properties of carbons for different utilizations of interest. In comparison to pure carbon catalysts, these catalysts demonstrate superior catalytic activity in many organic reactions. This review highlights the most recent progress in synthetic strategies to fabricate metal-free heteroatom-doped carbon catalysts including single and multiple heteroatom-doped carbons and the catalytic applications of these fascinating materials in various organic transformations such as oxidation, hydrogenation, hydrochlorination, dehydrogenation, etc.

Electrochemically active site-rich nanocomposites of two-dimensional materials as anode catalysts for direct oxidation fuel cells: new age beyond graphene

Direct oxidation fuel cell (DOFC) has been opted as a green alternative to fossil fuels and intermittent energy resources as it is economically viable, possesses good conversion efficiency, as well as exhibits high power density and superfast charging. The anode catalyst is a vital component of DOFC, which improves the oxidation of fuels; however, the development of an efficient anode catalyst is still a challenge. In this regard, 2D materials have attracted attention as DOFC anode catalysts due to their fascinating electrochemical properties such as excellent mechanical properties, large surface area, superior electron transfer, presence of active sites, and tunable electronic states. This timely review encapsulates in detail different types of fuel cells, their mechanisms, and contemporary challenges; focuses on the anode catalyst/support based on new generation 2D materials, namely, 2D transition metal carbide/nitride or carbonitride (MXene), graphitic carbon nitride, transition metal dichalcogenides, and transition metal oxides; as well as their properties and role in DOFC along with the mechanisms involved.

Highly efficient blackberry-like trimetallic PdAuCu nanoparticles with optimized Pd content for ethanol electrooxidation

The rational design of highly efficient catalysts for ethanol electrooxidation is extremely challenging for developing direct ethanol fuel cells (DEFCs). Herein, a facile one-pot method has been developed to prepare blackberry-like PdAuCu nanoparticles (NPs) with tunable composition and surface structures. Among PdAuCu NPs with different Pd contents (1.6-22 mass%), PdAuCu NPs-0.5 (contained Pd at 2.5 mass%) delivered one of the highest catalytic activities of Pd-based catalysts towards ethanol electrooxidation, exhibiting a mass activity of 23.0 A mg_Pd^-1. Kinetic analysis, electrochemical impedance spectroscopy and CO stripping test results suggested that the excellent electrocatalytic activity may originate from the optimized balance between Pd content and surface structure of PdAuCu NPs-0.5. The optimization of the balance between composition and surface structure would contribute to the further design of multimetallic nanoparticles for fuel cells and other applications.

Enhancement of Catalytic Activity and Durability of Pt Nanoparticle through Strong Chemical Interaction with Electrically Conductive Support of Magnéli Phase Titanium Oxide

In this study, we address the catalytic performance of variously sized Pt nanoparticles (NPs) (from 1.7 to 2.9 nm) supported on magnéli phase titanium oxide (MPTO, Ti₄O₇) along with commercial solid type carbon (VXC-72R) for oxygen reduction reaction (ORR). Key idea is to utilize a robust and electrically conductive MPTO as a support material so that we employed it to improve the catalytic activity and durability through the strong metal-support interaction (SMSI). Furthermore, we increase the specific surface area of MPTO up to 61.6 m² g⁻¹ to enhance the SMSI effect between Pt NP and MPTO. After the deposition of a range of Pt NPs on the support materials, we investigate the ORR activity and durability using a rotating disk electrode (RDE) technique in acid media. As a result of accelerated stress test (AST) for 30k cycles, regardless of the Pt particle size, we confirmed that Pt/MPTO samples show a lower electrochemical surface area (ECSA) loss (<20%) than that of Pt/C (~40%). That is explained by the increased dissolution potential and binding energy of Pt on MPTO against to carbon, which is supported by the density functional theory (DFT) calculations. Based on these results, we found that conductive metal oxides could be an alternative as a support material for the long-term fuel cell operation.

Platinum-Based Electrocatalysts for Direct Alcohol Fuel Cells: Enhanced Performances toward Alcohol Oxidation Reactions

In the past few decades, Pt-based electrocatalysts have attracted great interests due to their high catalytic performances toward the direct alcohol fuel cell (DAFC). However, the high cost, poor stability, and the scarcity of Pt have markedly hindered their large-scale utilization in commerce. Therefore, enhancing the activity and durability of Pt-based electrocatalysts, reducing the Pt amount and thus the cost of DAFC have become the keys for their practical applications. In this minireview, we summarized some basic concepts to evaluate the catalytic performances in electrocatalytic alcohol oxidation reaction (AOR) including electrochemical active surface area, activity and stability, the effective approaches for boosting the catalytic AOR performance involving size decrease, structure and morphology modulation, composition effect, catalyst supports, and assistance under other external energies. Furthermore, we also presented the remaining challenges of the Pt-based electrocatalysts to achieve the fabrication of a real DAFC.

Durable Tungsten Carbide Support for Pt-Based Fuel Cells Cathodes

In an effort to develop durable, corrosion-resistant catalyst support materials for polymer electrolyte fuel cells, modified polymer-assisted deposition method was used to synthesize tungsten carbide (WC, WC_1-x), which was later used as a support material for Pt-based oxygen reduction reaction catalyst, as an alternative for the corrosion-susceptible, carbon supports. The Pt-deposited tungsten carbide's corrosion-resistance, oxygen reduction reaction electrocatalysis, and durability were studied and compared to that of Pt/C. Bulk free carbon was found to be absent from the ceramic matrix which had particle size in the range of 2-25 nm. Tungsten carbide support appears to enhance the oxygen reduction activity on Pt, showing an increase in mass activity (nearly 2-fold at 0.85 V vs RHE) and specific activity (more than 7 times higher), alongside decrease in overpotential, in comparison to Pt/C. A significant increase in durability was also observed with the tungsten carbide-based system.

Pt nanoparticles supported on nitrogen-doped porous carbon as efficient oxygen reduction catalysts synthesized via a simple alcohol reduction method

Abstract

Pt nanoparticles supported on nitrogen-doped porous carbon (NPC) were investigated as both a highly active catalyst for the oxygen reduction reaction (ORR) and a suitable porous support structure. Pt/NPC catalysts with loadings of 8.8–35.4 wt.% were prepared via a simple alcohol reduction method and exhibited homogeneously dispersed Pt nanoparticles with a small mean size ranging from 1.90 to 2.99 nm. X-ray photoelectron spectroscopy measurement suggested the presence of strong interactions between the Pt nanoparticles and NPC support. 27.4% Pt/NPC demonstrated high catalytic activity for the ORR in a rotating disk electrode system and was also effectively applied to a gas diffusion electrode (GDE). A GDE fabricated using the Pt/NPC with a fine pore network exhibited excellent performance, especially at high current densities. Specific activity of Pt/NPC and Pt/carbon black catalysts for the ORR correlated with the peak potential of adsorbed OH reduction on Pt, which was dependent on the particle size and support.

Graphic abstract

Low-PGM and PGM-Free Catalysts for Proton Exchange Membrane Fuel Cells: Stability Challenges and Material Solutions

Fuel cells as an attractive clean energy technology have recently regained popularity in academia, government, and industry. In a mainstream proton exchange membrane (PEM) fuel cell, platinum-group-metal (PGM)-based catalysts account for ≈50% of the projected total cost for large-scale production. To lower the cost, two materials-based strategies have been pursued: 1) to decrease PGM catalyst usage (so-called low-PGM catalysts), and 2) to develop alternative PGM-free catalysts. Grand stability challenges exist when PGM catalyst loading is decreased in a membrane electrode assembly (MEA)-the power generation unit of a PEM fuel cell-or when PGM-free catalysts are integrated into an MEA. More importantly, there is a significant knowledge gap between materials innovation and device integration. For example, high-performance electrocatalysts usually demonstrate undesired quick degradation in MEAs. This issue significantly limits the development of PEM fuel cells. Herein, recent progress in understanding the degradation of low-PGM and PGM-free catalysts in fuel cell MEAs and materials-based solutions to address these issues are reviewed. The key factors that degrade the MEA performance are highlighted. Innovative, emerging material concepts and development of low-PGM and PGM-free catalysts are discussed.