Transition-Metal- and Nitrogen-Doped Carbide-Derived Carbon/Carbon Nanotube Composites as Cathode Catalysts for Anion-Exchange Membrane Fuel Cells

Surprising Route to a Monoazaporphyrin and Full Characterization of Its Complexes with Five Different 3d Metals

In the search for mild agents for the oxidative cyclization of tetrapyrromethane to the corresponding corrole, we discovered a route that leads to a monoazaporphyrin with three meso-CF3 groups. Optimization studies that allowed access to appreciable amounts of this new macrocycle paved the way for the preparation of its cobalt, copper, nickel, zinc, and iron complexes. All complexes were fully characterized by various spectroscopic methods and X-ray crystallography. Their photophysical and electrochemical properties were determined and compared to those of analogous porphyrins in order to deduce the effect of the peripheral N atom. Considering the global efforts for designing efficient alternatives to platinum group metal (PGM) catalysts, they were also absorbed onto a porous carbon electrode material and studied as electrocatalysts for the oxygen reduction reaction (ORR). The cobalt complex was found to be operative at a quite positive catalytic onset potential and with good selectivity for the desirable 4-electrons/4-protons pathway.

Recent Development of Anion Exchange Membrane Fuel Cells and Performance Optimization Strategies: A Review

Anion exchange membrane fuel cells (AEMFCs) are the most promising low-temperature fuel cells and have received extensive attention. Compared to PEMFCs, the cost per unit of power can be significantly reduced for AEMFCs because, in theory, they allow the usage of non-precious metal catalysts and low-cost cell components. Owing to the development of advanced materials and performance improvement strategies, AEMFCs have achieved new records in both initial performance and durability. However, the high performance currently achieved is contingent on certain conditions, e. g., high Pt loading, large gas flowrates, and operation in pure O2 , which are far from practical applications. Therefore, the transition to commercially relevant performance and durability is the next goal of AEMFCs. This paper reviews the performance data of H2 -fueled AEMFCs since 2010 and summarizes possible performance optimization schemes, which can provide useful insights for developing next-generation AEMFCs.

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

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

Size and Electronic Effects on the Performance of (Corrolato)cobalt-Modified Electrodes for Oxygen Reduction Reaction Catalysis

Considering the worldwide efforts for designing catalysts that are not based on platinum group metals while still reserving the many advantages thereof, this study focused on the many variables that dictate the performance of cathodes used for fuel cells, regarding the efficient and selective reduction of oxygen to water. This was done by investigating two kinds of porous carbon electrodes, modified by molecular cobalt(III) complexes chelated by corroles that differ very much in size and electron-withdrawing capability. Examination of the electronic effect uncovered shifts in the CoII/CoIII redox potentials and also large differences in the affinity of the cobalt center to external ligands. Spontaneous absorption of the catalysts was found to depend on the size of the corrole's substituents (C6F5 ≫ CF3 ≫ H) and the metal's axial ligands (PPh3 versus pyridine), as well as on the porosity of the carbon electrodes (BP2000 > Vulcan). The better-performing cobalt-based catalysts were almost as active and selective as 20% platinum on Vulcan in terms of the onset potential and the only 2-10% undesirable formation of hydrogen peroxide. Durability was also addressed by using the best-performing modified cathode in a proper anion-exchange membrane fuel cell setup, revealing very little voltage change during 12 h of operation.

Molten salt electrolytic synthesis of porous carbon from SiC and its application in supercapacitors

Nanoscale porous carbide-derived carbon (CDC) microspheres were successfully synthesized via the electrolysis etching of nano-SiC microsphere powder precursors with a particle diameter of 200 to 500 nm in molten CaCl₂. Electrolysis was conducted at 900 °C for 14 h in argon at an applied constant voltage of 3.2 V. The results show that the obtained product is SiC-CDC, which is a mixture of amorphous carbon and a small quantity of ordered graphite with a low degree of graphitization. Similar to the SiC microspheres, the obtained product retained its original shape. The specific surface area was 734.68 m² g^-1. The specific capacitance of the SiC-CDC was 169 F g^-1, and it exhibited excellent cycling stability (98.01% retention of the initial capacitance after 5000 cycles) at a current density of 1000 mA g^-1.

Interfacial assembly of binary atomic metal-Nx sites for high-performance energy devices

Anion-exchange membrane fuel cells and Zn-air batteries based on non-Pt group metal catalysts typically suffer from sluggish cathodic oxygen reduction. Designing advanced catalyst architectures to improve the catalyst's oxygen reduction activity and boosting the accessible site density by increasing metal loading and site utilization are potential ways to achieve high device performances. Herein, we report an interfacial assembly strategy to achieve binary single-atomic Fe/Co-N_x with high mass loadings through constructing a nanocage structure and concentrating high-density accessible binary single-atomic Fe/Co-N_x sites in a porous shell. The prepared FeCo-NCH features metal loading with a single-atomic distribution as high as 7.9 wt% and an accessible site density of around 7.6 × 10¹⁹ sites g^-1, surpassing most reported M-N_x catalysts. In anion exchange membrane fuel cells and zinc-air batteries, the FeCo-NCH material delivers peak power densities of 569.0 or 414.5 mW cm^-2, 3.4 or 2.8 times higher than control devices assembled with FeCo-NC. These results suggest that the present strategy for promoting catalytic site utilization offers new possibilities for exploring efficient low-cost electrocatalysts to boost the performance of various energy devices.

Fluorinated Poly(aryl piperidinium) Membranes for Anion Exchange Membrane Fuel Cells

Anion-exchange-membrane fuel cells (AEMFCs) are a cost-effective alternative to proton-exchange-membrane fuel cells (PEMFCs). The development of high-performance and durable AEMFCs requires highly conductive and robust anion-exchange membranes (AEMs). However, AEMs generally exhibit a trade-off between conductivity and dimensional stability. Here, a fluorination strategy to create a phase-separated morphological structure in poly(aryl piperidinium) AEMs is reported. The highly hydrophobic perfluoroalkyl side chains augment phase separation to construct interconnected hydrophilic channels for anion transport. As a result, these fluorinated PAP (FPAP) AEMs simultaneously possess high conductivity (>150 mS cm^-1 at 80 °C) and high dimensional stability (swelling ratio <20% at 80 °C), excellent mechanical properties (tensile strength >80 MPa and elongation at break >40%) and chemical stability (>2000 h in 3 m KOH at 80 °C). AEMFCs with a non-precious Co-Mn spinel cathode using the present FPAP AEMs achieve an outstanding peak power density of 1.31 W cm^-2 . The AEMs remain stable over 500 h of fuel cell operation at a constant current density of 0.2 A cm^-2 .

Candied Haws-Like Fe-N-C Catalysts with Broadened Carbon Interlayer Spacing for Efficient Zinc-Air Battery

As efficient nonprecious metal catalysts for oxygen reduction reaction (ORR), Fe-N-C materials are one of the most promising alternatives to Pt-based catalysts for fuel cells and metal-air batteries. However, the intrinsically low density of key active sites like FeN₄ moieties hampers their commercial applications. Herein, we provide a smart strategy to construct a candied haws-like Fe-N-C catalyst (CH-FeNC) with broadened carbon interplanar spacing (>4 Å), starting with trehalose as a structure-built brick coupled with a zinc-zeolite imidazole framework (ZIF-8) and polyaniline (PANI) and then followed by copyrolysis carbonization of them. The obtained CH-FeNC exhibits half-wave potentials of 0.92 and 0.90 V (vs RHE) before and after 10,000 cycles in 0.1 M KOH, which are superior to the 0.90 and 0.85 V obtained by commercial Pt/C for ORR. The power density of a homemade zinc-air battery equipped with the catalyst is up to 131 mW cm^-2, greater than that of Pt/C (124 mW cm^-2). The extended X-ray absorption fine structure (EXAFS) results and density functional theory (DFT) theoretical calculations reveal that there exists enriched zigzag or armchair edge-hosted FeN₄ active sites, located at the abundant interface between carbon components in this composite. Furthermore, the unique broadened carbon interlayer spacing plays a key role in deciding the ORR rate in alkaline but not in acidic environments because there exists a fifth ligand of active Fe in the form of FeN₄ centers coupled with SO₄^2- and ClO₄^- from acids.

Dual Fe, Zn single atoms anchored on carbon nanotubes inlaid N, S-doped hollow carbon polyhedrons for boosting oxygen reduction reaction

It is still challengeable but significant to rationally develop dual-metal single-atom catalysts with rich accessible active sites and excellent intrinsic catalytic activity towards oxygen reduction reaction (ORR). Herein, we present a novel dual-metal single-atom catalyst, Fe and Zn single atoms homogenously anchored on carbon nanotubes inlaid N, S-doped hollow carbon polyhedrons (FeZn-NSC), synthesized by facile iron-salt impregnation and high-temperature pyrolysis for zeolitic imidazolate framework-8. Due to the synergistic effects of the hierarchical porous nanoarchitecture with high specific surface area (795.48 m² g^-1), N, S co-doped hollow carbon polyhedrons, in-situ grown highly conductive carbon nanotubes, and high loading of dual-metal single-atoms of Fe (3.12 wt%) and Zn (3.71 wt%), the optimized FeZn-NSC delivers outstanding ORR performance with high half-wave potential of 0.87 V, low Tafel slope of 44.7 mV dec^-1, long-term durability, and strong tolerance of methanol crossover. This work provides a strategy to rationally design and facilely synthesize dual-metal single-atom catalysts with high ORR activity.

A perspective on the controlled synthesis of iron-based nanoalloys for the oxygen reduction reaction

The worsening ecological environment is calling for clean energy alternatives, among which hydrogen fuel cells have been one of the hot topics. The commercialized Pt/C catalyst for the oxygen reduction reaction (ORR) in the cathode of fuel cells is suffering from its high cost, serious scarcity and so on. Hence, the exploration on alternative ORR catalysts has attracted much attention. Iron(Fe)-based nanoalloys have shown advantages of low cost, high abundance, and pleasant ORR activity. In this feature, we have summarized Fe-based nanoalloy structures and our recent progress on controllable synthesis as well as their ORR performance, including iron-platinum (Fe-Pt), iron carbide (Fe-C), and iron nitride (Fe-N). Finally, the perspective on this type of ORR electrocatalyst is also discussed.

Cobalt-Containing Nitrogen-Doped Carbon Materials Derived from Saccharides as Efficient Electrocatalysts for Oxygen Reduction Reaction

The development of non-precious metal electrocatalysts towards oxygen reduction reaction (ORR) is crucial for the commercialisation of polymer electrolyte fuel cells. In this work, cobalt-containing nitrogen-doped porous carbon materials were prepared by a pyrolysis of mixtures of saccharides, cobalt nitrate and dicyandiamide, which acts as a precursor for reactive carbon nitride template and a nitrogen source. The rotating disk electrode (RDE) experiments in 0.1 M KOH solution showed that the glucose-derived material with optimised cobalt content had excellent ORR activity, which was comparable to that of 20 wt % Pt/C catalyst. In addition, the catalyst exhibited high tolerance to methanol, good stability in short-time potential cycling test and low peroxide yield. The materials derived from xylan, xylose and cyclodextrin displayed similar activities, indicating that various saccharides can be used as inexpensive and sustainable precursors to synthesise active catalyst materials for anion exchange membrane fuel cells.

Popcorn-like Co3O4 Nanoparticles Confined in Three-Dimensional Hierarchical N-doped carbon nanotubes Networks as Highly Efficient Trifunctional Electrocatalyst for Zinc-Air Batteries and Water Splitting Device

A novel unique popcorn-like three-dimensional (3D) hierarchical structural electrocatalyst is synthesized by the pyrolysis of ZIF-8/ZIF-67 and polyacrylonitrile fibers composites, where popcorn-like Co3O4 nanoparticles coated with nitrogen-doped amorphous carbon anchor...

Formamide-derived "glue" for the hundred-gram scale synthesis of atomically dispersed iron-nitrogen-carbon electrocatalysts

The distinct structure and maximum utilization of metal atoms on supported single-atom catalysts (SACs) represents a new frontier of heterogeneous catalysis, yet the low-cost mass production of high-performance SACs is still a key issue for practical applications. Herein, by coating a formamide-derived highly N-modified carbonaceous layer as a "glue" on commercially available activated carbon black (AC), a hundred-gram scale synthesis of atomically dispersed non-noble metal-nitrogen-carbon (MNC) materials was realized, including but not limited to Fe, Co, Ni, Mn, and Cu. The dispersion and coordination environments of Fe atoms on AC were initially revealed by XRD, HRTEM, and XPS, and further confirmed by HAADF-STEM and XANES analysis, presenting Fe atoms in a Fe-N₄ structure. The atomically dispersed metal species, though relatively low-loading grafted on AC (typical loading of 0.16 to 0.29 at%), are mostly distributed on the electrochemically accessible surface, resulting in improved metal utilization. The FeNC@AC-3 sample exhibited highly comparable catalytic performance to 20 wt% Pt/C for the alkaline oxygen reduction reaction, and superior Al-air battery performance. Our work may inspire the synthesis of other types of SACs for broad electrocatalysis applications at kilogram or even ton scale.

Engineering Active Sites of Gold-Cuprous Oxide Catalysts for Electrocatalytic Oxygen Reduction Reaction

Understanding how the catalyst morphology influences surface sites is crucial for designing active and stable catalysts and electrocatalysts. We here report a new approach to this understanding by decorating gold (Au) nanoparticles on the surface of cuprous oxides (Cu₂O) with three different shape morphologies (spheres, cubes, and petals). The Au-Cu₂O particles are dispersed onto carbon nanotube (CNT) matrix with high surface area, stability, and conductivity for oxygen reduction reaction. A clear morphology-dependent enhancement of the electrocatalytic activity is revealed. Oxygenated gold species (AuO^-) are found to coexist with Au⁰ on the cube and petal catalysts, whereas only Au⁰ species are present on the sphere catalyst. The AuO^- species function effectively as active sites, resulting in the improved catalytic performance by changing the reaction mechanism. The enhanced catalytic performance of the petal-shaped catalyst in terms of onset potential, half-wave potential, diffusion-limited current density, and stability is closely associated with the presence of the most abundant AuO^- species on its surface. Highly active AuO^- species are identified on the surface of the catalysts as a result of the unique structural characteristics, which is attributed to the structural origin of high activity and stability. This insight constitutes the basis for assessing the detailed correlation between the morphology and the electrocatalytic properties of the nanocomposite catalysts, which has implications for the design of surface-active sites on metal/metal oxide electrocatalysts.

Bifunctional Oxygen Electrocatalysis on Mixed Metal Phthalocyanine-Modified Carbon Nanotubes Prepared via Pyrolysis

Non-precious-metal catalysts are promising alternatives for Pt-based cathode materials in low-temperature fuel cells, which is of great environmental importance. Here, we have investigated the bifunctional electrocatalytic activity toward the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) of mixed metal (FeNi; FeMn; FeCo) phthalocyanine-modified multiwalled carbon nanotubes (MWCNTs) prepared by a simple pyrolysis method. Among the bimetallic catalysts containing nitrogen derived from corresponding metal phthalocyanines, we report the excellent ORR activity of FeCoN-MWCNT and FeMnN-MWCNT catalysts with the ORR onset potential of 0.93 V and FeNiN-MWCNT catalyst for the OER having EOER = 1.58 V at 10 mA cm-2. The surface morphology, structure, and elemental composition of the prepared catalysts were examined with scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The FeCoN-MWCNT and FeMnN-MWCNT catalysts were prepared as cathodes and tested in anion-exchange membrane fuel cells (AEMFCs). Both catalysts displayed remarkable AEMFC performance with a peak power density as high as 692 mW cm-2 for FeCoN-MWCNT.

Recent Insights on Catalyst Layers for Anion Exchange Membrane Fuel Cells

Anion exchange membrane fuel cells (AEMFCs) performance have significantly improved in the last decade (>1 W cm-2 ), and is now comparable with that of proton exchange membrane fuel cells (PEMFCs). At high current densities, issues in the catalyst layer (CL, composed of catalyst and ionomer), like oxygen transfer, water balance, and microstructural evolution, play important roles in the performance. In addition, CLs for AEMFCs have different requirements than for PEMFCs, such as chemical/physical stability, reaction mechanism, and mass transfer, because of different conductive media and pH environment. The anion exchange ionomer (AEI), which is the soluble or dispersed analogue of the anion exchange membrane (AEM), is required for hydroxide transport in the CL and is normally handled separately with the electrocatalyst during the electrode fabrication process. The importance of the AEI-catalyst interface in maximizing the utilization of electrocatalyst and fuel/oxygen transfer process must be carefully investigated. This review briefly covers new concepts in the complex AEMFC catalyst layer, before a detailed discussion on advances in CLs based on the design of AEIs and electrocatalysts. The importance of the structure-function relationship is highlighted with the aim of directing the further development of CLs for high-performance AEMFC.

The joint effect of electrical conductivity and surface oxygen functionalities of carbon supports on the oxygen reduction reaction studied over bare supports and Mn–Co spinel/carbon catalysts in alkaline media

Mn–Co spinel/carbon electrocatalyst performance exhibits a volcano-type shape which results from a trade-off between electrical conductivity and the amount of oxygen groups.

Fe–N–C single-atom catalysts with an axial structure prepared by a new design and synthesis method for ORR

FePc powder sublimates losing H atoms to form unstable fragments at 450 °C which self-assemble to form units with a double-layer structure. The self-assembly units are driven by argon gas at 70 °C to where the substrate is located and crystallize to form Fe-N5/C@G catalyst.