Achieving high catalytic performance with the lowest possible amount of platinum is critical for fuel cell cost reduction. Here we describe a method of preparing highly active yet stable ...electrocatalysts containing ultralow-loading platinum content by using cobalt or bimetallic cobalt and zinc zeolitic imidazolate frameworks as precursors. Synergistic catalysis between strained platinum-cobalt core-shell nanoparticles over a platinum-group metal (PGM)-free catalytic substrate led to excellent fuel cell performance under 1 atmosphere of O
or air at both high-voltage and high-current domains. Two catalysts achieved oxygen reduction reaction (ORR) mass activities of 1.08 amperes per milligram of platinum (A mg
) and 1.77 A mg
and retained 64% and 15% of initial values after 30,000 voltage cycles in a fuel cell. Computational modeling reveals that the interaction between platinum-cobalt nanoparticles and PGM-free sites improves ORR activity and durability.
In this work, the complex hydrogen sorption behaviors in a 3NaBH4/HoF3 composite prepared through mechanical milling were carefully investigated, including the reactions occurred during ball milling ...and de-/rehydrogenation processes. Different from other rear earth fluorides, the HoF3 can react with NaBH4 during ball milling, leading to the formations of Na–Ho–F and Na–Ho–BH4 complex compounds. The first dehydriding of the 3NaBH4/HoF3 composite can be divided into 4 steps, including the ion exchange between H− and F−, the formation of NaHo(BH4)4, the decomposition of NaHo(BH4)4 and reaction of NaBH4 with Na–Ho–F compounds. The final products, HoB4, HoH3 and NaF, can be rehydrogenated to generate NaBH4 and NaHoF4 with an absorption capacity of 2.3 wt% obtained at 400 °C. Based on the Pressure–Composition–Temperature measurements, the de-/rehydrogenation enthalpies of the 3NaBH4/HoF3 composite are determined to be 88.3 kJ mol−1 H2 and −27.1 kJ mol−1 H2, respectively.
•A Reversible hydrogen sorption of 2.3 wt% is achieved in a 3NaBH4/HoF3 composite.•A dual-cation borohydride NaHo(BH4)4 formed during first dehydrogenation process.•Onset dehydriding temperature of NaBH4 is lowered down to 86 °C by the HoF3 addition.•The ion exchanges play an important role for the reversible hydrogen sorption.
A gel and spray-drying method is demonstrated for large-scale preparation of Li3.98Al0.06Ti4.96O12/C secondary microspheres via optimizing various synthetic conditions. The electrochemical ...performances of the Li3.98Al0.06Ti4.96O12/C microspheres are investigated. Display omitted
•Various synthetic conditions are investigated.•Materials can be produced at ∼1kg scale by using our demonstrated synthesis method.•Li3.98Al0.06Ti4.96O12/C sample possesses high electronic conductivity and rate property, and excellent cycling performance.•Secondary micro-spherical Li3.98Al0.06Ti4.96O12/C sample has high tap density et al.
Poor electronic conductivity is one of the biggest obstacles for practical application of lithium titanate as lithium-ion battery anode material. Utilizing the advantages of coating and doping techniques to optimize the conductive and rate performances of lithium titanate was reported in this work. Herein, the effects of various synthetic conditions including calcination temperatures and holding times, lithium overdoses, carbon contents, doping contents and doping elements on phase, primary particles’ size and electrochemical performance were comprehensively investigated. The optimal Li3.98Al0.06Ti4.96O12/C secondary microspheres were synthesized, which possessed high electronic conductivity, tap density, reversible capacity and first columbic efficiency, and excellent rate performances. Furthermore, the synthesized samples were characterized by various techniques.
Finding inexpensive alternatives to platinum group metals (PGMs) is essential for reducing the cost of proton exchange membrane fuel cells (PEMFCs). Numerous materials have been investigated as ...potential replacements of Pt, of which the transition metal and nitrogen-doped carbon composites (TM/Nx/C) prepared from iron doped zeolitic imidazolate frameworks (ZIFs) are among the most active ones in catalyzing the oxygen reduction reaction based on recent studies. In this report, we demonstrate that the catalytic activity of ZIF-based TM/Nx/C composites can be substantially improved through optimization of synthesis and post-treatment processing conditions. Ultimately, oxygen reduction reaction (ORR) electrocatalytic activity must be demonstrated in membrane-electrode assemblies (MEAs) of fuel cells. The process of preparing MEAs using ZIF-based non-PGM electrocatalysts involves many additional factors which may influence the overall catalytic activity at the fuel cell level. Evaluation of parameters such as catalyst loading and perfluorosulfonic acid ionomer to catalyst ratio were optimized. Our overall efforts to optimize both the catalyst and MEA construction process have yielded impressive ORR activity when tested in a fuel cell system.
Discovery of earth-abundant electrocatalysts to replace iridium for the oxygen evolution reaction (OER) in a proton exchange membrane water electrolyzer (PEMWE) represents a critical step in reducing ...the cost for green hydrogen production. We report a nanofibrous cobalt spinel catalyst codoped with lanthanum (La) and manganese (Mn) prepared from a zeolitic imidazolate framework embedded in electrospun polymer fiber. The catalyst demonstrated a low overpotential of 353 millivolts at 10 milliamperes per square centimeter and a low degradation for OER over 360 hours in acidic electrolyte. A PEMWE containing this catalyst at the anode demonstrated a current density of 2000 milliamperes per square centimeter at 2.47 volts (Nafion 115 membrane) or 4000 milliamperes per square centimeter at 3.00 volts (Nafion 212 membrane) and low degradation in an accelerated stress test.
Carboxylate-based metal-organic frameworks (MOFs) have emerged as promising electrocatalyst candidates for the water splitting and metal-air batteries. Hierarchical porous structure and redox-active ...metal centers with unsaturated coordination sites in MOFs facilitate the enhanced catalytic activity of oxygen evolution reaction (OER). Herein, uniform hollow structured Fe-free bi-metal (Co, Ni) MOF-74 nanoprisms are successfully synthesized using a solvothermal method and (Co1Ni1)3(OH)(CH3COO)5 as the sacrificial templates, where Co and Ni are the metal nodes and 2,5-dihydroxyterephthalic acid (H4DOBDC) serves as the organic ligand. At an overpotential of 300 mV, CoNi MOF-74 shows a high electrocatalytic activity towards OER in 0.1 M KOH, where the current density is 10 mA cm−2 and the Tafel slope is 65.6 mV dec−1. Meanwhile, CoNi MOF-74 is durable that sustains in alkaline for 100 h with 83.25% retention of current density. The improved catalytic activity can be associated with the in-situ generated amorphous Ni–Co (oxy)hydroxide, as well as the electron transfer from Ni2+ to Co2+. This work elucidates the potential application of MOF materials as efficient electrocatalysts for OER.
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•A hollow structured bi-metal organic framework of CoNi MOF-74 is synthesized using a template sacrifice method.•Hierarchical porous structure and metal-metal interaction effectively improve OER catalytic performances.•The optimized MOF-74 exhibits high OER activity and retains 83.25% of the initial current density after 100 h in 0.1 M KOH.•The structure reconstruction of MOFs and partial oxidation of metal cations during OER process are carefully investigated.
A new high‐capacity reversible hydrogen‐storage material synthesized by the encapsulation of NaBH4 nanoparticles in graphene is reported. This approach effectively prevents phase agglomeration or ...separation during successive H2 discharge/recharge processes and enables rapid H2 uptake and release in NaBH4 under mild conditions. The strategy advanced here paves a new way for application in energy generation and storage.
For the promotion of lithium-oxygen batteries available for practical applications, the development of advanced cathode catalysts with low-cost, high activity, and stable structural properties is ...demanded. Such development is rooted on certain intelligent catalyst-electrode design that fundamentally facilitates electronic and ionic transport and improves oxygen diffusivity in a porous environment. Here we design a biphasic nitrogen-doped cobalt@graphene multiple-capsule heterostructure, combined with a flexible, stable porous electrode architecture, and apply it as promising cathodes for lithium-oxygen cells. The biphasic nitrogen-doping feature improves the electric conductivity and catalytic activity; the multiple-nanocapsule configuration makes high/uniform electroactive zones possible; furthermore, the colander-like porous electrode facilitates the oxygen diffusion, catalytic reaction, and stable deposition of discharge products. As a result, the electrode exhibits much improved electrocatalytic properties associated with unique morphologies of electrochemically grown lithium peroxides.
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