High Performance Anion Exchange Membrane Fuel Cells Enabled by Fluoropoly(olefin) Membranes

Although the peak power density of anion exchange membrane fuel cells (AEMFCs) has been raised from ≈0.1 to ≈1.4 W cm^?2 over the last decade, a majority of AEMFCs reported in the literature have not been demonstrated to achieve consistently high performance and steady‐state operation. Poly(olefin)‐based AEMs with fluorine substitution on the aromatic comonomer show considerably higher dimensional stability compared to samples that do not contain fluorine. More importantly, fluorinated poly(olefin)‐based AEMs exhibit high hydroxide conductivity without excessive hydration due to a new proposed mechanism where the fluorinated dipolar monomer facilitates increased hydroxide dissociation and transport. Using this new generation of AEMs, a stable, high‐performance AEMFC is operated for 120 h. When the fuel cell configuration is subjected to a constant current density of 600 mA cm^?2 under H₂/O₂ flow, the cell voltage declines only 11% (from 0.75 to 0.67 V) for the first 20 h during break‐in and the cell voltage loss is low (0.2 mV h^?1) over the subsequent 100 h of cell testing. The ease of synthesis, potential for low‐cost commercialization, and remarkable ex situ properties and in situ performance of fluoropoly(olefin)‐based AEM renders this material a benchmark membrane for practical AEMFC applications.     

In Situ Construction of Ni/Ni0.2Mo0.8N Heterostructure to Enhance the Alkaline Hydrogen Oxidation Reaction by Balancing the Binding of Intermediates

The inferior activity of hydrogen oxidation reaction (HOR) in alkali severely hampers the deployment of Ni catalysts in the promising anion exchange membrane fuel cells (AEMFCs), due to the unbalanced binding energies of hydrogen (HBE) and hydroxyl (OHBE) species. Ni-Mo alloy and nickel nitride have been proven to improve the Ni-based activities of HOR but they still can be further enhanced. Because it sacrifices the HBE for enlarging OHBE. Herein, it is reported that the activity can be further improved by constructing heterostructure between Ni nanoparticles (NPs) and nitride of Ni-Mo alloy (Ni_0.2Mo_0.8N) by an in situ synthetic strategy. The in situ prepared reduced graphene oxide (rGO) supported heterostructure (Ni/Ni_0.2Mo_0.8N/rGO) possesses the state-of-the-art activity (overpotential of 100 mV to achieve 2.9 mA cm⁻²), faster kinetics (kinetics current density of 11.20 mA cm⁻² and exchange current density of 2.74 mA cm⁻²), and ultrahigh durability (maintaining the current densities for over 40 h or 10000 cycles). Detailed characterizations together with density functional theory simulations reveal that the tuned d-band electronic structures optimize and balance the HBE and OHBE, facilitating the HOR process on the as-fabricated heterostructured catalyst.     

Very High Performance Alkali Anion‐Exchange Membrane Fuel Cells

Anion‐exchange membrane fuel cells (AEMFCs) have emerged as an alter‐native technology to overcome the technical and cost issues of proton‐exchange membrane fuel cells (PEMFCs). In this study, we describe a new electrocatalyst for AEMFCs composed of carbon nanotubes (CNTs), KOH‐doped polybenzimidazole (PBI) and platinum nanoparticles (Pt), in which the CNTs are wrapped by KOH‐doped PBI at a nanometer thickness and Pt is efficiently loaded on the wrapping layer. In the electrocatalyst, it is revealed that the CNTs and the KOH‐doped PBI layer function as electron‐ and hydroxide‐conductive paths, respectively, and the large exposed surface of the Pt allows an effective access of the fuel gas. Quantitative formation of the well‐defined interfacial structure formed by these components leads to an excellent mass transfer in the catalyst interface and realizes a high fuel‐cell performance. Membrane electrode assemblies fabricated with the electrocatalyst show a high power density of 256 mW cm^?2. To the best of our knowledge, this is the highest value for AEMFC systems measured in similar experimental conditions.     

PdAg Nanorings Supported on Graphene Nanosheets: Highly Methanol‐Tolerant Cathode Electrocatalyst for Alkaline Fuel Cells

Due to the high costs, slow reaction kinetics, and methanol poisoning of platinum‐based cathode catalysts, designing and exploring non‐Pt or low‐Pt cathode electrocatalysts with a low cost, high catalytic performance, and high methanol‐tolerance are crucial for the commercialization of fuel cells. Here, a facile method to fabricate a system of PdAg nanorings supported by graphene nanosheets is demonstrated; the fabrication is based on the galvanic displacement reaction between pre‐synthesized Ag nanoparticles and palladium ions. X‐ray diffraction and high‐resolution transmission electron microscopy show that the synthesized PdAg nanocrystals exhibit a ring‐shaped hollow structure with an average size of 27.49 nm and a wall thickness of 5.5 nm. Compared to the commercial Pd–C catalyst, the PdAg nanorings exhibit superior properties as a cathode electrocatalyst for oxygen reduction. Based on structural and electrochemical studies, these advantageous properties include efficient usage of noble metals and a high surface area because of the effective utilization of both the exterior and interior surfaces, high electrocatalytic performance for oxygen reduction from the synergistic effect of the alloyed PdAg crystalline phase, and most importantly, excellent tolerance of methanol crossover at high concentrations. It is anticipated that this synthesis of graphene‐based PdAg nanorings will open up a new avenue for designing advanced electrocatalysts that are low in cost and that exhibit high catalytic performance for alkaline fuel cells.     

Bifunctional Electrode Design Targeting Co-Enhanced Kinetics and Mass Transport for Hydrogen and Water Oxidation Reactions

Bifunctional catalysts based on noble metals have achieved practical-level performances in round-trip energy conversion systems. However, the required amount of noble metals should be substantially reduced via a new catalyst design that can pursue the synergy of the constituent materials’ intrinsic properties and architectural maneuver over reactant/product transport. In this study, cross-stacked Ir and Pt nanowires resolved the bottlenecks of two reactions essential for the hydrogen-based energy system: i) hydrogen spillover phenomenon between Pt and Ir nanowires to expedite the hydrogen oxidation reaction and ii) spacing Ir nanowires sufficiently to enhance the mass transport of the oxygen evolution reaction. Simultaneously accommodating the different strategies within the single catalyst layer, a new horizon to design a bifunctional electrode is proposed with the high performance of polymer electrolyte membrane unitized regenerative fuel cells: 47% of round-trip efficiency at 0.5 A cm⁻² with total noble metal loading < 0.3 mg cm⁻².     

A General Carboxylate-Assisted Approach to Boost the ORR Performance of ZIF-Derived Fe/N/C Catalysts for Proton Exchange Membrane Fuel Cells

An Fe/N/C catalyst derived from the pyrolysis of metal–organic frameworks, for example, a zeolitic-imidazolate-framework-8 (ZIF-8), has been regarded as one of the most promising non-precious metal catalysts toward oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). However, its ORR mass activity is still much inferior to that of Pt, partly because of the lack of general and efficient synthetic strategies. Herein, a general carboxylate-assisted strategy that dramatically enhances the ORR mass activity of ZIF-derived Fe/N/C catalysts is reported. The carboxylate is found to promote the formation of Fe/N/C catalysts with denser accessible active sites and entangled carbon nanotubes, as well as a higher mesoporosity. These structural advantages make the carboxylate-assisted Fe/N/C catalysts show a 2–10 fold higher ORR mass activity than the common carboxylate-free one in various cases. When applied in H₂–O₂ PEMFCs, the active acetate-assisted Fe/N/C catalyst generates a peak power density of 1.33 W cm⁻², a new record of peak power density for a H₂–O₂ PEMFC with non-Pt ORR catalysts.     

One‐Step Synthesis of W2C@N,P‐C Nanocatalysts for Efficient Hydrogen Electrooxidation across the Whole pH Range

Highly active and low‐cost non‐noble metal electrocatalysts for hydrogen oxidation reaction (HOR) are crucial for the large‐scale applications of fuel cells, which, unfortunately, are rarely documented up to now. Here, a facile one‐step strategy to fabricate W₂C nanoparticles (≈3 nm) encased in N, P‐doped few layer carbon materials (W₂C@N,P‐C, WNPC) as an efficient non‐noble metal HOR electrocatalyst simply by calcining the mixture of recrystallized phosphotungstic acid and dicyandiamide is reported. The obtained WNPC catalyst shows extraordinarily high HOR activities (1.03/0.91/0.84 mA cm^?2 at 0.05 V vs reversible hydrogen electrode in 0.1 m HClO₄/0.1 m KOH/0.1 m neutral phosphate buffered saline electrolytes, respectively), excellent durability during accelerated degradation tests for 10 000 cycles, and outstanding CO tolerance. These high performances are attributed to the uniform structure of WNPC, and more essentially, the synergistic effect among N, P, and C species which elevates the reducibility of WNPC, favoring the generation of abundant HOR active sites.     

Nanoporous Graphene Enriched with Fe/Co‐N Active Sites as a Promising Oxygen Reduction Electrocatalyst for Anion Exchange Membrane Fuel Cells

Here, a simple but efficient way is demonstrated for the preparation of nanoporous graphene enriched with Fe/Co–nitrogen‐doped active sites (Fe/Co‐NpGr) as a potential electrocatalyst for the electrochemical oxygen reduction reaction (ORR) applications. Once graphene is converted into porous graphene (pGr) by a controlled oxidative etching process, pGr can be converted into a potential electrocatalyst for ORR by utilizing the created edge sites of pGr for doping nitrogen and subsequently to utilize the doped nitrogens to build Fe/Co coordinated centers (Fe/Co‐NpGr). The structural information elucidated using both XPS and TOF‐SIMS study indicates the presence of coordination of the M–N (M = Fe and Co)‐doped carbon active sites. Creation of this bimetallic coordination assisted by the nitrogen locked at the pore openings is found to be helping the system to substantially reduce the overpotential for ORR. A 30 mV difference in the overpotential (η) with respect to the standard Pt/C catalyst and high retention in half wave potential after 10 000 cycles in ORR can be attained. A single cell of an anion exchange membrane fuel cell (AEMFC) by using Fe/Co‐NpGr as the cathode delivers a maximum power density of ≈35 mWcm^?2 compared to 60 mWcm^?2 displayed by the Pt‐based system.     

Hollow Multivoid Nanocuboids Derived from Ternary Ni–Co–Fe Prussian Blue Analog for Dual‐Electrocatalysis of Oxygen and Hydrogen Evolution Reactions

Hydrogen generation from electrochemical water‐splitting is an attractive technology for clean and efficient energy conversion and storage, but it requires efficient and robust non‐noble electrocatalysts for hydrogen and oxygen evolution reactions (HER and OER). Nonprecious transition metal–organic frameworks (MOFs) are one of the most promising precursors for developing advanced functional catalysts with high porosity and structural rigidity. Herein, a new transition metal‐based hollow multivoid nanocuboidal catalyst synthesized from a ternary Ni–Co–Fe (NCF)‐MOF precursor is rationally designed to produce dual‐functionality toward OER and HER. Differing ion exchanging rates of the ternary transition metals within the prussian blue analog MOF precursor are exploited to produce interconnected internal voids, heteroatom doping, and a favorably tuned electronic structure. This design strategy significantly increases active surface area and pathways for mass transport, resulting in excellent electroactivities toward OER and HER, which are competitive with recently reported single‐function nonprecious catalysts. Moreover, outstanding electrochemical durability is realized due to the unique rigid and interconnected porous structure which considerably retains initial rapid charge transfer and mass transport of active species. The MOF‐based material design strategy demonstrated here exemplifies a novel and versatile approach to developing non‐noble electrocatalysts with high activity and durability for advanced electrochemical water‐splitting systems.     

Dual 3D Ceramic Textile Electrodes: Fast Kinetics for Carbon Oxidation Reaction and Oxygen Reduction Reaction in Direct Carbon Fuel Cells at Reduced Temperatures

Direct carbon fuel cells (DCFCs) are an efficient energy‐conversion technology capable of generating electricity with carbon‐dioxide‐capture chemistry with solid carbon as fuels. The efficiency and performance of DCFCs depend on the kinetics of the carbon oxidation reactions (COR) and the oxygen reduction reactions (ORR), each occurring at anode and cathode, respectively. The limited active sites paired with reduced temperatures greatly decrease the efficiency of the electrochemical reactions. Ultraporous dual‐3D ceramic textiles (dual‐3DCT) are integrated into electrolyte‐supported DCFCs to enhance charge and mass transfer at the electrodes. Improved COR at the anode is achieved by the synergy between the 3DCT NiO–Ce_0.8Gd_0.2O_1.95 (GDC) structure and optimal carbon fuel choice. In a comparative study, DCFCs using graphitic carbon (GC) as fuel show the best COR performance when compared to DCFCs utilizing alternative fuels such as carbon black (CB) and activated carbon (AC). The 3DCT Sm_0.5Sr_0.5CoO_3‐δ–GDC (SSC–GDC) composite cathode shows electrochemical performance superior to that of the conventional screen‐printed SSC–GDC. A peak power density of 392 mW cm^?2 at 600 °C is obtained in a DCFC using the 3DCT‐anode/electrolyte/3DCT‐cathode configuration, an unprecedented value for any reported DCFC as of yet. This points toward promising applications of dual‐3DCT electrodes for reduced‐temperature DCFCs.     

Synergistically Tuning Electronic Structure of Porous β‐Mo2C Spheres by Co Doping and Mo‐Vacancies Defect Engineering for Optimizing Hydrogen Evolution Reaction Activity

The development of novel non‐noble electrocatalysts with controlled structure and surface composition is critical for efficient electrochemical hydrogen evolution reaction (HER). Herein, the rational design of porous molybdenum carbide (β‐Mo₂C) spheres with different surface engineered structures (Co doping, Mo vacancies generation, and coexistence of Co doping and Mo vacancies) is performed to enhance the HER performance over the β‐Mo₂C‐based catalyst surface. Density functional theory calculations and experimental results reveal that the synergistic effect of Co doping with Mo vacancies increases the electron density around the Fermi‐level and modulates the d band center of β‐Mo₂C so that the strength of the Mo? H bond is reasonably optimized, thus leading to an enhanced HER kinetics. As expected, the optimized Co₅₀‐Mo₂C‐12 with porous structure displays a low overpotential (η₁₀ = 125 mV), low‐onset overpotential (η_onset = 27 mV), and high exchange current density (j₀ = 0.178 mA cm^?2). Furthermore, this strategy is also successfully extended to develop other effective metal (e.g., Fe and Ni) doped β‐Mo₂C electrocatalyst, indicating that it is a universal strategy for the rational design of highly efficient metal carbide‐based HER catalysts and beyond.     

Fe Vacancies Induced Surface FeO6 in Nanoarchitectures of N‐Doped Graphene Protected β‐FeOOH: Effective Active Sites for pH‐Universal Electrocatalytic Oxygen Reduction

Developing high‐active, good‐stable, and cost‐effective electrocatalyst for oxygen reduction reaction (ORR) in all‐pH medium is highly desired for the application of various fuel cell systems. Here, a network architecture hybrid with porous nitrogen‐doped graphene encapsulated β‐FeOOH nanoparticals (β‐FeOOH/PNGNs) as ORR electrocatalyst, which exhibits remarkable enhancement ORR performance in terms of activity and stability in pH‐universal medium is reported. Systematic characterization combining with X‐ray absorption fine structure analysis and the first principles simulations reveal that the as‐formed surface FeO₆ active sites that induced by a mass of Fe vacancies in β‐FeOOH/PNGNs can significantly lower the thermodynamic barrier of the total reaction, and hence contribute to a remarkable enhancement in ORR activity.