Recent advances in fuel cell reaction electrocatalysis based on porous noble metal nanocatalysts

An iron and lanthanide heterobimetallic coordination polymer derived electrocatalyst showing enhanced activity and stability for the oxygen reduction reaction

It is desirable to develop noble-metal-free electrocatalysts with both excellent activity and stability for the oxygen reduction reaction (ORR) in clean energy conversion devices. Herein, we report an Fe, La co-doped FeLaNC electrocatalyst obtained by pyrolyzing a heterobimetallic coordination polymer [La2L3(CH3OH)4]∞ loaded on carbonized ZIF-8, which was synthesized from 1,1'-ferrocenedicarboxylic acid (H2L1) and lanthanum salt. The FeLaNC catalyst exhibited higher ORR activity with a half-wave potential (E1/2) of 0.874 V (vs. RHE) than the control catalyst FeNC (E1/2 = 0.864 V) without the La dopant as well as commercial 20 wt% Pt/C (E1/2 = 0.862 V) in 0.1 M KOH solution, and FeLaNC displayed excellent stability with negligible half-wave potential decay after 10 000 potential cycles. When FeLaNC was applied as a cathodic electrocatalyst in Zn-air batteries (ZABs), the open circuit voltage (OCV) and maximum power density (Pmax) of the FeLaNC-based ZABs reached 1.46 V and 130 mW cm-2, respectively, which were significantly higher than those of Pt/C (OCV = 1.41 V and Pmax = 120 mW cm-2). The introduction of La3+ into Fe-N-C catalysts not only promoted ORR activity by regulating the electron density of Fe-Nx sites but also enhanced the stability by eliminating the harmful radicals, which provided an effective approach to prepare high-performance and stable electrocatalysts to be applied in energy conversion devices.

Enhancing Li-O2 battery performance with conductive hierarchical metal-organic framework composite cathodes

Li-O2 batteries are recognized for their high theoretical capacity and energy density, positioning them as excellent candidates for next-generation energy storage. This study explores the use of Metal-Organic Frameworks (MOFs) with high specific surface areas and open metal sites as cathode materials to address existing challenges. We developed conductive "cactus-like" composites by employing hydroxylated graphene (G-OH) as a substrate to grow columnar M3(HHTP)2 and MxM3-x(HHTP)2 (M = Cu, Ni) in a one-pot synthesis, enhancing the structure's conductivity and order. The cathode, especially the [Cu1.5Ni1.5(HHTP)2]1-(G-OH)1 composition, demonstrated a specific capacity of up to 12 542 mA h g-1 at a current density of 50 mA g-1 and maintained stability over more than 40 cycles at a limited capacity of 500 mA h g-1 in an O2 atmosphere. This performance surpasses that of M3(HHTP)2, MxM3-x(HHTP)2, or G-OH alone, highlighting the potential of MOF-based composites in improving the efficiency and durability of Li-O2 batteries and opening new avenues for cathode material design.

Boosting the oxygen reduction activity of non-metallic catalysts via geometric and electronic engineering through nitrogen and chlorine dual-doping

The development of heteroatom dual-doped porous carbon frameworks with uniform doping is highly desirable for achieving highly efficient oxygen reduction reaction (ORR) activity, due to their tunable chemical and electronic structures. Herein, porous covalent triazine-based frameworks (CTFs) incorporating nitrogen/chorine dual-doped porous carbon networks were fabricated by selecting 1,3-bis(4-cyanophenyl) imidazolium chloride as a building block, in a facile and controllable way via a bottom-up strategy. The resulting nitrogen/chorine dual-doped catalyst CCTF-700 exhibits excellent ORR performance with a more positive onset and half-wave potential (0.85 V vs. RHE), higher diffusion-limited current density and significantly improved stability in comparison with the benchmark commercial 20 wt% Pt/C catalyst. It is worth mentioning that CCTF-700 shows one of the best ORR performances among all the reported metal-free electrocatalysts under alkaline conditions. This work paves the way for a controllable and reliable strategy to craft highly efficient heteroatom dual-doped carbon catalysts for energy conversion.

Coordination polymer derived Fe-N-C electrocatalysts with high performance for the oxygen reduction reaction in Zn-air batteries

Developing high performance noble-metal-free electrocatalysts as an alternative to Pt-based catalysts for the oxygen reduction reaction (ORR) in energy conversion devices is highly desirable. We report herein the preparation of a coordination-polymer (CP)-derived Fe/CP/C composite as an electrocatalyst for the ORR with excellent activity and stability both in solution and in Zn-air batteries. The Fe/CP/C catalyst was obtained from the pyrolysis of an iron porphyrin Fe(TPP)Cl (5,10,15,20-tetraphenyl-21H,23H-porphyrin iron(III) chloride) grafted Zn-coordination polymer with dangling functional groups 4,4'-oxybisbenzoic acid and 4,4'-bipyridine ligands. The Fe/CP/C catalyst showed much higher ORR activity with a half-wave potential (E1/2) of 0.90 V (vs. RHE) than the Fe/C catalyst (E1/2 = 0.85 V) derived from the carbon-black-supported Fe porphyrins in 0.1 M KOH solution. When Fe/CP/C was used as the cathode electrocatalyst in Zn-air batteries (ZABs), the ZABs achieved a significantly higher open circuit voltage (OCV = 1.43 V) and maximum power density (Pmax = 142.8 mW cm-2) compared with Fe/C (OCV = 1.38 V, Pmax = 104.5 mW cm-2) and commercial 20 wt% Pt/C (OCV = 1.41 V, Pmax = 117.6 mW cm-2). Using dangling functional groups in CP to increase the loading efficiency of iron porphyrins offered a facile method to prepare high-performance noble-metal-free electrocatalysts for the ORR, which may provide promising applications to energy conversion devices.

One-step pyrolysis synthesis of ternary (P,S,N)-doped graphene as an efficient metal-free electrocatalyst for the oxygen reduction reaction

Graphene-based materials have been regarded recently as a promising substance for electrochemical energy conversion and storage devices owing to their unique structure and extraordinary properties. Herein, an enormously facile one-step pyrolysis approach is reported for the fabrication of ternary (P,S,N)-doped graphene, which is further investigated as an efficient metal-free electrocatalyst for the oxygen reduction reaction (ORR). Furthermore, optimized ternary-doped graphene can deliver excellent ORR catalytic activity that favors the four-electron ORR process and outstanding long-term durability (90.54% current retention after 20000 s which is far superior to that of commercial Pt/C) owing to the preferable synergetic coupling effect between P, S and N. Density functional theory (DFT) calculations were performed to reveal the synergetic coupling effect between doping elements in the ORR process. This work provides an extremely simple one-step pyrolysis method for the synthesis of P,S,N-doped graphene for electrochemical energy conversion and storage devices.

Recent development and challenges in fuel cells and water electrolyzer reactions: an overview

Water electrolysis is the most promising method for the production of large scalable hydrogen (H2), which can fulfill the global energy demand of modern society. H2-based fuel cell transportation has been operating with zero greenhouse emission to improve both indoor and outdoor air quality, in addition to the development of economically viable sustainable green energy for widespread electrochemical applications. Many countries have been eagerly focusing on the development of renewable as well as H2-based energy storage infrastructure to fulfill their growing energy demands and sustainable goals. This review article mainly discusses the development of different kinds of fuel cell electrocatalysts, and their application in H2 production through various processes (chemical, refining, and electrochemical). The fuel cell parameters such as redox properties, cost-effectiveness, ecofriendlyness, conductivity, and better electrode stability have also been highlighted. In particular, a detailed discussion has been carried out with sufficient insights into the sustainable development of future green energy economy.

Single-atom palladium anchored N-doped carbon towards oxygen electrocatalysis for rechargeable Zn-air batteries

Herein, an atomically dispersed palladium catalyst on a hierarchical porous structure of N-doped carbon (Pd₁/N-C) is prepared using a facile freeze-drying-assisted strategy. Freeze-drying methods not only suppress the aggregation of Pd atoms but also successfully produce abundant nanopores. HAADF-STEM confirms that Pd single atoms are uniformly anchored on the N-C surface. The Pd₁/N-C electrocatalyst enhances the ORR and OER activity and durability compared to N-C and Pd-NPs/N-C. Rechargeable Zn-air batteries (ZABs) based on novel Pd₁/N-C exhibit a peak power density of 113.7 mW cm^-2 and maintain a voltage efficiency of 64.0% after 495 cycles at a discharge current density of 5 mA cm^-2. Besides, two ZABs in series can supply an LED light for at least 170 h.