Recent advances of bifunctional catalysts for zinc air batteries with stability considerations: from selecting materials to reconstruction

With the growing depletion of traditional fossil energy resources and ongoing enhanced awareness of environmental protection, research on electrochemical energy storage techniques like zinc-air batteries is receiving close attention. A significant amount of work on bifunctional catalysts is devoted to improving OER and ORR reaction performance to pave the way for the commercialization of new batteries. Although most traditional energy storage systems perform very well, their durability in practical applications is receiving less attention, with issues such as carbon corrosion, reconstruction during the OER process, and degradation, which can seriously impact long-term use. To be able to design bifunctional materials in a bottom-up approach, a summary of different kinds of carbon materials and transition metal-based materials will be of assistance in selecting a suitable and highly active catalyst from the extensive existing non-precious materials database. Also, the modulation of current carbon materials, aimed at increasing defects and vacancies in carbon and electron distribution in metal-N-C is introduced to attain improved ORR performance of porous materials with fast mass and air transfer. Finally, the reconstruction of catalysts is introduced. The review concludes with comprehensive recommendations for obtaining high-performance and highly-durable catalysts.

Water-activated formation of porous graphitic carbon nitride with carbon vacancies to boost photocatalytic hydrogen peroxide production

A facile water-activated method is developed for preparing porous graphitic carbon nitride with carbon vacancies by co-pyrolysis of melamine and water at a relatively low temperature under an Ar atmosphere, resulting in an increased specific surface area and the efficient separation of photo-generated electrons and holes. As expected, the optimal catalyst exhibited a high H₂O₂ yield of 180 μM within 4 h and good cycling stability. The reported template-free method may provide a reference for the preparation of high-performance photocatalysts in a facile way.

Phthalocyanine-induced iron active species in metal–organic framework-derived porous carbon for efficient alkaline zinc–air batteries

The FePc-encapsulated MOF can be thermally converted into a series of Fe-based active centers embedded into N-doped carbon nanomaterials. It shows good oxygen reduction activity in terms of mass activity, long-term durability and methanol tolerance.

Microwave-assisted rapid synthesis and activation of ultrathin trimetal-organic framework nanosheets for efficient electrocatalytic oxygen evolution

Ultrathin metal-organic frameworks (MOFs) nanosheets have been considered as one of promising electrocatalysts for oxygen evolution reaction due to their unique structures. However, the preparation process is subject to the low-yield exfoliation and time-consuming synthetic methods. And it still requires to be explored to develop an efficient strategy for regulation of surface electronic states of MOFs to enhance their electrocatalytic activities. Herein, we report a facile microwave-assisted synthesis of NiCoFe-based trimetallic metal-organic framework (MOF) nanosheets, which can achieve simultaneously rapid synthesis and activation of MOF for OER. The as-prepared Ni₄Co₄Fe₂-MOF nanosheets exhibit excellent OER activity and electrochemical stability, which can deliver a current density of 10 mA cm^-2 at a low overpotential of 243 mV with a small Tafel slope of 48.1 mV dec^-1. This work provides a facile method for the rapid synthesis of MOF nanosheets and also demonstrates an effective approach for regulating the electronic structure of MOFs for electrocatalysis.

Tunable engineering hollow carbon nanomaterial served as an excellent catalyst for oxygen reduction reaction and hydrogen evolution reaction

Fe and N functionalized hollow carbon spheres (Fe/N-HCS) with hierarchically porous structure are constructed. Remarkably, it is discovered that the pyrolysis temperature effects the chemical composition intensively. At 800 °C, only graphitic-N and oxidized-N are formed for all as-prepared samples. The surface area and pores can be precisely tuned, the surface area of all Fe/N-HCS samples is more than 500  m² g^-1 benefiting from the porous hollow structure. Thus, the optimized Fe/N-HCS exhibits excellent oxygen reduction reaction performance in term of onset potential (1.00 V vs. RHE), half-wave potential (0.87 V vs. RHE), good stability as well as methanol tolerance for oxygen reduction reaction, even surpassing the Pt in alkaline condition and more competitive in acidic condition; Furthermore, the optimized Fe/N-HCS displays better hydrogen evolution reaction activity in acidic condition with onset overpotential of 40 mV and overpotential to deliver 10 mA cm^-2 at 170 mV, indicating better active. It is found that Fe/N-HCS improve the hydrogen evolution reaction activity after electrodeposition trace quantity of Pt, which shows 170 mV of overpotential to deliver 100 mA cm^-2. X-ray photoelectron spectroscopy result indicates the loading of Pt is roughly 0.11 at%, thus, the improved performance is basically due to the synergistic effect between Pt and Fe/N-HCS.

Cobalt, Nitrogen-Doped Porous Carbon Nanosheet-Assembled Flowers from Metal-Coordinated Covalent Organic Polymers for Efficient Oxygen Reduction

The breakthrough of nonprecious metal catalysts replacing platinum-based catalysts toward the oxygen reduction reaction (ORR) is extremely urgent for the development of high-efficiency energy conversation systems. Herein, a solution-processed condensation polymerization using cyanuric chloride and piperazine as the monomers was proposed for the synthesis of a nitrogen-rich covalent organic polymer (COP). High contents of precisely tailored pyridinic-N within the COP facilitate the formation of the Co/N coordination between Co ions and N species. As a result, the subsequent carbonization of the Co-coordinated COP led to the formation of the cobalt, nitrogen dual-doped porous carbon nanosheet-assembled flowers (Co/N-PCNF). The as-obtained Co/N-PCNF catalyst with a nearly 4-electron oxygen reduction pathway exhibits an excellent ORR catalytic activity with a half-wave potential of 0.835 V comparable to the commercial Pt/C catalysts (0.865 V). Most impressively, the Co/N-PCNF catalyst displays a long-term stability and a much better resistance to methanol than the Pt/C catalyst because of its high surface area, well-defined porous structure, and homogeneous distributions of active sites within the carbon matrix. Therefore, this work establishes an operating rule for tailored synthesis of COP-derived nonprecious metal catalysts offering high activity for the ORR in electrochemical energy conversations.