Long-range interconnected nanoporous Co/Ni/C composites as bifunctional electrocatalysts for long-life rechargeable zinc-air batteries

One-Step Preparation of Trinary Co(III)/Co(II)/Co(0) Bifunctional Catalysts with Controllable Co Valence Distribution via Solution Combustion Synthesis and Its Application to Oxygen Reduction and Evolution in Zinc-Air Batteries

Regulating the electronic structure and chemical valence of the active site for electrocatalysis is highly crucial and challenging. In this work, a cost-effective and facile strategy for regulating the cobalt valence distribution through one-step solution combustion synthesis via cobalt nitrate (oxidizer) and glycine (fuel) was developed to prepare a bifunctional Co(III)/Co(II)/Co(0) catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in a zinc-air battery. Experimental findings show that when increasing the fuel/oxidizer ratio φ, the composition of the synthesized catalyst gradually changes from binary Co(III)/Co(II) to trinary Co(III)/Co(II)/Co(0), and the average Co valence keeps decreasing. As the content of Co(II)/Co(0) increases, the ORR performance of the sample gradually improves. The sample synthesized at φ = 1.2 shows the best bifunctional catalytic activity and was employed to assemble a rechargeable zinc-air battery. Overall, compared with the published data, the proposed catalyst has a comparable ORR/OER activity and catalyst stability, with an excellent battery life, efficiency, and stability (up to 160 h). This work provides a promising pathway for designing the valence state distribution of non-noble cobalt-based catalysts, which can be easily prepared on a large scale with low cost and used in various technologies involved in ORR and OER reactions.

Niobium-doped conductive TiO-TiO2 heterostructure supported bifunctional catalyst for efficient and stable zinc-air batteries

The development of highly active and durable nonprecious metal-based bifunctional electrocatalysts for oxygen reduction/evolution reaction (ORR/OER) is important for rechargeable zinc-air batteries. Herein, a three-dimensional conductive niobium-doped TiO-TiO2 heterostructure supported ZIF-67-derived Co-NC bifunctional catalyst was fabricated. In the Co-NC@Nb-TiOx catalyst, the Nb doping promoted the formation of TiO-TiO2 heterojunction support, enhanced its conductivity and stability and provided strong electron metal-support interaction between Co-NC and Nb-TiOx. Also, the supported Co-NC nanoparticles provided abundant active sites with excellent ORR/OER activity. Experimental analysis reveals that the high OER activity of Co-NC@Nb-TiOx can be attributed to the in-situ generated CoOOH species. It exhibits excellent ORR activity, as shown by its onset potential (0.95 V vs. RHE) and half-wave potential (0.86 V vs. RHE). Its OER overpotential at 10 mA cm-2 is 480 mV. The zinc-air battery realizes outstanding cycling stability over 225 h cycles tested at 10 mA cm-2. This work demonstrates the importance of designing highly stable metal oxide-supported catalysts in electrochemical energy conversion devices.

Engineering Molecular Heterostructured Catalyst for Oxygen Reduction Reaction

Introducing a second metal species into atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts to construct diatomic sites (DASs) is an effective strategy to elevate their activities and stabilities. However, the common pyrolysis-based method usually leads to substantial uncertainty for the formation of DASs, and the precise identification of the resulting DASs is also rather difficult. In this regard, we developed a two-step specific-adsorption strategy (pyrolysis-free) and constructed a DAS catalyst featuring FeCo "molecular heterostructures" (FeCo-MHs). In order to rule out the possibility of the two apparently neighboring (in the electron microscopy image) Fe/Co atoms being dispersed respectively on the top/bottom surfaces of the carbon support and thus forming "false" MHs, we conducted in situ rotation (by 8°, far above the critical angle of 5.3°) and directly identified the individual FeCo-MHs. The formation of FeCo-MHs could modulate the magnetic moments of the metal centers and increase the ratio of low-spin Fe(II)-N4 moiety; thus the intrinsic activity could be optimized at the apex of the volcano-plot (a relationship as a function of magnetic moments of metal-phthalocyanine complexes and catalytic activities). The FeCo-MHs catalyst displays an exceptional ORR activity (E1/2 = 0.95 V) and could be used to construct high-performance cathodes for hydroxide exchange membrane fuel cells and zinc-air batteries.

Preparation and bifunctional properties of the A-site-deficient SrTi0.3Fe0.6Ni0.1O3-δ perovskite

The development of efficient, non-noble metal electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for their application in energy storage devices, such as fuel cells and metal-air batteries. In this study, SrTi_0.3Fe_0.6Ni_0.1O_3-δ (STFN) perovskite was synthesized using the sol-gel method, and its electrocatalytic activity was evaluated using a rotating disk electrode (RDE) in an alkaline medium. STFN synthesized at the optimum synthesis temperature of 800 °C exhibited good ORR and OER performances. To further improve electrocatalytic activity, a series of Sr_1-x Ti_0.3Fe_0.6Ni_0.1O_3-δ (x = 0, 0.05, and 0.1) perovskites with A-site vacancies were synthesized at 800 °C. Material characterization results showed that the removal of the A-site from the perovskite led to an increase in surface oxygen vacancies, resulting in higher ORR and OER activities. The results of this study indicate that Sr_1-x Ti_0.3Fe_0.6Ni_0.1O_3-δ (x = 0.1) is a promising bifunctional oxygen electrocatalyst for Zn-air batteries.

Interfacial Electron Redistribution of FeCo2S4/N-S-rGO Boosting Bifunctional Oxygen Electrocatalysis Performance

Developing bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is essential for the development of zinc–air batteries (ZABs), but several challenges remain in terms of bifunctional activity. FeCo2S4/N-S-rGO was prepared by in situ homogeneous growth of bimetallic sulfide FeCo2S4 on N, S-doped reduced graphene oxide. FeCo2S4/N-S-rGO exhibits a half-wave potential of 0.89 V for ORR and an overpotential of 0.26 V at 10 mA cm−2 for OER, showing significantly bifunctional activity superior to Pt/C (0.85 V) and RuO2 (0.41 V). Moreover, the FeCo2S4/N-S-rGO assembled ZAB shows a superior specific capacity and a power density of 259.13 mW cm−2. It is demonstrated that the interfacial electron redistribution between FeCo2S4 nanoparticles and heteroatom-doped rGO matrix can efficiently improve the electrochemical performance of the catalyst. The results provide new insights into the preparation of high-capability composite catalysts combining transition metal sulfides with carbon materials for applications in ZABs.