Preparation and properties of an amorphous MnO2/CNTs-OH catalyst with high dispersion and durability for magnesium-air fuel cells

Insights into Nano- and Micro-Structured Scaffolds for Advanced Electrochemical Energy Storage

Adopting a nano- and micro-structuring approach to fully unleashing the genuine potential of electrode active material benefits in-depth understandings and research progress toward higher energy density electrochemical energy storage devices at all technology readiness levels. Due to various challenging issues, especially limited stability, nano- and micro-structured (NMS) electrodes undergo fast electrochemical performance degradation. The emerging NMS scaffold design is a pivotal aspect of many electrodes as it endows them with both robustness and electrochemical performance enhancement, even though it only occupies complementary and facilitating components for the main mechanism. However, extensive efforts are urgently needed toward optimizing the stereoscopic geometrical design of NMS scaffolds to minimize the volume ratio and maximize their functionality to fulfill the ever-increasing dependency and desire for energy power source supplies. This review will aim at highlighting these NMS scaffold design strategies, summarizing their corresponding strengths and challenges, and thereby outlining the potential solutions to resolve these challenges, design principles, and key perspectives for future research in this field. Therefore, this review will be one of the earliest reviews from this viewpoint.

Highly Exposed Active Sites of Fe/N Co-doped Defect-rich Graphene as an Efficient Electrocatalyst for Oxygen Reduction Reaction

A defect-rich interconnected hierarchical three-dimensional Fe and N co-doped graphene has been prepared by a facile synthesis with poly (2,5-benzimidazole) (ABPBI) as nitrogen and carbon sources and CaCO₃ as the template. ABPBI possesses abundant nitrogen, and pyrolysis of ABPBI is helpful to form graphene structure. CaCO₃ and its decomposition products CO₂ can promote the formation of interconnected hierarchical porous three-dimensional graphene, which possesses more defects and exposed active sites. Benefiting from the defective catalysis mechanism, rich defect catalysts are applied as electrode materials to enhance the catalytic performance for oxygen reduction reaction (ORR). Electrochemically, the half-wave potential (E_1/2 ) of Fe-3D-NG#800 is 0.84 V (vs. RHE), and the accelerated durability tests shows the E_1/2 of Fe-3D-NG#800 shifted by a 21 mV drop after cyclic voltammetry scanning for 5000 cycles. Therefore, Fe-3D-NG#800 has excellent activity and durability than 20 wt % Pt/C.

Bimetallic Sulfide with Controllable Mg Substitution Anchored on CNTs as Hierarchical Bifunctional Catalyst toward Oxygen Catalytic Reactions for Rechargeable Zinc-Air Batteries

The exploitation of high-efficiency and cheap bifunctional cathode electrocatalyst is of significant importance to rechargeable zinc-air batteries. In this paper, a bimetallic sulfide coupled with a CNT ((Co, Mg)S₂@CNTs) hybrid catalyst is developed via a proposed vulcanization process. The (Co, Mg)S₂@CNTs) with controllable Mg substitution has a tailored crystal structure (amorphous and crystalline), which catalyzes the oxygen reduction/evolution reaction (ORR/OER). The active sites of CoS₂@CNTs are activated by doping Mg ions, which accelerates the kinetics of the oxygen adsorption for ORR and oxygen desorption for OER. Meanwhile, the hybrid catalyst exhibits a unique hierarchal morphology and a "catalytic buffer", which further accelerate the mass transfer of catalytic processes. In addition, the outer wall of CNTs as substrate effectively avoid the agglomeration of (Co, Mg)S₂ particles by reasonably providing adsorption sites. The inner and outer walls of CNTs form a high-speed conduction pathway, quickly transferring the electrons produced by oxygen catalytic reactions. As a result, the (Co, Mg)S₂@CNTs exhibit an ORR performance comparable with commercial catalyst Pt/C-RuO₂ and remarkable OER performance (E_j=10 = 1.59 V). The high power density of 268 mW cm^-2 and long-term charge/discharge stability of the zinc-air battery proves the feasibility of (Co, Mg)S₂@CNTs application in high-power devices.

Controllable Hortensia-like MnO2 Synergized with Carbon Nanotubes as an Efficient Electrocatalyst for Long-Term Metal-Air Batteries

The exploitation of a high-activity and low-cost bifunctional catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) as the cathode catalyst is a research priority in metal-air batteries. Herein, a novel bifunctional hybrid catalyst of hortensia-like MnO₂ synergized with carbon nanotubes (CNTs) (MnO₂/CNTs) is controllably synthesized by reasonably designing the crystal structure and morphology as well as electronic arrangement. On the basis of these strategies, the hybrid accelerates the reaction kinetics and avoids the change of structures. As expected, MnO₂/CNTs exhibit a remarkable ORR and OER activity [low ORR Tafel slope: 71 mV dec^-1, low OER Tafel slope: 67 mV dec^-1, and small potential difference (Δ E): 0.85 V] and a long-term stability, which should be attributed to its unique morphology, K⁺ ions in the 2 × 2 tunnels, and synergistic effect between MnO₂ and CNTs. Notably, in zinc-air batteries (ZABs), aluminum-air batteries (AABs), and magnesium-air batteries (MABs), the composite shows high power density (ZABs: 243 mW cm^-2, AABs: 530 mW cm^-2, and MABs: 614 mW cm^-2) and large specific capacities (793 mA h g_Zn^-1, 918 mA h g_Al^-1, and 878 mA h g_Mg^-1). Importantly, the rechargeable ZABs reveal small charge-discharge voltage drop (0.81 V) and strong cycle durability (500 h), which are better than the noble-metal Pt/C + IrO₂ mixture catalyst (the voltage drop: 1.15 V at first and 2 V after 100 h). These superior performances together with the simple synthetic method of the hybrid reveal great promise in large-power energy storage and conversion equipment.

Spinel MnCo2 O4 Nanoparticles Supported on Three-Dimensional Graphene with Enhanced Mass Transfer as an Efficient Electrocatalyst for the Oxygen Reduction Reaction

The rational design of highly efficient and durable oxygen reduction reaction (ORR) catalysts is critical for the commercial application of fuel cells. Herein, three-dimensional graphene (3D-G) is synthesized by the template method, which used coal tar pitch as the carbon source and nano MgO as the template. Then, spinel MnCo₂ O₄ is in situ supported on the 3D-G by a facile hydrothermal method, giving MnCo₂ O₄ /3D-G. The resultant MnCo₂ O₄ /3D-G retains the multilayered mesoporous graphene structure where MnCo₂ O₄ nanoparticles are deposited on the inner walls of pores in the 3D-G. The catalyst MnCo₂ O₄ /3D-G shows high electrocatalytic activity with a half-wave potential of 0.81 V versus reversible hydrogen electrode, which is clearly superior to those of MnCo₂ O₄ /reduced graphene oxide (0.78 V), MnCo₂ O₄ /carbon nanotubes (0.74 V), MnCo₂ O₄ /C (0.72 V), and 20 wt % Pt/C (0.80 V). The electron transfer number of MnCo₂ O₄ /3D-G indicates a four-electron process of ORR. The durability test demonstrates that the MnCo₂ O₄ /3D-G catalyst has a much better durability than 20 wt % Pt/C. Our work makes an inspiring strategy to prepare high-performance electrocatalysts for the development of fuel cells.