Recent Advances on Two-Dimensional Nanomaterials Supported Single-Atom for Hydrogen Evolution Electrocatalysts.

Water electrolysis has been recognized as a promising technology that can convert renewable energy into hydrogen for storage and utilization. The superior activity and low cost of catalysis are key factors in promoting the industrialization of water electrolysis. Single-atom catalysts (SACs) have attracted attention due to their ultra-high atomic utilization, clear structure, and highest hydrogen evolution reaction (HER) performance. In addition, the performance and stability of single-atom (SA) substrates are crucial, and various two-dimensional (2D) nanomaterial supports have become promising foundations for SA due to their unique exposed surfaces, diverse elemental compositions, and flexible electronic structures, to drive single atoms to reach performance limits. The SA supported by 2D nanomaterials exhibits various electronic interactions and synergistic effects, all of which need to be comprehensively summarized. This article aims to organize and discuss the progress of 2D nanomaterial single-atom supports in enhancing HER, including common and widely used synthesis methods, advanced characterization techniques, different types of 2D supports, and the correlation between structural hydrogen evolution performance. Finally, the latest understanding of 2D nanomaterial supports was proposed.

Coordinated Co-NC/CoFe dual active centre embedded three-dimensional ordered macroporous framework as bifunctional catalyst for efficient and stable zinc-air batteries.

Developing efficient and stable multifunctional electrocatalyst is very important for zinc-air batteries in practical. Herein, semiconductive spinel CuFe2O4supported Co-N co-doped carbon (Co-NC) and CoFe alloy nanoparticles were proposed. In this strategy, the three-dimensional ordered macroporous CuFe2O4support provides rich channels for mass transmission, revealling good corrosion-resistance and durability at the same time. ZIF-67 derived Co-NC decoration improves the conductivity of the catalyst. Further, the uniformly distributed Co-NC and CoFe nanoparticles (C/CF) dramatically promote the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performance. Accordingly, C/CF@CuFe2O4catalyst shows remarkable bifunctional electrocatalytic activity, with an ORR half-wave potential of 0.86 V, and an OER over-potential of 0.46 V at 10 mA cm-2. The zinc-air battery using this catalyst exhibits a power density of 95.5 mW cm-2and a durable cyclability for over 170 h at a current density of 10 mA cm-2, which implies a great potential in practical application.

Zirconia-supported cobalt nanoparticles as high-performance sulfur cathode for lithium-sulfur batteries.

Lithium-sulfur (Li-S) battery is now a promising technology for energy storage. However, rapid capacity decay due to sulfur dissolution and shutting effect severely limit its commercial development. In this work, a NH2-UIO-66 metal organic framework-derived porous composite (Co-ZrO2@NC) consists of nitrogen-doped carbon (NC) and zirconium oxide (ZrO2) loaded with cobalt nanoparticles was prepared. The porous NC component not only increases the accommodation of sulfur in the cathode, but also benefits the charge transfer in sulfur electrochemistry. The Co and ZrO2would act as active centers to enhance the adsorption/conversion of lithium polysulfide and improve its electrochemical utilization. When used in sulfur cathode, the Co-ZrO2@NC electrode shows excellent electrochemical performance with an initial specific capacity of 1073 mAh g-1at a rate of 0.2 C and a reversible capacity of 1015 mAh g-1after 100 cycles, corresponding to a capacity retention of 94.6%. Furthermore, after 300 cycles at 1.0 C, corresponding to a capacity retention of 75.4%. Moreover, the cell also exhibits good rate performance (640 mAh g-1at 3.0 C). Even at high sulfur loading of 4.0 mg cm-2, the S/Co-ZrO2@NC cathode is able to deliver an areal specific capacity of 4.8 mAh cm-2.

ZIF-67-derived N-doped double layer carbon cage as efficient catalyst for oxygen reduction reaction.

The slow kinetic of oxygen reduction reaction (ORR) hampers the practical application of energy conversion devices, such as fuel cells, metal-air batteries. Here, an efficient ORR electrocatalyst consists of Co, Ni co-decorated nitrogen-doped double shell hollow carbon cage (Ni-Co@NHC) was fabricated by pyrolyzing Ni-doped polydopamine wrapped ZIF-67. During the preparation, polydopamine served as a protective layer can effectively prevent the aggregation of Co and Ni nanoparticles during the pyrolysis process, and at the same time forming a carbon layer to grow a double layer carbon cage. This unique hollow structure endows the catalyst with a high specific surface area as well as more exposed active sites. Also benefited from the synergistic effect between Ni and Co nanoparticles, the Ni-Co@NHC catalyst leads to an outstanding ORR performance of half-wave potential (E1/2, 0.862 V), outperforms that of commercial Pt/C catalyst. Additionally, when Ni-Co@NHC was used in the cathode for the zinc-air battery, the cell exhibits high power density (108 mW cm-2) and high specific capacity (806 mAh g-1) at 20 mA cm-2outperforming Pt/C. This work offers a promising design strategy for the development of high-performance ORR electrocatalysts.

Two-dimensional Co(OH)2/graphene oxide composite as an efficient multifunctional interlayer for high-performance lithium-sulfur batteries.

The shuttling-effect of soluble lithium-based polysulfides (LiPS) represents one of the main obstacles for practical application of lithium-sulfur (Li-S) batteries. Herein, to address this issue, a flexible Li-S interlayer consisting of a two-dimensional α-Co(OH)2 nano-plate and graphene oxide (GO) is developed using a simple vacuum filtration technique. In the interlayer, the α-Co(OH)2 and GO are assembled into a layered structure forming a physical barrier for the shuttling of LiPS. Additionally, the α-Co(OH)2 offers strong chemical adsorption and efficient catalysis performance towards LiPS conversion, further inhibiting its shuttling. Attributed to these beneficial features of the interlayer, the Li-S battery delivers an initial discharge capacity of 834 mAh g-1 at the current density of 1 C. More importantly, after 300 cycles, a high discharge capacity of 590 mAh g-1 was retained, corresponding to a low capacity fading rate of 0.1% per cycle. This work might be of great interest for the feasible and scalable preparation of multifunctional interlayers in Li-S batteries.

A COF-coated ordered porous framework as multifunctional polysulfide barrier towards high-performance lithium-sulfur batteries.

The practical application of lithium-sulfur batteries (LSBs) is still hindered by the shuttle effect of lithium polysulfides (LiPS) and slow sulfur conversion kinetics. Herein, a LiPS inhibited covalent organic framework (COF)-coated conductive porous metal oxide design strategy is proposed towards the development of efficient and durable sulfur cathode in LSBs. This strategy is demonstrated by coating a TpPa-1 COF layer on cobalt-decorated titanium oxynirtide (TiOxNy) with a three-dimensional ordered microporous framework (3DOM) structure. In this strategy, the oxygen-deficient TiOxNy framework ensures a good conductivity and structural stability of the cathode during the charge/discharge process. The 3DOM macrospores provide a high capacity for sulfur accommodation and exposes active interfaces, whereas the coated TpPa-1 COF featured with ultrafine microspore offer an effective confinement of LiPS within the 3DOM framework, mitigating its shuttling effect. At the same time, the Co embedded in 3DOM TiOxNy servers as efficient catalyst promoting the sulfur electrochemical reaction. Attributed to these structural superiorities, the 3DOM TpPa-1@Co/TiOxNy/S cathode exhibits excellent performance even under high sulfur loading and low electrolyte condition. This work of using microporous COF coating with conductive macroporous metal oxides offers an effective alternative strategy for the design of practical sulfur battery.

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.

Decorated Oxidation-resistive deficient Titanium oxide nanotube supported NiFe-nanosheets as high-efficiency electrocatalysts for overall water splitting.

In this study, oxidation-resistive deficient TiO2-x supported NiFe-based electrocatalysts were developed towards efficient and durable water splitting performance. The oxidation-resistive deficient TiO2-x support with oxygen vacancies ensures good stability and electrical conductivity of the catalyst. The decorated NiFe and NiFeP nanosheets serve as efficient catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. In 1 M KOH, the NiFe@TiO2-x and NiFeP@TiO2-x electrodes show low overpotential for OER (300 mV) and HER (273 mV) at 100 mA cm-2, respectively, and excellent stability performance in overall water splitting as well. In-situ Raman and theoretical analysis reveals that the in-situ formed Fe3+-doped NiOOH species are essential in catalyzing OER on NiFe@TiO2-x, particularly the electron localization of surface Fe-O bonds offers lower energy barriers for OER elemental reactions and thus enhance its catalytic activity. This work provides an oxide-based catalyst support strategy for the development of stable and active overall water splitting catalysts, and advances the insights on catalytic origin of NiFe-based catalysts as well.