Hydrogen fuel is considered as the cleanest renewable resource and the primary alternative to fossil fuels for future energy supply. Sustainable hydrogen generation is the major prerequisite to realize future hydrogen economy. The electrocatalytic hydrogen evolution reaction (HER), as the vital step of water electrolysis to H2 production, has been the subject of extensive study over the past decades. In this comprehensive review, we first summarize the fundamentals of HER and review the recent state-of-the-art advances in the low-cost and high-performance catalysts based on noble and non-noble metals, as well as metal-free HER electrocatalysts. We systemically discuss the insights into the relationship among the catalytic activity, morphology, structure, composition, and synthetic method. Strategies for developing an effective catalyst, including increasing the intrinsic activity of active sites and/or increasing the number of active sites, are summarized and highlighted. Finally, the challenges, perspectives, and research directions of HER electrocatalysis are featured.

Recent Advances in Electrocatalytic Hydrogen Evolution Using Nanoparticles.

Electrochemical water splitting is a promising technology for sustainable conversion, storage, and transport of hydrogen energy. Searching for earth-abundant hydrogen/oxygen evolution reaction (HER/OER) electrocatalysts with high activity and durability to replace noble-metal-based catalysts plays paramount importance in the scalable application of water electrolysis. A freestanding electrode architecture is highly attractive as compared to the conventional coated powdery form because of enhanced kinetics and stability. Herein, recent progress in developing transition-metal-based HER/OER electrocatalytic materials is reviewed with selected examples of chalcogenides, phosphides, carbides, nitrides, alloys, phosphates, oxides, hydroxides, and oxyhydroxides. Focusing on self-supported electrodes, the latest advances in their structural design, controllable synthesis, mechanistic understanding, and strategies for performance enhancement are presented. Remaining challenges and future perspectives for the further development of self-supported electrocatalysts are also discussed.

Self-Supported Transition-Metal-Based Electrocatalysts for Hydrogen and Oxygen Evolution

As one important component of sulfur cathodes, the carbon host plays a key role in the electrochemical performance of lithium-sulfur (Li-S) batteries. In this paper, a mesoporous nitrogen-doped carbon (MPNC)-sulfur nanocomposite is reported as a novel cathode for advanced Li-S batteries. The nitrogen doping in the MPNC material can effectively promote chemical adsorption between sulfur atoms and oxygen functional groups on the carbon, as verifi ed by X-ray absorption near edge structure spectroscopy, and the mechanism by which nitrogen enables the behavior is further revealed by density functional theory calculations. Based on the advantages of the porous structure and nitrogen doping, the MPNC-sulfur cathodes show excellent cycling stability (95% retention within 100 cycles) at a high current density of 0.7 mAh cm −2 with a high sulfur loading (4.2 mg S cm −2 ) and a sulfur content (70 wt%). A high areal capacity (≈3.3 mAh cm −2 ) is demonstrated by using the novel cathode, which is crucial for the practical application of Li-S batteries. It is believed that the important role of nitrogen doping promoted chemical adsorption can be extended for development of other high performance carbon-sulfur composite cathodes for Li-S batteries.

Nitrogen-Doped Mesoporous Carbon Promoted Chemical Adsorption of Sulfur and Fabrication of High-Areal-Capacity Sulfur Cathode with Exceptional Cycling Stability for Lithium-Sulfur Batteries

Zn-air batteries are becoming the promising power sources for portable and wearable electronic devices and hybrid/electric vehicles because of their high specific energy density and the low cost for next-generation green and sustainable energy technologies. An air electrode integrated with an oxygen electrocatalyst is the most important component and inevitably determines the performance and cost of a Zn-air battery. This article presents exciting advances and challenges related to air electrodes and their relatives. After a brief introduction of the Zn-air battery, the architectures and oxygen electrocatalysts of air electrodes and relevant electrolytes are highlighted in primary and rechargeable types with different configurations, respectively. Moreover, the individual components and major issues of flexible Zn-air batteries are also highlighted, along with the strategies to enhance the battery performance. Finally, a perspective for design, preparation, and assembly of air electrodes is proposed for the future innovations of Zn-air batteries with high performance.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201700691

Advanced Architectures and Relatives of Air Electrodes in Zn-Air Batteries.

A Metal-Organic Framework Host for Highly Reversible Dendrite-free Zinc Metal Anodes

Tungsten carbide is one of the most promising electrocatalysts for the hydrogen evolution reaction, although it exhibits sluggish kinetics due to a strong tungsten-hydrogen bond. In addition, tungsten carbide’s catalytic activity toward the oxygen evolution reaction has yet to be reported. Here, we introduce a superaerophobic nitrogen-doped tungsten carbide nanoarray electrode exhibiting high stability and activity toward hydrogen evolution reaction as well as driving oxygen evolution efficiently in acid. Nitrogen-doping and nanoarray structure accelerate hydrogen gas release from the electrode, realizing a current density of −200 mA cm−2 at the potential of −190 mV vs. reversible hydrogen electrode, which manifest one of the best non-noble metal catalysts for hydrogen evolution reaction. Under acidic conditions (0.5 M sulfuric acid), water splitting catalyzed by nitrogen-doped tungsten carbide nanoarray starts from about 1.4 V, and outperforms most other water splitting catalysts. Water electrolysis can generate carbon-neutral hydrogen gas from water, yet the required catalysts are often expensive, scarce, and poor at gas release. Here, the authors prepared nitrogen-doped carbon tungstide nanoarrays with high water-splitting activities and bubble-releasing surfaces.

/pdf/nitrogen-doped-tungsten-carbide-nanoarray-as-an-efficient-dz14bozo1n.pdf

Nitrogen-doped tungsten carbide nanoarray as an efficient bifunctional electrocatalyst for water splitting in acid.

Ternary NiCoFe-layered double hydroxide (NiCoIIIFe-LDH) with Co3+ is grafted on nitrogen-doped graphene oxide (N-GO) by an in situ growth route. The array-like colloid composite of NiCoIIIFe-LDH/N-GO is used as a bifunctional catalyst for both oxygen evolution/reduction reactions (OER/ORR). The NiCoIIIFe-LDH/N-GO array has a 3D open structure with less stacking of LDHs and an enlarged specific surface area. The hierarchical structure design and novel material chemistry endow high activity propelling O2 redox. By exposing more amounts of Ni and Fe active sites, the NiCoIIIFe-LDH/N-GO illustrates a relatively low onset potential (1.41 V vs reversible hydrogen electrode) in 0.1 mol L−1 KOH solution under the OER process. Furthermore, by introducing high valence Co3+, the onset potential of this material in ORR is 0.88 V. The overvoltage difference is 0.769 V between OER and ORR. The key factors for the excellent bifunctional catalytic performance are believed to be the Co with a high valence, the N-doping of graphene materials, and the highly exposed Ni and Fe active sites in the array-like colloid composite. This work further demonstrates the possibility to exploit the application potential of LDHs as OER and ORR bifunctional electrochemical catalysts.

NiCoFe-Layered Double Hydroxides/N-Doped Graphene Oxide Array Colloid Composite as an Efficient Bifunctional Catalyst for Oxygen Electrocatalytic Reactions

Flame-Engraved Nickel–Iron Layered Double Hydroxide Nanosheets for Boosting Oxygen Evolution Reactivity

Ultrathin 2D metal alloy nanomaterials have great potential applications but their controlled syntheses are limited to few noble metal based systems. Herein Ni x Co1-x alloy nanosheets with ultrathin (sub-3 nm) single-crystalline 2D structure are synthesized through a topochemical reduction method. Moreover, the optimized composition Ni0.6Co0.4 alloy nanosheets array exhibits excellent performances for hydrazine oxidation reaction and direct hydrazine fuel cells.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201600179

Single Crystalline Ultrathin Nickel-Cobalt Alloy Nanosheets Array for Direct Hydrazine Fuel Cells.

The rational structure and composition manipulation for the efficient, affordable bifunctional electrocatalysts of the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is critical for renewable-energy technologies including fuel cells and metal–air batteries. Metal oxide nanoarray electrodes are considered to be alternative options for this issue, although they always suffer from poor conductivity and limited ORR performance in terms of onset potential and diffusion current. In this study, we develop an alternative strategy to prepare a novel bifunctional catalyst by coating three-dimensional CoOx nanoarrays with a porous nitrogen-doped carbon layer (NC). The porous NC not only provides a conductive coating to benefit the charge transfer and retains the channels for electrolyte diffusion, but also greatly enhances the electrochemical surface area, which endows the electrode with higher activity for both the OER and ORR in terms of onset potential and diffusion current. When employed as an air cathode in rechargeable zinc–air batteries over 100 cycles, the electrode exhibits durable performance superior to those of Pt/C and IrO2/C. The results fully demonstrate the great potential of the strategy in electrode construction.

Boosting the bifunctional electrocatalytic oxygen activities of CoOx nanoarrays with a porous N-doped carbon coating and their application in Zn–air batteries

Nana Han

Papers

Nitrogen-doped tungsten carbide nanoarray as an efficient bifunctional electrocatalyst for water splitting in acid.

NiCoFe-Layered Double Hydroxides/N-Doped Graphene Oxide Array Colloid Composite as an Efficient Bifunctional Catalyst for Oxygen Electrocatalytic Reactions

Flame-Engraved Nickel–Iron Layered Double Hydroxide Nanosheets for Boosting Oxygen Evolution Reactivity

Single Crystalline Ultrathin Nickel-Cobalt Alloy Nanosheets Array for Direct Hydrazine Fuel Cells.

Boosting the bifunctional electrocatalytic oxygen activities of CoOx nanoarrays with a porous N-doped carbon coating and their application in Zn–air batteries