Hao-Fan Wang

Spatially Confined Hybridization of Nanometer-Sized NiFe Hydroxides into Nitrogen-Doped Graphene Frameworks Leading to Superior Oxygen Evolution Reactivity

Nanometer-sized hydroxide active centers are uniformly and strongly hybridized into a graphene framework by means of defect-anchored nucleation and spatially confined growth, resulting in a superior electrocatalyst for oxygen evolution reaction. This family of strongly coupled complexes and the topology-assisted fabrication strategy is expected to open up new avenues of research. It sheds light on a novel branch of advanced nano-architectured materials.

Topological Defects in Metal-Free Nanocarbon for Oxygen Electrocatalysis.

A bifunctional graphene catalyst with abundant topological defects is achieved via the carbonization of natural gelatinized sticky rice to probe the underlying oxygen electrocatalytic mechanism. A nitrogen-free configuration with adjacent pentagon and heptagon carbon rings is revealed to exhibit the lowest overpotential for both oxygen reduction and evolution catalysis. The versatile synthetic strategy and novel insights on the activity origin facilitate the development of advanced metal-free carbocatalysts for a wide range of electrocatalytic applications.

Defect Engineering toward Atomic Co–Nx–C in Hierarchical Graphene for Rechargeable Flexible Solid Zn-Air Batteries

Rechargeable flexible solid Zn-air battery, with a high theoretical energy density of 1086 Wh kg-1 , is among the most attractive energy technologies for future flexible and wearable electronics; nevertheless, the practical application is greatly hindered by the sluggish oxygen reduction reaction/oxygen evolution reaction (ORR/OER) kinetics on the air electrode. Precious metal-free functionalized carbon materials are widely demonstrated as the most promising candidates, while it still lacks effective synthetic methodology to controllably synthesize carbocatalysts with targeted active sites. This work demonstrates the direct utilization of the intrinsic structural defects in nanocarbon to generate atomically dispersed Co-Nx -C active sites via defect engineering. As-fabricated Co/N/O tri-doped graphene catalysts with highly active sites and hierarchical porous scaffolds exhibit superior ORR/OER bifunctional activities and impressive applications in rechargeable Zn-air batteries. Specifically, when integrated into a rechargeable and flexible solid Zn-air battery, a high open-circuit voltage of 1.44 V, a stable discharge voltage of 1.19 V, and a high energy efficiency of 63% at 1.0 mA cm-2 are achieved even under bending. The defect engineering strategy provides a new concept and effective methodology for the full utilization of nanocarbon materials with various structural features and further development of advanced energy materials.

Bifunctional Transition Metal Hydroxysulfides: Room-Temperature Sulfurization and Their Applications in Zn–Air Batteries

Bifunctional electrocatalysis for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) constitutes the bottleneck of various sustainable energy devices and systems like rechargeable metal-air batteries. Emerging catalyst materials are strongly requested toward superior electrocatalytic activities and practical applications. In this study, transition metal hydroxysulfides are presented as bifunctional OER/ORR electrocatalysts for Zn-air batteries. By simply immersing Co-based hydroxide precursor into solution with high-concentration S2- , transition metal hydroxides convert to hydroxysulfides with excellent morphology preservation at room temperature. The as-obtained Co-based metal hydroxysulfides are with high intrinsic reactivity and electrical conductivity. The electron structure of the active sites is adjusted by anion modulation. The potential for 10 mA cm-2 OER current density is 1.588 V versus reversible hydrogen electrode (RHE), and the ORR half-wave potential is 0.721 V versus RHE, with a potential gap of 0.867 V for bifunctional oxygen electrocatalysis. The Co3 FeS1.5 (OH)6 hydroxysulfides are employed in the air electrode for a rechargeable Zn-air battery with a small overpotential of 0.86 V at 20.0 mA cm-2 , a high specific capacity of 898 mAh g-1 , and a long cycling life, which is much better than Pt and Ir-based electrocatalyst in Zn-air batteries.

Dual-sized NiFe layered double hydroxides in situ grown on oxygen-decorated self-dispersal nanocarbon as enhanced water oxidation catalysts

The oxygen evolution reaction (OER) is extensively involved in various sustainable energy processes and systems, such as water splitting, fuel cells, and metal–air batteries. Towards superior OER performance, the wise integration of transition metal compounds with nanocarbon materials is a promising strategy. Herein, a mildly oxidized graphene/single-walled carbon nanotube hybrid was introduced to regulate and control the hybridization of nickel–iron layered double hydroxides into a nanocarbon scaffold. The oxygen functionalities and defects anchored the nucleation and in situ growth of dual-sized layered double hydroxides, leading to a hierarchical porous structure for smooth mass diffusion, intimate interfaces for rapid charge transfer, and efficiently utilized active sites. Attributed to the synergy of individual components and the unique structural features, the as-fabricated composites exhibited superior OER performance with a small onset overpotential (ca. 240 mV), a low overpotential required for 10 mA cm−2 (ca. 350 mV), and a decreased Tafel slope (ca. 54 mV dec−1) in 0.10 M KOH. This work provides a brilliant catalyst for water oxidation and more importantly, opens up new avenues for preparing nanocarbon-based multi-functional composites applicable in heterogeneous catalysis, energy conversion and storage, and so on.

Monolithic-structured ternary hydroxides as freestanding bifunctional electrocatalysts for overall water splitting

Efficient oxygen and hydrogen evolution electrocatalysts, based on low-cost and earth-abundant elements, are strongly required for sustainable hydrogen production through water splitting. Herein, we fabricated a monolithic-structured electrode by facilely electrodepositing NiCoFe ternary layered double hydroxides (LDHs) onto 3D conductive scaffolds, providing abundant fully exposed active sites for electrochemical reactions. The moderate Co dopant effectively improved the electrical conductivity of the LDH phase and substantially increased its intrinsic activity. When used for oxygen evolution, the as-obtained monolith LDH electrode exhibited superior kinetics with 275 mV overpotential required to achieve 10 mA cm−2 in 0.10 M KOH, as well as a very low activation energy of 21.0 kJ mol−1. Such a freestanding electrode was also able to catalyze hydrogen evolution efficiently in alkaline media, which further enabled a high-efficiency water electrolyzer delivering 10 mA cm−2 at a very low cell voltage of 1.62 V in 1.0 M KOH. This sheds fresh insight into the principle and process of practical water electrolysis through the rational design of precious-metal-free bifunctional electrodes with a monolithic configuration.

A ‘point–line–point’ hybrid electrocatalyst for bi-functional catalysis of oxygen evolution and reduction reactions

Both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) hold the core position in various sustainable energy systems. Attributed to their sluggish kinetics, the principle and concept to achieve efficient electrocatalysts for a sustainable catalytic process, especially bi-functional electrocatalysts with abundant active centers and 3D conductive scaffolds for both OER and ORR, are strongly considered. In this contribution, rather than physically mixing active catalyst flakes with conductive fillers, a hybrid electrocatalyst with ‘active point–conductive line–active point’ connections was proposed and validated. As a proof-of-concept, Co-based active sites embedded on layered double oxide (LDO) substrates interlinked with carbon nanotubes (CNTs) were realized and exhibited a superior bi-functional activity for both OER and ORR in an alkaline electrolyte. The LDO/CNT hybrids catalyzed the OER to reach 10.0 mA cm−2 at 1.64 V vs. RHE, and the ORR to reach 3.0 mA cm−2 at 0.65 V vs. RHE, with a potential gap of 0.99 V. Such model catalysts of LDO/CNT hybrids even delivered a better bi-functional performance than routine noble metal catalysts (e.g. Pt/C and IrO2). The novel strategy of combining metal compounds and carbon nanomaterials through ‘point–line–point’ configurations can be applied to other hierarchical composites with multi-building blocks, aiming at promising applications in energy storage and environmental protection.

Towards superior oxygen evolution through graphene barriers between metal substrates and hydroxide catalysts

The oxygen evolution reaction (OER) plays a key role in various sustainable energy systems, such as solar cells, fuel cells and metal–air batteries. The wise integration of transition metal compounds with macroscale current collectors is a promising strategy to achieve OER catalysts with high utilization efficiency. Herein, graphene with superior electron pathways, high surface area, excellent conductive and mechanical properties, and conjugate planes to anchor other phases with good dispersion was employed as the barrier between 3D Ni foam and NiFe-LDHs for 3D monolith electrodes. The as-obtained electrode exhibited a remarkably low onset overpotential of 240 mV, a small overpotential of 325 mV for 10 mA cm−2 and a substantially decreased Tafel slope of 44 mV dec−1 in 0.1 M KOH. The graphene barrier herein not only provides a novel hydrophobic substrate to modulate the growth of 3D NiFe-LDHs with good dispersion, but also delivers a strongly coupled interface between the active phase and current collectors. Consequently, the as-obtained graphene mediated 3D monolith electrode exhibited a very high electrochemically active surface area, an optimal interfacial junction, as well as superior OER performance. This work provides a novel catalyst for OER and more importantly, it reveals the role of graphene in modulating the interfacial features of various composites, which is general and can be employed in many material systems for satisfactory applications in heterogeneous catalysis, energy conversion and storage, sensors and so on.

An aqueous preoxidation method for monolithic perovskite electrocatalysts with enhanced water oxidation performance

Aqueous preoxidation strategy for in situ hybridization of oxidative perovskite and reductive conductive frameworks. Perovskite oxides with poor conductivity call for three-dimensional (3D) conductive scaffolds to demonstrate their superb reactivities for oxygen evolution reaction (OER). However, perovskite formation usually requires high-temperature annealing at 600° to 900°C in air, under which most of the used conductive frameworks (for example, carbon and metal current collectors) are reductive and cannot survive. We propose a preoxidization coupled electrodeposition strategy in which Co2+ is preoxidized to Co3+ through cobalt Fenton reaction in aqueous solution, whereas the reductive nickel framework is well maintained during the sequential annealing under nonoxidative atmosphere. The in situ–generated Co3+ is inherited into oxidized perovskites deposited on 3D nickel foam, rendering the monolithic perovskite electrocatalysts with extraordinary OER performance with an ultralow overpotential of 350 mV required for 10 mA cm−2, a very small Tafel slope of 59 mV dec−1, and superb stability in 0.10 M KOH. Therefore, we inaugurate a unique strategy for in situ hybridization of oxidative active phase with reductive framework, affording superb reactivity of perovskite electrocatalyst for efficient water oxidation.

Guest–host modulation of multi-metallic (oxy)hydroxides for superb water oxidation

Multi-metallic (oxy)hydroxide materials are some of the most promising alternatives for superb water oxidation electrocatalysts. Herein, a series of graphene/NiFe (oxy)hydroxides were fabricated to investigate the role of the Fe/Ni ratio in regard to the resultant physical structures, decoration styles, and combined catalytic activities. With the Fe content increasing, a phase evolution from Fe doped Ni(OH)2/NiO(OH) to Ni doped FeO(OH) was demonstrated. The moderate guest-metal substitution into the host-oxyhydroxide framework (Fe into Ni or Ni into Fe) substantially enhanced the oxygen evolution activity with a decrease in both the Tafel slope and overpotential. In addition, the NiO(OH) and FeO(OH) frameworks exhibited distinct properties and performances for the oxygen evolution reaction due to the different metal–oxygen bond lengths and adsorption energies of the intermediates.