Facile synthesis of CeO2-decorated W@Co-MOF heterostructures as a highly active and durable electrocatalyst for overall water splitting.

Rational coupling of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts is extremely important for practical overall water splitting; however, it is still challenging to construct such bifunctional heterostructures. Herein, a CeO2/W@Co-MOF/NF bifunctional electrocatalyst was prepared via a two-step in situ growth method involving an electrodeposition process. The incorporation of the W element enhanced the electronic interaction and enlarged the electrochemical surface area. After the electrodeposition of CeO2, the obtained CeO2/W@Co-MOF/NF possessed abundant heterointerfaces with a modulated local distribution, which promoted water dissociation and rapid electrocatalytic kinetics. In particular, it required very low overpotentials of 239 mV and 87 mV to reach a current density of 10 mA cm-2 in OER and HER, respectively. A corresponding alkaline electrolysis cell afforded a cell voltage of 1.54 V at 10 mA cm-2 to boost overall water splitting. This work provides a feasible strategy to fabricate MOF-based complexes and explores their possible use as bifunctional catalysts toward overall water splitting.

Three-dimensional flower-like Ni-S/Co-MOF grown on Ni foam as a bifunctional electrocatalyst for efficient overall water splitting.

Poor conductivity of the metal-organic frameworks (MOFs) limits their applications in overall water splitting. Surface sulfur (S) doping transition metal hydroxides would effectively improve the conductivity and adjust the electronic structure to generate additional electroactive sites. Herein, we fabricated a Ni-S/Co-MOF/NF catalyst by electroplating a Ni-S film on the 3D flower-like Co-MOF. Because the 3D flower-like structures are covered in Ni foam, the high exposure of active sites and good conductivity are obtained. Moreover, the synergistic effect between Ni-S and Co-MOF contributes to the redistribution of electrons in the catalyst, which can then optimize the catalytic performance of the material. The obtained 3D flower-like Ni-S/Co-MOF/NF demonstrates excellent activity toward both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in 1 M KOH, which only requires a low overpotential of 248 mV@10 mA cm-2 for the OER and 127 mV@10 mA cm-2 for the HER, respectively. At a current density of 10 mA cm-2, the Ni-S/Co-MOF/NF‖Ni-S/Co-MOF/NF requires a low cell voltage of 1.59 V to split overall water splitting.

Morphological control and performance engineering of Co-based materials for supercapacitors.

As one of the most promising energy storage devices, supercapacitors exhibit a higher power density than batteries. However, its low energy density usually requires high-performance electrode materials. Although the RuO2 material shows desirable properties, its high cost and toxicity significantly limit its application in supercapacitors. Recent developments demonstrated that Co-based materials have emerged as a promising alternative to RuO2 for supercapacitors due to their low cost, favorable redox reversibility and environmental friendliness. In this paper, the morphological control and performance engineering of Co-based materials are systematically reviewed. Firstly, the principle of supercapacitors is briefly introduced, and the characteristics and advantages of pseudocapacitors are emphasized. The special forms of cobalt-based materials are introduced, including 1D, 2D and 3D nanomaterials. After that, the ways to enhance the properties of cobalt-based materials are discussed, including adding conductive materials, constructing heterostructures and doping heteroatoms. Particularly, the influence of morphological control and modification methods on the electrochemical performances of materials is highlighted. Finally, the application prospect and development direction of Co-based materials are proposed.

A/B-Site Management Strategy to Boost Electrocatalytic Overall Water Splitting on Perovskite Oxides in an Alkaline Medium.

In this paper, Pr0.7Sr0.3Co1-xRuxO3 perovskite oxides were synthesized by the sol-gel method as bifunctional catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The overpotentials of PSCR0.05 against HER and OER at 10 mA cm-2 were 319 and 321 mV in alkaline medium, respectively. The Tafel slopes of HER and OER were 87.32 and 118.1 mV/dec, respectively. PSCR0.05 showed the largest electrochemical active area, the smallest charge transfer resistance, and excellent long-term durability. Meanwhile, the PSCR0.05 electrocatalyst was applied for overall water splitting and its cell voltage was maintained at 1.77 V at 10 mA cm-2. The super-exchange interaction between adjacent RuO6-CoO6 octahedra in perovskite made of PSCR0.05 contains sufficient active sites (such as Co2+/Co3+, Ru3+/Ru4+, and O22-/O-). The increase of surface oxygen vacancy and active site is the main reason for the improvement of difunctional catalyst performance. In this work, the electrocatalytic performance of perovskite-type oxides was further optimized by the method of A- and B-site cationic doping regulation, which provides a new idea for perovskite-type bifunctional electrocatalysts.

Synergistic Interfacial Engineering of Heterostructured Cobalt Phosphide Spheres/Cobalt Hydroxide Nanosheets for Overall Water Splitting.

Recently, transition metal phosphides (TMPs) have been widely explored for the hydrogen evolution reaction (HER) due to their advantaged activity. Nevertheless, the OER performance of TMPs in an alkaline medium is still unsatisfactory. Therefore, interfacial engineering of TMPs to enhance the OER performance is highly desirable. Herein, a Co(OH)2 nanosheet coupled with a CoP sphere supported on nickel foam (NF) is developed by a simple two-step electrodeposition. The large surface area derived from stacked nanosheets and the electronic regulation induced by heterostructure can significantly enhance charge/mass transfer and expose more active sites, thus accelerating the kinetics of the reaction. In addition, the strong electronic interaction between CoP and Co(OH)2 is conducive to the generation of a high valence cobalt center; thus, the electrocatalytic performances toward HER and OER are remarkably improved. Impressively, the optimized CoP/Co(OH)2@NF heterostructure obtains an excellent HER and OER performance with low overpotentials of 76 and 266 mV at 10 mA cm-2, respectively, superior to the commercial Pt/C and RuO2. Moreover, the optimized CoP/Co(OH)2@NF can afford the lowest cell voltage of 1.58 V to achieve 10 mA cm-2 for alkaline overall water splitting and shows outstanding long-term stability.

A carboxylate linker strategy mediated densely accessible Fe-N4 sites for enhancing oxygen electroreduction in Zn-air batteries.

Iron-nitrogen-carbon single-atom catalysts derived from zeolitic-imidazolate-framework-8 (ZIF-8) have presented its great potential for the oxygen reduction reaction (ORR) in Zn-air batteries (ZABs). However, due to insufficient active Fe-N sites, its ORR activity is inferior to Pt-based catalysts. Herein, a carboxylate (OAc) linker strategy is proposed to design a ZIF-8-derived FeNCOAc catalyst with abundant accessible Fe-N4 single-atom sites. Except that imidazole groups can coordinate with Fe ions, the OAc linker on the unsaturated coordination Zn nodes can anchor and coordinate with more Fe ions, resulting in a significant increase in Fe-N4 site density. Meanwhile, the corrosion of carbon skeleton by OAc oxidation during heat-treatment leads to improved porosity of catalyst. Benefitting from the highly dense Fe-N4 sites and hierarchical pores, the FeNCOAc endows superior performance in alkaline medium (E1/2 = 0.906 V), which is confirmed by density functional theory calculation results. Meanwhile, the assembled liquid ZAB delivers a favorable peak power density of 173.9 mW cm-2, and a high specific capacity of 770.9 mAh g-1 as well as outstanding durability. Besides, the solid-state ZAB also shows outstanding discharge performance.