MOF-directed templating synthesis of hollow nickel-cobalt sulfide with enhanced electrocatalytic activity for oxygen evolution

The development of efficient, universal and cheap electrocatalysts for the utilization in the oxygen evolution reaction (OER) by desired morphology and composition remains a great challenge. Herein, we report a facile and novel method to prepare the porous hollow nickel-cobalt sulfide (NiCoS) by using zeolitic imidazolate framework-67 (ZIF-67) as the template. The obtained NiCoS-3 polyhedron shows superior catalytic activity toward OER with a low overpotential of 320 mV at the current density of 10 mA cm^?2, a small Tafel slope of 58.8 mV dec^?1 and excellent stability. Benefiting from their structural and compositional merits, the as-synthesized NiCoS-3 polyhedron may be a good promising candidate electrocatalysts for water splitting. Present work provides a simple strategy to regulated composition, morphology and catalytic activity relationship, offer an effective way to design a low-cost and efficient electrocatalyst.     

Optimization of active sites by sulfurization of the core–shell ZIF 67@ZIF 8 for rapid oxygen reduction kinetics in acidic media

Achieving a highly active cathode surface with extremely efficient electrocatalysts for oxygen reduction kinetics is a fundamental necessity for durable fuel cells. Developing such active materials into desirable nanostructures is crucial in the pursuit of electrocatalysis. A facile preparation of cobalt decorated nitrogen and sulfur co-doped carbon nanostructures (Co–NSC) with the added advantage of Zeolitic imidazolate frameworks (ZIF), ZIF 67@ZIF 8 is reported here. The as-prepared Co, N, S, co-doped carbon (Co–NSC) electrocatalyst comes under platinum group metal-free (PGM-free) catalysts which intends to replace the scarce and highly expensive commercial Pt catalysts. Facile preparation of the Co–NSC catalyst that is scalable, low cost and highly ORR active makes the material advantageous. Co–doping sulfur has dramatically enhanced the intrinsic catalytic activity of the catalyst, and the degree of variation in sulfurization greatly influences the overall catalytic property. Co–NSC 200 with high sulfur doping exhibit a positively shifted onset potential of 0.81 V, and a high yielding current density of 5.5 mA cm^?2 at 20 mV s^?1.     

A dual ligand coordination strategy for synthesizing drum-like Co,N co-doped porous carbon electrocatalyst towards superior oxygen reduction and zinc-air batteries

We herein propose a dual ligand coordination strategy for deriving puissant non-noble metal electrocatalysts to substitute valuable platinum (Pt)-based materials toward oxygen reduction reaction (ORR), a key reaction in metal-air batteries and fuel cells. In brief, cobalt ions are firstly double-coordinated with proportionate 2-methylimidazole (2-MeIm) and benzimidazole (BIm) to obtain drum-like zeolitic imidazolate frameworks (D-ZIFs), which are then carbonized to output the final Co, N co-doped porous carbon (Co–N–PC_D) catalyst inheriting the drum-like morphology of D-ZIFs. The Co–N–PC_D is featured by mesopores and exhibits superb electrocatalytic behavior for ORR. Impressively, the half-wave potential of Co–N–PC_D catalysts is 0.886 V with finer methanol-tolerance and stability than those of commercial Pt/C. Additionally, a zinc-air battery assembled from the Co–N–PC_D displays an open-circuit voltage of 1.413 V, comparable to that of commercial Pt/C (1.455 V), suggesting the potentials of Co–N–PC_D in practical energy conversion devices.     

Palladium nanoparticles grown on β-Mo2C nanotubes as dual functional electrocatalysts for both oxygen reduction reaction and hydrogen evolution reaction

Developing efficient dual functional electrocatalysts for both oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is critical for boosting the performance of fuel cells and metal air batteries, as well as production of clean and sustainable energy source. Herein, Pd nanoparticles grown on Mo₂C nanotubes were prepared as dual functional electrocatalysts for both ORR and HER. A series of samples with different Pd loadings were fabricated, while the morphology and the structural features were well examined by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Interestingly, both the ORR activity and HER activity first increased then decreased with the increasing of Pd loading, and the sample of Mo₂C-Pd-9% exhibited the best performance among the series, superior than commercial Pd/C in both ORR and HER tests. Furthermore it also exhibited markedly higher long-term stability than Pd/C for both electrocatalytic reactions. The results may shed light on rational design of novel bi-functional electrocatalysts in the renewable energy field.     

Platinum nanoparticles decorated Nb2CTx MXene as an efficient dual functional catalyst for hydrogen evolution and oxygen reduction reaction

Here, a dual functional Nb₂CT_x@Pt nanocomposite has been synthesized by in situ reduction method. The Pt loading in the composite has been optimized to get minimum overpotential (141 mV at 10 mA/cm²) for hydrogen evolution reaction (HER) along with a promising Tafel slope of 46.3 mV/dec, while Pt/C shows an overpotential and Tafel slope of 104 mV and 32.4 mV/dec, respectively. The Pt mass activity for Nb₂CT_x@Pt3.8 composite at 100 mV overpotential was 3.44 A g^?1 while the Pt mass activity for conventional Pt/C was 0.7 A g^?1, which shows that the activity of Nb₂CT_x@Pt3.8 composite is approximately 5 times higher than Pt/C. In addition, the catalyst was found to be stable for continuous 500 cycles without any binder molecules. The oxygen reduction reaction (ORR) capability of the material was also evaluated and found that the catalyst exhibited a current density of ?4.28 mA/cm² in the diffusion limiting region in comparison with the current density of ?5.82 mA/cm² for Pt/C at 2600 revolutions per minute (RPM). The Pt mass activity of Nb₂CT_x@Pt3.8 composite for ORR is approximately 10 times higher than Pt/C. The Nb₂CT_x@Pt3.8 composite was able to reduce O₂ completely using the 4-electron pathway with very little peroxide production. From these results, the dual functionality of the Nb₂CT_x@Pt3.8 composite for both HER and ORR has been established.     

CoNi/CNTs composite as effective and stable electrode for oxygen evaluation reaction in alkaline media

Alloy structure can strongly enhance the chemical resistance of the bimetallic electrodes to be exploited as effective, low electrons transfer resistance and stable electrodes for the oxygen evolution reaction (OER) in the alkaline media. CoNi nanoparticles/carbon nanotubes (CNTs) composite was prepared by calcination of the physically well mixed nickel acetate, cobalt acetate and CNTs mixture under inert atmosphere at 850 °C. To ensure good mixing as well as strong adhesion of the metallic nanoparticles with the CNTs, the metals precursors were dissolved in ethanol first before addition of the carbonaceous partner. Due to the good absorbability of the CNTs to ethanol and the abnormal decomposition of the acetates precursors, oxides-free and well distributed CoNi nanoparticles/CNTs composite was obtained. Moreover, the XRD and TEM analyses affirmed formation of the alloy structure. The electrochemical measurements indicated that the proposed composites have very good catalytic activity toward the OER regardless the bimetallic nanoparticles composition. Numerically, the composites having bimetallic nanoparticles containing 0, 10 and 20 wt% cobalt revealed Tafel slops of 101, 159 and 111 mV dec^?1, and overpotentials of 415, 461 and 485 mV at 10 mA cm^?2 current density, respectively. However, the best stability in the alkaline medium was observed with the composite contains 10 wt% cobalt, while the other two formulations showed fast dissolution. Overall, the present study opens an avenue for the transition metals alloys to be invoked in the OER cells to overcome the high electron transfer resistance associated with the popularly used oxides-based electrodes.     

Co3O4 nanosheet arrays treated by defect engineering for enhanced electrocatalytic water oxidation

Due to its poor electrical conductivity and finite exposed active sites, the development of high activity Co₃O₄ oxygen evolution reaction (OER) electrocatalysts remains a major challenge. Oxygen vacancies can enhance the electrical conductivity of electrocatalysts and reduce the adsorption energy of H₂O molecules on surfaces, thereby significantly enhancing their electrocatalytic activity. Taking inspiration from this, we demonstrate a green and facile reduction strategy to prepare reduced Co₃O₄ nanosheet arrays (R-Co₃O₄ NSA) with large electrochemical surface area and rich in surface oxygen vacancies. Compared to pristine Co₃O₄ nanosheet arrays (P-Co₃O₄ NSA), R-Co₃O₄ NSA exhibits better OER performance, with a lower overpotential of 330 mV at a current density of 20 mA cm^?2 and a smaller Tafel slope of 72 mV dec^?1. Impressively, the excellent properties of R-Co₃O₄ NSA can rival to the state-of-the-art noble metal oxide electrocatalyst (IrO₂). This strategy of defect-engineering offers a briefness and cost-effective means for the development of highly efficient OER systems.