Chaojun Lei

An ultrathin cobalt-based zeolitic imidazolate framework nanosheet array with a strong synergistic effect towards the efficient oxygen evolution reaction

Development of highly efficient and stable zeolitic imidazolate framework (ZIF) nanosheets has recently received growing interest as alternatives to noble-metal electrocatalysts towards the oxygen evolution reaction (OER). Herein, we develop a novel vapor-phase hydrothermal growth strategy for in situ synthesis of ultrathin cobalt-based zeolitic imidazolate framework (ZIF-67) nanosheets grown on the surface of Co(OH)2 nanosheets vertically aligned on electrochemically exfoliated graphene (EG) foil. Ultrathin ZIF-67 nanosheets with a thickness of ∼5 nm are uniformly coated on the surface of a Co(OH)2 nanosheet array to form a 2D core–shell structure. Benefitting from the strong coupling and synergistic effects, the resulting EG/Co(OH)2/ZIF-67 hybrid exhibits excellent catalytic activity for the OER in alkaline electrolytes, which only requires an overpotential of 280 mV to attain a current density of 10 mA cm−2 with a low Tafel slope of 63 mV dec−1 and high stability. Such high OER performance for the EG/Co(OH)2/ZIF-67 hybrid is superior to that of commercial Ir/C catalysts and even better than that of all previously reported ZIF-based OER electrocatalysts. In situ Raman analyses demonstrate that under OER conditions, the Co–O species are converted into Co–OOH groups, which are responsible for the excellent catalytic activity of EG/Co(OH)2/ZIF-67 in the OER.

Efficient alkaline hydrogen evolution on atomically dispersed Ni–Nx Species anchored porous carbon with embedded Ni nanoparticles by accelerating water dissociation kinetics

Developing inexpensive and efficient electrocatalysts for hydrogen evolution reaction (HER) during alkaline water electrolysis is crucial for renewable and sustainable energy harvesting. Herein, we report a novel hybrid electrocatalyst comprising atomically dispersed Ni–Nx species anchored porous carbon (Ni–N–C) matrix with embedded Ni nanoparticles for HER. This new catalyst is synthesized via pyrolysis of hydrothermally prepared supermolecular composite of dicyandiamide and Ni ions followed by an acid etching treatment. The achieved hybrid exhibits superior catalytic performance toward HER with a small overpotential of 147 mV at 10 mA cm−2 and a low Tafel slope of 114 mV dec−1, comparable to those of state-of-the-art heteroatom-doped nanocarbon catalysts and even outperforming other reported transition-metal-based compounds in basic media. Experimental observations and theoretical calculations reveal that the presence of Ni nanoparticles can optimize surface states of Ni−Nx active centers and reduce energy barriers of dissociated water molecules, which synergistically improve OH− adsorption and promote HER kinetics. When served as electrodes for both cathode and anode, an alkaline water electrolyzer could afford a current density of 10 mA cm−2 at a low cell voltage of 1.58 V, rivalling the sufficiently high overpotentials of integrated Pt/C–Ir/C benchmark electrodes.

Porous Cobalt Oxynitride Nanosheets for Efficient Electrocatalytic Water Oxidation

Exploring efficient and cheap oxygen evolution reaction (OER) electrocatalysts is of great significance for electrochemical water splitting. Herein, we successfully prepared an efficient ternary electrocatalyst of porous cobalt oxynitride (Co3 Ox Ny ) nanosheets by a simple nitridation strategy. Specifically, CoON PNS-400 (cobalt oxynitride with porous nanosheet structure obtained at 400 o C) offered a low OER overpotential of 0.23 V to achieve the catalytic current density of 10 mA cm-2 and a small Tafel slope of 48 mV dec-1 in alkali media, outperforming most of the first-row transition-metal-based OER electrocatalysts. The calculated density of states (DOS) analysis and electron spin resonance (ESR) measurements revealed that the introduction of foreign N atoms into pristine Co3 O4 nanosheets can optimize the electronic structure and create more oxygen vacancies, thus leading to enhanced electrical conductivity. Density functional theory (DFT) calculations demonstrated that the foreign N atoms can also improve the energetics for OER by modulating the free energy for adsorbed intermediates (OOH*, OH*, O*), further improving the OER electrocatalytic activity of CoON PNS-400. This work provides a possibility for rationally designing ternary transition-metal compounds as advanced OER electrocatalysts.

Electrochemical activation of sulfate by BDD anode in basic medium for efficient removal of organic pollutants

Electrochemical advanced oxidation processes (EAOPs) based on hydroxyl radicals (OH) have some limitations when they are applied to real wastewater treatment, such like strict requirements on pH (acidic electrolyte) and high energy consumption. Compared to OH, Sulfate radicals (SO4-) have high redox potential in wider range of pH (2-9). In this study, SO4- were efficiently produced by electrochemical activation of SO42- at boron doped diamond (BDD) anode. The degradation rate of 2,4-DCP (k = 0.828 ± 0.05 h-1) in the presence of Na2SO4 was approximately 4 times than that without Na2SO4 (k = 0.219 ± 0.01 h-1), indicating that SO4- exhibited higher reactivity than OH at initial pH = 9. Moreover, the amount of O2 decreased by 65% after 100 min during electro-oxidation of 2,4-DCP and the specific energy consumption per unit TOC (ECTOC) was reduced by 70% when the concentration of Na2SO4 increased from 0.01 to 0.1 M. The impact of sulfate ions in the electrochemical advanced oxidation were investigated. Electron spin resonance (ESR) measurements were conducted to identify the formation of SO4-. Electrolysis of 2,4-DCP with specific radical scavengers (ethanol and tert-Butanol) were conducted and the results revealed that SO4- were mainly produced by one-electron loss of sulfate at basic condition. Electro-generation persulfate was measured and participation of non-radical activation of persulfate was investigated. O2 production was quantified and we found electrochemical activation of sulfate could inhibit water dissociation. Taken all findings, a mechanism of electrochemical activation of sulfate at BDD anode was summarized. This technology eliminates the requirement for pH adjustment for wastewater treatment and makes EAOPs more effective and economic as well.