Mesoporous molybdenum carbide for greatly enhanced hydrogen evolution at high current density and its mechanism studies

Currently the catalysis of hydrogen evolution reaction(HER)is mainly focused on the inherent electrocatalytic activity at relatively lower current densities while scarce at high current densities.Nevertheless,the latter is highly demanding in efficient mass-production of hydrogen.A SiO_(2) nanospheres template-synthesis is used to prepare mesoporous molybdenum carbide nanocrystals-embedded nitrogen-doped carbon foams(mp-Mo_(2)C/NC).The material shows much more excellent catalytic activity than the non-etched Mo_(2)C/NC toward hydrogen evolution reaction(HER)in acidic medium.More interestingly mp-Mo_(2)C/NC still has larger overpotential than Pt/C at lower current densities,but possess remarkably smaller overpotential than the latter at higher current densities for much better electrocatalytic performance.An approach is developed to investigate the electrode kinetics by Tafel plots,especially with eliminating the diffusion effect,indicating that Pt/C and mp-Mo_(2)C/NC display different reaction mechanisms.At low current densities the former presents reversible reaction,while the latter shows mixed electrochemical polarization/reversible electrode process.In the region of higher current densities,the former becomes totally gas-diffusion controlled with large overpotential,while the latter can still retain an electrode polarization process for much lower overpotential at the same current density.Result endorses that the meso-porously structured mp-Mo_(2)C/NC plays a critical role in avoiding gas diffusion control-resulting large overpotential at high current densities.This work holds great potential for an inexpensive catalyst better than Pt/C in practical applications of mass-production hydrogen at high current densities,while clearly shedding fundamental lights on designs of rational HER catalysts for the uses at high current densities.

Mechanisms of water oxidation on heterogeneous catalyst surfaces

Water oxidation,an essential step in photosynthesis,has attracted intense research attention.Understanding the reaction pathways at the electrocatalyst/water interface is of great importance for the development of water oxidation catalysts.How the water is oxidized on the electrocatalyst surface by the positive charges is still an open question.This review summarizes current advances in studies on surface chemistry within the context of water oxidation,including the intermediates,reaction mechanisms,and their influences on the reaction kinetics.The Tafel analyses of some electrocatalysts and the rate-laws relative to charge consumption rates are also presented.Moreover,how the multiple charge transfer relies on the intermediate coverage and the accumulated charge numbers is outlined.Lastly,the intermediates and rate-determining steps on some water oxidation catalysts are discussed based on density functional theories.

Electrodeposited transparent PEDOT for inverted perovskite solar cells:improved charge transport and catalytic performances

The acidic, corrosive effect of sodium polystyrene sulfonate(PSS) in poly 3,4-ethylenedioxythiophene:sodium polystyrene sulfonate(PEDOT:PSS) limits the stability of inverted perovskite solar cells(PSCs) based on the ITO/PEDOT:PSS/perovskite/PCBM/BCP/Ag structure. In this work, a poly 3,4-ethylenedioxythiophene(PEDOT) hole transport layer(HTL) with high hole mobility and good catalytic performance was prepared by electrochemical cyclic voltammetry(CV) method for inverted PSCs. By controlling the CV cycles(from 1 to 5 cycles) and EDOT monomer solution concentration(from0.5 to 2.0 mmol·L^(-1)) of electrochemical deposition, the thickness, morphology, optical and electrochemical properties of PEDOT could be accurately adjusted. The optimal photovoltaic performance with current density(J_(sc)) of 22.19 mA·cm^(-2), open circuit voltage(V_(oc)) of 0.94 V, fill factor(FF) of 0.65 and photoelectric conversion efficiency of 13.56% was obtained when deposition of PEDOT with 1 CV cycle and EDOT concentration of 0.5 mmol·L^(-1). At this point, the perovskite showed good crystallization,optimal optical, charge transport and recombination performance, resulting in better V_(oc)and photoelectric conversion efficiency(PCE) compared to the devices with higher CV cycle numbers and 3,4-ethylenedioxythiophene(EDOT) concentration. For comparison with spin-coated PEDOT:PSS, the device with electrodeposited PEDOT showed improved J_(sc)and comparable V_(oc), which may result from its better charge transport and catalytic ability.The device with spin-coated PEDOT:PSS showed photoelectric conversion efficiency of 12.25%, which was lower than that based on electrodeposited PEDOT(13.56%) with1 CV cycles and 0.5 mmol·L^(-1) EDOT concentration. And the device with electrodeposited PEDOT as HTLs showed more excellent air stability. In ambient air((32 ± 5) ℃ and RH: 70% ± 20%), it still maintained more than 80%of the initial photoelectric conversion efficiency after1000 h. In comparison, the photoelectric conversion efficiency of the device with PEDOT:PSS decreased to 20% of the initial value after storage for 500 h. From this study, a facial and low-cost way to prepare PEDOT HTL with high performances that better than the traditional PEDOT:PSS has been explored, which is expected to eliminate the acidic, corrosive effect of PSS in PEDOT:PSS.