Junyuan Xu

Revealing the Origin of Activity in Nitrogen-Doped Nanocarbons towards Electrocatalytic Reduction of Carbon Dioxide.

Carbon nanotubes (CNTs) are functionalized with nitrogen atoms for reduction of carbon dioxide (CO2 ). The investigation explores the origin of the catalysts activity and the role of nitrogen chemical states therein. The catalysts show excellent performances, with about 90 % current efficiency for CO formation and stability over 60 hours. The Tafel analyses and density functional theory calculations suggest that the reduction of CO2 proceeds through an initial rate-determining transfer of one electron to CO2 , which leads to the formation of carbon dioxide radical anion (CO2 (.-) ). The initial reduction barrier is too high on pristine CNTs, resulting in a very high overpotentials at which the hydrogen evolution reaction dominates over CO2 reduction. The doped nitrogen atoms stabilize the radical anion, thereby lowering the initial reduction barrier and improving the intrinsic activity. The most efficient nitrogen chemical state for this reaction is quaternary nitrogen, followed by pyridinic and pyrrolic nitrogen.

One‐Step Fabrication of Monolithic Electrodes Comprising Co9S8 Particles Supported on Cobalt Foam for Efficient and Durable Oxygen Evolution Reaction

A very easy and cost-effective approach to the fabrication of monolithic Co9 S8 water oxidation electrodes (Co@Co9 S8 ), fabricated by one-step hydrothermal treatment of commercially available cobalt foam in the presence of thiourea, is reported. The morphology, crystal structure, microstructure, and composition of as-fabricated Co@Co9 S8 electrodes were examined by using scanning electron microscopy (SEM), powder X-ray diffractometry (XRD), transmission electron microscope (TEM), and X-ray photoelectron spectroscopy (XPS), and their electrochemical properties were investigated by cyclic voltammetry (CV), chronopotentiometry (CP), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). When used to catalyze the oxygen evolution reaction (OER) in alkaline solution, the Co@Co9 S8 electrode with an optimal Co9 S8 loading exhibits outstanding catalytic activity, requiring a low overpotential of 350 mV to deliver an anodic current density of 10 mA cm-2 and showing fast kinetics for OER with a small Tafel slope (55 mV dec-1 ) and charge-transfer resistance (0.44 Ω cm-2 ), which outperforms many sulfide-based OER catalysts and some state-of-the-art noble metal catalysts recently reported in the literature. Importantly, the electrodes show excellent long-term stability, and are capable of operating at both a low current density and a high current density relevant to industrial water electrolysis up to 100 hours.

The Effect of Different Phosphorus Chemical States on an Onion-like Carbon Surface for the Oxygen Reduction Reaction

Two kinds of phosphorus-modified onion-like carbons dominated by C-O-P bonds and C-P bonds were fabricated and further used as catalysts in the oxygen reduction reaction (ORR). The results show that the bonding state of phosphorus has a significant effect on the ORR catalytic activity. The formation of C-O-P bonds improves ORR activity, whereas C-P bonds play an adverse role in stabilizing the key intermediates during the ORR owing to the distorted graphitic structure, as confirmed by the work function value.

Boosting the hydrogen evolution performance of ruthenium clusters through synergistic coupling with cobalt phosphide

In this article, we for the first time report the synthesis and electrocatalytic properties of ruthenium cobalt phosphide hybrid clusters for the hydrogen evolution reaction (HER). Two types of catalysts are investigated: wet chemical reduction of Ru3+ on pre-formed cobalt phosphide (CoP) nanoparticles results in ruthenium–cobalt phosphide side-by-side structures (Ru/CoP); while phosphorizing chemically reduced RuCo alloy leads to the formation of hybrid ruthenium cobalt phosphide (RuCoP) clusters. Compared to pristine Ru clusters, both Ru/CoP and RuCoP show significantly improved HER performance in both acidic and alkaline solutions. In particular, the hybrid RuCoP clusters demonstrate a considerably low overpotential (η10) of 11 mV at −10 mA cm−2 and a high turnover frequency (TOF) of 10.95 s−1 at η = 100 mV in 0.5 M H2SO4. Even in 1.0 M KOH the excellent HER activity of the RuCoP clusters remains, with a very low η10 of 23 mV and exceptionally high TOF value of 7.26 s−1 at η = 100 mV. Moreover, the RuCoP catalysts can sustain galvanostatic electrolysis in both acidic and alkaline solutions at −10 mA cm−2 for 150 h with little degradation, showing better catalytic stability than the state-of-the-art commercial Pt/C catalysts. Our density functional theory (DFT) calculations indicate that the RuCoP hybrid exhibits a hydrogen adsorption energy very close to that of Pt and water and –OH adsorption energies distinct from pristine Ru, which reasonably explain the experimentally observed excellent HER activities and highlight the importance of synergistic coupling with cobalt phosphide to boost the HER performance of ruthenium.

Highly-ordered silicon nanowire arrays for photoelectrochemical hydrogen evolution: an investigation on the effect of wire diameter, length and inter-wire spacing

Vertically-aligned, highly-ordered silicon nanowire (SiNW) array photocathodes are fabricated employing e-beam lithography followed by deep reactive ion etching (DRIE) of Si. The effect of structural parameters of SiNWs, including wire diameter, length and inter-wire spacing, on their photoelectrocatalytic hydrogen evolution performance has been systematically investigated. Within the range of dimensions under study, the SiNW photocathode with a wire diameter of 200 nm, a length of 1 μm and an inter-wire spacing of 175 nm shows the best performance exhibiting a maximal saturated photocurrent density of 52 mA cm−2 and an onset potential (@−1 mA cm−2) of −0.17 V versus reversible hydrogen electrode. These lithography-patterned SiNWs with homogeneous structural parameters can help establish an unobscured structure–activity relation and facilitate Si-based photoelectrode design.

Conformal and continuous deposition of bifunctional cobalt phosphide layers on p-silicon nanowire arrays for improved solar hydrogen evolution

Vertically aligned p-silicon nanowire (SiNW) arrays have been extensively investigated in recent years as promising photocathodes for solar-driven hydrogen evolution. However, the fabrication of SiNW photocathodes with both high photoelectrocatalytic activity and long-term operational stability using a simple and affordable approach is a challenging task. Herein, we report conformal and continuous deposition of a di-cobalt phosphide (Co2P) layer on lithography-patterned highly ordered SiNW arrays via a cost-effective drop-casting method followed by a low-temperature phosphorization treatment. The as-deposited Co2P layer consists of crystalline nanoparticles and has an intimate contact with SiNWs, forming a well-defined SiNW@Co2P core/shell nanostructure. The conformal and continuous Co2P layer functions as a highly efficient catalyst capable of substantially improving the photoelectrocatalytic activity for the hydrogen evolution reaction (HER) and effectively passivates the SiNWs to protect them from photo-oxidation, thus prolonging the lifetime of the electrode. As aconsequence, the SiNW@Co2P photocathode with an optimized Co2P layer thickness exhibits a high photocurrent density of–21.9 mA·cm−2 at 0 V versus reversible hydrogen electrode and excellent operational stability up to 20 h for solar-driven hydrogen evolution, outperforming many nanostructured silicon photocathodes reported in the literature. The combination of passivation and catalytic functions in a single continuous layer represents a promising strategy for designing high-performance semiconductor photoelectrodes for use insolar-driven water splitting, which may simplify fabrication procedures andpotentially reduce production costs.