Neil V. Rees

Enhancement of the Hydrogen Evolution Reaction from Ni-MoS2 Hybrid Nanoclusters

This report focuses on a novel strategy for the preparation of transition metal–MoS2 hybrid nanoclusters based on a one-step, dual-target magnetron sputtering, and gas condensation process demonstrated for Ni-MoS2. Aberration-corrected STEM images coupled with EDX analysis confirms the presence of Ni and MoS2 in the hybrid nanoclusters (average diameter = 5.0 nm, Mo:S ratio = 1:1.8 ± 0.1). The Ni-MoS2 nanoclusters display a 100 mV shift in the hydrogen evolution reaction (HER) onset potential and an almost 3-fold increase in exchange current density compared with the undoped MoS2 nanoclusters, the latter effect in agreement with reported DFT calculations. This activity is only reached after air exposure of the Ni-MoS2 hybrid nanoclusters, suggested by XPS measurements to originate from a Ni dopant atoms oxidation state conversion from metallic to 2+ characteristic of the NiO species active to the HER. Anodic stripping voltammetry (ASV) experiments on the Ni-MoS2 hybrid nanoclusters confirm the presence of Ni-doped edge sites and reveal distinctive electrochemical features associated with both doped Mo-edge and doped S-edge sites which correlate with both their thermodynamic stability and relative abundance.

Effect of catalyst carbon supports on the oxygen reduction reaction in alkaline media: a comparative study

Some carbon materials commonly used as electronically-conductive supports for catalysts in fuel cell research are Carbon Black (CB), multi-walled carbon nanotubes (MWCNT), Graphene Oxide (GO) and reduced graphene oxide (rGO). Here we present a comparative study into the relative effects of each of these on the performance towards the oxygen reduction reaction (ORR) in alkaline media. For the purposes of comparing the supports, a simple Pt catalyst is used and the performance is evaluated via Koutecky–Levich analysis and direct measurement of peroxide by rotating ring-disk electrode (RRDE) to determine the number of electrons (n) transferred in the ORR. It is found that Pt/CB follows a quasi 4-electron mechanism due to that the ORR takes place mainly on the active Pt particles, whereas Pt/MWCNT, Pt/GO and Pt/rGO exhibit a mixed behaviour between the two proposed mechanisms due to the higher activity of the graphene-derived supports towards the peroxide formation compared to CB. The effect of the oxide groups of GO and the metal impurities of MWCNT on the catalytic performance is also studied.

Electrocatalytic regeneration of atmospherically aged MoS2 nanostructures via solution-phase sulfidation

The performance of MoS2 as a hydrogen evolution catalyst is diminished by exposure to air. We demonstrate a solution phase technique to resulfidate MoSxO2-x using Na2S2O3. The success of the method was judged by performance as a H+ reduction catalyst. Following sulfidation samples displayed a favourable decrease in both onset potential and Tafel slope, with the best decreasing from -0.23 V to -0.18 V (vs. SHE), and 282 mV dec-1 to 87 mV dec-1 respectively. Ageing studies indicate that this method may be used to recycle the MoS2 repeatedly without losing catalytic performance, although repeated sulfidation did result in homogenisation of the nanostructure.

Electrochemistry Fundamentals: Nanomaterials Evaluation and Fuel Cells

The chapter will cover the fundamentals of electrochemistry of specific reference to fuel cells, and nanomaterials evaluation. Whilst these may be familiar to the trained electrochemist, a significant amount of fuel cells research is conducted by non-specialists who often use electrochemical methods as a tool without an appreciation of its nuances. The article covers electron transfer and mass transport, issues of the nanoscale (as compared to macroscale), the evaluation of electrocatalytic behaviour, introduction to mechanism, and the electrochemical characterisation of catalyst materials.