Investigation of ternary sulfides for hydrogen evolution reaction in an electrolysis cell or photoelectrolysis cell
Date of Issue2016-05-14
School of Materials Science and Engineering
The construction of viable photoelectrochemical (PEC) devices for solar-driven water splitting can be achieved by first identifying an efficient independent photoanode for water oxidation and a photocathode for hydrogen generation. These two photoelectrodes then shall be assembled with a proton exchange membrane within a complete coupled system. For constructing efficient PEC devices, there are two factors to be considered. One is semiconductor absorbers which determine the upper limit of PEC efficiency and the other is the surface catalysts which help to push the PEC efficiency approaching to the ideal state. For the materials used for catalyzing hydrogen evolution reactions (HER), development of noble-metal-free material is one of the most desired efforts during the past decade. Transition metal sulfide electrocatalysts (e.g. MoS2) are highly attractive and have been proven as very efficient catalysts in both electrolysis and photoelectrolysis cells. However, the catalytic activity of MoS2 could be further enhanced by introducing additional metal ions inside the structure. Unfortunately, these ternary sulfides are not well understood. Therefore, in this thesis, two categories of ternary sulfides were explored. We first investigated the photocatalytic activity of Cu2MoS4 on Cu2O and Si surface. The electrochemical study confirmed the catalytic activity of Cu2MoS4 on both semiconductors. Driven by drop-casted Cu2MoS4 particles on the surface, Cu2O and Si photocathodes could drive HER at the potential of only 0.45V and 0.25V vs. RHE. The result is very competitive compared with the reports in literature. However, by electrochemical impedance measurement, photoactivity of Cu2O and Si was also found to be limited by this drop-casted Cu2MoS4 film. We then explored electrodeposition of novel Co/NiMoSx for hydrogen evolution. For the first time in the scientific society, we were able to synthesize this material precisely by electrodeposition method. The intrinsic electrocatalytic activity of Co/NiMoSx for catalyzing HER was then carefully examined. It was found that CoMoSx required only ~100mV overpotential to start up hydrogen production compared with ~200mV of MoSx prepared by the same approach. In the effort of investigation of its atomic-structure, we successfully evidenced coordinated bonds between different species. Moreover, mechanism study of this material showed the catalytic activity is limited by 1 proton, 1 electron process, which seems to be a general case on molybdenum or tungsten based sulfides. Co/NiMoSx catalyst was also investigated in a photoelectrochemical cell by coupled with p-Si electrodes for solar hydrogen production. By novel photodeposition method, we successfully deposit Co/NiMoSx film on p-type silicon electrodes easily. The obtained Si/CoMoSx turned to perform excellently with the Jsc more than 17mA.cm-2. The result is very competitive among the best-performed photocathode cells reported in the literature. We then inspected its catalytic activity by impedance analysis. Compared with MoSx and NiMoSx, CoMoSx layer on illuminated Si surface acts not just as a very effective catalyst accelerating the H2 generation, but also as a very effective pacifying agent which accelerates photoelectrons’ injection and transfer.