Moybdenum based materials synthesis and energy applications
Date of Issue2015
School of Chemical and Biomedical Engineering
The content of this thesis includes most of my research work during my Ph.D., and it is categorized into two aspects– hydrogen oxidation and hydrogen production. Together with hydrogen transportation/storage, they are central to hydrogen economy. To realize hydrogen economy for the future, we aim to develop non-noble, cost-effective catalysts for efficient and stable hydrogen oxidation and production. The body of this thesis consists of 6 chapters: the first chapter is background introduction and literature review; the second and third are for hydrogen oxidation in SOFC; the last three chapters are the summary of electrochemical hydrogen production by using molybdenum based non-noble catalysts, including porous Mo2C rods, binary Mo2C/WC nanowires, and bulk MoP. Adopting the electrode configuration of the state-of-the-art Ni based anode, e.g., Ni‒YSZ and the concept of full-ceramic anode, e.g. Sr2M1-xMoxO6-δ, we propose to use highly conductive SrMoO3 (~103 S cm-1) to replace Ni, forming a composite anode with YSZ. Concerning the catalytic activity of pristine SrMoO3‒YSZ anode, different mass loadings of GDC were infiltrated. Regarding the improved performance for hydrogen oxidation, various characterizations are applied, e.g. EIS, linear polarization, and potentiostatic test etc., to examine its redox cycling stability, influence of water content in H2 for optimal power output. Subsequently, in chapter 3, dopant calcium is used to partially substitute for Sr in the A site of perovskite structure, forming a catalyst of Sr1-xCaxMoO3 for hydrogen oxidaion in SOFCs. Structural destablization is evoked by Ca substitution, causing Mo ex-solution on the surface of host anode Sr1-xCaxMoO3‒GDC. Characterizations, e.g. XRD and TGA, are employed to have verified the reversibility of Mo ex-solution, which ensures Sr1-xCaxMoO3‒GDC anode good redox cycling stability. Analysis through the Rietveld method to refine XRD data reveals that structure evolution is responsible for the ex-solution of metal Mo. The Mo ex-solution boosts the performance of H2 oxidation at 800 ºC, achieving 330 mW cm-2 compared to 280 mW cm-2 for SrMoO3‒GDC. For hydrogen production, we firstly synthesized porous Mo2C rods with different loadings of metal Mo via the route of anisotropic growth of anilium trimolybddate. The investigation of electrochemical activity and measurement of conductivity show that porous Mo2C rod is an excellent catalyst for HER both in acidic and alkaline media, and possesses high conductivity (~30 S cm-1). It is also found that nano-sized metal Mo on the surface has negative contribution to the performance, and Ni-impregnated porous Mo2C rods exhibit nearly zero onset potential in alkaline, like Pt. Then, we integrated Mo2C with WC in a nanowire structure. The electrochemical test results indicate that cyclic voltammetry scans (0 to 0.63V vs. RHE) would induce a hydrophilic interface that leads to much enhanced performance in catalysing HER in acid. We employ XPS, TEM coupled EDX, to characterize the binary nanowire, and find that the high activity for HER is ascribed to the features of high conductivity because of WC, well-dispersed Mo2C nano particles as the active sites, and electrochemically induced carboxyl interface. Finally, we adopt MoP, a good catalyst for HDS. For the first time, we intuitively examine its activity of HER in acid. The results of electrochemical tests suggest that even bulk MoP performs well in catalysing HER. Experimental investigation of the catalytic activity of metal Mo, Mo3P, and MoP points out that the degree of phosphorization plays an important role in making a good catalyst for HER. Theoretical study attests that phosphorus site in MoP functions as a ‘hydrogen deliverer’, achieving zero Gibbs free energy at certain H coverage, and is responsible for the high performance obtained for HER in both acid and alkaline.