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Title: Transitional metal based nanomaterials as catalysts for water electrolysis
Authors: Zhang, Yongqi
Keywords: DRNTU::Engineering::Materials::Energy materials
Issue Date: 2018
Source: Zhang, Y. (2018). Transitional metal based nanomaterials as catalysts for water electrolysis. Doctoral thesis, Nanyang Technological University, Singapore.
Abstract: Growing concerns about fossil-fuel crisis and global warming make it urgent to explore renewable energy sources as alternatives to fossil fuels. Finding a green and efficient way to harvest, store and use renewable energy is highly necessary. As an energy carrier, hydrogen, has attracted massive attention due to its highest gravimetric energy density and free of CO2 emissions. In addition, electrochemical water splitting provides a feasible and environmental method to produce hydrogen. However, the sluggish kinetics in both side electrodes makes it not economic. To decrease the energy consumption and speed up the reaction rate, efficient and earth abundant catalysts are urgently needed. In this thesis, we focus on the rational design and facile preparation of transition metal based nanomaterials, which are one group of promising catalysts. Radio frequency (RF) plasma is emerging as a very efficient and environmentally friendly technology for surface modification and conversion reaction of solid electrode materials. In this thesis, we first applied the RF N2 plasma to fabricate metal nitrides via the conversion reaction and studied their superior electrochemical performances as catalysts for water splitting. Transition metal nitrides possess high chemical stability and functional physical properties, such as superior corrosion resistance, high conductivity and high melting points. In addition, the introduced N atoms strongly affect the electronic structure of the metal by concomitant structural modification and/or charge transfer processes. They show excellent catalytic activities in various areas due to their distinct electronic structure. However, in most previous reports, metal nitrides are prepared via annealing precursors under caustic and hazardous ammonia (NH3) flow. Even worse, it requires long processing duration and high reaction temperature. In this method, earth abundant and nontoxic N2 is used as nitrogen source and the processing duration is only a few minutes. We successfully converted dense NiMo alloy and Ni metal films into 3D porous nickel molybdenum nitride (NiMoN) and nickel nitride (Ni3N), respectively. Attributed to the synergistic effect of Ni, Mo and N, high roughness factor and electron transport, the obtained NiMoN catalyst exhibits outstanding hydrogen evolution reaction (HER) performance, reaching the current density of 10 mA cm-2 at a small overpotential (~109 mV) with a long-term stability under different current densities. In addition to dense metal precursors, nanostructured metal (hydr)oxides could also be converted into corresponding metal nitrides. For example, Co3O4 nanowire arrays were converted into CoN with the nanowire nanostructure preserved at room temperature in one minutes under N2 plasma. We studied in detail their superior electrochemical performances for OER and compared to four control samples with different plasma treatment durations. The 1-min sample show best performance – small overpotential (290 mV) at 10 mA cm-2 and small tafel slope (70 mV Dec-1) due to the complete conversion and well-preserved morphology. This method is new and should be applicable to a wide range of metal nitrides that can be useful in supercapacitors, Na-ion batteries, OER/HER, etc. Transitional metal oxides, as a large and important class of chemical compounds, are easily available materials with various nanostructures, which endows them large specific surface areas. We found that there is a slow self-activation of metal oxides during the constant hydrogen evolution process due to the reduction of intermediate hydrogen. But the self-activation effect is only temporary and not stable. Hence pre-reducing of metal oxides should be a feasible method to improve their catalytic activity. Metal oxides (NiMoO4, Co3O4 and NiO) were pre-reduced by H2 and C-plasma, respectively. Compared with traditional H2 annealing reduction method, the carbon plasma treatment has a “One stone, two birds” effect - it not only creates lower-valence active sites on the surface, but also deposits a thin graphitic carbon shell simultaneously. This carbon shell protects the surface from re-oxidation and can maintain the catalytic activity for long time. Our C-plasma method opens a new door to make cheap metal oxides more catalytic efficient and stable for HER in harsh conditions. In chapter 5, benefiting from the uniform dip coating and in-situ reduction of precursor, a series of ultrafine transition metal-based nanoparticles (Ni-Fe, Ni-Mo) embedded in N-doped carbon have been successfully fabricated as replacements for noble metal-based catalysts in electrolytic water splitting. The diameter of metal-based nanoparticles is around 2 nm, which increases the availability of active sites for electrocatalysis. The as-prepared catalysts demonstrate outstanding catalytic activities rendered by the synergistic effect of bimetal elements and N-dopants, the improved electrical conductivity and hydrophilism. Ni/Mo2C@N-doped porous carbon (NiMo-PVP) and NiFe@N-doped carbon (NiFe-PVP) produce low overpotential of 130 and 297 mV at a current density of 10 mA cm-2 as catalysts for HER and OER, respectively. In addition, the binder-free electrodes make them show long-term stability. The overall water splitting is also demonstrated based on the couple of NiMo-PVP||NiFe-PVP. Finally, the achievements in each chapter are summarized in chapter 6. In addition, the plans for further research are also proposed.
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