Pulsed laser deposition of porous N-carbon supported cobalt (oxide) thin films for highly efficient oxygen evolution

Identification of efficient non-precious metal catalysts for the oxygen evolution reaction (OER) remains a great challenge. Here we report robust cobalt (oxide) nanoparticles deposited on a porous nitrogendoped carbon (N-carbon) film prepared by pulsed laser deposition under a reactive background gas, which exhibit highly efficient OER performance with a low overpotential and high stability

Highly active nickel–cobalt/nanocarbon thin films as efficient water splitting electrodes

Developing low cost, highly active and stable electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) using the same electrolyte has remained a major challenge. Herein, we report a novel and robust material comprised of Nickel-Cobalt nanoparticles coated on a porous nitrogen-doped carbon (NC) thin film synthesized via a two-step pulsed laser deposition technique. The optimized sample (Ni0.5Co0.5/NC) achieved lowest overpotentials of 176 mV and 300 mV at a current density of 10 mAcm-2 for HER and OER, respectively. The optimized OER activity might be attributed to the available metal oxide nanoparticles with effective electronic structure configuration and enhanced mass/charge transport capability. At the same time, the porous nitrogen doped carbon incorporated with cobalt and nickel species can serve as an excellent HER catalyst. As a result, the newly developed electrocatalysts manifest high current densities and strong electrochemical stability in overall water splitting, outperforming most of the previously reported non-precious metal-based catalysts

Fabrication of carbon-based nanomaterials for energy conversion

Nowadays, energy is one of the most important challenges facing mankind due to its supply and demand issues and global warming. Replacing unsustainable energy sources such as fossil fuels is among the most critical issues in the 21st century. Among different solutions for the energy challenges, electrochemistry which studies the conversion between electricity and energy stored in chemical bonds can be used to solve these issues without any significant impact to the environment. One of the energy conversion limitations in electrochemical processes is extra energy requirements to overcome the high activation barriers. To overcome this problem, electrocatalysts are always utilized to improve electrode efficiency in order to decrease activation energy and increase energy conversion. Electrocatalysts should be lowcost, durable, efficient and sustainable. One of the main categories of electrocatalysts in energy conversion reactions is precious metal- based materials which have high performance. However, they cannot be applied at large scales due to their high cost, scarcity, limited supply and weak durability. Thus, other alternative electrocatalysts based on lower cost material such as non-precious metal or metal free catalysts have been widely developed. Carbon-based materials are one of the most important materials which have been playing a significant role in the development of energy conversion and storage devices because of their abundance, low cost, stability, easy accessibility, good recycling, and relatively environmentally friendly characteristics with high durability, especially in alkaline medium. This thesis aims to design and fabricate a series of advanced electrocatalysts for the different range of electrochemical reactions including oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) which are principals of various types of energy conversion devices. The first part of the thesis focuses on the development of metal-free nitrogen-doped mesoporous carbon spheres prepared via soft-templating procedure by tuning different nitrogen precursor contents and carbonization temperature. The synthesized electrocatalyst showed a favourable catalytic activity in ORR with high kinetic current and positive onset potential due to its high surface area, high pore volume, narrow mesopore size distribution, high conductivity and high nitrogen content. In the second part, carbon-based composites co-doped with nitrogen and trace amount of metallic cobalt have been developed as electrocatalysts for water splitting system at low overpotential and high current density. An excellent electrochemical activity of newly developed electrocatalyst originates from its graphitic nanostructure and highly active Co-Nx sites. Based on the spectroscopic and electrochemical investigations the newly identified Co- Nx sites in the carbon framework are responsible for the high electrocatalytic activity of the Co, N-doped carbon. The third research project is to utilize the physical synthesis technique to ensure high control and tunability of morphology, structure and composition of multi-component materials. In this context, pulsed laser deposition (PLD) is particularly versatile in the tuning of properties of deposited materials which is based on ablating a target material by laser pulses and has been applied to develop new carbon-based thin films. Thus, cobalt oxide nanoparticles deposited on porous nitrogen -doped carbon films were successfully developed via a two-step pulsed laser deposition technique. The synthesised material behaves as an efficient OER electrocatalyst with superior activity in alkaline electrolyte. The excellent catalytic activity of prepared electrodes could be attributed to the surrounding N-carbon framework, and it was found that a higher ratio of Co2+/Co3+ yields better catalytic activity towards the OER. Transport of reactants and products involved in electrochemical reactions was also facilitated by the porous structure of material. Additionally, the carbon framework, comprising carbons adjacent to cobalt (oxide) nanoparticles, increases catalytic sites and prevents the aggregation or dissolution of nanoparticles. The last part of the thesis aims to design an efficient and stable bifunctional electrocatalyst for both HER and OER in the same electrolyte for overall water electrolysis. Thus a novel type of robust binary Ni-Co nanoparticles coated on porous N-carbon thin film with low overpotential has been reported. The efficient OER activity might be contributed to the available metal oxide nanoparticles with effective electronic structure configuration, enhanced mass/charge transport capability. At the same time, the porous nitrogen doped carbon incorporated with cobalt and nickel species might serve as an excellent HER catalyst. As a result, the newly developed electrocatalysts manifest high current densities and strong electrochemical stability in overall water splitting, outperforming most of the previously reported non-precious metal-based counterparts.Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering, 201

Significant enhancement of water splitting activity of N-carbon electrocatalyst by trace level Co doping

Replacement of precious metal electrocatalysts with highly active and cost efficient alternatives for complete water splitting at low voltage has attracted a growing attention in recent years. Here, this study reports a carbon-based composite co-doped with nitrogen and trace amount of metallic cobalt (1 at%) as a bifunctional electrocatalyst for water splitting at low overpotential and high current density. An excellent electrochemical activity of the newly developed electrocatalyst originates from its graphitic nanostructure and highly active Co-N-x sites. In the case of carefully optimized sample of this electrocatalyst, 10 mA cm(-2) current density can be achieved for two half reactions in alkaline solutions-hydrogen evolution reaction and oxygen evolution reaction-at low overpotentials of 220 and 350 mV, respectively, which are smaller than those previously reported for nonprecious metal and metal-free counterparts. Based on the spectroscopic and electrochemical investigations, the newly identified Co-N-x sites in the carbon framework are responsible for high electrocatalytic activity of the Co,N-doped carbon. This study indicates that a trace level of the introduced Co into N-doped carbon can significantly enhance its electrocatalytic activity toward water splitting

Electrosynthesis of HKUST-1 with Flow-Reactor Post-Processing

Electrochemical synthesis has been proposed as an efficient method for cost-effective and large-scale production of metal-organic frameworks (MOFs). This work investigates the combined electrochemical synthesis with flow synthesis post-treatment for the production of high surface area HKUST-1. The electrochemical synthesis process used in the experimental work did not require additional electrolytes or washing of the synthesis product. Batch electrosynthesis and electrosynthesis with flow synthesis were compared for the quality of the product using Brunauer–Emmett–Teller (BET) surface area, X-ray diffraction (XRD), and scanning electron microscopy (EIS). Batch electrosynthesis in 0.01 M benzene-1,3,5-tricarboxylic acid (H3BTC) solution produced HKUST-1 with BET surface area of 1550 m2/g which was increased further to 1716 m2/g with post-flow-synthesis treatment. The greatest change in surface area after flow processing was observed when using 0.78 M H3BTC, with corresponding surface areas of 481 m2/g and 1531 m2/g. According to SEM and BET results, the product purity improved during the post-flow-synthesis treatment. The proposed method enables continuous flow synthesis of high-quality MOFs with minimal purification steps