Use of regenerative fuel cells requires efficient bifunctionality in oxygen electrocatalysis: oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Commonly used noble metals like Pt and its alloys (Pt/Ir or Pt/Ru) are often used for their catalytic activity, selectivity and stability in harsh environments. However, Pt can degrade during operation from catalyst agglomeration and poisoning. Therefore, researchers have used non-precious transition metal oxides (TMO) including Fe3O4, MnOx and Co3O4 and/or nanocarbon structures (NC) as potential catalyst. Composite structures where TMO nanoparticles are deposited onto a NC, derived from either graphene oxide (GO) or metal-organic frameworks (MOFs), have often been used. NCs have high surface area and excellent electronic conductivity, and while many studies assert these types of composite materials exhibiting synergistic effects in oxygen electrocatalysis, efforts to elucidate the origin of the synergy is lacking. This doctoral research explores how functional groups present on the surface of NCs affect synergy (reaction route and kinetics) of these electrocatalysis. To incur catalytically active sites between the metal oxides and carbon, the NCs basal plane were functionalized using acid treatments, after which various types of TMO/NC hybrids were synthesized using either wet process or vacuum deposition techniques. The hydroxylated CeO2/graphene hybrids showed the best ORR and OER performance in both alkaline and acidic media, in terms of onset/half-wave potential, electron transfer number, and current density when compared to the performance of benchmark catalysts: Pt/C (for ORR) and IrO2 (for OER). From a series of material and electrochemical analyses, it was determined that a strong tethering of TMOs on graphene's basal plane prohibited restacking and particle-carbon interfaces dictates the performance and reaction route, as indicated in density functional theory calculations. In addition, a hybrid catalyst of TiO2 nanodots, uniformly anchored on phosphorylated carbon by atomic layer deposition (ALD), showed even better ORR and OER performance in alkaline media when compared the aforementioned CeO2/graphene hybrid. Materials characterization emphasized a strong adhesion of TMOs on MOF structures; thus providing ample surface interactions for a favorable reaction route. Therefore, an activation of catalytic sites can be realized by proper engineering of interfaces in each hybrid system.

Metal Oxide–Carbon Hybrid Materials: Synthesis, Properties and Applications reviews the advances in the fabrication and application of metal oxide–carbon-based nanocomposite materials. Their unique properties make them ideal materials for gas-sensing, photonics, catalysis, opto-electronic, and energy-storage applications. In the first section, the historical background to the hybrid materials based on metal oxide–carbon and the hybridized metal oxide composites is provided. It also highlights several popular methods for the preparation of metal oxide–carbon composites through solid-state or solution-phase reactions, and extensively discusses the materials’ properties. Fossil fuels and renewable energy sources cannot meet the ever-increasing energy demands of an industrialized and technology-driven global society. Therefore, the role of metal oxide–carbon composites in energy generation, hydrogen production, and storage devices, such as rechargeable batteries and supercapacitors, is of extreme importance. These problems are discussed in in the second section of the book. Rapid industrialization has resulted in serious environmental issues which in turn have caused serious health problems that require the immediate attention of researchers. In the third section, the use of metal oxide–carbon composites in water purification, photodegradation of industrial contaminants, and biomedical applications that can help to clean the environment and provide better healthcare solutions is described. The final section is devoted to the consideration of problems associated with the development of sensors for various applications. Numerous studies performed in this area have shown that the use of composites can significantly improve the operating parameters of such devices. Metal Oxide–Carbon Hybrid Materials: Synthesis, Properties and Applications presents a comprehensive review of the science related to metal oxide–carbon composites and how researchers are utilizing these materials to provide solutions to a large array of problems. Reviews the fundamental properties and fabrication methods of metal-oxide–carbon composites Discusses applications in energy, including energy generation, hydrogen production and storage, rechargeable batteries, and supercapacitors Includes current and emerging applications in environmental remediation and sensing

It is well known that renewable energy, e.g., wind and solar power, are intermittent energy sources. This means that energy storage devices are needed to store the energy for when it is needed. Currently Li-ion batteries are used as these energy storage devices, not only for alternative energy plants but in vehicles and electronics. There are several drawbacks with using Li-ion batteries, such as low safety, harmful Li mining practices, and high material costs. Rechargeable zinc-air batteries (ZABs) have gained a lot of traction recently due to their low cost, high safety, low environmental impact, and high theoretical energy density. However, a major obstacle is the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the air electrode, which have hindered practical applications of ZABs. Precious metal catalysts have been applied to help mitigate the slow reaction kinetics; however, these are expensive and complicate manufacturing practices since two different precious metals are needed to achieve a bifunctional catalyst. Therefore, a low-cost bifunctional catalyst is needed to improve the slow reaction kinetics at the air electrode. This work focuses on further investigating a previously developed impregnation technique for air electrode preparation using an array of transition metal (Zn, Ni, Mn, and Co) oxide combinations. Various electrochemical and microstructural characterization techniques, e.g., linear sweep voltammetry, electrochemical impedance spectroscopy, electron microscopy, and energy dispersive X-ray spectroscopy, are used to examine each sample. The first study involved fabricating several catalysts by decorating nitrogen doped carbon nanotubes (N-CNTs) with either tri-metallic (Ni-Mn-Co) or tetra-metallic (Zn-Ni-Mn-Co) oxides, through a simple impregnation method into carbon-based, gas diffusion layers (GDL). Metal oxide compositions were selected based on previous results, preliminary electrochemical testing, and statistical design of experiments (DOE). Microstructural characterization was done using electron microscopy and X-ray photoelectron spectroscopy (XPS), and determined that the oxides fabricated were spinel oxides. Samples were electrochemically tested and the best candidates were subjected to full cell testing and bifunctional cycling for 200 charge/discharge cycles at 10 mA/cm2. The overall bifunctional efficiency, after cycling, of the best NiMnCoOx/N-CNT and ZnNiMnCoOx/N-CNT catalysts was 53.3% and 56.4%, respectively; both outperformed Pt-Ru/C in both overall bifunctional efficiency (38%) and cycling stability. The maximum power density of one of the tetra-metallic oxides exceeded that of Pt-Ru/C (110 mW/cm2) at 134 mW/cm2. The addition of Zn with Ni-Mn-Co oxide particles showed improved cycling stability and overall bifunctional efficiency. The second study investigated the effect of co-doping of carbon nanotubes with nitrogen and sulfur (N,S-CNTs), combined with tri-metallic and tetra-metallic oxides, on the ORR and OER reaction kinetics at the air electrode. The best tri-metallic (Ni-Mn-Co) oxide and tetra-metallic (Zn-Ni-Mn-Co) oxide from the first study were used in this investigation. Microstructural characterization analysis revealed that the Co and Mn valences increased for the Ni-Mn-Co and Zn-Ni-Mn-Co oxides, respectively. Electrochemical testing revealed that the Ni-Mn-Co oxide was comparable to the Pt-Ru/C catalyst with a power density of ~95 mW/cm2 and Zn-Ni-Mn-Co oxide was comparable to the Pt-Ru/C catalyst with an efficiency of 56.0% at 20 mA/cm2. The addition of sulfur to the N-CNTs positively impacted the Ni-Mn-Co oxide, leading to a round trip bifunctional cycling efficiency of 55.1% for 200 charge-discharge cycles at 10 mA/cm2. The impact of sulfur did not have a positive impact on the Zn-Ni-Mn-Co oxide; the LSV results were significantly worse than the equivalent oxide on N-CNTs and the full cell testing was comparable to the N-CNT oxide. Both tri-metallic and tetra-metallic oxides outperformed Pt-Ru/C during bifunctional cycling.

Hydrogen fuel, storing solar energy by splitting water, is of great potential as efficient energy storage due to its sustainability, carbon-neutrality and high energy density per mass. One of major bottlenecks for the solar-driven energy storage into hydrogen, however, is oxygen evolution reaction (OER) because of its high overpoential and the complexity of surface structures and reaction mechanisms. To overcome these obstacles, researchers have approached in two ways: (i) searching for the best materials with the highest efficiency and (ii) devising schemes that can yield a higher efficiency, given materials. Considering that the efficiency improvement with inexpensive materials would be ultimately beneficial for future global energy requirements, we pursue the second approach and examine nanoscale heterojunctions of earth abundant materials. In this thesis, we employ density functional theory (DFT) calculations to investigate nanoscale heterojunctions of transition metal oxide and silicon (Si), which are commonly used for photo/photoelectocatalytic and photovoltaic materials, respectively. In particular, the heterojunction of anatase titanium dioxide (TiO2) and Si is of our best interest. The heterojunctions of TiO2 and Si have not only exhibit great synergies based on the bulk properties, but also have improved the photoelectrocatalytic efficiency experimentally. However, the mechanism for this improvement is unclear. Optimizing the catalytic activity of such systems requires a deeper understanding of the detailed atomic and electronic structure of the TiO2/Si interface, the OER mechanism on TiO2 surface, and how the TiO2/Si interface affects the active TiO2 surface, thus changing the OER overpotential. This thesis examines mainly four aspects of the heterojunctions of anatase TiO2(001) and Si: (i) the thermodynamic stability of different local stoichiometry at the TiO2/Si interface, (ii) the electronic structures induced by the different TiO2/Si interface, (iii) how the TiO2/Si interface influences OER on TiO2 surface and its rate-limiting overpotential, and (iv) whether this scheme is transferrable to other oxides such as strontium titanate (SrTiO3 perovskite) to improve the OER efficiency. We also propose a new OER pathway on anatase (001) surface that is plausible under realistic experimental conditions and compare it with the OER pathways that have been proposed earlier. This work, thus, has potential to deepen our understanding and insights of interface physics, surface chemistry and energy conversion.

Highly active, low cost and durable electrocatalysts are desired for the development and commercialization of fuel cells and metal-air batteries. The efficiency of these devices is significantly limited by the activation of oxygen-involved reactions, namely oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Precious metals (such as Pt) and metal oxides (such as RuO2) are traditional electrocatalysts for ORR and OER, respectively. However, the electrocatalytic performance of these precious metal-based nanomaterials is still hindered by their scarcity, high cost, insufficient activity and poor durability. Recently, developing cost-efficient and highly active electrocatalysts to replace the precious metals and oxides have obtained increasing attentions. To enhance the performance of ORR electrocatalysis, formation of PtM (M=Fe, Co, Ni, Cu) is one of most widely used strategies. The utilization of PtM can not only decrease the overall cost but improve the catalytic activity due to the synergistic effect between Pt and M. In addition, porous carbon-based nanomaterials, such as heteroatom-doped carbon, metal-nitrogen-carbon (M-N-C) nanostructures and carbon/nonprecious metal hybrids, have also been demonstrated to be promising candidates for ORR catalysis in alkaline media. These porous catalysts can effectively reduce the cost because of the absence of precious metals. Besides, the unique porous structures are favorable for mass transport and electron transfer, thus improving ORR catalytic performance. For OER electrocatalysis, a multitude of efforts have been devoted to investigate earth-abundant and highly active catalysts, such as transition metal-based nanomaterials (alloys, oxides, phosphides, phosphates, hydroxides, etc.). The corresponding OER catalytic performance can be effectively improved by tailoring the intrinsic nature of the catalysts as well as forming sufficient active sites, which can be achieved by tuning the elemental composition and increasing the surface area. Herein, a large variety of nanostructured electrocatalysts with different composition and morphology were designed and synthesized. Thanks to their compositional and morphological advances, these catalysts have been demonstrated to be active for ORR or OER.