Rational Design of Advanced Functional Materials for Electrochemical Devices

In recent years, there has been a fast-growing trend in developing urea (CO(NH2)2) as a substitute H2 carrier in energy conversion due to its high energy density, nontoxicity, stability, and nonflammability. Urea, a byproduct in the metabolism of proteins and a frequent contaminant in wastewater, is an abundant compound that has demonstrated favorable characteristics as a hydrogen-rich fuel source with 6.7 wt % gravimetric hydrogen content. Also, there is 2-2.5 wt % urea from mammal urine; therefore, 0.5 million ton of additional fuels will be produced per year just from human urine (240 million ton each year). Electrochemical oxidation has been recognized as an efficient strategy for urea conversion and wastewater remediation. Thus, the chemical energy harvested from urea/urine can be converted to electricity via urea oxidation reaction (UOR). Moreover, the removal of urea from water is a priority for improving drinking water quality and presents an opportunity for UOR. However, the transition of UOR from theory and laboratory experiments to real-world applications is largely limited by the conversion efficiency, catalyst cost, and feasibility of wide-spread usage. Therefore, utilization of urea using electrochemical method is a ‘two birds with one stone’ strategy which convert wastewater to electricity via anodic urea oxidation reaction (Seen in Chapter 2). Developing efficient and low-cost urea oxidation reaction (UOR) catalysts is a promising but still challenging task for environment and energy conversion technologies such as wastewater remediation and urea electrolysis. NiO nanoparticles that incorporated graphene as the NiO@Graphene composite were constructed to study the UOR process in terms of density functional theory. The single-atom model, which differed from the previous used heterojunction model (Chapter 2), was employed for the adsorption/desorption of urea and CO2 in the alkaline media. As demonstrated from the calculated results, NiO@Graphene prefers to adsorb the hydroxyl group than urea in the initial stage due to the stronger adsorption energy of the hydroxyl group. After NiOOH@Graphene was formed in the alkaline electrolyte, it presents excellent desorption energy of CO2 in the rate-determining step. Electronic density difference and the d band center diagram further confirmed that the Ni(III) species is the most favorable site for urea oxidation while facilitating charge transfer between urea and NiO@Graphene. Moreover, graphene provides a large surface for the incorporation of NiO nanoparticles, enhancing the electron transfer between NiOOH and graphene and promoting the mass transport in the alkaline electrolyte. Notably, this work provides theoretical guidance for the electrochemical urea oxidation work (As presented in Chapter 3). In addition, urea oxidation reaction (UOR) has been known as a typical energy conversion reaction but is also a viable method for renal/liver disease diagnostic detection. Here, we reported the three-dimensional nickel oxide nanoparticles decorated on the carbonized eggshell membrane (3D NiO/c-ESM) as a modified electrode toward urea detection. The electrocatalysts are characterized by XRD, SEM, and EDX to confirm its structural and morphological information. NiO/c-ESM modified electrode exhibits an outstanding performance for urea determination with a linear range from 0.05 to 2.5 mM, and limit detection of ~20 μM (3σ). This work offered a green approach for introducing 3D nanostructure through employing biowaste ESMs as templates, providing a typical example for producing new value-added nanomaterials with urea detection (Presented in Chapter 4). Generally, urea oxidation reaction happens on the anode, less attention is paid on the cathode. In fact, hydrogen evolution reaction happens on cathode during water/urea electrolysis. Therefore, in this chapter (Chapter 5), we focus our attention on the cathodic reaction, as follows: Transition metal oxides (TMOs), especially nickel oxide (NiO), are environmentally benign and cost-effective materials, and have recently emerged as potential hydrogen evolution reaction (HER) electrocatalysts for future industrial scale water splitting in alkaline environment. However, their applications in HER electrocatalysts remain challenging because of poor electronic conductivity and unsatisfactory activity. Besides, the disposal of eggshell waste is also an environmentally and economically challenging problem because of food industry. Here, we report the synthesis of NiO nanoparticles (NPs) encapsulated in the carbonization of eggshell membrane via a green and facile approach for HER application. Noteworthy to mention here that the active carbon was made from the waste, eggshell membrane (ESM), meanwhile, the eggshell was used as a micro-reactor for preparation of electrocatalyst, NiO/C nanocomposite. Then, the as-prepared NiO/C nanocomposite was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS). The SEM, EDS and TEM images reveal that NiO nanoparticles distributed on the carbon support, and XRD patterns confirm the presence of the nanoparticles are NiO and C hybrids. The catalytic activity and durability of NiO/C nanocomposite was examined for HER in 1 M KOH solution. It has been observed that NiO/C nanocomposite showed the better catalytic activity with the smallest Tafel slope of 77.8 mV dec−1 than single component\u27s result, NiO particles (112.6 mV dec−1) and carbonization of ESM (94.4 mV dec−1). It indicates that the HER performance of electrocatalyst can be enhanced by synergistic effect between NiO particles and carbonization of ESM, with better durability after 500 CV cycles. Furthermore, such design principle for developing interfaces between TMOs and C by a green and facile method can offer a new approach for preparing more efficient electrocatalysts (Seen in Chapter 5). Differed from other chapters, Chapter 4 focuses on the electroanalytical application of advanced nanomaterials. In this chapter, the sweep wave voltammetry (SWV) method was used for molecule detection. It is noted that we also developed several methods to detect small molecules, including differential pulse voltammetry (DPV) and chronopotentiometry (i-t). Therefore, several novel nanomaterials like gold nanoparticles and ZIF-8, two-dimensional nickel phthalocyanine-based metal-organic framework compounds were synthesized, respectively, and then used for the electroanalytical application, listed as Appendix A and B avoiding breaking the logistic of the whole manuscript