Developing clean and sustainable energy technology is critical for human society. Abundant electrical energy can now be generated by solar cells. Two approaches are promising in using this clean energy. One is to store it in rechargeable Zn air batteries. The other is to use them to produce hydrogen in electrolyzers as an energy carrrier. The efficiency of these two approaches depends several electrochemical reactions: oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER). In my thesis, I focus on the design of high performance electrocatalysts for these reactions with the aid of novel carbon nanomaterials for rechargeable zinc-air batteries and water electrolyzer.
In the first project, a highly efficient bifunctional O2 electrocatalyst was synthesized by incorporating mixed metal oxide (NiFeOx) nanoparticles into N (8.5 at.%) and P (1.9 at.%) dually doped carbon materials derived from milk powders. In Australia, lots of expired milk powders are generated yearly, which are regularly threw awy as food wastes. This catalyst demonstrates an onset potential of 0.90 V vs. reversible hydrogen electrode (RHE), a kinetic limiting current density of 12.9 mA cm-2 at 0.4 V vs. RHE, and an electron transfer number of 3.86 for ORR, it also demonstrates an onset potential of 1.48 V vs. RHE, an overpotential of 0.32V at 10 mA cm-2, a Tafel slope of 59.03 mV/dec in 1.0 M KOH for OER. Its catalytic performance is comparable to that of the best metal-free carbon-based ORR electrocatalysts and OER electrocatalysts based on hybrids of metal oxides and carbon materials. Rechargeable Zn-air batteries were also fabricated using air electrodes made of this catalyst, demonstrating an open-circuit potential of 1.39 V, a specific capacity of 688 mAhg-1 (corresponding to an energy density of 853 Wh kgZn-1), and excellent rechargeability over 150 cycles with a small performance loss. The milk powder-derived carbon materials can be used to produce eco-friendly and high value-added electrocatalysts for energy conversion and storage applications while reducing food wastage.
In order to improve the stability of electrocatalysts, it is desriable to encapsulate catalytically-active nanoparticles in porous carbon cages. Thus, in the second project, a high-performance bifunctional oxygen electrocatalyst was synthezied by confining cobalt (Co) nanoparticles in N-doped porous carbon cages derived from a metal-organic framework, i.e., ZIF-8. Co precursors were first impregnated into ZIF-8 by a double solvent method. Afterward, carbonization produces highly dispersed Co nanoparticles (with the average diameter of 6.2 nm) confined in N (11.4 at.%) doped porous carbon cages (434.5 m2 g−1). This electrocatalyst exhibits excellent catalytic activity for both ORR and OER with long-term stability. It delivers a half-wave potential of 0.837 V vs. reversible hydrogen electrode, an electron transfer number of 3.9 for ORR, an overpotential of 0.411 V at 10 mA cm−2, and a Tafel slope of 71.2 mV dec-1 for OER. Rechargeable Zn-air batteries assembled using this electrocatalyst demonstrates an open-circuit potential of 1.48 V, a specific capacity of 731.1 mAh g-1, and good rechargeability. The simple and efficient method can confine metal nanoparticles in porous carbon cages, which can be further explored to synthesize novel electrocatalysts for various energy conversion applications.
Recently, transition metal boride (TMB) materials have gained interests as a new class of electrocatalysts. However, their catalytic performance is often poor due to their poor electrical conductivity and limited specific surface area. In the third project, small-diameter multi-walled carbon nanotubes (MWCNTs) were used as substrates for TMB materials because their inner graphitic walls remain largely intact after surface functionalization, providing a conductive network. Meanwhile, the functionalized outermost walls of mildly oxidized MWCNTs can enhance the coupling with actual catalyst particles. Besides, the small-diameter MWCNTs are quite affordable yet providing high surface area close to single-walled carbon nanotubes, making them an attractive substrate candidate. Ultrathin nickel boride (NixB) sheets were anchored on the surface of functionalized small-diameter MWCNTs (f-MWCNTs). The electrochemically active surface area and charge transfer resistance of the resulting hybrid materials (NixB/f-MWCNT) is 3.4 and 0.24 times that of the NixB nanosheets, and NixB/f-MWCNT exhibited superior catalytic activities and stability toward both oxygen evolution and hydrogen evolution reactions. For the overall water splitting, it requires a cell voltage of 1.60 V to reach the current density of 10 mA cm−2, outperforming all existing metal boride catalysts as well as commercial IrO2//Pt/C catalysts. Further, X-ray photoelectron spectroscopy revealed the strong chemical coupling between NixB and f-MWCNTs and the in situ formation of highly active NiOOH/NixB and Ni(OH)2/NixB heterojunctions, which contributes to the superior activity. The developed design concept can serve as a general approach to improve other electrocatalysts with low electrical conductivity and specific surface area, such as metal oxides, metal hydroxides, and metal-organic framework-derived materials.
In conclusion, this thesis shows that carbon nanomaterials can be used to create novel electrocatalysts. These electrocatalysts can facilitate the development of rechargeable zinc-air batteries and water electrolyzers for practical applications in renewable energy storage and conversion.