Investigation of Transition Metal-Based Catalysts for Electrochemical Energy Storage and Conversion
Creators: Wu, Shuwen
Publisher: name: University of Alberta Library
Year: 2023

<p>Electrochemical energy conversion and storage, such as electrocatalytic hydrogen generation, metal-air batteries, and fuel cells, are one of the most efficient and reliable systems for renewable energy storage, which address the intermittency of renewable energy systems. However, the energy conversion efficiency is severely restricted by the oxygen redox catalysis, including the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), because of the sluggish kinetics. To date, state-of-the-art oxygen electrocatalysts are noble-metal-based materials (such as Pt-, Ru- and Ir-), but the limited resources and high cost impede their practical applications. Thus, the development of efficient, durable, and low-cost noble-metal-free oxygen electrocatalysts is significantly important to realize the widespread applications and advance those electrochemical technologies. In this thesis, I aim to develop earth-abundant transition-metal-based catalysts with high electrocatalytic activity and durability and reveal the reaction mechanisms that lead to the enhancement effects of the developed catalysts.<br>
For transition-metal-based layered double hydroxides (LDHs) to become more competitive OER catalysts, substantial progress is required to advance the catalytic activity and durability. I developed a robust electrode composed of Ni3S2-embedded NiFe LDH heterostructured nanosheets with a porous structure supported on nickel foam (NF) via a one-pot solution approach at room temperature in 15 min. The as-prepared Ni3S2-NiFe LDH/NF catalyst delivers 50, 500, and 1000 mA cm−2 with an overpotential of only 230, 285 and 303 mV for oxygen evolution reaction, respectively. In situ and ex-situ analysis reveal that the Ni3S2 was in situ partially transformed under an electrooxidation environment into NiOOH over an equally important electrically conductive Ni3S2 to drive proficient catalysis. This strategy can be extended to fabricate Ru-Ni3S2-NiFe LDHs/NF electrocatalyst for high active hydrogen evolution reaction. <br>
The coupled Ni3S2-NiFe LDHs/NF-2 ‖ Ru-Ni3S2-NiFe LDHs/NF electrodes showed a significantly boosted overall water splitting activity with low voltages of 1.47, 1.71 and 1.85 V to deliver high js of 10, 100 and 500 mA cm-2, respectively, far surpassing current commercial requirements (1.8-2.40 V for 200-400 mA cm−2).<br>
Apart from boosting the water electrolysis by improving the anodic OER catalytic activity, replacing OER with a more efficient reaction is compelling because the anodic product O2 is not of high value. I developed the nanowires composed of CoF2/CoP heterostructure grown on nickel foam as a robust bifunctional electrocatalyst for 5-hydroxymethylfurfural oxidation reaction (HMFOR) and hydrogen evolution reaction (HER). The developed CoF2/CoP-2 exhibits excellent HMFOR activity with a working potential of 1.33 V to deliver 100 mA cm-2 and a Tafel slope of 21.1 mV dec-1. Meanwhile, the FDCA yield achieved 98.8 % and the faradic efficiency is 98 %. In addition, CoF2/CoP-2 delivers a current density of 10 mA cm−2 at an overpotential of 59 mV with a Tafel slope of 59.8 mV dec−1 toward HER. Furthermore, bifunctional CoF2/CoP-2 exhibits excellent full-cell electrocatalytic activity when employing CoF2/CoP-2 for cathodic H2 and anodic FDCA production, which only requires the cell voltage of 1.33 V at 10 mA cm−2, superior to the voltage of 1.54 V at 10 mA cm−2 for pure water splitting.<br>
The energy conversion efficiency of metal-air batteries and fuel cells is also severely restricted by the kinetically sluggish oxygen catalysis. Highly efficient and low-cost electrocatalysts for cathodic ORR are highly desirable. I demonstrated an effective strategy of Cu‒N4 sites functionalizing atomic Fe clusters on hierarchical porous carbon nanofibers (Fex/Cu‒N@CNF) for high-performance ORR. Fex/Cu‒N@CNF exhibits superior catalytic performances with an onset potential (Eonset) of 1.03 V, an ultrahigh half-wave potential (E1/2) of 0.944 V, and remarkable durability in alkaline medium, outperforming commercial Pt/C and most recently reported  <br>
transition-metal-based catalysts. The theoretical calculations indicate that the single Cu‒N4 sites assist the activation of O2 to reduce the energy barrier of O2* protonation and facilitate O‒O bond cleavage, boosting the ORR activity. I used Fex/Cu‒N@CNF as a cathode for zinc-battery application, it achieves an impressive specific capacity (1110.4 mA h g‒1 at 100 mA cm‒2) and long-cycling over 400 h, exceeding those Pt/C-based devices.</p>