Surface design of transition-metal based electrocatalysts for high energy storage devices

Abstract

High energy storage devices (e.g. Li-O2 batteries and hydrogen fuel cell) have much larger theoretical specific energy over the commercial Li-ion batteries. However, majority of the energy losses incurred in these high energy storage devices are induced by the overpotential in electrocatalysis due to the sluggish reaction kinetics. Therefore, there is an urge to enhance the reaction kinetics so as to achieve maximum energy efficiency. The aim for this research is to embellish the reaction kinetics of electrochemical reaction, curtail the activation polarization of the electrochemical reaction and improve the energy efficiency of these high energy storage devices by rational-design of the transition-metal based electrocatalysts (TMBE) via the scope as listed in the following: (1) improve the wetting property of TMBE by deposition of metal oxide (e.g. IrO2 and CuO) onto noble metal (e.g. Pt) for Li-O2 battery; (2) accelerate the proton diffusion rate by fabricating microporous TMBE (e.g. microporous Mo2C) for hydrogen evolution reaction (HER); (3) introduce synergistic effect by co-doping (e.g. nitrogen and sulfur) TMBE (e.g. Mo2C) to manipulate the metal-adsorbate electronic structure for HER. Previously, the fabrication of IrO2 nanoparticles (NPs) deposited on Pt nanowires (NWs) was employed as an electrocatalyst to establish the “wetting” behavior between metal oxides and the Li2O2 for Li-O2 battery applications. With the presence of IrO2, the deposition of Li2O2 thin layer on the IrO2/Pt cathode during the discharging process was being observed for the first time. IrO2 appeared to exhibit a strong affinity with the Li2O2 on the electrode. Without such metal oxide-Li2O2 interfacial interaction for bare Pt, large Li2O2 NPs were formed when Li-O2 battery was being discharged. The formation of Li2O2 thin layer was vital in Li-O2 battery as it facilitated fast electron transport for oxygen evolution reaction (OER), therefore lowered the charging potential of Li-O2 battery. In the later stages of study, we explored the flexibility of this “wetting” system by replacing expensive IrO2 with other low-cost metal oxide (e.g. CuO). Interestingly, such “wetting” phenomenon of CuO-Li2O2 could also be noticed on the CuO/Pt electrode during discharging. Pt is a preeminent electrocatalyst for numerous electrochemical reactions, such as OER and HER due to its low overpotential in driving the electrochemical reactions. However, being a scarce natural resource, Pt is expensive which confines its practical implementation. Therefore, finding an economical and efficient non-noble metal electrocatalysts is advantageous to achieve scalable production. Recently, Mo2C has emerged to be an alternative non-Pt electrocatalyst due to its similar d-band electronic structure with Pt. Based on the literatures, Mo2C material also exhibited excellent electrical conductivity and chemical stability. Inspired by these advantages of the intrinsic properties of Mo2C material, we have fabricated microporous Mo2C versus mesoporous Mo2C for HER. For the first time, we observed that the microporous Mo2C exhibited much lower HER overpotential as compared to mesoporous Mo2C electrocatalyst. We could therefore propose that the microporous Mo2C facilitated a faster proton transport than the latter and thus improved the HER kinetics. Lastly, we modified the electronic structure of the Mo2C electrocatalysts by chemically co-doped with heteroatoms, such as nitrogen (N) and sulfur (S). The N/S co-doped Mo2C exhibits better HER activity as compared to singly doped Mo2C (e.g. S-doped Mo2C and N-doped Mo2C) and pure Mo2C. We therefore proposed that the duel dopants (N/S) in the Mo2C co-activated the Mo center, exhibiting a synergistic effect (σ-donation and π-back donation), which improved the catalyst-adsorbate interaction, destabilized the H-O bond of H3O+ and promoted the formation of H2 molecules. In conclusion, the overpotential of the electrochemical reactions could be lowered by the aforementioned rational-designs of the electrocatalysts. Thus, the energy efficiency of their respective energy storage devices could be improved.