Laser Assisted Synthesis of Precious Metal Electrocatalysts

Abstract

Precious metal (e.g. Pt, Ru, Pd) as a promising class of metal electrocatalysts are widely used in various energy conversion and storage scenarios, e.g. oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) etc. However, the conventional synthesis routes through wet chemistry generally led to several issues: using harsh chemicals and stabilizer, which requires post-process purification and the ligand may cover the catalyst's surface and hinder further application; a relatively long processing time; and lack of universality. The laser-assisted fabrication stands out as a promising approach for electrocatalysts synthesis, for its free of ligands coverage, which guarantees nanoparticles (NPs) with bare surface exposure and avoids extensive post-synthesis purification and cleaning. Additionally, the laser-assisted approaches are not materials specific, i.e. the methods are not designed for specific elements/materials, which can be easily extended to a wide variety of nanomaterials fabrication. In this thesis, the laser fabrication technique as a fast, facile and versatile tool for the synthesis of precious metal electrocatalysts was studied. Moreover, the thesis dedicates to explore the potential of laser fabrication methods for effective size reduction of nanomaterials, as the catalysts dimension has a significant impact on the active site exposure and precious metal utilization. In this work, benefiting by the monochromaticity, high brightness and wide range of wavelengths, various laser-assisted synthesis routes are developed to fabricate platinum group metal catalysts. The main contributions include: (i) Laser ablation of a Pt target in liquid and further size refinement using an ultraviolet (UV) Excimer laser. Monodispersed Pt nanoparticles (NPs) with an average size of 4.7 nm were successfully synthesized, and the laser refined Pt NPs possesses enhanced ORR performance. The Pt NPs size tailoring through the exposure of 248 nm laser irradiation was simulated by considering the size-dependent absorption of the incident beam, heat accumulation and dissipation, and phase changes. The mechanism of Pt NPs size refinement through selective surface vaporization was proposed. (ii) A laser-assisted facile, continuous, solution route for simultaneously reducing graphene oxide and metal precursors was developed. Benefit by the semiconductor characteristic of graphene oxide, and high photon energy together with a high laser fluence of the 248 nm Excimer laser, the obtained RuO2 and Pt nanoparticles with average sizes of 2.8 and 2.0 nm, respectively, were rapidly deposited on the reduced graphene oxide (GO) flakes. Further electro-catalytical tests demonstrated the promising performance of RuO2 and Pt functionalized graphene composites in OER, HER, ORR and supercapacitor applications. Based on the experimental findings, the photo-deposition mechanism was deduced to be the photo-induced electron-hole pairs on the GO flakes under UV irradiation. (iii) Graphene-based Pt single-atom catalysts (SACs) fabrication through fast photothermal reduction of both GO and Pt precursor using 1064 nm picosecond laser was demonstrated for the first time. Several key aspects were addressed to successfully fabrication Pt SACs through the solid-phase laser synthesis strategy, including the laser wavelength, pulse overlaps, selection of metal precursors etc. To understand the effect of laser process parameters on the single-atom synthesis, a numerical model was built to study the temporal evolution of heat. In the later electrochemical tests, the graphene-based Pt SACs exhibits comparable HER activity to the commercial catalyst but with ultrahigh precious metal utilization. Owing to the rapid scanning process, the productivity in current lab-scale can reach ~ 2 mg/s (i.e. ~ 7.2 g/h). The non-contact and fast SACs synthesis process is deemed compatible with roll-to-roll fabrication, thereby demonstrating its flex

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Lin Li (Supervisor) & Zhu Liu (Supervisor)