Laser Synthesis of Pt-based Alloy Electrocatalysts for Oxygen Reduction Reaction

Abstract

Traditional precious metal Pt catalysts have been extensively utilized in various energy conversion and storage applications, notably in PEMFCs and water splitting. However, the intrinsic scarcity of Pt significantly escalates the commercialization cost for these applications. The amalgamation of Pt with other transition metal elements to form Pt-based alloy catalysts has unveiled a novel dimension of electrocatalytic performance, surpassing their pure-metal counterparts. This combination allows for the reduction of Pt content without compromising on catalytic activity, thereby optimizing Pt utilization. Although the conventional wet-chemical synthesis of these catalysts is prevalent, it is frequently fraught with challenges such as the use of ligands and toxic chemicals. These not only pose environmental concerns but often result in need of further purification steps to eliminate the hinder of byproducts on catalytic active site. In light of this, this study delves deep into the potential of laser-assisted fabrication as a revolutionary synthesis method for Pt-based alloy electrocatalysts. Moreover, the study aims to harness laser-assisted method to generate Pt-based electrocatalysts with high ORR activity and durability by tailoring the laser synthesis routes, parameters, alloy compositions, and metal-substrate interactions. This study initially focused on a comparative investigation of laser synthesis of Pt-based binary alloy NPs in both liquid and solid environments, employing the pulsed laser ablation in liquids (PLAL) method and laser scanning techniques respectively. Utilizing an infrared nanosecond pulse laser (1064nm in wavelength and 5 ns in pulse duration) to ablate Pt-Fe solid alloy targets immersed in a liquid environment result in monodispersed PtFe alloy colloidal NPs. Within this process, the impacts of varying laser parameters and liquid atmospheres on the nanoparticle size, size distribution, yield, and structure were meticulously examined. Unfortunately, the PtFe/CB catalyst, made of the 20 wt% PtFe alloy colloidal NPs and 80 wt% carbon black, showed a bad ORR activity due to its poor size distribution. Subsequently, by using an ultraviolet picosecond pulse laser (355nm in wavelength and 10 ps in pulse duration) to scan multi-walled carbon nanotubes (MWCNTs) loaded with metal precursors, we successfully synthesized sub-5 nm PtFe, PtNi, and PtCo alloy NPs by flexibly selecting metal salt solutions. Although this approach effectively addresses the challenge of oversized particle dimensions and uneven size distribution frequently observed in PLAL-produced alloy NPs, the catalytic performance of obtained PtM/CNT catalysts was still not as good as expected in the subsequent electrocatalytic tests. This can be attributed to the severe surface oxidation of the sample during the laser processing and sample storage. Compared with the PtFe/CB catalyst produced by PLAL, the ORR activity of the PtM/CNT catalyst obtained from laser scanning has been greatly improved, with a comparable mass activity to that of the commercial Pt/C catalyst, underscoring the potential of laser synthesis for Pt-based electrocatalysts. Expanding upon the methodologies previously explored, this study also involved with the laser synthesis of Pt-based (PtPdFeCoNi) high-entropy alloy (HEA) catalysts loaded on laser-induced graphene (LIG). Firstly, through a coagulation bath process, we pre-constructed PBI films (a type of graphene precursor) with a porous structure and uniformly distributed the required diverse metal precursors on their surfaces. Then, the laser scanning approach can not only facilitate the formation of evenly distributed HEA nanoparticles (NPs), but also transform the PBI substrate into LIG at the same time. Interestingly, when comparing the products produced using an infrared pulsed laser (1064nm, 5 ns) and an ultraviolet pulsed laser (355nm, 10 ps), we observed that while both lasers can generate NPs and LIG, only the infrared laser could produce uniform HEA NPs without segregation, attributed to its much higher induced temperatures. In the subsequent electrochemical tests, this uniformed HEA/LIG catalyst exhibited excellent ORR activity and stability that significantly surpassed that of commercial Pt/C catalysts.

Date of Award	16 Apr 2024
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Lin Li (Supervisor) & Jiashen Li (Supervisor)