Platinum-free Electrocatalysts

QC 2023-02-06

Abstract

Pt is one of the most commonly used catalysts for green energy applications, such as fuel cells, electrolyzers, and dye-sensitized solar cells. Although enormous effort has been put into improving the catalytic activity and minimizing the usage of Pt in the catalysts, the low abundance and high price of Pt still limit the large-scale commercialization of green energy devices and facilities. Developing cost-effective and highly efficient Pt-free catalysts is urgent and imperative. γ-radiation induced synthesis approach is known for its mild reaction condition, good controllability, and upscaling production capability. In this thesis, Pt-free nanocatalysts of various types: monometallic (Ni, Ag nanoparticles), bimetallic (Ag-Ni core-shell and heterostructures, Pd-Ni nanoframe, Ni-Co alloys), metal oxides (MnO_x, CeO₂), and hybrids (Pd-CeO₂, Ag-ionomer), are designed and fabricated using γ-radiation induced synthesis method. The structural, chemical and electrocatalytic properties of the obtained nanocatalysts are analyzed. 

First, Ni nanoparticles (NPs) were synthesized. It was found that the freestanding Ni NPs are small (~3 nm) and tend to agglomerate to larger clusters (Paper I). Based on the results obtained for the Ni NPs, binary nanocatalysts Ni-Co, Ni-Ag, and Ni-Pd are produced. It has been shown for Ni-Co alloy nanoparticles that, by varying the Ni-to-Co ratio, one can tune their electrocatalytic performance for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) (Paper IV). By introducing the less expensive noble metal Ag to the catalysts, bimetallic Ni-Ag NPs with different structures are fabricated: heterostructure (Ag-Ni) and core-shell (Ag@NiO). A comparative study of these two different nanocatalysts allowed us to separate the contributions of ligand and strain effects on the ORR activity of the Ni-Ag catalysts (Paper II). By mixing up the highly active but expensive metal, Pd, with a less expensive transition metal, Ni, Ni-Pd nanocatalysts with a framework (PdNi-NF) and nanochain (PdNi-NC) morphology are obtained. PdNi-NF nanocatalysts deposited on commercial carbon black exhibit superior ORR activity and durability due to their framework morphology and Pd-enriched surface layer (Paper III). Since poly(vinyl alcohol) (PVA) has often been used as a surfactant to control the size of nanoparticles in γ-radiation induced synthesis, it is found that residual PVA on the catalyst surface may inhibit its activity. Therefore, an anion-exchange ionomer is tried to be used as the surfactant, and it turns out to be a superior, efficient size regulator for the synthesis of Ag NPs. The mechanism of Ag particle size control was studied. The prepared Ag NPs covered by the ionomer are applied as model catalysts to investigate the kinetics of ORR (Paper VIII). 

In addition to metal nanoparticles, metal oxides, including manganese oxide (MnO_x) and Ceria (CeO₂), have also been synthesized and investigated. Two types of MnO_x with different compositions and morphologies were produced using reducing and oxidizing synthetic pathways. Thereafter, the ORR performance of the two types of MnO_x nanostructures is compared (Paper V). In Paper VI, the nucleation and growth mechanism of CeO₂ mesocrystals are investigated and proposed. Considering the results in Paper VI, the Pd-CeO₂ nanocatalysts have been developed. First, the CeO₂ mesocrystals were pretreated with ascorbic acid to create more Ce³⁺ defects on the surface. After that, using pretreated CeO2 as a substrate, the Pd is deposited with various sizes (single atom, cluster, and nanoparticle). A comparative study of different Pd/CeO₂ catalysts reveals a relationship between size, metal-support interactions (MSI), and ORR activity (Paper VII).

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