Precious metal Pt-based electrocatalysts have been widely used in the methanol catalytic oxidation anodes in direct methanol fuel cells. However, decreasing their cost and improving their efficiency and durability have still been challenging. Herein, ternary PtPdCu nanocatalysts are synthesized through a facile one-step hydrothermal synthesis method. When KI is present with a suitable amount in the synthesis, PtPdCu nanospheres with surface-embedded CuI clusters (CuI/PtPdCu) are fabricated. However, without KI, the prepared PtPdCu catalysts show a distinct hollow structure (h-PtPtCu). CuI/PtPdCu displays the highest specific activity with enhancement 4 times higher than commercial Pt/C for the methanol oxidation reaction in an alkaline medium. The superior activity can be attributed to two aspects: 1) the electronic effect originating from the highly alloyed PtPdCu; 2) the synergetic effect resulting from surface inlaid CuI clusters, which can promote the CO intermediate removal. Furthermore, because of the stable Pt-Pd-rich surface and its special linked hollow structure, the h-PtPtCu catalyst exhibits good durability with only a 3.6% decay in the specific activity.

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).

Platina är en av de mest använda katalysatorerna i gröna energikällor somexempelvis bränsleceller, elektrolysörer och grätzelsolcell. Stora ansträngningar har dock gjorts för att reducera mängden platina som används i dessa energikällor, på grund av dess sällsynthet och höga kostnad. Att utveckla platinafria katalysatorer som även är kostnadseffektiva och kraftfulla äridag mycket aktuellt. Gammastålningsinducerande syntes är känd för dessmilda reaktionsförhållnaden, är lätt att kontrollera samt så finns det utrymme för uppskalning av produktionskapaciteten. I denna avhandling designas och produceras följande typer av platinafria katalysatorer med hjälp av gammastålningsinducerande syntes: Monometalliska (Ni, Ag nanopartiklar), bimetalliska (Ag-Ni core-shell och heterostrukturer, Pd-Ni nanoram, Ni-Co legeringar), metalloxider (Manganocid, Ceriumdioxid) och hybrider (Pd-CeO₂), Ag-jonomer. Sedan analyseras nanokatalysatorernas strukturella, kemiska samt elektrokatalytiska egenskaper.

Först syntetiseras nanopartiklarna. Det upptäcktes att fristående nickel-nanopartiklar är små (ca 3 nm) och brukar samla ihop sig i större kluster (Artikel I). Baserat på resultaten som sågs för nickel-nanopartiklarna, skapas binära nanokatalysatorer Ni-Co, Ni-Ag och Ni-Pd. Det visade sig att Ni-Co legeringens elektrokatalytiska prestationsförmåga för syrereduktionsreaktion, syreutvecklingsreaktion samt väteutvecklingsreaktion kan justeras genom att förhållandet mellan Ni och Co ändras (Artikel IV). Genom att lägga till silver, som är en billigare ädelmetall till katalysatorerna, tillverkas bimetalliska Ni-Ag nanopartiklar med olika strukturer: Heterostrukturen (Ag-Ni) och core-shell (Ag@NiO). En studie som jämförde dessa två nanokatalysatorer möjliggjorde det för oss att urskilja ligand- och deformationseffekt påverkan på Ni-Ag katalysatorerna (Artikel II). Genom att blanda den högreaktiva men den dyra metallen palladium med den billigare övergångsmetallen, Nickel, skapas Ni-Pd nanokatalysatorer med formerna nanoramen (PdNi-NF) och nanokedjan (PdNi-NC). PdNi-NF nanokatalysatorer som deponeras på kommersiellt kol uppvisar större syrereduktionsreaktionsförmåga och hållbarhet på grund av dess nätliknande struktur och Pd-berikade ytlager (Artikel III). Eftersom polyvinylalkohol (PVA) ofta har används som tensid för att begränsa storleken på nanopartiklarna under en gammastålningsinducerande syntes, är det möjligt att överflödigt PVA på katalysatorn kan hämma dess aktivitet. Därför gjordes försök där en anjonbytarjonomer användes som tensid och den visade sig en effektivare storleksreglerare för syntesen av silvernanopartiklar. Mekanismen för reglering av silverpartikelstrolek studerades. De färdiga silvernanopartiklarna som var täckta med jonomer användes som modellkatalysatorer för att granska syrereduktionsreaktionens kinetik (Artikel VIII).

Utöver metallnanopartiklarna, syntetiserades och granskades även metalloxiderna manganoxid (MnO_x) samt ceria (CeO₂). Två typer av manganoxid med olika sammansättning och morfologi tillverkades genom syntesmetoderna reduktion och oxidation. Därefter jämförs prestationsförmågan för syrereduktionsreaktionen för de två olika typerna av manganoxid (Artikel V). I Artikel VI undersöks och föreslås mekanismen för kärnbildning och tillväxt av ceria. Baserat på resultatet i Artikel VI, togs Pd-CeO₂ nanokatalysatorer fram. Först förbehandlades CeO₂ mesokristallerna med askorbinsyra för att skapa mer Ce³⁺ defekter på ytan. Sedan deponeras palladium i olika storlekar (atom, kluster och nanopartikel), genom att förbehandlad ceria används som substrat. En studie som jämför olika Pd/CeO₂ katalysatorer visar på en relation mellan storlek, metall-stöd interaktioner (MSI) och aktivitetsnivån för syrereduktionsreaktion (Artikel VII).

Transition metal-based catalysts show great potential to replace Pt-based material in energy conversion devices thanks to their low cost, reasonable intrinsic activity, thermodynamic stability, and corrosion resistance. The electrochemical performance of such catalysts is sensitive to their composition and structure. Here, it is demonstrated that homogeneous alloy nanoparticles with varying Ni-to-Co ratio and controlled structure can be synthesized from aqueous Ni(Co) acetate solutions using a facile γ-radiolytic reduction method. The obtained samples are found to possess defects that are ordered to form polytypes structures. The concentration of these defects depends on the Ni-to-Co ratio, as supported by the results of ab initio calculations. It is found that structural defects may influence the activity of catalysts toward the oxygen evolution reaction, while this effect is less pronounced with respect to the oxygen reduction reaction. At the same time, the activity of Ni–Co catalysts in the hydrogen evolution reaction is affected by formation of Ni-OH bonds on the surface rather than by the presence of structural defects. This study demonstrates that the composition of Ni-Co nanoparticles is an essential factor affecting their structure, and both composition and structure can be tuned to optimize electrochemical performance with respect to various catalytic reactions.

Ag nanoparticles (Ag NPs) are among the most promising candidates to replace Pt as the catalyst for the oxygen reduction reaction (ORR) in anion exchange membrane fuel cells (AEMFCs). However, synthesizing size-controlled Ag NPs with efficient catalytic performance is still challenging. Herein, uniform Ag NPs are produced through a γ-radiation induced synthesis route in aqueous solutions, using the ionomer PTPipQ100 as both an efficient size regulator in the synthesis and a conductor of hydroxide ions during the ORR process. The origin of the size control is mainly attributed to the affinity of the ionomer to metallic silver. The resulting Ag NPs covered with ionomer layers can be applied as model catalysts for ORR. The nanoparticles that were prepared using 320 ppm ionomer in the reaction solution turned out to be coated with a ∼ 1 nm thick ionomer layer and exhibited superior ORR activity as compared to other Ag NPs of similar size studied here. The improved electrocatalytic performance can be attributed to the optimal ionomer coverage that enables fast oxygen diffusion, as well as interactions at the Ag-ionomer interface which promote the desorption of OH intermediates from the Ag surface. This work demonstrates the advantage of using an ionomer as the capping agent to produce efficient ORR catalysts.

To reach commercial viability for fuel cells, one needs to develop active and robust Pt-free electrocatalysts. Silver has great potential to replace Pt as the catalyst for the oxygen reduction reaction (ORR) in alkaline media due to its low cost and superior stability. However, its catalytic activity needs to be improved. One possible solution is to fabricate bimetallic nanostructures, which demonstrate a bifunctional enhancement in the electrochemical performance. Here, two types of bimetallic silver-nickel nanocatalysts, core-shells (Ag@NiO) and heterostructures (Ag/Ni), are fabricated using γ-radiation induced synthesis. The Ag@NiO nanoparticles consist of an amorphous, NiO layer as a shell and a facetted crystalline Ag particle as a core. Meanwhile, the Ag/Ni heterostructures comprise Ag particles decorated with Ni/Ni(oxy-hydro)-oxide clusters. Both materials demonstrate similar and increased alkaline ORR activity as compared to monometallic catalysts. It was revealed that the enhanced catalytic activity of the core-shells is mainly attributed to the electronic ligand effect. While in the Ag/Ni heterostructures, a lattice mismatch between the Ni-based clusters and Ag implies a significant lattice strain, which, in turn, is responsible for the increased activity of the catalyst. Also, the Ag/Ni samples exhibit good stability under operating conditions due to the existence of stable Ni3+ compounds on the surface.

Precious metal Pt-based electrocatalysts have been widely used in methanol catalytic oxidation of anodes in direct methanol fuel cells (DMFCs). However, it has been still the challenge to decrease their cost and improve their efficiency and stability. In this study, ternary PtPdCu nanocatalysts were synthesized through a facile one-step hydrothermal synthesis method. When KI was present with a suitable amount in the synthesis, PtPdCu nanospheres with surface embedded CuI clusters (CuI/PtPdCu) were obtained. While without using KI, the prepared PtPdCu catalysts show a distinct hollow structure (h-PtPtCu). CuI/PtPdCu displays the highest specific activity with a 4 times enhancement than commercial Pt/C for methanol oxidation reaction (MOR) in an alkaline medium. The superior activity can attribute to the two aspects: i) Electronic effect originated from the highly alloyed PtPdCu. ii) synergetic effect due to the surface inlayed CuI clusters which can promote the CO intermediate removal. Furthermore, owing to the stable Pt-Pd-rich surface and its special linked hollow structure, h-PtPtCu catalyst exhibits the best durability with only a 3.6 % decay in specific activity.

Exploration of efficient water oxidation catalysts (WOCs) is the primary challenge in conversion of renewable energy into fuels. Here we report a molecularly well-defined heterogeneous WOC with Aza-fused, pi-conjugated, microporous polymer (Aza-CMP) coordinated single cobalt sites (Aza-CMP-Co). The single cobalt sites in Aza-CMP-Co exhibited superior activity under alkaline and near-neutral conditions. Moreover, the molecular nature of the isolated catalytic sites makes Aza-CMP-Co a reliable model for studying the heterogeneous water oxidation mechanism. By a combination of experimental and theoretical results, a pH-dependent nucleophilic attack pathway for O-O bond formation was proposed. Under alkaline conditions, the intramolecular hydroxyl nucleophilic attack (IHNA) process with which the adjacent -OH group nucleophilically attacks Co4+=O was identified as the rate-determining step. This process leads to lower activation energy and accelerated kinetics than those of the intermolecular water nucleophilic attack (WNA) pathway. This study provides significant insights into the crucial function of electrolyte pH in water oxidation catalysis and enhancement of water oxidation activity by regulation of the IHNA pathway.

The development of Pt-free nanocatalysts with enhanced electrocatalytic activity and durability is essential for the commercialization of fuel cells. Herein, we report a new class of active and durable PdNi nanocatalysts for the oxygen reduction reaction (ORR) in an alkaline medium. PdNi nanoframework (PdNi-NF), consisting of interconnected ultrathin ridges was synthesized through a galvanic replacement reduction (GRR) strategy using Ni nanoparticles, obtained by radiolytic reduction, as seeds. PdNi nanochain (PdNi-NC), composed of aggregated nanoparticles, was produced by one-pot γ-radiation induced reduction. It was found that the carbon-supported PdNi-NF catalysts exhibit higher ORR activity and enhanced durability compared to PdNi-NC/C (PdNi-NC on carbon), Pd-NP/C (Pd nanoparticle on carbon), and commercial Pt/C catalysts. The superior activity of PdNi-NF/C catalyst may originate from the nanoframework morphology that facilitates the O2 diffusion and the electronic effect induced by the sub-surface PdNi alloy; the improved durability can be ascribed to the stable Pd-enriched surface layer. 

The outer coordination sphere of metalloenzyme often plays an important role in its high catalytic activity, however, this principle is rarely considered in the design of man-made molecular catalysts. Herein, four Ru-bda based molecular water oxidation catalysts with well-defined outer spheres are designed and synthesized. Experimental and theoretical studies showed that the hydrophobic environment around the Ru center could lead to thermodynamic stabilization of the high-valent intermediates and kinetic acceleration of the proton transfer process during catalytic water oxidation. By this outer sphere stabilization, a 6-fold rate increase for water oxidation catalysis has been achieved. 

The role of intermediate phases in CeO2 mesocrystal formation from aqueous CeIII solutions subjected to γ-radiation was studied. Radiolytically formed hydroxyl radicals convert soluble CeIII into less soluble CeIV. Transmission electron microscopy (TEM) and X-ray diffraction studies of samples from different stages of the process allowed the identification of several stages in CeO2 mesocrystal evolution following the oxidation to CeIV: (1) formation of hydrated CeIV hydroxides, serving as intermediates in the liquid-to-solid phase transformation; (2) CeO2 primary particle growth inside the intermediate phase; (3) alignment of the primary particles into “pre-mesocrystals” and subsequently to mesocrystals, guided by confinement of the amorphous intermediate phase and accompanied by the formation of “mineral bridges”. Further alignment of the obtained mesocrystals into supracrystals occurs upon slow drying, making it possible to form complex hierarchical architectures.