Development of Metal-Embedded Conductive Polymer Composites for Efficient Catalyst Systems

Abstract

Precious metals are widely used as active materials in catalytic systems, offering outstanding performance across a range of catalytic reactions. However, their scarcity and high cost present significant challenges in developing cost-effective catalytic systems. Consequently, reducing the dependence on precious metals is critical. While non-precious metal-based catalysts have been explored as alternatives, they often suffer from low activity and poor stability. Therefore, it is essential to not only reduce the amount of precious metal used but also maximize its efficiency. One key approach to minimizing precious metal usage is to reduce the size of the metal particles, thereby increasing the surface area available for catalytic reactions. Additionally, the incorporation of appropriate support materials can enable the development of highly active and stable catalysts even with small amounts of precious metals. Among the various potential supports, conductive polymers stand out due to their high electrical conductivity, large surface area, ease of fabrication, and cost-effectiveness. Despite these advantages, the use of conductive polymers as catalyst supports has been largely underexplored. Moreover, their application in aqueous electrolyte environments is limited by their tendency to swell. This study focuses on developing metal-embedded conductive polymer composites as catalysts to enhance catalytic system efficiency. The structural, electrical, and electrochemical properties of these composites were thoroughly analyzed, highlighting the potential of conductive polymer matrices as effective catalyst supports. Chapter 1 provides a comprehensive understanding of catalytic systems by describing the fundamentals of catalysis along with various electrocatalytic reactions. It also explains both precious metal and non-precious metal catalysts, highlighting the importance of support catalysts for stable utilization. finally, it discusses key considerations and existing development strategies for designing support catalysts to achieve high activity and stability in various (electrochemical) catalytic reactions. In Chapter 2, we developed a crystallized poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)–platinum nanoparticles (Pt NPs) composite electrode by electroplating Pt NPs within a crystallized PEDOT:PSS nanofibrillar matrix and demonstrated its electrochemical catalytic function in the hydrogen evolution reaction (HER) and methanol oxidation. We systematically investigated the optical, structural, electrical, and electrochemical properties of crystallized PEDOT:PSS–Pt NP composites. The produced catalyst electrode showed uniform impregnation of Pt NPs in the entire volume of the crystallized PEDOT:PSS films. Thus, a volumetric electrochemical catalyst was formed with high aqueous stability, high reactant permeability, large electrochemically active surface area of 20 m2 gPt −1, and enhanced catalytic activity (overpotential of 54.7 mV at 10 mA cm−2) and mass activity of 2 A mgPt −1 at an overpotential of 50 mV for HER compared to that of the Pt NP electrode without the crystallized PEDOT:PSS. In Chapter 3, highly flexible and uniform PEDOT:PSS-engineered membranes at a reduced thickness have been fabricated on track-etched poly(ethylene terephthalate) (PET) substrates. The membranes were functionalized and embedded with Pt NPs having a higher affinity toward H2 gas. The materials and fabricated membranes were characterized by using high-resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM) techniques for morphological and structural analysis. FTIR and Raman characterizations were performed to study the characteristic bonds. The uniformity and quantification of Pt nanoparticle binding were tested through inductively coupled plasma mass spectrometry (ICP–MS) studies and FESEM with EDS mapping. The gas separation performance was studied using H2, N2, and CO2 gases in pure and mixed (H2/CO2 in 50:50) states. It was observed that the PEDOT:PSS-Pt based membrane showed a 116% increment in H2 permeability and 82% and 107% increments in H2/CO2 and H2/N2 selectivity values with pure gas, while a 121% increment in H2 permeability and 156% increment in H2/CO2 selectivity using mixed gas. The separation performance in pure and mixed gas states with repeated experiments conspicuously highlighted their prospective viability as prime contenders for gas separation applications. In Chapter 4, a high efficient HER catalyst constructed from Pt decorated MXene and polymer matrix is presented. Benefiting from the hydrophilicity and reducibility of MXene, Pt cations are spontaneously reduced into metallic nanoscale Pt anchored on MXene flakes without additional reductants. Pt decorated nanoflakes further hybrid with conductive PEODT:PSS leading to the composite catalyst. PEDOT:PSS functions not only as binder improving catalyst’s stability, but also as pillar alleviating restacking of MXene flakes that enlarge catalytic active sites exposure. Meanwhile, the swellable PEDOT:PSS further guarantees ions accessible path to active sites. The fabricated catalyst shows excellent performance toward hydrogen evolution reaction. A very low overpotential of 31.6 mV is required to drive the HER to reach a current density of 10 mA cm-2. The catalyst also exhibits super high mass activity of 28.2 A mgPt -1, a low Tafel slope of 21.8 mV dec⁻¹, and excellent stability. In the meantime, the catalytic performance increases with the increase of composite film thickness, revealing the volumetric behavior. This work offers a rational pathway to designing and preparing high-performance HER catalysts with low Pt loading and high utilization efficiency. In Chapter 5, a bulk and porous catalyst was fabricated by impregnating platinum into a PEDOT:PSS foam. The macropores observed in the foam structure of PEDOT:PSS facilitate not only the penetration of reactive ions but also the diffusion of gas bubbles, which are the products of the reaction. Additionally, it was confirmed through XPS and ICP-MS that platinum clustering within PEDOT:PSS can occur without a reducing agent, and the PEDOT:PSS foam impregnated with platinum exhibited better mechanical strength compared to the foam without platinum. As a result, platinum was successfully embedded into the porous PEDOT:PSS foam using a simple method, demonstrating its potential as an electrocatalyst in aqueous environments. In summary, this study presents strategies for developing a catalyst with high activity and stability while significantly reducing the loading amount of noble metals by introducing PEDOT:PSS, one of the conductive polymer materials, as a platinum support. The crystallized PEDOT:PSS used as a support exhibits high nanoporosity, network structure, excellent electrical conductivity, and ion permeability. By utilizing these properties, reactants can more easily access the active sites, enabling the formation of a 3-D reaction zone and efficient electron transfer pathways. The results of this study have the potential to create cost-effective alternatives to noble metal-based catalysts and achieve high catalytic efficiency, contributing to the development of sustainable energy technologies.