Abstract
Palladium (Pd) and platinum (Pt) nanoparticles are invertible and are the most active electrocatalysts in alcohol oxidation reactions in fuel cells. In this work, due to their exorbitant prices, the Pd and Pt nanoparticles were supported by various carbon nanomaterials, which include activated carbon, mesoporous carbon (MC), and carbon nanofibers (CNFs) hybridized with metal oxides, phosphides, and selenides were prepared. The activated carbons were prepared from orange peel, seaweed (macroalgae-chlorophyta) from the Harare River, and graphite, CNFs from waste chicken fat. The ultimate anchoring of the Pd, Pt, and PtRu nanoparticles was done using a modified sodium borohydride-ethylene glycol internment microwave irradiation synthesis method, a modified polyol method. The structural characterization of all the support materials was conducted using Fourier Transform Infrared (FTIR) Spectroscopy and the Brunauer-Emmett-Teller (BET) Technique. The FTIR results revealed that all the support materials contain moieties that serve as the anchoring sites for the deposition of nanoparticles. The structural characterization of the mono-supported Pd catalysts, hybrid-supported Pd catalysts, and binary (Pt-Ru) electrocatalysts was done using X-ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HR-TEM). The XRD confirmed that all the electrocatalysts are crystalline and exhibit face-centered crystal (fcc) structures of Pd, crystalline face-centered cubic Pt, and Ru, mainly existing in an amorphous phase. The HR-TEM images showed that the modified microwave method can control the nanoparticle's morphology, as we saw majorly spherical electrocatalyst nanoparticles dispersed on the surfaces of the various hybrid support materials. The particle size of the prepared electrocatalysts was determined using HR-TEM images by ImageJ software. From HR-TEM images, the particle size of the mono-supported Pd catalysts for Pd/ fMC-NiO was approximately 7 nm, for Pd/ V2O5-fAC 1→5% Pd at 10% V2O5, 20% V2O5, and 30% V2O5 are approximate ~6 nm, ~7 nm, and ~8 nm. The notable trend shows a general increase of ~1 nm in Pd NPs for every 10% increase in V2O5, while the particle sizes for the PtRu/Co3O4-C electrocatalysts series range between 1.9 and 11 nm. The particle sizes proved that what is important is not the particle size but rather the exposure of the electrocatalytically active site. Of all these electrocatalyst series for the Pd-based, ICP-OES showed that we achieved the desired low loading of 5%, whereas, for the PtRu/Co3O4-C, we achieved ~1% for the Pt and ~3% for the Ru. As different geographical locations can produce different alcohol at low costs, the synthesized electrocatalysts were tested for their ability to electrooxidize different monohydric [methanol (PtRu/Co3O4-C) and ethanol (Pd/Ni2P-MoS2)] and polyhydric alcohol [ethylene glycol (Pd/fMC-NiO) and glycerol (Pd/V2O5-fAC and Pd/Co3Se4–CNFs)]. The electrochemical characterization of all the electrocatalysts was carried out using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry (CA), and Linear sweep voltammetry (LSV). The CV results reveal that Pd/V2O5-fAC, Pd/fMC-NiO, PtRu/Co3O4-C, Pd/Ni2P-MoS2 and Pd/Co3Se4–CNFs electrocatalysts exhibited the
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highest electroactive surface area (ECSA) of 103.59 m2 g-1, 119.77 m2 g-1, 102.1 𝑚2𝑔𝑃𝑡−1, 113.38 m2 g-1 and 98.10 m2 g-1, respectively compared to commercial Pd/C. The calculated ECSA was within the same magnitude. These ECSA values complemented the electrochemical activity examined using cyclic voltammetry (CV). The CV results show that the Pd/V2O5-fAC, Pd/fMC-NiO, PtRu/Co3O4-C, Pd/Ni2P-MoS2 and Pd/Co3Se4–CNFs exhibited better electrocatalytic activity towards the respective alcohols with mass activity current densities of 2163 mA mg-1Pd, 4475.5 mA mg-1Pd, 6709 𝑚𝐴 𝑚𝑔𝑃𝑡−1 157.93 mA mg-1Pd, and 2716.7 mA mg-1Pd respectively, relative to commercial Pd/C. This excellent electrochemical performance can be attributed to the high electroactive surface area, metal oxides promotional effect, Pd, Pt, and PtRu synergy with the hybrid support system, strong metal support interaction, and Pd electronic coupling with the support system, and cooperation and induced effects within the hybrid support system. The electrochemical impedance spectroscopy (EIS) proved that the carbonaceous material increases the conductivity of the electrocatalyst based on the small charge transfer resistance (Rct); thus, upon the addition of the activated carbon, mesoporous carbon (MC), and carbon nanofibers (CNFs) to the respective V2O5, NiO, Co3O4, Ni2PMoS2, and Co3Se4, specifically Pd/V2O5-fAC (57.67 Ω), Pd/fMC-NiO (52.21 Ω), PtRu/Co3O4-C (33.9 Ω), Pd/Ni2P-MoS2 (68.36 Ω), and Pd/Co3Se4–CNFs (12.6 Ω). The reported Rct values prove to support better electrochemical kinetic properties relative to the commercial Pd/C (~170.60, 411.1, 56.1, 125.13, 34.4 Ω) and Pt/C (56.1 Ω) in glycerol, ethylene glycol, methanol, ethanol, ethylene glycol respectively. The realized Rct values can be attributed to the synergistic interaction between the metal nanoparticles and the hybrid support materials, leading to an easy flow of electrons.