Siyabonga Patrick Mbokazi | University of Johannesburg, South Africa (original) (raw)

Papers by Siyabonga Patrick Mbokazi

Fuel, 2025

The study involved the conversion of Palladium (Pd) and Ruthenium (Ru) metal-supported carbon bla... more The study involved the conversion of Palladium (Pd) and Ruthenium (Ru) metal-supported carbon black (CB) into a hybrid nano electrocatalyst using the microwave synthesis method. Among the four tested catalysts (Pd/CB, Pd1Ru1, Pd1Ru2, and Pd2Ru1), Pd2Ru1 electrocatalyst demonstrated the most effective performance for ethylene glycol electrooxidation (−0.51 V onset potential, 0.13 mA/cm2 current density and −0.21 V anodic peak potential). The physical and chemical properties of the hybrid electrocatalysts were characterized using optical and electrochemical techniques. Additionally, the electrochemical test indicated that Pd2Ru1 exhibited superior poisoning resistance and improved electrochemical stability compared to other tested materials. Also, the catalyst generated close to 0.055 A hydrogen production current while the presence of ethanol enhanced the oxidative current. The performance of the cell at two different temperatures (22 and 56 °C) was also elucidated with up to 0.87 and 0.57 V voltage outputs respectively. The study revealed the potential of Pd2Ru1 as an efficient anodic electrocatalyst in the ethylene glycol direct fuel cell assemblies.

Heliyon, 2024

The study presents for the first time complex spinel NiFe2O4 nanoparticles supported on nitrogen ... more The study presents for the first time complex spinel NiFe2O4 nanoparticles supported on nitrogen and phosphorus co-doped carbon nanosheets (NPCNS) prepared using sol gel and the carbonization of graphitic carbon nitride with lecithin as a highly active and durable electrocatalyst for oxygen reduction reaction. The physicochemical properties of complex spinel NiFe2O4 on NPCNS and subsequent nanomaterials were investigated using techniques such as X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. The electrochemical activity of the electrocatalysts was evaluated using hydrodynamic linear sweep voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy, and chronoamperometry. The electrocatalytic performance of the NiFe2O4/NPCNS nanohybrid electrocatalyst is dominated by the 4e− transfer mechanism, with an onset potential of 0.92 V vs. RHE, which is closer to that of the Pt/C, and a current density of 7.81 mA/cm2 that far exceeds that of the Pt/C. The nanohybrid demonstrated the best stability after 14 400 s, outstanding durability after 521 cycles, and the best ability to oxidize methanol and remove CO from its active sites during CO tolerance studies. This improved catalytic activity can be attributed to small nanoparticle sizes of the unique complex spinel nickel ferrite structure, N-Fe/Ni coordination of nanocomposite, high dispersion, substantial ECSA of 47.03 mF/cm2, and synergy caused by strong metal-support and electronic coupling interactions.

Chemistry of Materials, 2024

Renewable energy systems have gained remarkable attention as potential green energy sources with ... more Renewable energy systems have gained remarkable attention as potential green energy sources with escalating energy demand and environmental issues. Direct alcohol fuel cells are potential energy sources with quick start-up, zero emissions, and high power density. However, current electrocatalysts' poor efficiency and catalytic activity hinder their commercialization. In this study, Pd−Nb metal nanoparticles (MNPs) supported on carbon nano-onions (CNOs) were synthesized using the polyol method for the electro-oxidation of isopropanol and ethanol in an alkaline medium. An inexpensive CNO support was synthesized using the soot-based approach. High-resolution transmission electron microscopy analysis confirmed the successful synthesis of CNOs with a quasi-spherical structure and concentric rings resembling an onion. The Fourier transform infrared spectroscopy analysis confirmed the presence of oxygen moieties on the surface of the CNOs, which were used to anchor the MNPs to the surface of the support. The X-ray photoelectron spectroscopy analysis confirmed the composition of the electrocatalysts and the presence of Pd and Nb in different oxidation states. The synthesized Pd−Nb/CNOs exhibited high catalytic activity and stability for isopropyl alcohol and ethanol electro-oxidation. The addition of Nb to Pd reduced the loading of Pd, thus reducing the cost of the electrocatalyst and improving the physicochemical properties and electrocatalytic activity of Pd toward isopropanol and ethanol electro-oxidation. The increased electrocatalytic activity of Pd−Nb/CNOs is attributed to the increased active sites on the surface of the MNPs and the synergistic effects arising from the CNO support and the Pd−Nb MNPs.

ChemElectroChem, 2024

Direct alcohol fuel cells (DAFCs) face several challenges such as carbon support corrosion, poor ... more Direct alcohol fuel cells (DAFCs) face several challenges such as carbon support corrosion, poor kinetics, and long-term stability, requiring improved electrocatalyst support development. We synthesized 5 %Pd/fMCÀ NiO using a microwave-assisted sodium borohydride-enhanced polyol method. X-ray photoelectron spectroscopy, transmission electron microscope, and X-ray diffractometry probed the material's surface composition, morphology, and structure. ICP-OES is employed to quantify palladium loading. Fourier Transform Infrared Spectroscopy mapped the functional groups. Cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy, and chronoamperometry show that the 5 %Pd/fMCÀ NiO has the lowest activation energy, and with that, the best electroactivity, which is~16 times higher compared to commercial Pd/C; additionally, the catalyst shows anti-poisoning properties, and long-term durability. This is merited to the cooperation and promotional effect of Pd/fMCÀ NiO. The electrocatalysts' electroactivity improved via enhanced electron movement instigated by NiO. This study introduced the parallelism effect concept borrowed from the graphite structure for controlled electron channeling the 5 % Pd/fMCÀ NiO electrocatalyst. The theoretical calculations corroborated the experimental findings that our approach favors anchoring and dispersing Pd NPs uniformly, demonstrating NiO's cooperative and promotional effects. Thus, opening new opportunities for the development of electrocatalysts for high-performance DAFCs.

Energy Technology, 2023

Direct alcohol fuel cells are the next-generation energy sources of the future due to their high ... more Direct alcohol fuel cells are the next-generation energy sources of the future due to their high power density. Palladium electrocatalysts are promising prospects for enhancing alcohol oxidation in alkaline media, but the higher cost and susceptibility to CO poisoning limit their application and commercialization. Thus, there is a need to improve the performance of the Pd electrocatalysts by utilizing a double supporting system. A microwave-assisted polyol method is used to synthesize the palladium nanoparticles supported on iron oxide-carbon black material (Pd/Fe2O3-CB). Physiochemical and electrochemical characterization of the obtained electrocatalysts materials is conducted to study morphology and the electrochemical behaviour of the as-synthesized electrocatalysts. The Pd/Fe2O3-CB displayed higher kinetics approved by a higher current density of 58.7 mA cm−2, stability, and durability, owing to Fe2O3, and CB incorporation. Density functional theory (DFT) proves that C from CO has more robust interactions with surface Pd, thereby explaining the stronger C-O binding property of Pd. The orbital analysis revealed that 3d orbitals of Pd participate in the hybridization with 2p orbitals of C and O. As a result, the overlap between C2p and Pd3d/Fe3d orbitals significantly broadened, leading to solid adsorption of CO over Pd/Fe2O3.

International Journal of Hydrogen Energy, 2023

The instability of carbon support materials has motivated the development of metal oxides support... more The instability of carbon support materials has motivated the development of metal oxides supports which are stable under the fuel cell environment. In this study, tungsten (VI) oxide (WO3) is utilized as a secondary support and cocatalyst for the electrooxidation of methanol and ethanol. Functionalized carbon nanodots employed as primary supports were blended with WO3 nanoparticles to form a composite support onto which Pd nanoparticles were deposited by a borohydride reduction method. The synthesized Pd/fCNDs-WO3 electrocatalysts were characterized by Transmission Electron microscopy (TEM), X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy. XRD results proved that incorporating WO3 into Pd/fCNDs electrocatalyst shifts the Pd diffraction peaks to lower 2Ɵ value due to lattice relaxation. XPS results revealed that W exist in oxidised form and confirmed the strong interaction between the support material and the catalyst. The Pd/fCNDs-WO3 electrocatalysts exhibited a remarkable catalytic activity towards methanol and ethanol oxidation. High current densities of 87.24 mA cm−2 and 44.23 mA cm−2 were obtained for ethanol and methanol oxidation, respectively, using a catalyst with 2.5% Pd loading. EIS, CA and stability tests revealed that the presence of WO3 in Pd/fCNDs electrocatalyst improves the kinetics, tolerance to poisoning and long-term durability in alkaline conditions. This superior performance is attributed to the electronic coupling between Pd and WO3 nanoparticles.

Fuel, 2023

In this work, we report PtRu alloy system supported on Co 3 O 4-activated carbon (Co 3 O 4-C) pre... more In this work, we report PtRu alloy system supported on Co 3 O 4-activated carbon (Co 3 O 4-C) prepared by direct reduction of H 2 PtCl 6 and RuCl 3 solutions as a highly active and durable electrocatalyst for methanol oxidation reaction. The electrochemical activity of the electrocatalysts was evaluated using electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), cyclic voltammetry (CV), and chronoamperometry (CA). The formation of PtRu alloy on the Co 3 O 4-C matrix, as well as their electronic interactions, is confirmed by employing X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and X-ray diffraction (XRD) techniques. The electrocatalysts showed distinct electrocatalytic activity in alkaline medium, depending on whether the catalysts contained PtRu alloy and whether they were supported on the hybrid Co 3 O 4-C, proving their unique contributions to the electrocatalytic reaction. Benefiting from the strong metal-support interaction (SMSI) and electronic interaction between Pt and Ru, the PtRu/Co 3 O 4-C displayed a highly efficient electrocatalytic performance with a mass activity of 6709 mAmg − 1 Pt , a substantial improvement compared to the commercial Pt-based benchmark catalyst, which achieved only 212 mAmg − 1 Pt. Furthermore, the PtRu/Co 3 O 4-C showed excellent stability after 10 000 s, high durability after 500 cycles retaining 94 % of current density, and higher tolerance for CO. This enhanced catalytic performance can be attributed to the small nanoparticle sizes, high dispersion, large ECSA of 102.1 m 2 /g, and the synergy resulting from electronic coupling interactions.

Small Science, 2023

In direct alcohol fuel cells (DAFCs), energy conversion co-occurs at the anode (alcohol oxidation... more In direct alcohol fuel cells (DAFCs), energy conversion co-occurs at the anode (alcohol oxidation reaction [AOR]) and cathode (oxygen reduction reaction [ORR]). The sluggishness of AOR and ORR needs highly electrocatalytically active and stable electrocatalysts that boost electrokinetics, which is central in electrocatalysts’ architectural design and modulation. This design entails enhanced engineering synthesis protocols, heteroatomic doping, metallic doping/alloying, and deliberate introduction of defective motifs within the electrocatalyst matrix. The electrocatalyst activity and behavior depend on the electrocatalysts’ nature, type, composition, and reaction media, acidic or alkaline. Alkaline media permits cheap nonplatinum group metals. This review elucidates the roles and electrocatalytic pathways on different AOR and ORR electrocatalysts and outlines the aspects distinguishing ORR in alkaline and acidic media. It gives up-to-date and ultramodern strategies, protocols, and underlying mechanisms pointing to the efficacy and efficiency of electrocatalysts. The focus centers on heteroatomic, metallic dopants, defects effects correlated to electrocatalytic properties and experimental and theoretical findings. For the advancement in the field, the present study discusses critical parameters for improving the performances of electrocatalysts for DAFCs and breakthroughs on the horizon. Conclusively, knowledge gaps and prospects of these materials for industrial viability and reigning futuristic research directions are presented.

ChemCatChem, 2023

A series of Pd nanoparticles supported on V2O5 immobilized on functionalized carbon, %Pd (1, 3, a... more A series of Pd nanoparticles supported on V2O5 immobilized on functionalized carbon, %Pd (1, 3, and 5) and %V2O5 (10, 20, and 30), were prepared by sodium borohydride-assisted microwave polyol synthesis for glycerol oxidation reaction (GlyOR) in an alkaline medium. Electrocatalysts loading, temperature, V2O5 immobilization, and their synergistic effect on the electrocatalytic performance are systematically studied. The electrocatalysts' morphology and electronic properties were investigated using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, Transmission electron microscopy, and X-ray photoelectron spectroscopy. A significantly improved GlyOR is observed with increased V2O5 content and Pd percentage. The 5%Pd/30%V2O5–fAC showed the highest mass activity of 2157.3 mA.mg-1Pd, a more negative onset potential of 0.62 VRHE, versus the commercial equivalent, and possessed high stability and durability. The increase in electrocatalytic activity is attributed to the effective immobilization of V2O5 on fAC efficient synergism between Pd and V2O5, strong metal support interaction (SMSI), and great exposure of the electroactive sites. The results herein contribute significantly to the understanding of the physicochemical and electrochemical effects of metal oxide immobilization, microwave irradiation, %Pd/%Metal oxide optimization, and SMSI on metal oxide-carbon hybrid electrocatalysts for GlyOR, opening new avenues for fabricating high-performance direct alkaline glycerol fuel cells.

CRC Press eBooks, Jul 6, 2022

ChemElectroChem, 2023

Due to their low cost, accessibility of resources, and improved stability and durability, carbon-... more Due to their low cost, accessibility of resources, and improved stability and durability, carbon-based nanomaterials have attracted significant attention as cathode materials for oxygen reduction reactions. These materials also exhibit intrinsic physical and electrochemical features. However, their potential for use in fuel cells is constrained by low ORR activity and slow kinetics. Carbon nanomaterials can be functionalized and doped with heteroatoms to change their morphologies and generate a large number of oxygen reduction active sites to lessen the problems. Doping the carbon lattice with heteroatoms like N, S, and P and functionalizing the carbon structure with À OCH 3 , À F, À COO À , À O À are two of these modifications that can change specific properties of the carbon nanomaterials like expanding interlayer distance, producing a large number of active sites, and enhancing oxygen reduction activity. When compared to pristine carbon-based nanomaterials, these doped and functionalized carbon nanomaterials, including their composites, exhibit accelerated rate performance, outstanding stability, and higher methanol tolerance. This article summarizes the most recent developments in heteroatom-doped and functionalized carbonbased nanomaterials, covering different synthesis approaches, characterization methods, electrochemical performance, and oxygen reduction reaction mechanisms. As cathode materials for fuel cell technologies, the significance of heteroatom codoping and transition metal heteroatom co-doping is also underlined.

Catalysts, 2022

Carbon nanofibers (CNFs) supported by Pd and Pd-Sn electro-catalysts were prepared by the chemica... more Carbon nanofibers (CNFs) supported by Pd and Pd-Sn electro-catalysts were prepared by the chemical reduction method using ethylene glycol as the reducing agent. Their physicochemical characteristics were studied using high resolution-transmission electron microscopy (HR-TEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and Bruanaer-Emmett-Teller (BET) analysis. FTIR revealed that oxygen, hydroxyl, carboxylic and carbonyl functional groups facilitated the dispersion of Pd and Sn nanoparticles. The doping of Pd with Sn to generate PdSn alloy was also confirmed by XPS data. The amorphous nature of CNFs was confirmed by XRD patterns which exhibited the Pd diffraction peaks. When Sn was added to Pd/CNFs, the diffraction peaks moved to lower angles. HRTEM images revealed that the CNFs with cylindrical shape-like morphology and also Pd-Sn nanoparticles dispersed on carbon support. The cata...