Hybrid palladium-ceria nanorod electrocatalysts applications in oxygen reduction and ethanol oxidation reactions in alkaline media

Fossil fuel alternatives are being increasingly studied, and alkaline direct ethanol fuel cells (ADEFC) have acquired importance, as to ethanol is a renewable fuel. In this context, the aims of the present study were to synthesize, characterize and evaluate electrocatalytic activity in oxygen reduction reaction (ORR) and ethanol oxidation reaction (EOR) using hybrid electrocatalysts based on Pd nanoparticles and CeO₂ nanorods supported on carbon black for application in ADEFC. The highest OCV, maximum current and power densities obtained using Pd₁₅(CeO₂ NR)₁₀(Vn)₇₅ as the cathode and Pd₁₀(CeO₂ NR)₂₀(Vn)₇₀ as the anode were 1270 mV, 190 mA cm^?2 and 65 mW cm^?2, respectively. These interesting results are justified by the highest I_D/I_G ratio and ECSA, which suggest a high number of oxygenated species, defects and vacancies in these electrocatalysts and by the synergistic effect between CeO₂ NR and Pd nanoparticles. Therefore, these hybrid electrocatalysts are promising for ADEFC applications.     

Preparation and electrocatalytic characteristics of the Pt-based anode catalysts for ethanol oxidation in acid and alkaline media

The current study reports the preparation and investigation of several Pt-based anode catalysts loaded on reduced graphene oxide (rGO) as electrocatalysts in both acid and alkaline media for ethanol electrooxidation. The synthesized catalysts are evaluated by the method of XRD, Raman spectroscopy, XPS and TEM. Electrocatalytic properties of these catalysts for ethanol oxidation were investigated by cyclic voltammetry and chronoamperometry. It was found that the as-prepared nanocatalysts doped by metals and oxide metals showed the improvement of catalytic performance compared to Pt-only supported on graphene catalyst. The results indicated that the presence of Al favoured Pt nanoparticles dispersing on the surface of rGO sheets. Indeed, the PAG catalyst exhibits the highest mass activity for the ethanol oxidation of 1194 mA mg^?1_Pt in acid medium and 3691 mA mg^?1_Pt in alkaline medium. In addition, the PAG catalyst also shows good antipoisoning ability for ethanol electrooxidation in both media. This catalyst could be a potential catalyst for direct ethanol fuel cell (DEFC).     

Electrochemical and fuel cell evaluation of PtAu/C electrocatalysts for ethanol electro-oxidation in alkaline media

PtAu/C electrocatalysts in different atomic ratios and supported on Vulcan XC 72 carbon were tested for ethanol electro-oxidation in alkaline media. The electrocatalysts were prepared using borohydride as reducing agent. PtAu/C X-ray diffraction (XRD) patterns showed peaks characteristic of Pt face-centered cubic (fcc) structure and carbon. PtAu/C (70:30) and (50:50) XRD patterns showed a shift to lower values of 2θ when compared to Pt/C, this way suggesting the formation of PtAu alloy. Transmission electron micrographs showed the nanoparticles with particle size between 4 and 6.5 nm for all PtAu/C electrocatalysts. Electrochemical characterization of the PtAu/C materials suggested the PtAu/C (50:50) as the most promising material for ethanol electro-oxidation while experiments in single fuel cell suggested PtAu/C (70:30). The discrepancy in the results obtained can be explained by the electrode construction since PtAu/C (50:50) yields a much thicker electrode than PtAu/C (70:30), due to the Pt load is the same. The best results obtained with PtAu/C electrocatalysts could be explained by the presence of Pt and Au in close contact (alloy) associated to the extend in the platinum lattice parameters since these properties could contribute to the C–C breaking bond.     

Design of electrocatalysts with reduced Pt content supported on mesoporous NiWO4 and NiWO4-graphene nanoplatelets composite for oxygen reduction and hydrogen oxidation in acidic medium

Herein, a new direct synthesis route leading to a mesoporous NiWO₄ with crystalline framework and NiWO₄ - graphene nanoplatelets (GNP) composite is reported. Ni and W assembled into a mesoporous tungstate type of symmetry by co-precipitation synthesis route and its composite with GNP were used as supports for electrocatalysts, with reduced Pt content (8 wt.%), in oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) in acidic medium. A comprehensive assessment of the modifications related to the crystalline and porous structures, morphological aspects as well as the surface chemistry aiming to explain the electrochemical properties was performed. It was found that the presence of GNP during the synthesis process leads, mainly, to the enhanced growth of NiWO₄ nanocrystallites, as well as induces changes in the surface chemistry. The electrochemical results show that the introduction of GNPs into the NiWO₄ composite support leads to a significant improvement in the activity of the Pt electrocatalyst in ORR and HOR compared to both initial NiWO₄ and Pt/NiWO₄ samples, as well as mechanical mixtures of these catalysts with carbon. Mass activity for hydrogen oxidation, determined in a mixed kinetic-diffusion controlled region, obtained on the 8 wt.% Pt/NiWO₄-GNP catalyst was significantly higher compared to the commercial 20 wt.% Pt/C Quintech catalyst. Our comprehensive structural and surface chemistry assessments indicate this composite material as a viable electrocatalyst for PEMFCs using a broader type of fuels.     

Carbon supported MnOx–Co3O4 as cathode catalyst for oxygen reduction reaction in alkaline media

The electroreduction of oxygen of MnO_x–Co₃O₄/C was firstly studied in alkaline media. The MnO_x–Co₃O₄/C showed better electrocatalytic activity towards ORR than MnO_x/C and Co₃O₄/C. Compared to Pt/C, MnO_x–Co₃O₄/C showed better methanol tolerance and durability in alkaline solution. Thus, the MnO_x–Co₃O₄/C catalyst had potential for applications in metal–air batteries and alkaline fuel cells.     

(IrOx – Pt)/Ti bifunctional electrodes for oxygen evolution and reduction

Mixed Ir–Pt electrocatalytic films on Ti metal supports were prepared via a galvanic deposition process. Two types of (Ir – Pt)/Ti electrodes were prepared with different Ir–Pt compositions (Ir/Pt atomic composition ratios of 1.74 and 0.44, based on ICP-MS measurements) and of a similar total metal loading (0.15 and 0.12 mg cm^?2). The simultaneous deposition of both metallic Ir and Pt occurred spontaneously upon immersion of a freshly etched Ti metal substrate into a composite solution of Ir(IV) and Pt(IV) complexes of variable concentration. This was followed by electrochemical anodization to convert Ir to IrO_x. Both electrodes showed homogeneous Ir and Pt dispersion on the Ti surface. The bifunctional electrocatalytic performance of (IrO_x/Ir – Pt)/Ti electrodes has been tested towards the oxygen evolution (OER) and reduction (ORR) reactions in acidic solutions. The thus prepared Ti-supported Ir–Pt film electrodes exhibited satisfactory performance towards both reactions, with mass-specific currents for OER being higher than those at a single component IrOx/Ir/Ti electrode and the ones for ORR being comparable to those at a single component Pt/Ti electrode.     

Microbial synthesis of highly dispersed nano-Pd electrocatalyst for oxygen reduction reaction

Microbial fabrication is eco-friendly for nobel-metal catalysts typically used in proton exchange membrane fuel cell (PEMFC). In our study, nano-Pd electrocatalysts were successfully prepared by using three Shewanellas as precursors through hydrogen reduction (200 °C) and carbonization (800 °C). The analysis revealed that the catalysts showed outstanding ORR electrocatalytic performance via a predominant four-electron oxygen reduction pathway in alkaline medium. The best performance was obtained for Pd/HNC-32, which showed a mass activity at 0.526 A mg^?1, 3.78 times higher than that of commercial Pd/C. Shewanella putrefaciens CN-32 was a more effective Pd-adsorbent. The enhanced performance can be ascribed to the small Pd-particle size and uniform dispersion on microbial support, which results from stronger hydrophilicity of Shewanella putrefaciens CN-32. The content of nitrogen is another key to the performance of Pd/HNC-32. This study developed a promising strategy for screening microbial strains for electrocatalyst fabrication.     

Evaluation of the catalytic activity of Pd-Ag alloys on ethanol oxidation and oxygen reduction reactions in alkaline medium

Pd-Ag alloys containing different amounts of Ag (8, 21 and 34 at.%) were prepared in order to evaluate their catalytic activity towards the ethanol oxidation (EOR) and oxygen reduction (ORR) reactions. A sequential electroless deposition of Ag and Pd on a stainless steel disc, followed by annealing at 650 °C under Ar stream, was used as the alloy electrode deposition process.From half-cell measurements in a 1.0 M NaOH electrolyte at ≅20 °C, it was found that alloying Pd with Ag leads to an increases of the ORR and EOR kinetics, relative to Pd. Among the alloys under study, the 21 at.% Ag content alloy presents the highest catalytic activity for the EOR and the lowest Ag content alloy (8 at.% Ag) shows the highest ORR activity. Moreover, it was found that the selectivity of Pd-Ag alloys towards ORR is sustained when ethanol is present in the electrolyte.     

Facile preparation of biomass-derived bifunctional electrocatalysts for oxygen reduction and evolution reactions

The development of inexpensive and efficient bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is still remained a challenge in wide range of renewable energy technologies. Herein, biomass-derived nitrogen self-doped porous carbon nanosheets (NPCNS) are produced by a facile and green pyrolysis of Euonymus japonicus leaves at controlled temperature and then the nitric acid pickling was carried out to remove the excess metal ingredients. The obtained NPCNS exhibits a hierarchically porous distribution, high BET surface area and uniform nitrogen doping. Electrochemical measurements show that the NPCNS possess a high electrocatalytic activity for both ORR and OER. Among these NPCNS catalysts, the sample carbonized at 900 °C (NPCNS-900) with the highest concentration of pyridinic nitrogen shows the best ORR and OER activity. According to our DFT calculations, the high content of pyridinic nitrogen with the moderate O and OH adsorption energies among the three types of nitrogen should be the critical factor for the efficient catalytic performance of NPCNS-900 toward ORR and OER. This work demonstrates that the facile prepared NPCNS-900 is a potential candidate material with excellent performance in electrocatalytic applications such as fuel cells or metal-air batteries.     

Two step synthesis of TiO2–Co3O4 composite for efficient oxygen evolution reaction

For an active hydrogen gas generation through water dissociation, the sluggish oxygen evolution reaction (OER) kinetics due to large overpotential is a main hindrance. Herein, a simple approach is used to produce composite material based on TiO₂/Co₃O₄ for efficient OER and overpotential is linearly reduced with increasing amount of TiO₂. The scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) investigations reveal the wire like morphology of composite materials, formed by the self-assembly of nanoparticles. The titania nanoparticles were homogenously distributed on the larger Co₃O₄ nanoparticles. The powder x-ray diffraction revealed a tetragonal phase of TiO₂ and the cubic phase of Co₃O₄ in the composite materials. Composite samples with increasing TiO₂ content were obtained (18%, 33%, 41% and 65% wt.). Among the composites, cobalt oxide-titanium oxide with the highest TiO₂ content (CT-20) possesses the lowest overpotential for OER with a Tafel slope of 60 mV dec^?1 and an exchange current density of 2.98 × 10^?3A/cm². The CT-20 is highly durable for 45 h at different current densities of 10, 20 and 30 mA/cm². Electrochemical impedance spectroscopy (EIS) confirmed the fast charge transport for the CT-20 sample, which potentially accelerated the OER kinetics. These results based on a two-step methodology for the synthesis of TiO₂/Co₃O₄ material can be useful and interesting for various energy storage and energy conversion systems.