Ag-MnxOy on Graphene Oxide Derivatives as Oxygen Reduction Reaction Catalyst in Alkaline Direct Ethanol Fuel Cells

Influence of the Electrode Deposition Method of Graphene-Based Catalyst Inks for ADEFC on Performance

The utilization of graphene as a catalyst support has garnered significant attention due to its potential for enhancing fuel cell performance. However, a critical challenge in electrode production still lies in the electrode preparation technologies and the restacking of graphene sheets, which can greatly impact the fuel cell performance alongside with catalyst development. This study aimed to investigate the impact of different electrode deposition methods for N-rGO-based catalyst inks on catalyst layer morphology, with a specific focus on graphene sheet orientation and its influence on the performance of alkaline direct ethanol fuel cells (ADEFCs). The dispersion behavior and ink stability of the catalysts were assessed using ultraviolet-visible light (UV-vis), ζ potential, and dynamic light scattering techniques. The morphology and physical properties of the gas diffusion electrodes (GDEs) were analyzed through Brunauer-Emmett-Teller measurements, contact angle measurements and scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy. The electrochemical behavior was evaluated both ex-situ, utilizing half-cell GDE measurements, and in situ, through single-cell tests. The N-rGO-based membrane electrode assembly, comprising Pt-free catalysts and a biobased membrane, exhibited outstanding performance in ADEFCs, as evidenced by high maximum power density values and long-term durability. The N-rGO-based membrane electrode assembly has demonstrated remarkable potential for high-performance fuel cells, presenting an exciting avenue for further exploration.

Study on Commercially Available Membranes for Alkaline Direct Ethanol Fuel Cells

This study provides a comparison of different commercially available low-cost anion exchange membranes (AEMs), a microporous separator, a cation exchange membrane (CEM), and an anionic-treated CEM for their application in the liquid-feed alkaline direct ethanol fuel cell (ADEFC). Moreover, the effect on performance was evaluated taking two different modes of operation for the ADEFC, with AEM or CEM, into consideration. The membranes were compared with respect to their physical and chemical properties, such as thermal and chemical stability, ion-exchange capacity, ionic conductivity, and ethanol permeability. The influence of these factors on performance and resistance was determined by means of polarization curve and electrochemical impedance spectra (EIS) measurements in the ADEFC. In addition, the influence of two different commercial ionomers on the structure and transport properties of the catalyst layer and on the performance were analyzed with scanning electron microscopy, single cell tests, and EIS. The applicability barriers of the membranes were pointed out, and the ideal combinations of membrane and ionomer for the liquid-feed ADEFC achieved power densities of approximately 80 mW cm^-2 at 80 °C.

Mixed Transition-Metal Oxides on Reduced Graphene Oxide as a Selective Catalyst for Alkaline Oxygen Reduction

The development of highly efficient, stable, and selective non-precious-metal catalysts for the oxygen reduction reaction (ORR) in alkaline fuel cell applications is essential. A novel nanocomposite of zinc- and cerium-modified cobalt-manganese oxide on reduced graphene oxide mixed with Vulcan carbon (ZnCe-CMO/rGO-VC) was prepared. Physicochemical characterization reveals uniform distribution of nanoparticles strongly anchored on the carbon support resulting in a high specific surface area with abundant active sites. Electrochemical analyses demonstrate a high selectivity in the presence of ethanol compared to commercial Pt/C and excellent ORR activity and stability with a limiting current density of -3.07 mA cm^-2, onset and half-wave potentials of 0.91 and 0.83 V vs reversible hydrogen reference electrode (RHE), respectively, a high electron transfer number, and an outstanding stability of 91%. Such a catalyst could be an efficient and cost-effective alternative to modern noble-metal ORR catalysts in alkaline media.

Influence of the electrocatalyst layer thickness on alkaline DEFC performance

Determining the optimum layer thickness, for the anode and cathode, is of utmost importance for minimizing the costs of the alkaline direct ethanol fuel cell (DEFC) without lowering the electrochemical performance. In this study, the influence of layer thickness on the performance of the ethanol oxidation reaction (EOR) and oxygen reduction reaction (ORR) in an alkaline medium and resistance was investigated. The prepared gas diffusion electrodes (GDEs) were fully characterized, with scanning electron microscopy to determine the layer thickness and electrochemically in half-cell configuration. Cyclic voltammetry and polarization curve measurements were used to determine the oxidation and reduction processes of the metals, the electrochemical active surface area, and the activity towards the ORR and EOR. It was demonstrated that realistic reaction conditions can be achieved with simple and fast half-cell GDE measurements. Single cell measurements were conducted to evaluate the influence of factors, such as membrane or ethanol crossover. In addition, electrochemical impedance spectra investigation was performed to identify the effect of layer thickness on resistance. This successfully demonstrated that the optimal layer thicknesses and high maximum power density values (120 mW cm^-2) were achieved with the Pt-free catalysts and membranes used.

Protein-Carbonized Engineering to Construct Three-Dimensional Sponge-like Carbons for Oxygen Reduction Electrocatalysis

The low specific surface area and insufficient exposure of active sites are usually the key reasons for the poor oxygen reduction reaction of catalysts. Here, we update a new method, using NaCl as a template, egg white as a carbon source and nitrogen source, adding FeCl3 as an iron source, and adopting a two-step pyrolysis method to synthesize a sponge-like porous Fe-N-C catalyst. This kind of three-dimensional sponge-like catalyst exhibits more defective structures, so it shows an excellent electrochemical performance with a half-wave potential of 0.73 V and onset potential of 0.88 V. Additionally, the catalyst has amazing stability, which proves that it is a promising candidate for green energy devices. Our research provides an innovative method to synthesize high-performance Fe-N-C catalysts using low-cost common biomaterials.

Carboxymethyl-Cellulose-Containing Ag Nanoparticles as an Electrochemical Working Electrode for Fast Hydroxymethyl-Furfural Sensing in Date Molasses

Novel biosensors based on carboxymethyl cellulose extract from date palm fronds containing Ag nanoparticles as an electrochemical working electrode for fast hydroxymethylfurfural (HMF) sensing in date molasses were prepared. The morphological, structural, and crystallinity characteristics of the prepared Ag@CMC were described via SEM, DLS, TEM, and XRD. In addition, Raman spectroscopy and UV-VIS spectroscopy were performed, and thermal stability was studied. The investigated techniques indicated the successful incorporation of AgNPs into the CMC polymer. The sensing behavior of the prepared AgNPs@CMC electrode was studied in terms of cyclic voltammetry and linear scan voltammetry at different HMF concentrations. The results indicated high performance of the designed AgNPs@CMC, which was confirmed by the linear behavior of the relationship between the cathodic current and HMF content. Besides, real commercial samples were investigated using the novel AgNPs@CMC electrode.

Ce-modified Co-Mn oxide spinel on reduced graphene oxide and carbon black as ethanol tolerant oxygen reduction electrocatalyst in alkaline media

Electrocatalyst development for alkaline direct ethanol fuel cells is of great importance. In this context we have designed and synthesized cerium-modified cobalt manganese oxide (Ce-CMO) spinels on Vulcan XC72R (VC) and on its mixture with reduced graphene oxide (rGO). The influence of Ce modification on the activity and stability of the oxygen reduction reaction (ORR) in absence and presence of ethanol was investigated. The physicochemical characterization of Ce-CMO/VC and Ce-CMO/rGO-VC reveals CeO₂ deposition and Ce doping of the CMO for both samples and a dissimilar morphology with respect to the nature of the carbon material. The electrochemical results display an enhanced ORR performance caused by Ce modification of CMO resulting in highly stable active sites. The Ce-CMO composites outperformed the CMO/VC catalyst with an onset potential of 0.89 V vs. RHE, a limiting current density of approx. -3 mA cm^-2 and a remaining current density of 91% after 3600 s at 0.4 V vs. RHE. In addition, remarkable ethanol tolerance and stability in ethanol containing electrolyte compared to the commercial Pt/C catalyst was evaluated. These outstanding properties highlight Ce-CMO/VC and Ce-CMO/rGO-VC as promising, selective and ethanol tolerant ORR catalysts in alkaline media.

Influence of the ultrasound cavitation intensity on reduced graphene oxide functionalization

Graphene is a valuable and useful nanomaterial due to its exceptionally high tensile strength, electrical conductivity and transparency, as well as the ability to tune its materials properties via functionalization. One of the most important features needed to integrate functionalized graphene into products via scalable processing is the effectiveness of graphene dispersion in aqueous and organic solvents. In this study, we aimed to achieve the functionalization of reduced graphene oxide (rGO) by sonication in a one-step process using polyvinyl alcohol (PVA) as a model molecule to be bound to the rGO surface. We investigated the influence of the sonication energy on the efficacy of rGO functionalization. The correlation between the performance of the high-intensity ultrasonic horn and the synthesis of the PVA functionalized rGO was thoroughly investigated by TGA coupled with MS, and IR, Raman, XPS, Laser diffraction, and SEM analysis. The results show that the most soluble PVA-functionalized rGO is achieved at 50% of the ultrasonic horn amplitude. Analysis of cavitation dynamics revealed that in the near vicinity of the horn it is most aggressive at the highest amplitude (60%). This causes rGO flakes to break into smaller domains, which negatively affects the functionalization process. On the other hand, the maximum of the pressure pulsations far away from the horn is reached at 40% amplitude, as the pressure oscillations are attenuated significantly in the 2-phase flow region at higher amplitudes. These observations corelate well with the measured degree of functionalization, where the optimum functionalized rGO dispersion is reached at 50% horn amplitude, and generally imply that cavitation intensity must be carefully adjusted to achieve optimal rGO functionalization.