Thin-film Rotating Disk Electrode Technique Research Articles

Pd-Based Bimetallic Electrocatalysts for Hydrogen Oxidation Reaction in 0.1 M KOH Solution.

A series of carbon black-supported 7.5 wt.% Pd-2.5 wt.% M/C (M: Ag, Ca, Co, Cu, Fe, Ni, Ru, Sn, Zn) electrocatalysts, synthesized via the wet impregnation method, and reduced at 300 °C, were compared in terms of their hydrogen oxidation reaction (HOR) activity in a 0.1 M KOH solution using the thin-film rotating-disk electrode technique. Moreover, 10 wt.% Pd/C and 10 wt.% Pt/C electrocatalysts were prepared in the same manner and used as references. The 7.5 wt.% Pd-2.5 wt.% Ni/C electrocatalyst exhibited the highest HOR activity among the Pd-based electrocatalysts, although it was lower than that of the 10 wt.% Pt/C. Its activity was also found to be higher than that of Pd-Ni electrocatalysts of the same total metal loading (10 wt.%) and reduction temperature (300 °C) but of different Pd to Ni atomic ratio. It was also higher than that of 7.5 wt.% Pd-2.5 wt.% Ni/C electrocatalysts that were reduced at temperatures other than 300 °C. The superior activity of this electrocatalyst was attributed to an optimum value of the hydrogen binding energy of Pd, which was induced by the presence of Ni (electronic effect), as well as to the oxophilic character of Ni, which favors adsorption on the Ni surface of hydroxyl species that readily react with adsorbed hydrogen atoms on neighboring Pd sites in the rate-determining step.

Reduced graphene oxide as efficient carbon support for Pd-based ethanol oxidation catalysts in alkaline media

The sluggish kinetics of the ethanol oxidation reaction (EOR) and the related development of low-cost, highly active and stable anode catalysts still remains the major challenge in alkaline direct ethanol fuel cells (ADEFCs). In this respect, we synthesized a PdNiBi nanocatalyst on reduced graphene oxide (rGO) via a facile synthesis method. The prepared composite catalyst was physicochemically characterized by SEM, STEM, EDX, ICP-OES and XRD to analyze the morphology, particle distribution and size, elemental composition and structure. The electrochemical activity and stability towards EOR in alkaline media were examined using the thin-film rotating disk electrode technique. The results reveal well-dispersed and strongly anchored nanoparticles on the rGO support, providing abundant active sites. The PdNiBi/rGO presents a higher EOR activity and stability compared to a commercial Pd/C ascribed to a high ECSA and synergistic effects between Pd, Ni and Bi and the rGO material. These findings suggest PdNiBi/rGO as a promising anode catalyst in ADEFC applications.

Pd–Zn/C bimetallic electrocatalysts for oxygen reduction reaction

A series of eight 10 wt% Pd–Zn/C electrocatalysts were synthesized via wet impregnation and compared concerning their activity for the oxygen reduction reaction (ORR) in 0.1 M HClO4 and room temperature, using the thin-film rotating disk electrode technique. The electrocatalysts differed in Pd:Zn mass ratio and reduction temperature. Pt/C, Pd/C and Zn/C electrocatalysts of 10 wt% metal loading, also prepared via wet impregnation and reduced at 300 °C, were used as reference. The highest activity among the Pd–Zn/C, Pd/C and Zn/C electrocatalyts was exhibited by Pd–Zn/C reduced at 300 °C and with a Pd:Zn mass ratio equal to 3:1. Its specific activity was higher than that of 10 wt% Pt/C (by ca. 3.5 times at 0.5 V vs. Ag/AgCl), whereas their mass activities were similar. On the contrary, the ORR specific activity of a 29 wt% Pd–Zn/C electrocatalyst reduced at 300 °C and with Pd:Zn mass ratio 3:1 was lower than that of a 29 wt% Pt/C electrocatalyst prepared in the same manner, by ca. 3.7 times at 0.5 V vs. Ag/AgCl, although their mass activities were similar above this potential. Both these catalysts were clearly less active than a commercial (TKK) 29 wt% Pt/C electrocatalyst.

Comparison of the Activity of Pd–M (M: Ag, Co, Cu, Fe, Ni, Zn) Bimetallic Electrocatalysts for Oxygen Reduction Reaction

Carbon-supported 7.5 wt% Pd–2.5 wt% M (M: Ag, Co, Cu, Fe, Ni, Zn) bimetallic catalysts were synthesized via wet impregnation and assessed as electrocatalysts for the oxygen reduction reaction (ORR) in acidic solution at room temperature, using the thin-film rotating disk electrode technique. Monometallic 10 wt% Pt/C and 10 wt% Pd/C catalysts, prepared via the same method, were used as reference materials. The highest activity for ORR among the tested electrocatalysts was exhibited by PdZn/C and the lowest by Pd/C and PdCu/C. The activity of the rest Pd-based electrocatalysts followed the descending order: PdNi/C > PdAg/C ≥ PdCo/C > PdFe/C. The specific activity of PdZn/C was higher than that of Pt/C (more than 3 times higher for potentials 0.35–0.5 V versus Ag/AgCl), whereas their mass activities were similar. PdNi/C and PdAg/C also exhibited higher specific activity than Pt/C for potentials lower than ca. 0.4 V versus Ag/AgCl, but their mass activity was lower. The high ORR activity of PdZn/C, which renders it a promising alternative to Pt-based cathodic electrocatalysts in PEMFCs, was associated with the presence of Pd–Zn alloy in the active phase, as revealed via XRD and TEM.

Organo-Metallic Chemical Deposition of Metal Oxide Supported Electrocatalysts for ORR with High Noble Metal Utilization

Oxygen electrocatalysis is a key enabling technology for a sustainable future. The oxygen reduction reaction (ORR) has received extensive research attention as it is central to the development of several renewable energy technologies such as fuel cells and metal-air batteries [1]. Governed by sluggish kinetics, this reaction requires expensive noble metal based catalysts that require an efficient utilization in regard of the economic competitiveness of the respective technologies. In the case of the polymer electrolyte fuel cell (PEFC), Pt supported on high surface area carbon is at present the state-of-the-art material. However, the carbon support is thermodynamically unstable at high oxidative potentials leading to Pt detachment and severe fuel cell performance losses [2]. This presents a major hurdle to the widespread availability of commercial applications. In order to overcome the problem of carbon support corrosion, chemically robust, conductive metals oxides in their thermodynamically favorable oxidation states have been shown to be a promising alternative [3]. However, conventional wet-chemical methods of Pt nanoparticle deposition on high-surface-area support materials yield much lower specific Pt surface areas when applied to metal oxide supports as compared to the results for carbon supports. The resultant reduction in Pt utilization thwarts the performance of such metal oxide supported Pt catalysts. In this study, we have developed an organo-metallic chemical deposition method to produce Pt nanoparticles supported on several TiO2 and SnO2 based materials with a high Pt utilization for PEFC applications. The resultant Pt particle size distribution and dispersion on the support was determined by transmission electron microscopy. The Pt metal loading was determined by inductively-coupled plasma optical emission spectroscopy. The ORR activity and durability of the metal oxide supported Pt nanoparticles was tested by cyclic voltammetry (CV) in oxygen saturated 0.1 M HClO4 using the thin-film rotating disk electrode technique [4]. Here, a known aliquot of catalyst ink is drop-coated onto a glassy carbon disk as the working electrode. The Pt utilization of the catalyst was evaluated in terms of the electrochemically active surface area (ECSA) by hydrogen underpotential deposition [5]. For the durability studies, CV measurements were performed over 1000 potential cycles between 0.5 and 1.5 V versus a hydrogen reference electrode (RHE) using a scan rate of 50 mV s-1 at room temperature. The performance and stability of the prepared catalysts for the ORR will be discussed. Acknowledgements We thank the South African Department of Science and Technology for financial support in the form of HySA/Catalysis Centre of Competence programme funding and HySA/Catalysis student bursary. The financial assistance of the National Research Foundation (DAAD-NRF) towards this research is hereby acknowledged. Opinions expressed and conclusions arrived at, are those of the authors and are not necessarily to be attributed to the DAAD-NRF. Financial support from the Competence Center for Energy and Mobility (CCEM) Switzerland and Umicore AG & Co. KG is greatly acknowledged.

The Effect of the Voltage Scan Rate on the Determination of the Oxygen Reduction Activity of Pt/C Fuel Cell Catalyst

The oxygen reduction reaction (ORR) is one of the most important chemical reactions. Besides others, it plays a crucial role for the development of a sustainable energy scenario, as it is the decisive reaction in proton exchangemembrane fuel cell (PEMFC) [1, 2]. Improving the catalysis of the ORR can lead to a breakthrough in electrochemical energy conversion; therefore, the fundamental understanding of the reaction processes as well as the rapid assessment of the kinetic activity of different catalyst materials is of utmost important [3]. So far, it is well established that the surface coverage of electrosorbed oxygenated species like H2O, OH, and O determines platinum ORR activity, although the true nature of the potential dependent oxygenated species has yet to be resolved [4, 5]. Under the assumption that the Pt surface is almost fully covered at low overpotentials also referred as kinetic region, where Θad is the surface coverage of any adsorbate species, the ORR kinetic current becomes directly proportional to the pre-exponential factor (1-Θad) of the rate expression [6]. This know-how has helped to interpret effects introduced for instance by the particle size of Pt catalysts [7–9] or the development of catalysts with enhanced activities like Ptalloys with lower Θad compared to pure Pt [5, 6, 10–13]. The advancement in ORR fundamental understanding and performance evaluation has benefited to a great extent from informative half-cell kinetic survey studies of applied catalysts, as for instance performed with the thin-film rotating disk electrode technique (TF-RDE) [2, 14–16]. Such electrochemical model studies enable the determination of true kinetic current densities even of porous materials without complex interference with mass-transport or catalyst utilization effects, which are additional important factors for the final performance in electrochemical reactors [17]. In order to perform these ORR half-cell measurements accurately, certain practical guidelines have been shown to be essential. These include especially the cleanliness of the experimental setup [18], catalyst film preparation on the electrode [14], appropriate potentiostat sampling mode, evaluation of kinetic data [15], IR compensation [19], correction for true reversible hydrogen electrode (RHE) potential, and background currents, later especially at high scan rates [15]. Interestingly, although already reported in literature for polycrystalline and high surface area Pt catalysts [2, 16, 20], the effect of the voltage scan rate on the activity determination is often underestimated or not clearly separated from the effect of impurities. In the present work, the impact of the voltage scan rate on the activity determination of a high surface area Pt catalyst is systematically studied. We take special care in order to work extra clean and avoid the effect of impurities. We confirm that the relatively slow platinum surface oxidation process significantly influences the kinetic evaluation [20, 21], which is one reason behind varying ORR-specific activities in literature. Moreover, we describe the scan rate-dependent influence of chloride ions, as well as discuss the relationship to surface coverage and the implications of the results for practical applications. * Nejc Hodnik n.hodnik@mpie.de

Electrocatalysis of Perovskites: The Influence of Carbon on the Oxygen Evolution Activity

Single and double perovskite oxides have been reported to be amongst the most active electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolyte. Although a detailed study of the bulk electronic structure-activity relationship towards oxygen evolution on these oxides has been reported, the influence of carbon on the behavior of these oxides as OER catalysts is not yet clearly understood. In the present work we study the influence of functionalized acetylene black (ABf) carbon in the electrode composition on the OER activity for perovskite oxides using the thin-film rotating disk electrode technique. It was found that the addition of ABf significantly enhances the OER activity of the single perovskite oxides, namely Ba0.5Sr0.5Co0.8Fe0.2O3-δ, (BSCF) and La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF). The activity of the double perovskite oxide, (Pr0.5Ba0.5)CoO3-δ (PBCO), was relatively unaffected by the addition of ABf in the electrode. Ex situ impedance spectroscopy measurements at room temperature showed that BSCF powder displayed a lower resistivity compared to PBCO with LSCF displaying the highest resistivity of the three materials. These results therefore suggest that the ABf present can more than simply improve the conductive pathway in the perovskite/carbon composite electrodes but can also significantly enhance the electrocatalytic activity of selected perovskites towards the OER.

Bi-Functional Water/Oxygen Electrocatalyst Based on PdO-RuO2 Composites

One of the main challenges toward reversible metal-air batteries and regenerative fuel cells is the bi-functional water/oxygen electrocatalyst component. At present, no single metal oxide can catalyze both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with low overpotential between them. Here, the composite concept is employed to address this problem in conjunction with well-defined quantification of the onset of both reactions utilizing thin-film rotating disk electrode technique. This work is built up on the idea of utilizing a good ORR catalyst (e.g. PdO) and a good OER catalyst (e.g. RuO2) in alkaline solution. Four electron mechanism and two electron mechanism are implied for the ORR catalyzed using PdO containing compounds and RuO2, respectively.

Bi-functional oxygen electrocatalysts based on Palladium oxide-Ruthenium oxide composites

ABSTRACTBi-functional oxygen electrodes are an enabling component for rechargeable metal-air batteries and regenerative fuel cells, both of which are regarded as the next-generation energy devices with zero emission. Nonetheless, at the present, no single metal oxide component can catalyze both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with high performance which leads to large overpotential between ORR and OER. This work strives to address this limitation by studying the bi-functional electrocatalytic activity of the composite of a good ORR catalyst compound (e.g. palladium oxide, PdO) and a good OER catalyst compound (e.g. ruthenium oxide, RuO2) in alkaline solution (0.1M KOH) utilizing a thin-film rotating disk electrode technique. The studied compositions include PdO, RuO2, PdO/RuO2 (25wt.%/75wt.%), PdO/RuO2 (50wt.%/50wt.%) and PdO/RuO2 (75wt.%/25wt.%). The lowest overpotential (e.g. E (2 mA cm−2) - E (-2 mA cm−2)) of 0.82 V is obtained for PdO/RuO2 (25wt.%/75wt.%) (versus Ag|AgCl (3M NaCl) reference electrode).

Bimetallic Pd–Cu Oxygen Reduction Electrocatalysts

A series of Vulcan carbon-supported Pd–Cu catalysts with various molar ratios of Pd to Cu was prepared by co-impregnation followed by a reduction in a hydrogen atmosphere at three different temperatures. The degree of alloying between the two metals, alloy composition, and particle size and size distribution were characterized by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The electrocatalytic activity for the oxygen reduction reaction (ORR) for these various compositions was determined using the thin-film rotating disk electrode technique. Our study reveals that the Pd–Cu bimetallic electrocatalysts, with a suitable degree of alloying, offer a greatly enhanced ORR activity compared to the Pd monometallic electrocatalyst. The best electrocatalytic activities were observed for the bimetallic catalysts that showed alloy nanoparticles with a Pd–Cu molar ratio of approximately 1:1.