Electrochemical characterisation of Pd modified ceramic  carbon electrodes: partially flooded versus wetted channel hydrophobic gas electrodes

https://doi.org/10.1016/S0022-0728(99)00118-7Get rights and content

Abstract

Inert metal modified composite ceramic  carbon electrodes (CCE) were recently introduced and found potential applications as biosensors and gas electrodes. The electrodes comprise graphite powder dispersed in hydrophobically modified silicate xerogels. The interconnected graphite network provides percolative conductivity. The silicate backbone provides rigidity and porosity. The hydrophobic moieties reject water and thus limit the thickness of the electrochemically active portion of the electrodes. Inert metal dispersion is introduced for catalysis. The characterisation of the geometric configuration of the wetted section of the gas electrodes and the inert metal dispersion in porous electrodes poses an interesting challenge since these cannot be resolved by spectroscopic or microscopic techniques or by gas adsorption isotherms. It is demonstrated that electrochemical techniques provide a means to characterise the morphology of the wetted section of Pd-modified gas electrodes. The surface area of the palladium dispersion in CCEs was characterised by cathodic stripping of adsorbed oxygen and underpotential copper deposition (upd), and the active volume of the palladium in the CCE was estimated by electrochemical formation of β-phase palladium hydride. Finally, the parameters that were obtained by the electrochemical characterisation were used in order to fit the potential–current polarisation curves of oxygen reduction on CCEs of different compositions and preparation protocols.

Introduction

Hydrophobic gas electrodes are comprised of noble metal impregnated graphite particles imbedded in hydrophobic porous substrates. Fluoroplastic polymers such as PVDF (poly(vinyl difluoride)), PTFE (poly(tetrafluorethylene)) or Teflon are widely used matrices. These electrodes, which were first introduced by Niedrach and Alford [1], revolutionised the practice of gas electrodes. The outstanding efficiency of the hydrophobic electrodes stems from the channelled structure of their wetted surface in aqueous electrolytes. Water rejection by the hydrophobic thermoplast creates extended thin layers of an aqueous phase covering patches of the inner conductive surface. The very thin liquid layer between the electrolyte and the gas phase minimises the mass transport barrier. Recently, we have introduced catalyst and biocatalyst- modified ceramic  carbon electrodes comprised of sol–gel derived hydrophobic silicate – graphite composite materials. Sol–gel technology offers specific advantages for the production of gas electrodes [2], [3]. We and others have demonstrated that CCEs can be used for electrochemical processes in which only dissolved reactants are involved [4], [5], [6], [7], [8], [9], [10], in processes where the rate-determining step involves diffusion from the gas phase [10], [11], [12] and for biochemical and other processes that require the participation of both dissolved and gas phase feeds [13], [14], [15], [16], [17], [18], [19], [20]. Each of these diverse applications requires a different structure of the wetted section of the electrode.

Elucidation of the morphology of the wetted section of the CCEs and other partially wetted electrodes and the characterisation of the metal nanodispersion catalyst in porous electrodes pose interesting challenges. Electron microscope techniques cannot probe the configuration of porous materials, and electron spectroscopies are hampered by the presence of the carbon particles. The structure of the solid section of porous supports can be resolved by a variety of nitrogen and other adsorption techniques, but none of these methods can distinguish between wet and dry surfaces. This publication presents a detailed investigation and modelling of palladium-modified CCEs by an electrochemical approach. Electrochemistry has an inherent capability to distinguish between dry and wet surfaces, because only the latter are active electrochemically. Besides gaining better information on the structure of the wetted section of catalyst modified CCEs and the way to alter this structure, the study illustrates an electrochemical characterisation approach for partly wetted electrodes. As far as we know, there was no previous attempt to elucidate the morphology of nanodispersed metal catalyst in porous modified electrodes with the combination of electrochemical tools used here, though each of these techniques was used separately for particle characterisation. A recent review of electrochemical characterisation of inert metal catalyst dispersion in porous electrodes details some of the analytical techniques used in this study [21].

Scheme 1 demonstrates two different structures of the wetted section of CCEs. 1A shows a flooded electrode in which the wetted thickness is constant throughout the cross section of the matrix. So, the diffusion pathlength for moieties penetrating from the back – gas phase – side of the electrode and from the bottom-electrolyte side is symmetrical within the flooded section. In contradistinction, in the channelled electrode (1B), the wetted section penetrates deep into the electrode in segregated, ribbon-like channels. Here, gas phase reactants can access the very close proximity of the catalytic site and then diffuse through a thin liquid barrier to the carbon surface and the dispersed catalyst. The 1A configuration is most useful for catalytic reactions in which liquid phase reagents control the reaction rate, but penetration of gas reactants is also required. Thus for example, biocatalysed glucose conversion is dominated by mass transport of dissolved glucose to catalytic sites (imbedded enzymes and noble metal catalysts) in the wetted section of the electrode, while oxygen supply from the gas phase is still beneficial for eliminating the dependence on the dissolved oxygen concentration. In contrast, for oxygen reduction (in fuel cells or air batteries) the primary reactant is supplied from the gas side (proton transport is considered to be very fast). For such applications, the 1B configuration is ideal, because the thickness of the liquid phase barrier is minimised while the wetted surface area is still kept large. Since CCEs were recently proposed for both applications (e.g. [11], [12], [13] vs. [14], [15]), it is important to control the structure of the active section of the electrode (as much as possible) in order to provide favourable configurations for each application.

The methodology employed here for the elucidation of the dependence of the active microstructure on the composition and preparation protocol of CCEs was based on fitting the polarisation curves of oxygen reduction to reaction–diffusion models that take into account the penetration of the gas reactants and their potential dependent conversion on the noble metal dispersion. In order to fit the experimental results with a minimal number of free variables, we had to elucidate the kinetic and structural parameters of the palladium dispersion. The total surface area of the palladium metal in the wetted section of the electrode was determined by monolayer oxygen adsorption and its coulometric stripping. The surface area of the palladium was determined independently also by copper underpotential deposition. Now, since only part of the palladium in the wetted section of the electrode is connected electrically to the percolating graphite powder, we had to estimate the active palladium content by an independent method. Hydrogen β-phase Pd–hydride formation was used to this end. The kinetic parameters of oxygen reduction (i.e. the Tafel slope and the kinetic coefficient, k0) were estimated by measuring the current–potential curves of palladium wire in the same test solution. Using these tests, we could obtain all the parameters that were needed for modelling the oxygen reduction in a set of flooded electrodes of different palladium loading, except for one: the specific capacitance of the graphite particles per unit electrode volume. This parameter is common to all the palladium-loaded CCEs and thus its estimate can be based on several polarisation curves of different electrodes. For the channelled electrodes, which involve diffusion barriers in two phases, we needed an additional parameter, the mass transport coefficient, which is also indifferent to the palladium loading. Thus, despite the fact that the wetted thickness of the electrodes depends on the palladium loading, only one parameter was adjusted in order to fit all the polarisation curves of flooded electrodes, and only two free parameters were needed to fit all the polarisation curves of the more complex three-phase electrodes. Podlovchenko [21] described a similar approach for modelling palladium metal dispersion on carbon cloth electrodes, but that approach did not employ the diffusion–reaction schemes developed here or the hydrogen β-phase Pd–hydride quantitation for the wetted palladium dispersion.

In the following section we outline the derivation of two simple models that account for reaction-diffusion in flooded (1A) and wetted-channel palladium-modified CCEs. The rest of the paper outlines the procedures used to fit the models for the response of oxygen reduction on Pd-modified CCEs and to evaluate the parameters that are required for minimal-parameter mathematical modelling.

Section snippets

Theory

The diffusion–reaction models outlined below are based on several simplifying assumptions:

  • 1.

    The oxygen reduction rate on the graphite surface is negligibly slow compared to the reaction rate on the palladium surface. This assumption holds for Pd/CCEs with >10−4 (wt%) palladium loading as confirmed by comparison of the polarisation curves of blank CCE and Pd/CCEs containing more than 10−4 (wt%) catalyst.

  • 2.

    External diffusion at the liquid and gas side of the active section is negligible. This

Materials

Analytical grade reagents and triply distilled water (resistivity>20 MΩ cm−1) were used. Methyltrimethoxysilane (MTMOS) was purchased from ABCR (Karlsruhe, Germany). Palladium (II)-chloride was obtained from Reidel-de-Haen (Seelze, Germany). High purity carbon powder (ca. 70–80 μm) was purchased from Bay Carbon (Bay City, MI). Oxygen and nitrogen gases were of 99.9% purity.

Apparatus

An EG&G PARC model 263A galvanostat-potentiostat with PARC M270 electrochemical software was used for voltammetric studies.

Electrochemical characterisation of the palladium dispersion

The palladium dispersion was characterised by the following electrochemical methods:

  • 1.

    voltammetric quantitation of an adsorbed oxygen monolayer on the surface of the palladium dispersion;

  • 2.

    underpotential copper deposition and its coulometric stripping;

  • 3.

    formation of β-phase palladium hydride and coulometric anodic stripping of the hydrogen. These tests are often used for surface area determination and characterisation of palladized palladium electrodes. Since these tests are less frequently used for

Oxygen reduction tests

Steady state polarisation curves for oxygen reduction were used to characterise the electrocatalytic performance and to probe the structure of the wetted section of the composite electrodes. Fig. 5, Fig. 6 present typical polarisation curves obtained at 0.1 mV s−1 in 0.5 M H2SO4, for Pd  CCEs and Pd–C  CCEs, respectively. The back side of the electrodes was exposed to a constant flow of air. Preliminary tests showed that changing the air flow rate or a 5-fold increase of the scan rate had no

Acknowledgements

The authors thank the Ministry of Science for financial support of this research. We gratefully thank J. Gun and A. Modestov for their help.

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