Electrochemical impedance studies of methanol oxidation on GC/Ni and GC/NiCu electrode

doi:10.1016/j.ijhydene.2008.10.067

International Journal of Hydrogen Energy

Volume 34, Issue 2, January 2009, Pages 859-869

https://doi.org/10.1016/j.ijhydene.2008.10.067 Get rights and content

Abstract

The electro-oxidation of methanol on nickel and nickel–copper alloy modified glassy carbon electrodes (GC/Ni and GC/NiCu) in a 1 M NaOH solution at different concentrations of methanol was studied by the method of ac-impedance spectroscopy. Two semicircles in the first quadrant of a Nyquist diagram were observed for electro-oxidation of methanol on GC/Ni corresponding to charge transfer resistance and adsorption of intermediates. Electro-oxidation of methanol on GC/NiCu shows negative resistance in impedance plots as signified by semi-circles terminating in the second quadrant. The impedance behavior shows different patterns at different applied anodic potential. The influence of the electrode potential on impedance pattern is studied and a mathematical model was put forward to quantitatively account for the impedance behavior of methanol oxidation. At potentials higher than 0.49 V vs. Ag/AgCl, a pseudoinductive behavior is observed but at higher than 0.58 V, impedance patterns terminate in the second quadrant. The conditions required for this behavior are delineated with the use of the impedance model.

Introduction

Direct methanol fuel cells (DMFC) have recently received extensive attention for both mobile and stationary applications [1], [2]. Methanol as a fuel has numerous advantages such as simplicity of operation and ease of fuel storage and distribution. However, compared to hydrogen based fuel cells, DMFCs still remain to be further developed. One of the problems still unsolved is the slow kinetics of the anodic methanol oxidation [3]. Considerable efforts have been directed towards the study of methanol electro-oxidation at high pH. The use of alkaline solutions in a fuel cell has many advantages such as increasing its efficiency [4], [5], wider selection of possible electrode materials, higher efficiency of both anodic and cathodic processes, almost no sensitivity to the surface structure [6] and negligible poisoning effects [7], [8].

Various materials involving nickel have been used as catalysts in fuel cells. Ni has commonly been used as an electro-catalyst for both anodic and cathodic reactions in organic synthesis and water electrolysis [9], [10], [11], [12]. One of the very important uses of nickel as a catalyst is for the oxidation of alcohols. Several studies on the electro-oxidation of alcohols on Ni have been reported [13], [14], [15].

The study of alloy electrodes is motivated primarily from the anticipation of a synergistic electrocatalytic benefit from the combined properties of the component metals of alloys. Furthermore, the use of pre-anodized alloy electrodes offers the advantages of ease of preparation and long-term stability in comparison with thermally-prepared and electrolytically-deposited mixed-oxide film electrodes. Kuwana and co-workers investigated the alloy electrodes in their studies of carbohydrate reaction at Ni-based alloys containing high percentages of Cu and Cr [16], [17]. One promising characteristic of such alloy electrodes is the resistance to corrosion in alkaline media.

The fact that pure Ni and Cu metals have the same face-centered cubic structure with similar lattice parameters (a = 3.523 for Ni and 3.616 for Cu) makes it possible to have a wide range of composition for Ni–Cu alloys. Numerous papers have described chemical and physical properties of Ni–Cu alloys. An excellent review was presented by Khulbe et al. on the behavior of Ni–Cu alloys in a variety of catalytic processes including hydrogenation reactions, ortho–para hydrogen conversion and H₂/D₂ exchange reaction [18].

Electrochemical impedance spectroscopy (EIS) is a good tool for the study of the kinetics of electrode reactions. The advantage of EIS over DC techniques is that this steady-state technique is capable of probing relaxation phenomena over a wide frequency range. The measured impedance can be presented in the form of imaginary vs real parts at various measurement frequencies, Nyquist plots, which appear as a multitude of semi-circles and lines [19], [20], [21], [22]. Equivalent electrical circuits corresponding to potential stimuli to generate the same impedance plots are used for the interpretations that associate kinetics and transport properties with the circuit elements. Often, discrepancies and ambiguities hamper the analysis [20]. Many equivalent circuits can show the same impedance characteristics and supplementary data as well as chemical intuition help to select the most relevant one. Also, it may be difficult to find the electrochemical equivalent for some electrical circuit elements and vice versa [21], [22], [23], [24]. Inductive loops are often difficult to account for and are related to desorptive generation of sites for the charge transfer processes of electro-active constituents [23], [24], [25], [26].

The purpose of this work is the analysis of impedance characteristics of electro-oxidation of methanol on Ni and NiCu modified glassy carbon in NaOH solution in different methanol concentrations and potentials, aiming at the elucidation of the reaction mechanism. The analysis of the theoretical impedance function provides important information on the kinetic parameters. This information allows EIS spectrum simulation and therefore predicts the system behavior with regard to the variation of the methanol concentration and overpotential.

Section snippets

Materials and methods

Sodium hydroxide, nickel sulfate, sodium citrate and methanol used in this work were from Merck (Darmstadt, Germany), products of analytical grade and were used without further purification. Doubly distilled water was used throughout. Electrochemical studies were carried out in a conventional three electrode cell powered by an electrochemical system comprising of an EG&G model 273 potentiostat/galvanostat and Solartron model 1255 frequency response analyzer. The system is run by a PC through

Results and discussion

Fig. 1 presents consecutive cyclic voltammograms (CV) of a nickel electrode in 1 M NaOH solution recorded at a potential sweep rate of 100 mV s⁻¹. In the first sweep a pair of redox peaks appear at 500 and 405 mV vs. Ag/AgCl that are assigned to the Ni²⁺/ Ni³⁺ redox couple according to: ${Ni(OH)}_{2} + {OH}^{-} \Leftrightarrow NiOOH + H_{2} o + e^{-}$ In the subsequent cycles both the anodic and cathodic peaks shift negatively and stabilize pointing to higher energies (potential) required for nucleation of NiOOH in the first cycle. The

Conclusion

The nickel oxide film was formed electrochemically on electrodeposited nickel and nickel–copper alloy in a regime of cyclic voltammetry on a glassy carbon electrode and tested for electro-oxidation of methanol in alkaline media. Electrochemical impedance studies of methanol oxidation on GC/Ni and GC/NiCu electrodes demonstrate the potentialities of this method as a tool for investigating the mechanism of methanol oxidation. Different impedance patterns are observed for Ni and NiCu alloy

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Citation Excerpt :
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Electrochemical impedance studies of methanol oxidation on GC/Ni and GC/NiCu electrode

Abstract

Introduction

Section snippets

Materials and methods

Results and discussion

Conclusion

J. Power Sources

J. Electroanal. Chem.

J. Electroanal. Chem.

Electrochim. Acta

J. Electroanal. Chem.

J. Electroanal. Chem

J. Electroanal. Chem.

Anal. Chim. Acta

Solid State Ion

Electrochim. Acta

Electrochim. Acta

J. Power Sources

J. Power Sources

Electrochim. Acta

J. Electroanal. Chem.

Corros. Sci.

J. Alloys Compd

Electrochim. Acta

Electrochim. Acta

J. Electroanal. Chem.

J. Electroanal. Chem.

Electrochim. Acta

Electrochim. Acta