Electrolytically fabricated nickel microrods on screen printed graphite electrodes: Electro-catalytic oxidation of alcohols

Abstract

Nickel modified graphite screen printed electrodes are explored towards the sensing of alcohols in alkaline solutions. Electrolytically formed nickel microrods with average lengths and diameters of 12 μM and 2 μM respectively are shown to be readily formed on the surfaces of graphite screen printed electrodes. This is the first example of electrolytically formed nickel nanorods which exhibit electro-catalysis towards the sensing of ethanol over the range 2.6–23 mM and glycol over the range 230–1840 μM with limits of detection of 1.4 mM and 186 μM respectively.

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1. Introduction

The electrochemical oxidation of alcohols is of wide importance in high performance fuel cells which hold promise as renewable and low cost alternative fuels sources.^1,2 A plethora of electro-catalytic materials have been explored with platinum based materials which are the current contenders, with research into other possible materials a highly active area in order to reduce the associated cost.³ One such potential electro-catalyst is nickel oxide which has been explored as a potential catalyst towards alcohols.^1,4–7 While research is heavily focused with a view to develop fuel cell technologies, the analytical utilisation of such a system should not be overlooked and novel low cost, simple and disposable analytical tools are urgently required. For example, this might be utilised as a sensor for the monitoring of operational safety in the form of fuel cell leaks.

Towards these goals, recently nickel nano-particle modified boron-doped diamond electrodes have been explored towards the sensing of ethanol and glycerol⁸ as well as a nickel micro-particle modified boron-doped diamond electrode for methanol sensing.⁹ There is considerable merit in this approach over that of a solid electrode consisting completely of nickel which includes reduced cost, ease of fabrication and a renewable surface and an improvement in the accessible linear ranges towards the target analyte due to improvements in diffusion to effectively a nickel micro/nano array. An alternative to commercially available solid electrodes such as boron-doped diamond and glassy carbon are disposable and cost effective screen printed electrodes.^10–13

In this paper we explore the fabrication of electrolytically fabricated nickel microstructures using graphite screen printed graphite electrodes. It is found that nickel microrods can be readily formed at the surface of a screen printed electrode with average lengths and diameters of 12 μM and 2 μM respectively. While screen printed electrodes have been routinely used as templates for the electro-deposition of different nanoparticles compositions for a range of analytical targets,^14–16 we find no literature reports of producing microrod structures in this way. Additionally we note that nickel hydroxide microrods have seldom been reported with fabrication methods involving solvothermal approaches¹⁷ and vertically aligned nickel oxide microrods on silicon substrates have also been reported via thermal heating.¹⁸ The electrolytically fabricated nickel microrods are explored as a potential electro-catalyst, in particular, towards the electro-catalytic oxidation of alcohols.

2. Experimental section

All chemicals used were of analytical grade and were used as received without any further purification from Sigma-Aldrich. All solutions were prepared with deionised water of resistively not less than 18.2 MΩ cm. All solutions were vigorously degassed with nitrogen to remove oxygen.

Voltammetric measurements were carried out using a μ-Autolab III (Eco Chemie, The Netherlands) potentiostat/galvanostat and controlled by Autolab GPES software version 4.9 for Windows XP. Screen printed graphite macroelectrodes which have a 3 mm diameter working electrode were fabricated as reported previously.¹⁹ These electrodes have been characterised electrochemically in a prior paper and have heterogeneous electron transfer rate constants ∼ 1.7 × 10⁻³ cm s⁻¹.¹² In addition to the screen printed graphite electrodes being used as the working electrode, a large surface area platinum wire as a counter electrode and a Saturated Calomel Electrode (SCE) as the reference electrode were utilised. Connectors for the efficient connection of the screen printed electrochemical sensors were purchased from Kanichi Research Services Ltd.²⁰

Scanning electron microscopy (SEM) images were obtained using a JEOL JSM-5600LV model.

3. Results and discussion

Screen printed graphite macroelectrodes were electrolytically modified with nickel using a previously reported methodology⁸ from a solution containing 1 mM nickel (II) in a 0.1 M sodium acetate solution (pH 5) employing a deposition potential of −1.2 V for 300 s. Fig. 1 depicts SEM images of the modified electrode which indicates that the electrode surface has been modified with unique structures which have average lengths of 12 μm with diameters of 2 μm. It is interesting to note that using identical electrochemical parameters and solutions as reported in the literature⁸ that utilising boron-doped diamond electrodes results in the formation of nanoparticles which indicates that the nucleation dynamics are quite different to that observed at boron-doped diamond electrode surfaces. Given the wide range in the size of the microrods, as observed in Fig. 1, a progressive growth processes arising from continued nucleation of metallic nickel to the active sites of the screen printed surface is highly likely.²¹ A survey of the literature reveals that nickel microrods have been fabricated but never electrochemically as presented here.^17,18

Fig. 1 SEM images of nickel modified screen printed platforms.

We next turn to exploring the nickel microrod modified screen printed graphite macroelectrodes towards the electrochemical sensing of alcohols. Fig. 2A depicts voltammetric profile of the nickel microrod modified electrodes in 1 M sodium hydroxide where an oxidation wave at +0.44 V (vs.SCE) and a reduction wave at +0.35 V (vs.SCE) are observed. EDAX analysis of the nickel microrods exhibited three peak corresponding to nickel, an oxygen peak and of course a carbon peak from the underlying electrode surface. It is well established in the literature that deposited nickel metal spontaneously forms Ni(OH)₂ in alkaline solutions,⁸ likely surface oxide rather than complete bulk transformation and we observe an electrochemical oxidation wave at ∼+0.4 V which is due to the Ni²⁺/Ni³⁺ signal as described by eqn (1). Previous research has shown that the nickel can exist in two crystallographic forms, hydrated α-Ni(OH)₂ and anhydrous β-Ni(OH)₂ which produces two voltammetric oxidation peaks.⁸ However in our case the observation of a single voltammetric wave indicates that the nickel microrods are highly likely to be in the β-Ni(OH)₂ form.⁸Fig. 2A also depicts the voltammetric profiles resulting from the additions of 1.3 mM ethanol using a nickel microrod modified electrode where analysis of the anodic wave, as depicted in Fig. 2B increases with additions of ethanol coupled with the position of the voltammetric peak moving to more anodic potentials with each addition. It is clear at this concentration level a linear response over the range 4 to 10 mM (Fig. 2B) is observed. The observed voltammetric profiles are due to the Ni²⁺/Ni³⁺ redox couple which is the electro-catalytic origin of the voltammetric response undergoing the following process:

Ni(OH)₂ ⇄ NiOOH + H⁺ + e⁻ (1)

with the adsorption of the target alcohol on the electrode surface occurring with the NiOOH and the adsorbed target alcohol undergoing a hydrogen-abstraction reaction producing an intermediate and starting material (Ni(OH)₂) with the subsequent oxidation of the adsorbed intermediate.^4,7,22 Also observed in Fig. 2A is the increase of the cathodic wave in a linear fashion, the analysis of the voltammetric peak as a function of concentration is depicted in Fig. 2C where a linear response is observed and while the exact origin of this is unclear, it is evident that it may be used as a possible indirect sensing methodology. The analytical response of the nickel microrods was further explored towards the sensing of ethanol using chronoamperometry. Ethanol additions were made into a 0.1 M sodium hydroxide solution using the nickel microrod modified screen printed sensor with analysis of the current at +0.4 V (vs.SCE) as a function of concentration revealing a linear response (I/A = 0.08 AM⁻¹ + 2.9 × 10⁻⁶A; R² = 0.983; N = 17), as depicted in Fig. 3, over the range 2.6 mM to 23 mM beyond which the plot plateaus. The limit of detection, based on 3-sigma²³ is found to correspond to 1.4 mM. This linear range and detection limit is identical to state-of-the-art nickel hydroxide nanoparticles modified boron-doped diamond electrodes.⁸ To understand further the electrochemical oxidation mechanism using the nickel microrods, the Langmuir adsorption isotherm is employed.^3,24 Assuming that the catalytic peak current (I_cat) is proportional to the surface concentration of the alcohol under investigation, the electrochemical adsorption equilibrium constant, β, can be determined according to the Langmuir equation:A plot of against [alcohol] allows the adsorption equilibrium constant, β, to be estimated as 356 (±3) M⁻¹. Thus the Gibbs energy changes due to adsorption may be deduced from:

ΔG⁰ = −RTInβ (3)

which was found to correspond to −14.7 kJ mol⁻¹ suggesting that the overall electrochemical oxidation reaction is governed by adsorption-controlled kinetics rather than diffusion processes.

Fig. 2 Cyclic voltammetric profiles from 1.3 mM ethanol additions into a 0.1 M sodium hydroxide obtained using a nickel microrod modified screen printed electrochemical platform. All scans recorded at 50 mVs⁻¹vs.SCE. The analysis of the oxidation peak height (B) and reduction peak height (C) as a function of added ethanol concentrations are also shown.

Fig. 3 Analysis of chronoamperometric response of the nickel microrod modified screen printed graphite electrode resulting from ethanol additions over the range of 0 to 26 mM. The electrode was held at a potential of +0.4 V (vs.SCE).

We turn now to exploring the nickel microrod modified screen printed platforms for the sensing of glycerol. Fig. 4 depicts typical voltammetric profiles obtained from additions of glycerol into a 0.1 M NaOH solution where a linear type response is observed (Fig. 4B). To access the analytical response of the nickel microrods towards the sensing of glycerol, chronoamperometry was employed. As shown in Fig. 4C, analysis of the limiting current as a function of added glycerol concentration exhibits a linear response (I/A = 0.004 AM⁻¹ + 1.3 × 10⁻⁶A; R² = 0.991; N = 8) over the range of 230 μM to 1840 μM with a limit of detection (3-sigma²³) found to correspond to 186 μM. Unfortunately this limit of detection is not as competitive as nickel nanoparticle modified boron-doped diamond electrodes or a nickel macroelectrode.⁸ Given that a very good analytical output is observed for the case of ethanol (see above) this suggests that the nickel microrods likely have a different amount of oxide on the nickel metal which might reduce the rate of reaction in the rate determining hydrogen-abstraction step in the electrochemical oxidation mechanism. While this is speculation, the unexpectedly poor analytical performance might still have analytical utility in niche applications.

Fig. 4 Cyclic voltammetric profiles (A) recorded showing 230 μM glycerol additions into a 1M sodium hydroxide solution using the nickel microrod modified screen printed graphite electrode. All scans recorded at 50 mVs⁻¹vs.SCE. Part B depicts the analysis of the oxidation peak height as a function of glycerol concentrations. Part C is the analysis of chronoamperometric responses using the nickel microrod modified screen printed graphite electrode. The electrode was held at a potential of +0.4 V (vs.SCE).

4. Conclusions

The first electrochemical fabrication of nickel microrods is reported which have been shown to be electroanalytical useful for the sensing of ethanol which perform analytically similar to current state-of-the-art using nickel nanoparticle modified electrodes and to a lesser extent the microrods can be utilised for glycerol sensing. The simple and low cost fabrication process of the nickel microrods suggest their use in the electroanalytical quantification of ethanol and glycol, for example in the monitoring for potential fuel cell leaks. The potential use of the nickel microrods in alcohol fuel cells as a electro-catalyst should also not be overlooked and are also being explored in other sensing applications.

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