A facile approach for the synthesis of copper(II) myristate strips and their electrochemical activity towards the oxygen reduction reaction

Abstract

The usage of an inexpensive and non-noble metal-based material as the electrocatalyst in the oxygen reduction reaction has gained great attention for fuel cell application. In this report, a facile approach has been developed for the synthesis of copper myristate strips. The as-synthesized material has demonstrated its electrocatalytic activity towards oxygen reduction reaction. This study opens up a new scope for the use of straight chain copper complexes as electrocatalysts. The interesting performance of the copper myristate strips holds promising application in energy conversion devices.

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Introduction

Study of the growth and application of low dimensional materials has been motivating researchers as it is linked with the solution of challenging problems concerning energy and the environment. Previous studies have explored numerous materials as pollutant removers,¹ energy generators,² sensors,³ nano-bio conjugates⁴ etc. The facile synthesis of low dimensional materials with tunable size and morphology is one of the important subjects of research as cost and time effective material production is the very first step of device making. Like nanorods,⁵ nanotubes,⁶ nano-columns⁷ nanosprings⁸ etc., nano-strips have also been explored for their interesting properties in sensing^9–11 and photodegradation of organic dyes,¹² as well as their transport mechanism,¹³ structural stability¹⁴ etc. It is always preferable to grow nanostructures by soft chemical approaches and fabrication of nano-strips is no exception. Fabrications of nanostrips of different functional materials are reported following calcinations of carbonate precursors,¹² microwave technique,¹⁵ co-precipitation method etc. Herein, we report the synthesis of self-assembled strips of copper myristate (CM-Ss) through a one step solution-immersion process at room temperature. Micro-flowers of copper myristate (flower size ≈ 50 μm) have been explored for their application in anti-corrosion coating.^16,17 In this work, we aim to study the utilization of as-grown copper myristate strips (CM-Ss) as a cathode material towards electro-reduction activity of oxygen. The dissociation of molecular oxygen (O₂) into water (H₂O) with the aid of electron and proton transfer reaction is in general referred as the electro-reduction of oxygen. It has become a subject of huge interest in recent times due to its widespread applicability in energy converting systems like fuel cells^18–20 and air batteries.²¹ However, due to the high dissociation energy of O₂ (494 kJ mol⁻¹), electro-reduction of oxygen follows a slow kinetics, which makes it difficult for practical applicability. Usage of electro-catalysts can lower the activation energy and reduce the overpotential of the reduction reaction.²² Pt-based catalysts^23–25 are known to be the most efficient and preferred ones, but there are certain limitations associated with this which compels the researchers to find for new electro-active catalysts. Using Pt as catalysts, the onset of electro-reduction of oxygen occurs at 1.00 V, which is much lower as compared to the reversible potential for O₂ reduction (1.23 V) w.r.t. RHE. In addition to that, high cost of Pt also limits its commercialization. Hence effort has been made for the development of non-Pt catalysts, which are comparatively cheap and can be effective in oxygen electro-reduction. Other than Pt, copper-based macrocyclic systems are found to be active catalysts in electro-reduction of oxygen.^26–29 One of the best onset potential of 0.68 V was reported for a substituted phenanthroline Cu complex by McCrory et al.²⁹ Other than this, Cu(II) complexes with aromatic N-donor ligands have been reported to exhibit onset potentials of 0.1 V vs. RHE.³⁰ Highest catalytic activity is shown by lacasse, which is a Cu containing oxidase enzyme consisting of a Cu trinuclear active site³¹ but its huge size hinders its practical usage. At the same time, it is needless to mention that all these systems are having complicated molecular frameworks and are synthesized following critical procedures.^32,33 In general, straight chain Cu–alkyl complexes are reported to be poor active towards oxygen electro-reduction. This scenario intrigued us to explore the electrocatalytic activity of copper myristate with a rather simple molecular framework towards oxygen reduction. The probable structure of copper myristate is shown in Fig. 1.

Fig. 1 Representative copper myristate structure.

Till date, to the best of our knowledge, the synthesis of CM-Ss and study of their electrocatalytic activity towards oxygen reduction reaction has not been exploited. We hope that the detailed study of physico-chemical properties of CM-Ss and their electrocatalytic activity will attract researchers to investigate the usefulness of this material in other practical fields of researches.

Experimental methods

Growth of copper myristate-strips (CM-Ss) using solution-immersion technique

Commercially available Cu powder (Johnson Matthey, 99.9%) was used as a precursor material and 0.05 g of copper powder was mixed with 50 mL of 0.01 M ethanolic solution of myristic acid (Sigma, 99%). After mixing, the brown coloured suspension was stirred continuously for 72 h in a sealed container at room temperature. The brown suspension was transformed to deep blue and finally bluish green after 72 hours of time. The product was thereafter centrifuged and thoroughly washed with ethanol for several times to separate out as-synthesized material from copper powder. The powder was then dried at ambient temperature to remove excess ethanol. The process sequence is displayed schematically in Fig. 2.

Fig. 2 Schematic representations of the reaction sequence leading to the synthesis of strips of copper myristate starting from copper powder and myristic acid.

Characterizations

The powder X-ray diffraction (XRD) data was recorded using a PANalytical X'pert Pro diffractometer. The surface area of the samples was measured using a Micromeritics ASAP 2020 surface analyzer. About 100 mg of sample was degassed at 10⁻⁶ torr and 70 °C prior to N₂ adsorption. The Barret–Joyner–Halenda (BJH) method was used for determination of pore size distribution. The diffuse reflectance UV-visible absorbance spectrum was recorded on a Shimadzu UV-2450, Japan spectrophotometer. The morphology of the as-prepared samples was investigated using field emission SEM (FESEM) instrument (Zeiss Ultra 55 Plus) and a transmission electron microscope (TEM) instrument (Carl Zeiss LIBRA 120). FT-IR spectra were recorded using HATR-FTIR (Perkin-Elmer). The chemical composition of as-grown samples was examined by X-ray photoelectron spectroscopy (XPS) using Mg-Kα X-ray source (Prevac model no. 10001, Poland) where binding energy was scaled with respect to carbon peak. The XPS measurement was performed on the as grown samples i.e. no ion cleaning was done prior to final data collection.

Electrochemical study

The electrochemical measurements were executed in a two-compartment three-electrode system.²⁰ The CM-Ss modified glassy carbon electrode (GCE) was used as working electrode. A bare platinum wire and Ag/AgCl (3 M KCl) was used as auxiliary and reference electrode, respectively. Prior to the electrochemical measurement, the GCE were well polished with alumina slurry of 1, 0.3 and 0.05 μm size sequentially. After that, the as-polished electrodes were washed properly with ultrasonication in double distilled water and dried with argon gas to get a mirror-finished surface. The as-prepared ink of CM-Ss/5% Nafion/ethanol was drop casted onto the well-polished GCE and vacuum dried prior to the experiment. The rotating disc electrode (RDE) was polished, washed and modified with the sample as in the case of GCE. Then, the linear sweep voltammograms were recorded at various rotation rate of the electrode with a sweep rate of 10 mV s⁻¹. All the electrochemical data were recorded using a computer controlled CHI 660C electrochemical workstation (CHI, Austin, USA). The control experiment and oxygen reduction data were collected in the argon and oxygen saturated 0.1 M KOH electrolyte, respectively. All the electrochemical data recorded were converted to the reversible hydrogen electrode (RHE) scale and presented here.

Results and discussion

A bright green coloured suspension was formed after 72 h of reaction time from a dark maroon suspension of Cu powder via a bluish-green suspension after ≈40 h (Fig. 2). After thorough washing with ethanol, the morphology of the as-prepared dry powder was studied with FESEM measurement. Interestingly, it shows the formation of strip-like structure (Fig. 3(a)) with strip-length of 7 ± 2 μm and width 300 ± 50 nm. Surprisingly, the structures are entirely different than the mother precursor (Cu powder) which is polycrystalline and contains the spheroidal grains of size ≈10 μm (Fig. S1 in ESI†). Fig. 3(b) represents TEM images of individual strips, which indicate that the average aspect ratio of the strips ≈40. The low angle XRD (Fig. 3(c)) confirms the presence of series of diffraction lines in the ratio 1 : 1/2 : 1/3⋯1/n, that identify the presence of layered structure in the as-grown material.³⁴ Fig. 3(d) represents the typical nitrogen adsorption–desorption isotherm obtained from CM-NSs. The mesoporous structure of the sample was confirmed from the nature of hysteresis in the isotherm (type IV, IUPAC) at a high relative pressure (P/P₀).³⁵ It can be seen that there is an inflection showing at P/P₀ ≈ 0.9. This may be attributed to slit-type porosity,³⁶ that may be arising from the gaps between adjacent strips. The BET surface area of the strip sample is 14.4 m² g⁻¹. The average pore size or gap width between the strips as calculated from the BJH desorption branch, was found to be approximately 50 nm. The gap width distribution is shown in the Fig. S2 in ESI.† Fourier transform infrared (FTIR) spectroscopy is done to determine the structure of the as-prepared CM-NSs. The FTIR spectrum (Fig. 3(e)) shows major bands at 1727 cm⁻¹ that corresponds to aldehyde C=O stretching, band at 1448 cm⁻¹ corresponds to C–C stretching. The bands due to C–O stretching of carboxylic acid or ester and C–H bending vibrations are obtained at 1310 cm⁻¹ and 711 cm⁻¹. A broad signal in the range 2000–3000 cm⁻¹ can be assigned to the methylene and methyl groups. A strong signal is seen at 1587 cm⁻¹ that can be assigned to the coordinated COO band, which indicates the formation of CM-NSs. To further confirm the formation of copper myristate, FTIR of free myristic acid is recorded and compared with that of copper myristate (zoomed in part of Fig. 3(e)). In the as-grown CM-Ss, the band of free COO moiety (1702 cm⁻¹) is absent and the band of co-ordinated COO moiety (1587 cm⁻¹) is clearly visible. This observation supports the formation of copper myristate starting from copper and myristic acid precursor. The as-grown powder appears to be bright green that intrigued us to check the optical absorption of the material. The diffuse reflectance UV-visible spectrum of the strips is shown in Fig. S3 in ESI.† The spectrum is dominated by the characteristic fundamental absorption in the visible region. The broad absorption peak in the range of 500 nm to 1100 nm arises probably due to d-electron transitions in the Cu(II) complex.³⁶ The other two prominent absorption peaks in the UV region (275 nm and 395 nm) may arise from n–π* and π–π* electronic transitions in myristate residue. The strong visible light absorption of the as-grown material may find its application in optics and photocatalysis.

Fig. 3 (a) Scanning electron micrographs, (b) transmission electron micrograph (c) X-ray diffraction patterns (d) nitrogen adsorption–desorption isotherm, (e) FTIR spectrum of copper myristate strips. (f) Zoomed in FTIR spectra shows the shifting of free COO band from 1702 cm⁻¹ (present in free myristic acid) to coordinated COO band at 1586 cm⁻¹ (present in copper myristate).

XPS analysis is performed to study the elemental composition and the electronic states of the as-prepared sample. Fig. 4(a) represents the X-ray photoelectron spectrum of copper (2p) which show two major peaks at around 933.3 and 953.1 eV that can be assigned to the binding energies of 2p_1/2 and 2p_3/2 states of Cu²⁺.^35,37 The photo-electron spectra of carbon (1s) is deconvoluted into three major peaks at 284.6 eV, 285.9 eV and 288.3 eV that can be assigned to the contribution of –CH₂–, –C–O and –C=O respectively (Fig. 4(b)).^35,38 Fig. 4(c) represents the photo-electron peak corresponding to oxygen (1s) state. The peak at binding energy of 531.2 eV is originating due to organic C–O bond and peak at 533 eV from organic C=O bond.³⁸ The structural (XRD, FTIR), morphological (SEM and TEM) and compositional analyzes (XPS) of as-grown samples altogether indicate the formation of CM-NSs with layered structure.

Fig. 4 (a) Cu 2p, (b) C 1s and (c) O 1s spectra of as-synthesized copper myristate strips.

Glassy carbon electrodes coated with CM-Ss are tested for oxygen reduction reaction for fuel cell applications (Fig. 5). In the first step, the electrocatalytic activity of pristine glassy carbon electrode is examined in inert argon-saturated electrolyte as well as oxygen saturated electrolyte (0.1 M KOH) using sweep rate of 100 mV s⁻¹. No characteristic reduction peak is observed in case of pristine glassy carbon electrode, which is in accordance with the earlier observations of Gorlin et al.³⁸ Next, under the same experimental conditions, the electrocatalytic activity of strip modified GCE is explored. In Ar saturated electrolyte, no characteristic reduction peak is observed but interestingly, in the presence of oxygen, a strong cathodic wave at 0.56 V (vs. RHE) is observed with an onset potential of 0.88 V (vs. RHE) (Fig. 5(a)). This preliminary result indicates that the as grown CM-Ss can be used as the non-noble metal electrocatalyst for the oxygen reduction reaction. Further, the cyclic voltammograms are recorded on CM-Ss modified electrode at different scan rates to deduce reduction process of oxygen (Fig. 5(b)). The electro-reduction of oxygen is monitored by varying sweep rates from 100 mV s⁻¹ to 400 mV s⁻¹. From Fig. 5(c) it is observed that reduction current increases with increasing sweep rate. The plot of i vs. ν^1/2 shows linear in nature indicating the diffusion controlled process for the reduction of oxygen on the electrode surface.³⁹ Moreover the reaction kinetics involved in the ORR by the Cu myristate is evaluated by measuring the limiting current density value. Fig. 6(a) shows the LSV by the Cu myristate modified RDE at a sweep rate of 10 mV s⁻¹ with various rotation rates. It clearly shows that on increasing the rotation rate of the electrode, the limiting current density value goes on increasing showing good agreement with the previous reports.^19,20 To go more insight into the reaction mechanism, we have derived the Koutecky–Levich (K–L) equation from the limiting current density value.

Fig. 5 (a) Cyclic voltammetry of the glassy carbon modified copper myristate strips in argon/oxygen saturated 0.1 M KOH at a sweep rate of 100 mV s⁻¹, (b) cyclic voltammetry of the glassy carbon modified copper myristate strips in oxygen saturated 0.1 M KOH at different sweep rates, (c) the plot showing the linearity behaviour of current to square root of scan rate.

Fig. 6 (a) Linear sweep voltammograms of the RDE modified copper myristate strips in argon/oxygen saturated 0.1 M KOH at different rotation rate of the electrode, (b) the corresponding Koutecky–Levich plot.

The number of electrons involved in the ORR by the Cu myristate strips is analysed by the following Koutecky–Levich equation.

j⁻¹ = j_k⁻¹ + j_dl⁻¹ (1)

j_dl = Bω^1/2 (2)

Here j_k and j_dl are the kinetic and limiting current densities. “ω” is the rotation speed of the RDE.

B = 0.62nFC_O2D_O2^1/2η^−1/6 (3)

Here, n is the number of electrons involved in the ORR. F is the Faraday constant (96 485.33C), C_O2 is bulk concentration of O₂ (1.2 × 10⁻⁶ mol s⁻¹), D_O2 is diffusion coefficient of O₂ (1.9 × 10⁻⁵ cm² s⁻¹), and η is the kinematic viscosity of the electrolyte (for H₂O it is 0.01 cm² s⁻¹).^19,20

The non-zero intercepts in the Koutecky–Levich plots suggest the kinetic character of the measured currents. The number of electrons involved in the reaction is calculated from the slope of the linear fitted K–L plot and found to be 3.7–3.82 that is nearly equal to 4. This observation indicates that the ORR by the Cu myristate proceeds via a four-electron reaction path in which the O₂ is directly converted to the OH⁻ ion without the formation of intermediate peroxide ion. To the best of our knowledge, this is the first report on the electrochemical activity towards oxygen reduction of as grown CM-Ss.

Conclusions

In summary, a facile single step synthetic method for growth of strips of copper myristate is presented in this work. The strips are found to exhibit electrocatalytic activity towards oxygen reduction reaction. In this work, we show that copper myristate with simple straight chain structure can be used as a potential electro-catalyst in ORR. This report provides a scope to produce straight chain copper complex electrocatalyst for oxygen reduction reaction.

Acknowledgements

S. Chatterjee and I. Mukherjee are grateful to DST (IFA12-CH-65), India for the financial assistance. The authors are grateful to Prof. B. K. Mishra, Director, IMMT Bhubaneswar for providing the support to undertake this work. Dr B. K. Jena acknowledges BRNS, Mumbai, India (No. 2013/37p/67/BRNS) and MNRE, New Delhi, India (No. 102/87/2011-NT), CSIR, New Delhi, India (Project (YSP-02, P-81-113) and MULTIFUN (CSC-0101)) for the financial support. A. K. Samantara acknowledges CSIR, India for the fellowship. S. Sarkar and Prof. P. Ayyub are acknowledged for valuable discussions, suggestions and extending experimental facilities.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22986c

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