Electrochemical deposition of manganese oxide–phosphate–reduced graphene oxide composite and electrocatalysis of the oxygen evolution reaction

Abstract

A composite of manganese oxide and reduced graphene oxide (rGO) is prepared in a single step electrochemical reduction process in a phosphate buffer solution for studying as an electrocatalyst for the oxygen evolution reaction (OER). The novel composite catalyst, namely, MnO_x–Pi–rGO, is electrodeposited from a suspension of graphene oxide (GO) in a neutral phosphate buffer solution containing KMnO₄. The manganese oxide incorporates phosphate ions and deposits on the rGO sheet, which in turn is formed on the substrate electrode by electrochemical reduction of GO in the suspension. The OER is studied with the MnO_x–Pi–rGO catalyst in a neutral phosphate electrolyte by linear sweep voltammetry. The results indicate a positive influence of rGO in the catalyst. By varying the ratio of KMnO₄ and GO in the deposition medium and performing linear sweep voltammetry for the OER, the optimum composition of the deposition medium is obtained as 20 mM KMnO₄ + 6.5% GO in 0.1 M phosphate buffer solution of pH 7. Under identical conditions, the MnO_x–Pi–rGO catalyst exhibits 6.2 mA cm⁻² OER current against 2.9 mA cm⁻² by MnO_x–Pi catalyst at 2.05 V in neutral phosphate solution. The Tafel slopes measured for OER at MnO_x–Pi and MnO_x–Pi–rGO are similar in magnitude at about 0.180 V decade⁻¹. The high Tafel slopes are attributed to partial dissolution of the catalyst during oxygen evolution. The O₂ evolved at the catalyst is measured by the water displacement method and the positive role of rGO on catalytic activity of MnO_x–Pi is demonstrated.

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1. Introduction

Water splitting is one of the promising routes for energy storage in the form of hydrogen. The process involves reduction of H⁺ ions to produce H₂ at the cathode and oxidation of OH⁻ ions to O₂ at the anode. In these reactions, the oxygen evolution reaction (OER) has sluggish kinetics and requires an active and stable electrocatalyst.¹ The most commonly used catalysts in acidic media involve compounds of iridium^2,3 or ruthenium,^3,4 which are rare, expensive and unstable. On the other hand, oxides or hydroxides of many non-precious elements, such as iron,^5,6 cobalt^7,8 and nickel^9,10 have been explored as oxygen evolution catalysts (OECs) in alkaline media. Manganese is the preferred choice owing to its low price, high abundance, low toxicity and the possibility to design functional models of biological catalysts.

Several manganese oxides such as MnO₂,^11,12 Mn₂O₃,^13,14 Mn₃O₄ ^15,16 and MnFe₂O₄ ^17,18 exhibit catalytic activity towards the OER in alkaline solutions. Ohsaka and co-workers reported that electrochemically deposited MnO_x nanorods exhibited high performance in basic solutions and the activity decreased in acidic and neutral media.¹⁹ In another study, Tamura et al., demonstrated that an overpotential of 300 mV at a current density of 1 mA cm⁻² at pH 14 increased considerably to 610 mV at pH 7 on MnO₂.²⁰ Several other manganese compounds including MnOOH²¹ and MnO₂–Mn₂O₃ composite,²² also require overpotentials in the range 500–700 mV in neutral electrolytes. According to a report by Takashima et al.,²³ Mn³⁺ species in the catalyst play a key role in water oxidation, and the decreased activity under neutral or acidic conditions is due to the instability of Mn³⁺. At higher pH, Mn³⁺ is stable whereas Mn³⁺ ions disproportionate to produce Mn²⁺ and Mn⁴⁺ ions at pH < 9. Recently, amorphous manganese oxides (MnO_x) were deposited by anodic oxidation of Mn²⁺ ions in phosphate (MnO_x–Pi) and acetate (MnO_x–Ac) buffer solutions and used as electrocatalysts for OER.^24,25

Electronic conductivity of Mn compounds is generally low. The electronic conductivity can be improved by interfacing the catalyst with a highly conductive material such as graphene,^26,27 carbon nanotubes^28,29 or activated carbons.³⁰ Recently, graphene has emerged as an attractive support for catalysts because of its high surface area, excellent charge transport, good mechanical properties and superior chemical as well as electrochemical stability.^31,32 In the present work, a composite of MnO_x and reduced graphene oxide (rGO) is prepared by electrochemical deposition in a single step in a neutral phosphate solution. An analysis of the electrodeposited material suggests the presence of phosphorous in a large quantity. A high catalytic activity of Co oxide prepared in phosphate solution is reported in the literature and it is referred to as Co–Pi.^33,34 Similar to these reports, the MnO_x prepared in phosphate solution in the present study is referred to as MnO_x–Pi. Generally electrooxidation of Mn²⁺ ions to deposit MnO_x is well known.^35,36 Instead, electrochemical reduction of MnO₄⁻ ions is adopted in the present study for the purpose of codepositing MnO_x and rGO. Electrochemical reduction of graphene oxide (GO) to form rGO on a substrate electrode is also reported in the literature.^37,38 Thus, the simultaneous electrochemical reduction of MnO₄⁻ ions and GO in phosphate buffer solution produces MnO_x–Pi–rGO, which exhibits a high catalytic activity towards the OER in neutral phosphate buffer solution. The results of experiments conducted under identical conditions using MnO_x–Pi and MnO_x–Pi–rGO suggest that rGO imparts a substantial improvement on the catalytic activity of MnO_x–Pi towards the OER.

2. Experimental section

2.1. Reagents, materials and characterization

Analytical grade KH₂PO₄, K₂HPO₄ and KMnO₄ (all from Merck) were used as received. All solutions were prepared using double distilled water. Phosphate buffer solution was prepared by mixing calculated volumes of 0.1 M KH₂PO₄ and 0.1 M K₂HPO₄ solutions. The solution pH was maintained at 7.0, unless otherwise stated. The physical characterization studies of the materials were carried out by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDXA), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. For SEM images, an Ultra 55 scanning electron microscope equipped with an EDXA system at 20 kV was used. TEM images were recorded using an FEI Tecnai T-20 200 kV transmission electron microscope. Surface compositions of the electrodeposits were analyzed by XPS using a SPECS GmbH spectrometer (Phoibos 100 MCD Energy Analyzer) with Mg K_α radiation. Raman spectra were collected using a LabRam HR (UV) system at an excitation wavelength of 532 nm. Sheet resistance was measured by the four-probe van der Pauw method using an Agilent Device Analyzer B1500A. Electrochemical deposition and OER studies were conducted using a PARC EG&G potentiostat/galvanostat model Versastat II. A Pt foil substrate (area: 0.5 cm²) for the working electrode, two large Pt foil auxiliary electrodes and a saturated calomel reference electrode (SCE) in a glass cell were used. Oxygen produced during electrolysis was detected by monitoring its reduction on a Pt-rotating disc electrode (d – 3 mm) in an air-tight glass cell. Oxygen gas was quantitatively measured in a burette by the water displacement method using a large area (5 cm²) electrode. Mass variation during electrochemical depositions was monitored by using a CH Instruments potentiostat/galvanostat model 400A with an electrochemical quartz crystal microbalance (EQCM). For these experiments, a small Teflon cell with Pt-coated quartz crystal (8 M Hz, area – 0.205 cm², sensitivity – 0.146 Hz ng⁻¹ cm²) as the working electrode, Pt wire counter electrode and Ag/AgCl, Cl⁻ (3 M) reference electrode was used. Electrochemical impedance spectroscopy (EIS) measurements were performed at 10 mV ac excitation signal over the frequency range of 100 kHz to 0.10 Hz. Current density values are reported on the basis of the geometrical area of the electrode. All potential values are converted to the reversible hydrogen electrode (RHE) scale using the equation, E_RHE = E_SCE + E⁰_SCE + 0.059 × pH, where E_SCE is the measured potential using SCE reference and E⁰_SCE is the standard potential of the SCE at 25 °C (0.242 V), unless otherwise stated.

2.2. Preparation of graphene oxide dispersion

Graphite powder was converted into graphite oxide (GtO) by a modified Hummers and Offemann method.³⁹ In a typical synthesis, 3.0 g graphite powder was added to 69 ml conc. H₂SO₄ containing 1.5 g NaNO₃. The mixture was stirred for 1 h at ambient temperature. The container was cooled in an ice-bath, and 9.0 g KMnO₄ was added slowly under stirring. The container was removed from the ice-bath and allowed to reach room temperature. Double-distilled water was added slowly in two aliquots of 138 ml and 420 ml at about 15 min intervals. Then, 7.5 ml of 30% H₂O₂ was added and the color of the suspension changed from yellow to brown, indicating the oxidation of graphite to graphite oxide (GtO). The product was separated by centrifugation, washed with warm water and ethanol several times, and dried at 50 °C. About 300 mg of GtO was added to 300 ml double-distilled water and sonicated for 3 h using a Misonix ultra sonicator model S4000-010. The dispersion was allowed to settle for 24 h. The supernatant solution was collected for further use as graphene oxide (GO) dispersion and the settled residue was discarded. GO concentration of 0.7 mg ml⁻¹ was obtained by this scheme.

2.3. Preparation of MnO_x–Pi–rGO catalyst

MnO_x–Pi was deposited on a Pt foil electrode at −1.05 V vs. SCE for 30 min from 0.1 M phosphate solution (pH 7.0) containing 20 mM KMnO₄ under stirring condition. To study the effect of rGO towards OER activity, MnO_x–Pi–rGO was also deposited under identical conditions, except that the deposition bath contained GO additionally in it. The electrodes were washed well with double distilled water and air dried. The OER activity of the deposit was measured by subjecting it to linear sweep voltammetry at 5 mV s⁻¹ in phosphate buffer (pH 7.0) solution.

3. Results and discussion

3.1. Simultaneous electrochemical deposition

Electrochemical deposition of MnO_x–Pi catalyst was performed on a Pt foil electrode from 0.1 M phosphate buffer (pH 7.0) solution containing 20 mM KMnO₄ at −1.05 V vs. SCE (Fig. 1(i)). In neutral aqueous conditions, reduction of MnO₄⁻ proceeds via the following reaction,⁴⁰

MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻ (1)

Fig. 1 Current density versus time plot during deposition at −1.05 V vs. SCE from 0.1 M phosphate solutions (pH 7.0) containing (i) 20 mM KMnO₄, (ii) GO and (iii) 20 mM KMnO₄ + GO. Concentration of GO was 0.2 mg ml⁻¹.

Since the electrolyte has a large amount of phosphate ions, incorporation of phosphate leading to the formation of a manganese-oxide–phosphate catalyst (MnO_x–Pi) is expected. The process is similar to the deposition of MnO_x–Pi by the oxidation of Mn²⁺ ions in phosphate solution,²⁴ but for the reduction of Mn⁷⁺ ions in the present study. It is reported that Mn³⁺ is stabilized by the phosphate group in the catalyst structure.⁴¹ As described below in the XPS section, the majority of Mn is present in the 3+ oxidation state due to the presence of phosphate in the deposit. Furthermore, phosphate in the catalyst is known to play a crucial role in the proton coupled electron transfer (PCET) mediated mechanism of the OER and stability of the catalyst.^42,43 Recent studies on the structure and activity of a Mn-phosphate catalyst suggested the role of phosphates as bridging ligands interconnecting two or more manganese ions.⁴⁴ The amperogram recorded during the deposition of MnO_x–Pi (Fig. 1(i)) exhibits three distinct regions: (i) a sudden rise in the current followed by a rapid decay, (ii) a rise and fall in the current to form a peak and (iii) a steady decrease in the current to reach a limiting current. This indicates that MnO_x–Pi deposition is not a simple diffusion-controlled process but, proceeds via a nucleation-growth pathway as reported in the case of anodically deposited MnO_x–Pi catalyst.⁴⁵ The initial rise and fall in the current to form a peak in the amperogram is due to primary nucleation on the electrode surface. The current peak is the result of depletion of MnO₄⁻ ions in the vicinity of the electrode. As the depletion zone expands, current falls as function of t^−1/2, which is characteristic of a diffusion-controlled growth of the deposited material. However, the current does not decay to zero, due to the parasitic hydrogen evolution reaction. Application of a higher negative voltage causes vigorous H₂ evolution and consequently hinders the electrodeposition of MnO_x–Pi. Electrochemical reduction of GO to rGO was reported as a convenient and fast route to synthesize high quality graphene nanosheets on a large scale.^37,38 The applied negative potentials facilitate reduction of oxygen functionalities (–OH, C–O–C on the planes and –COOH on the edges) of GO adsorbed on the substrate electrode resulting in the deposition of rGO sheets.³⁷ Cyclic voltammograms of a glassy carbon electrode recorded in a suspension of GO exhibited a reduction current peak at −1.20 V vs. SCE which was attributed to the reduction of GO to rGO.³⁷ Fig. 1(ii) shows the current–time curve for the GO reduction at −1.05 V vs. SCE in 0.1 M phosphate buffer solution (pH 7.0) containing GO of concentration 0.2 mg ml⁻¹ as a suspension. A gradual decrease in the current density from 7.0 mA cm⁻² to a steady state value of 2.9 mA cm⁻² is observed. A black coating on the electrode surface was visually observed, indicating the reduction of GO, and concomitant deposition of rGO on the substrate. Since both MnO_x–Pi and rGO are deposited at −1.05 V vs. SCE from phosphate solutions containing KMnO₄ and GO, respectively, simultaneous deposition and formation of hybrid MnO_x–Pi–rGO from a mixed KMnO₄–GO solution in the phosphate buffer is expected. Fig. 1(iii) shows the variation in the current as a function of time during the deposition of MnO_x–Pi–rGO from 0.1 M phosphate solution (pH 7.0) containing 20 mM KMnO₄ and GO (amount of GO was adjusted to 6.5% with respect to the weight of KMnO₄). The current–time trace in Fig. 1(iii) shows the nucleation-growth features as observed in the case of deposition of MnO_x–Pi (Fig. 1(i)), but with a higher magnitude of the current. The increased value of current indicates that the reaction involves not only MnO_x–Pi deposition but GO reduction also. Thus, these experiments indicate that the MnO_x–Pi–rGO hybrid material can be obtained by the simultaneous electrochemical reduction of MnO₄⁻ and GO in neutral phosphate buffer solution.

MnO₂ is generally prepared by electrochemical oxidation of Mn²⁺ ions in the electrolyte,^35,36 similar to the procedures used for electrodeposition of MnO_x-based catalysts for OER.^24,25 However, the catalyst in the present study is prepared by reducing Mn⁷⁺ ions. This method is adopted to facilitate compatibility with the preparation of rGO. It is reported^37,38 that an aqueous suspension of graphene oxide can be reduced electrochemically to produce rGO deposit on electrodes. Therefore, a graphene oxide suspension in KMnO₄ solution is used to reduce simultaneously Mn⁷⁺ ions and graphene oxide. The electrodeposition of MnO_x–Pi–rGO composite is achieved successfully, for the first time in the literature.

3.2. EQCM study

It was intended to monitor the electrodeposition process in situ by EQCM. This technique allows us to quantitatively measure the mass of the deposit on the electrode surface as a function of time. It is seen from Fig. 2(i) that the cathodic polarization of Pt coated quartz crystal electrode to −1.05 V vs. SCE in MnO₄⁻ solution causes an increase in the mass of the electrode. The mass increases rapidly to 4.5 μg cm⁻² within the first 10 s and thereafter increases gradually to 10.8 μg cm⁻² at 60 s. This also suggests an initial rapid nucleation followed by a gradual growth of the catalyst. Similar experiments were carried out for the deposition of MnO_x–Pi–rGO and the corresponding massogram is shown in Fig. 2(ii). In this case, the mass increases to 5.3 μg cm⁻² in the first 10 s and attains 14.5 μg cm⁻² after 60 s. Thus, the net mass gain after 60 s of polarization at −1.05 V vs. SCE in the presence of rGO deposition is 14.5 μg cm⁻² (Fig. 2(ii)) whereas that in the absence of rGO is 10.8 μg cm⁻² (Fig. 2(i)). Assuming similar kinetics of deposition for MnO_x–Pi in the presence and absence of GO in the deposition bath, the mass of rGO in the MnO_x–Pi–rGO is calculated as 3.7 μg cm⁻², which is about 25.5% of the total mass.

Fig. 2 Variation in mass during deposition at −1.05 V vs. SCE from 0.1 M phosphate solutions (pH 7.0) containing (i) 20 mM KMnO₄ and (ii) 20 mM KMnO₄ + 0.2 mg ml⁻¹ GO.

3.3. Morphology

The morphology of the electrodeposit was studied by using SEM and TEM. As shown in Fig. 3(a), electrochemically deposited rGO sheets exhibit a typical wrinkled structure with folding. These geometric wrinklings and ripplings are the result of nanoscale interlocking of rGO sheets. Corrugation and folding are inherent features of graphene, because the 2D membrane structure is thermodynamically stable via bending. These features of rGO provide enhanced mechanical properties, reduced surface energy, increased surface roughness and high density of electrochemically active sites.⁴⁶ It is also reported that corrugations in graphene sheets make electron transfer 10 times faster than at the basal plane of graphite.⁴⁷ Morphology of the electrodeposited MnO_x–Pi is shown in Fig. 3(b). Several particles of 100–200 nm size are coalesced to form catalytic islands. Hence, it is proposed that MnO_x–Pi deposits as small nuclei in the initial stages of electrolysis and then grows slowly to bigger particles. These bigger particles finally merge together to form catalytic islands. Fig. 3(c) shows an image of MnO_x–Pi–rGO. The deposit preserves the nanosheet features of rGO together with MnO_x–Pi catalyst particles embedded in it. It is to be noted that rGO and MnO_x–Pi are not deposited in separate zones, but both are mixed, resulting in a strong interaction between them at the molecular level. This is achieved due to the simultaneous deposition of MnO_x–Pi and rGO by the present method. It is seen from the TEM image (Fig. 3(d)) that the electrodeposited rGO nanosheets are folded and overlapped. The deposition of rGO in the presence of MnO₄⁻ leads to the formation of hybrid MnO_x–Pi–rGO composite (Fig. 3(e) and (f)). The rGO sheets are decorated with nanoparticles as well as large particles of MnO_x–Pi. The high resolution image for MnO_x–Pi–rGO (Fig. 3(f)) shows the lattice fringes of rGO. Poor crystalline features obtained in the SAED pattern (inset in Fig. 3(f)) suggests the amorphous nature of the MnO_x–Pi deposit on the surface of rGO sheets.

Fig. 3 SEM micrographs for (a) rGO, (b) MnO_x–Pi and (c) MnO_x–Pi–rGO. TEM images for (d) rGO and (e, f) MnO_x–Pi–rGO. Inset in (f) shows the SAED pattern for MnO_x–Pi–rGO.

3.4. Composition

The elemental composition of MnO_x–Pi–rGO was determined from the EDXA spectra. The spectrum in Fig. 4(a) shows Mn, P, C, O and K as the major elements. The Mn : P ratio varies from 2 : 1 to 1.5 : 1, indicating significant incorporation of phosphate in the catalyst layer. Also, the Mn : K ratio is found to be about 3 : 1. Although the role of K is not clear, it is likely that K⁺ ions are adsorbed on the surface of the catalyst. The presence of the C peak indicates the deposition of rGO along with MnO_x–Pi during the electrochemical reduction of MnO₄⁻ ions in phosphate solution containing GO. The distribution of elements on the substrate surface is shown by X-ray maps (Fig. 4(c)–(e)). Elements are distributed uniformly without agglomeration. It is anticipated that the defects in the rGO sheets may act as the nucleation site for the deposition of MnO_x–Pi, thus preventing agglomeration.

Fig. 4 (a) EDXA spectra and (b) SEM image of MnO_x–Pi–rGO on Pt. X-ray maps showing the distribution of (c) Mn, (d) P and (e) C.

The chemical composition of the MnO_x–Pi–rGO was also analyzed by XPS and the results are shown in Fig. 5. The survey spectrum (Fig. 5(a)) identifies Mn, P, O, K and C as the major elements present. This is in agreement with the EDXA results. The Mn 2p high resolution spectrum (Fig. 5(b)) is divided into Mn 2p_1/2 at 653.6 eV and Mn 2p_3/2 at 641.8 eV.⁴⁸ These peaks are further de-convoluted into two peaks. In the Mn 2p_3/2 branch, the intense peak at 641.6 eV is assigned to Mn³⁺ while the smaller peak at 643.8 eV is due to Mn⁴⁺.⁴⁸ The Mn³⁺/Mn⁴⁺ ratio of about 4.5 is obtained from the ratio of areas under the peaks. This suggests that the Mn present in MnO_x–Pi–rGO is largely in 3+ oxidation state, which is reported to play a key role in water oxidation.²³ The peak at 132.8 eV in the P 2p spectrum (Fig. 5(c)) is consistent with phosphate. This is comparable to the binding energy of the P 2p peak observed in the case of Co–Pi³³ and Ir–Pi⁴⁹ catalysts. The high resolution C 1s peak (Fig. 5(d)) is de-convoluted into three peaks. The main peak at 284.6 eV is due to graphitic sp² C atoms while the small ones at 286.3 and 287.8 eV correspond to C atoms bonded with oxygenated groups.⁵⁰ It is seen that the percentage of oxygenated carbon is less than 20% of the total carbon, indicating a significant removal of oxygen functional groups during electrochemical reduction of graphene oxide in phosphate medium. The XPS peak in the O 1s region (Fig. 5(e)) is fitted to Mn–O–Mn (529.7 eV), Mn–O–H (530.8 eV) and C–OH (532.9 eV).⁵⁰ A small amount of K is also detected in the XPS survey spectrum similar to the EDXA data. Hence, high resolution K 2p was also recorded (Fig. 5(f)). The spectrum shows a doublet with spin–orbit splitting of 2.8 eV. The peaks at 295.7 and 292.5 eV are assigned to K 2p_3/2 and 2p_1/2, respectively.⁴⁹

Fig. 5 XPS spectra for MnO_x–Pi–rGO. (a) Survey spectrum, and high resolution spectra for (b) Mn 2p, (c) P 2p, (d) C 1s, (e) O 1s and (f) K 2p.

3.5. Raman spectroscopy

The Raman spectrum of rGO (Fig. 6(i)) shows peaks at 1344 cm⁻¹ and 1596 cm⁻¹, which are assigned to D and G bands, respectively. The G band corresponds to the optical E_2g phonons at the Brillouin zone center due to bond stretching of the sp² carbon pairs, whereas the D band is associated with the second-order of zone-boundary phonons, which are activated by defects.^51,52 Thus, the intensity ratio between D and G peak (I_D/I_G) is usually used as a measure for the degree of disorder. In the present case, I_D/I_G ratio for rGO is 1.4, indicating significant disorder in the rGO sheets. However, the I_D/I_G does not always correlate well with the electronic quality of rGO.⁵³ After electrochemical reduction, the intensity ratio was found to be increased in most of the electrochemically prepared rGO in the literature.⁵³ This is attributed to a decrease in the average size of sp² domains. Electrochemical deposition involves the formation of new graphitic domains with smaller size but in larger number.⁵³ The Raman spectrum of MnO_x–Pi–rGO (Fig. 6(ii)) shows D and G peaks of rGO at 1352 cm⁻¹ and 1604 cm⁻¹. These values are slightly shifted in comparison with that of bare rGO. Interestingly, the I_D/I_G ratio is reduced to 1.1. More importantly, the spectrum in Fig. 6(ii) shows a broad peak centered at 625 cm⁻¹. This can be assigned to Mn–O stretching vibrations of the MnO₆ group.^50,54 This clearly indicates the formation of MnO_x–Pi along with rGO on the electrode surface during the electrochemical reduction process.

Fig. 6 Raman spectra for (i) rGO and (ii) MnO_x–Pi–rGO. Excitation wave length: 532 nm.

3.6. OER activity

The catalytic activity of MnO_x–Pi with and without rGO was studied using linear sweep voltammetry at 5 mV s⁻¹ in neutral phosphate buffer solutions. Phosphate buffer was chosen for comparing the present results with the reported values on anodically deposited MnO_x–Pi.^24,25 Also, the buffer solution annuls any change in the pH by proton release during the water oxidation reaction. In the case of MnO_x–Pi catalyst (Fig. 7(i)), a negligibly small capacitive current flows until the potential reaches 1.65 V, the onset potential of the OER. Then the current increases to 2.8 mA cm⁻² at 2.05 V due to the OER. Thus, the overpotential required to reach the onset potential of the OER is about 420 mV, which is considered as large. One of the reasons for such poor catalytic activity of MnO₂ towards the OER is low conductivity. Hence, it is anticipated that incorporation of a highly conducting matrix such as rGO can improve the overall catalytic activity. The linear sweep voltammogram recorded for MnO_x–Pi–rGO (Fig. 7(ii)) shows that the onset of the OER is 1.55 V, which is 100 mV less than that on bare MnO_x–Pi. Moreover, the current density at 2.05 V for MnO_x–Pi–rGO is 6.2 mA cm⁻², which is about 2.2 times higher in comparison with that of MnO_x–Pi catalyst without rGO. Thus, the hybrid MnO_x–Pi–rGO is superior to MnO_x–Pi in catalytic activity in terms of both the onset potential of OER and current density at any potential in the OER region.

Fig. 7 Linear sweep voltammograms at 5 mV s⁻¹ in 0.1 M neutral phosphate buffer solution using (i) MnO_x–Pi and (ii) MnO_x–Pi–rGO electrodes.

3.7. Optimization of deposition conditions

It was intended to optimize the deposition parameters for MnO_x–Pi–rGO in view of the maximum catalytic activity. At first, several electrodes were made by keeping the concentration of KMnO₄ at 13 mM in 0.1 M phosphate buffer solution, while the weight percentage of GO in the electrolyte was gradually varied. Deposition was carried out at −1.05 V vs. SCE for 30 min and the OER activity of each electrode was studied by linear sweep voltammetry at 5 mV s⁻¹ in neutral phosphate buffer solution. The current density obtained at 2.05 V is plotted as a function of weight percentage of GO in the electrolyte (Fig. 8(a)). It is seen that the current is 3.3 mA cm⁻² at 0% GO and it increases to a maximum value of 5.5 mA cm⁻² for 6.5% GO. The current density values of 4.4 and 4.8 mA cm⁻² were obtained at intermediate GO concentration of 3.4% and 4.9%. Since the deposition of rGO increases with an increase in GO concentration, it indicates the favorable role of rGO towards the OER. However, increasing GO concentration more than 6.5% results in a decrease in the OER current. It is expected that at higher concentration of GO, coagulation of GO increases leading to its reduced concentration in the phosphate buffer solution. Therefore, there is a decrease in rGO in the catalyst resulting in reduced catalytic activity. This is supported by the EDXA analysis of MnO_x–Pi–rGO deposits prepared at different GO concentrations that the carbon content in the catalyst decreases by increasing GO concentration beyond a particular limit (ESI, Fig. S1†). In another set of experiments for the optimization of concentration of KMnO₄, the percentage of GO in the deposition bath was fixed to 6.5%, and then the concentration of KMnO₄ was varied. The deposition of the catalyst and the OER analysis were carried out as above. As shown in Fig. 8(b), the OER current increases with an increase in concentration of KMnO₄ in the electrolyte. It reaches a maximum value of 6.2 mA cm⁻² at 2.05 V for 20 mM concentration. The increase in current with concentration of KMnO₄ is attributed to the enhanced rate of deposition and thus a greater loading of the MnO_x–Pi at higher concentration. On the other hand, material loading beyond an optimum range diminishes the catalytic performance. This is ascribed to poor conductivity of Mn-based catalysts, which is a decisive factor at higher loading level. Also, rapid nucleation and growth of the catalyst at very high concentration causes the agglomeration of particles, which in turn reduce the available electrochemically active surface area for the catalysis. Hence, a composition of 20 mM KMnO₄ and 6.5% of GO is considered as the optimum for the deposition of MnO_x–Pi–rGO. Although the deposition potential is another parameter which requires optimization, it is restricted to −1.05 V vs. SCE as the deposition rate is lower at higher potentials, whereas excessive hydrogen evolution occurs at lower potentials. It was then planned to deposit MnO_x–Pi–rGO at −1.05 V vs. SCE from the electrolyte with optimum concentration of GO and KMnO₄ at different time intervals. Results are shown in Fig. 8(c). It is seen that current increases from 4.2 mA cm⁻² for 5 min of deposition to 4.9 mA cm⁻² and 5.6 mA cm⁻² for 10 and 20 min, respectively. Current density reaches a maximum of 6.2 mA cm⁻² for 30 min of deposition. A further increase in the deposition time leads to a gradual fall in the OER performance due to low electronic and ionic conductivity of the thick MnO_x–Pi catalyst. These experiments (Fig. 8(a)–(c)) establish that the best MnO_x–Pi–rGO catalyst is obtained by deposition at −1.05 V vs. SCE for 30 min from 20 mM KMnO₄ + 6.5% GO in 0.1 M phosphate buffer (pH 7.0) solution. The highest OER performance on this catalyst is thus summarized to be 6.2 mA cm⁻² current at 2.05 V in neutral phosphate solution.

Fig. 8 (a) Current density at 2.05 V obtained for the electrodes deposited from 13 mM KMnO₄ + 0.1 M phosphate as the function of weight percentage of GO in the electrolyte. Similar plots for the electrodes (b) deposited from 6.5% GO + 0.1 M phosphate solutions (pH 7.0) at different concentrations of KMnO₄ and (c) deposited from 20 mM KMnO₄ + 6.5% GO + 0.1 M phosphate solutions (pH 7.0) for different time intervals.

The OER activity of MnO_x–Pi–rGO catalyst on Pt is compared with the performance of similar Mn based catalysts reported in the literature (ESI, S2†). For instance, hydrated Mn₃(PO₄)₂ was recently reported as an efficient catalyst for OER.⁵⁵ This catalyst exhibited greater catalytic activity of about 0.3 mA cm⁻² of OER current at 1.5 V vs. NHE in relation to the low currents measured on MnO, Mn₂O₃, MnO₂ and Mn₃O₄ catalysts. The OER current measured on MnO_x–Pi–rGO on Pt substrate at 2.0 V vs. RHE (i.e., 1.59 V vs. NHE) is about 6.0 mA cm⁻² in the present study. Similarly, the catalytic activity of MnO_x–Pi–rGO is found to be superior to electrochemically deposited amorphous Mn-oxides.²⁵ The higher catalytic activity of composite MnO_x–Pi–rGO in the present work is attributed to the presence of rGO and method of preparation of the catalyst.

3.8. Mechanism of OER

For conducting Tafel polarization experiments, MnO_x–Pi and MnO_x–Pi–rGO were deposited under the optimized conditions and linear sweep voltammograms were recorded at 0.05 mV s⁻¹. The phosphate buffer solution was kept under stirring by a magnetic pellet. A very low scan rate was used to maintain the steady state conditions and stirring to avoid mass transfer limitations. The corresponding Tafel plots for MnO_x–Pi and MnO_x–Pi–rGO are shown in Fig. 9. At any current density value, the overpotential requirement is lower for MnO_x–Pi–rGO compared to MnO_x–Pi, indicating the superior OER activity of the former. By extrapolating these curves to zero over potential, the exchange current density for the OER is obtained. The exchange current density on MnO_x–Pi–rGO is 1 × 10⁻⁴ mA cm⁻² whereas it is only 3.1 × 10⁻⁵ mA cm⁻² on MnO_x–Pi. Nevertheless, both the catalysts show Tafel slope of 170 mV decade⁻¹, signifying a similar mechanism for OER. In neutral and alkaline medium, OER can be written as,

4OH⁻ ⇌ 2H₂O + O₂ + 4e⁻ (2)

with a reversible electrode potential (E^r) of 1.23 V vs. RHE. The generally accepted mechanism for OER is as follows.⁵⁶

S + OH⁻ ⇌ S–OH + e⁻ (3)

S–OH + OH⁻ ⇌ S=O + H₂O + e⁻ (4)

2S=O ⇌ 2S + O₂ (5)

where S is the substrate site, which facilitates adsorption of OH. The current density (i) and potential (E) relationship of electron-transfer controlled step may be written as,

i = i₀(1 − θ)e^{(1−α)nF(E−Er)/RT} (6)

where i₀ is exchange current density, θ is surface coverage by OH, α is transfer coefficient, E^r is reversible potential and other symbols have their usual meanings. If either (3) or (4) is the rate determining step, n = 1 and then the Tafel slope (dE/dlog i) is,

(dE/dlog i) = 2.3RT/{(1 − α)F} (7)

= (0.059/{1 − α}) V at 25 °C (8)

Fig. 9 Tafel plots for (i) MnO_x–Pi and (ii) MnO_x–Pi–rGO in 0.1 M neutral phosphate solution.

Assuming α = 0.5, the value of Tafel slope thus expected is 118 mV. The mechanism of OER on several perovskite catalysts was reported by Bockris and Otagawa.⁵⁷ While Tafel slopes in the range of 40–120 mV decade⁻¹ are reported for a majority of catalysts, values in the range of 175–235 mV decade⁻¹ are measured for LaVO₃ and SrVO₃. The unusually high Tafel slope is attributed to the oxidation of V³⁺ and Cr³⁺ in the catalysts.⁵⁷ It is likely in the present case also that Mn³⁺ present in the MnO_x–Pi–rGO and MnO_x–Pi catalysts undergoes oxidation to Mn⁴⁺, which occurs simultaneously with OER resulting in anomalous Tafel slope of 170 mV decade⁻¹.

3.9. RDE study

The O₂ evolved during water oxidation on the MnO_x–Pi–rGO catalyst was detected by reduction on a Pt rotating disc electrode (RDE) in an air tight glass cell. The MnO_x–Pi–rGO coated electrode was prepared at −1.05 V vs. SCE for 30 min from 20 mM KMnO₄ + 0.1 M phosphate buffer solution (pH 7.0) containing 6.5% GO. An electrochemical cell was assembled in phosphate buffer solution with both MnO_x–Pi–rGO coated Pt and a Pt RDE as working electrodes. The electrolyte was saturated with Ar and the potential of Pt RDE was swept at 10 mV s⁻¹ from 0.95 to 0.15 V at 2500 rpm (Fig. 10(a)(i)). A negligibly small current flows, indicating the absence of dissolved O₂ in the electrolyte. Then, electrolysis was conducted at the MnO_x–Pi–rGO electrode at 1.95 V. After 1 h, the electrolysis was terminated and the Pt RDE was again subjected to linear sweep voltammetry. It is seen (Fig. 10(a)(ii)) that the current due to ORR increases to −0.43 mA cm⁻² at 0.10 V. This indicates that the O₂ has evolved on the MnO_x–Pi–rGO electrode during electrolysis and the dissolved O₂ undergoes reduction subsequently at the RDE. Electrolysis was again continued at the MnO_x–Pi–rGO and the voltammogram of ORR was recorded after every 1 h of electrolysis. As observed in Fig. 10(a)((iii)–(v)), the ORR current increases to −0.7, −0.8 and −0.9 mA cm⁻², respectively, after 2, 3 and 4 h of OER. It is worth noticing that ORR current varies almost linearly with the time of electrolysis on MnO_x–Pi–rGO (Fig. 10(b)(i)), indicating a constant rate of oxygen evolution. Experiments were also repeated with bare MnO_x–Pi without rGO (Fig. 10(b)(ii)). As evident from Fig. 10(b)(ii), the ORR current is smaller during the entire period of the experiment compared to the results obtained in the case of MO_x–Pi–rGO (Fig. 10(b)(i)). These experiments clearly show that the MnO_x–Pi–rGO is a better catalyst for the OER than MnO_x–Pi. Furthermore, the quantitative oxygen gas measurement using the water displacement method also confirmed the higher activity of MnO_x–Pi–rGO than MnO_x–Pi (ESI, S3†). Both the catalysts exhibited Faradaic efficiency above 90% (ESI, S3†). The MnO_x–Pi–rGO has moderate stability in the phosphate electrolyte under prolonged electrolysis (ESI, Fig. S4†). There is a decrease in the OER current by about 30% in 1 h electrolysis at 1.95 V. The decrease in activity is probably due to the formation of less active Mn⁴⁺ oxides by the oxidation of active Mn³⁺ species at higher anodic potential.

Fig. 10 (a) Linear sweep voltammetry for ORR on a Pt RDE after oxygen evolution reaction on a MnO_x–Pi–rGO electrode at 2.05 V for (i) 0, (ii) 1, (iii) 2, (iv) 3 and (v) 4 h. Scan rate: 10 mV s⁻¹ and rotation speed: 2500 rpm. (b) Variation in the ORR current density at 0.10 V versus electrolysis time for (i) MnO_x–Pi–rGO and (ii) MnO_x–Pi.

3.10. Electrochemical impedance spectroscopy

The superior catalytic activity of MnO_x–Pi–rGO over MnO_x–Pi was further confirmed from electrochemical impedance spectroscopy (EIS) studies. Nyquist plots for MnO_x–Pi and MnO_x–Pi–rGO electrodes at 1.95 V in 0.1 M phosphate electrolytes (pH 7.0) are shown in Fig. 11(a). In both the cases, two broad semicircles are seen. The corresponding electrical equivalent circuits are provided in Fig. 11(a) inset. The capacitive elements are replaced by constant phase elements (CPEs), denoted by Q to account for the non-uniformity of the electrode surface. The electrolyte resistance is indicated by R_s. R_f and Q_f denote the resistance and CPE of the catalyst whereas R_ct and Q_dl represent the charge transfer resistance of OER and CPE corresponding to double layer capacitance, respectively. The values of parameters obtained by fitting the impedance spectra are given in Table 1. Notably, the R_ct value obtained for MnO_x–Pi–rGO electrode is 103 Ω which is smaller than the R_ct value obtained for MnO_x–Pi electrode (137 Ω) without rGO. Since, the electrocatalytic activity is inversely related to the charge transfer resistance, it predicts a higher OER activity for MnO_x–Pi–rGO. These results are in agreement with the voltammetric studies.

Fig. 11 (a) Nyquist plots at 1.95 V and (b) Mott–Schottky plots at 1 kHz for (i) MnO_x–Pi and (ii) MnO_x–Pi–rGO electrodes in 0.1 M neutral phosphate solution.

Table 1 Impedance parameters

(a) MnOx–Pi–rGO catalyst
Parameter	Value	Error (%)
Rs (Ω)	2.5	4.1
Qf − Y0 (F s^n−1)	0.0002	3.0
Qf − n	0.80	0.6
Rf (Ω)	18.4	0.9
Qdl − Y0 (F s^n−1)	6.7 × 10^−7	9.3
Qdl − n	0.90	0.8
Rct (Ω)	103	0.7

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(b) MnOx–Pi catalyst
Parameter	Value	Error (%)
Rs (Ω)	2.5	4.1
Qf − Y0 (F s^n−1)	0.0002	3.0
Qf − n	0.80	0.6
Rf (Ω)	18.8	1.1
Qdl − Y0 (F s^n−1)	8.7 × 10^−7	10
Qdl − n	0.90	1.0
Rct (Ω)	137	0.8

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Furthermore, the Mott–Schottky plots at 1 kHz for MnO_x–Pi and MnO_x–Pi–rGO are compared in Fig. 11(b). According to the Mott–Schottky equation,⁵⁸

1/C² = [2/(ε_rε₀eN_d)][E − E_fb − (k_BT/e)] (9)

where C is the capacitance of the space charge layer, e is the electronic charge, ε_r is the dielectric constant of the electrode material, ε₀ is the permittivity of air, N_d is the carrier density, E is the applied potential and E_fb is the flat band potential. Thus, the Mott–Schottky plot (1/C² vs. E) provides the flat band potential and charge carrier density. The flat band potential (Fig. 11(b)) for MnO_x–Pi is 0.74 V whereas that of MnO_x–Pi–rGO is 0.79 V. Both of them show positive slope which is typical for n-type semiconductors. However, the MnO_x–Pi electrode shows a slightly higher slope in comparison with MnO_x–Pi–rGO. Since, the charge carrier (electrons for n-type semiconductor) density is inversely related to the slope of the Mott–Schottky plot, it is concluded that the carrier density is larger for MnO_x–Pi–rGO catalyst compared to MnO_x–Pi. Similarly, MnO_x–Pi–rGO exhibited a smaller sheet resistance value in comparison with that of MnO_x–Pi under identical deposition conditions (ESI, S5†).

4. Conclusions

A Mn based composite catalyst, MnO_x–Pi–rGO was prepared from a suspension of GO in neutral phosphate buffer solution consisting of KMnO₄. The manganese oxide incorporated phosphate ions and was deposited on rGO sheets, which in turn was formed on the substrate electrode by electrochemical reduction of GO in the suspension. By varying the ratio of KMnO₄ and GO in the deposition medium and performing linear sweep voltammetry for the OER, the optimum composition of the deposition medium was obtained as 20 mM KMnO₄ + 6.5% GO in 0.1 M phosphate buffer electrolyte of pH 7. Under identical conditions, the MnO_x–Pi–rGO catalyst exhibited 6.2 mA cm⁻² OER current against 2.9 mA cm⁻² by MnO_x–Pi catalyst at 2.05 V in neutral phosphate solution. The Tafel slopes measured for OER at MnO_x–Pi and MnO_x–Pi–rGO were similar in magnitude at about 0.180 V decade⁻¹. The high Tafel slopes were attributed to partial dissolution of the catalyst during oxygen evolution. The positive influence of rGO on catalytic activity of MnO_x–Pi towards OER is demonstrated.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01217e

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