T. Pandiarajan,
S. Ravichandran* and
L. J. Berchmans
CSIR – Central Electrochemical Research Institute, Karaikudi-630006, India. E-mail: Srravi371@gmail.com
First published on 18th November 2014
Alkaline water electrolysis (AWE) is the simplest way of producing hydrogen, an attractive fuel for the future. In view of their cost-effectiveness and durability, non-noble metal oxides are promising catalysts for AWE. Here, we studied the effect of Ce substitution on the OER activity of manganese ferrite. Ce substituted MnFe2O4 (0 ≤ x ≤ 0.8) was synthesized by a combustion method. Characterization techniques such as SEM, XRD, EDAX and XPS were used to analyse the surface morphology and the chemical composition of CexMnFe2−xO4. Substitution of Ce3+ in the cubic lattice of MnFe2O4 increases the conductivity of CexMnFe2−xO4, which results in the negative shift in the OER onset potential. Among all the Ce3+ substituted manganese ferrites, Ce0.2MnFe1.8O4 was found to be more active for OER in terms of current and stability. Notably, Ce0.2MnFe1.8O4 affords a current density of 10 mA cm−2 at a small overpotential of ∼310 mV and a Tafel slope value of 31 mV per decade, these values are comparable to the well-investigated non-noble metal oxides.
Although electrolytic water splitting was first identified in an acidic medium, because of cost-effective electrolyser fabrication and the simple control of corrosion, alkaline water electrolysis has dominated large-scale hydrogen production technology for several decades. Still, it has been challenge to find non-noble metal catalysts that exhibit a current density of j > 0.5 A cm−2 at an over potential of ηO2 < 0.3 V with long-term stability.5,6
The exhaustive search for non-noble catalysts has identified that the mixed metal oxides of transition elements, spinel ferrites, oxy hydroxides (such as MnO2, NiOx, MnFe2O4, NiFe2O4, CoFe2O4, Co3O4, etc.) and perovskites oxides, as catalysts for OER reaction.2,6–10 Importantly, inspired by the oxygen evolution centres (OEC) of photosystem II, several investigations of low-cost manganese oxides for electrochemical water splitting revealed the efficiency of MnO2 and Mn3O4 for OER, particularly the α-Mn2O3 phase. While MnO2 is a potential bifunctional catalyst in both acidic and basic mediums, changing the pH from alkaline to neutral results in a sharp increase in the onset potential.11–14
Surprisingly, spinel ferrites and mixed metal oxides NPs supported with graphene or carbon nanotube potentially support the ORR in alkaline solutions.2,9 However, their performance also drops significantly on conventional carbon supports. But, Spinel ferrites (MxFe3−xO4) NPs supported with conventional carbon exhibit better ORR activity compared to commercial Pt NPs in alkaline solution.15
However, the spinel ferrites possessing significant electro catalytic activity and stability in high pH, the low electrical conductivity, restricts its application as a catalyst. In general, the practical applications of ferrites are mainly depends on electronic conductivity, results from thermal activation of electrons or positive holes along chains of neighbouring cations in the ionic lattice. Due to the bimetallic nature of spinels, the conductivity of ferrites can be tuned by substituting with a foreign metal, using different preparation conditions or using heat treatment.16–18
Previous studies of MnFe2O4 employed several synthesis methods, mostly to examine structural, electrical conductivity, magnetization and OER properties.19,20 Recently, Shouheng Sun and his co-works found that similar ferrite system exhibits the ORR activity in alkaline solution, comparable to that of Pt.15 The focus of our current study involves determining the catalytic behaviour of cerium substituted manganese ferrite, for OER in alkaline solution. We prepared manganese ferrite and cerium-substituted manganese ferrites by a combustion method.18 Electro catalytic analysis showed that Ce0.2MnFe1.8O4 is a promising catalyst for OER in alkaline solutions.
Fig. 1 SEM images of as prepared CexMnFe2−xO4(X=0.0,0.2) samples. SEM images of sintered samples at 1200 °C. |
Fig. 3 (a) XRD patterns of (i) MnFe2O4 (ii) Ce0.2MnFe1.8O4 (iii) Ce0.4MnFe1.6O4 (iv) Ce0.6MnFe1.4O4 (v) Ce0.8MnFe1.2O4. (b) The variation of lattice constant (a) with x of CexMnFe2−xO4. |
Fig. 2 shows the chemical composition of Pt/MnFe2O4 electrodes (sintering at 1200 °C, in the presence of air for 2 hours) estimated by energy-dispersive X-ray (EDX) analysis, which revealed that the catalysts contained only Mn, Fe and O atoms in a ratio of 1:2:4. The absence of Pt in the EDX profile shows that no diffusion of platinum occurs during the high temperature treatment (Fig. 5). Moreover, the EDS measurements confirm the presence of Ce in the MnFe2O4 lattice in the calculated ratio (Fig. 2). The XPS spectrum, shown in Fig. 4, was also used to verify the presence of Ce3+. The characterized peaks at 845 eV and 905 eV in the XPS spectra occurs due to the presence of Ce3+.22 Based on the above results, we conclude that Ce3+ is present in our samples.
Fig. 5 Cyclic voltammograms of Pt supported MnFe2O4 and CexMnFe2−xO4 electrodes at a potential scan of 10 mV s−1 in 1 M KOH at 25 °C. |
The morphology of the as-prepared MnFe2O4 and substituted MnFe2O4 are confirmed by the SEM images Fig. 1a. All the particles have discrete crystals with a cube shape. Each sample is then dispersed in isopropyl alcohol (IPA) and coated on Pt (1 cm × 1 cm) electrode. As shown in Fig. 1b, sintering the electrode to 1200 °C leads to the melting of discrete MnFe2O4 particles, resulting in the agglomeration of ferrite particles together to form a single, thin, film-like layer on the Pt surface.10 The sintering process favours the formation of a thin flim of spinel ferrites on Pt surface; however, no characterise peak for Pt is observable in XRD analysis, which accounts for absence of Pt diffusion into the spinel ferrites. Thin flim layer insulates the Pt surface completely. Also, no redox peak appeared for Pt on cyclic voltammetry studies of samples in KOH solution.
Fig. 5 shows the corresponding cyclic voltammograms of Pt/CexMnFe2−xO4 with x = 0.0, 0.2, 0.4, 0.6, 0.8 at a potential scan rate of 10 mV s−l in 1 M KOH solution at 25 °C. The electrode surface of the Pt/CexMnFe2−xO4 did not show any redox peaks either for Pt or for catalyst on cyclic voltammetry studies.19 It implies that ferrite does not undergo further oxidation. R. N. Singh and co-workers reported the similar behaviour for Mn-substituted ferrites coated on Pt and Ti substrates.19
However, with each subsequent CV cycle, OER current increases and OER onset potential shifted towards more negative region due to incorporation of Ce3+. It is often considered that the effect of Ce3+ incorporation on MnFe2O4 enhances the electrical conductivity of materials in turn influencing the OER activity.
The polarisation curves of Pt/CexMnFe2−xO4 electrodes were performed in 1 M KOH solution Fig. 7. The Pt/MnFe2O4 electrode prepared by current method shows an over potential of 360 mV for OER which is lower than the reported value of 580 mV for MnFe2O4 electrodes by Iwakura et al.23 Related to pure MnFe2O4 a negative shift in the OER onset potential is noticed with all Ce substituted electrodes. Among all, the Ce0.2MnFe1.8O4 exhibits a maximum negative shift of 60 mV for OER lower than MnFe2O4. Comparison of over potentials of the various catalysts required to achieve a current density of 10 mA cm−2 for CexMnFe2−xO4 are shown in Fig. 7. In a similar condition, Iwakura et al.23 observed an over potential of 440 mV and 580 mV for CoFe2O4 and MnFe2O4 in 1 M KOH respectively. Similarly, Orehotsky et al. obtained the same current density at the over potential of 340 mV for NiFe2O4 in 30 wt% KOH. In addition, our results are comparable to well investigated Cobalt-based electrocatalysts (Table 2).
Fig. 6 Electrochemical impedance spectra of catalysts as a function of Ce substitution, measured at 1.6 V during oxygen evolution. |
Fig. 7 Polarisation curves of CexMnFe2−xO4 sintered on Pt (with catalyst loading of 1 mg cm2) measured in 1 M KOH. |
S. no. | Catalyst | Tafel slope (b) mV per decade |
---|---|---|
1 | MnFe2−O4 | 38 |
2 | Ce0.2MnFe1.8O4 | 31 |
3 | Ce0.4MnFe1.6O4 | 33 |
4 | Ce0.6MnFe1.4O4 | 35 |
5 | Ce0.8MnFe1.2O4 | 37 |
S. no. | Electrodes | KOH concentration & temperature | OER potential (mV) at 10 mA cm−2 | Tafel slope value (b) mV per decade | Ref. |
---|---|---|---|---|---|
1 | Pt/Ce0.2MnFe1.8O4 | 1 M, 25 °C | 310 | 31 | This work |
2 | Ni/MnFe2O4 | 1 M, 25 °C | 300 | 36–42 | 19 |
3 | Pt/MnFe2O4 | 1 M, 25 °C | 580 | 110–115 | 23 |
4 | Pt/CoFe2O4 | 1 M, 25 °C | 440 | 110–115 | 23 |
5 | NiFe204 | 30 wt%, 25 °C | 340 | — | 19 |
6 | Ni/Co3O4/N-rmGO | 1 M, 25 °C | 310 | 67 | 2 |
7 | NiLaOx | 1 M, 25 °C | 400 | — | 3 |
A negative shift in the OER onset is associated with incorporation Ce3+ in the MnFe2O4 spinel lattice. As shown in Fig. 6 the electrochemical impedance plots of CexMnFe2−xO4 exhibit low charge transfer resistance compared to pure MnFe2O4 electrodes. Because of the reduced charge transfer resistance the electronic conductivity increases and favours in the enhancement of electro catalytic activity Ce3+ substituted MnFe2O4. Typically, manganese ferrite has inverse spinel structure in which 80% of Mn2+ occupies site A and 20% occupies site B. Site B can also be occupied by Fe2+ and Fe3+ ions.24,25 The electrical conductivity of ferrites are explained by Verwey-de Boer mechanism and the polaron effect. Although the electrical conduction in ferrites originates between Fe2+ and Fe3+ ions at site B, conduction mainly depends on the Fe2+ concentrations. In the sintering process, some lattice oxygen escapes from the oxides, causing an oxygen deficiency in the crystal lattice. Therefore, to balance the electrical charge created in the lattice unit, Fe3+ is reduced to Fe2+.24,26 The X-ray photoelectron spectroscopy (XPS) spectrum, shown in Fig. 4, confirms the existence of Fe2+ ions in the spinel lattice, and the presence of two main peaks with binding energies of 710.4 and 724.8 eV, which clearly proves that Fe2+ coexists with Fe3+.27,28 Moreover, it was found that the created Fe2+ ions prefer to occupy the octahedral B-sites. This reduction facilitates the formation of excess Fe2+ ions in the system (as shown in Fig. 4(b)) such that the hopping rate of electrons in ferrites increases which is the cause of their higher electronic conductivity.24,29 If it is the only cause then, we could expect a gradual increase in catalytic activity of substituted MnFe2O4 with higher concentrations of Ce3+. In contrast, the OER onset potential increased at concentrations higher than 0.2 M. Because, the replacement of rare earth ions forms grain boundaries in ferrites;30 if high molar concentration is used, then it leads to the formation of secondary phases (i.e. ABO3) on the materials.16,17 Most importantly, the secondary phase exists at concentration >0.4. Although the density of these grain boundaries is reduced upon treating the powders at high temperature, secondary phase are still present in the catalyst layer at high concentrations of cerium; which may hinder the mobility of charge carriers and increases the electrical resistivity.31 Similar behaviour was found in Al doped manganese ferrites.19 This coincides with the negative shift of OER onset potential.
As a result, of the cooperative effect32 between the electron-rich Mn2+ and Ce3+ centres in the cubic spinel lattice also enhances the OER activity of Ce0.2MnFe1.8O4.17 Nevertheless, both effects work simultaneously and increase the electro catalytic activity of the substituted manganese ferrite. It is clear from the above discussion that the electronic and structural properties of spinels play an important role in the OER.
The iR corrected Tafel slope values of the CexMnFe2−xO4 are calculated from the steady state polarization plots shown in Fig. 8 using the following equation:
η = a + blogj → | (1) |
We also obtained a range of Tafel slope values varying from 31–45 mV per decade for all oxides, which is similar to the previous reports (Table 1).19,23
The reaction order (p) of Ce0.2MnFe1.8O4 and pure MnFe2O4 with respect to [OH−] are calculated by the method described in ref. 18. The calculated p and b values are in good agreement with recently reported spinel ferrites such as MnFe2O4 (b = 36 mV per decade, p = 1.9), CrxMnFe2−xO4 (b = 35 mV per decade, p = 2) and CrxCoFe2−xO4 (b1 = 40–51, b2 = 60–89 mV per decade, p = 1.2–1.4). Besides, similar electrode kinetic parameters also exhibited by NiCo2O4 film (b = 42 mV per decade, p = 1.7) and CuCo2O4 (b = 40 mV per decade) sprayed on Ni.19,23
Based on the Tafel slope value and oxygen evolution potential, the sequential order of the electro catalytic activity of the electrodes is given as Ce0.2MnFe1.8O4 > Ce0.4MnFe1.6O4 > Ce0.6MnFe1.4O4 > Ce0.8MnFe1.2O4 > MnFe2O4.
In addition, the Tafel slope measurements attribute to the common mechanism for OER reaction as well:
S + OH− ↔ SOH + e− | (2) |
SOH + OH− ↔ SO− + H2O | (3) |
SO− ↔ SO + e− | (4) |
2SO ↔ 2S + O2 | (5) |
R. N. Singh and co-workers19 observed a tafel slope (b) value of 36–47 mV per decade in 1 M KOH at 25 °C, suggesting that MnxFe2−xO4 follows second order kinetics with respect to [OH−] and step (3) as the rate determining step (RDS) for OER. We also observed a similar trend in tafel slope values and reaction rate in case of MnFe2O4 (b = 38 mV per decade; p = 2.0) and Ce0.2MnFe1.8O4 (b = 31 mV per decade; p = 1.9). The long-term stability of Ce0.2MnFe1.8O4 under OER conditions was determined using chronoamperometric studies. The electrode is kept at a constant potential of 1.8 V for 10 h in 1 M KOH solution, while the current was measured as a function of time. Stability of the electrode found to be unchanged throughout the constant-potential studies with a constant current density of 130 mA cm−2 (Fig. 9).
Fig. 9 Chronoamperometric curves of MnFe2O4 and Ce0.2MnFe1.8O4 loaded on Pt under potential of 1.8 V. |
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