Introduction

The development of new functional oxide materials is important given their applications in different fields, such as solid oxide fuel cells (SOFCs) [1], oxygen sensors [2], and electrocatalysis [3, 4]. In particular, oxides with perovskite-related structure have received much attention due to their diverse range of properties [5, 6]. In addition, they can form several different structure types due to the possibility of oxygen removal [7] that is used to promote various properties, such as oxygen diffusivity and surface exchange kinetics at intermediate temperatures [8, 9]. There are also layered structures related to perovskites, such as Ruddlesden-Popper type structure, which has shown interesting properties, such as electrocatalysis [10]. In 1950s, Ruddlesden and Popper explored a series of oxide compounds with this type of layered structure, including those with the composition, Srn+1TinO3n+1 (n = 1, 2, and 3) [11, 12]. The Ruddlesden-Popper oxides can be represented by the general formula, An+1BnO3n+1 (n = 1, 2, 3…), where A represents a rare-earth or alkaline-earth metal and B is usually a transition metal. The general formula can also be expressed as (AO)·(ABO3)n, where n perovskite (ABO3) layers are sandwiched between AO rock-salt layers along the crystallographic c-axis. The simplest member of the series, A2BO4 (n = 1), adopts the K2NiF4 structure [13]. Other Ruddlesden-Popper phases can be realized when the thickness of perovskite stacks is increased to n = 2 (A3B2O7), n = 3 (A4B3O10), etc. As n increases from 1 to infinity (\(\infty\)), the structure transitions from two-dimensional RP to three-dimensional perovskite structure, with n = \(\infty\) representing a simple perovskite phase (ABO3) [10]. The crystal structure for a typical n = 2 Ruddlesden-Popper oxide (A3B2O7) is shown in Fig. 1b. Some examples of such compounds are Sr3Fe2O7 [14] and Sr3MnO7 [15]. It is possible to remove some of the oxygen atoms of the perovskite slabs, which will lower the coordination number of the transition metal. For example, partial reduction of Sr3Fe2O7 transforms the FeO6 octahedra into square pyramids [14, 16]. This indicates the structural versatility of Ruddlesden-Popper compounds, making them suitable for different applications, including electrocatalysis [10].

Fig. 1
figure 1

a Crystal structure of Sr3Mn2O6 and Sr3FeMnO6. The (Fe/Mn)O5 square-pyramids are shown in purple and Sr atoms are in orange. b Typical Ruddlesden-Popper structure is shown for comparison

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Among electrocatalytic processes, where Ruddlesden-Popper oxides can be utilized, is water-splitting [17], which involves two half-reactions, oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). However, both OER and HER are kinetically slow and require considerable overpotentials (η). To minimize the overpotential and increase the efficiency of these processes, various catalysts, including different oxides, have been used [18, 19]. There are multiple examples of Ruddlesden-Popper oxides as OER catalysts, such as La1.7Ca0.3Ni0.75Cu0.25O4 and LaSr3Fe3O10 [20, 21]. Similarly, the HER catalysis has been done using some Ruddlesden-Popper oxides, such as Sr2RuO4 [22]. Several factors have been investigated in an effort to achieve enhanced catalytic activity in these materials, such as A-site cation substitution in Sr2.6La0.4Fe2O7 [23], nanostructuring in La2NiO4+δ particles [24], variation of dimensionality (n) in Lan+1NinO3n+1 [25], and composite/nanohybrid formation in NiO–(La0.613Ca0.387)2NiO3.562 [26]. While Ruddlesden-Popper materials exhibiting either OER or HER activity have been studied earlier, bifunctional RPs that can catalyze both reactions are less common.

Following our recent work on oxide electrocatalysts for OER and HER [3, 27,28,29,30,31,32], in this article we demonstrate bifunctional electrocatalytic activity in oxygen-deficient RP oxides, Sr3Mn2O6, and Sr3FeMnO6. Structural studies on similar systems, Sr3Mn2O7-δ [33] and Sr3FeMnO7-δ [34], have been reported, where the degree of oxygen deficiency is lower (δ = 0–0.5). In our work, the careful control of synthesis conditions using inert atmosphere has resulted in a greater degree of oxygen-deficiency, to form Sr3Mn2O6 and Sr3FeMnO6, similar to the previously reported materials Sr3Co2O6 [35] and Sr3Fe2O6 [16]. In addition, we show the significant enhancement of electrocatalytic activity toward both OER and HER for Sr3FeMnO6 as compared to Sr3Mn2O6. The observation of bifunctional electrocatalytic properties for Sr3FeMnO6 is remarkable.

Experimental

Both materials, Sr3Mn2O6 and Sr3FeMnO6, were synthesized using the solid state synthesis method in argon atmosphere. The powders of precursor compounds SrCO3 (Aldrich, 99.9%), Fe2O3 (Alfa Aesar, 99.998%), and Mn2O3 (Alfa Aesar, 99.7%) were mixed and ground together using an agate mortar and pestle. The mixture was pressed into a pellet and heated in argon at 1250 °C for 24 h. The samples were allowed to cool down to room temperature under argon and were reground immediately after being removed from the furnace and refired under the same conditions, in argon at 1250 °C for 24 h, to ensure the formation of pure products. The heating and cooling rates were 100 °C/h in all cases. For comparison, Sr3FeMnO7 was also synthesized by a similar method, i.e., two heating runs at 1200 °C for 24 h each, but in air and using MnO2 as manganese precursor. The structures of products were determined by powder X-ray diffraction using Cu Kα1 radiation (λ = 1.54056 Å). The GSAS software [36] with EXPEGUI interface [37] was used for Rietveld refinements. High-resolution field-emission scanning electron microscopy (SEM) was used to study the microstructure of the materials. Iodometric titrations were performed under an argon atmosphere as reported earlier [38,39,40], by dissolving about 50 mg of the sample and excess KI (∼2 g) in 100 mL of 1 M HCl. A total of 5 mL of the solution was then pipetted out into a conical flask with 20 mL of water. The solution was titrated against 0.025 M Na2S2O3. Near the endpoint of the titration, 0.2 mL of a starch solution was added to act as an indicator. The iodometric titrations were done on three different samples for each compound, and the measurement on each sample was repeated three times to ensure reproducibility. Electrocatalytic activities were measured in a three-electrode electrochemical workstation. The working electrode was prepared by the dropcast method for which the catalyst ink was prepared as described previously [31, 41, 42], by mixing 35 mg of the sample with 20 µL of nafion and 7 mg of carbon black. Then, 7 mL of THF was added and stirred for few minutes, followed by sonication for 5 min. The catalyst ink (4 coats of 10 µL each) was loaded onto a glassy carbon electrode (GCE) with a diameter of 5 mm (area = 0.196 cm2) and was dried in air for 24 h. Before starting each measurement, the KOH electrolyte was purged by argon for at least 30 min. The GCE coated with catalyst ink and an Ag/AgCl electrode were used as working and reference electrodes, respectively. The counter electrode was a platinum electrode for OER and a graphite electrode for HER. Potentiostatic electrochemical impedance spectroscopy in the frequency range of 100 kHz to 1 Hz was used to record the resistance (R) before each electrocatalytic experiment. Then, iR-corrected potential was converted to the potential versus reversible hydrogen electrode (RHE) according to the Nernst equation: ERHE = EAg/AgCl + 0.059 pH + E°Ag/AgCl.

Here, E°Ag/AgCl = 0.21 V for 3 M NaCl and 0.197 V for saturated KCl, used for OER and HER measurements, respectively. The stability tests of catalysts were performed using chronopotentiometry. A two-electrode setup was used for chronopotentiometry, as described in the literature [43]. Briefly, the electrodes were fabricated by loading 100 µL of the catalyst ink, described above, on a 1-cm2 nickel foam and dried overnight to obtain a total mass loading of ~ 1 mg/cm2. Two Ni foam electrodes connected to gold leads and gold wires were sandwiched together and separated by a glass fiber filter paper to prevent short-circuiting and crossover. X-ray photoelectron spectroscopy (XPS) data were obtained at room temperature using Al Kα radiation (1486.7 eV).

Results and discussion

Structural characterization

Since both materials were synthesized under argon atmosphere using oxides of trivalent manganese and iron, the ideal formulas should be Sr3Mn2O6 and Sr3FeMnO6, to maintain the charge neutrality. Iodometric titrations were used to reliably determine the oxygen content. These experiments indicate oxygen stoichiometries of Sr3Mn2O6.04(2) and Sr3FeMnO5.96(2). Rietveld refinements using powder X-ray diffraction data show that the two materials are isostructural and have a tetragonal structure with space group I4/mmm, similar to that reported for several analogous oxygen-deficient Ruddlesden-Popper systems, such as Sr3Co2O6 [35], Sr3Fe2O6 [16], and La1.9Ca1.1Cu2O6 [44].

Figure 2 shows the Rietveld refinement profiles for both compounds, and Tables 1 and 2 list the refined structural parameters. Compared to a typical Ruddlesden-Popper structure (Fig. 1b) that has the general formula A3B2O7, these two materials feature an oxygen-deficiency (Fig. 1a), leading to the formula A3B2O6. A typical Ruddlesden-Popper structure with I4/mmm space group has oxygen atoms on three crystallographic sites, 0,½,z; 0,0,z; and 0,0,0. In oxygen-deficient systems, the latter site is commonly vacant or partially occupied [16, 35, 44, 45]. As a consequence of this, the coordination geometry around Fe and Mn changes from octahedral to square-pyramidal. This leads to 2-dimensional layers of corner-sharing (Fe/Mn)O5 square-pyramids, which alternate in orientation and are separated by Sr atoms (orange spheres in Fig. 1a).

Fig. 2
figure 2

Rietveld refinement profile for powder X-ray diffraction data of a Sr3Mn2O6 and b Sr3FeMnO6. Black crosses, red line, vertical green tick marks, and lower magenta line represent experimental data, the model, peak positions, and difference plot, respectively

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Table 1 Refined structural parameters for Sr3Mn2O6 using powder X-ray diffraction data. Space group: I4/mmm, a = 3.84393 (8) Å, b = 3.84393 (8) Å, c = 20.2105 (5) Å, Rp = 0.0558, wRp = 0.0785, χ2 = 2.589
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Table 2 Refined structural parameters for Sr3FeMnO6 using powder X-ray diffraction data. Space group: I4/mmm, a = 3.8446 (4) Å, b = 3.8446 (4) Å, c = 20.123 (2) Å, Rp = 0.0399, wRp = 0.0512, χ2 = 1.596
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Scanning electron microscopy (SEM) was used to investigate the microstructure of both materials. As observed in Fig. 3, the SEM micrographs show that grain sizes are larger for Sr3FeMnO6 as compared with those of Sr3Mn2O6.

Fig. 3
figure 3

Scanning electron microscopy images for a Sr3Mn2O6 and b Sr3FeMnO6

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Electrocatalytic activity for hydrogen-evolution reaction

The electrocatalytic activity for hydrogen-evolution reaction (HER) was studied for both compounds in 1 M KOH, as commonly utilized for HER [46, 47]. The onset potential and the overpotential at 10 mA/cm2 are the two most important parameters for evaluation of the electrocatalytic activity for HER during heterogeneous catalysis [48,49,50]. The onset potential corresponds to the start of the Faradaic process, where a rise of current begins. An onset potential of almost 0.0 V versus RHE is observed for the benchmark Pt/C (20 wt. % Pt) catalyst [51, 52]. The HER polarization curves of Sr3Mn2O6 and Sr3FeMnO6 are shown in Fig. 4a, where the respective onset potential values of ~ − 0.38 V and − 0.30 V are observed. Similarly, the corresponding overpotential (η10) values are − 0.59 V and − 0.45 V at − 10 mA/cm2 (Fig. 4a) for both catalysts. The best performing catalyst, Sr3FeMnO6, is also very stable, as shown by the chronopotentiometry data in the inset of Fig. 4a. While there are some oxide, such as PrBaCo2O5+δ (η10 = − 0.356 V) [53] and CaSrFeMnO6-δ (η10 = − 0.39 V) [29] which show better catalytic performance, the overpotential values of the catalysts in this work are better than those of some other oxide catalysts in alkaline electrolyte, such as the perovskite oxide Ba(Fe0.7Ta0.3)O3-δ (η10 = ~ − 0.70 V) [54].

Fig. 4
figure 4

a HER polarization curves in 1 M KOH. The inset shows chronopotentiometry data for Sr3FeMnO6. b The Tafel plot showing Tafel slopes for both compounds

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The evaluation of the reaction kinetics is done using Tafel slopes. The Tafel equation, η = a + b log j (where η is overpotential and j is current density), is utilized to determine the Tafel slope from the linear fit to the plot of η versus log j (Fig. 4b), where the data from the curved region of the polarization curve are considered [30, 55, 56]. We note that the cathodic and anodic scans in polarization curves of these materials are very close to each other. Nevertheless, the average of both scans is commonly used for evaluation of the electrocatalytic activity [19, 41, 57,58,59]. Faster electron transfer during the HER process is indicated by a smaller value of the Tafel slope. As shown in Fig. 4b, Tafel slopes for Sr3Mn2O6 and Sr3FeMnO6 are determined to be 240 mV/dec and 215 mV/dec, respectively. These are in the same range as the values reported for some other HER catalysts, such as MgCr2O4 (217.51 mV/dec) [60] and CuO (243 mV/dec) [61]. The smaller Tafel slope for Sr3FeMnO6 compared to that of Sr3FeMnO6 indicates faster reaction kinetics and is consistent with the higher electrocatalytic activity of the former material.

Electrocatalytic activity for oxygen-evolution reaction

The electrocatalytic activity for oxygen-evolution reaction (OER) was studied for both compounds in 0.1 M KOH, as commonly used for OER [62,63,64]. Figure 5a shows the polarization curves from cyclic voltammetry for both materials. Similar to the HER, the onset potential, overpotential at 10 mA cm−2, and Tafel slope were evaluated. The corresponding onset potentials for Sr3FeMnO6 and Sr3Mn2O6 are 1.51 V and 1.63 V, respectively. The onset potential of Sr3FeMnO6 is close to that of the well-known OER catalyst Ba0.5Sr0.5Co0.8Fe0.2O6-δ (BSCF), ⁓1.5 V [65, 66]. The OER overpotential (η10) is evaluated as the potential, beyond the ideal 1.23 V, which is needed to deliver the current density of 10 mAcm−2 [67, 68]. Sr3FeMnO6 has an overpotential of η10 = 0.59 V vs RHE, while the data from Sr3Mn2O6 does not even reach the current density of 10 mA cm−2 (Fig. 5a). The overpotential of Sr3FeMnO6 is not as low as some other electrocatalysts, such as RuO2 (η10 = 0.42 V) [69] and BSCF (η10 = 0.51 V) [70]. However, it is comparable to those of several other oxide catalysts, such as CoFe2O4 (η10 = 0.59 V), Co3O4 (η10 = 0.60 V) [71], and Ca2FeMnO6-δ (η10 = 0.56 V) [29].

Fig. 5
figure 5

a OER polarization curves in 0.1 M KOH. The inset shows chronopotentiometry data for Sr3FeMnO6. b The Tafel plot showing Tafel slopes for both compounds

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The Tafel slopes for OER were obtained from the linear fit of the plot of η versus log j [30, 55, 56] to evaluate the reaction kinetics, which is related to the electron and mass transport [72, 73]. The Tafel slope values of 187 mV/dec and 95 mV/dec (Fig. 5b) were determined for Sr3Mn2O6 and Sr3FeMnO6, respectively. This is consistent with the enhanced OER activity of the latter material. Moreover, Sr3FeMnO6 is very stable under OER conditions, as shown by the chronopotentiometry data in the inset of Fig. 5a.

We have also calculated electrochemically active surface area (ECSA) for these two materials. The ECSA is estimated from double-layer capacitance, Cdl, in the non-Faradic region [74]. In this region, the current is considered to originate mainly from the electrical double layer charge and discharge, which does not have an electron transfer contribution [74, 75]. The ESCA is calculated from the relation ECSA = Cdl/Cs [74, 76], where Cs is specific capacitance [74, 76]. Therefore, ECSA is directly proportional to Cdl. Hence, it is common practice to use the value of Cdl as an indication of the magnitude of ECSA [77,78,79]. The Cdl value can be obtained from the equation Cdl = javerage/ν [80, 81], where javerage is the average of the absolute values of anodic and cathodic current densities in non-Faradic region. The slope of the plot of javerage versus ν gives the Cdl value.

Figure 6a and b show the CVs in non-Faradic region, measured at the different scan rates of 10, 20, 40, and 80 mV/s, from which double-layer capacitance (Cdl) is obtained. Figure 6c shows comparative plots of javerage versus ν, which indicate the Cdl values of 494 µF for Sr3FeMnO6 and 194 µF for Sr3Mn2O6. This is consistent with the greater electrocatalytic activity of the former material, which also showed lower values of overpotential and faster Tafel kinetics compared to the latter compound.

Fig. 6
figure 6

a, b Cyclic voltammetry data in the non-Faradaic region in 1 M KOH. c Plot of javerage versus scan rate. The double-layer capacitance (Cdl) is obtained as the slope of the line of best fit

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It is possible that the enhanced electrocatalytic properties of Sr3FeMnO6 compared with Sr3Mn2O6 are related to the electronegativity effect, given the significantly greater electronegativity of Fe relative to Mn. It has been shown previously that the increase in electronegativity results in the lowering of the energy of d orbitals [82]. The lowering of the metal d-band in oxides can result in an improvement of the overlap between metal d and oxygen p bands, leading to enhanced covalency and hybridization [83, 84], which serve to boost the electrocatalytic activity [83, 84].

Additionally, the XPS data (Fig. 7) are consistent with trivalent Fe and Mn for both compounds, in line with previous reports [85, 86]. This is expected, given the oxygen stoichiometry, i.e., 6 oxygen per formula unit, obtained from iodometric titrations. To highlight the effect of the oxygen stoichiometry, we conducted further HER and OER experiments using Sr3FeMnO7, which is a Ruddlesden-Popper system without oxygen-deficiency. The activity of Sr3FeMnO7 is compared with Sr3FeMnO6 in Fig. 8, indicating the enhanced performance of the latter due to the presence of oxygen-deficiency.

Fig. 7
figure 7

X-ray photo electron spectroscopy data. a and b show the Fe and Mn spectra for Sr3FeMnO6, and c shows the Mn spectrum for Sr3Mn2O6

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Fig. 8
figure 8

Comparison of a HER and b OER activity of Sr3FeMnO6 with the parent compound Sr3FeMnO7

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Conclusions

The 2-dimensional oxides Sr3Mn2O6 and Sr3FeMnO6 have been synthesized and their structural and electrocatalytic properties have been investigated. Their structures consist of square-pyramidal (Fe/Mn)O5 units that form 2D layers, separated by strontium ions. There is a persistent enhancement of electrocatalytic properties for Sr3FeMnO6 as compared to Sr3Mn2O6. The former shows enhanced activity toward both OER and HER, exhibiting improved overpotential, Tafel kinetics, and electrochemically active surface area. The significantly enhanced electrocatalytic performance is attributed to the effect of electronegativity, given the considerably higher electronegativity of Fe compared with Mn, which can lead to the lowering of the d-band energy, resulting in better overlap with oxygen p bands.