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Article

Co- and Sn-Doped YMnO3 Perovskites for Electrocatalytic Water-Splitting and Photocatalytic Pollutant Degradation

by
Paula Sfirloaga
1,2,
Szabolcs Bognár
3,
Bogdan-Ovidiu Taranu
1,*,
Paulina Vlazan
1,
Maria Poienar
4 and
Daniela Šojić Merkulov
3
1
National Institute of Research and Development for Electrochemistry and Condensed Matter, Dr. A. Paunescu Podeanu Street, No. 144, 300569 Timisoara, Romania
2
Spin-Off Nattive-Senz SRL, Dr. A.P. Podeanu Street, No. 144, 300569 Timisoara, Romania
3
Department of Chemistry, Biochemistry and Environmental Protection, University of Novi Sad Faculty of Sciences, Trg Dositeja Obradovića 3, 21000 Novi Sad, Serbia
4
Institute for Advanced Environmental Research, West University of Timisoara (ICAM-WUT), Oituz Str., No. 4, 300086 Timisoara, Romania
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(4), 475; https://doi.org/10.3390/coatings15040475
Submission received: 25 February 2025 / Revised: 9 April 2025 / Accepted: 15 April 2025 / Published: 16 April 2025
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Abstract

:
The current environmental pollution and energy crises are global concerns that must be addressed. Considering this background, three perovskites (YMnO3, Co-doped YMnO3, and Sn-doped YMnO3) were synthesized via a sol–gel method and characterized by XRD, SEM, and EDX. Their water-splitting electrocatalytic activity was evaluated in a strongly alkaline medium. The highest activity was observed during hydrogen evolution reaction (HER) experiments on a glassy carbon electrode coated with a catalyst ink containing the Co-doped material. Initially, the HER overpotential value at −10 mA/cm2 was 0.59 V, and the Tafel slope was 115 mV/dec. Following a chronoamperometric stability test, the overpotential became 0.46 V and the Tafel slope 119 mV/dec. The higher HER activity of the modified electrode is ascribed to a higher number of catalytic sites exposed to the electrolyte solution and the presence of Carbon Black. The photocatalytic activity of the perovskites was investigated as well, using different experimental conditions and simulated solar irradiation. The results show that the photocatalytic activity can be improved by doping, and the highest removal efficiency is achieved in the presence of the Co-doped YMnO3 when ~60% of 17α-ethynylestradiol is degraded. Furthermore, the initial pH has no favorable effect on the degradation efficiency. The reusability of Co-doped YMnO3 was also tested and minimal activity loss was found after three photocatalytic cycles.

Graphical Abstract

1. Introduction

The issues that humanity is presently facing include the continuously increasing global energy demand, the rapid depletion of fossil fuels, and the negative impact that using non-renewable energy sources has on the environment [1]. These problems have prompted scientists to focus on environmentally friendly renewable energy sources as an attempt to replace the reliance on oil, coal, and natural gas to generate electricity with a reliance on solar, tidal, and wave power [2,3]. However, these latter sources cannot be relied upon uninterruptedly, as they manifest substantial spatial and temporal variability [4]. The efforts of the scientific community have revealed hydrogen, which has been termed the “fuel of the future”, as a potential solution to the above-mentioned issues [5]. Once generated from renewable resources, environmentally compatible H2 can be exploited to obtain electricity, or it can be stored for later use [6]. In terms of available technologies, water electrolysis is considered the only environmentally friendly approach for the immediate and large-scale production of H2 [7]. It allows the use of renewable energy sources to obtain the electricity needed to split the water molecule, and it requires water, which is abundantly available [8]. The water-splitting process involves two half-cell reactions—the H2 and O2 evolution reactions (HER and OER)—occurring at the cathode and anode, respectively. The unfolding of the water-splitting reaction depends on the pH of the electrolyte solution [9]. For an alkaline electrolyte, the half-reactions occurring at the anode (R1) and cathode (R2), together with the full reaction (R3), are presented below [10].
2OH− → H2O + 1/2O2 + 2e−
2H2O + 2e− → H2 + 2OH−
H2O → H2 + 1/2O2
Low overpotential values have to be ensured to streamline this process, which requires materials with high OER and HER electrocatalytic activity [11]. The state-of-the-art electrocatalysts are Pt-based materials for HER and Ru and Ir oxides for OER. However, because noble metals are scarce and costly, they cannot be employed for large-scale water-splitting [12]. To resolve this issue, substantial research is being carried out to identify low-cost, earth-abundant, stable, and highly active water-splitting catalysts [13]. As a result, many studies have been reported thus far concerning the synthesis and testing of materials with electrocatalytic water-splitting properties aimed at improving the performance of water electrolyzers [14,15,16,17]. Because of the simplicity of alkaline water electrolyzers and the numerous electrocatalysts identified for them [18,19,20,21,22], this type of device has received particular attention [23,24].
Perovskites are a class of materials shown to display electrocatalytic water-splitting activity when used in the abovementioned type of electrolyzer [25,26]. The crystal structure of a perovskite has the general stoichiometry ABX3. The A- and B-sites are occupied by cations, while an anion is located in the X-site [27]. In the case of perovskite oxides, their structure follows the ABO3 formula with rare or alkaline Earth metals usually in the A-site and transition metals in the B-site [28,29]. Because of the various cations employed in synthesizing perovskites, and since these ions can be partially substituted for each site, many perovskites are developed via metal combinations [30]. This compositional flexibility, their high-tolerance crystal structures, adjustable electronic properties, proven activity for water-splitting [31], as well as relatively low cost, place perovskites among the promising electrocatalysts for alkaline water-splitting [32,33,34].
Some perovskite catalysts have been shown to achieve comparable and even lower onset-potential and Tafel slope values than the benchmark Pt/C and IrO2 electrocatalysts [35]. Furthermore, it has been reported that the overall water-splitting current density reached by an electrolyzer with a perovskite-based anode and Pt/C cathode at a 2 V cell voltage was as high as the Ir-based anode and Pt/C cathode pair [36].
The study reported by Ji et al. [37] is an example meant to outline the current relevance of perovskite oxides for the water-splitting domain. The researchers employed a simple and effective strategy for enhancing the electrocatalytic properties of the O2 evolution reaction of a perovskite oxide via A-site acceptor doping. The electrocatalytic activity and stability experiments performed in 1 M KOH solution revealed the optimum composition. Its activity was comparable to other electrocatalysts found in the scientific literature, but it also exhibited exceptional long-term stability.
When the doping strategy is applied to perovskites possessing electrocatalytic water-splitting properties, the aim is to improve their intrinsic catalytic activity. The advantages of this approach, such as easy synthesis and an electron transfer that regulates H2 adsorption and enhances the HER kinetics, are accompanied by disadvantages, such as the heterogeneous distribution of the doping atom and difficulties with identifying the reaction mechanism and model construction [38]. However, reported studies have shown that doping can have a highly desirable impact on the water-splitting electrocatalytic properties of catalysts despite the aforementioned drawbacks [38,39].
The applicative potential of perovskites is not limited to the electrocatalysis field but extends to other domains, such as photocatalysis. Of particular importance to the present work is the use of perovskite materials in the photocatalytic removal of organic pollutants. Organic pollutants, such as pharmaceutically active ingredients, cause serious environmental issues, mainly impacting water resources [40]. One example is the synthetic steroid estrogen 17α-ethynylestradiol, widely utilized in the curing of various disorders or illnesses. In humans, the presence of this pollutant is linked to conditions like prostate cancer and reduced sperm production [41].
The prevalence of organic contaminants in aquatic environments, coupled with concerns about the contamination of drinking water sources, has spurred significant interest in developing various techniques to mitigate water pollution [40]. Heterogeneous photocatalysis is an advanced, powerful, and sustainable oxidation technique for organics removal. It consists in harvesting the freely available sunlight energy in the presence of adequate photocatalysts [42]. In recent years, perovskite materials have garnered considerable attention as promising photocatalysts. This is primarily attributed to their distinctive optoelectronic characteristics and remarkable light-harvesting capabilities [40]. For example, co-doped Ce–Mn perovskite materials were efficiently applied as photocatalysts to remove tetracycline (TC) [43]. In addition, NiZrO3 perovskite was successfully produced by the solvent-free ball-mill technique, and it was also employed in the degradation of TC using a photo-Fenton-like process [44]. In another study, CuAl2O4-modified BaSnO3 perovskite nanoplatelets, prepared using a sol–gel-based process, were examined in the removal of atrazine [45].
Herein, three perovskite materials synthesized using a sol–gel method—YMnO3, Sn-doped YMnO3, and Co-doped YMnO3—are investigated in terms of their water-splitting electrocatalytic activity and their photocatalytic activity for the degradation of the organic pollutant 17α-ethynylestradiol from an aqueous environment. The electrochemical study is a contribution to the ongoing efforts aimed at identifying new materials with applicability in alkaline water electrolysis. Out of the evaluated modified electrodes, the sample obtained by coating the GC substrate with a catalyst ink containing the Co-doped perovskite was the most electrocatalytically active for the HER. The photocatalytic activity experiments were carried out under simulated solar irradiation, and the highest removal efficiency, when ~60% of the pollutant is degraded, is achieved in the presence of YMO-Co. Catalyst reusability was performed as well.

2. Materials and Methods

2.1. Materials and Reagents

The three perovskite materials—YMnO3 (YMO), Sn-doped YMnO3 (YMO-Sn), and Co-doped YMnO3 (YMO-Co)—were synthesized using the citrate precursor sol–gel route. The following precursors were used: manganese (II) nitrate tetrahydrate, Mn(NO3)2·4H2O, ≥98% (Merck, Darmstadt, Germany); yttrium (III) acetate hydrate, Y(CH3COO)3·xH2O, 99.9%, (Sigma-Aldrich, Saint Louis, MO, USA); cobalt (II) nitrate hexahydrate, Co(NO3)2·6H2O, 99.9% (Sigma-Aldrich), as dopant; and tin (II) chloride dihydrate, SnCl2·2H2O (Sigma-Aldrich), also as dopant. In the first stage, the precursors of Mn2+ and Y3+ in a molar ratio of 1:1 were dissolved in 40 cm3 of double-distilled water. Subsequently, 0.01 mmol of the cobalt nitrate dopant or the tin chloride dopant (Co2+ or Sn2+) was added. Citric acid monohydrate, C6H8O7·H2O, 99.9% (Merck), and ethylene glycol, C2H6O2, ≥99.5% (Sigma-Aldrich), were added to the obtained solutions. The molar ratio between total metal ions, citric acid, and ethylene glycol was 1:1:2. The solutions were stirred on a magnetic stirrer with a heating plate. The initial working temperature was 80 °C. After 30 min, the temperature was increased to 120 °C to ensure complete solvent evaporation and the obtaining of gels. Subsequently, the gels were dried in an oven at 120 °C for 4 h. Each dry gel was ground in an agate mortar, and the obtained powder was calcined at 800 °C into a muffle furnace with a heating rate of 5 °C/min, in air, for 6 h. The crystal structure and phase composition of the samples were determined using an X’Pert PRO MPD diffractometer (PANalytical, Almelo, The Netherlands), with Cu-Kα radiations (λ = 0.15406 nm) in a 2θ range from 15° to 70° and a scan rate of 2°/min. The surface morphology characterization and elemental analysis of the samples were performed using a scanning electron microscope, Inspect S type (FEI Company, Hillsboro, OR, USA), equipped with an EDX module. A Raman spectroscopy analysis was carried out on the most catalytically active sample identified during the electrochemical experiments with a Shamrock 500i Spectrograph (Andor, Essex, UK) that was part of a MultiView-2000 system (Nanonics Imaging Ltd., Jerusalem, Israel) and used a 514 nm laser as the excitation source.
The materials and reagents utilized during the electrochemical experiments are described in what follows. Glassy carbon (GC) pellets (Andreescu Labor & Soft SRL, Bucharest, Romania) were employed as substrates in the modified electrode manufacturing process since carbon materials possess properties desirable in a wide range of applications [46]. Their surface was covered with inks containing the perovskite materials and Nafion solution and/or Carbon Black. Nafion® 117 solution (Sigma-Aldrich) served as a binder to ensure the adherence of the catalyst ink to the GC surface, while Carbon Black—Vulcan XC 72 (Fuell Cell Store, Bryan, TX, USA) was selected to improve the charge transfer at the interface between the modified electrode and the electrolyte solution. KOH and KNO3 (Merck), K3[Fe(CN)6] (Sigma-Aldrich), C2H5OH, and C3H6O (Chimreactiv, Bucharest, Romania) were also used during the investigation. All aqueous solutions were prepared with laboratory-produced double-distilled water.
The selected pollutant in investigating the photocatalytic activity of YMO, YMO-Sn, and YMO-Co was 17α-ethynylestradiol (CAS No 57–63–6, C20H24O2, Mr = 296.40 g/mol, >98.0% VETERNAL™, analytical standard, Sigma-Aldrich). To determine the photocatalytic efficiency, the samples were analyzed after irradiation via liquid chromatography with the following components of mobile phase: acetonitrile (99.9%, Sigma-Aldrich) and orthophosphoric acid (85%, pro analysis, Sigma-Aldrich). The initial pH values were set using 0.1 M HClO4 (70% (w/w), >99.99%, Sigma-Aldrich) and 0.1 M NaOH (pro analysis, MOSS & HeMOSS, Belgrade, Serbia).

2.2. Electrochemical Setup and Testing

The electrochemical setup consisted of a Voltalab PGZ 402 potentiostat (Radiometer Analytical, Lyon, France), three electrodes, and a glass cell. A Pt plate with a 0.8 cm2 geometrical surface (Sg) served as the auxiliary electrode, and an Ag/AgCl (sat. KCl) electrode was utilized as reference. The GC pellets (bare or modified) were used as the working electrode. Before the experiments, they were inserted into a polyamide-based support that ensured a fixed surface exposed to the electrolyte (Sg = 0.28 cm2).
The electrode manufacturing procedure included the following steps: the GC pellets were cleaned with detergent and water and subsequently rinsed with double-distilled water, acetone, and ethanol; after drying at 23 ± 2 °C, a 10 µL volume of catalyst ink was drop-casted on their surface and a second drying stage (of 24 h at 23 ± 2 °C) led to the obtaining of the modified electrodes. Afterwards, the samples were inserted into the inert support to ensure that during the experiments the modified surface came into contact with the electrolyte solution. The catalyst inks were prepared based on a previously reported protocol [47]. Some inks were obtained by adding 5 mg catalyst (YMO, YMO-Sn, or YMO-Co) and 50 µL Nafion solution to 0.45 cm3 C2H5OH, followed by ultrasonication for 30 min. Other inks were formulated by adding 5 mg catalyst, 5 mg Carbon Black, and 50 µL Nafion solution to 0.9 cm3 C2H5OH. The same ultrasonication treatment was applied. The tags for identifying the electrodes employed in the study, together with the ink compositions utilized to obtain them, are presented in Table 1.
All water-splitting experiments were carried out in 1 M KOH electrolyte solution. All iR-corrected polarization curves were recorded at the scan rate v = 5 mV/s, in agreement with previously published work [48]. The electrolyte solution was thoroughly degassed by bubbling N2 in the electrolyte solution before each HER experiment. Equation (1) was used to express the electrochemical potential (E) values in terms of the Reversible Hydrogen Electrode (RHE) [49]. Unless differently indicated, the current density (i) values refer to the geometric current density. Equation (2) was employed to determine the HER overpotential (ηHER). The Tafel slope was calculated with Equation (3) [50], and Equation (4) is the Randles–Sevcik equation [51] utilized to estimate the diffusion coefficient of ferricyanide ions and the electroactive surface area (EASA) for the modified electrode identified during water-splitting experiments as the most electrocatalytically active.
E R H E = E A g / A g C l ( s a t . K C l ) + 0.059 × p H + 0.197
η H E R = E R H E − 0
η = b × log ⁡ i + a
I p = 2.69 × 10 5 × n 3 / 2 × A × D 1 / 2 × C × v 1 / 2
where: ERHE = reversible hydrogen electrode potential (V); EAg/AgCl(sat. KCl) = potential vs. the Ag/AgCl (sat. KCl) reference electrode (V); ηHER = hydrogen evolution overpotential (V); η = overpotential (V); i = current density (mA/cm2); b = Tafel slope; Ip = peak current (A); n = the number of electrons involved in the redox process at T = 298 K (n = 1 for the ferrocyanide/ferricyanide redox system); A = working electrode surface (cm2); D = diffusion coefficient of the electroactive species, with a theoretical value for the ferrocyanide/ferricyanide system of 6.7 × 10−6 cm2/s [52,53]; C = concentration of the electroactive species (M) and v = scan rate (V/s).

2.3. Sample Preparation and Photocatalytic Activity Experiments

The photocatalytic experiments were conducted in eight photochemical quartz glass cells (total volume of about 100 cm3) placed in a factory-produced batch photoreactor (Topiton-V, Topiton, Xi’an, China) shown in Figure 1. A Xe lamp was employed as a source of SSI. The utilized cells were placed in a circle around the Xe lamp to achieve equal exposure to the irradiation source. The lamp was placed in a quartz cold trap equipped with water-circulating jackets and linked to a cooler to ensure constant temperature inside the photoreactor (25 ± 3.0 °C). The UV energy fluxes were measured using a portable photo-radiometer, model HD2102.2 (Delta Ohm, Padova, Italy). The radiometer was equipped with two sensors: one for UV (LP 471 UVA, spectral range 315–400 nm) and one for visible (LP 471 RAD, spectral range 400–1050 nm) radiation. The photon flux for the 300 W Xe lamp (Toption-V) was 57.52 W/m2 for the UVA region and 320.85 W/m2 for the visible radiation.
A total of 50 cm3 17α-ethynylestradiol (EE2) was measured in the cells. After that, 25 mg of each newly synthesized perovskite was added to the solution (γ = 0.5 mg/cm3). Prior to the photocatalytic experiments, all samples were stirred for 30 min in the dark to achieve uniform distribution of the photocatalyst particles and reach the adsorption–desorption equilibrium. During the photocatalytic degradation, samples were taken in the time interval of 0 to 120 min (0, 5, 10, 15, 30, 45, 60, and 120 min) under SSI. All samples were filtered through Millipore (Millex-GV, 0.22 µm) membrane filters and subsequently used to study their photocatalytic degradation efficiency. There were experiments carried out under natural pH (9), as well as with different initial values (11 and 7) set by a portable pH meter (ProfiLine pH 3110, WTW, London, UK).
The reutilization study of YMO-Co in the photocatalytic removal of EE2 from ultrapure water (pH = 9) under SSI consisted of three consecutive runs, each lasting 120 min. After each run, the suspension was kept overnight in the dark to ensure YMO-Co precipitation. After supernatant removal, the photocatalyst was oven-dried for 2 h at 60 °C. The powder was subsequently used in the photocatalytic degradation of a fresh amount of EE2 solution under identical experimental conditions.
To study the photocatalytic degradation efficiency of EE2 under different experimental conditions, a high-pressure liquid chromatograph with a fluorescence detector (UFLC-RF, Shimadzu Nexera, Tokyo, Japan) and equipped with InertSustain® C18 column (150 mm × 4.6 mm, i.d., particle size 5 µm, 40 °C) was used. The excitation and emission wavelengths of EE2 were 220 nm and 310 nm, respectively. The prepared samples (10 µL) were injected and analyzed. The mobile phase (flow rate 0.7 cm3/min) was a mixture of acetonitrile and water (80:20, v/v, pH 2.56), while the water was acidified with phosphoric acid so that the acid’s mass fraction was 0.1%.
The ultrapure water for the preparation of all solutions used in the photocatalytic activity experiments was provided by an Adrona water purification system (LPP Equipment AG, Uster, Switzerland).

3. Results and Discussion

3.1. XRD Analysis

Figure 2 shows XRD patterns of the YMO, YMO-Sn, and YMO-Co perovskites synthesized by the sol–gel method. The XRD diffraction spectra reveal well-defined peaks, indicating that the samples are well crystallized. The X-ray diffraction data confirm the YMnO3 phase crystallized in a hexagonal P63cm structure, according to reference pattern JCPDS: 00-025-1079, and also outline some peaks of a secondary phase of YMn2O5, in accordance with JCPDS: 00-034-0667. Crystallographic data were obtained by Rietveld refinement using the PANalytical X’Pert HighScore Plus software (version 4.0) and are presented in Table 2.
Based on the analysis of the X-ray diffraction results, the structural parameters show a slight decrease in the unit cell volumes and crystallite size for the doped samples.
The formation of a secondary phase in the case of the perovskite material doped with Sn but not in the case of the Co-doped sample can be explained via the larger ionic radius of Sn compared to Co, as well as the fact that Sn exhibits multiple oxidation states.
Profile matching refinement (LeBail fit) [54] has been performed for the perovskite materials using the FullProf software from the FullProf Suite (version 5.10) [55]. Figure 3 shows the observed, calculated, and difference XRD profiles for the perovskites. Unfortunately, the quality of the X-ray laboratory data and the presence of a small quantity level of impurities did not allow the performing of “proper” Rietveld refinements. Rietveld refinements with one phase (YMnO3) and three phases—YMnO3, YMn2O5 (orthorhombic), and Mn3O4 (P m a b)—have been tested, but they have been unsuccessful. The calculated unit cell parameters along the agreement factors from the LeBail fit are specified in Table 3. Considering the previously presented Rietveld refinement (carried out with the HighScore Plus software), it can be said that both sets of obtained unit cell values are in agreement. The main conclusion of the X-ray diffraction study is that the data indicate all samples are well crystallized within the main phase—YMnO3 (P63mc space group).

3.2. SEM Micrographs and EDX Analysis

Scanning electron microscopy was employed to analyze the synthesized perovskites. The recorded micrographs are presented in Figure 4. As can be seen, the surface morphology varies from one material to another. The particles of the undoped perovskite agglomerated into asymmetric micrometric formations. Doping with Co leads to a sponge-like morphology, while uniformly dispersed particles are observed in the case of YMO-Sn.
The EDX spectra for Co-doped and Sn-doped YMnO3 are presented in Figure 5. Based on spectra analysis and quantification, the elements making up the synthesized materials are oxygen, magnesium, yttrium, and in the case of doped perovskites, cobalt or tin. The elemental quantification shows that the stoichiometry of the ABO3-type compounds is preserved.

3.3. Electrochemical Studies

In the first part of the electrochemical investigation, anodic polarization curves were recorded in 1 M KOH electrolyte solution for all modified electrodes described in Table 1 in order to evaluate their OER electrocatalytic activity. However, the experiments revealed the poor OER activity of the samples, and therefore, the rest of the electrochemical study focused on the assessment of their HER electrocatalytic properties.
Figure 6 shows the cathodic polarization curves traced on the electrodes. As can be seen in Figure 6a, out of the GC0, GCYMO, GCYMO-Sn, and GCYMO-Co specimens, it is the latter that displays the lowest HER overpotential value (of 0.73 V) at i = −10 mA/cm2—which is the current density often used when measuring the HER catalytic performance [56,57]. Better overall results were obtained for the samples modified with compositions containing the electroconductive Carbon Black (Figure 6b). Out of the tested electrodes, GCCB-YMO-Co exhibited the smallest ηHER value of 0.59 V, at i = −10 mA/cm2. The HER electrocatalytic properties of this electrode were further investigated.
Firstly, the EASA and diffusion coefficient values for the GCCB-YMO-Co electrode were calculated with Equation 4, using experimental data obtained by recording cyclic voltammograms at scan rate values in the 50–350 mV/s range, in increments of 50 mV/s. The curves were traced in the 0–0.8 V potential range in 1 M KNO3 and 1 M KNO3 + 4 mM K3[Fe(CN)6] electrolyte solutions, respectively. The same procedure was employed for the GC0 and GCCB samples. Table 4 shows the average values of the specified parameters (together with the standard deviation), calculated based on data acquired from three electrodes of each type.
A high EASA and a high diffusion coefficient are advantageous electrochemical properties due to the direct proportionality between the former parameter and the active centers involved in an electrochemical reaction [58] and because a higher diffusion coefficient indicates a faster diffusion rate [59]. As can be seen in Table 4, the highest values for these parameters belong to the GCCB-YMO-Co electrode. It should also be pointed out that the three average diffusion coefficient values are comparable to previously reported values from studies performed using GC and modified GC electrodes [60,61].
The voltammetry data collected from experiments carried out in the electrolyte solution containing K3[Fe(CN)6] can be used to obtain further information about the GCCB-YMO-Co electrode. Figure 7a shows a graphical representation that reveals the increase with the scan rate of the absolute values of the peak current densities belonging to the anodic and cathodic features observed on the cyclic voltammograms and corresponding to the ferrocyanide/ferricyanide redox couple. This behavior indicates a diffusion-controlled charge transfer process [62].
Electrochemical stability is another parameter relevant to the study of the water-splitting electrocatalytic properties of electrodes [17]. Figure 7b presents the current density vs. time curve recorded on the GCCB-YMO-Co electrode. The experiment lasted 6 h and was carried out at the constant E value corresponding to i = −10 mA/cm2. With a few exceptions, the current density kept decreasing and became relatively stable at the end of the test, when it reached i = −18 mA/cm2. However, despite the sample’s electrochemical stability limitations, the recording of a cathodic polarization curve following the experiment revealed a significant improvement in the HER overpotential value (at i = −10 mA/cm2), which changed from 0.59 V to 0.46 V (inset in Figure 7b). This is not the first time a chronoamperometric stability test is responsible for the increase in the water-splitting electrocatalytic activity of a modified electrode [63].
To identify the potential cause of the observed change in the catalytic properties of the sample, Raman spectroscopy was employed to analyze it before and after the stability experiment. The bands of the Raman spectra shown in Figure 8a,b at 1355 cm−1 (D band), 1596 cm−1 (G band), 2697 cm−1 (2D band), and 2948 cm−1 (D + G band) correspond to the glassy carbon substrate and the Carbon Black material [64,65]. The small band at 681 cm−1—better outlined after the electrochemical stability test—belongs to the perovskite material. A strong D band, such as the one observed in both spectra, indicates structural defects [65]. This band became even stronger following the testing, indicating the occurrence of some additional surface-level structural defects [66]. The intensity ratio between the D and G bands (ID/IG) is >1, which is specific for the glassy carbon material. No band shifts are revealed when the spectrum of GCCB-YMO-Co is compared to that of GCCB-YMO-Co’. The change in intensity of the 681 cm−1 band, characteristic of the perovskite material, may indicate structural changes occurring on the sample surface [67,68]. Given these results, the increased water-splitting electrocatalytic activity of the considered electrode following the chronoamperometric experiment can be attributed to the evidenced spectral changes.
The further study of GCCB-YMO-Co focused on the HER kinetics at the interface between its surface and the alkaline medium in which it was immersed. The Tafel plots presented in Figure 9 were obtained from the polarization curves recorded before and after the stability test and by normalizing the i values by the calculated EASA (iEASA).
The Tafel slope is a parameter relevant to the water-splitting catalytic activity of electrodes [69] and its value for the studied electrode was found to be 115 mV/dec (R2 = 0.9995) before the chronoamperometric test and 119 mV/dec (R2 = 0.9998) after the test. Electrocatalytic HER kinetics can be explained in terms of the Volmer–Tafel or Volmer–Heyrovsky mechanisms [70]. A Tafel slope value within the 40 to 120 mV/dec range indicates the unfolding of the HER on the electrode surface via a Volmer–Heyrovsky pathway [71,72]. Since the specified Tafel slope values are very close to 120 mV/dec, it is probable that in the case of GCCB-YMO-Co, the HER occurs through the Volmer–Heyrovsky mechanism. Considering the alkaline nature of the electrolyte solution, the first step of this pathway leads to a surface-adsorbed hydrogen atom, and in the second step, this atom combines with an H2O molecule and an electron to generate the H2 molecule [73,74].
Based on the results of the electrochemical experiments, the GCCB-YMO-Co electrode exhibits the highest HER electrocatalytic activity in the strong alkaline medium. While its particular structural and transport effects are the probable reason for its water-splitting properties [75], there are a few observations to be made. As previously specified, the EASA value estimated for this electrode is higher than the values calculated for the GC0 and GCCB specimens, and the higher the EASA value is the more catalytic sites are implicated in the HER. Furthermore, in Co-containing perovskites with water-splitting electrocatalytic activity, the cobalt cation has been identified as an active element in the H2O decomposition process [76]. However, the EASA values are not very different from each other, indicating the potential contribution of a synergistic effect between the Co-doped perovskite and Carbon Black. Such an effect between a Co-containing perovskite and an electroconductive carbon material has been previously reported [77]. The water-splitting catalytic activity of an electrocatalyst can be affected by its morphological properties. In the present case, the sponge-like morphology of the Co-doped YMnO3 allows for a higher surface area to be exposed to the electrolyte solution compared to the bead-like morphology of the Sn-doped YMnO3, which means that more catalytic sites are implicated in the electrochemical process.
Available literature data concerning Co-based HER electrocatalysts in alkaline electrolyte solutions reveal a variety of ηHER and Tafel slope values [78,79,80]. The ηHER value obtained for the GCCB-YMO-Co electrode following the electrochemical stability experiment is higher than those values, but almost all of the Tafel slope values presented in Table S1 (see Supplementary Material), together with the modified electrodes for which they were obtained, are either higher than or comparable with the value determined for the specified electrode. Future studies will consider the use of more complex electrode manufacturing protocols, such as a 3D printing-based one [81,82].

3.4. Photocatalytic Degradation Studies

Apart from the electrocatalytic investigation, experiments were also carried out to examine the photolytic/photocatalytic removal activity of EE2 under SSI and in the presence of YMO, YMO-Sn, and YMO-Co perovskites.
Firstly, based on adsorption studies, it can be concluded that the adsorption–desorption equilibrium is achieved after stirring in the dark for 30 min. Therefore, in all experiments, the reaction suspension was stirred for 30 min prior to irradiation. However, considering the direct photolysis results (Figure 10), the process can be improved, and therefore, the newly synthesized catalysts were investigated under SSI. The results show that the highest removal efficiency of EE2 is reached in the presence of the YMO-Co material. In the investigated system, 70% of EE2 is successfully degraded, and the efficiency is lower when a different photocatalyst is used. Furthermore, it can be seen that the cobalt doping process improved the photocatalytic activity of the pristine YMO material. The higher photocatalytic activity of the YMO-Co perovskite can be explained by its structure. The recorded SEM images show a sponge-like morphology with a higher surface area compared to the uniformly dispersed YMO-Sn and agglomerated YMO. The greater available surface can result in higher photocatalytic activity [83].
Since the highest removal efficiency is accomplished in the presence of the YMO-Co catalyst, the studies regarding the influence of initial pH on the photocatalytic activity were conducted in the presence of this perovskite. The obtained results are shown in Figure 11. Based on the findings, the highest degradation efficiency is achieved without modifying the initial pH value of ~9. In the strongly basic medium (pH~11), higher adsorption is observed before irradiation, and the photodegradation efficiency during the treatment is decreased. In addition, during the first 45 min of irradiation, only the adsorption–desorption of EE2 is recognized, while the degradation occurs after this period. When the pH is set to 7, the adsorption level is lower, but the degradation efficacy of EE2 is still decreased compared to the unchanged experimental conditions. The improved photocatalytic activity at pH~9 can be explained by the favorable interactions between YMO-Co and EE2. The pKa of EE2 is ~10.4 [84], whereas the isoelectric point of materials with rare earths is considered to be in the pH range between 1 and 8.7 [85]. According to this data, at pH~9, the selected pollutant was in cationic form, while the catalyst surface was negatively charged. Thus, attractive forces were present between them, resulting in higher removal efficiency.
Photocatalyst reutilization experiments were conducted in three consecutive runs with EE2 as the pollutant of choice, in the presence of YMO-Co, in ultrapure water, at the pH value of 9, and under SSI. A minor loss (up to 5%) in the photocatalytic potential was observed, but nonetheless, the photocatalyst remained effective even after three photocatalytic cycles.
The higher photocatalytic activity of the YMO-Co material can be attributed to the fact that Co ions can introduce mid-gap states that facilitate electron transfer, reducing the recombination rates of photogenerated charge carriers. On the other hand, the Sn ions may only affect charge mobility and surface redox reactions without having a serious influence on the photocatalytic activity [86].

4. Conclusions

In this work, perovskite materials based on undoped and Co- or Sn-doped YMnO3 were successfully synthesized using the sol–gel technique. Their water-splitting properties were investigated in a strongly alkaline medium. While the electrodes manufactured using the perovskites display poor electrocatalytic activity for the OER, the sample obtained by coating a GC support with a catalyst ink containing the Co-doped material and Carbon Black exhibits the lowest ηHER value during the HER investigation. At i = −10 mA/cm2, the ηHER value for this electrode, tagged GCCB-YMO-Co, is 0.59 V. In contrast, for the electrodes manufactured with the same method but with the other perovskites, tagged GCCB-YMO-Sn and GCCB-YMO, the ηHER values are 0.81 V and 0.82 V. Additional experiments performed on GCCB-YMO-Co revealed that its overpotential can be further decreased via a chronoamperometric stability test. Its Tafel slope value is close to 120 mV/dec, indicating that the HER occurs via a Volmer–Heyrovsky mechanism, and it suffers no significant changes during the stability experiment. The water treatment applicative potential of the newly synthesized perovskites was examined as well. The obtained data indicate that the materials possess conceivable photocatalytic efficiency in the degradation of EE2 under SSI. The highest removal efficiency (70%) occurs in the presence of Co-doped YMnO3, while in the systems with undoped and Sn-doped YMnO3, lower photocatalytic activity is observed. Furthermore, a reutilization study shows a minimal activity loss after three photocatalytic cycles. However, further experiments should be carried out to achieve enhanced activity under natural conditions.
The results from the electrochemical study contribute to the constantly expanding scientific understanding regarding materials with water-splitting catalytic properties as part of the widespread effort to resolve some of humanity’s pressing problems. Additionally, the promising photocatalytic activity of the fabricated perovskites makes them suitable for the removal of organic pollutants, such as EE2, from aqueous environments by employing innovative, sustainable, and powerful heterogeneous photocatalysis. On the other hand, additional investigations should be performed to gather more information about the studied perovskites since there is a lack of understanding concerning their isoelectric point, morphology, etc. These data are needed to adequately set up a photocatalytic system and reach the highest removal efficiency of organic pollutants.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings15040475/s1, Table S1: Tafel slope values obtained during HER studies performed in alkaline solutions on the GCCB-YMO-Co electrode and on some reported Co-based electrodes. References [77,80,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107] are cited in the Supplementary Material.

Author Contributions

Conceptualization, P.S., B.-O.T. and P.V.; methodology, P.S., S.B., D.Š.M., M.P. and B.-O.T.; software, M.P., B.-O.T. and D.Š.M.; validation, P.S., S.B., M.P. and P.V.; formal analysis, B.-O.T., P.V. and D.Š.M.; investigation, P.S., S.B. and B.-O.T.; resources, P.S.; data curation, B.-O.T. and P.S.; writing—original draft preparation, P.S., S.B., D.Š.M. and B.-O.T.; writing—review and editing, P.S., D.Š.M. and B.-O.T.; visualization, S.B., P.V. and D.Š.M.; supervision, B.-O.T. and P.S.; project administration, P.S.; funding acquisition, P.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data supporting the findings of this study are available within the article and Supplementary Material.

Acknowledgments

The authors thank D. Ursu from the National Institute of Research and Development for Electrochemistry and Condensed Matter Timisoara for recording the XRD spectra. The authors also gratefully acknowledge the financial support of the Ministry of Science, Technological Development and Innovation of the Republic of Serbia (Grant No. 451-03-137/2025-03/200125 and 451-03-136/2025-03/200125).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Factory-produced batch photoreactor.
Figure 1. Factory-produced batch photoreactor.
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Figure 2. XRD patterns for the undoped (YMO), Co-doped (YMO:Co), and Sn-doped (YMO:Sn) perovskite materials.
Figure 2. XRD patterns for the undoped (YMO), Co-doped (YMO:Co), and Sn-doped (YMO:Sn) perovskite materials.
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Figure 3. The observed, calculated, and difference XRD profiles for the perovskite materials.
Figure 3. The observed, calculated, and difference XRD profiles for the perovskite materials.
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Figure 4. SEM images of (a) YMO, (b) YMO-Co, and (c) YMO-Sn perovskite materials.
Figure 4. SEM images of (a) YMO, (b) YMO-Co, and (c) YMO-Sn perovskite materials.
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Figure 5. EDX spectra and quantification of YMO-Co and YMO-Sn perovskite materials.
Figure 5. EDX spectra and quantification of YMO-Co and YMO-Sn perovskite materials.
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Figure 6. (a) Cathodic polarization curves recorded in 1 M KOH solution, at v = 5 mV/s, on the bare GC electrode (GC0) and on the following modified electrodes: GCYMO, GCYMO-Sn, and GCYMO-Co. (b) Cathodic polarization curves recorded in 1 M KOH solution, at v = 5 mV/s, on the GCCB, GCCB-YMO, GCCB-YMO-Sn, and GCCB-YMO-Co modified electrodes.
Figure 6. (a) Cathodic polarization curves recorded in 1 M KOH solution, at v = 5 mV/s, on the bare GC electrode (GC0) and on the following modified electrodes: GCYMO, GCYMO-Sn, and GCYMO-Co. (b) Cathodic polarization curves recorded in 1 M KOH solution, at v = 5 mV/s, on the GCCB, GCCB-YMO, GCCB-YMO-Sn, and GCCB-YMO-Co modified electrodes.
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Figure 7. (a) The graphical representation of the anodic and cathodic peak current densities as a function of the square root of the scan rate for the GCCB-YMO-Co electrode. (b) HER chronoamperogram obtained on GCCB-YMO-Co in 1 M KOH solution and inset containing the cathodic polarization curves recorded on the same electrode, in 1 M KOH solution, at v = 5 mV/s, prior to the stability test (GCCB-YMO-Co) and following the respective test (GCCB-YMO-Co′).
Figure 7. (a) The graphical representation of the anodic and cathodic peak current densities as a function of the square root of the scan rate for the GCCB-YMO-Co electrode. (b) HER chronoamperogram obtained on GCCB-YMO-Co in 1 M KOH solution and inset containing the cathodic polarization curves recorded on the same electrode, in 1 M KOH solution, at v = 5 mV/s, prior to the stability test (GCCB-YMO-Co) and following the respective test (GCCB-YMO-Co′).
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Figure 8. Raman spectra recorded on the GCCB-YMO-Co electrode before (GCCB-YMO-Co) and after (GCCB-YMO-Co′) the electrochemical stability test. (a) Range: 200–2500 cm−1. (b) Range: 1000–3100 cm−1.
Figure 8. Raman spectra recorded on the GCCB-YMO-Co electrode before (GCCB-YMO-Co) and after (GCCB-YMO-Co′) the electrochemical stability test. (a) Range: 200–2500 cm−1. (b) Range: 1000–3100 cm−1.
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Figure 9. Tafel plots for the GCCB-YMO-Co electrode obtained from the polarization curves recorded in 1 M KOH solution, at v = 5 mV/s, before the stability test (GCCB-YMO-Co) and following the respective test (GCCB-YMO-Co′).
Figure 9. Tafel plots for the GCCB-YMO-Co electrode obtained from the polarization curves recorded in 1 M KOH solution, at v = 5 mV/s, before the stability test (GCCB-YMO-Co) and following the respective test (GCCB-YMO-Co′).
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Figure 10. Photocatalytic removal of EE2 (0.05 mmol/dm3) in the presence/absence of various YMO-based materials (γ = 0.5 mg/cm3) under SSI.
Figure 10. Photocatalytic removal of EE2 (0.05 mmol/dm3) in the presence/absence of various YMO-based materials (γ = 0.5 mg/cm3) under SSI.
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Figure 11. Influence of the initial pH on the photocatalytic removal efficiency of EE2 (0.05 mmol/dm3) in the presence of YMO-Co (γ = 0.5 mg/cm3) under SSI.
Figure 11. Influence of the initial pH on the photocatalytic removal efficiency of EE2 (0.05 mmol/dm3) in the presence of YMO-Co (γ = 0.5 mg/cm3) under SSI.
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Table 1. Electrode tags and ink compositions.
Table 1. Electrode tags and ink compositions.
Electrode TagPerovskite MaterialCarbon Black
(mg)		Nafion Solution 5% (µL)Ethanol
(µL)
GC0----
GCYMO5 mg YMO-50450
GCYMO-Sn5 mg YMO-Sn-50450
GCYMO-Co5 mg YMO-Co-50450
GCCB-550450
GCCB-YMO5 mg YMO550900
GCCB-YMO-Sn5 mg YMO-Sn550900
GCCB-YMO-Co5 mg YMO-Co550900
Table 2. Crystallographic data and average crystallite size obtained by Rietveld refining for nanocrystalline YMO, YMO-Co, and YMO-Sn.
Table 2. Crystallographic data and average crystallite size obtained by Rietveld refining for nanocrystalline YMO, YMO-Co, and YMO-Sn.
SampleLattice Parameters (Å)Space GroupUnit Cell Volume
(Å3)		Crystallite Size
(nm)
abc
YMO6.149106.1491011.375 (2)P63cm372.4984027.3
YMO-Co6.151 (2)6.151 (2)11.336 (5)P63cm371.4879023.07
YMO-Sn6.128 (2)6.128 (2)11.358 (3)P63cm369.3606024.74
Table 3. Data obtained from the LeBail fit in space group P63mc for XRD diffractograms of YMO, YMO-Co, and YMO-Sn.
Table 3. Data obtained from the LeBail fit in space group P63mc for XRD diffractograms of YMO, YMO-Co, and YMO-Sn.
SamplesYMOYMO-CoYMO-Sn
a (Å)6.14517 (6)6.14978 (2)6.14624 (5)
c (Å)11.316 (8)11.33198 (3)11.30616 (6)
Volume (Å3)370.077 (6)371.156 (2)369.883 (4)
X23.103.112.75
RBragg6.861.691.88
Rf factor5.201.882.05
Table 4. Estimated EASA and diffusion coefficient values.
Table 4. Estimated EASA and diffusion coefficient values.
Electrode TagEASA (cm2)Diffusion Coefficient (cm2/s)
GC00.123 ± 0.00771.31 × 10−6 ± 0.161 × 10−6
GCCB0.135 ± 0.0091.573 × 10−6 ± 0.214 × 10−6
GCCB-YMO-Co0.19 ± 0.00873.108 × 10−6 ± 0.284 × 10−6
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Sfirloaga, P.; Bognár, S.; Taranu, B.-O.; Vlazan, P.; Poienar, M.; Šojić Merkulov, D. Co- and Sn-Doped YMnO3 Perovskites for Electrocatalytic Water-Splitting and Photocatalytic Pollutant Degradation. Coatings 2025, 15, 475. https://doi.org/10.3390/coatings15040475

AMA Style

Sfirloaga P, Bognár S, Taranu B-O, Vlazan P, Poienar M, Šojić Merkulov D. Co- and Sn-Doped YMnO3 Perovskites for Electrocatalytic Water-Splitting and Photocatalytic Pollutant Degradation. Coatings. 2025; 15(4):475. https://doi.org/10.3390/coatings15040475

Chicago/Turabian Style

Sfirloaga, Paula, Szabolcs Bognár, Bogdan-Ovidiu Taranu, Paulina Vlazan, Maria Poienar, and Daniela Šojić Merkulov. 2025. "Co- and Sn-Doped YMnO3 Perovskites for Electrocatalytic Water-Splitting and Photocatalytic Pollutant Degradation" Coatings 15, no. 4: 475. https://doi.org/10.3390/coatings15040475

APA Style

Sfirloaga, P., Bognár, S., Taranu, B.-O., Vlazan, P., Poienar, M., & Šojić Merkulov, D. (2025). Co- and Sn-Doped YMnO3 Perovskites for Electrocatalytic Water-Splitting and Photocatalytic Pollutant Degradation. Coatings, 15(4), 475. https://doi.org/10.3390/coatings15040475

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