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. 2026 Jul 8;16(36):38608–38621. doi: 10.1039/d6ra03751h

Engineering A-site vacancies and Mo substitution in La2CuO4 for high-performance bifunctional water-splitting electrocatalysis

Islam Saad a,✉, Rafat M Amin a, Amel Haouas b, S I El-Dek c
PMCID: PMC13390739  PMID: 42488804

Abstract

Designing electrocatalysts that concurrently offer high activity, long-term durability, and low cost remains a major challenge in sustainable energy conversion technologies. In particular, bifunctional catalysts capable of efficiently catalyzing both the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are highly desirable for practical water-splitting systems. Herein, La1.9Cu0.9Mo0.1O4−δ (d-LCMO-Ar) (“d” refers to deficiency) is synthesized via a citric-acid-assisted method followed by argon annealing, using a Ruddlesden–Popper parent phase as a structural platform to engineer enhanced bifunctional performance. The optimized catalyst achieves current densities of 10 and 100 mA cm−2 at overpotentials of 258 and 500 mV for the OER and −117 and −323 mV for the HER, respectively, in 1 M KOH. Moreover, d-LCMO-Ar maintains a stable current density of 50 mA cm−2 over 50 h, demonstrating excellent durability, and exhibits a large electrochemical surface area (183.7 cm2), low Tafel slopes of 67 mV dec−1 (OER) and 157 mV dec−1 (HER), as well as high turnover frequencies of 6.51 s−1 (OER) and 13 s−1 (HER). Collectively, these attributes highlight d-LCMO-Ar as a promising and cost-effective alternative to noble-metal based catalysts for alkaline water-splitting applications.

A schematic of OER/HER measurements and RP phase crystal structures modeled via VESTA. Structures are shown along the (113) plane in ball- and stick/polyhedral styles alongside XRD lattice constant variations.graphic file with name d6ra03751h-ga.jpg

1. Introduction

Many efforts have focused on developing sustainable alternative energy sources to address the rapid depletion of fossil fuels and environmental concerns. Commercial alkaline electrolyzers, while practical for large scale, high purity hydrogen production, still suffer from inefficiencies.1 Several electrochemical devices, such as water electrolyzers, rechargeable metal air batteries, and electrochemical synthesizers, hold promise for clean energy conversion and fuel transformation. However, significant overpotentials arise mainly from the sluggish kinetics of the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode.2 To drastically lower the overpotential, extremely active OER and HER electrocatalysts must be developed. Over the last 10 years, a lot of work has gone into finding novel OER and HER bifunctional catalysts with the necessary activity, durability, and selectivity. Numerous catalytic materials, such as carbon compounds, perovskites, spinel metal oxides, and layered double hydroxides, have been explored for their remarkable catalytic performances in alkaline electrolytes.3,4 Iridium/ruthenium oxides (IrO2/RuO2) and platinum (Pt) metal are now the state of the art electrocatalysts for the OER and HER, respectively; however, their limited availability, high cost, and poor stability have prevented their widespread use. Consequently, it is desired to create OER/HER catalysts with high activity and stability in alkaline solutions that are both affordable and plentiful in the earth.5,6 A continuous challenge is to optimize the architectures of the most active surface catalytic sites to serve as synthetic targets. Accurate mechanistic research that may provide a deeper comprehension of OER and HER catalysis and open up new avenues for the creation of better catalysts has been hampered by the same problem. Thus, how to locate and build well-defined catalytic centers for effective water-splitting catalysis and how to clearly define the atomistic mechanism and kinetics of the running reaction and competing reactions at this center are the two unresolved issues that are essential to heterogeneous OER/HER catalysis.7

La2CuO4, which has the general formula Lan+1CunO3n+1, belongs to the family of layered Ruddlesden–Popper (RP) phases. The perovskite type La2CuO4 and rock salt type layers alternate along the crystallographic axis to form the layered structure of La2CuO4 (also known as the K2NiF4 type structure). The capacity to accept overstoichiometric oxygen in the interstitial places in the rock salt type layers is a distinctive property of the RP structure.6 RP has been reported for widespread applications in the energy and environment fields. Their functions as bifunctional electrocatalysts have drawn more attention recently.8,9 However, their instability during the electrochemical process continues to be a significant obstacle. An efficient method of increasing catalytic efficiency is to build hybrids with interfacial contact between the constituents. Due to the presence of charge transfer and lattice tension between the various compounds, the interface may, on the one hand, function as new catalytic actives with the optimum electronic structure.10 Conversely, the hybrid systems have the capacity to provide certain synergistic effects that allow for the stabilization of catalytic sites and the achievement of a certain level of electrocatalytic activity and stability.11 La3+ has an ionic radius that is well-suited for the A-site of the Ruddlesden–Popper La2CuO4. The ionic radius of La allows it to stabilize the K2NiF4-type structures. Rare earth metal oxides, in particular, demonstrate outstanding catalytic performance by boosting material conductivity, increasing surface area, and providing improved pathways for ion and molecule diffusion. In addition to improving the ionic conductivity, carrier transportability, and catalytic performance.12,13 In order to preserve the compound's electroneutrality, the B element may be forced into a higher oxidation state or the creation of oxygen vacancies by replacing the A and/or B cations with a lower valence cation.12 These produced oxygen vacancies increase the lattice oxygen's mobility, which in turn raises the catalysts' oxidation activity.13 Since their simple metal oxides are very active for the complete oxidation reaction, Cu, Co, Fe, and Ni are often employed as B-site cations when La is in the A site. The equilibrium excess oxygen content fluctuation of B-site doped LCO. LCO based oxides may exhibit significant oxygen excess composition, with the quantity of excess oxygen being highly dependent on dopant species and might result from an imbalance in the charge.14

Several reports the A-site defects, which generate negative charges to the perovskite crystal and improve the catalytic activity by raising the concentration of oxygen vacancies and holes. Others reported that an A-site deficit causes the material's oxygen nonstoichiometry and hole concentration to rise. Improved conductivity and catalytic activity were the outcomes of introducing an A-site deficit.15 In catalysts, additional surface oxygen vacancies and Cu species were created by adjusting the quantity of A-site La ions in the perovskite structure (La1.9CuO4 catalyst), surpassing those of other transition metal based catalysts previously documented in the literature.16 Furthermore, it is technologically significant and urgently needed to develop a catalyst with high OER and HER electrolyte and low cost in practical applications.17 Although there has been considerable progress in this direction, there are still numerous obstacles to overcome before low-cost, highly effective catalysts for alkaline water splitting can be developed.2,16 This work aims to provide an innovative, high-stability electrocatalyst to be used in the water-splitting application by tuning the crystal structure and inducing defects using synergetic effects collected between A-site defects, doping, and reducing conditions such as Ar annealing medium.

2. Materials and methods

La(NO3)3·6H2O (Alfa Aesar, 99.9%), Cu(NO3)2·3H2O (Sigma-Aldrich, 99%), and (NH4)2MoO4 (Sigma-Aldrich, 99.98%) were used as raw materials. Ethanol (99.5%, Alfa Aesar) and citric acid, anhydrous (Thermo Scientific, 99.5%), were utilized as solvent and combustion agent. Commercial epoxy resin adhesive, Ag wires, and Ni foam sheets of thickness 0.5 mm were purchased from SRNewTech Company, China.

2.1. Sample preparation

La2CuO4 catalysts were prepared by the citric acid combustion technique. Stoichiometric quantities of La(NO3)3·6H2O, Cu(NO3)2·3H2O, and (NH4)2MoO4 were dissolved in 0.1 M citric acid and stirred magnetically at 150 °C for 5 hours until gel formation. The grinded powder was calcined for 3 h at 800 °C, 5 °C min−1, as heating and cooling conditions were applied during the experiment. The final powders were grinded by pestle and mortar as well as sieved (using a fine mesh of 250 microns) for a homogenous distribution of the powders.

The same previous steps were applied for La1.9CuO4 (d-LCO), La1.9Cu0.9Mo0.1O4-Air (d-LCMO-Air), and La1.9Cu0.9Mo0.1O4-Ar (d-LCMO-Ar). “Air” and “Ar” refer to the annealing atmosphere.

2.2. Sample characterization

Using PANalytical Empyrean and under 40 kV accelerating voltage, 35 mA current, CuKα source, and λ = 0.15406 nm, the crystal structures of the prepared powders were detected. Using Quanta FEG 250 field emission scanning electron microscopy (FESEM) the microstructures and morphologies were determined, bulk elemental quantification and compositional mapping were performed via an integrated Energy Dispersive X-ray (EDX) spectrometer operated at an accelerating voltage of 20 kV. Fourier Transform Infrared Spectroscopy (FTIR), manufactured by PerkinElmer, was utilized for the surface functional group identification. The electrode preparation conditions: a mixture of 400 µl of high grade ethanol and 3% Nafion (by volume) was prepared, and 10 mg of the targeted electrocatalysts were added to the previously mentioned mixture. To confirm the optimum distribution of particles, they were ultrasonicated and magnetically stirred for 30 min on each device. 1 cm2 of 0.5 mm thickness of Ni foam electrode (NF) was welded with Ag wire and isolated by epoxy as a working electrode. The working electrode was immersed in the mixture (Nafion, electrocatalyst, and ethanol) for 20 s. The loaded electrode was dried at 60 °C for 15 min before the electrochemical tests. Electrochemical measurements: the potentiostat (Metrohom) and three electrode setup working electrode (NF), counter electrode (Pt), and reference electrode (calomel) were used to conduct all the tests for both OER and HER. Using 100 ml of 1 M KOH as a measurement medium for each measurement. Chronopotentiometry, electrochemical impedance spectroscopy (EIS), and linear sweep voltammetry (LSV) were among the electrochemical testing methods. The LSV for OER and HER were carried out with a scan rate of 10 mV s−1 and a fixed applied range of voltage. The long term stability of chronopotentiometry was examined for 50 h, yielding 50 mA cm−2 for the optimized sample. The EIS was performed with a 10 mV amplitude and a 5–100 000 Hz frequency range. The detailed experimental procedures are illustrated in Fig. 1.

Fig. 1. A schematic diagram for the OER and HER measurement. In addition to the RP phase crystal structure. The RP phase image was reproduced using VESTA software. Both of the structures were viewed in the plane 113 in ball and stick and polyhedral styles. The XRD structures and the lattice constant a, b, and c variations are shown in the left column.

Fig. 1

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3. Results and discussion

3.1. Structural characterization

The XRD diffraction patterns of the various produced samples are shown in Fig. 2(A). It matches JCPDS card no. 01-082-2142 of lanthanum copper oxide (La2CuO4), indicating that the orthorhombic phase, with the space group Bmab.18 The reflection (113) assures the orthorhombic structure of the various catalysts. The broadening of the peaks and low intensities, which verify the successful integration of the targeted samples. The lattice parameters a, b, and c were estimated using eqn (1).

3.1. 1

where the Miller indices are represented by (hkl) and the interplanar distance (d). The metal's radii determine the process of replacement between Cu ions and Mo ions. The closer the radii, the better the substitution. Ion radius values are reported to be Cu2+ (0.57 Å and 0.73 Å for four coordinate and six coordinate, respectively), Cu3+ (0.54 Å and 0.60 Å for six coordinate and eight coordinate, respectively), Mo5+ (0.61 Å, six coordinate), and Mo6+ (0.59 Å, six coordinate). These reported values support the substitution between Cu2+,3+ and Mo5+,6+, as Mo3+ (0.69 Å, six coordinate) and Mo4+ (0.65 Å, six coordinate) have quite large ionic radii and may be less suitable for the substitution process.19 The variation of cell volume due to lattice parameters/dimensions as d-LCO exhibits A-site deficiency due to vacancies reducing the radius of the A-site, leading to lattice contraction.20 A possible charge compensation occurs in the B-site in the ions to Cu3+, shrinking the CuO6 octahedral.21 Possible oxygen vacancies may partially offset contraction, but Cu3+ dominates the structural effect.22,23 Mo doping reverses the contraction observed in d-LCO due to Mo5+ or Mo6+ substitution at Cu sites, which have a suitable ionic radius. Oxygen vacancy dynamics compensate for the contraction, with Mo6+ substitution increasing the B-site charge and oxygen vacancy formation relaxing the lattice strain.24 In addition to Ar-calcination, which promotes more oxygen vacancies, enhancing expansion.25

Fig. 2. (A) X-ray diffraction of LCO, d-LCO, d-LCMO-Air, and d-LCMO-Ar (B) Williamson Hall plot, (C) lattice parameters a and b. (D) Lattice parameter c.

Fig. 2

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Because of its significance in the XRD investigation, the lattice microstrain values were calculated using the Williamson–Hall relation (W–H). Peak broadening is mostly caused by microstrain. Lattice strain measures the lattice constant distribution that is caused by crystal flaws such as lattice dislocations.26 From the W–H for XRD data processing, we determined the lattice strain and crystallite size. The W–H plot between 4 sin θ and β cos θ is shown in Fig. 2(C). Eqn (2) was used to compute the strain resulting from crystal imperfections and distortions in the samples that were generated.

3.1. 2

where D is the size of the crystallite, λ is the wavelength of XRD, k is the shape constant (0.94), θ is the angle of diffraction, β is the corrected full width at half maximum in “radians,” and ε is the microstrain denoted. From the linearity of the plot, the microstrain equals the slope, and the intercept = 0.94λ/D. The detailed structural parameters are recorded in Table 1.

Table 1. XRD parameters of LCO, d-LCO, d-LCMO-Air, and d-LCMO-Ar. a, b & c: lattice parameters; V: volume; ρ: density; crystallite size, microstrain and δ: dislocation density.

Parameters LCO d-LCO d-LCMO-Air d-LCMO-Ar
a calculated (Å) 5.33 5.32 5.36 5.36
b calculated (Å) 5.37 5.36 5.41 5.41
c calculated (Å) 13.07 13.05 13.02 13.15
a/c 0.4078 0.4076 0.4116 0.4076
V calculated (Å3) 374.9 372.8 378 381.9
ρ calculated (g cm−3) 7.17 6.97 6.93 6.86
Crystallite size WH (nm) 118.6 267.8 59.08 33.98
Microstrain 1.5 × 10−3 2.4 × 10−3 −1.1 × 10−4 −1.7 × 10−4
δ (lines per m2) (×1014) 0.71 0.13 2.86 8.66
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Contact/sintered stresses, faults due to stacking, and coherency stresses are the possible sources of strain formation in the pure samples (host material).27 For further prepared samples, the air calcination promotes crystallite growth by reducing defects and grain boundaries, and the reason for positive microstrain occurs due to A-site La3+ vacancies in d-LCO, causing tensile strain and lattice expansion.28 In the Mo doped and Ar-calcined samples, oxygen vacancies are highly pronounced, disrupting crystallite growth and reducing size. Smaller crystallites indicate inhibited grain growth due to vacancy induced fragmentation. Negative microstrain occurs due to smaller ions of Mo substitution in addition to the compressive strain of the Ar medium.29

Using eqn (3), one can determine the dislocation density (δ) (lines per m2).

3.1. 3

The properties of crystalline materials rely mostly on the number of dislocations, which are structural defects.30 Dislocation density may be indirectly estimated by XRD by examining the broadening of diffraction peaks in the pattern.31 Peak broadening can also result from various factors, one of which is the presence of dislocation density within the crystal structure.

3.1. 4

where ρ: the theoretical density, Z is the number of atoms in the unit cell, Mw is the molecular weight (or atomic weight), Vc is the volume of the unit cell, and NA is Avogadro's number (6.023 × 1023 atoms per mol). Overall, the changes over a, b, and c axes can be justified as follows. In the d-LCO sample, there is a slight shrinkage of the a axis due to A-site defects (vacancies), while Mo doping in air causes a noticeable lattice expansion from distortion linked to oxidation state variation. As for d-LCMO-Ar, extra expansion occurs, possibly due to the dominance of a lower oxidation state in the reducing Ar atmosphere. A similar trend appears in the b-axis. Owing to the orthorhombic phase and axis variations, anisotropic microstrain arises, especially because of Mo doping, along with oxygen vacancies induced during preparation. The structural change in the d-LCMO-Ar sample indicates internal reorganization. The a/c ratio remains near constant, indicating balanced lattice changes, with d-LCMO-Air showing a slightly higher value (0.4116), likely due to the Mo oxidation state. Lattice expansion may reduce packing density; consistent with this, the calculated density trends downward.

For crystallite size, d-LCO (267.8 nm) shows a clear increase compared to LCO, as A-site defects favor growth of fewer nucleations with larger crystallites. Both Mo doped samples have smaller values due to growth disruption. Mo doping causes lattice distortion with negative strain (compression), amplified in the Ar atmosphere by oxygen vacancies.

Post-electrochemical characterization was carried out on the optimized sample loaded over nickel foam to evaluate the structural integrity and phase preservation under long-term operational stress (Fig. S1). The fundamental peaks at 44.66 and 51.57 are due to porous NF. The main planes due to the electrocatalyst still confirm the orthorhombic crystalline configuration with the characteristic space group Bmab. The primary low-intensity structural reflections, including the key (111), (004), (113), (020), (200), (220), (117), (224), (133), and (313) crystallographic planes, remain visible and slightly shifted in position, signifying outstanding structural robustness against active phase dissolution or severe lattice collapse in alkaline environments. Of note, these peaks are shifting to a higher two theta after the test. A minor attenuation in peak clarity and slight background elevation is ascribed to amorphous polymer binder matrix features. Nafion and trace surface hydroxylation layer formations occurring due to the test operations. Although plane (212) complies with JCPDS card no. 01-082-2142 of lanthanum copper oxide, it was absent prior to the stability test, which could be due to internal rearrangements or the binder matrix.

3.2. Morphology and compositional analysis

Fig. 3 illustrates the morphology of the various synthesized samples, as observed through SEM micrographs. A basic nanostructure with visible changes, including the generated porosity, especially with the Mo doping using the auto-combustion method. This porosity possibly resulted from gas discharges, such as nitrogen, carbon dioxide, and oxygen.32 Also, intergranular pores can be observed from the micrographs, which is useful in various applications. From the micrographs, closely spaced nanoclusters can be detected. Various factors such as defects at the A-site, differences in ionic radii between the dopants and the host material, and internal strain lead to a reduction in particle size. Due to uniform electron scattering within each grain, the grains appear as distinct regions with consistent brightness and texture. Grain boundaries appear as narrow or overlapping areas where noticeable changes in texture or brightness occur, marking the transition between grains and indicating the presence of both adhesive and agglomerated grains. Grain boundaries directly affect the material's various properties, such as mechanical, electrical, and magnetic properties.

Fig. 3. FESEM micrographs: (A and B) for LCO, (C and D) for d-LCO, (E and F) for d-LCMO-Air, and (G–I) for d-LCMO-Ar.

Fig. 3

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The structural morphology of d-LCMO-Ar catalyst loaded over NF was evaluated via FESEM analysis before and after the 50 h stability test (Fig. S2). The material exhibits a uniform distribution of granular nanoclusters on the NF surface. After the 50 h stability test, the surface underwent a significant, localized morphological rearrangement. These micrographs reveal the self reconstruction into highly dense, interconnected structures extending across the NF skeleton, which is possible to increase the active catalyst surface area directly exposed to the electrolyte.

A representative EDX spectrum taken from an even, uniform region (region 2) is shown in Fig. S3. Also, the average atomic percentage and weight percentage of the contributing elements are listed in Table S1. The EDX spectrum confirms the successful integration of lanthanum, copper, and molybdenum within the layered oxide host matrix. Table S1 values show exceptional agreement with the targeted theoretical stoichiometry of d-LCMO-Ar.

3.3. FTIR

Fig. S4 illustrates the FT-IR spectrum in the 4000–400 cm−1 range for the different prepared samples. Various functional groups were found in the perovskite system. The wide band centered at 3433 cm−1 is the stretching vibration of adsorbed O–H of H2O molecules. Additionally, bands at 1625, 1586, and 1473 cm−1 are attributed to the bond vibration of H–O–H (symmetric stretching, asymmetric stretching, and bending (scissoring) mode). Absence of the signals in the range of 1100 cm−1 to 1150 cm−1 confirms no residual nitrogen groups and complete combustion, confirming sample purity. Moreover, the bands at 620 cm−1 and 524 cm−1 show the stretching modes of La–O and Cu–O of the orthorhombic phase of LCO and modified LCO samples, which is related to the perovskite structure. The signal that appears at 625 cm−1 is the La–O stretching mode and may shift higher or lower based on sample preparation technique, measurement conditions, and specific structural characteristics. The absorption bands at 758.9 cm−1 and 899.7 cm−1 represent the stretching vibrations of O ions in Mo–O stretching vibrations and Mo Created by potrace 1.16, written by Peter Selinger 2001-2019 O stretching vibrations, respectively. The 438.8 cm−1 and 515.6 cm−1 bands show the stretching vibrations of O atoms in the O–Mo–O units, while around 570.9 cm−1 for the bending vibration of O ions in Mo–O–Mo.33,34 These observations confirm the perovskite structure's stability upon various modifications, including the induced defects and Mo doping, and the successful integration of the contributing elements as a compound.

3.4. Electrochemical results

3.4.1. OER & HER performance

Using the Ni foam (NF) substrate as an electrode, the OER and HER of the different prepared catalysts were tested. Using 100 ml of 1 M KOH solution, a three electrode cell system, and 10 mV s−1, all the data were measured. The various tests showed excellent catalytic activity. The prepared d-LCMO-Ar catalytic activity in OER and HER is much enhanced compared to the other prepared samples due to the material manipulation appearing in the internal induced defects arising from the A-site defects, doping, and the Ar combustion medium, as shown in Fig. 4(A). For our optimized sample, the required overpotential for 10 mA cm−2 and 100 mA cm−2 for OER are 258 mV and 500 mV, and for HER are −117 mV and −323 mV, respectively. Despite the overall samples' overpotential (η) increasing after 10 mA cm−2 due to NF use and the preparation method, they still give excellent overpotential values at the high current density comparable with the state of the art for OER and HER. The main hidden novelty is clearly abbreviated in the high current growth with a sharp slope after 1.58 V (η = 350 mV). 200 mA, 300 mA, and 400 mA can easily be obtained before 1.9 V for the d-LCMO-Ar sample. Since no plateau is seen in Fig. 4(A), indicating a straight proportional connection, it is expected that increasing the potential would increase the current density.35 It suggests that more potential is provided, and the current will grow linearly. This implies that the electrochemical process being studied does not exhibit a clear saturation point within the investigated potential range. The enhanced HER electrochemical activity in the d-LCMO-Air and d-LCMO-Ar samples is attributed to the doping process, as evidenced by their significantly higher current densities compared to the other samples (LCO and d-LCO), which exhibited very low current densities below 10 mA cm−2. A comparison with reported literature, including various perovskite based electrocatalysts on the state of the art of noble metal results, is presented in Table 2. In Fig. 4(B), the results of the logarithmic relation of Tafel plots are plotted and written on the graphs to show the excellent rate of kinetics for OER and HER. Tafel eqn (5)

η = a + b log(j) 5

η and j are the overpotential and current density, and a and b are constants, respectively. The Tafel slope values for LCO (180 mV dec−1), d-LCO (156 mV dec−1), d-LCMO-Air (123 mV dec−1), and d-LCO-Ar (67 mV dec−1). The values show a 40% enhancement in the Tafel slope of the d-LCO-Ar sample if compared to the as-prepared sample, suggesting much quicker reaction kinetics.

Fig. 4. (A and B) LSV curves (C and D) Tafel plots, and (E and F) chronopotentiometry (for d-LCMO-Ar) of OER and HER, respectively.

Fig. 4

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Table 2. OER behavior of several perovskite based electrocatalysts. The present investigation compares the bifunctional catalytic activity of the optimized sample with that of other high quality perovskite based functional catalysts reported in the literature, together with catalysts based on precious metals.
Electrocatalyst η@current density mA cm−2 (mV) OER η@current density mA cm−2 (mV) HER Tafel slope (mV dec−1) OER/HER R ct (Ω) TOF (s−1) ECSA (cm2) Stability (h) Scan rate (mV s−1) Electrode/electrolyte Ref. electrode Ref.
d-LCMO-Ar 258@10 −117@10 67/157.9 23.8 6.51 183.7 50 10 NF/1 M KOH HgO This work
500@100 −323@100
d-LCMO-Air 268@10 −148@10 119/185.7 168.4 3.24 120.6 50 10 NF/1 M KOH HgO
564@100 −394@100
Ni1.8Cu0.8P/NF — 78@10 —/70 ∼3.8 — — 30 — NF/1 M KOH RHE 35
Cu2Se/Cu foam (hydrothermal) 850@750 —/(32–35) 0.32–0.6 — — — — Cu foam/0.5 M H2SO4 Hg/HgO 36
La5Ni3CoO13−δ 450@55.5 — 35 — — — 10 GCE/0.1 M KOH Hg/HgO 37
La5Ni3FeO13−δ 450@10.2 89
Ir/C 450@28.2 43
Sr4Ir3O10 205@10 — 66.1 50 10 —/1 M KOH Ag/AgCl 38
275@100
IrO2 330@10 88.1 5
1% La-doped CuO 322@10 −173@10 88/310 — — 1.07 10 Stainless substrate/1 M KOH Ag/AgCl 39
CoSe/NiFe LDH 270@150 −260@10 57/125 — — — 10 — Exfoliated graphene foil/1 M KOH RHE 40
Cu/CoSx 230@10 −125@10 — 10 — Cu foam/1 M KOH 41
500@40
CuO@Ni/NiFe hydroxide 230@10 −125@10 86 4.5 1350 12–16 2 Cu plate/1 M KOH Hg/HgO 42
RuO2 79
Pt/C 50
SrCo0.5Fe0.4Mo0.1O3−δ (SCFM10) 310@10 200@10 72/- — — — 100 — 1 M KOH RHE 43
LaMoO3/GO 560@10 210@10 87 ∼550 1.08 5 —/0.5 M H2SO4 Ag/AgCl 44
186 —/1 M KOH
La0.9NiO3/NiSex 297.6@10 — — — — — GCE/1 M KOH
Pt/C 48 —/0.5 M H2SO4
RuO2 75 —/1 M KOH
La1−xCexFeO3 257@10 — 49 12 — — 200 NF/1 M KOH Ag/AgCl 45
312@100
BaTi0.9Ni0.1O3/rGO 350@80 93 305 — — 1.5 10 GCE/1 M KOH RHE 46
Cu3P@NF 320@10 105@10 54 4 100 — NF/1 M KOH RHE 47
Cu3N–CuO 286@10 118@10 118.5/122 0.48 10 days 1 NF/1 M KOH RHE 48
IRO2/NF 287@10 <120@10 127.9/139 — — — —
RuO2/NF <300@10 <120@10 120/112 — — — —
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Additionally, Fig. 4(A and D) shows the significant OER and HER activity of the various samples. Fig. 4(B and E) Tafel Plot of OER and HER activity. The stability test (chronopotentiometry) of OER and HER with LSV after the stability test for 50 h. In addition to activity, stability is a crucial factor in determining the usefulness of electrocatalysts in real world applications. To evaluate this, chronopotentiometry was carried out for η@50 mA cm−2, almost unchanged, as shown in Fig. 5(C). A clear, stunning stability was found and was supported with LSV and Nyquist plots after the 50 h of the stability test. At the conclusion of cycling, both the polarization and Nyquist curves indicate only a minimal loss in current. Overall, the defect engineering in the sample of LCMO-Ar shows exceptional activity and resilience in alkaline conditions.

Fig. 5. (A–D) CVs with different scan rates for LCO, d-LCO, d-LCMO-Air, and d-LCMO-Ar. (E) Estimation of Cdl values by plotting the current against the scan rate for the prepared samples.

Fig. 5

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From eqn (6),28 the electrochemical surface area (ECSA) was estimated.

3.4.1. 6
3.4.1. 7

For KOH, Cs is 0.040 mF cm−2 (specific capacitance), and Cdl is the capacitance of the double layer. The current density in this relation is the change between anodic and cathodic current density (Δj = ja − jc) vs. RHE in the non-faradaic range (as the current is exclusively due to the double layer charging) by applying different scan rates from 20 mV s−1 to 100 mV s−1.49Cdl values were obtained from the current density against the scan rate, as shown in Fig. 5. ECSA of the various electrocatalysts was 70.2 cm2, 100.6 cm2, 120.6 cm2, and 183.7 cm2 for LCO, d-LCO, d-LCMO-Air, and d-LCMO-Ar samples, respectively.

Exchange current density from EIS.

3.4.1. 8

R is the universal gas constant (8.314 J K−1 mol−1), T is the reaction temperature, n is the number of electrons transferred (4 for OER and 2 for HER), and θ is the resistance calculated from the EIS. The estimated values of Iex are 1.9 × 10−2 mA cm−2, 1.2 × 10−2 mA cm−2, 3.7 × 10−2 mA cm−2, and 0.27 mA cm−2 for LCO, d-LCO, d-LCMO-Air, and d-LCMO-Ar, respectively.50

3.4.1.1. Determination of TOF from the integrated OER/HER polarization curve

One of the highly important parameters for the intrinsic activity is the turnover frequency (TOF), which can be calculated based on eqn (9). Several factors will be estimated to finally calculate it.

3.4.1.1.1. Determination of surface concentration from CV

The electrochemically active surface sites were estimated from the integrated area of the Ni3+/Ni2+ redox peaks in the cyclic voltammograms.49 The calculated charges for the different samples were 0.53, 2.55, 3.86, and 9.74 mA s, respectively. Based on these values, the total number of electrons transferred was determined using Faraday's law, assuming one electron redox process for the Ni3+/Ni2+ couple. Under this assumption, the number of electrochemically active surface atoms is considered equivalent to the number of transferred electrons. Accordingly, the estimated numbers of surface active atoms for the samples were 2.31 × 1016, 3.31 × 1015, 1.92 × 1015, and 2.41 × 1015.

3.4.1.1.1. 9

where i = current in ampere, NA = Avogadro's number, A = electrode surface area, F = Faraday constant, n = number of electrons, and Γ = surface concentration of atoms. The calculation of TOF assumes all the catalytically active sites are accessible during the reaction. Thus, TOFOER = 0.36 s−1, 1.24 s−1, 3.24 s−1, and 6.51 s−1 and TOFHER = 0.73 s−1, 2.48 s−1, 6.48 s−1, and 13 s−1 for LCO, d-LCO, d-LCMO-Air, and d-LCMO-Ar, respectively.

To evaluate the intrinsic electrocatalytic performance of the prepared materials and remove the contributions of physical surface area variations, the polarization curves for both OER and HER were normalized by their respective ECSA.51,52 As illustrated in Fig. S5, the ECSA-normalized specific current density (jECSA) maintains a clear superiority for the d-LCMO-Ar catalyst over the parent phases. For OER, d-LCMO-Ar exhibits the highest specific current density. As for HER, d-LCMO-Air and d-LCMO-Ar are comparable in intrinsic activity. These results indicate that performance enhancement is driven by accelerated intrinsic kinetics rather than a simple increase in geometric active site exposure.

As shown in Fig. 6(A), the EIS of the samples LCO, d-LCO, d-LCMO-Air, and d-LCMO-Ar was carried out by applying a frequency range of 5 to 100 000 Hz. The behavior changed much from sample to sample; the d-LCMO-Ar samples showed a nonhomogeneous semicircle. The value of charge transfer electrons (Rct) can be determined by the circle/semicircle radius, which suggests enhanced conductivity and better charge transfer.53,54

Fig. 6. (A) Nyquist plots of the various samples. (B) Nyquist plots of d-LCMO-Ar before and after the stability test.

Fig. 6

Open in a new tab

Fig. 6(A) indicates the various electrocatalysts' EIS spectrum matching with the catalytic activity. The smaller values of resistance refer to better conductivity and higher catalytic activity. The equivalent circuit consists of Rct, Rs, and CPE, where Rct a resistance expressing the flow of electrons between ions throughout an electrochemical process. Rct is the semicircle diameter. For the different catalysts LCO, d-LCO, d-LCMO-Air, and d-LCMO-Ar, the values of Rct are 487.4 Ω, 574.1 Ω, 168.4 Ω, and 23.81 Ω, respectively. Rs is the resistance of the ohmic electrolyte between the working and reference electrodes.46 For better fitting, the constant phase element (CPE) is used, representing a non-ideal capacitor resulting from the heterogeneities and porosity of the surface. In the absence of frequency dispersion, the capacitance value of perfect capacitors is expressed as CPE-P, and the exponent of CPE-T = 1.55 By checking Table 3. The values of each parameter for the various electrocatalysts were tabulated.

Table 3. Equivalent circuit parameters of LCO, d-LCO, d-LCMO-Air, and d-LCMO-Ar.
Material R s (Ω) R p (Ω) CPE
CPE-T (F) CPE-P
LCO 0.97 487.4 0.008 0.74
d-LCO 0.85 574.1 0.029 0.66
d-LCMO-Air 0.89 168.4 0.03 0.67
d-LCMO-Ar 0.76 23.81 0.002 0.74
Open in a new tab

To assess the electronic durability of d-LCMO-Ar, the post stability test Nyquist was carried out and compared with the Nyquist before the stability test (Fig. 6(B)). Even after a 50 h stability test, it retains a similar Nyquist with minor enlargement in the semicircle profile. This enlargement refers to possible changes such as reconstruction in the electrocatalyst and confirms the high stability of the prepared electrocatalyst.

The optimized sample d-LCMO-Ar showed a low onset potential comparable to the reported samples, even the precious metal IrO2. By checking the stability curves, our sample d-LCMO-Ar showed stunning stability over 50 h and was better than precious metals. The d-LCMO-Ar shows promising results, as seen in the stability, current density, Tafel slope value, and ECSA, and represents a step towards replacing precious metals as OER and HER electrocatalysts. It's a fertile area that still needs improvements to lower its potential and increase its current density.

4. Discussion

The crystallite volume of LCO was 118.6 nm, more than double, and sharply inclined to lower the half and continued decreasing even in the tested samples under electrochemical tests for the samples d-LCO, d-LCMO-Air, and d-LCMO-Ar. The microstrain in LCO (1.5 × 10−3) was positive and increased to be about double (in sample d-LCO), then kept negative and increased in the negative with doped Mo samples, whether in the air or Ar; refer to Fig. 2 and Table 1. It meant that great changes occurred with small changes, such as tuning the structure with 10% of A-sited defects or doping with small amounts under different media. Any of these changes representing a driving force could assist the charge transfer via vacancies to the catalytic site beneficially. The a/c value was quite the same value even when the changes occurred in the fourth digit, except for the sample d-LCMO-Air that confirms a non-pseudo lattice due to the various effects in the prepared compounds and under the high temperature effect for 3 h. It is believed that the diffusion occurred between the contributing particles, whether La, Cu, or Mo, due to an A-site defect or doping, resulting in lattice distortion and particle surface defect creation, and then catalytic active sites increase. These slight changes in the d-LCMO-Ar caused the greatest effective electrocatalytic area. Porous structures formed in the d-LCMO-Air and d-LCMO-Ar with higher homogeneities in the pore size distribution. The influence on electrochemical properties seemed too much of an increment, as mesopores enhance electrolyte access to active sites.

The eqn (10)–(13) discuss the adsorbate evolution mechanism (AEM), which is the closest explanation to the occurring reaction over the perovskite oxide, especially in alkaline medium for OER. Starting with the adsorption of OH− ions of water molecules on the perovskite active sites, causing the formation of several intermediates such as peroxide formation, hydroxide deprotonation, then peroxide deprotonation, and hydroxide reformation. These intermediates, such as OH*, OOH*, and O*, are all common, and electrons are described in the referred equations.56 The X in the following refers to the catalytic site.

X + OH− → X–OH* + e− 10
X–OH* + OH− → X–O* + H2O + e− 11
X–O* + OH− → X–OOH* + e− 12
X–OOH* + OH− → X + O2 + H2O + e− 13

As for HER, the next equations will discuss the possible steps of HER, dissociation of water molecules producing H+, to be adsorbed on the electrocatalyst surface (Volmer reaction).

H2O + e− → OH− + Had 14

This is followed by one of two possible pathways, both of which may occur in alkaline or acidic media. The first is the Heyrovsky step, involving electrochemical desorption.

Had + H2O + e− → OH− + H2 15

The second is the Tafel step, involving the recombination of two adsorbed hydrogen atoms:

Had + Had → H2 16

Overall, the HER in alkaline media can be summarized by the net reaction.

2H2O + 2e− → 2OH− + H2 17

From Fig. 5 (CV) and Fig. 6 (Nyquist) analyses, the double-layer capacitance of the Cdl value increase means the layer thickness becomes thinner to facilitate the adsorption and desorption of adsorbate faster. Then, the electron transfer mechanism in eqn (12)–(15) will be accelerated; it also can be proven by Tafel and Nyquist plots (Fig. 4 and 6). Table 2 compares parts of the published work in different parameters, such as overpotential, current density, stability, Tafel slope, and the used electrodes. The optimized sample showed a low onset potential comparable to the most reported samples, even the noble metals. This is a novel nano-compound, representing a step towards replacing noble metals as OER and HER electrocatalysts. It's a fertile area that still needs improvements to lower its overpotential and increase its current density.

5. Conclusions

In summary, the optimized sample d-LCMO-Ar showed excellent activity at low onset potential towards OER and HER coupled with the superior stability of 50 h at 50 mA cm−2 current density. Also, 10 mA cm−2 is achieved at 258 mV and −117 mV overpotentials. By checking the ECSA, Nyquist, and Tafel slope measurements, the same trend of data was found. The highest ECSA was 183 cm2, with the lowest Tafel slope value and the smallest semicircle of Nyquist, i.e., the largest double layer capacitance. The optimized sample is strongly recommended in OER and HER scenarios. It's a chance for both OER and HER electrocatalysts and a significant advancement in the research and development of inexpensive, naturally occurring catalysts for general water-splitting. Overall, d-LCMO-Ar showed a synergistic effect resulting from the lattice manipulation by controlling defects, which was revealed in the current density, stability, charge transfer, and adsorption/desorption of the oxygen gas molecules on the working electrode. The greatest output of this work is the importance of the minor ratio of defective sites, and doping by 0.1 of both of them plays a main role in enhancing the current density and reducing the overpotential of the LCO as a pristine sample. This strongly reflects the promising importance of material engineering for inspiring the researchers' work toward a permanent solution for the electrocatalysis process.

Conflicts of interest

There are no conflicts to declare.

Supplementary Material

RA-016-D6RA03751H-s001

Acknowledgments

The authors extend their appreciation to the Deanship of Scientific Research at Northern Border University, Arar, KSA for funding this research work through the project number “NBU-FFR-2026-2723-05”.

Data availability

Data will be made available on request.

Supplementary information (SI): post-stability XRD patterns and FESEM micrographs of the d-LCMO-Ar electrocatalyst loaded over NF, EDX elemental profiles, an FTIR spectrum comparison, and ECSA normalized polarization curves for OER and HER. See DOI: https://doi.org/10.1039/d6ra03751h.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-016-D6RA03751H-s001
RA-016-D6RA03751H-s001.pdf (699.5KB, pdf)

Data Availability Statement

Data will be made available on request.

Supplementary information (SI): post-stability XRD patterns and FESEM micrographs of the d-LCMO-Ar electrocatalyst loaded over NF, EDX elemental profiles, an FTIR spectrum comparison, and ECSA normalized polarization curves for OER and HER. See DOI: https://doi.org/10.1039/d6ra03751h.

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