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Article

Green Synthesis of Manganese-Cobalt Oxyhydroxide Nanocomposite as Electrocatalyst for Enhanced Oxygen Evolution Reaction in Alkaline Medium

by
Rajeh Alotaibi
*,†,
Mabrook S. Amer
†,
Prabhakarn Arunachalam
* and
Saad G. Alshammari
Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2024, 14(6), 369; https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060369
Submission received: 16 May 2024 / Revised: 31 May 2024 / Accepted: 2 June 2024 / Published: 6 June 2024
(This article belongs to the Section Electrocatalysis)
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Abstract

:
Using green synthetic methods, a manganese-cobalt oxyhydroxide (MnCo-OOH) nanocomposite for electrocatalysis was prepared. Electrocatalysts were examined using powder X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FT-IR), and field-emission scanning electron microscopy (FESEM). In an alkaline medium, cyclic voltammetry and chronoamperometric analysis were applied to assess the electrocatalytic features of the MnCo-OOH nanocomposite. A strong correlation existed between MnCo-OOH’s morphology, crystallinity, and electrochemical activity. Upon examining the electrochemical characteristics, the as-deposited MnCo-OOH catalyst demonstrated a significantly lower overpotential, achieving 75 mA·cm−2 OER current density at 370 mV, four times larger than 19.7 mA·cm−2 for CoOOH catalysts, signifying that the MnCo-OOH catalyst exhibits a higher electrocatalytic OER features. In addition, the MnCo-OOH nanocomposite demonstrated a high current density of 30 and 65 mA·cm−2 at 1.55 and 1.60 VRHE for 12 h in 1.0 M KOH aqueous electrolyte. As a result of this study, it was determined that the fabricated MnCo-OOH nanocomposite would be an appropriate electrocatalyst in water electrolysis.

Graphical Abstract

1. Introduction

Globally, alternative energy sources as well as sustainable energy storage and conversion have been highly active topics of research [1,2]. A major focus of research is finding ways to reduce our dependence on fossil fuels and developing more efficient and renewable energy sources [3,4]. For electronic devices and vehicles, molecular hydrogen is proposed as a primary energy storage medium [1,5]. Moreover, the transportation sector needs energy-efficient storage technologies and environmentally friendly charging systems. Electrochemical water splitting limitations can be overcome by cheap and reliable electrochemical methods [6,7]. The thermodynamically unfavorable oxygen evolution reaction (OER) is a key issue for electrochemical water splitting. Hence, it is highly imperative to develop OER electrocatalysts with improved water oxidation kinetics and reduced overpotential to enhance energy conversion.
As of yet, no electrocatalytic material has reached the equilibrium potential of 1.23 VRHE [8,9], which corresponds to the minimum reaction energy. Furthermore, noble metal oxides (e.g., IrO2 and RuO2) are needed electrode materials for acidic environments owing to their admirable electrochemical properties [10]. Commercially, electrocatalytic water splitting is not feasible because of high prices and scarcity of electrocatalysts. As a result, efficient, robust, and cost-effective non-noble metal electrochemical OER catalysts are essential. Recently, nanostructured materials found on earth have gained attention for applications in catalysis, sensing, and energy conversion [11,12,13,14,15,16,17]. As a result, bare metal oxide electrocatalytic performances toward OER were relatively low, with NiOx > CoOx > FeOx > MnOx [18]. Despite this, it appears that alloying influences, such as the integration of multiple active alloy elements, can promote OER performance [19,20]. In recent years, cobalt oxyhydroxide (CoOOH) has drawn the greatest attention as a heterogeneous catalyst among cobalt-containing materials [21]. There is a high electron transfer rate, multiple surface active sites, a large number of phase reactions, and extensive presence on the Katanga Copperbelt [22]. Further, CoOOH occurs naturally as a stable and multifunctional material and has been engaged as a CO sensor [23], can serve as an electrode candidate for energy storage systems, and can be applied as a super-capacitor [24,25,26]. Recently, researchers discovered that integrating a different transition metal into bimetallic CoOx might enhance its intrinsic OER properties [27,28]. Several recent research reports have shown that mixed metal oxides and hydroxides form CoFe [29], NiFe [30,31], NiFeCo [32], and NiFeCoCe [33] electrocatalysts materials that can be used for electrocatalytic OER. In particular, the incorporation of transition metal ions over cobalt oxides/hydroxides is attractive to OER. Often, these transition metal ions are used in OER processes as catalysts to increase efficiency and reduce energy consumption.
It is traditional to synthesize metal oxide nanoparticles by first reducing them with a strong base, followed by a capping or reducing agent. It is sometimes necessary to dissolve these capping agents in organic solvents. It is also likely to reduce unnecessary toxic chemicals by using a green synthesis approach [34]. The green synthesis method uses plant-derived metabolites to synthesize metal oxide nanoparticles [35]. As a result of coating metal oxide nanoparticles with these metabolites, they are resistant to extensive aggregation and produce tunable particle sizes and well-defined morphologies [36]. Since plants contain diverse phytochemical profiles, the choice of plant species and their individual parts determines NP morphology and particle size [37]. Thus, green synthesis provides a cost-efficient and eco-friendly alternative to conventional nanoparticle synthesis. In addition, manganese with cobalt hydroxide can have synergistic effects leading to enhanced activity as a result of charge transfer between the metal and substrate.
Herein, we demonstrate manganese-doped cobalt oxyhydroxide catalysts using a green synthesis method using alkanna root extract as an electrocatalyst in alkaline media, followed by studies on electrocatalysis of OER reactions. Through FT-IR, XRD, N2 sorption, and microscopic analysis, we determined the structural and textural features of the Mn-incorporated CoOOH electrocatalysts. As a result, the Mn-promoted CoOOH electrodes (surface area 75 m2/g) were significantly more efficient than Mn or CoOOH electrodes and analogous to benchmark IrO2 electrodes in terms of their onset potential shifting by 98 mV, small Tafel slope, and reduced charge transfer resistance. Due to their facile synthesis, characteristic structure, and remarkable electrocatalytic features, these ternary-based electrocatalysts could be used in industrialized water splitting.

2. Results and Discussion

2.1. Crystalline, Structural, and Surface Features of Mn-CoOOH

The manganese-incorporated CoOOH catalysts were obtained by green synthetic approach with an alkanna plant root extract. Figure 1 reveals the different phases for the development of crystalline manganese-incorporated CoOOH catalysts. The acquired material was cleaned with DI water several times to eliminate the impurities. In the final step, the resulting products were obtained by thermosetting processes to obtain the formation of MnCo-OOH materials.
Figure 2a displays the powder diffraction patterns of MnCo-OOH materials compared to CoOOH and Mn(OH)2. As displayed in Figure 2a, the CoOOH and MnCo-OOH catalysts show the characteristic CoOOH trigonal PXRD pattern (PDF # 01-073-0497). The sharp diffraction lines clearly indicate a relatively higher crystallinity for these products. For reference, the bare Mn(OH)2 catalysts showed the characteristic Mn(OH)2 hexagonal powdered XRD pattern (PDF # 01-073-5018). Additionally, Figure 2a displays the diffraction patterns of the MnCo-OOH electrodes that match the diffraction lines of CoOOH and Mn(OH)2 (PDF # 01-073-0497 and 01-073-5018, respectively). During the green synthetic process, no other XRD peaks were observed, suggesting there were no crystalline phases. Overall, the results confirm that the synthetic process is successful in forming MnCo-OOH.
A comprehensive FTIR study was conducted on MnCo-OOH, CoOOH, and Mn(OH)2 samples, varying from 4000 to 400 cm−1, respectively. CoOOH materials exhibit a solid peak (Figure 2b) due to stretching vibrations of hydrogen-bound O-H groups [38,39], υ (O-H). There were two signals at (1613 cm−1 and 583 cm−1) in CoOOH that were related to vibrations of the double and single bonds between Co and O, respectively [40]. The FTIR spectrum of the Mn(OH)2 sample (Figure 2b) showed two strong peaks (612 and 502 cm−1) originating from the stretching vibrations of Mn-O and Mn-O-Mn bonds [41], which indicate that MnO was formed. As Mn elements are introduced, vibrational modes shifted marginally to the lower wavenumber region. There were two distinctive M-O stretching vibration bands at 577 cm and 656 cm−1. Hydrogen bonds between hydroxides of layers, interlayer water, and anions in the interlayer gap also contribute to this broad feature of the hydroxyl band. In addition, there was a weak distinctive band at 1613 cm−1, related to the HOH-bending frequencies of adsorbed water molecules. It was verified by these FTIR results that MnCo-OOH catalysts can be produced effectively.
BET N2 sorption isotherms were used to regulate the pore size and specific area of the nanomaterials. As shown in Figure 2c, MnCo-OOH and CoOOH, as well as Mn(OH)2 nanomaterials have different BET isotherms. The BET specific area values of Mn(OH)2, CoOOH, and MnCo-OOH were evaluated to be 87.48, 104.19, and 75.84 m2/g, respectively. Table 1 summarizes the BET results of fabricated materials. In accordance with IUPAC classifications, the N2 sorption curves closely resemble type-IV isotherm hysteresis loops. It was detected at a relative pressure of greater than 0.7 that hysteresis loops were distinct, indicating capillary condensation of mesopores [42,43]. Based on BJHs, PSD curves were calculated for each fabricated material (Figure 2d). The pore sizes of these samples were 3.0 to 10 nm. As shown in Table 1, all samples had similar textural parameters such as pore volume, specific area, and diameter. The mesoporosity of the material can be attributed to its thermal decomposition, which could lead to weight loss and volume shrinkage. This characteristic of MnCo-OOH samples provides easy access for ions to electrode/electrolyte interfaces, allowing surface redox reactions to occur.
The morphology and fine structure of CoOOH, Mn(OH)2, and MnCo-OOH catalysts were evaluated using electron microscopy measurements (FESEM/HRTEM). Particularly, the stepwise pyrolysis in the air is exposed to maintain nanoparticle structure. FESEM images of the annealed in air CoOOH materials (Figure 3a,b) clearly show interconnected nanoparticles. Figure 3b shows high-resolution images of CoOOH nanoflakes. Further, it illustrates the morphology of assembled microblocks of nanoparticle aggregates with mean particle sizes ranging from 15 to 30 nm. As a comparison, Figure 3c,d shows that Mn(OH)2 nanoparticles had a flower-like morphology and spherical shapes. High magnification SEM images (Figure 3d) clearly show that microplates of different sizes were stacked together. There was no noticeable change in the morphology of nanoflake CoOOH films after incorporating manganese (Figure 3e,f). According to Figure 3f, the incorporation of Mn into CoOOH nanoparticles hampers the agglomeration of nanoparticles, resulting in smaller particle sizes than bare CoOOH nanoparticles. Furthermore, we observed nanoflakes and spherical nanoparticles that provided electrode materials with more reaction sites. Further examination of the morphology of the CoOOH, Mn(OH)2, and MnCo-OOH NP catalysts was performed using TEM. As can be seen in Figure 4a,b, CoOOH is largely composed of hexagonal plates with smaller irregularly shaped particles. Figure 4c,d show that the bare Mn(OH)2 consists largely of ultra-small nanoplate-like structures with numerous small pores between the particles. Figure 4e,f illustrate that the MnCo-OOH composite material consists of hexagonal plates that are fused together in nanoaggregates, forming continuous networks. Additionally, the TEM image of MnCo-OOH clearly shows that the flakes have multiple interconnected nanoparticles and, therefore, several pores between them.
Using energy-dispersive X-ray spectroscopy (EDAX), Mn loading in catalysts was evaluated. A detailed examination of EDAX mapping revealed the attendance of Mn, Co, and O components across the entire MnCo-OOH structure (Figure 5a–g). The EDAX mapping of CoOOH materials showed a homogenous distribution of Mn, Co, and O elements. Based on the EDAX analysis, MnCo-OOH catalysts consist of 7.61, 72.61, and 19.72 wt%, individually, for Mn, Co, and O elements (Figure 5g).

2.2. Electrochemical Performance of MnCo-OOH for OER

In Figure 6, LSV in alkaline media was used to determine the electrocatalytic features of OER materials. In a three-electrode system, Mn(OH)2, CoOOH, and MnCo-OOH-loaded glassy carbon electrodes were engaged as working electrodes. Figure 6a illustrates the LSV activities of bare CoOOH, bare Mn(OH)2, state-of-the-art IrO2, and MnCo-OOH electrodes in 1.0 M KOH. The LSV showed that the as-prepared Co-based electrocatalysts exhibited Co2+/Co3+ oxidation between 1.35 and 1.48 V vs. RHE [44,45]. Additionally, MnCo-OOH-loaded electrodes were the best-optimized electrode candidates for OER, with enhanced current density between 1.50 and 1.6 V compared to RHE electrodes. In electrocatalysis, the onset potential can be roughly determined by measuring the x-coordinate of the point where the two linear parts cross. Additionally, the MnCo-OOH catalyst’s OER onset potential was shifted by about 90 mV to a more negative position than the bare CoOOH catalyst, showing much faster OER kinetics. As seen in Figure 6b, the Tafel slope of MnCo-OOH (99 mV dec−1) is suggestively lesser than those of IrO2 (115 mV dec−1), and CoOOH (64 mV dec−1). Based on the smaller Tafel slope for MnCo-OOH compared to Mn(OH)2, it is evident that the simultaneous influence of co-doped Mn and Co elements significantly enhances the electrochemical kinetics of OER [46].
An ECSA was employed in order to examine the effect of Mn incorporation on the enhancement of Mn-CoOOH catalysts towards OER activity. The capacitive currents revealed in Figure 6c of the obtained electrode materials demonstrate that the linear slope for the attained catalysts, CoOOH and MnCo-OOH, is 44.05, and 64.4 mF cm−2, correspondingly. Based on the results, MnCo-OOH catalysts have a larger ECSA compared to catalysts without Mn incorporation, confirming that Mn generates more active sites. Among all fabricated electrodes, MnCo-OOH catalysts have the best performance (Figure 6c). The OER features of electrocatalysts are enhanced with increased Mn loading in alkaline conditions. An MnCo-OOH catalyst exhibits the best electrocatalysis performance among the synthesized CoOOH, IrO2, and MnCo-OOH catalysts (Figure 6d), with a lower onset potential and an overpotential of 259 mV.
To gain a better quantitative understanding of the OER process, we studied the kinetics and charge transfer process using EIS. In Figure 7, Nyquist plots are shown for pure CoOOH and MnCo-OOH electrodes at 1.48 V (vs. RHE), and impedance parameters are shown in Table 2. In Figure 7a, the Nyquist plots best match Randle’s equivalent circuits. In Figure 7b, the same data are also plotted in Bode format. It is confirmed that the RCT of the catalysts examined corresponds to the lower frequency zone and its equivalent circuit. After Mn was introduced into a CoOOH framework, R1 and R2 values decreased. As shown in Table 2, the MnCo-OOH, CoOOH, and Mn(OH)2 R2 values are 8.32, 31.45, and 4154 Ω, respectively. Most noticeably, the arc radius of the Nyquist curve of MnCo-OOH electrodes is smaller than that of CoOOH electrodes. Accordingly, MnCo-OOH has smaller R2 values, indicating more significant electrochemical features, as described earlier. Similarly, MnCo-OOH electrodes in Figure 7b show lower resistance than CoOOH electrodes in Bode plots. MnCo-OOH has an excellent charge transfer capability, which contributes to the electrode’s higher intrinsic OER properties. It has been demonstrated that ternary transition metal oxides are best formed using doping approaches, and that Mn’s synergetic effect on CoOOH integration greatly enhances the performance of the OER electrocatalytic system.
The durability of OER on MnCo-OOH was investigated using CA measurements at 1.55 V vs. RHE in 1.0 M KOH. Additionally, CoOOH (Figure 8a) and MnCo-OOH (Figure 8b) samples were also tested for 12 h under a nearly constant voltage of 1.55 VRHE and 1.60 VRHE in alkaline mediums. During the electrochemical reaction, when a sufficient voltage is applied, oxygen gas is produced at the electrode surface. The oxygen ions then combine to form oxygen bubbles, which detach from the electrode and rise to the surface. In particular, at 1.60 V compared to RHE, oxygen bubbles at the electrode surface are definitely responsible for the fluctuations in current. According to the results, MnCo-OOH electrodes maintain a nearly constant current density over a long period of time. In comparison with other MnCo-OOH catalysts, CoOOH exhibited much lower CA currents and the acquired results agree with the LSV curve (Figure 6a). After calcination, MnCo-OOH electrodes create metal oxide networks that make them suitable for OER because they can be transported easily, have high electronic conductivity, and can be transported in mass [47]. Furthermore, MnCo-OOH electrodes show good durability and stability, with no sign of degradation even after 12 h of operation. Ultimately, MnCo-OOH electrodes prove to be a promising and cost-efficient choice for OER applications. In Table 3, we compared the electrocatalytic properties of MnCo-OOH@GC with those of other catalysts found in the literature. In particular, our catalyst exhibited a significantly higher OER activity, achieving a lower overpotential and a higher current density. Due to its improved OER features, MnCo-OOH@GC electrode materials offer great potential for energy storage and conversion systems.

3. Experimental Methods

3.1. Materials

Cobalt (II) nitrate hexahydrate (Co(NO3)2·6H2O, ≥98.0%), manganese dichloride (MnCl2, ≥98.0%), and sodium hydroxide (NaOH, ≥85.8%) were acquired from Fisher Scientific, Hampton, NH, USA. Potassium hydroxide (KOH, ≥84.5%) was obtained from PanRaeac AppliChem, Darmstadt, Germany. All chemicals were used without any modification.

3.2. Preparation of Alkanna Tinctorial Root Extract

To remove impurities, alkanna dye root was washed several times with deionized (DI) water. In total, 10 g of alkanna tinctorial root was heated in 100 mL of DI water at 60 °C for 3 h. The aqueous extract was filtered. The filtration was used as a reducing agent.

3.3. Preparation of Co/Mn Oxide Nanoparticles Synthesis

A total of 100 mL of 0.02 M Co3(NO3)2·6H2O and 0.002 M MnCl2 were mixed with 20 mL of alkanna tinctorial root extract. Afterward, the reaction mixture was left to stir for three hours. The pH was maintained at 10 by adding 1 M of NaOH. The reaction left to continue stirring for 3 h. The nanoparticles were collected using a Buckner funnel. The product was washed several times with DI water to remove impurities. Afterwards, the acquired product was calcined at 200 °C for three hours. As control catalysts, CoOOH and MnOOH samples were prepared using the same steps, but in the presence of Mn or Co precursors.

3.4. Catalyst Characterization

The crystalline nature of the materials was evaluated with XRD on a Bruker D8-advance diffractometer (λ = 1.5418 Å) (Bruker, Ettlingen, Germany). Using linear sweep voltammetry, chronoamperometry, and EIS, we measured the electrochemical characteristics of the obtained materials using a three-electrode Pyrex glass cell with a computerized potentiostat/galvanostat (Autolab, Bloomfield Hills, MI, USA, PGSTAT30). A Pt counter electrode and an Ag/AgCl (3.0M KCl) reference electrode were employed, while glassy carbon supported catalysts were engaged as working electrodes. As a reference electrode, an Ag/AgCl (1 M KCl) reference electrode was used, which was modified according to Nernst equation to obtain the reversible hydrogen electrode (RHE). OER was conducted at 25 °C with 1.0 M KOH at a sweep rate of 10 mVs−1. Using the electrode materials at fixed potentials, CA characterization was performed in order to determine their durability.

4. Conclusions

We developed a simple green synthetic method for ternary manganese-integrated CoOOH electrocatalysts. Further, the resulting MnCo-OOH catalyst displayed excellent OER performance and reduced charge transfer resistance. MnCo-OOH composite electrodes also demonstrated better conductivity, electrocatalytic activity, and stability than CoOOH electrocatalysts and benchmark IrO2 electrocatalysts. The electrocatalytic properties of MnCo-OOH are attributed to its high electronic conductivity, allowing rapid reaction reactants and product diffusion and interfacial electron transport. As a result of these results, we demonstrated the importance of green synthetic approaches in the preparation of CoOOH-based electrocatalysts that demonstrate electrochemical activity for water splitting in alkaline environments.

Author Contributions

R.A.: Conceptualization, supervision, methodology, investigation, data curation, formal analysis, project administration, funding acquisition. M.S.A.: Conceptualization, data curation, formal analysis writing—review and editing. P.A.: Writing—review and editing, data curation. S.G.A.: Data curation, formal analysis. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Researchers Supporting Project number (RSPD2024R644) at King Saud University, Riyadh, Saudi Arabia.

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 14 00369 g001
Figure 1. Schematic depiction of the fabrication of MnCo-OOH electrocatalysts obtained with alkanna plant root extract.
Figure 1. Schematic depiction of the fabrication of MnCo-OOH electrocatalysts obtained with alkanna plant root extract.
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Catalysts 14 00369 g002
Figure 2. (a) The XRD patterns for pure CoOOH, Mn(OH)2, and MnCo-OOH catalysts shown below are indexed using reference peaks; trigonal phase CoOOH (JCPDS # 01-073-0497), hexagonal Mn(OH)2 (JCPDS # 01-073-1604), all the peaks are indexed via standard peaks, (b) FTIR patterns of pure CoOOH, Mn(OH)2, and manganese incorporated CoOOH catalysts prepared by the green synthetic approach using alkanna plant root extract, (c) N2 sorption isotherms, and (d), its agreeing BJH desorption pore size distributions of CoOOH, Mn(OH)2, and MnCo-OOH catalyst.
Figure 2. (a) The XRD patterns for pure CoOOH, Mn(OH)2, and MnCo-OOH catalysts shown below are indexed using reference peaks; trigonal phase CoOOH (JCPDS # 01-073-0497), hexagonal Mn(OH)2 (JCPDS # 01-073-1604), all the peaks are indexed via standard peaks, (b) FTIR patterns of pure CoOOH, Mn(OH)2, and manganese incorporated CoOOH catalysts prepared by the green synthetic approach using alkanna plant root extract, (c) N2 sorption isotherms, and (d), its agreeing BJH desorption pore size distributions of CoOOH, Mn(OH)2, and MnCo-OOH catalyst.
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Catalysts 14 00369 g003
Figure 3. Morphological characteristics of samples. FE-SEM photographs of the obtained (a,b) CoOOH, (c,d) Mn(OH)2, and (e,f) MnCo-OOH samples.
Figure 3. Morphological characteristics of samples. FE-SEM photographs of the obtained (a,b) CoOOH, (c,d) Mn(OH)2, and (e,f) MnCo-OOH samples.
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Catalysts 14 00369 g004
Figure 4. TEM analysis of CoOOH, Mn(OH)2, and Co-Mn-OOH samples. TEM photographs of crystalline Co-OOH at different amplifications, (a,b) and Mn(OH)2 samples at different amplifications (c,d), and MnCo-OOH (e,f) samples obtained through a green synthetic approach via alkanna tinctorial root extract.
Figure 4. TEM analysis of CoOOH, Mn(OH)2, and Co-Mn-OOH samples. TEM photographs of crystalline Co-OOH at different amplifications, (a,b) and Mn(OH)2 samples at different amplifications (c,d), and MnCo-OOH (e,f) samples obtained through a green synthetic approach via alkanna tinctorial root extract.
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Catalysts 14 00369 g005
Figure 5. EDAX elemental mapping of Co, Mn, and O elements in MnCo-OOH. SEM and EDAX profiles of Mn, Co, and O elements in MnCo-OOH (a–g).
Figure 5. EDAX elemental mapping of Co, Mn, and O elements in MnCo-OOH. SEM and EDAX profiles of Mn, Co, and O elements in MnCo-OOH (a–g).
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Catalysts 14 00369 g006
Figure 6. Electrochemical OER characteristics of films. (a) LSV plots of Mn(OH)2, CoOOH, and MnCo-OOH materials on GC substrate in 1.0 M KOH at 10 mV s−1. (b) Tafel curves acquired from the LSV plots in (a). (c) Linear curves of capacitive currents of the obtained electrodes against scan rate. (d) A comparison of catalyst OER properties and current properties at varied potentials.
Figure 6. Electrochemical OER characteristics of films. (a) LSV plots of Mn(OH)2, CoOOH, and MnCo-OOH materials on GC substrate in 1.0 M KOH at 10 mV s−1. (b) Tafel curves acquired from the LSV plots in (a). (c) Linear curves of capacitive currents of the obtained electrodes against scan rate. (d) A comparison of catalyst OER properties and current properties at varied potentials.
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Catalysts 14 00369 g007
Figure 7. Nyquist plots of electrodes (a) and phase angle bodes (b) derived from EIS analysis in 1.0 M KOH at 1.48 VRHE of bare Mn(OH)2, CoOOH and MnCo-OOH catalysts.
Figure 7. Nyquist plots of electrodes (a) and phase angle bodes (b) derived from EIS analysis in 1.0 M KOH at 1.48 VRHE of bare Mn(OH)2, CoOOH and MnCo-OOH catalysts.
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Figure 8. Durability tests. Chronoamperometry responses of CoOOH (a) and MnCo-OOH (b) electrocatalysts evaluated in 1.0 M KOH at 1.55 V vs. RHE and 1.60 V vs. RHE.
Figure 8. Durability tests. Chronoamperometry responses of CoOOH (a) and MnCo-OOH (b) electrocatalysts evaluated in 1.0 M KOH at 1.55 V vs. RHE and 1.60 V vs. RHE.
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Table 1. Textural properties of specific surface area, total pore volume, and the pore size of the MnO, MnCo-OOH, and CoOOH catalysts derived from nitrogen adsorption and desorption data.
Table 1. Textural properties of specific surface area, total pore volume, and the pore size of the MnO, MnCo-OOH, and CoOOH catalysts derived from nitrogen adsorption and desorption data.
Materials Surface
Area (m2/g)		Pore
Size (nm)		Pore
Volume (cc/g)
Mn(OH)287.492.740.471
MnOCoO75.841.600.337
CoO104.201.960.484
Table 2. EIS parameters were used to calculate equivalent circuits of impedance spectra in 1.0 M KOH solutions. Rs = solution resistance, R1 = film resistance, R2 = charge-transfer resistance, Q1 = CPE of electrode layer, Q2 signifies the double layer capacitance.
Table 2. EIS parameters were used to calculate equivalent circuits of impedance spectra in 1.0 M KOH solutions. Rs = solution resistance, R1 = film resistance, R2 = charge-transfer resistance, Q1 = CPE of electrode layer, Q2 signifies the double layer capacitance.
SamplesRs (ohm)Q1 (µF)R1 (ohm)Q2 (µF)R2 (ohm)
Mn(OH)210.9617.6327,36542.244154
CoOOH10.28884212616,52031.45
MnCo-OOH9.8411,83010533,5008.32
Table 3. Comparison of our catalyst MnCo-OOH electrochemical performance with those published for Mn/Co-based catalysts in alkaline medium.
Table 3. Comparison of our catalyst MnCo-OOH electrochemical performance with those published for Mn/Co-based catalysts in alkaline medium.
Anodic CatalystsOverpotential at 10 mA·cm−2, mVCurrent Density, mA·cm−2
at 1.6 VRHE		Tafel Slope, mV·dec−1Electrolyte Conc., MRef.
5%Ni-Co@PG carbon38148730.1 M KOH[48]
Cu–doped
Co(OH)2 nanosheets@GC		300102471.0 M KOH[49]
Ni-doped Co3O4@GC380-631.0 M KOH[50]
Ni/Ni0.2Mo0.8N/MoO3@NF318751001.0 M KOH[51]
Fe-CoOOH/G @GC33025371.0 M KOH[52]
Fe0.33Co0.67OOH PNSAs@CFC266-301.0 M KOH[53]
Cu-CoOOH@CFP234-791.0 M KOH[54]
FeV/m-Co3Se4@GC28063411.0 M KOH[55]
Ce-doped
NiCo LDH@NF		236-441.0 M KOH[56]
WCoO-NP@GC27044921.0 M KOH[57]
MnCo-OOH@GC25078991.0 M KOHThis work
PG—pyrolytic graphite carbon electrode; CFC—carbon fiber paper; NF—nickel foam; GC—glassy carbon.
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Alotaibi, R.; Amer, M.S.; Arunachalam, P.; Alshammari, S.G. Green Synthesis of Manganese-Cobalt Oxyhydroxide Nanocomposite as Electrocatalyst for Enhanced Oxygen Evolution Reaction in Alkaline Medium. Catalysts 2024, 14, 369. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060369

AMA Style

Alotaibi R, Amer MS, Arunachalam P, Alshammari SG. Green Synthesis of Manganese-Cobalt Oxyhydroxide Nanocomposite as Electrocatalyst for Enhanced Oxygen Evolution Reaction in Alkaline Medium. Catalysts. 2024; 14(6):369. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060369

Chicago/Turabian Style

Alotaibi, Rajeh, Mabrook S. Amer, Prabhakarn Arunachalam, and Saad G. Alshammari. 2024. "Green Synthesis of Manganese-Cobalt Oxyhydroxide Nanocomposite as Electrocatalyst for Enhanced Oxygen Evolution Reaction in Alkaline Medium" Catalysts 14, no. 6: 369. https://0-doi-org.brum.beds.ac.uk/10.3390/catal14060369

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