Self-Assembly of Copper Oxide Interfaced MnO2 for Oxygen Evolution Reaction

Designing efficient electrocatalytic systems through facile synthesis remains a formidable task. To address this issue, this paper presents the design of a combination material comprising two transition metal oxides (copper oxide and manganese oxide (CuO/MnO2)), synthesized using a conventional microwave technique to efficiently engage as an active oxygen evolution reaction (OER) catalyst. The structural and morphological properties of the composite were confirmed by the aid of X-ray diffraction (XRD) studies, field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive spectrometry (EDS). FESEM clearly indicated well-aligned interlacing of CuO with MnO2. The OER performance was carried out in 1 M KOH. The assembled CuO/MnO2 delivered a benchmark current density (j = 10 mA cm−2) at a minimal overpotential (η = 294 mV), while pristine CuO required a high η (316 mV). Additionally, the CuO/MnO2 electrocatalyst exhibited stability for more than 15 h. These enhanced electrochemical performances were attributed to the large volume and expanded diameter of the pores, which offer ample surface area for catalytic reactions to boost OER. Furthermore, the rate kinetics of the OER are favored in composite due to low Tafel slope (77 mV/dec) compared to CuO (80 mV/dec).


Introduction
In the pursuit of achieving a carbon-neutral society and to alleviate the harmful effects of conventional fossil fuels, there is a pressing need to develop sustainable, eco-responsive, and green energy sources. To cater to fossil fuel resource depletion and its associated environmental impacts, in addition to the dynamic geopolitical instabilities of fossil fuel production, energy researchers have explored numerous sustainable energy sources such as wind, biomass, tidal, and solar energy. Among these available energy resources, solar radiation is one of the most abundant natural energy sources, capable of meeting the entire global energy demand, but it suffers from intermittent availability due to geographical variations [1,2]. Furthermore, the sun shines only for a part of the day and its power density varies as the function of geographical altitude imposes another complexity to its wider applicability. Additionally, with the recent progress in photovoltaic devices, the power convergence efficiency hardly reached~26% [3], further jeopardizing the adequacy of solar power [4]. Among several available renewable energy sources, splitting water (2H 2 O = 2H 2 + O 2 ) to generate useful hydrogen as a fuel and oxygen as a by-product has drawn substantial attraction, as the generated hydrogen can be stored and can be transported at the required site. However, water electrolysis is an energy-intensive process, and its efficiency is limited by the sluggish kinetics of the oxygen evolution reaction (OER) at the anode. The OER proceeds via a four-electron process and involves the formation of two oxygen-oxygen bonds, and thus determines the overall rate kinetics of the water electrolysis [5]. Theoretically, OER requires a thermodynamic potential of 1.23 V (25 • C/1 atm); however, the commercial catalysts require much higher overpotentials (η). To date, Irand Ru-based electrocatalysts have been the best-performing OER electrocatalysts in aqueous media. However, their scarcity and prohibitive cost pose a major obstacle towards widespread use of electrolysis technologies. Therefore, significant efforts have been made in recent years to develop efficient and cost-effective OER catalysts that can enhance the performance of water electrolysis and enable the large-scale production of green hydrogen [6][7][8]. Of note, several 3D transition metal (TM)-based layered oxide electrocatalysts requiring lower η and demonstrating improved electrochemical performance toward OER have been reported recently [9,10].
Recently, larger interest has been shown in first-row TM-oxides (for instance, Cu, Co, Ni, and Fe) due to their enhanced electrochemical water splitting [11]. In particular, there has been significant interest in Cu-based oxidation electrocatalysts due to their natural abundance, low cost, rich redox chemistries, and non-toxic nature. Additionally, due to their low bandgap, they have been employed in several applications spanning from semiconductor, gas sensing, solar cells, and several biomedical applications [12][13][14]. However, their η values are still higher between 320-450 mV, which needs to be reduced significantly [15]. Another promising candidate is oxide of Mn (MnO 2 ), which displays enhanced electrochemical performances due to its unsaturated edges and possesses a wide variety of electronic structures, and its natural abundance is an added advantage [15]. This material has been widely used in other energy domains such as energy storage and electrochemical sensors for detecting metal ions [16][17][18]. Inheriting the benefit of CuO and MnO 2 by combining (CuO/MnO 2 ), which is prepared hydrothermally, has been employed in catalytic applications for CO oxidations [19]. It is reported that CuO-MnO 2 interfaces serve as the active sites for CO oxidations and the chemisorbed CO on CuO reacts with the oxygen species in MnO 2 . This combination has also been reported to enhance electrochemical performances during the oxidation of quinolone at supercritical conditions and the proposed model dictates that the catalytic reaction strongly follows the temperature dependency [20][21][22].
Interfacial systems of similar structural and chemical complexity and their structural, chemical, and adsorption properties, including in relation to real-world applications, have been studied by using high-level DFT and ab initio molecular dynamics calculations [23]. Molecular modeling is regularly used for guiding and controlling the synthesis of various materials [24]. The nanosheet arrays of metal oxide/carbon (MOx/C; M = Fe, Ag, and Mn) fabricated and employed in the covered two-dimensional (2D) metal-organic frameworks (2D-MOFs) exhibiting significant electrocatalytic activity and durability [25]. Nb-doped TiO 2 rod-like particles were synthesized using molten salt as a reaction medium and applied as a catalyst support in ORR/OER bifunctional gas diffusion electrodes for use in metal-air batteries [26]. Ultrathin amorphous MnO 2 modified prawn-shell-derived porous carbon (U-MnO 2 /PSNC) was designed and synthesized via a self-template-assisted pyrolysis coupling in situ redox reaction strategy to create robust oxygen electrocatalysts [27]. Fe 2 O 3 decorated on carbon nanotubes as a promising architecture was utilized in OER reactions [28]. Pt alloy integrated in a cobalt-nitrogen-nanocarbon matrix 11.7 times higher Nanomaterials 2023, 13, 2329 3 of 10 than that of a commercial Pt catalyst retained a stability of 98.7% after 30,000 potential cycles [29]. A carbon/rGO composite, as an efficient palladium electrocatalyst for formic acid oxidation reaction, was reported [30]. Developments in electrocatalyst corrosion chemistry, including corrosion mechanisms, mitigation strategies, and corrosion syntheses/reconstructions based on typical materials and important electrocatalytic reactions, have been extensively studied [31]. These interesting reports encouraged us to produce a CuO/MnO 2 system for electrocatalytic studies.
In the present study, we proposed a microwave-assisted synthetic protocol for the successful preparation of CuO and its interlaced architecture with MnO 2 . The structural reliability and morphological properties of the composites were confirmed with the aid of XRD, FESEM, XPS, and EDS mapping. The surface property of CuO/MnO 2 was determined by BET analysis. As a proof-of-concept demonstration, we explored the virgin CuO and its composite CuO/MnO 2 for application in the oxygen evolution reaction. The CuO/MnO 2 exhibited an OER of 294 mV, which was evaluated with virgin CuO electrocatalyst (316 mV) at 10 mA cm −2 and remained stable for 15 h. The synthetic procedure and the OER property of the prepared materials demonstrate its potential applications in electrocatalysis for the design of new hybrid materials.

Materials and Reagents
The following materials of the highest purity were acquired from Sigma-Aldrich chemical (Korea) and applied without any further cleansing: CuSO 4 ·5H 2 O, KMnO 4 , Mn(CH 3 COO) 2 ·4H 2 O, and NaOH. Double-deionized water (DDW) was used for the preparation.

Syntheses of CuO
The protocol reported in [22] was utilized for preparing the CuO. Here, 20 mL of 0.2 M CuSO 4 ·5H 2 O (20 mL) and 0.8 M NaOH (20 mL) solution were combined in a crucible and subjected to microwave irradiation for 10 min at 300 W. After completion of the reaction, the crude product was filtered, sequentially cleansed with deionized water to remove the water-soluble impurities, and ultimately dried at 80 • C. The obtained CuO was then utilized for electrocatalytic studies.

Syntheses of CuO/MnO 2
First, 0.2 M Mn(CH 3 COO) 2 ·4H 2 O (20 mL), 0.2 M KMnO 4 (20 mL), and 100 mg of CuO were placed in a crucible. The reaction mass was subjected to microwave irradiation for 15 min at 300 W. The solid product was then filtered, washed, and dried at 80 • C. Calcination of the resultant product for 6 h at 400 • C at a heating level of 5 • C/min resulted in pure CuO/MnO 2 , which was then utilized for electrocatalytic studies.

Results and Discussion
Pristine CuO and its composite CuO/MnO 2 were prepared using a microwave-assisted synthetic protocol. The precursors CuSO 4 .5H 2 O, Mn(Ac) 2 ·4H 2 O, and KMnO 4 were used for the formation of CuO/MnO 2 as illustrated in Figure 1. The suggested imitation route was ecologically friendly, as CuO is interlaced with MnO 2 . Growth mechanism for the formation of CuO/MnO 2 structure, can be divided into three stages. Firstly, the MnO 4nuclei are produced and adsorbed on surfaces of CuO, forming MnO 2 nuclei. With the increase of reaction time, the MnO 2 nuclei are aggregated and transformed to nanosheets and nanorods. The MnO 2 nanoroads are compact and totally cover surfaces of CuO, resulting in the formation of the hierarchical CuO/MnO 2 nanocomposites. Such a process is supported by the morphological evolution at different growth stages via tuning the reaction time. It is worth noting that the interconnected MnO 2 nanosheets and CuO nanostructure give rise to a highly porous morphology, which can offer very high surface area and many active sites for electrochemical transportation. X-ray diffraction (XRD) was used to study the structural advancement and phase purity of the CuO and its composite CuO/MnO 2 . As illustrated in Figure 2a, the phase analysis was conducted using XRD at Bragg's diffraction angle range was adjusted between 5 • -65 • during the measurement, and the scan rate was set to 2 • /minute. The signals for planes (110) , which is crystalline [32,33]. The surface areas (BET) and pore size distribution (BJH) parameters were studied using N 2 adsorption−desorption isotherms as displayed in Figure 2, which demonstrated a characteristic IV isotherm that was ascribed to the mesoporous architecture of the aggregated CuO sphere, as displayed in Figure 2b. The CuO/MnO 2 revealed an enhanced volume and diameter of the pores, as shown in Figure 2c. The specific surface area for CuO/MnO 2 was found to be 54.50 m 2 g -1 , compared to 19.8 m 2 g -1 for pure CuO. The enhancement in surface area of the CuO/MnO 2 was a result of the combined porous MnO 2 nanorods. Nanomaterials 2023, 13, x FOR PEER REVIEW 4 of 10 nanostructure give rise to a highly porous morphology, which can offer very high surface area and many active sites for electrochemical transportation. X-ray diffraction (XRD) was used to study the structural advancement and phase purity of the CuO and its composite CuO/MnO2. As illustrated in Figure 2a, the phase analysis was conducted using XRD at Bragg's diffraction angle range was adjusted between 5°-65° during the measurement, and the scan rate was set to 2°/minute. The signals for planes (110) , which is crystalline [32,33]. The surface areas (BET) and pore size distribution (BJH) parameters were studied using N2 adsorption−desorption isotherms as displayed in Figure 2, which demonstrated a characteristic IV isotherm that was ascribed to the mesoporous architecture of the aggregated CuO sphere, as displayed in Figure 2b. The CuO/MnO2 revealed an enhanced volume and diameter of the pores, as shown in Figure  2c. The specific surface area for CuO/MnO2 was found to be 54.50 m 2 g -1 , compared to 19.8 m 2 g -1 for pure CuO. The enhancement in surface area of the CuO/MnO2 was a result of the combined porous MnO2 nanorods.   nanostructure give rise to a highly porous morphology, which can offer very high surface area and many active sites for electrochemical transportation. X-ray diffraction (XRD) was used to study the structural advancement and phase purity of the CuO and its composite CuO/MnO2. As illustrated in Figure 2a, the phase analysis was conducted using XRD at Bragg's diffraction angle range was adjusted between 5°-65° during the measurement, and the scan rate was set to 2°/minute. The signals for planes (110) , which is crystalline [32,33]. The surface areas (BET) and pore size distribution (BJH) parameters were studied using N2 adsorption−desorption isotherms as displayed in Figure 2, which demonstrated a characteristic IV isotherm that was ascribed to the mesoporous architecture of the aggregated CuO sphere, as displayed in Figure 2b. The CuO/MnO2 revealed an enhanced volume and diameter of the pores, as shown in Figure  2c. The specific surface area for CuO/MnO2 was found to be 54.50 m 2 g -1 , compared to 19.8 m 2 g -1 for pure CuO. The enhancement in surface area of the CuO/MnO2 was a result of the combined porous MnO2 nanorods.  The valency and chemical binding on the surface of the combined material were studied using X-ray photoelectron spectroscopy (XPS) analyses. The XPS spectra of CuO/MnO 2 Nanomaterials 2023, 13, 2329 5 of 10 unveiled the manifestation of three elements (copper at 932.6 eV, manganese at 641.6 eV, and oxygen at 532.9 eV) on the exterior of the fused material. The binding powers of 932.6 and 952.4 eV referenced Cu 2p 3/2 and Cu 2p 1/2 , and a spin-orbit separation of 19.8 eV was exhibited for Cu 2p, as displayed in Figure 2d. Furthermore, two peaks were detected at 942.7 and 960.9 eV, which further confirmed that the sample contained CuO. The Mn 2p demonstrated peaks at 652.9 and 641.6 eV with a partition of 11.3 eV, as shown in Figure 2e, which is in agreement with the literature [34][35][36]. The CuO/MnO 2 presented a divided energy of 5.  Figure S1.
The microstructure analysis of the CuO and CuO/MnO 2 was determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the findings were reviewed in Figures 3 and 4. As illustrated in Figure 3a  The valency and chemical binding on the surface of the combined material were studied using X-ray photoelectron spectroscopy (XPS) analyses. The XPS spectra of CuO/MnO2 unveiled the manifestation of three elements (copper at 932.6 eV, manganese at 641.6 eV, and oxygen at 532.9 eV) on the exterior of the fused material. The binding powers of 932.6 and 952.4 eV referenced Cu 2p3/2 and Cu 2p1/2, and a spin-orbit separation of 19.8 eV was exhibited for Cu 2p, as displayed in Figure 2d. Furthermore, two peaks were detected at 942.7 and 960.9 eV, which further confirmed that the sample contained CuO. The Mn 2p demonstrated peaks at 652.9 and 641.6 eV with a partition of 11.3 eV, as shown in Figure 2e, which is in agreement with the literature [34][35][36]. The CuO/MnO2 presented a divided energy of 5.8 eV due to Mn−O in the existence of Cu 2+ . The O 1s exhibited three peaks (Figure 2f) at 532.4, 531.3, and 530.4 eV. The peaks of 532.4 and 531.3 were assigned to oxygen and moisture absorption on the composite surface. However, the peak at 530.4 eV agreed with the O 2-bonds with copper and manganese. Subsequently, the XPS results authorized the establishment of CuO/MnO2 composite, and it was endorsed by XRD results. The survey spectra for CuO/MnO2 are displayed in Figure S1.
The microstructure analysis of the CuO and CuO/MnO2 was determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the findings were reviewed in Figures 3 and 4. As illustrated in Figure 3a   As shown in Figure 4a,b, CuO/MnO 2 was comprised of regular roads with negligible sheets at a 500 nanometer scale with accumulation. Moreover, at a greater enlargement, as shown in Figure 4b, the CuO/MnO 2 structures demonstrated a plane surface with clearly visible nanorods. Furthermore, the images of CuO@MnO 2 also confirm that MnO 2 was clearly interlaced with CuO. The elemental arrangement of the CuO/MnO 2 nanorod is exemplified in Figure 4c-f by energy-dispersive spectrometry (EDS) mapping, which indicated the presence of O, Cu, and Mn validating the effective construction of the composite.
The LSV behavior of the CuO and CuO/MnO 2 electrodes for OER was determined at a scan rate of 5 mV/s. Figure 5a displays the LSV curves of the CuO and CuO/MnO 2 electrodes. The overpotentials of the CuO and CuO/MnO 2 electrodes were 316 and 294 mV at a current density of 10 mA/cm 2 , respectively ( Figure S1). Moreover, the CuO/MnO 2 electrode exhibited a lower overpotential than that of the CuO electrode because of the enhanced intrinsic reaction kinetics and high catalytically active sites of the composite system of CuO/MnO 2 . Furthermore, the overpotential of the prepared catalysts was closely related to the Tafel slope values. Figure 5b displays the Tafel curves of the prepared CuO and CuO/MnO 2 electrodes. The Tafel values of the CuO and CuO/MnO 2 electrodes were determined from the following equation: where b, a, and η are the Tafel slope, arbitrary fitting parameter, and overpotential, respectively. The Tafel values of the CuO and CuO/MnO 2 electrodes were determined as 80 and 77 mV/dec, respectively. Compared to the CuO electrode, the CuO/MnO 2 electrode exhibited a low overpotential and a small Tafel value, due to its large number of catalytic active sites and the improved intrinsic reaction kinetics. The noted excellent electrocatalytic behavior had an impact on the electrochemical charge transfer kinetics, which can be seen through electrochemical impedance spectroscopy (EIS) measurement. Figure 5c presents the Nyquist plots of the CuO and CuO/MnO 2 electrodes, which were performed in a frequency range of 1 Hz to 10 kHz.  Table S1, revealing that the electron transport in the material interior, particularly at the CuO and MnO 2 interface, was increased compared to utilizing pure CuO, causing positively advanced electrocatalytically active sites that helped to improve the electrochemical reaction kinetics and yield better catalytic OER activity. Long-term potential stability is a crucial factor in catalysts for industrial purposes. Figure 5d presents the stability curve of the CuO/MnO 2 electrode at 10 mA/cm 2 . The CuO/MnO 2 electrode clearly exhibited a stable static voltage curve at 15 h, indicating the excellent stable nature of the prepared materials. The Over potential, Tafel slope of CuO and CuO/MnO 2 is shown in Figure S2. Furthermore, the SEM images and XRD after OER confirms that the structure is retained ( Figure S3). The embroidering layer of CuO/MnO 2 proposes conduction channels which speed up electron transport and fundamentally enrich the electrochemical properties [37][38][39][40][41][42][43]. A comparative table for the electrochemical OER performances of copper-based materials is shown in Table S2. The LSV polarization curve for IrO 2 catalyst is shown in Figure S4.  As shown in Figure 4a,b, CuO/MnO2 was comprised of regular roads with negligible sheets at a 500 nanometer scale with accumulation. Moreover, at a greater enlargement, as shown in Figure 4b, the CuO/MnO2 structures demonstrated a plane surface with clearly visible nanorods. Furthermore, the images of CuO@MnO2 also confirm that MnO2 was clearly interlaced with CuO. The elemental arrangement of the CuO/MnO2 nanorod is exemplified in Figure 4c-f by energy-dispersive spectrometry (EDS) mapping, which The Over potential, Tafel slope of CuO and CuO/MnO2 is shown in Figure S2. Furthermore, the SEM images and XRD after OER confirms that the structure is retained ( Figure  S3). The embroidering layer of CuO/MnO2 proposes conduction channels which speed up electron transport and fundamentally enrich the electrochemical properties [37][38][39][40][41][42][43]. A comparative table for the electrochemical OER performances of copper-based materials is shown in Table S2. The LSV polarization curve for IrO2 catalyst is shown in Figure S4.

Summary and Conclusions
In summary, CuO and CuO/MnO2 nano catalysts were synthesized using a green microwave-assisted protocol for utilization in OER. The prepared nano catalyst demonstrated the interlacing of CuO with MnO2, required a minimal η of 294 mV to achieve a benchmark current density (j = 10 mA cm −2 ), and a superior stability of more than 15 h. These enhanced electrochemical performances are attributed to the large volume and expanded diameter of the pores, which offer ample surface area for catalytic reactions to boost OER. Furthermore, the rate kinetics of the OER were favored in composites due to a low Tafel slope (77 mV/dec) compared to CuO (80 mV/dec). The improved OER and long duration stability, along with the easy and scalable fabrication process of the CuO/MnO2, offer a promising approach for the potential use of the electrode as an inexpensive catalyst material for electrochemical water-splitting applications.

Summary and Conclusions
In summary, CuO and CuO/MnO 2 nano catalysts were synthesized using a green microwave-assisted protocol for utilization in OER. The prepared nano catalyst demonstrated the interlacing of CuO with MnO 2 , required a minimal η of 294 mV to achieve a benchmark current density (j = 10 mA cm −2 ), and a superior stability of more than 15 h. These enhanced electrochemical performances are attributed to the large volume and expanded diameter of the pores, which offer ample surface area for catalytic reactions to boost OER. Furthermore, the rate kinetics of the OER were favored in composites due to a low Tafel slope (77 mV/dec) compared to CuO (80 mV/dec). The improved OER and long duration stability, along with the easy and scalable fabrication process of the CuO/MnO 2 , offer a promising approach for the potential use of the electrode as an inexpensive catalyst material for electrochemical water-splitting applications.