View PDF Version
 
DOI: 10.1039/D5NR02150B (Paper) Nanoscale, 2025, Advance Article

ORR, OER, and HER activity promotion in hierarchical yolk–shell structures based on Co-glycerate@cobalt carbonate hydroxide by dual doping with manganese and boron

Maryam Shamloofard and Saeed Shahrokhian*
Department of Chemistry, Sharif University of Technology, Tehran 11155-9516, Iran. E-mail: shahrokhian@sharif.edu

Received 21st May 2025 , Accepted 21st July 2025

First published on 22nd July 2025

Abstract

Nowadays, exploring efficient electrocatalysts is crucial for the tri-functional oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). In this study, we designed a novel electrocatalyst based on Mn- and B-doped Co-glycerate@cobalt carbonate hydroxide for the tri-functional OER, HER and ORR under alkaline conditions. Electrochemical measurements demonstrate overpotentials of 318 and 75 mV at a current density of 10 mA cm−2 for the OER and HER, respectively. Moreover, a two-electrode device based on the prepared electrocatalyst required low cell voltages of 1.61 V and 1.86 V to achieve current densities of 10 and 100 mA cm−2, respectively. The prepared electrocatalyst exhibits an onset potential of 0.96 V and a half-wave potential of 0.89 V as a robust electrocatalyst for the ORR. The improved electrocatalytic performance of the prepared electrocatalyst can be attributed to the increased electrical conductivity enabled by heteroelement doping and synergistic effects.

1. Introduction

With the growing concerns of extreme air pollution and clean energy demands, the development of renewable energy sources has attracted increasing attention.1 Electrochemical water splitting with water as the feedstock has been identified as a robust approach to generate clean energy because of its many advantages, such as low cost and low pollutant generation without the emission of greenhouse gases. However, the slow kinetics of water splitting greatly hinder its application in actual hydrogen production due to the large overpotential of the overall water splitting reaction.2–4 In order to resolve the obstacle of the sluggish and complex electron-transfer process of water splitting, the development of highly efficient catalysts is required.5–8

The oxygen reduction reaction (ORR) plays a decisive role in many energy-converting systems, ranging from fuel cells to metal–air batteries.9 Generally, the ORR mechanism can occur via two pathways in an aqueous electrolyte: a direct four-electron mechanism without formation of the peroxide and a two-electron reduction mechanism with the production of peroxide.10 However, due to the sluggish kinetics of the cathodic ORR, it is extremely imperative to develop an efficient electrocatalyst for accelerating the ORR.11–13

Pt, Ru or Ir-based materials are the most advanced materials for HER and OER, respectively, in alkaline electrolyte. Also, the compounds containing Pt have indicated the best enhancement for ORR performance like HER.14 The large-scale practical application and commercial implementation of noble-metal-based catalysts are limited by their high price, inferior durability and an acute shortage of resources. Consequently, it is attractive to fabricate non-precious metal water-splitting catalysts with low overpotential, compatible durability and low cost.15–17 Over the past years, great progress has been made in the design of catalysts based on earth-abundant elements for the HER, OER and ORR. Among different electrocatalysts, metal alkoxides, which are the derivatives of metal hydroxides, can be used as intermediate templates to synthesize the desired metal compounds.18 Metal glycerates have a layered structure with stacked metal–oxygen sheets separated by anions of glycerate. The layered structure, which consists of an interlayer spacing for reactant accommodation, is similar to that of anion-intercalated hydroxides. The metal glycerate structures are open, which can allow the rapid transport of the reactants and provide more catalytically active sites for catalysts. Moreover, with the formation of –OOH groups and active sites, optimized availability is expected under OER conditions due to the similarity between the hydroxide and metal glycerates.18–20

The rational design of the structure and morphology of catalysts is an important factor that can improve the electrochemical reaction kinetics of OER, HER and ORR. Yolk–shell structures are a type of hybrid material that features an outer shell that surrounds a core inside a hollow cavity.21 The yolk–shell structures are beneficial for a range of applications, including catalysis, energy storage, and biomedicine.22 Usually, the unique morphological features of the yolk–shell structures result in large surface areas, synergistic properties of core and shell materials, improved functionalizability, and other physicochemical properties.23–25 The yolk–shell materials, as a hybrid of core/shell and hollow structure, benefit from the properties of both frameworks. It should also be noted that the shell in the yolk–shell materials can help protect the core from harsh conditions and prevent the core from agglomerating. Given the mentioned points, the synthesis of yolk–shell materials as catalysts for OER, HER and ORR has been investigated extensively.26–28 Recently, structures with diverse hierarchical architectures have been developed that can reduce the electron diffusion distance in electron transport processes.29–31 The hierarchical architectures, with attractive features such as high porosity, can improve ion transport and lead to more exposed active sites, which are necessary for increased electrocatalytic performance.32,33 Based on the features of the yolk–shell materials and the hierarchical structures, the design of the hierarchical yolk–shell architectures can be used to enhance the catalytic performance in OER, HER and ORR.34–36

Introducing heteroatoms into electrode materials through interfacial engineering and electronic structure manipulation of catalysts can lead to crucial modifications.37,38 Various heteroatom doping elements, including nitrogen, phosphorus, sulfur and boron, can be used to efficiently enhance the electrochemical performance of OER, HER and ORR.38 In 2025, Huo et al.39 reported a study on amorphous MnO2 lamellae encapsulated covalent triazine polymer-derived multi-heteroatoms-doped carbon for ORR/OER bifunctional electrocatalysis and showed that the oxygen electrocatalytic performance can be enhanced by the heteroatom doping.40 In 2024, Liu et al. studied heteroatom-doped Fe–N–C single-atom catalysts for the ORR and showed that the electrocatalytic performance can be enhanced by heteroatom doping.43 Recently, the unique electronegativity of boron (B) has drawn considerable attention owing to its ability to reduce the negative charge density of nearby metal atoms.40–42 The B heteroatom in the catalyst structure can play the role of a Lewis acid in the electrocatalytic process. Investigations have shown that the introduction of B atoms can increase the performance of the catalyst by modulating the d-band center of metals and, as a result, increase the interactions between B and metal atoms.41–43 However, to date, the use of B heteroatom in the structure of OER, HER and ORR electrocatalysts has rarely been reported. In this work, we investigated a facile approach to the synthesis of boron-doped electrocatalysts for tri-functional OER, HER and ORR in an alkaline electrolyte. Moreover, doping with suitable amounts of metal atoms, such as Mn, can enhance the electrocatalytic activity by modulating the electronic structure and reducing the intermediate adsorption energy of the catalyst.37

Inspired by the abovementioned observations, herein, a novel electrocatalyst for OER, HER and ORR with the architecture of hierarchical yolk–shell spheres based on Mn and B dual-doped Co-glycerate (CoG) solid spheres is achieved through a simple Co-glycerate-template strategy. The transformation of the solid spheres as the starting templates into the hierarchical yolk–shell sphere is achieved by hydrothermal and room temperature treatment. Benefitting from their unique structural advantages, the target catalyst based on Mn and B dual-doped Co-glycerate solid spheres@Co-carbonate hydroxide nanosheets (MnB-CoGSS@CoCHNs) structure showed good potential as a catalyst for OER, HER and ORR in comparison with other samples without a dopant. The hierarchical yolk–shell sphere based on MnB-CoGSS@CoCHNs generated 10 mA cm−2 current density in 1.0 M KOH with an overpotential of 318 and 75 mV for OER and HER, respectively. Moreover, a two-electrode device based on the MnB-CoGSS@CoCHNs required low cell voltages of 1.61 and 1.86 V to achieve 10 and 100 mA cm−2 current densities with appropriate stability. The prepared catalyst exhibits an onset potential (Eonset) of 0.98 V and a half-wave potential (E1/2) of 0.87 V for ORR, demonstrating the good potential of the MnB-CoGSS@CoCHNs as a catalyst. The robust activity of the electrocatalyst toward OER, HER and ORR can be attributed to the enhanced ion transportation and electrical conductivity induced by heteroelement doping and synergistic effects. To our knowledge, no studies have examined hierarchical yolk–shell structures based on the MnB-CoGSS@CoCHNs. Although Chai et al. studied manganese-doped hollow cobalt oxide catalysts in 2024, the advantage of our work is the trifunctional behavior of the catalyst and the presence of boron in the catalyst, which improves the electrocatalytic activity.44

2. Experimental

2.1. Materials

Cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O), manganese nitrate tetrahydrate (Mn(NO3)2·4H2O), glycerol (C3H8O3), 2-propanol (C3H8O, >99.5%), ethanol (C2H6O, 99.5%), urea (CO(NH2)2), sodium borohydride (NaBH4), Pt/C (20 wt%), commercial RuO2, potassium hydroxide (KOH), acetone (C3H6O) and Nafion (5 wt%) were purchased from Merck and Sigma-Aldrich chemical companies.

2.2. Synthesis of solid spheres of the Co-glycerate template (CoG)

In a typical procedure, cobalt glycerate was prepared by a simple solvothermal reaction. To synthesize the cobalt glycerate solid spheres, 7.5 mL of glycerol was dispersed in 52.5 mL of 2-propanol and stirred for 5 min to form a transparent solution. Subsequently, 0.625 mmol of Co(NO3)2·6H2O was added to the prepared solution under vigorous stirring for 5 min. The resulting mixture solution was then transferred to a Teflon-lined stainless-steel autoclave and heated at 180 °C for 6 h to perform the hydrothermal reactions. The brownish precipitate was then filtered by centrifugation, washed three times with ethanol and then dried overnight.

2.3. Hydrothermal transformation of the Co-glycerate solid spheres into hierarchical yolk–shell Co-glycerate solid spheres@Co-carbonate hydroxide nanosheet structures (CoGSS@ CoCHNSs)

Briefly, 40 mg of Co-glycerate was added to 40 mL of a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 (v/v) mixture of ethanol and water. Then, 100 mg urea was added under vigorous stirring, and the prepared solution was subsequently transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated at 120 °C for 6 h. The resulting precipitate was harvested through centrifugation and rinsed with water and ethanol, then dried overnight in an oven.

2.4. Preparation of Mn-doped hierarchical yolk–shell Co-glycerate solid spheres@Co-carbonate hydroxide nanosheet structures (Mn-CoGSS@ CoCHNSs)

For the synthesis of the Mn-doped hierarchical yolk–shell CoGSS@CoCHNSs structure, the procedure was similar to the preparation of the hierarchical yolk–shell CoGSS@CoCHNSs structure, with the difference that 0.04 mmol of Mn(NO3)2·4H2O was added to the initial solution. The prepared sample is named Mn(0.04)-CoGSS@CoCHNSs. To investigate the effect of Mn content, different amounts of Mn(NO3)2·4H2O were used, including 0.02 mmol, 0.08 mmol, and 0.12 mmol; these samples are named Mn(0.02)-CoGSS@CoCHNSs, Mn(0.08)-CoGSS@CoCHNSs, and Mn(0.12)-CoGSS@CoCHNSs, respectively.

2.5. Synthesis of the hierarchical yolk–shell sphere based on the Mn and B dual-doped CoGSS@CoCHNSs structure (MnB-CoGSS@CoCHNs)

The hierarchical yolk–shell sphere based on Mn and B dual-doped CoGSS@CoCHNSs (MnB-CoGSS@CoCHNSs) was prepared via a simple process. 50 mg of the prepared Mn(0.04)-CoGSS@CoCHNSs precursor was dispersed in 15 mL of water, then 15 mL of 0.3 M NaBH4 solution was added to the solution under constant stirring for 0.5 h to form the MnB-CoGSS@CoCHNSs catalyst.

2.6. Materials characterizations

To analyze the morphology and the elemental distribution of the samples, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) were employed on the same FESEM equipment (FE-SEM, MIRA 3 TESCAN, 15 kV, Czech) with an acceleration voltage of 5 kV in a high vacuum. For a better view of the morphology, transmission electron microscopy (TEM) (JEOL, JEM-2010) was conducted. High-resolution transmission electron microscopy (HRTEM) was conducted with a Tecani G2 F20 to analyze the structure of the catalyst. The chemical states of the surface elements were monitored by X-ray photoelectron spectroscopy (XPS) with an Al Kα anode X-ray source (hν = 1486.6 eV). X-ray diffraction (XRD) patterns were collected with a GBCMMA instrument in the 2θ range from 10° to 80° and Cu Kα radiation (λ = 1.5406 Å) to investigate the phase composition and crystal structure of the catalysts. Fourier-transform infrared (FT-IR) spectra were recorded with a PerkinElmer FT-IR spectrometer (spectrum 100, USA) from 400 cm−1 to 4000 cm−1.

3. Results and discussion

The procedure for the synthesis of the hierarchical yolk–shell sphere based on the MnB-CoGSS@CoCHNSs structure is shown in Scheme 1. The MnB-CoGSS@CoCHNSs catalyst was synthesized through a facile solvothermal method at room temperature. The preparation started with the synthesis of the Co-glycerate as self-engaged templates through the hydrothermal process. The Mn-doped hierarchical yolk–shell CoGSS@CoCHNSs structure was formed by a second hydrothermal reaction between the Co-glycerate precursor and manganese. After the preparation of the Mn-doped hierarchical yolk–shell CoGSS@CoCHNSs, room temperature reaction with sodium borohydride was conducted to generate the MnB-CoGSS@CoCHNSs as a robust tri-functional catalyst for OER, HER and ORR.
image file: d5nr02150b-s1.tif
Scheme 1 Schematic of the synthesis of the hierarchical yolk–shell sphere-based MnB-CoGSS@CoCHNSs structure.

X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface valence states of the Mn(0.04)B-CoGSS@CoCHNSs. Two spin–orbit doublets and two shake-up satellites can be seen for the Co 2p deconvoluted spectrum.19 The peaks at approximately 779.1 and 781.9 eV and the 786.8 eV satellite peak are attributed to Co 2p3/2 and the peaks at approximately 795.9 and 798.8 eV and the 804.3 eV satellite peak are attributed to Co 2p1/2 (Fig. 1A). The deconvoluted Mn 2p spectrum showed peaks at approximately 641.4 and 653.8 eV for Mn2+ and at approximately 642.9 and 654.6 eV for Mn3+ (Fig. 1B). The peak at 192.6 eV is attributed to the B in the structure of the Mn(0.04)B-CoGSS@CoCHNSs (Fig. 1C).41 The O 1s spectra of the Mn(0.04)B-CoGSS@CoCHNSs can be deconvoluted into three peaks at approximately 529.3, 531.9, and 533.2; the observed peaks are attributed to the M–O, hydroxyl groups (M–OH) or absorbed water, and the oxygen vacancies or defects, respectively (Fig. 1D).35


image file: d5nr02150b-f1.tif
Fig. 1 XPS of the Mn(0.04)B-CoGSS@CoCHNSs catalyst.

XRD analysis was used to monitor the crystal structure of the prepared catalysts. Fig. 2A shows the XRD pattern of the Co-glycerate precursor. The spectrum shows a sharp peak at approximately 12°, which is attributed to the metal glycerates and the interlayer spacing in a lamellar structure.45–47 Therefore, the XRD results confirm that the synthesized substance can be identified as the Co-glycerate solid sphere (CoGSS). The XRD spectrum of the CoGSS precursor after further hydrothermal reaction with water, ethanol and urea for 6 hours at 120 °C is shown in Fig. 2A. The XRD spectrum of the as-prepared catalyst revealed a considerable decrease in intensity of the CoGSS precursor characteristic peaks, which can be related to the formation of the hierarchical yolk–shell CoGSS@CoCHNSs. From the obtained results, it can be suggested that the lamellar structure of the Co-glycerate precursor is destroyed by the solvothermal treatment in ethanol/water as a mixed solvent because of the instability of the glycerate under the reaction conditions.48 The XRD pattern of the cobalt carbonate hydroxide indicated the main diffraction peaks of the crystal structure at 17.6°, 33.7°, 35.2°, 36.4° and 39.5°, which can be attributed to the (020), (221), (040), (301) and (231) planes, respectively (Fig. 2A) and are in good agreement with the structure of Co(CO3)0.5OH0.11H2O (PDF#48-0083).49,50 By reacting the manganese nitrate with the cobalt glycerate spheres, the XRD pattern of the sample was negatively shifted to lower angles, which can be due to the doping of manganese atoms onto the structure and the substitution of Co2+ ions in some planes with higher-radius Mn2+ ions (Fig. 2A).51 When the Mn-doped hierarchical yolk–shell CoGSS@CoCHNSs structure reacted with NaBH4, the XRD patterns showed no new peak and only a slight shift to the direction of low angles was observed (Fig. 2A). The observed negative shift is due to the larger radius of the B atom (∼0.082 nm) compared to that of the O atom (∼0.074 nm). The results confirm the formation of the Mn- and B-doped CoGSS@CoCHNSs catalyst.52 To reveal the morphology of the CoGSS, CoGSS@CoCHNSs, Mn-CoGSS@CoCHNSs and MnB-CoGSS@CoCHNSs, field emission scanning electron microscopy (FE-SEM) was carried out. Fig. 2(B–D) shows the FE-SEM images of the CoGSS as the self-engaged template at different magnifications. The observations reveal the smooth surface of the solid microspheres. It can be noted that the cobalt isopropoxide can be formed by dissolving the cobalt(II) nitrate in 2-propanol as the solvent through an interchange process. After that, due to the lower boiling point of the 2-propanol compared to glycerol, the exchange of cobalt isopropoxide to Co-glycerate can occur through a ligand exchange process.53 The uniform distribution of the Co-glycerate components can be observed from the energy-dispersive X-ray spectroscopy (EDX) (Fig. 2E) and elemental mapping analysis (Fig. S1).


image file: d5nr02150b-f2.tif
Fig. 2 (A) XRD analysis of the different catalysts. (B–D) The FE-SEM images. (E) Energy-dispersive X-ray spectrum of CoGSS.

Fig. 3(A and D) shows the FE-SEM images of the CoGSS@CoCHNSs at different magnifications. The images reveal the formation of the hierarchical yolk–shell structure for CoGSS@CoCHNSs. Based on the morphology evolution, it can be proposed that the Co-glycerate self-sacrificing template acts as a water-soluble precursor to form the Co-glycerate microsphere as a core and carbonate hydroxide nanosheets as the shell. The use of ethanol/water as a mixed solvent helps to reduce the dissolution rate of the glycerate and retains the spherical structure. Furthermore, the OH− ions formed from water dissociation can gradually react with a portion of the Co-glycerate and, in the presence of the CO32− anions produced by the hydrolysis of urea, form the carbonate hydroxide nanosheets of the shell.54,55 The presence of all the CoGSS@CoCHNSs components was further confirmed by energy-dispersive X-ray spectroscopy (EDX) (Fig. S2A) and elemental mapping analysis (Fig. S2B). Fig. S3 shows a comparison of FE-SEM morphologies of catalysts prepared with different amounts of manganese doping in the Co-glycerate precursor. By reacting the manganese nitrate with Co-glycerate precursor during the hydrothermal process, Mn-CoGSS@CoCHNSs can be obtained. The formation of the Mn-CoGSS@CoCHNSs can be explained as follows. First, the OH− ions released from water cause the hydrolysis of the Co-glycerate precursor, and the nanosheets can be formed in the presence of the CO32− anions on the surface of the Co-glycerate spheres. As the reaction proceeds, the Mn2+ ions react with the released Co2+ to form Mn-CoGSS@CoCHNSs (Fig. 3(B and E)).54–56 The presence of the components in the structure of the Mn-CoGSS@CoCHNSs was confirmed by energy-dispersive X-ray spectroscopy (EDX) (Fig. S4) and elemental mapping analysis (Fig. S5). The FE-SEM images of the MnB-CoGSS@CoCHNSs are shown in Fig. 3(C and F). The FE-SEM images confirm that the MnB-CoGSS@CoCHNSs catalyst maintained the original structure of the hierarchical yolk–shell sphere precursor with increasing porosity. The presence and the uniform distribution of the elements can be seen in the elemental mapping analysis (Fig. 3G) and energy-dispersive X-ray spectrum (EDX) (Fig. 4A). Moreover, the yolk–shell structure of the Mn(0.04)B-CoGSS@CoCHNSs catalyst was confirmed by the TEM images (Fig. 4(B and C)). As can be seen in Fig. 3(B and C), the outer surfaces of the CoGSS are covered with nanosheets, which confirms the hierarchical nature of these spheres.


image file: d5nr02150b-f3.tif
Fig. 3 FE-SEM images of (A and D) CoGSS@CoCHNSs (B and E) Mn-CoGSS@CoCHNSs and (C and F) MnB-CoGSS@CoCHNSs, and (G) elemental mapping analysis of the MnB-CoGSS@CoCHNSs. Scale bar: (A, B and C) 1 μm, (D, E and F) 500 nm.

image file: d5nr02150b-f4.tif
Fig. 4 (A) Energy-dispersive X-ray and (B and C) TEM images of the Mn(0.04)B-CoGSS@CoCHNSs at different magnifications. (D) FT-IR spectra of the catalysts. (E and F) HRTEM images of the Mn(0.04)B-CoGSS@CoCHNSs (inset of E shows the SAED pattern).

Fourier-transform infrared (FT-IR) analysis was carried out to monitor the phase conversion of CoGSS into CoGSS@CoCHNSs, Mn-CoGSS@CoCHNSs and MnB-CoGSS@CoCHNSs. For the pristine CoGSS (Fig. 4D), the broad IR absorption peak in the region of approximately 3200 to 3600 cm−1 can be indexed to the O–H stretching vibration and the observed peaks between 2850–2950 cm−1 can be attributed to the –CH3 group in IPA (CH3CH(OH)CH3) and the –CH2– group in glycerol.57–59 The absorption bands in the range of approximately 1640–1650 cm−1 can be attributed to the C[double bond, length as m-dash]O stretching bands, whereas the peaks in the wavenumber regions below 2000 cm−1 are characteristic of Co–O, C–C, C–C–O and C–O–Co.60,61 After the hydrothermal reaction and the formation of the CoGSS@CoCHNSs and Mn-CoGSS@CoCHNSs, peaks in the region of approximately 3200 to 3600 cm−1 are observed due to the O–H stretching vibrations of the adsorbed molecular water in the structure of the catalysts (Fig. 4D).62 The bands at 1500 cm−1 in the spectra of CoGSS@CoCHNSs and Mn-CoGSS@CoCHNSs, are assigned to the CO32− ions. Moreover, the presence of IR bands between 520 and 900 cm−1 indicates Mn–OH and Co–OH bending modes.61 According to Fig. 4D, by introducing the B atoms in the structure of the sample and formation of the MnB-CoGSS@CoCHNSs, the FT-IR spectrum showed peaks attributed to M–B in the region of 600 to 800 cm−1, where M is cobalt or manganese metal.62

The HRTEM was used to investigate the structure of Mn(0.04)B-CoGSS@CoCHNSs in further detail (Fig. 4(E and F)). The images showed small pits in the structure of the Mn(0.04)-CoGSS@CoCHNSs (red circles), which may be formed as a result of the oxygen vacancies or defects in the structure. The spacing distance of 0.266 nm was revealed in the HRTEM images of the Mn(0.04)B-CoGSS@CoCHNSs, which can be related to the (221) crystal face of the catalyst. Also, the minor changes in the structure can be due to the Mn and B doping in the structure (Fig. 4(E and F)). The selected-area electron diffraction (SAED) pattern is consistent with a single-crystalline nature (inset of Fig. 4E).

3.2. Electrocatalytic performance of the prepared catalysts

3.2.1 Electrocatalytic OER and HER performance. The catalytic performance of the CoGSS, CoGSS@CoCHNSs, Mn-CoGSS@CoCHNSs and MnB-CoGSS@CoCHNSs catalysts toward OER was systematically analyzed in 1 M KOH by iR compensated LSV curves at a scan rate of 5 mV s−1. For performance comparison, the RuO2 catalyst toward OER was also evaluated as a benchmark under the same conditions. Fig. 5(A and B) shows the polarization tests for these catalysts toward OER. To investigation of the effect of the amount of Mn on the catalytic behavior of the catalyst, Mn-CoGSS@CoCHNSs was prepared with different amounts of Mn (Fig. 5A). The results reveal that the electrocatalytic activity of the Mn-CoGSS@CoCHNSs is increased with increasing amounts of manganese from 0 mmol to 0.04 mmol; however, when the amount of manganese doping exceeded 0.04 mmol, the electrocatalytic activity of the Mn-CoGSS@CoCHNSs toward OER decreased. As shown in Fig. 5A, the Mn(0.04)-CoGSS@CoCHNSs required a lower overpotential of 430 mV to gain 10 mA cm−2 current density than other catalysts; i.e., CoGSS@CoCHNSs (445 mV), Mn(0.02)-CoGSS@CoCHNSs (440 mV), Mn(0.08)-CoGSS@CoCHNSs (448 mV), and Mn(0.12)-CoGSS@CoCHNSs (480 mV). Furthermore, comparison of the electrocatalytic behavior of CoGSS, CoGSS@CoCHNSs, Mn(0.04)-CoGSS@CoCHNSs and Mn(0.04)B-CoGSS@CoCHNSs revealed that the inclusion of manganese and boron can enhance the electrocatalytic activity of the catalyst toward OER, as shown in Fig. 5B and Table 1. The Mn(0.04)B-CoGSS@CoCHNSs showed a lower overpotential of 318 mV to achieve 10 mA cm−2 current density compared to others; i.e., CoGSS (450 mV), CoGSS@CoCHNSs (445 mV), and Mn(0.04)-CoGSS@CoCHNSs (430 mV). Although the overpotential of the Mn(0.04)B-CoGSS@CoCHNSs at 10 mA cm−2 is higher than the overpotential of RuO2, the synthesized catalyst showed better electrocatalytic behavior for OER at higher current densities. For example, at 50 mA cm−2, the Mn(0.04)B-CoGSS@CoCHNSs exhibited an overpotential of 410 mV, which is higher than that of RuO2 (475 mV) and other prepared samples; i.e., CoGSS (580 mV), CoGSS@CoCHNSs (550 mV), and Mn(0.04)-CoGSS@CoCHNSs (510 mV).
image file: d5nr02150b-f5.tif
Fig. 5 Polarization curves of the catalysts in 1.0 M KOH with a scan rate of 5 mV s−1 during (A and B) the OER and (C and D) HER.
Table 1 Comparison of some electrochemical parameters of OER catalysts
Catalysts η at j = 10 mA cm−2 (mV) η at j = 50 mA cm−2 (mV) Tafel slope (mV dec−1)
CoGSS 450 580 179
CoGSS@CoCHNSs 445 553 153
Mn(0.04)-CoGSS@CoCHNSs 430 511 150
Mn(0.04)B-CoGSS@CoCHNSs 318 410 97

Moreover, the catalytic performance of the catalysts was monitored toward HER. As shown in Fig. 5C, the Mn(0.04)-CoGSS@CoCHNSs exhibited a higher electrocatalytic performance for HER (overpotential of 290 mV at 10 mA cm−2) compared to electrocatalytic performance of CoGSS@CoCHNSs (326 mV), Mn(0.02)-CoGSS@CoCHNSs (320 mV), Mn(0.08)-CoGSS@CoCHNSs (319 mV), and Mn(0.12)-CoGSS@CoCHNSs (355 mV). Furthermore, comparing the electrocatalytic performance of CoGSS, CoGSS@CoCHNSs, Mn(0.04)-CoGSS@CoCHNSs and Mn(0.04)B-CoGSS@CoCHNSs toward HER (Fig. 5D and Table 2) indicated an activity trend of Mn(0.04)B-CoGSS@CoCHNSs > Mn(0.04)-CoGSS@CoCHNSs > CoGSS@CoCHNSs > CoGSS. The Mn(0.04)B-CoGSS@CoCHNSs was the best catalyst for HER with an overpotential of 75 mV at 10 mA cm−2 current density compared to others; i.e., CoGSS (350 mV), CoGSS@CoCHNSs (326 mV), and Mn(0.04)-CoGSS@CoCHNSs (290 mV). Although the overpotential of the Mn(0.04)B-CoGSS@CoCHNSs at 10 mA cm−2 was higher than that of Pt/C (20 wt%) at the same current density, the overpotential of the Mn(0.04)B-CoGSS@CoCHNSs was lower than that of Pt/C (20 wt%) for HER at higher current density. For example, at 150 and 250 mA cm−2, Mn(0.04)B-CoGSS@CoCHNSs exhibited overpotentials of 271 and 324 mV, respectively, which are lower than the overpotentials of Pt/C (20 wt%) (354 mV at 150 mA cm−2 and 503 mV at 250 mA cm−2) and other prepared samples. The obtained results of the OER and HER indicate that the presence of Mn and B in the catalyst can improve the performance of the catalyst toward OER and HER. According to some studies, a defective coordination environment can be created in the structure of the catalyst by doping with Mn and B and, as a result, the electronic structure of the catalyst can be manipulated. Furthermore, the hierarchical 2D nanosheets that form the shell of the Mn(0.04)B-CoGSS@CoCHNSs catalyst may provide more active sites on the catalyst surface and accelerate the charge transport.19,24,31

Table 2 Comparison of some electrochemical parameters of HER catalysts
Catalysts η at j = 10 mA cm−2 (mV) η at j = 250 mA cm−2 (mV) Tafel slope (mV dec−1)
CoGSS 350 — 188
CoGSS@CoCHNSs 326 530 170
Mn(0.04)-CoGSS@CoCHNSs 290 434 108
Mn(0.04)B-CoGSS@CoCHNSs 75 324 96

To obtain more insights on the kinetics of the prepared catalysts toward OER, Tafel slopes of the samples were calculated from Tafel plots of the linear region of the overpotential versus log (current density) and fitted to the Tafel equation (η = a + b[thin space (1/6-em)]log[thin space (1/6-em)]j), where j is the current density, η is the overpotential and b is the Tafel slope.26 The Tafel slope of the Mn(0.04)B-CoGSS@CoCHNSs is 92 mV dec−1, which is lower than those of CoGSS (179 mV dec−1), CoGSS@CoCHNSs (153 mV dec−1), and Mn(0.04)-CoGSS@CoCHNSs (150 mV dec−1) (Fig. 6A). The results (Table 1) indicate a smaller Tafel slope for Mn(0.04)B-CoGSS@CoCHNSs, which confirmed the kinetics of the OER process at the interface of the electrode/electrolyte is faster than those of the other prepared catalysts.


image file: d5nr02150b-f6.tif
Fig. 6 Tafel plots for (A) the OER and (B) HER. EIS Nyquist plots for (C) the OER at 1.55 V (vs. RHE) and (D) HER at −0.1 V (vs. RHE).

The Tafel slope, as an important parameter to analyze the kinetics of the electrode process, was also obtained for the prepared catalysts in HER. Generally, in alkaline media, the HER pathway can proceed through the Volmer–Heyrovsky process or Volmer–Tafel mechanism.16 Depending on the rate-determining step, the slopes of 120 mV dec−1 (Volmer), 40 mV dec−1 (Heyrovsky), or 30 mV dec−1 (Tafel) should be observed.53 In this regard, the Mn(0.04)B-CoGSS@CoCHNSs, with the small Tafel slope of 96 mV dec−1, can be a more effective catalyst for HER in comparison to CoGSS (188 mV dec−1), CoGSS@CoCHNSs (170 mV dec−1), and Mn(0.04)-CoGSS@CoCHNSs (108 mV dec−1) (Fig. 6B). The obtained Tafel slope for Mn(0.04)-CoGSS@CoCHNSs suggested that the HER over Mn(0.04)-CoGSS@CoCHNSs was dominated by the Volmer–Heyrovsky mechanism.

To determine the effective kinetics of the Mn(0.04)B-CoGSS@CoCHNSs toward OER, electrochemical impedance spectroscopy (EIS) measurements were recorded in 1.0 M KOH at a potential of 1.5 V (vs. RHE) to understand the charge transfer resistance of the samples. The EIS measurements and the simulated equivalent circuit are presented in Fig. 6C. The terms of Rs and Rct are assigned to the series resistance and charge transfer resistance, respectively, while the semicircle can be related to the amount of charge transfer resistance.23 Among these prepared catalysts, the Mn(0.04)B-CoGSS@CoCHNSs catalysts exhibits the smallest diameter of the Nyquist semicircle and charge transfer resistance (Rct = 3.7 Ω) as well as the lowest value of the series resistance (Rs = 4.1 Ω). The series and charge transfer resistances for other samples are given in Fig. 6B, which all have larger values than the series and charge transfer resistance values of the Mn(0.04)B-CoGSS@CoCHNSs. Based on the EIS results, it can be suggested that the highest electron transfer rate occurred at the interface of the electrolyte and the Mn(0.04)B-CoGSS@CoCHNSs catalyst. The effective connection between the prepared catalyst and graphite paper as a substrate can improve the electron conductivity and provide a good electron pathway without significant kinetic limitation. Moreover, the EIS measurements of the prepared catalysts were conducted at −0.1 V (vs. RHE) in 1.0 M KOH for HER kinetics investigation. As expected, the results demonstrated the smallest Rs of 4.2 and Rct of 4.8 Ω for the Mn(0.04)B-CoGSS@CoCHNSs catalyst in comparison to CoGSS, CoGSS@CoCHNSs and Mn(0.04)-CoGSS@CoCHNSs (Fig. 6D and 7B). The obtained results indicate a lower kinetic limitation for Mn(0.04)B-CoGSS@CoCHNSs at HER compared to the other prepared catalysts.


image file: d5nr02150b-f7.tif
Fig. 7 (A) Variance of current density as a function of the scan rate for different catalysts. (B) Comparison of charge transfer resistances of the catalysts in the OER and HER and chronoamperometry investigation of Mn(0.04)B-CoGSS@CoCHNSs (C) for the OER and (D) for HER.

Moreover, the electrochemically active surface area (ECSA) of the samples was assessed to elucidate the catalytic performance of the prepared catalysts in OER and HER. The ECSA of the samples was analyzed by cyclic voltammetry (CV) in a potential window of 1 to 1.1 V (vs. RHE) as a non-faradaic region and with a scan rate range of 5 to 100 mV s−1 (Fig. S6). By plotting the current density differences (Δj = ja − jc) at 1.05 V (vs. RHE) versus different scan rates, a linear relationship can be obtained, the slope of which is proportional to the ECSA of the samples. In other words, the slope of the plot is twice the electrochemical double-layer capacitance (Cdl), and the Cdl is proportional to the ECSA.19 An increase in the slope of the plot indicates an increase in the ECSA, which is more favorable for catalytic activity. As expected, the plot slope for the Mn(0.04)B-CoGSS@CoCHNSs is higher than those of CoGSS, CoGSS@CoCHNSs, and Mn(0.04)-CoGSS@CoCHNSs. The higher slope for Mn(0.04)B-CoGSS@CoCHNSs can be related to a larger ECSA, which provides more active sites on the surface of the catalyst (Fig. 7A).

In addition to its high performance, the durability of the catalyst is another essential factor for OER. To monitor the stability of the Mn(0.04)B-CoGSS@CoCHNSs, chronoamperometric (CA) tests were carried out in 1.0 M KOH at 1.55 V (vs. RHE) for 10 h. As presented in Fig. 7C, the current density of 10 mA cm−2 is maintained after 10 h for Mn(0.04)B-CoGSS@CoCHNSs between the start and the end of the chronoamperometric test. From the obtained results, it can be concluded that Mn(0.04)B-CoGSS@CoCHNSs exhibits good long-term stability. Furthermore, the stability of the Mn(0.04)B-CoGSS@CoCHNSs was tested by chronoamperometric (CA) measurement for HER in 1.0 M KOH at a constant potential of −0.1 V (vs. RHE) for 10 h. As expected, the obtained results showed that, as observed in the OER process, the stability of the Mn(0.04)B-CoGSS@CoCHNSs catalyst in the HER process is acceptable (Fig. 7D), and the current density of 10 mA cm−2 was maintained for 10 h.

To investigate the changes in morphology of Mn(0.04)B-CoGSS@CoCHNSs during HER and OER tests, FE-SEM images of the prepared catalyst after HER and OER tests were compared with the initial images. No noticeable change was observed in the images, which suggests acceptable stability of the Mn(0.04)B-CoGSS@CoCHNSs structure during HER and OER (Fig. S7). XPS characterization of Mn(0.04)B-CoGSS@CoCHNSs after the OER stability test (Fig. S8) showed a slight decrease in the intensity of the B 1s, Co3+ and Mn3+ peaks, which may be a result of conversion of the active substances into metal oxides or hydroxides.35–41 TEM images after the stability test showed no significant change, which also suggested the good stability of Mn(0.04)B-CoGSS@CoCHNSs (Fig. S9).

Using a two-electrode device, the electrocatalytic performance of Mn(0.04)B-CoGSS@CoCHNSs for overall water splitting in 1 M KOH was monitored. As shown in Fig. 8A, the polarization curve of Mn(0.04)B-CoGSS@CoCHNSs was characterized, and the obtained cell voltages were 1.61 and 1.86 V at 10 and 100 mA cm−2, respectively. Moreover, the Mn(0.04)B-CoGSS@CoCHNSs catalyst indicated acceptable stability after 6 h for overall water splitting (Fig. 8B).


image file: d5nr02150b-f8.tif
Fig. 8 (A) Polarization curve for overall water splitting at a scan rate of 5 mV s−1 (inset shows the overall water splitting device). (B) Stability test of the Mn(0.04)B-CoGSS@CoCHNSs catalyst at a 1.6 V cell voltage for overall water splitting in a two-electrode system. The electrolyte was 1.0 M KOH.
3.2.2 Electrocatalytic ORR performance. The ORR performance of the Mn(0.04)B-CoGSS@CoCHNSs catalyst was initially evaluated by cyclic voltammetry (CV) measurements in 0.1 M KOH electrolyte at a scan rate of 50 mV s−1. Fig. 9A shows the CV plots recorded for Mn(0.04)B-CoGSS@CoCHNSs with Ar or O2-saturated aqueous electrolyte. No significant reduction peak was observed in Ar-saturated electrolyte for Mn(0.04)B-CoGSS@CoCHNSs catalyst. In contrast, the CV plot for Mn(0.04)B-CoGSS@CoCHNSs with O2-saturated aqueous electrolyte showed a well-defined oxygen reduction peak with a peak potential (Epeak) of 0.83 V as a prominent ORR catalyst. The catalytic activities of the catalysts were further compared with LSV curves at a scan rate of 5 mV s−1 using a RDE at a rotation rate of 1600 rpm in O2-saturated 0.1 M KOH (Fig. 9B). Typical LSV curves of the samples showed that the Mn(0.04)B-CoGSS@CoCHNSs exhibit an onset potential (Eonset) of 0.96 V and half-wave potential (E1/2) of 0.89 V, which are much more positive than the corresponding potentials of CoGSS (Eonset = 0.77 V, E1/2 = 0.69 V), CoGSS@CoCHNSs (Eonset = 0.81 V, E1/2 = 0.71 V), and Mn(0.04)-CoGSS@CoCHNSs (Eonset = 0.84 V, E1/2 = 0.73 V). Meanwhile, Mn(0.04)B-CoGSS@CoCHNSs exhibited the highest current density of −5.45 mA cm−2 of the prepared samples.
image file: d5nr02150b-f9.tif
Fig. 9 (A) CV measurements for the Mn(0.04)B-CoGSS@CoCHNSs catalyst in an Ar and O2-saturated aqueous electrolyte of 0.1 M KOH at a scan rate of 50 mV s−1. (B) LSV curves of the samples and Pt/C at 1600 rpm in an O2-saturated aqueous electrolyte of 0.1 M KOH at a scan rate of 5 mV s−1. (C) LSV curves of the Mn(0.04)B-CoGSS@CoCHNSs catalyst at various rotation speeds and (D) K–L plots of the catalysts at various potentials.

Based on the rotating-disk electrode (RDE) measurements, the ORR kinetics of the samples were revealed at various rotating speeds (Fig. 9C). From the LSVs presented in Fig. 8C, the current density with Mn(0.04)B-CoGSS@CoCHNSs increases with increasing RDE rotation rates, suggesting a faster oxygen flux to the electrode surface. The linearity of the Koutechy–Levich (K–L) plots obtained from the LSV curves and near-parallel fitted lines confirmed that the kinetics of the reaction is first-order towards the concentration of O2 in the electrolyte, and the number of electron transfer toward the ORR is similar at different potentials.30 According to the slopes of the K–L plots, the electron transfer number (n) for the ORR on the surface of the Mn(0.04)B-CoGSS@CoCHNSs catalyst was obtained in the range of 3.79 to 4.23, indicating a four-electron oxygen reduction pathway (Fig. 9D).

To compare the catalytic activity of the various prepared catalysts toward ORR, the Tafel slopes of the CoGSS, CoGSS@CoCHNSs, Mn(0.04)-CoGSS@CoCHNSs and Mn(0.04)B-CoGSS@CoCHNSs catalysts were determined from the Tafel plots (Fig. 10A). The results show that the Tafel slope for Mn(0.04)B-CoGSS@CoCHNSs (62 mV dec−1) is smaller than those of CoGSS (102.1 mV dec−1), CoGSS@CoCHNSs (98.3 mV dec−1), and Mn(0.04)-CoGSS@CoCHNSs (88.6 mV dec−1), demonstrating that the electron transfer rate on the surface of the this electrode is faster than those on the other prepared catalytic electrodes. The turnover frequency (TOF) and ECSA, as the key factors for ORR that relate to the activity of the catalysts, showed the best performance for Mn(0.04)B-CoGSS@CoCHNSs (Table S1).


image file: d5nr02150b-f10.tif
Fig. 10 (A) Tafel plots of the prepared catalysts for the ORR. (B) Stability test for Mn(0.04)B-CoGSS@CoCHNSs (inset shows methanol tolerance evaluation of Mn(0.04)B-CoGSS@CoCHNSs and Pt/C with the addition of 1 M methanol).

The stability of the optimal sample was checked in O2-saturated 0.1 M KOH by conducting chronoamperometric measurements. The chronoamperometric curves of the Mn(0.04)B-CoGSS@CoCHNSs and Pt/C catalyst were first investigated with a rotating disk electrode (RDE) at 1600 rpm and by the addition of 1.0 M methanol to the electrolyte of 0.1 M KOH. The chronoamperometric curves of Mn(0.04)B-CoGSS@CoCHNSs and Pt/C are compared in the inset of Fig. 10B. It is clear that the chronoamperometric curve of the Pt/C catalyst showed a significant current drop in the presence of methanol. In contrast, no current drop related to methanol oxidation was observed in the chronoamperometric curve of Mn(0.04)B-CoGSS@CoCHNSs, which confirms the higher ORR selectivity on the surface of Mn(0.04)B-CoGSS@CoCHNSs compared to that of the commercial Pt/C catalyst. Furthermore, the Mn(0.04)B-CoGSS@CoCHNSs exhibits an appropriate stability for the ORR, as demonstrated in Fig. 10B. The durability of the prepared catalyst was evaluated at 0.8 V (vs. RHE) in O2-saturated 0.1 M KOH for 18[thin space (1/6-em)]000 s, which showed that approximately 91.2% of the current density was retained after the test.

4. Conclusions

This work has elaborated a facile method for constructing a tri-functional catalyst with a hierarchical yolk–shell structure based on manganese and boron dual-doped CoGSS@CoCHNSs using highly uniform cobalt glycerate solid spheres as self-sacrificial templates. The as-prepared CoGSS@CoCHNSs plays an important role in boosting the overall water splitting and oxygen reduction reaction as an active catalyst. The optimal prepared catalyst exhibited overpotentials of 318 and 75 mV at 10 mA cm−2 in 1.0 M KOH solution with Tafel slopes of 97 and 96 mV dec−1 for OER and HER, respectively. A small cell voltage of 1.61 V was required to achieve 10 mA cm−2 current density in the two electrode system based on Mn(0.04)B-CoGSS@CoCHNSs. Moreover, the prepared catalyst exhibits an onset potential of 0.96 V and a half-wave potential of 0.89 V as an active electrocatalyst for ORR. Due to the unique structure and suitable composition of the MnB-CoGSS@CoCHNSs, increasing the ion transportation and electrical conductivity by heteroelement doping and synergistic effects, the as-prepared catalyst exhibits excellent application potential as an electrocatalyst for ORR, OER, and HER. This work provides a simple and self-templating strategy for the fabrication of hierarchical yolk–shell structures based on MnB-CoGSS@CoCHNSs as an active trifunctional electrocatalyst for the ORR, OER, and HER, the performance of which is comparable to that of other catalysts under the same conditions (Tables S2 and S3).

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI.

Electrochemical investigations, FE-SEM images, EDX and elemental mapping data, CVs of ECSA, comparison of overall water splitting, and ORR activity of catalysts. See DOI: https://doi.org/10.1039/d5nr02150b

Acknowledgements

We are grateful for the financial support from the Iran National Science Foundation (INSF, under postdoctoral grant no. 4024803).

References

  1. X. Guo, M. Li, L. Qiu, F. Tian, L. He, S. Geng, Y. Liu, Y. Song, W. Yang and Y. Yu, Chem. Eng. J., 2023, 453, 139796 CrossRef CAS .
  2. N. L. W. Septiani, Y. V. Kaneti, K. B. Fathoni, Y. Guo, Y. Ide, B. Yuliarto, X. Jiang, N. Nugraha, H. K. Dipojono, D. Golberg and Y. Yamauchi, J. Mater. Chem. A, 2020, 8, 3035–3047 RSC .
  3. S. Rezaee, S. Shahrokhian and Q. Li, J. Colloid Interface Sci., 2024, 671, 394–409 CrossRef CAS PubMed .
  4. M. Abedi, S. Rezaee and S. Shahrokhian, J. Colloid Interface Sci., 2024, 666, 307–321 CrossRef CAS PubMed .
  5. M. Guo, S. Song, S. Zhang, Y. Yan, K. Zhan, J. Yang and B. Zhao, ACS Sustainable Chem. Eng., 2020, 8, 7436–7444 CrossRef CAS .
  6. D. Xu, T. Xia, H. Xu, W. Fan and W. Shi, Chem. Eng. J., 2020, 392, 124838 CrossRef CAS .
  7. D. Lai, Q. Kang, F. Gao and Q. Lu, J. Mater. Chem. A, 2021, 9, 17913–17922 RSC .
  8. W. Yuan, S. Wang, Y. Ma, Y. Qiu, Y. An and L. Cheng, ACS Energy Lett., 2020, 5, 692–700 CrossRef CAS .
  9. L. Zhang, X. Li, Y. Chen, H.-T. Wang, Y. Chen, K. H. Chen, Y.-C. Shao, W. Lin, C.-W. Pao, H. Wang, W.-F. Pong, J. Luo and L. Han, ACS Sustainable Chem. Eng., 2024, 12, 405–415 CrossRef CAS .
  10. M. Shamloofard, S. Shahrokhian and M. K. Amini, J. Colloid Interface Sci., 2021, 604, 832–843 CrossRef CAS PubMed .
  11. H. Liu, C. Xi, J. Xin, G. Zhang, S. Zhang, Z. Zhang, Q. Huang, J. Li, H. Liu and J. Kang, Chem. Eng. J., 2021, 404, 126530 CrossRef CAS .
  12. K. Tang, L. Chen, Y. Xiong, L. Zhang and M. Wu, ACS Appl. Nano Mater., 2023, 6, 11553–11560 CrossRef CAS .
  13. L. Xiao, W. Yu, J. Liu, S. Luan, S. Li, X. Cui and L. Jiang, ACS Sustainable Chem. Eng., 2024, 12, 4709–4717 CrossRef CAS .
  14. Z. Cheng, W. Cheng, X.-N. Lin, R.-H. Zhang, L.-Y. Yan, G.-X. Tian, X.-Y. Shen and X.-W. Zhou, Nanoscale, 2024, 16, 373–383 RSC .
  15. C. Xu, Y. Li, D. Li, Y. Zhang, B. Liu, M. D. H. Akhon and P. Huo, Nanoscale, 2024, 16, 8286–8306 RSC .
  16. R. Jiang, D. R. Baker, D. T. Tran, J. Li, A. C. Leff and S. S. Zhang, ACS Appl. Nano Mater., 2020, 3, 7119–7129 CrossRef CAS .
  17. S. Jiang, G. Ye, W. Zhu, S. Liu, Z. He and G. Jin, ACS Sustainable Chem. Eng., 2023, 11, 16926–16934 CrossRef CAS .
  18. H. Liu, H. Qin, J. Kang, L. Ma, G. Chen, Q. Huang, Z. Zhang, E. Liu, H. Lu, J. Li and N. Zhao, Chem. Eng. J., 2022, 435, 134898 CrossRef CAS .
  19. H. Yuan, S. Wang, Z. Ma, M. Kundu, B. Tang, J. Li and X. Wang, Chem. Eng. J., 2021, 404, 126474 CrossRef CAS .
  20. M. Ghaemmaghami and Y. Yamini, ACS Appl. Mater. Interfaces, 2024, 16, 18782–18789 CrossRef CAS PubMed .
  21. J. Nie, M. Dong, G. Chen, N. Wang, J. Nie and G. Ma, ACS Sustainable Chem. Eng., 2023, 11, 11161–11171 CrossRef CAS .
  22. F. H. Gourji, T. Rajaramanan, Ø. Frette and D. Velauthapillai, Nanoscale, 2023, 15, 16178–16187 RSC .
  23. Y. Song, X. Zheng, Y. Yang, Y. Liu, J. Li, D. Wu, W. Liu, Y. Shen and X. Tian, Adv. Mater., 2024, 36, 2305835 CrossRef CAS PubMed .
  24. F.-X. Ma, H. Hu, H. B. Wu, C.-Y. Xu, Z. Xu, L. Zhen and X. W. David Lou, Adv. Mater., 2015, 27, 4097–4101 CrossRef CAS PubMed .
  25. R. Purbia and S. Paria, Nanoscale, 2015, 7, 19789–19873 RSC .
  26. S. Li, C. Li, W. K. Pang, Z. Zhao, J. Zhang, Z. Liu and D. Li, ACS Appl. Mater. Interfaces, 2019, 11, 27805–27812 CrossRef CAS PubMed .
  27. L.-S. Lin, J. Song, H.-H. Yang and X. Chen, Adv. Mater., 2018, 30, 1704639 CrossRef PubMed .
  28. M. Shirvani and S. S. Hosseiny Davarani, Nanoscale, 2023, 15, 11115–11130 RSC .
  29. H. Wang, E. Feng, Y. Liu and C. Zhang, J. Mater. Chem. A, 2019, 7, 7777–7783 RSC .
  30. R. Liu, X. Gao, Y. Xie, Q. Liu, K. Zhang, Y. Sun, H. Bai, F. Yao and H. Yue, Nanoscale, 2023, 15, 16795–16802 RSC .
  31. L. Zhi, M. Zhang, J. Tu, M. Li and J. Liu, J. Colloid Interface Sci., 2024, 659, 82–93 CrossRef CAS PubMed .
  32. J. Chen, C. Cai, Y. Sun, B. Sun, H. Wang, Y. Tang and Y. Chen, Electrochim. Acta, 2024, 493, 144443 CrossRef CAS .
  33. P. Li, J.-H. Wen, Y. Xiang, M. Li, Y.-X. Zhao, J.-M. Dou, S. Wang, Y.-W. Li, H. Ma and L. Xu, Inorg. Chem. Front., 2024, 12, 11–38 RSC .
  34. S. Zhang, Y. Xiao, Q. Feng and Z. Lei, ACS Appl. Nano Mater., 2024, 7, 8978–8987 CrossRef CAS .
  35. H. Zhang, Y. Wang, X. Li, K. Deng, H. Yu, Y. Xu, H. Wang, Z. Wang and L. Wang, Appl. Catal., B, 2024, 340, 123236 CrossRef CAS .
  36. J. Gu, F. Duan, S. Liu, W. Cha and J. Lu, Chem. Rev., 2024, 124, 1247–1287 CrossRef CAS PubMed .
  37. Y.-N. Zhou, Y.-W. Dong, Y. Wu, B. Dong, H.-J. Liu, X.-J. Zhai, G.-Q. Han, D.-P. Liu and Y.-M. Chai, Chem. Eng. J., 2023, 463, 142380 CrossRef CAS .
  38. Y. Wu, Q. Yuan, Y. Zhao, X. Xu, J. Xu, Y. Wang, K. Sun, A. Wang, H. Sun, B. Li, R. Xu, Z. Wang, J. Jiang and M. Fan, ACS Sustainable Chem. Eng., 2023, 11, 13363–13373 CrossRef CAS .
  39. L. Huo, M. Lv, M. Li, X. Ni, J. Guan, J. Liu, S. Mei, Y. Yang, M. Zhu, Q. Feng, P. Geng, J. Hou, N. Huang, W. Liu, X. Y. Kong, Y. Zheng and L. Ye, Adv. Mater., 2024, 18, 2312868 CrossRef PubMed .
  40. M. Shi, M. Zhao, Q. Zheng, F. Li, L. Jiao, Z. Su, M. Li and X. Song, Energy Fuels, 2024, 38, 1525–1537 CrossRef CAS .
  41. S. Gupta, M. K. Patel, A. Miotello and N. Patel, Adv. Funct. Mater., 2020, 30, 1906481 CrossRef CAS .
  42. T. Zhang, F. Song, Y. Qian, H. Gao, J. Shaw and Y. Rao, ACS Appl. Energy Mater., 2021, 4, 5434–5442 CrossRef CAS .
  43. T. Liu, C. Xu, Y. Guo, P.-Z. Li, S. Yuan and M. Lin, ACS Appl. Nano Mater., 2024, 7, 9062–9067 CrossRef CAS .
  44. J. Y. Xie, F. L. Wang, X. J. Zhai, X. Li, Y. S. Zhang, R. Y. Fan, R. Q. Lv, Y. M. Chai and B. Dong, Chem. Eng. J., 2024, 482, 148926 CrossRef CAS .
  45. J. M. Gonçalves and J. G. Ruiz-Montoya, J. Mater. Chem. A, 2023, 11, 20872–20885 RSC .
  46. S. Chen, X. Yang, X. Tong, F. Zhang, H. Zou, Y. Qiao, M. Dong, J. Wang and W. Fan, ACS Appl. Mater. Interfaces, 2020, 12, 34971–34979 CrossRef CAS PubMed .
  47. X. Guo, X. Yang, M. Zhang, B. Yue, M. Zhang, J. Zhang and Z. Jin, ACS Appl. Mater. Interfaces, 2024, 16, 16399–16407 CrossRef .
  48. M. R. Kandel, U. N. Pan, P. P. Dhakal, R. B. Ghising, S. Sidra, D. H. Kim, N. H. Kim and J. H. Lee, Small, 2024, 20, 2307241 CrossRef CAS PubMed .
  49. M. Shamloofard and S. Shahrokhian, Inorg. Chem., 2023, 62, 1178–1191 CrossRef CAS PubMed .
  50. Y. Fang, C. Dai, X. Liu, Y. Wang, C. Ju, S. He, R. Shi, Y. Liu, J. Zhang, Y. Zhu and J. Wang, Nano Energy, 2024, 127, 109754 CrossRef CAS .
  51. X. Cui, X. Yang, Z. Liu, W. Jiang, J. Wan, Y. Liu and F. Ma, J. Colloid Interface Sci., 2024, 661, 614–628 CrossRef CAS PubMed .
  52. X. Zhang, W. Lu, C. Li, P. Mi, Z. Liu, Y. Li, Y. Tian, X. Ma, F. Liu and X. Jin, Mater. Today Chem., 2024, 35, 101826 CrossRef CAS .
  53. T. Dong, X. Zhang, P. Wang, H. S. Chen and P. Yang, Carbon, 2019, 149, 222–233 CrossRef CAS .
  54. Y. V. Kaneti, R. R. Salunkhe, N. L. W. Septiani, C. Young, X. Jiang, Y.-B. He, Y.-M. Kang, Y. Sugahara and Y. Yamauchi, J. Mater. Chem. A, 2018, 6, 5971–5983 RSC .
  55. Y. Guo, C. Wu, N.-W. Li, S. Yuan and L. Yu, J. Mater. Chem. A, 2019, 7, 25247–25253 RSC .
  56. A. Bahadoran, J. Roshan De Lile, F. Hasanvandian, M. Sharghi, B. Kakavandi, Y. K. Mishra, S. Giannakis and S. Rama Krishna, J. Mater. Chem. A, 2023, 11, 19465–19482 RSC .
  57. X. Guo, J. Liang, L. Wang, Z. Feng, T. Yu, Z. Zhang, Y. Shao, C. Hao and G. Li, Int. J. Hydrogen Energy, 2018, 2034–2042 CrossRef CAS .
  58. M. Shamloofard and S. Shahrokhian, Nanoscale, 2021, 13, 17576–17591 RSC .
  59. N. Kumar, T. R. S. Kamalakannan, M. Prakash, B. Viswanathan and B. Neppolian, ACS Appl. Energy Mater., 2022, 5, 2104–2111 CrossRef .
  60. M. Ravipati and S. Badhulika, ACS Appl. Nano Mater., 2024, 7, 7277–7288 CrossRef CAS .
  61. B. Jiang, J. Zhu, Z. Xia, J. Lyu, X. Li, L. Zheng, C. Chen, S. Chaemchuen, T. Bu, F. Verpoort, S. Mu, J. Wu, J. Wang and Z. Kou, Adv. Mater., 2024, 36, 2310699 CrossRef CAS PubMed .
  62. R. Sadhukhan, A. Kumar, P. K. Prasanna, A. Guha, R. Arenal, S. Chakraborty and T. N. Narayanan, ACS Appl. Mater. Interfaces, 2024, 16, 8627–8638 CrossRef CAS PubMed .
This journal is © The Royal Society of Chemistry 2025
Click here to see how this site uses Cookies. View our privacy policy here.