View PDF VersionPrevious ArticleNext Article
 
DOI: 10.1039/D3TA04768G (Paper) J. Mater. Chem. A, 2023, 11, 25332-25344

Ultrafine Co3O4 nanoparticles-engineered binary metal nitride nanorods with interfacial charge redistribution for enhanced water splitting†

Dinh Chuong Nguyen a, Thi Luu Luyen Doan b, Xinfeng Zhu a, Nam Hoon Kim *ad and Joong Hee Lee *acd
aDepartment of Nano Convergence Engineering, Jeonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea. E-mail: nhk@jbnu.ac.kr; jhl@jbnu.ac.kr
bDivision of Mechanical Design Engineering, School of Engineering, Jeonbuk National University, Jeollabuk-do Jeonju 54896, Republic of Korea
cCarbon Composite Research Center, Department of Polymer-Nano Science and Technology, Jeonbuk National University, Jeonju, Jeonbuk 54896, Republic of Korea
dAHES Co., 445 Techno Valley-ro, Bongdong-eup, Wanju-gun, Jeonbuk 55314, Republic of Korea

Received 10th August 2023 , Accepted 8th October 2023

First published on 12th October 2023

Abstract

Herein, an efficient bifunctional electrocatalyst based on binary metal nitride (Zn3N2–Co2N) nanorod arrays decorated with ultrathin Co3O4 nanoparticles is prepared to improve overall water splitting performance. Notably, a charge redistribution process occurs at the heterointerface between the host metal nitride and the guest Co3O4, resulting in the regulation of both their electronic structures to facilitate the reaction kinetics of hydrogen/oxygen evolution reaction (HER/OER). Using the material to catalyze the HER, current density of 10 and 50 mA cm−2 are delivered at small overpotential of 80.5 and 121.7 mV, respectively. Similarly, for OER, under the action of the material, overpotential of only 271.7 and 335.7 mV, respectively, is required to achieve 10 and 50 mA cm−2. Thus, an electrolyzer is assembled with the material as both cathode and anode, and shows high performance with a small cell voltage of 1.59 V at 10 mA cm−2, and excellent stability.

Introduction

Hydrogen (H2) as a promising alternative to traditional fossil fuel has attracted considerable attention, since the global energy crisis and environmental pollution associated with the widespread combustion of fossil fuel are radically increasing nowadays.1–4 The production of H2 based on electrochemical water splitting is considered an effective technique, both in terms of the environment, and the economy.5–7 During water electrolysis, H2 gas is produced at the cathodic electrode by an electrochemical reaction, namely the hydrogen evolution reaction (HER), while oxygen (O2) gas is produced at the anodic electrode via another reaction, namely the oxygen evolution reaction (OER).8–10 The sluggish kinetics of both HER and OER form the unavoidable bottleneck in water splitting, lowering its real performance well below that predicted in theory.11,12 The previous studies demonstrated that integrating the noble metal-based electrocatalysts, such as Pt/C for HER and RuO2 or IrO2 for OER, to form an electrolyzer, can yield the expected water-splitting performance.13–15 However, the scarcity and high cost of these noble metals make it difficult to commercialize the water electrolyzer.16,17 Accordingly, exploring cost-effective, highly efficient, and stable bifunctional electrocatalysts for both HER and OER is a critical strategy to develop water splitting toward the large-scale industrial production of green H2. Earth-abundant transition metal nitrides (TMNs), such as cobalt nitride (Co2N) and zinc nitride (Zn3N2), have drawn much research attention as efficient bifunctional electrocatalysts for overall water splitting, on account of their advantages of metallic conductivity, high corrosion resistance, special electronic structure, and their multi-valence states.18 Unfortunately, the single-phase nitrides still show unsatisfactory catalytic performance for HER and OER as compared to those of the noble metals-based electrocatalysts, causing the consumption of rather high electrical power to establish overall water splitting.19 By combining two TMNs to form a binary nitride system, the catalytic activity can be significantly improved considering both the large inheritance from the merits of individual components, and the chemical coupling effects of these nitride components. For example, Chen et al. reported on the accurate activation of MoN catalyst for HER by incorporating Ni atoms, which resulted in the formation of Mo–Ni binary nitride.20 The findings proved that after the introduction of Ni, the synthesized Mo–Ni binary nitride showed a proper bonding strength between Mo active site and the H intermediate, thus its HER catalytic activity was much better than that of the MoN alone. Furthermore, the Mo–Ni binary nitride outperformed the mono-metal nitride in terms of structural stability as well. Recently, the Guo group proposed the Mo2N/CoxN heterojunction nanosheets with interfacial electron redistribution as bifunctional electrocatalysts for overall water splitting.21 The theoretical calculations demonstrated that the proposed Mo2N/CoxN had favorable water dissociation kinetics, which could be attributed to the electron redistribution at the heterointerface between Mo2N and CoxN. This, coupled with the structural advantages of large accessible surface and abundant pore exposure to enhance mass transport capability, endowed the Mo2N/CoxN with excellent electrocatalytic activity for both HER and OER. In addition to TMNs, transition metal oxides (TMOs), such as cobalt oxides (Co3O4), have also been widely employed for water-splitting electrocatalysts, owing to their loosely bonded d electrons.22 The problem is that the electrical conductivity of cobalt oxides is rather poor, leading to the slow electron transfer to catalytically active sites and thereby an unsatisfactory electrocatalytic activity.23 This drawback can be overcome by constructing a hybrid structure containing TMOs and conductive components because there are also synergistic effects. Liu et al. reported that in the Co3O4@MoS2 heterostructure, strong electronic interaction between Co3O4 and MoS2 components caused the charge redistribution, which resulted in a change in the Gibbs free energy of the reaction intermediates for significant improvement of overall water splitting performance.24 Elsewhere, Zhou et al. demonstrated that the existence of electronic interactions between CuOx and Co3O4 components in CuOx@Co3O4 heterostructure led to improved electronic conductivity of the entire CuOx@Co3O4 material.25 Regarding hybrid structure of TMNs and TMOs, TMNs with their metallic highly electron conductive characteristic can promote electron transfer from the conductive support to the catalytic active sites,26 while in the introduction of TMOs, the water dissociation can be expedited to boost the overall kinetics of water splitting.27 It is well known that in addition to the optimization of adsorption/desorption energies of intermediates, the water dissociation must be focused to enhance the efficiency of overall water splitting.28,29 Liu et al. proved that Co3O4 with the thermal equilibrium of OH adsorption on Co atoms promoted the break of the H–OH bond, and thus the incorporation of Co3O4 into host materials could provide active sites for adsorption of OH group that is beneficial for water dissociation.24 Moreover, the presence of TMOs over the surface of other transition metal compounds can increase the adsorption of nearby H intermediates, which is conducive to the HER process.30 It is also demonstrated that the hybrid structure of TMOs and TMNs can endow the catalyst with charge redistribution at the heterointerface between the components, also benefiting the intermediate adsorption, and thereby further enhancing the reaction kinetics of both HER and OER.31 Yan et al. found that the heterostructure CoOx/CoNy nanoparticles was beneficial to the catalytic activity of the entire CoOx/CoNy embedded in carbon–nitride nanosheets associated with the exposure of multiple active sites, and synergistic effect between the CoNy as OER active site and favorable charge transfer on CoOx with abundance of oxygen vacancies.32 In another report, Sun et al. showed that petal-like MoO2@MoN core–shell nanosheets exhibited high HER and OER efficiency; this mainly contributed to the chemical coupling effect and strong electronic interaction between MoO2 and MoN to enhance the electronic conductivity and charge transfer ability.33 Nevertheless, despite the benefits mentioned, no binary metal nitride (Zn3N2–Co2N) hybridized TMOs (Co3O4) material has yet been reported in the catalysis field for overall water splitting. Herein, we propose an advanced nanostructured material based on earth-abundant binary cobalt–zinc nitride nanorod arrays decorated with ultrafine Co3O4 nanoparticles supported on three-dimensional (3D) Ni foam (NF) (labeled as Co3O4 NPs/Co–Zn nitride), as an ideal electrocatalyst for HER/OER toward achieving remarkable performance of overall water splitting in alkaline conditions. Emphatically, the Co3O4 NPs/Co–Zn nitride catalyst enables a HER and OER current density of 10 mA cm−2 at very low overpotentials of 80.5 and 271.7 mV, respectively. Further, the findings demonstrate the rather small Tafel slope values of Co3O4 NPs/Co–Zn nitride catalyst for HER and OER of 54.4 and 52.4 mV dec−1, respectively, which are solid evidence for the favorable HER and OER reaction kinetics under the help of our proposed catalyst. The result is that the electrolyzer with Co3O4 NPs/Co–Zn nitride as catalytic electrodes delivers current densities of 10 and 50 mA cm−2 at rather low cell voltages of 1.59 and 1.81 V, respectively, and is highly stable – at least, it can continuously operate for more than 60 h without damage to the catalytic performance.

Experimental

Chemical reagents

The (CH3COO)2Co·4H2O, Zn(NO3)2·6H2O, urea, NH4F, HCl (37%), NaOH, Nafion (5 wt%), KOH, acetone, Pt/C (20 wt% loading), ethanol, RuO2 (99.9%), and Co(NO3)2·6H2O were purchased from Sigma-Aldrich.

Synthesis of the Co3O4 NPs/Co–Zn nitride

The precursor cobalt–zinc carbonate hydroxide nanorod arrays supported on NF (labeled as Co-Zn-(OH-CO3)) was prepared by a hydrothermal method (see the ESI† for the synthetic details). The as-prepared Co-Zn-(OH-CO3) was then subjected to thermal nitridation under NH3 atmosphere for 3 h at 500 °C with a heating rate of 2 °C min−1 to convert into nitride sample (labeled as Co–Zn nitride). The NH3 flow rate applied was 50 sccm. Next, the obtained Co–Zn nitride was consecutively immersed in 1 mM mM (CH3COO)2Co and 1 mM NaOH solutions for 30 min each, prior to calcination in Ar gas (100 sccm) for 2 h at 300 °C with a heating rate of 2 °C min−1, to finally form Co3O4 NPs/Co–Zn nitride sample. The cobalt–zinc oxide nanorod arrays supported on NF (labeled as Co–Zn oxide) was also synthesized for comparison (see the ESI for the synthetic details†).

Material characterization

Field-emission scanning electron microscopy (FE-SEM) images and energy dispersive X-ray spectroscopy (EDS) patterns of the products were conducted on Zeiss-Gemini 500. Transmission electron microscopy (TEM) measurements, high-resolution TEM (HR-TEM), and EDS mapping data were carried out on JEOL-JEM 2010 at the Jeonju Center of the Korea Basic Science Institute. Powder X-ray diffraction (XRD) patterns of the products were collected by PANalytical-X’Pert PRO Multi-Purpose X-ray Diffractometry with Cu-Kα radiation (λ = 1.54 Å) at the Center for University-wide Research Facilities, Jeonbuk National University, Korea. The X-ray photoelectron spectroscopy (XPS) spectra of all products were recorded in Thermo Fisher Scientific-Theta Probe. The Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution of the products were acquired on Micromeritics Instrument-ASAP 2020 Plus system (USA). In situ Raman measurements were performed by A Raman spectrometer (Raman-touch, Nanophoton) with an excitation laser wavelength of 532 nm and a power of ca. 5 mW.

Electrochemical measurements

The electrochemical characterization of the products was conducted on CH660D workstation. Linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronoamperometric measurements for HER and OER under alkaline conditions (1.0 M KOH electrolyte) were performed using a three-electrode test configuration, wherein the developed samples (1 cm × 1 cm), graphite rod, and Ag/AgCl were the working, counter, and reference electrodes, respectively. All LSV curves for HER and OER were compensated for ohmic resistance. In addition, all potentials were calibrated with respect to reversible hydrogen electrode (RHE). To evaluate the overall water splitting performance, the Co3O4 NPs/Co–Zn nitride was used as both catalytic cathode and catalytic anode to assemble an Co3O4 NPs/Co–Zn nitride (−, +) cell. The comparative cell based on Pt/C as a cathode catalyst and RuO2 as an anode catalyst was also built for comparison (see the ESI† for the synthetic detail of the Pt/C- and RuO2-based electrodes).

Results and discussion

Morphological and structural characterization

Scheme 1 depicts the synthetic protocol of the Co3O4 NPs/Co–Zn nitride. First, the Co-Zn-(OH-CO3) nanorod arrays was deposited on the NF surface via a hydrothermal reaction, which is recognized as an effective method of nanomaterial fabrication owing to its facility and flexibility in morphology control. In this method, the reaction rate can be speeded up, accompanying by the occurrence of the unusual reaction, causing the formation of crystalline materials at rather low temperatures.34 Fig. S1† reveals the quite smooth surface of the pre-treated NF, which contrasts with that of the Co-Zn-(OH-CO3) sample (Fig. S2†). The SEM images at low magnifications (Figs. S2a and b†) reveal that the hydrothermal reaction gives the whole surface of the NF a coat of a secondary material. At high magnification (Fig. S2c and d†), the secondary material presents arrays of solid nanorod features with average diameter of ∼70 nm. The powder XRD analysis was then performed for Co-Zn-(OH-CO3) sample to investigate its crystalline structure. As shown in Fig. S3,† there are some obvious diffraction peaks observed from the XRD pattern of the Co-Zn-(OH-CO3), demonstrating its rather high crystallinity. The diffraction peaks centered at two theta of 28.4, 36.2, 39.1, 58.6, and 63.3° match well with the (021), (510), (−420), (−622), and (−821) planes of the Zn5(CO3)2(OH)6 (JCPDS no. 19-1458),35 and the other peaks centered at two theta of 33.8, 35.5, 36.5, 47.3, 59.9, and 62.2° should be assigned to the (221), (040), (231), (340), (412) and (450) planes of Co2CO3(OH)2 (JCPDS no. #48-0083) without any impurity.36 Then, Co–Zn nitride was synthesized by the nitridation treatment of Co-Zn-(OH-CO3) at high temperature in NH3 atmosphere. Although the obtained Co–Zn nitride still retains the solid nanorod arrays feature of its parent (Fig. S4a and b†), the surface of individual nanorods changes from smooth to relatively rough (Fig. S4c and d†). Also, the nanorods tend toward connection with each other, which will provide effective pathways for electron transport along the whole proposed material, thereby significant improving the catalytic performance. After nitridation, the Co–Zn nitride sample is found to be comprised of two dominant phases, such as Zn3N2 (JCPDS no. #035-0762),37 corresponding to the diffraction peaks at two theta of 31.6, 34.2, 36.7, 43.3, and 70.7°, and Co2N (JCPDS no. #072-1368),38 showing the diffraction peaks at two theta of 41.5, 42.6, 56.8, and 69.5° (Fig. S5†). Finally, the Co3O4 NPs/Co–Zn nitride was acquired through the growth of Co3O4 nanoparticles (NPs) over the surface of Co–Zn nitride by precipitation method, combined with thermal treatment in air. The SEM images of Co3O4 NPs/Co–Zn nitride in Fig. 1a–c clearly show a large amount of uniform nanorod structure with apparent rough surface and uniform diameter of ∼60 nm, which is very similar in the appearance of the Co–Zn nitride alone, suggesting that there is no influence on the pristine morphology, even under the harsh conditions of the growth process of the Co3O4 NPs. To explore the micromorphology of Co3O4 NPs/Co–Zn nitride, TEM measurement was also performed. Fig. 1d and e show the TEM images of the Co3O4 NPs/Co–Zn nitride at different magnifications. Clearly, the solid nanorod morphology with the rather rough surface feature of Co3O4 NPs/Co–Zn nitride is also confirmed by TEM observations. Such rough feature is anticipated to provide a larger number of catalytic active sites and endow the entire catalyst with a larger specific surface area than those of the smooth surface. The diameter of individual Co3O4 NPs/Co–Zn nitride nanorods is found to be about ∼60 nm, which is consistent with the SEM result. Interestingly, the surface of the Co3O4 NPs/Co–Zn nitride nanorod is decorated with dark pots highlighted in white cycles, which can be assigned to the Co3O4 NPs, as displayed in the magnified TEM (Fig. 1e), HR-TEM, and EDS mapping images (Fig. S6†). The content of Co3O4 NPs in the Co3O4 NPs/Co–Zn nitride sample was determined by XPS analysis, which exhibits that the Co percentage of Co3O4 NPs in the proposed sample is around 14.08 at% (Table S1†).
image file: d3ta04768g-s1.tif
Scheme 1 A schematic of the synthetic procedure of the Co3O4 NPs/Co–Zn nitride.

image file: d3ta04768g-f1.tif
Fig. 1 (a)–(c) SEM images, (d) and (e) TEM images, (f) HR-TEM images, (g)–(j) EDS mapping images, and (k) XRD pattern of the Co3O4 NPs/Co–Zn nitride.

To determine the primary phases in the developed material, we also carried out high-resolution TEM (HR-TEM) analysis at the surface of individual Co3O4 NPs/Co–Zn nitride nanorods (Fig. 1f). Three different values are detected, including the lattice fringes of 0.21, 0.28, and 0.28 nm corresponding to the (111) plane of Co2N, (222) plane of Zn3N2, and (220) plane of Co3O4, respectively, suggesting the existence of these three phases in the hybrid material. In addition to the Co2N and Zn3N2 phases originating from the Co–Zn nitride template as mentioned above, the appearance of new phase proves that the dark pots over the surface of Co–Zn nitride is Co3O4. Indeed, according to the EDS mapping images represented in Fig. 1g–j, the individual Co3O4 NPs/Co–Zn nitride nanorod was confirmed to be a compound of four elements, namely Zn, Co, N, and O. Further, the XRD pattern in Fig. 1k corroborates that the Co3O4 NPs/Co–Zn nitride contains three separated phases, such as Co2N, Zn3N2, and Co3O4 (JCPDS no. #080-1537),39 which matches well with the diffraction peaks at two theta of 30.9, 36.4, 48.5, 55.0 and 76.5°. The transformation of chemical composition from the pure NF to the Co3O4 NPs/Co–Zn nitride was observed from EDS analysis; the EDS spectra of the pure NF, Co-Zn-(OH-CO3), Co–Zn nitride alone, and Co3O4 NPs/Co–Zn nitride are represented in Fig. S7.† Only the Ni element can be observed from the EDS spectrum of the NF (Fig. S7a†), while after hydrothermal reaction, the signals of the Zn, Co, C, and O elements were detected from the Co-Zn-(OH-CO3) (Fig. S7b†), consistent with the Zn5(CO3)2(OH)6 and Co2CO3(OH)2 phases detected by XRD. Regarding the Co–Zn nitride (Fig. S7c†), in addition to the Zn and Co elements, the appearance of N element instead of C demonstrates the complete conversion into nitride phases (Zn3N2 and Co2N) after the nitridation treatment. Finally, the EDS spectrum of the Co3O4 NPs/Co–Zn nitride shows the presence of Zn, Co, N, and O elements, which is in line with the XRD and EDS mapping results.

Next, we performed XPS analysis for the Co3O4 NPs/Co–Zn nitride to further investigate the chemical environment of the surface element. More importantly, the XPS is a useful measurement to confirm whether or not there has been charge redistribution within the heterostructure of the Co3O4 NPs/Co–Zn nitride. Fig. 2a shows the XPS survey spectra of the Co3O4 NPs/Co–Zn nitride and Co–Zn nitride, which reveals that all the required elements of Zn, Co, N, and O are detected. To evidence the electron interaction occurring in the Co3O4 NPs/Co–Zn nitride heterostructure, we compare the high-resolution XPS spectra of the Co3O4 NPs/Co–Zn nitride and Co–Zn nitride (Fig. 2b–e). The Zn 2p spectrum of the pristine Co–Zn nitride was fitted, and the result in Fig. 2b shows two peaks at binding energies of 1022 and 1045.2 eV that can be ascribed to the Zn 2p3/2 and Zn 2p1/2 characteristic peaks of the Zn3N2 phase, respectively.40 After the introduction of the Co3O4 NPs, the binding energies of the Zn 2p3/2 and Zn 2p1/2 peaks in the Co3O4 NPs/Co–Zn nitride hybrid structure exhibit a slightly positive shift of around 0.3 eV, suggesting an increase in positive charges of Zn ion.41 In the Co 2p XPS spectrum of the Co–Zn nitride (Fig. 2c), there are two dominant peaks centered at binding energies of 780.8 and 796.0 eV, which index to the Co 2p3/2 and Co 2p1/2 characteristic peaks of the Co2N phase.42 Additionally, two shake-up satellite peaks at higher binding energies indicate that the Co2+ is the main oxidation state of Co in this heterostructure.43 Regarding the Co3O4 NPs/Co–Zn nitride, the Co 2p XPS spectrum is rather different from that of its counterpart. The new peaks at binding energies of 779.5 and 794.8 eV for Co 2p3/2 and Co 2p1/2, respectively, originate from the Co3+ of Co3O4 phase; in addition, those at binding energies of 781.2 and 795.9 eV for Co 2p3/2 and Co 2p1/2 belong to the Co2+ of Co3O4, demonstrating the formation of new phase Co3O4 in the hybrid material.40 Like Zn 2p, the characteristic peaks of Co2N show a slight shift to higher binding energies, evidencing the modulation of electronic structure of Co atoms after engineering by the guest Co3O4. As represented in Fig. 2d, the N 1s XPS spectrum of the Co–Zn nitride sample is deconvoluted into six peaks at 395.4, 396.1, 397.7, 399.6, 401.4, and 403.2 eV, which are due to the Zn–N bond, Co–N bond, pyridinic-N, pyrrolic-N, graphitic-N, and oxidized-N, respectively,40,44,45 whereas the N 1s XPS peaks of the Co3O4 NPs/Co–Zn nitride are positively shifted, compared to the pristine sample. The shifts of the Zn 2p, Co 2p, and N 1s peaks of the Co3O4 NPs/Co–Zn nitride in the direction of higher binding energies than those of the pristine Co–Zn nitride demonstrate the presence of a strong electron interaction between the host Co–Zn nitride and the guest Co3O4. The deposition of Co3O4 over the surface of Co–Zn nitride causes charge redistribution between Co3O4 and Zn3N2/Co2N, which can effectively regulate the electronic structure of both Co and Zn across the heterointerface, and thereby significantly improve their electrocatalytic activity.46,47 For the O 1s XPS spectrum of the Co3O4 NPs/Co–Zn nitride in Fig. 2e, the peak at binding energy of 529.7 eV corresponds to the Co–O bond belonging to Co3O4 phase, while the peaks at binding energies of 531.5 and 532.6 eV are assigned to the hydroxyl group and weakly adsorbed water, respectively.48,49 According to the XPS results, the incorporation of Co3O4 and Co–Zn nitride brings a strong electron interaction between them, and thus electronic structure of the atoms centered near the heterointerface region is tailored. Such charge redistribution can be anticipated to endow the entire Co3O4 NPs/Co–Zn nitride material with excellent catalytic activity for both HER and OER toward effective boosting of the overall water splitting.


image file: d3ta04768g-f2.tif
Fig. 2 (a) XPS survey spectra and (b)–(e) high-resolution XPS spectra of the Zn 2p, Co 2p, N 1s, and O 1s for the Co–Zn nitride and Co3O4 NPs/Co–Zn nitride. (f) N2 adsorption–desorption isotherm of the Co3O4 NPs/Co–Zn nitride.

In addition to the optimal electronic structure, the Co3O4 NPs/Co–Zn nitride with multi-dimensional structure, including zero-dimensional structure of Co3O4 NPs, one-dimensional structure of Co–Zn nitride nanorod arrays, and 3D structure of NF substrate, is expected to provide a large surface area with abundant exposure of catalytic active sites, which can effectively support the catalysis of electrochemical reactions on a geometric basis. The specific surface area is estimated using N2 adsorption–desorption isothermal test on Co3O4 NPs/Co–Zn nitride. As exemplified in Fig. 2f, the Co3O4 NPs/Co–Zn nitride possesses a relative high BET specific surface area of 57.2 m2 g−1, which exceeds that of other earth-abundant transition metal-based electrocatalysts reported elsewhere ZnO/AuNP-30 with 10.5 m2 g−1, Co–FeOOH@Ir–Co(OH)F with 30 m2 g−1, and 1T-VS2@N with 47 m2 g−1.50–52 It is also worth noting that all features of the obtained isotherm match well with the typical IUPAC type IV pattern with the existence of hysteresis loop, suggesting the existence of mesopores with the hybrid structure. Indeed, the pore size distribution in Fig. S8† reveals the highest peaks at pore diameter range 1.2 to 10 nm, which belong to the meso-range. The Co3O4 NPs/Co–Zn nitride with large amount of mesopores can expose more active sites to contact more reactant molecules, and facilitate the charge/mass transfer at the contacting interfaces, which is more beneficial to the reaction kinetics of HER and OER in the process of electrochemical water splitting.53

Electrocatalytic performance

Electrochemical activity for HER. The proposed Co3O4 NPs/Co–Zn nitride sample is expected to effectively catalyze the HER, OER, and overall water splitting processes, owing to its twin advantages of physicochemical property and structure. We firstly assessed the electrocatalytic property of the Co3O4 NPs/Co–Zn nitride for alkaline HER using a standard three-electrode configuration. The control samples, such as Co–Zn nitride, Co–Zn oxide, and Co-Zn-(OH-CO3), and commercial Pt/C catalyst, were also tested for comparison. As seen in Fig. 3a, the Co3O4 NPs/Co–Zn nitride shows a highly catalytic property for HER, with current densities of 10 and 50 mA cm−2 at low overpotentials of 80.5 and 121.7 mV, respectively. Notably, the Co3O4 NPs/Co–Zn nitride activity is negligibly poorer than that of the Pt/C with overpotential of 51.7 and 119.0 mV at the same current densities; however, it displays much better activity than those of the Co–Zn nitride, Co–Zn oxide, and Co-Zn-(OH-CO3), supporting by the much higher overpotentials of these counterparts at the same current densities.
image file: d3ta04768g-f3.tif
Fig. 3 HER performance: (a) LSV curves, (b) over potentials at 10 and 50 mA cm−2 and (c) Tafel plots of the Co3O4 NPs/Co–Zn nitride (4) and its counterparts: Co–Zn nitride (3), Co–Zn oxide (2), and Co-Zn-(OH-CO3) (1), and Pt/C commercial catalyst (5); (d) chronoamperometric curve of the Co3O4 NPs/Co–Zn nitride at constant applied potential of 121.7 mV. (e) XRD patterns, (f) XPS survey spectra, and (g)–(j) high-resolution XPS spectra of the Zn 2p, Co 2p, N 1s, and O 1s for the post-HER Co3O4 NPs/Co–Zn nitride and the fresh sample.

The Co–Zn nitride, Co–Zn oxide, and Co-Zn-(OH-CO3) afford current density of 10 mA cm−2 at overpotential of 117.8, 154.5, and 195.1 mV, respectively (Fig. 3b). Thus, the Co3O4 NPs/Co–Zn nitride catalyzes the alkaline HER the most effectively among its counterparts; furthermore, our material is also among the best noble-metal-free HER electrocatalysts, as listed in Table S2.†Fig. 3c depicts the Tafel plots for the Co3O4 NPs/Co–Zn nitride, Co–Zn nitride, Co–Zn oxide, Co-Zn-(OH-CO3), and Pt/C. As seen, the Pt/C commercial catalyst gives the smallest Tafel slope of 39.2 mV dec−1. For the Co3O4 NPs/Co–Zn nitride, its Tafel slope value of 54.4 mV dec−1 is lower than those of the Co–Zn nitride, Co–Zn oxide, and Co-Zn-(OH-CO3) of 88.3, 89.5, 92.1 mV dec−1, respectively. Such smaller value indicates that the Co3O4 NPs/Co–Zn nitride can bring more favorable and faster reaction kinetics to the HER process, compared to the controlled samples. Furthermore, we also used the Tafel slope values to determine the predominant mechanism of the HER under the action of the Co3O4 NPs/Co–Zn nitride. The measured Tafel slope of 54.4 mV dec−1 suggests that the HER taking place on the active surface of the Co3O4 NPs/Co–Zn nitride follows the Volmer–Heyrovsky mechanism, as described by the following equations:54

 
H2O + e− → Hads + OH− (Volmer reaction)(1)
 
Hads + H2O + e− → H2 + OH− (Heyrovsky reaction)(2)

Next, we carried out multi-step chronopotentiometric measurement for the Co3O4 NPs/Co–Zn nitride under HER to probe its stability. Under HER conditions (Fig. S9†), when the applied potential increases from 80.5 to 143.4 mV, the response current density also increases from 10 to 100 mA cm−2; interestingly, these response current densities seem to switch rather quickly, and maintain their stable state for at least 500 s at each potential, suggesting remarkable stability, high catalytic activity, and excellent mass transport character. Interestingly, such excellent mass transport nature accelerates the diffusion of reactant, electrolyte, and products along its structure, which is beneficial to the HER performance, even in the harsh conditions of continuously changing applied potentials.54 The durability of the Co3O4 NPs/Co–Zn nitride was further evaluated under alkaline HER conditions by recording the chronoamperometric curve at constant potential for 60 h. Fig. 3d reveals that the electrocatalytic activity of the Co3O4 NPs/Co–Zn nitride is retained well, with current density retention of 98.9%, after 60 h. The high stability of the Co3O4 NPs/Co–Zn nitride supports the stable release of H2 and O2 bubbles over its surface for long reaction time, as seen in the inset of Fig. 3d. The stable catalytic activity of the Co3O4 NPs/Co–Zn nitride is further proven by the overlap of the fresh LSV curve and that recorded after long-term testing (Fig. S10a†). Even after 60 h operation, the overpotentials of the post-HER Co3O4 NPs/Co–Zn nitride at current densities of 10 and 50 mA cm−2 are negligibly higher than those of the fresh one (Fig. S10b†). Also, the SEM, TEM, and EDS elemental mapping images in Fig. S11† evidence the remaining morphology and microstructure, as well as the unchanged chemical composition, of the post-HER Co3O4 NPs/Co–Zn nitride. We then performed XRD measurement to determine the phase of post-HER sample. As shown in Fig. 3e, all diffraction peaks observed from the XRD pattern of the post-HER Co3O4 NPs/Co–Zn nitride are in line with the phases of Co2N, Zn3N2, and Co3O4, reconfirming the chemical stability of our proposed material. XPS measurement was also employed to identify the valence state of the Co3O4 NPs/Co–Zn nitride after long-term catalysis of the HER. The obtained XPS survey spectrum in Fig. 3f reveals that the Zn, Co, N, and O elements are the primary components in the post-HER sample. By fitting the Zn 2p, Co 2p, N 1s, and O 1s XPS spectra of the post-HER Co3O4 NPs/Co–Zn nitride (Fig. 3g–j), all obtained peaks index well to the Co–N bond in Co2N phase, Zn2+ oxidation state in Zn3N2 phase, and Co2+ and Co3+ oxidation states of Co3O4 phase, highly consistent with the XPS result of the fresh sample. Therefore, all experimental results mentioned above demonstrate that the Co3O4 NPs/Co–Zn nitride serves as a highly active and stable electrocatalyst for the HER in alkaline conditions.

Electrochemical activity for the OER. In addition to the excellent catalytic activity of the Co3O4 NPs/Co–Zn nitride for alkaline HER, it is anticipated that the Co3O4 NPs/Co–Zn nitride can effectively catalyze the anodic electrochemical reaction: OER of the water splitting owing to its unique advantages of geometric and electronic structure. The catalytic performance of the Co3O4 NPs/Co–Zn nitride was thus examined for OER in 1.0 M KOH electrolyte. Also, the OER catalytic performance of its counterparts and the commercial RuO2 was probed for comparison. From the LSV curves in Fig. 4a, the overpotentials at current densities of 10 and 50 mA cm−2 are determined for all these products, and Fig. 4b presents the results. The Co3O4 NPs/Co–Zn nitride can deliver 10 and 50 mA cm−2 at overpotentials of 271.7 and 335.8 mV, respectively, which are much lower values than those of the Co–Zn nitride with overpotentials of 284.6 and 363.5 mV, Co–Zn oxide with overpotentials of 293.4 and 368.5 mV, or Co-Zn-(OH-CO3) with overpotentials of 337.8 and 396.6 mV at the same current densities. Obviously, the successful conversion into nitride state and the integration of Co3O4 NPs resulting in charge redistribution account for the better OER performance of the Co3O4 NPs/Co–Zn nitride than those of the control samples. Importantly, the Co3O4 NPs/Co–Zn nitride outperforms the RuO2 commercial catalyst; the overpotentials of RuO2 are 335.4 and 438.9 mV at current densities of 10 and 50 mA cm−2, respectively (Fig. 4b). Furthermore, the Co3O4 NPs/Co–Zn nitride shows the higher catalytic activity for OER than OER electrocatalysts based on the non-noble metals reported elsewhere, as listed in Table S3.† Then, we calculated the Tafel slope of the products and RuO2 catalyst according to the LSV results to elucidate the OER reaction kinetics (Fig. 4c). The Tafel slope of 52.4 mV dec−1 is obtained for the Co3O4 NPs/Co–Zn nitride, which is lower than those of the Co–Zn nitride, Co–Zn oxide, Co-Zn-(OH-CO3), and RuO2 with 87.3, 87.6, 87.8, and 111.8 mV dec−1, respectively. Such small Tafel slope value indicates the most favorable kinetics of the OER under the help of the Co3O4 NPs/Co–Zn nitride, which allow the oxygen evolution on the Co3O4 NPs/Co–Zn nitride surface to occur at the lowest overpotential, compared to those on the comparative samples, even on the commercial RuO2 catalyst. To evaluate the stability and durability of the Co3O4 NPs/Co–Zn nitride under OER conditions, multi-step chronopotentiometric measurement was performed (Fig. S12†). As under the HER conditions, the current densities instantly respond to the changed the applied potentials (from 1.50 to 1.60 V), and the corresponding current densities are stable for at least 500 s at each step. This phenomenon demonstrates that the Co3O4 NPs/Co–Zn nitride possesses by nature an extraordinary mass transport for OER process, which leads to the high OER performance, and good OER stability. Next, its stability for OER was deeply investigated by chronoamperometric measurement at constant voltage of 335.8 V for 60 h. Fig. 4d shows the stable state of the resultant current density throughout the stability testing; the Co3O4 NPs/Co–Zn nitride exhibits a very high current density retention of 87.6% after long-term operation. The inset of Fig. 4d presents the surface of Co3O4 NPs/Co–Zn nitride catalytic electrode during the chronoamperometric testing; it clearly reveals the formation of a large amount of O2 bubbles over the electrode surface, reconfirming the superior OER activity of our proposed material. Further, the LSV curve recorded after stability testing shows a subtle difference, compared to the fresh one (Fig. S13a†). The comparison of overpotentials at current densities of 10 and 50 mA cm−2 obtained before and after stability testing also shows a subtle difference (Fig. S13b†). The SEM and TEM images of the post-OER Co3O4 NPs/Co–Zn nitride in Fig. S14a–c† show a negligible aggregation of nanorods; however, the morphology feature of nanorod is still similar to the fresh sample. Turning to the chemical composition, the EDS elemental mapping images (Fig. S14d–g†) demonstrate that all the required elements, such as Zn, Co, N, and O, appear in the post-OER Co3O4 NPs/Co–Zn nitride. Notably, the density of O element becomes higher, compared to the fresh Co3O4 NPs/Co–Zn nitride, suggesting that the nitride phase may be partially oxidized on their surface, resulting in the formation of in situ oxidation phases, which is regarded as real active sites for OER. The magnified TEM images of the post-OER sample in Fig. S15a and b† reveal that there is a thin layer covering the Co3O4 NPs/Co–Zn nitride surface after long-term catalysis of the OER. Corresponding HR-TEM images of the shell layer in Fig. S15c and d† exhibit the estimated lattice fringe of 0.198 nm matching with the (104) plane of CoOOH, respectively. The findings further suggest that the CoOOH phase is formed over the catalyst surface after stability testing under the OER conditions.
image file: d3ta04768g-f4.tif
Fig. 4 OER performance: (a) LSV curves, (b) overpotentials at 10 and 50 mA cm−2 and (c) Tafel plots of the Co3O4 NPs/Co–Zn nitride (4) and its counterparts: Co–Zn nitride (3), Co–Zn oxide (2), and Co-Zn-(OH-CO3) (1), and RuO2 commercial catalyst (5); (d) chronoamperometric curve of the Co3O4 NPs/Co–Zn nitride at constant applied potential of 1.56 V. (e) XRD patterns, (f) XPS survey spectra, and (g)–(j) high-resolution XPS spectra of the Zn 2p, Co 2p, N 1s, and O 1s for the post-OER Co3O4 NPs/Co–Zn nitride, and the fresh sample.

XRD and XPS measurements were then performed for the post-OER Co3O4 NPs/Co–Zn nitride to gain insight into its changes after the stability testing. All characteristic peaks of the Co3O4, Co2N, and Zn3N2 phases are observed from the XRD pattern of the post-OER sample, as shown in Fig. 4e. Furthermore, the XRD pattern of the post-OER Co3O4 NPs/Co–Zn nitride does not significantly change in comparison with the fresh sample, reconfirming the high structural stability of the Co3O4 NPs/Co–Zn nitride during long-term operation. The XPS survey spectrum of the post-OER Co3O4 NPs/Co–Zn nitride (Fig. 4f) reveals the typical peaks for Zn 2p, Co 2p, N 1s, and O 1s, which is similar in appearance to the fresh sample. In the Zn 2p XPS spectrum of the post-OER Co3O4 NPs/Co–Zn nitride (Fig. 4g), the Zn 2p3/2 and Zn 2p1/2 typical peaks of the Zn3N2 phase can be observed even after the stability testing, which are unchanged in comparison with that Zn 2p of the fresh sample, confirming the good chemical surface of the Zn3N2. In contrast, in the Co 2p XPS spectrum of the post-OER Co3O4 NPs/Co–Zn nitride (Fig. 4h), the Co 2p3/2 peak belonging to Co2N phase has a slight shift in the direction of lower binding energy than that of the fresh sample, suggesting the partial transformation of Co2+ for Co2N phase into higher oxidation state, such as Co3+ for the cobalt oxide/hydroxide phases.55,56 Indeed, the increase in intensity of Co3+ peak in the Co 2p of the post-OER sample compared to the fresh one, is further supported by the supposition mentioned above. Also, Fig. 4i displays the decrease in intensities of the typical peak of Co–N bond in the N 1s XPS spectrum after the stability testing, while Fig. 4j shows the enhancement of peak intensity belonging to the O ion in the crystal lattice in the O 1s XPS spectrum of the post-OER sample. These observations are additive evidence for the in situ formation of oxidation phases over the Co3O4 NPs/Co–Zn nitride catalyst after long-term water oxidation.

Characterization of phase reconstruction during the HER and OER processes. In situ Raman measurements were conducted at various applied potentials to investigate the dynamic phase reconstruction of Co3O4 NPs/Co–Zn nitride under the HER and OER conditions toward deeply understanding the long-term dynamic structure–performance correlation. The in situ Raman data of the Co3O4 NPs/Co–Zn nitride sample under HER conditions is showed in Fig. 5a. As seen, three types of vibration peaks corresponding to the three compounds are observed from the Raman spectra of the Co3O4 NPs/Co–Zn nitride. The peaks at 265 cm−1 are concerned with the Raman-active photo modes in the Zn3N2 phase,57 while the peaks at 473, 513, and 667 cm−1 are in line with the cobalt nitride phase.58 In addition, the other smaller peaks at 482 and 512 cm−1 can be attributed to the Eg, F2g, and A1g vibration modes of Co3O4, respectively.59 Thus, the in situ Raman results reconfirm the coexistence of Zn3N2, Co2N, and Co3O4 phases in the Co3O4 NPs/Co–Zn nitride sample. Moreover, in situ Raman spectra under the HER conditions show the slight difference in intensity and shape of peaks between applied potential of 0 V and the others. This can be attributed to the formation of gas bubbles, which cover the surface of Co3O4 NPs/Co–Zn nitride sample, during the analysis of in situ Raman at different applied potentials, except for 0 V (Fig. S16†).60,61 More importantly, the Raman spectra shows that the characteristic peaks of Zn3N2, Co2N, and Co3O4 phases are unchanged during catalysis of the HER process at applied potentials from −0.080 to −0.132 V, suggesting that there is no phase reconstruction occurring on the surface of the Co3O4 NPs/Co–Zn nitride heterostructure under the HER circumstance event at high applied voltages; in other words, the in situ Raman analysis corroborates the excellent structural endurability of our proposed materials under HER, which is consistent with the XRD and XPS results of the post-HER Co3O4 NPs/Co–Zn nitride.
image file: d3ta04768g-f5.tif
Fig. 5 In situ Raman spectra of the Co3O4 NPs/Co–Zn nitride under (a) HER conditions, and (b) OER conditions. (c) A schematic of the phase transformation of the Co3O4 NPs/Co–Zn nitride surface during catalysis of the OER process.

The slight change in intensity of the peaks (<600 cm−1) at applied potentials of −0.098 V and −0.115 V is assigned to the difference in density of gas bubbles covering the sample surface at different applied potentials. Regarding the OER conditions (Fig. 5b), all typical vibration peaks of the main phases such as Zn3N2, Co2N, and Co3O4 are detected by in situ Raman analysis of Co3O4 NPs/Co–Zn nitride. Like the HER, the difference in intensity and shape of the detected peaks between applied potential of 0 V and the others is observed from in situ Raman spectra of Co3O4 NPs/Co–Zn nitride under OER conditions. This may be due to the influence of evolved gas bubbles. Interestingly, Fig. 5b also reveals that when the applied voltage exceeds 0 V, an additive Raman peak at 750.9 cm−1, which is ascribed to the CoOOH phase, is detected in addition to the typical vibration peaks of the Zn3N2, Co2N, and Co3O4.62 The vibration peak belonging to the CoOOH phase tend to enhance in terms of its intensity on increasing the applied voltages. At the same time, the intensity of peak belonging to Co2N phase at 667 cm−1 tends to reduce when increasing the applied voltage and this peak almost disappears at applied potential of 0.302 V. This suggest that during OER, the cobalt nitride phase gradually converts to CoOOH, which is a really active site for the OER process. The findings are solid evidence for the surface oxidation of Co3O4 NPs/Co–Zn nitride during the OER catalysis, consistent with the XRD and XPS results of the post-OER Co3O4 NPs/Co–Zn nitride. Therefore, in situ Raman analyses clarify the phase reconstruction of the Co3O4 NPs/Co–Zn nitride during the OER. It can be imagined that the cobalt nitride phase on the surface of the proposed heterostructure is partially oxidized to form oxide/hydroxide phases over the catalyst surface, as illustrated in Fig. 5c. These findings are in line with most research results regarding Co-based electrocatalysts for OER.63 These additive electrocatalytic centers will contribute to the excellent performance of the Co3O4 NPs/Co–Zn nitride for OER process toward further enhanced water-splitting performance, in addition to its twin merits of optimized electronic structure, and unique morphology.

Electrochemical activity for overall water splitting. Given its promising electrocatalytic performance and high stability for both the HER and OER, the Co3O4 NPs/Co–Zn nitride is employed as both cathode and anode in a lab-made electrolyzer device (Fig. 6a), and we performed a series of experimental tests to evaluate its overall water splitting performance. The LSV curves in Fig. 6b show that the electrolyzer using Co3O4 NPs/Co–Zn nitride electrode has exceptional electrocatalytic performance, which is rather analogous to the electrolyzer using the Pt/C and RuO2 commercial electrodes. Our Co3O4 NPs/Co–Zn nitride-based electrolyzer can afford water-splitting current densities of 10 and 50 mA cm−2 at cell voltages of 1.59 and 1.81 V, respectively. Meanwhile, the electrolyzer based on Pt/C and RuO2 can deliver water-splitting current densities of 10 and 50 mA cm−2 at cell voltages of 1.62 and 1.84 V, respectively, which are slightly higher than those of our device (Fig. 6c). The comparison of cell voltage at 10 mA cm−2 between our device and the other devices based on non-noble metal bifunctional electrocatalysts in the literature supports the better water-splitting performance of the Co3O4 NPs/Co–Zn nitride (Fig. 6d), thus demonstrating its better potential for large-scale applications. Moreover, the stability of these electrolyzers was investigated through recording their chronoamperometric curves (Fig. 6e). These show that the Co3O4 NPs/Co–Zn nitride-based electrolyzer is highly stable under overall water splitting conditions; it can run stably for at least 60 h with a current density retention of 88.3%. In contrast, the comparative electrolyzer using Pt/C and RuO2 can operate stably for merely 30 h, then its activity degrades rapidly, resulting in a low current density retention of 72.1% after 60 h operation. The LSV recorded after stability testing of the Co3O4 NPs/Co–Zn nitride-based electrolyzer is quite the same as the initial one, along with a little change in cell voltages after long-term running (Fig. 6f, and S17†). These results further support the good maintenance of the catalytic performance of the Co3O4 NPs/Co–Zn nitride, even after continuously catalyzing both the HER and OER under overall water splitting conditions. Fig. 6g shows that when using the Co3O4 NPs/Co–Zn nitride as both cathode and anode in the electrolyzer, the hydrogen and oxygen produced over its surface have a ratio of approximately 2[thin space (1/6-em)]:[thin space (1/6-em)]1. In addition, the amount of the produced hydrogen and oxygen is rather analogous to the theoretical values, suggesting that the Co3O4 NPs/Co–Zn nitride can afford a very high faradaic efficiency (FE) for the HER and OER. Indeed, the FE values of 98.9 and 99.1% are reached for hydrogen and oxygen production, respectively, emphasizing the high-efficiency electrochemical reactions for HER and OER under the action of the Co3O4 NPs/Co–Zn nitride catalyst. Therefore, small operating cell voltages, great potential for long-term operation without much degradation of electrocatalytic activity, and high FE values for hydrogen and oxygen productions are observed for the electrolyzer based on the Co3O4 NPs/Co–Zn nitride electrode, corroborating the great capability of our proposed material to meet the targeted replacement of noble metal electrocatalysts.
image file: d3ta04768g-f6.tif
Fig. 6 (a) Photograph of the lab-made electrolyzer device using the Co3O4 NPs/Co–Zn nitride as both cathode and anode. (b) LSV curves of electrolyzer based on the Co3O4 NPs/Co–Zn nitride, and commercial Pt/C//RuO2 catalysts. (c) Cell voltages of the electrolyzer based on the Co3O4 NPs/Co–Zn nitride at current densities of 10 and 50 mA cm−2, compared to the electrolyzer based on Pt/C and RuO2. (d) Cell voltages of the electrolyzer based on the Co3O4 NPs/Co–Zn nitride at current density of 10 mA cm−2, compared to the other devices in the literature. (e) Chronoamperometric curve of the electrolyzer based on the Co3O4 NPs/Co–Zn nitride. (f) Cell voltages of the electrolyzer based on the Co3O4 NPs/Co–Zn nitride at current densities of 10 and 50 mA cm−2 after stability testing. (g) The amount of gas theoretically calculated and experimentally measured vs. time for HER and OER of the Co3O4 NPs/Co–Zn nitride electrode. (h) Nyquist plots of the Co3O4 NPs/Co–Zn nitride compared to its counterparts (the inset of (h) is an equivalent circuit model). (i) Current densities as a function of the scan rates of the Co3O4 NPs/Co–Zn nitride, Co–Zn nitride, Co–Zn oxide, and Co-Zn-(OH-CO3) with the corresponding Cdl values, which are equal to the square roots of the corresponding slope values.

Subsequently, to further examine the charge-transfer property of the Co3O4 NPs/Co–Zn nitride under overall water splitting conditions, its EIS plot was recorded, along with the EIS plots of its counterparts for comparison. It can be clearly seen in Fig. 6h that the EIS semicircle diameter of the Co3O4 NPs/Co–Zn nitride is much smaller than those of the Co–Zn nitride, Co–Zn oxide, and Co-Zn-(OH-CO3), suggesting that compared to the others, the Co3O4 NPs/Co–Zn nitride possesses a much smaller charge-transfer resistance (Rct). Through fitting the EIS data by an equivalent circuit model (inset of Fig. 6h), the Rct values are estimated to be 0.32, 0.81, 1.03, and 1.06 Ω for the Co3O4 NPs/Co–Zn nitride, Co–Zn nitride, Co–Zn oxide, and Co-Zn-(OH-CO3), respectively. Thus, the electrochemical reactions under the action of the Co3O4 NPs/Co–Zn nitride will be faster kinetics than those of its counterparts, which can be attributed to the better electron mobility of nitride phase than that of oxide/hydroxide phases, as well as the synergy of the Co3O4 NPs and Co–Zn nitride. In other words, the Co3O4 NPs/Co–Zn nitride has greatly enhanced the charge transferability, and thus shows excellent performance toward overall water splitting.

Thereafter, the electrochemically active surface area (ECSA) was determined by electrochemical double-layer capacitance (Cdl) measurement to assess the number of catalytically active sites on the surface of the developed catalysts. To calculate the Cdl value, CV analysis was performed at various scan rates in the non-faradaic potential region, as seen in Fig. S18.† The result in Fig. 6i displays that the Co3O4 NPs/Co–Zn nitride can give a Cdl value of 46.0 mF cm−2, which is around 1.4, 3.8, and 7.4 times more remarkable than those of the Co–Zn nitride, Co–Zn oxide, and Co-Zn-(OH-CO3), respectively. The ECSA value being in direct proportion to the Cdl value implies that the ECSA of the Co3O4 NPs/Co–Zn nitride is highest among the others. The tendency for decrease in the Cdl values corresponds well to the difference in the electrochemical activity of these materials, highlighting that the higher the ECSA, the more superior the electrochemical activity that can be achieved. These results prove that the greater abundance of catalytic active sites constitutes one of the most important reasons for the higher electrocatalytic activity of the Co3O4 NPs/Co–Zn nitride than those of its comparative samples.

Conclusion

In summary, we successfully developed a novel nanostructured material based on binary metal nitride (Zn3N2–Co2N) integrated with ultrathin Co3O4 nanoparticles. In addition to the advantages of morphological structure, the introduction of the guest Co3O4 endows the Co3O4 NPs/Co–Zn nitride material with a strong charge redistribution at the interfacial region, leading to further improve catalytic activity for both HER and OER toward overall water splitting. Thus, the Co3O4 NPs/Co–Zn nitride shows good catalytic behavior with very low overpotential of 80.5 and 217.7 mV at current density of 10 mA cm−2 for HER and OER, respectively. Remarkably, under overall water splitting conditions, the Co3O4 NPs/Co–Zn nitride electrode requires a small cell voltage of 1.59 and 1.81 V to gain 10 and 50 mA cm−2, which approach that of the commercial Pt/C and RuO2 catalysts, and surpasses those of the other bifunctional water splitting catalysts reported elsewhere. In addition, the Co3O4 NPs/Co–Zn nitride-based electrolyzer demonstrates a superior stability for water splitting, supported by the rather high current retention of 72.1% after long-term electrolysis.

Conflicts of interest

The authors declare no competitive financial and other conflicts of interest.

Acknowledgements

This research was supported by the Basic Science Research Program (2022R1I1A1A01065291) and the Regional Leading Research Center Program (2019R1A5A8080326) through the National Research Foundation funded by the Ministry of Science and ICT of Republic of Korea. And was supported by the Technology Innovation Program (20020872, Development of high crystalline graphitic structure-based high-efficiency and high-performance catalyst for water splitting) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea)

Notes and references

  1. X. Han, X. Wu, Y. Deng, J. Liu, J. Lu, C. Zhong and W. Hu, Adv. Energy Mater., 2018, 8, 1800935 CrossRef .
  2. T. L. L. Doan, D. C. Nguyen, P. M. Bacirhonde, A. S. Yasin, A. I. Rezk, N. Y. Dzade, C. S. Kim and C. H. Park, Energy Environ. Mater., 2023, 6, e12407 CrossRef CAS .
  3. T. L. L. Doan, D. C. Nguyen, S. Prabhakaran, D. H. Kim, D. T. Tran, N. H. Kim and J. H. Lee, Adv. Funct. Mater., 2021, 31, 2100233 CrossRef CAS .
  4. J. Wang, J. Zhang, Y. Hu, H. Jiang and C. Li, Sci. Bull., 2022, 67, 1890–1897 CrossRef CAS PubMed .
  5. D. C. Nguyen, T. L. L. Doan, S. Prabhakaran, D. H. Kim, N. H. Kim and J. H. Lee, Appl. Catal., B, 2022, 313, 121430 CrossRef CAS .
  6. C. Huang, D. Wu, P. Qin, K. Ding, C. Pi, Q. Ruan, H. Song, B. Gao, H. Chen and P. K. Chu, Nano Energy, 2020, 73, 104788 CrossRef CAS .
  7. J. Zhang, Q. Xu, J. Wang, Y. Hu, H. Jiang and C. Li, Sci. China Mater., 2023, 66, 634–640 CrossRef CAS .
  8. D. C. Nguyen, D. T. Tran, T. L. Luyen Doan, N. H. Kim and J. H. Lee, Chem. Mater., 2019, 31, 2892–2904 CrossRef CAS .
  9. X. Zhu, D. C. Nguyen, S. Prabhakaran, D. H. Kim, N. H. Kim and J. H. Lee, Mater. Today Nano, 2023, 21, 100296 CrossRef CAS .
  10. Q. Xu, J. Zhang, H. Zhang, L. Zhang, L. Chen, Y. Hu, H. Jiang and C. Li, Energy Environ. Sci., 2021, 14, 5228–5259 RSC .
  11. J. Zhang, Q. Zhang and X. Feng, Adv. Mater., 2019, 31, 1808167 CrossRef PubMed .
  12. C. Hu, L. Zhang and J. Gong, Energy Environ. Sci., 2019, 12, 2620–2645 RSC .
  13. D. C. Nguyen, T. L. Luyen Doan, S. Prabhakaran, D. T. Tran, D. H. Kim, J. H. Lee and N. H. Kim, Nano Energy, 2021, 82, 105750 CrossRef CAS .
  14. T. L. Luyen Doan, D. T. Tran, D. C. Nguyen, H. Tuan Le, N. H. Kim and J. H. Lee, Appl. Catal., B, 2020, 261, 118268 CrossRef .
  15. H. Over, ACS Catal., 2021, 11, 8848–8871 CrossRef CAS .
  16. D. C. Nguyen, D. T. Tran, T. L. L. Doan, D. H. Kim, N. H. Kim and J. H. Lee, Adv. Energy Mater., 2020, 10, 1903289 CrossRef CAS .
  17. S. Singh, D. C. Nguyen, N. H. Kim and J. H. Lee, Chem. Eng. J., 2022, 442, 136120 CrossRef CAS .
  18. X. Peng, C. Pi, X. Zhang, S. Li, K. Huo and P. K. Chu, Sustainable Energy Fuels, 2019, 3, 366–381 RSC .
  19. J. K. Nørskov, T. Bligaard, A. Logadottir, J. R. Kitchin, J. G. Chen, S. Pandelov and U. Stimming, J. Electrochem. Soc., 2005, 152, J23 CrossRef .
  20. W.-F. Chen, K. Sasaki, C. Ma, A. I. Frenkel, N. Marinkovic, J. T. Muckerman, Y. Zhu and R. R. Adzic, Angew. Chem., Int. Ed. , 2012, 51, 6131–6135 CrossRef CAS PubMed .
  21. H. Guo, A. Wu, Y. Xie, H. Yan, D. Wang, L. Wang and C. Tian, J. Mater. Chem. A, 2021, 9, 8620–8629 RSC .
  22. C. Bai, S. Wei, D. Deng, X. Lin, M. Zheng and Q. Dong, J. Mater. Chem. A, 2017, 5, 9533–9536 RSC .
  23. W. Xia, R. Zou, L. An, D. Xia and S. Guo, Energy Environ. Sci., 2015, 8, 568–576 RSC .
  24. J. Liu, J. Wang, B. Zhang, Y. Ruan, H. Wan, X. Ji, K. Xu, D. Zha, L. Miao and J. Jiang, J. Mater. Chem. A, 2018, 6, 2067–2072 RSC .
  25. Q. Zhou, T.-T. Li, J. Qian, Y. Hu, F. Guo and Y.-Q. Zheng, J. Mater. Chem. A, 2018, 6, 14431–14439 RSC .
  26. Z. Liu, H. Tan, D. Liu, X. Liu, J. Xin, J. Xie, M. Zhao, L. Song, L. Dai and H. Liu, Adv. Sci., 2019, 6, 1801829 CrossRef PubMed .
  27. X. Wu, J. Li, Y. Li and Z. Wen, Chem. Eng. J., 2021, 409, 128161 CrossRef CAS .
  28. L. Tian, Z. Li, X. Xu and C. Zhang, J. Mater. Chem. A, 2021, 9, 13459–13470 RSC .
  29. L. Yang, R. Liu and L. Jiao, Adv. Funct. Mater., 2020, 30, 1909618 CrossRef CAS .
  30. Y. Tan, H. Wang, P. Liu, Y. Shen, C. Cheng, A. Hirata, T. Fujita, Z. Tang and M. Chen, Energy Environ. Sci., 2016, 9, 2257–2261 RSC .
  31. J. Wang, G. Lv and C. Wang, Appl. Surf. Sci., 2021, 570, 151182 CrossRef CAS .
  32. Y. Sun, Y. Zhou, Y. Zhu, Y. Shen and A. Xie, ACS Sustain. Chem. Eng., 2019, 7, 9153–9163 CrossRef CAS .
  33. J. Liu, C. Wang, H. Sun, H. Wang, F. Rong, L. He, Y. Lou, S. Zhang, Z. Zhang and M. Du, Appl. Catal., B, 2020, 279, 119407 CrossRef CAS .
  34. M. Fang, G. Dong, R. Wei and J. C. Ho, Adv. Energy Mater., 2017, 7, 1700559 CrossRef .
  35. L. B. Staal, S. S. Charan Pushparaj, C. Forano, V. Prevot, D. B. Ravnsbæk, M. Bjerring and U. G. Nielsen, J. Mater. Chem. A, 2017, 5, 21795–21806 RSC .
  36. Y. Zhang, B. Cui, Z. Qin, H. Lin and J. Li, Nanoscale, 2013, 5, 6826 RSC .
  37. W. S. Khan, C. Cao, Z. Ali, F. K. Butt, N. Ahmad Niaz, A. Baig, R. ud Din, M. H. Farooq, F. Wang and Q. ul Ain, Mater. Lett., 2011, 65, 2127–2129 CrossRef CAS .
  38. F. Jiao, J. Wang, Y. Lin, J. Li, X. Jing and Y. Gong, Appl. Surf. Sci., 2021, 553, 149440 CrossRef CAS .
  39. S. Podila, H. Driss, S. F. Zaman, A. M. Ali, A. A. Al-Zahrani, M. A. Daous and L. A. Petrov, Int. J. Hydrogen Energy, 2020, 45, 873–890 CrossRef CAS .
  40. R. Adalati, A. Kumar, M. Sharma, P. Tiwari and R. Chandra, Appl. Phys. Lett., 2020, 117, 123904 CrossRef CAS .
  41. M. Moloudi, A. Noori, M. S. Rahmanifar, Y. Shabangoli, M. F. El-Kady, N. B. Mohamed, R. B. Kaner and M. F. Mousavi, Adv. Energy Mater., 2023, 13, 2203002 CrossRef CAS .
  42. J. Jiang, P. Yan, Y. Zhou, Z. Cheng, X. Cui, Y. Ge and Q. Xu, Adv. Energy Mater., 2020, 10, 2002214 CrossRef CAS .
  43. Y. Xi, B. Dong, Y. Dong, N. Mao, L. Ding, L. Shi, R. Gao, W. Liu, G. Su and L. Cao, Chem. Mater., 2016, 28, 1355–1362 CrossRef CAS .
  44. Y. Wen, J. Qi, P. Wei, X. Kang and X. Li, J. Mater. Chem. A, 2021, 9, 10260–10269 RSC .
  45. K. R. Yoon, K. Shin, J. Park, S.-H. Cho, C. Kim, J.-W. Jung, J. Y. Cheong, H. R. Byon, H. M. Lee and I.-D. Kim, ACS Nano, 2018, 12, 128–139 CrossRef CAS PubMed .
  46. C. T. Cao, S.-W. Kim, H. J. Kim, R. Purbia, S. H. Kim, D. Kim, K. J. Choi, H. Park and J. M. Baik, Nano Energy, 2022, 96, 107117 CrossRef CAS .
  47. B. Zhang, H. Luo, B. Ai, Q. Gou, J. Deng, J. Wang, Y. Zheng, J. Xiao and M. Li, Small, 2023, 19, 2205431 CrossRef CAS PubMed .
  48. Y. Gao, D. Zheng, Q. Li, W. Xiao, T. Ma, Y. Fu, Z. Wu and L. Wang, Adv. Funct. Mater., 2022, 32, 2203206 CrossRef CAS .
  49. T. L. L. Doan, D. T. Tran, D. C. Nguyen, D. H. Kim, N. H. Kim and J. H. Lee, Adv. Funct. Mater., 2021, 31, 2007822 CrossRef CAS .
  50. J. Yu and J. Kim, Chemosphere, 2022, 287, 132168 CrossRef CAS PubMed .
  51. Z. Xu, Y. Jiang, J.-L. Chen and R. Y.-Y. Lin, ACS Appl. Mater. Interfaces, 2023, 15, 16702–16713 CrossRef CAS PubMed .
  52. P. P. Dhakal, U. N. Pan, D. R. Paudel, M. R. Kandel, N. H. Kim and J. H. Lee, Mater. Today Nano, 2022, 20, 100272 CrossRef CAS .
  53. W.-H. Huang, X.-M. Li, X.-F. Yang, H.-B. Zhang, F. Wang and J. Zhang, Chem. Commun., 2021, 57, 4847–4850 RSC .
  54. Y. Wu, G.-D. Li, Y. Liu, L. Yang, X. Lian, T. Asefa and X. Zou, Adv. Funct. Mater., 2016, 26, 4839–4847 CrossRef CAS .
  55. D. C. Nguyen, T. L. L. Doan, S. Prabhakaran, D. H. Kim, N. H. Kim and J. H. Lee, Nano Energy, 2021, 89, 106420 CrossRef CAS .
  56. B. K. Kang, S. Y. Im, J. Lee, S. H. Kwag, S. Bin Kwon, S. Tiruneh, M.-J. Kim, J. H. Kim, W. S. Yang, B. Lim and D. H. Yoon, Nano Res., 2019, 12, 1605–1611 CrossRef CAS .
  57. A. J. Addie, M. A. Mohammed and R. A. Ismail, Mater. Sci. Semicond. Process., 2022, 145, 106664 CrossRef CAS .
  58. T. Varga, G. Ballai, L. Vásárhelyi, H. Haspel, Á. Kukovecz and Z. Kónya, Appl. Catal., B, 2018, 237, 826–834 CrossRef CAS .
  59. C. Liu, G. Bai, X. Tong, Y. Wang, B. Lv, N. Yang and X.-Y. Guo, Electrochem. Commun., 2019, 98, 87–91 CrossRef CAS .
  60. M. Chen, D. Liu, L. Qiao, P. Zhou, J. Feng, K. W. Ng, Q. Liu, S. Wang and H. Pan, Chem. Eng. J., 2023, 461, 141939 CrossRef CAS .
  61. Y.-H. Wang, S. Zheng, W.-M. Yang, R.-Y. Zhou, Q.-F. He, P. Radjenovic, J.-C. Dong, S. Li, J. Zheng, Z.-L. Yang, G. Attard, F. Pan, Z.-Q. Tian and J.-F. Li, Nature, 2021, 600, 81–85 CrossRef CAS PubMed .
  62. T. Begildayeva, J. Theerthagiri, S. J. Lee, Y. Yu and M. Y. Choi, Small, 2022, 18, 2204309 CrossRef CAS PubMed .
  63. J. Zhang, W. He, H. B. Aiyappa, T. Quast, S. Dieckhöfer, D. Öhl, J. R. C. Junqueira, Y. Chen, J. Masa and W. Schuhmann, Adv. Mater. Interfaces, 2021, 8, 2100041 CrossRef CAS .

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ta04768g
This journal is © The Royal Society of Chemistry 2023
Click here to see how this site uses Cookies. View our privacy policy here.