Hollow Nanocages of NixCo1−xSe for Efficient Zinc–Air Batteries and Overall Water Splitting
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A facile strategy for fabricating NixCo1−xSe hollow nanocages was developed, and the formation mechanism was well explained.
Ni0.2Co0.8Se outperformed a Pt/C + RuO2 catalyst in rechargeable and all-solid-state Zn–air battery tests, as well as in overall water splitting.
The hydrogen adsorption onto NixCo1−xSe was simulated, and Gibbs free energies were calculated.
KeywordsNixCo1−xSe hollow nanocages Oxygen evolution reaction Hydrogen evolution reaction Rechargeable/all-solid-state zinc–air battery Overall water splitting
The rapid depletion and heavy reliance on fuel cells and associated global environmental concerns have motivated extensive research on the development of eco-friendly and sustainable energy technologies in the past decade. Among these technologies, Zn–air batteries (ZABs) and water-splitting devices have become viable eco-friendly energy technologies owing to recent advances in the preparation of highly active electrocatalysts [1, 2, 3]. The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are the key reversible reactions occurring at the cathode of ZABs and largely determine the energy-conversion efficiency of ZABs . The OER and the hydrogen evolution reaction (HER) are the two electrochemical reactions for catalyzing overall water splitting [5, 6, 7]. Pt-based materials have been widely considered as state-of-the-art electrocatalysts for the ORR and HER [8, 9, 10, 11, 12], while RuO2 and IrO2 are the standard high-efficiency OER catalysts [13, 14]. However, the large-scale commercial implementation of both Pt-based and Ru-/Ir-based materials has been significantly hampered by their scarcity, high cost, and poor long-term durability. Therefore, it is imperative to develop Earth-abundant, cost-effective, high-efficiency, and robust electrocatalysts [12, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24].
Among the various alternative materials, transition metal chalcogenides have been attracting increasing research attention, mainly owing to their high availability, low cost, and eco-friendliness [25, 26, 27, 28, 29, 30, 31, 32]. In particular, because of the high conductivity of metallic Se compared with O and S, transition metal selenides (MSe, M = transition metal) have superior electrocatalytic performance to transition metal oxides and sulfides [30, 31]. Therefore, transition metal selenides have received tremendous research attentions from the electrocatalytic community. For instance, Zheng et al. developed a novel hot-injection process to precisely control the phase and composition of a series of NixSe nanocrystals and discovered that Ni0.5Se nanoparticles exhibited superior OER activity comparable to that of RuO2, that Ni0.75Se nanoparticles exhibited the best performance for the HER and ORR, and that both could be engineered for efficient rechargeable ZABs and water splitting . Cao et al. demonstrated a facile strategy for in situ coupling of ultrafine Co0.85Se nanocrystals with N-doped C, and the as-prepared Co0.85Se@NC was employed as a trifunctional catalyst for the HER, ORR, and OER, exhibiting great potential for ZABs and water splitting . Rather than using only one transition metal, recent studies showed that superior electrocatalytic performance for ZABs and water splitting could be achieved by employing mixed transition metal selenides. For example, Xu et al.  prepared a Ni–Fe diselenide (NixFe1−xSe2) and used it as a templating precursor to form ultrathin nanosheets of the corresponding oxide, which exhibited a very low overpotential of only 195 mV in an alkaline solution at 10 mA cm−2 for the OER. Recently, Lv et al.  have designed Ni–Fe selenide (NiFeSe2) hollow nanoparticles, hollow nanochains , and Co–Fe selenide (CoFeSe2) nanosheets  for the OER, and the Co0.4Fe0.6Se nanosheets not only exhibited superior OER performance with a low overpotential of 217 mV at 10 mA cm−2 and a small Tafel slope of 41 mV dec−1 but also had a ultrahigh durability. There have been several reports of NiCoSe2-based materials for electrochemical energy storage and conversion. Yuan et al. reported monodisperse metallic NiCoSe2 hollow sub-microspheres for electrochemical supercapacitors . NiCoSe2−x/N-doped C mushroom-like core/shell nanorods on N-doped C fiber were prepared by Li et al.  for overall water splitting, and a low cell voltage of 1.53 V to obtain a current density of 10 mA cm−2 was observed. Recently, Chen and Tan have directly grew ultrathin ternary selenide (CoNiSe2) nanorods on Ni foam, which delivered a current density of 100 mA cm−2 with an overpotential as low as 307 and 170 mV for the OER and HER, respectively, and eventually reduced the cell voltage in the full water-splitting reaction to 1.591 V to obtain a current density of 10 mA cm−2 . Chen and Wang groups prepared a three-dimensional Ni–Co selenide (NiCoSe2) nanonetwork for the OER, and the overpotential at 10 mA cm−2 was 274 mV, exhibiting room for improvement .
Despite the progress regarding NiCoSe2, direct preparation of NiCoSe2 with precise manipulation of the morphology for ZABs and overall water splitting remains largely unexplored. Moreover, the stoichiometric ratio of Ni to Co has not been optimized for enhancing the synergistic catalytic effects. In light of the significant effects of the morphology, crystal structure, and stoichiometry on the electrocatalytic performance, a systematic investigation of NiCoSe2 with a well-defined surface structure and an optimized Ni/Co stoichiometry for establishing the structure–function relationship of NiCoSe2 materials is of great importance. This was the primary goal of the present study.
In this study, we employed a facile strategy to prepare a series of NixCo1−xSe samples with hollow cages and investigated them as trifunctional electrocatalysts for the OER, ORR, and HER. A novel process with Cu2O cubes as the starting material was developed to fabricate the NixCo1-xSe nanocages, and a reasonable formation mechanism was proposed. In electrochemical tests, Ni0.2Co0.8Se exhibited higher OER and HER activity than the other samples in the NixCo1−xSe series. To investigate the applications of the Ni0.2Co0.8Se sample, it was used as an air–cathode of a self-assembled rechargeable ZAB and an all-solid-state ZAB and employed as a catalyst for overall water splitting in an alkaline solution.
2 Experimental Section
Copper (II) chloride dihydrate (CuCl2·2H2O, 99%), sodium hydroxide (NaOH, ≥ 96.0%), L-ascorbic acid (AA, ≥ 99.7%), nickel (II) chloride hexahydrate (NiCl2·6H2O, ≥ 98.0%), cobalt (II) chloride hexahydrate (CoCl2·6H2O, ≥ 99.0%), polyvinylpyrrolidone (PVP, K30, 99%), anhydrous sodium thiosulfate (Na2S2O3, 99%), sodium selenite (Na2SeO3, ≥ 99.7%), absolute ethanol (≥ 99.7%), and ethylene glycol (EG, 99.0%) were used. Water was obtained from a Barnstead Nanopure water system (resistivity: 18.3 MΩ cm). All the chemicals were used as received, without further purification.
2.2 Synthesis of Cu2O Cubes
Cu2O cubes were synthesized by following a previously reported procedure . Typically, 341 mg of CuCl2·2H2O was first dissolved in 200 mL of Nanopure water. Then, the solution was heated to 55 °C and stirred for 30 min. Subsequently, 20 mL of a 2 M NaOH solution was slowly added to the aforementioned solution, forming a brown suspension. After 10 min of stirring, 20 mL of 0.6 M AA was added dropwise to the solution. The solution gradually changed from dark red to brick red, and the mixture was aged for 3 h. The formed precipitates were collected via suction filtration, washed with copious distilled water and ethanol 3–5 times, and eventually dried in vacuum at 35 °C overnight.
2.3 Synthesis of Ni0.2Co0.8(OH)2 Nanocages
In a typical procedure, 100 mg of cuprous oxide was dissolved into a mixed solvent of absolute ethanol and Nanopure water (100 mL, volume ratio = 1:1) with 30 min of ultrasonic treatment. Then, 34 mg of NiCl2·6H2O and CoCl2·6H2O (molar ratio of 2:8) was added to the solution, with stirring. Subsequently, 3.33 g of PVP was dispersed in the resulting suspension under another 30 min of ultrasonic treatment. Then, 40 mL of 1 M Na2S2O3 was slowly added to the mixture. Upon the addition of an excessive amount of sodium thiosulfate solution, the mixture changed from orange–red to transparent green, indicating that cuprous oxide was converted into Ni0.2Co0.8(OH)2. The reaction was conducted for 10 min to ensure that it was complete. The product was then collected via centrifugation, washed with copious Nanopure water and ethanol 3–5 times, and eventually dried in a vacuum at 35 °C overnight. For the synthesis of Ni0.5Co0.5(OH)2, Ni0.8Co0.2(OH)2, Ni(OH)2, and Co(OH)2, the same procedure was adopted, but the molar ratio of Ni to Co was changed to 5:5, 8:2, 1:0, and 0:1, respectively.
2.4 Synthesis of Ni0.2Co0.8Se Nanocages
In a typical procedure, 37 mg of Na2SeO3 was dissolved in a mixed solvent of Nanopure water and EG (10.0 mL, volume ratio = 1:1). Then, 10 mg of Ni0.2Co0.8(OH)2 was added to the solution, with 30 min of ultrasonication to ensure uniform dispersion. Subsequently, the mixture was transferred into an autoclave and kept at 200 °C for 6 h. Finally, after cooling to room temperature, the product was collected via centrifugation.
The morphologies and surface structures of the samples were observed via field emission scanning electron microscopy (SEM, Hitachi S-4800) and high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F30). X-ray diffraction (XRD) patterns in the Bragg’s angle (2θ) range of 10°–90° were recorded using a Bruker D8 diffractometer with Cu Kα radiation (λ = 0.1541 nm). X-ray photoelectron spectroscopy (XPS) was conducted using an ESCALAB 250 photoelectron spectrometer (Thermo Fisher Scientific, USA).
Electrochemical measurements were taken using a CHI 750E electrochemical workstation (CHI Instruments Inc.) in a 1 M KOH aqueous solution at ambient temperature. A three-electrode system was utilized in both HER and OER tests. Here, Ag/AgCl was used as the reference electrode [44, 45, 46], and C rod and C cloth electrodes were employed as the counter electrode and the working electrode, respectively. The catalyst ink was prepared as follows. Firstly, 10 mg of the catalyst was ultrasonically dispersed in 1000 μL of absolute ethanol, followed by the sequential addition of 900 μL of Nanopure water and 100 μL of Nafion (5%, Sigma-Aldrich), yielding a uniform suspension. Then, 20 μL of the suspension was cast dropwise onto a single-sided C cloth (1.5 × 0.5 cm2, load area of 0.5 cm2), followed by drying at room temperature. The catalyst loading was calculated as ~ 200 μg cm−2. The solution was saturated with N2 or O2 at least 30 min before each measurement. For the HER, the cyclic voltammetry (CV) test potential range was − 0.077 to 0.623 V (vs. reversible hydrogen electrode (RHE)), and the scan rate was 100 mV s−1. In addition, linear sweep voltammetry (LSV) was conducted in a potential range of − 0.477 to 0.323 V (vs. RHE), at a scan rate of 10 mV s−1. OER measurements were taken in the same manner as the HER measurements. LSV was performed in a N2-saturated 1 M KOH solution within the potential range of + 1.023 to + 2.023 V (vs. RHE), at a scan rate of 10 mV s−1. A relatively simple two-electrode system was used in the water-splitting test, where the same catalyst was loaded on two clip electrodes: an anode and a cathode. The operation method was similar to that for the OER test, but the LSV test voltage was 1.0–2.0 V. We recorded the chronoamperometric responses in a 1 M KOH solution for 40,000 s and performed an accelerated durability test (ADT), where the catalyst was cycled 1000 times in the potential range of + 1.023 to + 1.423 V (vs. RHE) for the OER and from − 0.077 to 0.623 V for the HER. The scan rates for the HER and OER were 100 and 50 mV s−1, respectively. Details regarding the calculation of the electrochemically active surface area (EASA) are presented in Supplementary Material.
2.7 Measurements of Liquid ZAB and All-Solid-State ZAB
3 Results and Discussion
3.1 Preparation of NixCo1−xSe Nanocages, Formation Mechanism, and Electron Microscopy
A possible mechanism for the formation of hollow NixCo1−xSe nanocages is described as follows. According to the Pearson’s hard and soft acid–base principle, cuprous oxide can react with sodium thiosulfate to form a soluble complex, accompanied by the release of hydroxide ions (Eq. 3). In the presence of hydroxide ions, upon the introduction of Ni and Co ions, the immediately formed NiCo hydroxide precipitates can aggregate in situ, leading to the formation of NixCo1−x(OH)2 nanocages (Eq. 4). The selenization proceeds via an anion exchange mechanism. Under a high temperature and high pressure, EG can react with SeO32−, generating elemental Se, hydroxide ions, and oxalic acid (Eq. 5). In the presence of hydroxide ions, the elemental Se can undergo the disproportionation reaction, forming SeO32− and Se2− (Eq. 6). Finally, the generated Se2− and the OH− ions complete the anion exchange reaction, forming the NixCo1−xSe nanocages (Eq. 7).
3.2 XRD and XPS Analyses
Subsequently, the chemical states of the composites were investigated via XPS, and the spectra are shown in Fig. 3b–d. In Fig. 3b, the two striking peaks with binding energies of 873.1 and 855.2 eV and the two satellite peaks can be assigned to the Ni 2p1/2 and Ni 2p3/2 electrons, respectively, strongly indicating that elemental Ni existed as Ni(II) . In Fig. 3c, the Co 2p1/2 and Co 2p3/2 signals (797.4 and 781.3 eV) and two satellite peaks are characteristics of Co(II) . The high-resolution Se 3d spectra can be deconvoluted into two peaks at 55.6 and 54.8 eV, which correspond well to the Se 3d3/2 and Se 3d5/2 electrons, respectively. Interestingly, the peak at 59.3 eV indicates the formation of SeOx, which was probably due to the surface oxidation of selenide .
The high-resolution XPS spectra of the Ni 2p, Co 2p, and Se 3d electrons of Ni0.5Co0.5Se, Ni0.8Co0.2Se, NiSe, and CoSe are shown in Fig. S5. In the Ni 2p spectrum (S5c1) and the NiSe and Co 2p (S5d1) spectrum for CoSe, the binding energies correspond to the Ni(II) and Co(II) species. For Ni0.5Co0.5Se, there are two distinctive peaks at 855.6 and 873.4 eV in the Ni 2p spectrum (S5a1), and both binding energy values correspond to the chemical valences exhibited by the Ni element in Ni0.2Co0.8Se, suggesting the presence of Ni(II). In the Co 2p spectrum (S5a2), the two sharp peaks (Co 2p3/2 and Co 2p1/2) at 781.2 and 797.9 eV are attributed to the Co(II) species. For Ni0.8Co0.2Se, the core-level Ni 2p spectrum (S5b1) exhibits two peaks at 855.4 and 873.3 eV, which are indexed to the Ni 2p3/2 and Ni 2p1/2 electrons, respectively, and there are two corresponding shakeup satellite peaks at 861.6 and 880.1 eV. The Co 2p spectrum (S5b2) exhibits a similar feature to the Co 2p spectrum for CoSe, implying that the Co exists as Co(II). Furthermore, the core-level Se 3d spectra (S5a3, b3, c2, and d2) of Ni0.5Co0.5Se, Ni0.8Co0.2Se, NiSe, and CoSe all exhibit two distinct characteristic peaks around 55.1 and 59.3 eV. The peak at 55.1 eV can be fitted into two sub-peaks representing the Se 3d5/2 and Se 3d3/2 electrons from the Se element. The other peak at 59.3 eV is probably due to the oxidation of surface Se and the formed Se–O bonds .
3.3 OER Performance
OER and HER activity for the NixCo1−xSe series, including the results of OER and HER tests in 1 M KOH, as well as the electrochemical properties
OER (1 M KOH)
HER (1 M KOH)
@ 10 mA cm−2 (mV)
@10 mA cm−2 (mV)
3.4 HER Performance
3.5 EASA Analysis and ORR Performance
The OER and HER performance of the Ni0.2Co0.8Se sample is comparable, if not superior, to that of most recently reported transition metal selenide-based materials, and the comparison results are presented in Table S1. For instance, in 1 M KOH for the OER, to obtain a current density of 10 mA cm−2, the required overpotential of Ni0.2Co0.8Se was 280 mV, which is lower than those for CoSe2/Mn3O4 (450 mV) , NixSe (330 mV) , Co0.85Se (320 mV) , and NiSe2/Ti (295 mV)  and comparable to those for Co(S0.22Se0.78)2 (283 mV) , Ni0.75Fe0.25Se2 (272 mV) , and NiSe/NF (270 mV) . In the HER test, at 10 mA cm−2, the overpotential was 73 mV for Ni0.2Co0.8Se, which is significantly lower than those for NixSe (233 mV) , Co0.85Se (230 mV) , Co(S0.22Se0.78)2 (175 mV) , and NiSe/NF (96 mV)  and comparable to that for NiSe2/Ti (70 mV)  under the same conditions. These comparison results indicate that Ni0.2Co0.8Se is a superior multifunctional catalyst for the OER and HER.
To determine the reason for the difference in electrocatalytic activity among the samples, EASA measurements were taken . The EASA values were estimated according to the electrochemical double-layer capacitance (CDL) of the catalyst, and the CDL was measured via cyclic voltammograms (Fig. S9) within a potential range where no apparent Faradaic process occurred. The detailed calculations are presented in Supplementary Material, and the calculation results are presented in Table 1. The EASA values well explain the trend of the electrocatalytic performance, as they are in good accordance with the OER activity order of the series (Ni0.2Co0.8Se > CoSe > Ni0.8Co0.2Se > Ni0.5Co0.5Se > NiSe). The EASA of Ni0.2Co0.8Se was the largest among the samples and was approximately 10 times larger than those of Ni0.8Co0.2Se and Ni0.5Co0.5Se.
In addition, the ORR performance of the NixCo1−xSe series was evaluated in an alkaline solution. Figure S10 shows the LSV polarization curves obtained with a rotation rate of 1600 rpm in 0.1 M KOH. The ORR activity matched the trend of the EASA values. As expected, the Ni0.2Co0.8Se sample had the best activity in the series. Its onset potential was 0.87 V, and its diffusion-limiting current density was 4.45 mA cm−2. Although its half-wave potential (0.769 V) was inferior to that of the commercial Pt/C (0.86 V), its limiting current density was higher than that of Pt/C (4.32 mA cm−2). A large anode peak appeared at approximately 1.0 V, which was mainly due to the oxidation of the metal ions. A corresponding reduction peak appeared at approximately 0.7 V, which is ascribed to the reduction of Ni3+ (Co3+) to Ni2+ (Co2+) . Additionally, there was a broad peak caused by the oxidation or reduction of surface Se, which was observed in previous studies [57, 58].
3.6 ZAB Performance
The performance of Ni0.2Co0.8Se in both the liquid ZAB and the all-solid-state ZAB was superior to that of the recently reported Co-based nanostructures. The comparison results are presented in Table S2. In the liquid ZAB test under the same conditions, the open-circuit potential for Ni0.2Co0.8Se was higher than those for Co-NDC  and NGM-Co , and the power density was higher than those for CoN4/NG , NGM-Co , and Co-NDC . In the all-solid-state ZAB test, the open-circuit potential for Ni0.2Co0.8Se was higher than those for NC-Co/CoNx , Co-NDC , and Co3O4/N-rGO ; the round-trip efficiency was higher than that for CoN4/NG ; and the power density was higher than those for CoN4/NG , NGM-Co , and Co3O4/N-rGO . The outstanding performance of Ni0.2Co0.8Se in the ZAB test is largely attributed to its excellent electrocatalytic performance, as discussed previously.
3.7 Overall Water-Splitting Test
We demonstrated the facile fabrication of a series of NixCo1−xSe samples with well-defined cages and investigated their catalytic performance for OER, HER, and ORR electrocatalysis. Among the NixCo1−xSe compounds, Ni0.2Co0.8Se exhibited the best performance, as indicated by the lowest overpotential of 280 and 73 mV to obtain a current density of 10 mA cm−2 for the OER and HER, respectively. Moreover, Ni0.2Co0.8Se was engineered as an air–cathode of both a rechargeable ZAB and an all-solid-state ZAB and employed as a catalyst for overall water splitting. It endowed both ZAB devices with outstanding performance, including a long cycling lifetime, high round-trip efficiency, and high power density, and achieved total water splitting with excellent efficiency at a low cell voltage. The study paves a pathway for preparing transition metal selenides with a well-defined morphology and optimized stoichiometric ratio as promising catalysts for renewable energy technologies, such as rechargeable and all-solid-state metal–air batteries and water-splitting devices.
Z. Tang thanks the Guangzhou Science and Technology Plan Projects (No. 201804010323), the Guangdong Natural Science Funds for Distinguished Young Scholars (No. 2015A030306006), the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2014ZT05N200), and the Fundamental Research Funds for the Central Universities (SCUT Grant No. 2018ZD022). Y. Tian is grateful for the Project for Natural Science Foundation of Guangdong Province (No. 2018A030313178), and W. Gao thanks the funding support from the Natural Science Foundation of Guangdong Province (No. 2015A030310176).
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