3D Porous Amorphous γ-CrOOH on Ni Foam as Bifunctional

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

3D Porous Amorphous γ‑CrOOH on Ni Foam as Bifunctional Electrocatalyst for Overall Water Splitting Zemin Sun,†,§ Mengwei Yuan,†,§ Han Yang,† Liu Lin,† Heyun Jiang,‡ Shengsong Ge,‡ Huifeng Li,† Genban Sun,*,† Shulan Ma,† and Xiaojing Yang† †

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China ‡ College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China Inorg. Chem. Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 03/07/19. For personal use only.

S Supporting Information *

ABSTRACT: The development of novel and highly efficient bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is an ongoing challenge. The Cr3+ cation has a special electronic configuration (t32ge0g), which facilitates charge transfer and electron capture. However, Cr-based materials applied on water-splitting electrocatalysis is still a research void up to now. Herein, a novel amorphous γ-CrOOH was developed as a bifunctional electrocatalyst toward overall water splitting for the first time. It shows extraordinary HER activity with an ultralow overpotential of only 149 mV at 50 mA cm−2. Meantime, there is a small overpotential of 334 mV at 50 mA cm−2 for the OER. Importantly, the bifunctional electrocatalyst for overall water-splitting electrocatalysis can work with a cell voltage of merely 1.56 V at 10 mA cm−2. Amorphous γ-CrOOH has effectively enhanced the intrinsic electrochemical activity via density functional theoretical calculations. Therefore, this work not only provides a new method for preparation of amorphous γ-CrOOH but also expands the types of catalysts for water splitting.

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been tremendous study for HER performance,22,23 but Cr has rarely been explored and they could also have the possibility to catalyze the water splitting. Just recently, it has been reported that the addition of Cr3+ can increase the OER performance of Ni-based LDH and Co-based LDH catalysts.24,25 However, the pure Cr-based catalysts for water splitting are still a research blank. Encouraged by these findings, the novel amorphous γCrOOH has been prepared via a facile hydrothermal method, and their activity about OER and HER was further measured for the first time. As a result, amorphous γ-CrOOH exhibits highly efficient catalytic activity for both OER and HER in alkaline solution. The overpotential of OER is as low as around 334 mV at 50 mA cm−2, and the HER current density at 50 mA cm−2 was ultralow to 149 mV. Furthermore, utilizing γCrOOH as a bifunctional electrocatalyt, it shows a low cell voltage of 1.56 V at 10 mA cm−2. Therefore, this work develops new types of γ-CrOOH as bifunctional catalysts, and provides new insight into the role of Cr-based catalysts for promoting the electrochemical activities.

INTRODUCTION One of the ways to keep the balance of development and environmental protection is to find an efficient, friendly, and sustainable energy. Hydrogen energy, as green fuel, is one of the most attractive measures to reduce greenhouse gas emission.1−3 The sustainable production of H2 via water electrolysis would be one of the main ways for hydrogen production.4 During the water electrolysis, there are two halfcell reactions: one is the process of HER at the cathode (2H+(aq) + 2e−→ H2(g)); another is OER at the anode (2H2O(l) → O2(g) + 4H+(aq) + 4e−).5,6 Whether HER or OER process, both required effective and robust catalysts to overcome the large overpotential and realize an ultralong stability. Nowadays, Pt-based noble metal catalysts are still the best catalysts for HER;7,8 meantime, noble metal oxides such as RuO2 and IrO2 are the state-of-the-art catalysts for OER.9−12 However, the noble metal catalysts suffer from scarcity and high cost which limited their widespread applications. Besides, the single noble metal cannot meet the requirement for both HER and OER catalysis. Much effort has been devoted toward non-noble metals bifunctional catalysts. Nowadays, lots of works have been carried out in high gear, and most of the works are based on Fe, Co, Ni, Mn, Mo, and W compound materials for water splitting.13−21 Chrome, molybdenum, and tungsten all belong to the Group VI B. The Mo- and W-based compounds have © XXXX American Chemical Society

Received: January 13, 2019

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DOI: 10.1021/acs.inorgchem.9b00112 Inorg. Chem. XXXX, XXX, XXX−XXX

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reported literature to further verify the existence of γCrOOH.30 The bands at 1150 and 1050 cm−1 in the red line corresponded to typical frequencies of γ-CrOOH.13 And the bands at 630 and 540 cm−1 in the black line were the typical absorption of the Cr-O band in Figure 1d.31 In order to avoid the introduction of adhesives and better enhance the connection between current collector and catalyst, we make the γ-CrOOH in situ grow on the Ni foam (NF) via hydrothermal reaction. The XRD profile of γ-CrOOH/NF shows no obvious crystalline diffraction peaks on Figure S2 except the diffraction peak of metal Ni from the substrate (Figure S3). After annealing, typical Cr2O3 diffraction peaks were obtained in Figure S2. Besides, the XPS for γ-CrOOH/ NF was also measured in Figure S4. The Cr3+ of γ-CrOOH/ NF showed the same with γ-CrOOH, which further proved the catalysts were successfully grown on the Ni foam. The morphology of the γ-CrOOH samples was characterized by SEM. The γ-CrOOH nanoparticles are shown in Figure 2a; the nanoparticles are ginger-shaped. And from the

RESULTS AND DISCUSSION The amorphous γ-CrOOH was synthesized through hydrothermal reaction with ammonium dichromate and hydrazine hydrate at 120 °C. The obtained emerald-green γ-CrOOH powder is shown as Figure S1a. And the emerald-green powder γ-CrOOH was calcined at high temperature, and then we get the blackish-green Cr2O3 powder as shown in Figure S1b. In order to further confirm the structure and composition of asprepared samples, a series of characterizations have been performed, which are shown in Figure 1. The X-ray

Figure 1. XPS of Cr 2p (a) and O 1s (b) for Cr2O3 and γ-CrOOH. XRD (c) and IR spectra (d) for Cr2O3 and γ-CrOOH.

photoelectron spectroscopy (XPS) was measured to study the surface chemical states. As for Cr 2p shown in Figure 1a, the maximum of the Cr 2p3/2 peak for as-prepared Cr2O3 at 576.7 eV is typical for Cr3+ in Cr2O3, which was also consistent with the reported literature for Cr2O3.26 Compared with asprepared Cr2O3, the maximum for the as-prepared γ-CrOOH of Cr 2p3/2 shifts to a high value at 577.4 eV, which was in accordance with the reported Cr3+ in CrOOH.27 Besides, the peaks of O 1s are illustrated in Figure 1b, according to the peak at 531.3 eV corresponds to OH− in oxyhydroxide and the third peak at 532.2 eV corresponds to O2− in water,28 much different from Cr2O3, in which only one peak in 530.2 eV corresponds to O2− in Cr2O3. According to the XPS, it can preliminarily prove we have successfully synthesized the γ-CrOOH and Cr2O3. The structure patterns were measured via X-ray powder diffraction (XRD). From the Figure 1c, the γ-CrOOH shows no obvious crystalline diffraction peaks; after annealing, it showed typical crystalline Cr2O3 diffraction peaks, which were matched well with the standard Cr2O3 pattern (JCPDS No. 381479).11 According to XPS and XRD image, after extensive literature review, there are three types for CrOOH which have been reported up to now. The first is rhombohedral αCrOOH, the second is orthorhombic β-CrOOH, and the third is amorphous γ-CrOOH.29 During the hydrothermal reaction, Cr2O72− was converted into amorphous γ-CrOOH via N2H4· H2O reduction. In order to further confirm the γ-CrOOH, the characteristic of the Fourier transform infrared spectroscopy (FT-IR) shown in Figure 1d has been measured; the FT-IR images were well-matched with γ-CrOOH spectra in the

Figure 2. SEM (a), HRTEM (b), and inset is SAED of γ-CrOOH. SEM (c), HRTEM (d), and inset is SAED of Cr2O3.

low magnification image in Figure S5, it grew on the Ni foam uniformly. And the corresponding energy dispersive X-ray spectroscopy (EDS) analysis of γ-CrOOH/NF also implied the chemical compositions as shown in Figure S6. It showed that O, Cr, and Ni were homogeneously distributed on γ-CrOOH/ NF. Furthermore, according to high-resolution transmission electron microscopy (HRTEM) images (Figure 2b) and the selected area electron diffraction (SAED) patterns (the inset of Figure 2b), both can confirm the amorphous structure due to the deficiency of fringe lattices. The Cr2O3 was also measured via SEM in Figure 2c; there was no obvious effect on the morphology compared with γ-CrOOH/NF. From the HRTEM in Figure 2d, the clear finger spacing is 0.25 nm, which matches well with the d spacing of the (110) plane of Cr2O3. And from SAED in the inset of Figure 2d, there was an obvious scattered diffraction pattern of Cr2O3. All the characterizations verified that the amorphous γ-CrOOH was formed via one-step hydrothermal reaction. And after annealing, the amorphous γ-CrOOH was resolved into Cr2O3. In addition, the chemical compositions of the loading of γ-CrOOH were analyzed by inductively coupled plasma B

DOI: 10.1021/acs.inorgchem.9b00112 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry mass spectrometry (ICP-MS) in Table S1. The loading of γCrOOH was ca. 0.41 mg cm−2. To investigate the electrochemical water-splitting performances of the γ-CrOOH/NF, a series of measurements were obtained in 1.0 M KOH. The digital photo of the electrolytic cell is shown in Figure S7. Before testing, the potentials have been calibrated, as shown in Figure S8. For the HER performance at the cathode, the linear sweep voltammetry at 5 mV s−1 is shown in Figure 3a. Meantime, the bare NF,

Figure S10, and Table S1, respectively. The SEM (Figure S9a) and XPS (Figure S10) for the sample after the chronopotentiometric measurements further reveal that the morphology and chemical state of γ-CrOOH can still keep well after the HER catalysis. Besides, the loading of γ-CrOOH remained almost unchanged in Table S1. It further implied the good stability of γ-CrOOH for HER in basic media. The OER performance of γ-CrOOH was also obtained in O2-saturated 1 M KOH. The linear sweep voltammetry of γCrOOH/NF, Cr2O3/NF, bare NF, IrO2/NF, and RuO2/NF at 5 mV s−1 is shown in Figure 4a. The Ni foam shows large

Figure 3. (a) Polarization curves of NF, Pt/C/NF, Cr2O3/NF, and γCrOOH/NF for the HER in 1.0 M KOH solution at 5 mV s−1. (b) The overpotential (η) at 50 mA cm−2 of the different samples. (c) Tafel plots of NF, Pt/C/NF, Cr2O3/NF, and γ-CrOOH/NF. (d) Chronoamperometric response at a fixed −1.03 V; inset is the LSV curves of initial and 1000th cycles of γ-CrOOH/NF.

Figure 4. (a) Polarization curves of NF, IrO2/NF, RuO2/NF, Cr2O3/ NF, and γ-CrOOH/NF for the OER in 1.0 M KOH solution (scan rate 2 mV s−1) and (b) a bar graph of the corresponding overpotential (η) at 50 mA cm−2 of the different samples. (c) Tafel plots of NF, IrO2/NF, RuO2/NF, Cr2O3/NF, and γ-CrOOH/NF. (d) Chronoamperometric response at a fixed 1.45 V; inset is the LSV curves of initial and 1000th cycles of γ-CrOOH/NF.

Cr2O3/NF, and Pt/C/NF were also measured under the same conditions as comparison samples. The Ni foam shows large hydrogen evolution overpotential ca. 650 mV at 50 mA cm−2 and almost no catalytic activity. In contrast, γ-CrOOH/NF exhibited excellent HER performance of only 79 mV at 10 mA cm−2, and ultralow overpotential of 149 mV at 50 mA cm−2, better than Cr2O3/NF about 183 mV at 10 mA cm−2 and 300 mV at 50 mA cm−2 listed in Figure 3b and Table S2. Furthermore, it was lower than almost all the metal (Ni, Co, Mn, and Fe, etc.) based catalysts which have been reported (Table S2). The excellent performance suggests that the γCrOOH/NF possessed more effective reaction kinetics during the HER. The Tafel slopes can be further compared to evaluate the samples’ reaction kinetics in Figure 3c. The Tafel slope of the γ-CrOOH/NF is approximately 89.1 mV dec−1, which is considerably lower than that of the Cr2O3/NF nanoparticles about 118 mV dec−1 and the Ni foam about 166 mV dec−1; besides, it was very close to that of Pt/C/NF. This result also suggests that the γ-CrOOH activate for hydrogen evolution reaction kinetics. In addition, the durability of catalysts also was an important judgment. The continuous cyclic voltammetry measurements were carried out. From the inset of Figure 3d, after 1000 CV cycles, the electrode exhibited stable performance with negligible loss. Besides, it also can keep a stable current density of 15 mA cm−2 with minimal change in overpotential in Figure 3d. In order to confirm the stability, the SEM, XPS, and ICP-MS have been conducted in Figure S9,

overvoltage of 725 mV at 50 mA cm−2. Compared with NF and Cr2O3/NF, γ-CrOOH/NF exhibited outstanding OER activity of only 334 mV at 50 mA cm−2, which was significantly better than Cr2O3/NF about 590 mV at 50 mA cm−2, even better than noble metal catalysts IrO2/NF (517 mV) and RuO 2 /NF (358 mV) electrode shown in Figure 4b. Furthermore, it was lower than almost all of the metal oxyhydroxides and hydroxides which have been reported (Table S3). The Tafel slopes also were calculated to evaluate reaction kinetics in Figure 4c. The Tafel slope of the γCrOOH/NF is approximately 41.4 mV dec−1, which is considerably lower than that of the Cr2O3/NF nanoparticles about 139.5 mV dec−1 and the Ni Foam about 182.2 mV dec−1. And it was also better than IrO2/NF (83.2 mV dec−1) and RuO2/NF (115.1 mV dec−1) electrodes. The result indicated that the γ-CrOOH possessed well reaction kinetics. Electrochemical double layer capacitances (Cdl) were also measured through CV scans. The cyclic voltammograms of Cr2O3/NF and γ-CrOOH/NF were collected in the region of 1.07−1.23 V versus the reversible hydrogen electrode (RHE) in Figure S11a,b, respectively. Besides, the Cdl of γ-CrOOH/ NF and Cr2O3/NF are shown in Figure S12a,b, respectively, and the Cdl of γ-CrOOH/NF was calculated as 5.78 mF cm−2, better than that of Cr2O3/NF (2.1 mF cm−2). The higher Cdl C

DOI: 10.1021/acs.inorgchem.9b00112 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry of γ-CrOOH revealed that it had the largest effective surface area for the OER process. Besides the high activity, the durability of catalysts can be judged through continuous cyclic voltammetry and the chronopotentiometric measurements. From the inset of Figure 4d, after 1000 CV cycles, the electrode exhibits stable performance with negligible loss. Besides, it also can keep a stable current density of 36.5 mA cm−2 with minimal change for 20 h at 1.45 V in Figure 4d. The XPS (Figure S10) results of γ-CrOOH can also still keep well after the OER catalysis. However, in addition, the loading of γCrOOH has been slightly decreased according to SEM (Figure S9b) and ICP-MS (Table S1), which was one of the reasons for the performance degradation. EIS measurement was conducted as shown in Figure S13. The Nyquist semicircle loop diameter for the high-frequency region represented the charge-transfer resistance. The Nyquist semicircle of the γCrOOH/NF electrode was much lower than that of Cr2O3/ NF and nickel foam, indicting the lower charge-transfer impedance and the faster reaction kinetics during the process of catalytic reaction. In order to further explore the reason why electrochemical activity has been increased, the periodic density functional theoretical (PDFT) calculations were carried out for idealized models of γ-CrOOH. According to the calculation results of the total and partial electronic densities of state (TDOS) in Figure 5a and the band structure in Figure 5b, the band gap of

requires a cell voltage of merely 1.56 V to deliver a current density of 10 mA cm−2 in Figure 5c. This number is very close to that of Pt/C//RuO2 (1.52 V). Besides, it is worth noting that the proportion of γ-CrOOH/NF//γ-CrOOH/NF was better than the Pt/C/NF//RuO2/NF when the potential was higher than ca. 1.70 V electrode. Such a low overpotential for overall water splitting is impressive and comparable to the recently reported overall water-splitting materials listed in Table S4. More worth emphasizing is that the γ-CrOOH catalysts of the two-electrode configuration showed much outstanding durability more than 20 h long-term measurement with only slightly decreased in Figure 5d.

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CONCLUSION In conclusion, we have synthesized amorphous γ-CrOOH via a hydrothermal method and investigated its applications for water-splitting electrolysis for the first time. It shows extraordinary HER activity with an ultralow overpotential of 79 mV at 10 mA cm−2, compared with almost all of the reported Ni-based, Fe-based oxide hydroxides. Meantime, there is the small overpotential of 334 mV at 50 mA cm−2 for the OER. Importantly, the bifunctional electrocatalyst for overall water-splitting electrocatalysis can work with a cell voltage of merely 1.56 V at 10 mA cm−2. Amorphous γCrOOH has effectively enhanced the intrinsic electrochemical activity via density functional theoretical calculations. More importantly, this work provides a new idea toward Cr-based electrocatalysts for bifunctional water splitting.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00112.

Figure 5. TDOS (a) and band structure (b) for γ-CrOOH. (c) Polarization curve of the γ-CrOOH/NF//γ-CrOOH/NF and Pt/C// RuO2 for comparison. (d) Chronoamperometric response at a fixed 1.60 V.

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Experimental section, materials characterization, electrochemical measurements, and calculation methods. Photographs of amorphous γ-CrOOH and Cr2O3; XRD and XPS of γ-CrOOH/NF and Cr2O3/NF; the SEM and HAADF-STEM image and element mapping images of γ-CrOOH/NF at low magnification. Digital image of electrolytic cell with electrode; SEM image and XPS of γ-CrOOH/NF after stability test for 20 h in 1.0 M KOH for HER and OER; LSV curves of hydrogen electrode reactions on Pt wire; CVs curves; EIS curves; ICP-MS; comparison of alkaline HER, OER, and overall water-splitting performance (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

γ-CrOOH is just 1.95 eV, much lower than that of γ-NiOOH (3.75 eV) and γ-FeOOH (3.50 eV), which indicated that the γCrOOH possessed well-conductive properties.32,33 The DFT result was in accordance with UV−vis diffuse-reflectance spectroscopy (UV−vis DRS) in Figure S14. The calculated band gaps are almost in accordance with the experimental value of 1.94 eV. The electrochemical activity would be increased with conductivity enhanced. Inspired by the good OER and HER activity of γ-CrOOH, the overall splitting of water has been measured. The γ-CrOOH worked as both the anode and the cathode for overall water splitting in 1 M KOH. It is observed that the γ-CrOOH bifunctional electrocatalyst

ORCID

Huifeng Li: 0000-0002-0257-3967 Genban Sun: 0000-0001-9005-8123 Shulan Ma: 0000-0002-8326-3134 Xiaojing Yang: 0000-0003-3620-3507 Author Contributions §

Z.S. and M.Y. contributed equally. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. D

DOI: 10.1021/acs.inorgchem.9b00112 Inorg. Chem. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21771024, 51572031, and 21871028).

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DOI: 10.1021/acs.inorgchem.9b00112 Inorg. Chem. XXXX, XXX, XXX−XXX