NiOOH Nanosheets

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Morphology Dynamics of Single-Layered Ni(OH)2/NiOOH Nanosheets and Subsequent Fe Incorporation Studied by in-situ Electrochemical Atomic Force Microscopy Jiang Deng, Michael Randall Nellist, Michaela Burke Stevens, Christian Dette, Yong Wang, and Shannon W. Boettcher Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03313 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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Morphology Dynamics of Single-Layered Ni(OH)2/NiOOH Nanosheets and Subsequent Fe Incorporation Studied by in-situ Electrochemical Atomic Force Microscopy Jiang Deng,1 Michael R. Nellist,2 Michaela Burke Stevens,2,# Christian Dette,2 Yong Wang,1 and Shannon W. Boettcher2,* 1

Advanced Materials and Catalysis Group, Center for Chemistry of High-performance and Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou, 310028, P. R. China. 2

Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403, United States *E-mail: [email protected] #

Present address: Department of Chemical Engineering, Stanford University, Stanford, CA 94305, United States Abstract Nickel (oxy)hydroxide-based (NiOxHy) materials are widely used for energy storage and conversion devices. Understanding dynamic processes at the solid-liquid interface of nickel (oxy)hydroxide is important to improve reaction kinetics and efficiencies. In this study, in-situ electrochemical atomic force microscopy (EC-AFM) was used to directly investigate dynamic changes of single-layered Ni(OH)2 nanosheets during electrochemistry measurements. Reconstruction of Ni(OH)2 nanosheets, along with insertion of ions from the electrolyte, results in an increase of the volume by 56% and redox capacity by 300%. We also directly observe Fe cations adsorb and integrate heterogeneously into/onto the nanosheets as a function of applied potential, further increasing apparent volume. Our findings are important for the fundamental understanding of NiOxHy-based supercapacitors and oxygen-evolution catalysts, illustrating the dynamic nature of Ni-based nanostructures under electrochemical conditions. Keywords: Ni(OH)2, EC-AFM, supercapacitor, oxygen evolution reaction, Fe incorporation

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Main Text The discovery and improvement of materials for energy storage and conversion are important for providing society clean, renewable power. Nickel (oxy)hydroxide-based materials (NiOxHy) are widely employed for pseudocapacitors and alkaline batteries because of their high theoretical specific capacitance, reversible redox properties, and low cost.1,2 Although many studies have been dedicated specifically to Ni(OH)2, the activation process of the material in pseudosupercapacitors and batteries, i.e. the increase of redox capacity after initial cyclic voltammetry or charge-discharge measurements, is not well understood.3,4 It has been hypothesized that this activation is due to increased porosity and exposure to active sites, but no clear experimental evidence for this hypothesis has been presented.5,6 The properties of Ni(OH)2-based materials can also be tuned by incorporating other transition metals.7 If Fe impurities in the electrolyte are included into the Ni(OH)2 matrix, it becomes an oxygen evolution reaction (OER) catalyst.8 Fe incorporation apparently results in moreoptimized adsorption energies with OER intermediates.9 With intentional Fe incorporation, Ni(Fe)OxHy is among the best catalysts for OER in alkaline conditions.10,11 The chemical composition and the coordination properties can be identified by X-ray absorption fine structure and X-ray absorption spectroscopy characterization.9 However, a significant debate continues regarding the role of Fe for enhancing the activity of Ni(Fe)OxHy based catalysts.12,13,14 While Xray techniques have been powerful to measure the changes in lattice and bond lengths, direct measurements of morphology changes are critical to fully understand the role of Fe species in enhancing catalytic activity. Here, we present an in-operando study of the dynamic reactions that affect the solid-liquid interface in Ni(OH)2 as a function of applied potential and the amount of added Fe using electrochemical atomic force microscopy (EC-AFM). Starting from well-defined single layer nickel hydroxide (SL-Ni(OH)2) nanosheets, we quantify via depth histograms, mean heights, surface areas, and volumes to evaluate changes of the nanoscale morphology. The observation that SL-Ni(OH)2 nanosheets transformed into small nanoparticles after hundreds electrochemical cycles was reported by Ida et al. as illustrated through ex-situ AFM experiments.15 Here, we demonstrate that the activation process observed during initial cycles can be attributed to the increase of volume and effective surface area, which is correlated directly to an increase in redox 2 ACS Paragon Plus Environment

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capacity. Also, we found a dramatic morphology change occurs even after one linear voltammetry sweep and the onset potential for the morphology change is consistent with the onset potential of Ni(OH)2 oxidation. After adding Fe2+ impurities to the electrolyte, further dramatic morphological changes were observed with Fe rapidly integrating in and depositing on the NiOxHy matrix. Preparation and in-situ imaging of single-layer nanosheets. The SL-Ni(OH)2 nanosheets were prepared by exfoliation of layered bulk Ni(OH)2 intercalated with dodecyl sulfate (DS-Ni(OH)2), which was synthesized through a hydrothermal process as reported in previous publications.15 Xray diffraction (XRD) patterns of the as-obtained DS-Ni(OH)2 show the formation of a αNi(OH)2 phase with extended basal spacing due to the intercalated dodecyl sulfate (Figure S1). The SL-Ni(OH)2 was then exfoliated from DS-Ni(OH)2 by formamide and deposited on a Si wafer by spin-casting. X-ray fluorescence analysis was used to show that the dodecyl sulfate surfactant was removed during the exfoliation process (Figure S2 and Table S1). The deposited Ni(OH)2 was then annealed at 180 °C for 1 h to remove solvent (see Methods in supplemental information). Images obtained from scanning electron microscopy (SEM) and ex-situ AFM (Figure S3 and Figure S4) show that SL-Ni(OH)2 on Si has a well-defined hexagonal shape with an average height of 0.90 ± 0.05 nm and diameter of 600 ± 100 nm. Based on the observed morphology, the Ni(OH)2 nanosheets appear to form through self-assembly and an oriented particle-attachment crystallization mechanism during the 24 h hydrothermal reaction, as has been discussed previously.15,16 For the in-situ experiment, the SL-Ni(OH)2 nanosheets were spin-coated onto highly oriented pyrolytic graphite (HOPG) and annealed in air at 180 °C for 1 hr. The sample was mounted in a homemade electrochemical cell as the working electrode with a Pt wire counter electrode and a saturated Ag/AgCl reference electrode (Figure S5). Figure 1a and Figure S6 show an individual SL-Ni(OH)2 nanosheet in 0.1 M KOH with a height of 1.16 nm and a diameter of ~400 nm at the open-circuit voltage (OCV). To monitor morphological changes as a function of the applied potential, the nanosheet was imaged using AFM while stepping the potential anodically (~500 s per image and voltage step). Figures 1b-d and S7a-k show the morphology evolution. The nanosheet is smooth at the OCV with little changes with potential until 1.41 V vs RHE. At this voltage, the surface of the nanosheet 3 ACS Paragon Plus Environment

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roughens as the size of the particles on the surface increases (Figure 1b). Deposition of nanoparticles can also be found in the immediate surroundings of the nanosheet. The transition from a single sheet into disordered nanoparticles continues at higher applied voltages with individual features increasing dramatically in size (Figure 1c, 1d, and S7). The measured current increases at potentials above 1.56 V vs RHE due to the onset of OER current8,17 (Figure S7l).

Figure 1. AFM images of SL-Ni(OH)2 (a) at the open-circuit voltage, and after the chronopotentiometry measurement (for 500 s) at (b) 1.41 V vs RHE, (c) 1.46 V vs RHE, and (d) 1.61 V vs RHE (scale bars = 200 nm). (e) Mean height of the nanosheet at varying potentials. A voltammogram (scan rate of 10 mV s-1) of the HOPG/SL-Ni(OH)2 system is presented as the background (grey). The data shows the dramatic restructuring is associated with the onset of Ni oxidation and driven more rapidly as the OER current turns on. Using mean height and depth histograms of the obtained AFM images, we can quantify the morphology evolution of the nanosheet. When the potential increases, the size distribution of the nanosheets shifts to higher values, for example going from 1.06 V to 1.61 V results in a change in mean height from 1.1 nm to 3.2 nm (Figure 1d and Figure S8); changes in morphology cease only when the electrode is returned to the OCV. The onset potential of the morphology change of the SL-Ni(OH)2 is 1.41 V vs RHE, which is consistent with the onset potential of Ni(OH)2 oxidation based on the cyclic voltammetry (CV) data (Figures 1e and S7). The morphology changes appear to accelerate at high potentials associated with the onset of OER current.


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We further investigated the potential-dependent changes in morphology by linear sweep voltammetry (LSV). After an initial linear sweep, the SL-Ni(OH)2 nanosheet transforms into nanoparticles increasing its surface area (Figure S9a and b, from 0.76 µm2 to 0.78 µm2). The surface area continued to increase as multiple CVs are collected (Figure 2b). After ~130 CV cycles the surface area has increased to 0.85 µm2. Additional changes are not observed with further cycling. Although no new redox peaks appear in the CV with cycling, the Ni(OH)2/NiOOH integrated redox wave capacity grows from 0.07 mC cm-2 to 0.28 mC cm-2 (Figure 2c). Mechanisms of restructuring. The increased surface area is associated with an increased volume of the nanosheet with cycling, for example from 0.008 µm3 to 0.014 µm3 after 250 cycles (Figure 2a). To study whether this volume expansion is due to deposition of Ni species that have dissolved from other areas or due to restructuring of the existing Ni nanosheet, we measured the total Ni content in the same micron-scale region where EC-AFM is conducted using energy dispersive X-ray spectroscopy (EDS). We note that EDS is usually considered semi-quantitative for elemental analysis. Here, however, we compare raw counts of the Ni fluorescence before and after electrochemistry and find little change (Figure S12c). This suggests the Ni content does not change within the sensitivity limit of the EDS measurement. There may be, however, Ni dissolution/redeposition that may occur at levels below this limit. From the EDS analysis, we conclude that the potential-driven activation process leads to introduction of significant porosity. This is consistent with the increases in redox activity and the electrolyte permeability of the resulting material.18,19 Furthermore, K+ ions can intercalate stabilizing the stacked nanoparticles; K is observed in EDS spectra (Figure S12c) of the SLNi(OH)2 after cycling. Hence, the height and volume of SL-Ni(OH)2 increases dramatically because of the reconstruction of Ni(OH)2 as well as the insertion of the K+, OH- and H2O. In general, this process is similar to Ni(OH)2-based supercapacitors where the capacitance (i.e. the integrated charge of the Ni redox wave) often increases with cycling.5 Figure 2 shows the correlated increases in surface area, volume (for a single nanosheet) and redox capacity (for the whole electrode) with cycling. This increase is consistent with the above microscopic picture.


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Figure 2. (a) Volume and (b) surface area derived from AFM images in Figure S10 collected during the multiple CVs. (c) The redox capacity calculated based on the CV curves during the cycling process. This data shows quantitatively the complete restructuring of the single nanosheet during redox cycling. We next discuss possible effects that play a role in the morphology changes triggered by the applied potential. As suggested by Ida et al.15 the origin could be simple dissolution and deposition of Ni. This might explain the observation of nanoparticle growth in the nearby region of the nanosheet. However, nanoparticle formation and sheet restructuring are not observed when the Ni is reduced and in the Ni(OH)2 form – even though Ni2+ is more soluble than Ni3+ and the solution would be expected to be saturated with Ni2+ under the initial conditions. The applied anodic potential therefore appears required to drive the nucleation Ni oxyhydroxide, perhaps from sparingly soluble Ni species that are generated locally from the nanosheet (Figure 1b shows such nucleation highlighted with a red circle). The anodic charging itself may also play a role in driving the restructuring of the nanosheet. When the applied potential is > 1.41 V vs RHE, Ni(OH)2 transitions to NiOOH by removing one proton and electron (Figure 3b). While the exact mechanism for this transition is unknown,21 the initial removal of an electron22 is expected to be fast relative to the slower ionic processes responsible for screening the injected charge (Figure 3c). This local charging could lead to a “Coulomb repulsion” effect, which together with geometric frustrations that go along with the Ni(OH)2 to NiOOH transition drives a rapid restructuring of the nanosheet. The Ni(OH)2 nanosheet is also expected to be only kinetically stable. Thermodynamically, the single sheet should be unstable relative to stacked structures that yield lower interfacial energies along the basal plane due to nanosheet-nanosheet interactions.


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Figure 3. Schematic of the dynamics of a SL-Ni(OH)2 during constant voltage measurements in 0.1 M KOH. (a) SL-Ni(OH)2 sheet adsorbed on HOPG in 0.1 M KOH. (b) Ni(OH)2 gets locally charged and transforms into NiOOH at 1.41 V vs RHE. (c) The nanosheet reconstructs. (d) Oxygen evolution occurring at 1.61 V vs RHE. This model explains the dramatic increase in sheet volume (for nominally the same Ni concentration) that is observed experimentally.

The effects of Fe incorporation. The low OER activity of Ni(OH)2 can be enhanced dramatically through Fe incorporation, and Ni(Fe)OxHy is among the most-active OER catalysts in alkaline media.8 We identified the importance of intentional and incidental Fe incorporation in the NiOxHy,8 but the mechanism of incorporation and how it affects the OER activity remains debated. Here we explore the dynamics of the Fe incorporation process. The Fe species were observed to deposit in inhomogeneously by EC-AFM. Figures 4a and b show that after 252 CV cycles in 0.1 M KOH (with only trace unintentional Fe impurities) the initial nanosheet has transformed into a collection of nanoparticles, as discussed above. When 3 ppm of Fe(NO3)3 is added to the electrolyte (Figure 4c), the Fe preferentially incorporates into the existing Ni (oxy)hydroxide nanoparticles, which expand in volume by ~ 19%. This is consistent with incorporation as a Ni-Fe oxyhydroxide phase, as is the anodic shift in the Ni redox wave observed (Figure 4e). When 6 ppm Fe is added to the electrolyte, the volume increases by 51% over the Fe-free state and the redox wave shifts further anodic. When 12 ppm of Fe(NO3)3 is added (Figure 4d), Fe is additionally deposited in regions where little Ni (oxy)hydroxide formerly existed. Elemental analysis by inductively coupled plasma optical emission spectrometry of the nanosheets after introduction of 12 ppm Fe in the electrolyte shows an Ni:Fe ratio of 1.1. If this Fe were all incorporated into a Ni-Fe oxyhydroxide phase it would have a


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composition exceeding the proposed solubility limit of Fe in Ni-Fe (oxy)hydroxide (~25%).9 This suggests the possibility that part of the Fe is depositing as a second FeOOH phase, which would lead to a variation in OER activity that might be mapped using other scanning probe techniques.23

Figure 4. AFM images and structural schematics of the SL-Ni(OH)2 in 0.1 M KOH (a) after one LSV in 0.1 M KOH and (b) after 252 CV cycles, and in presence of (c) 3 ppm Fe(NO3)2 and (d) 12 ppm Fe(NO3)2 after three CV cycles (scale bars = 200 nm). (e) Top: CV curves of the SLNi(OH)2 on HOPG in 0.1 M KOH at a scan rate of 50 mV s-1. Bottom: CV curves of the electrode in the presence of different amounts of Fe at a scan rate of 10 mV s-1. (f) Variation of the volume during the cycling process and subsequent addition of different amounts of Fe. The large volume increase >25% for 12 ppm Fe suggests that a separate FeOOH phase deposits on top of the NiOOH nanosheet, in addition to Fe incorporating into the NiOxHy structure. The Fe incorporation dynamics were also explored as a function of potential by using another SL-Ni(OH)2 nanosheet that was already activated. In the absence of Fe, the SL-Ni(OH)2 8 ACS Paragon Plus Environment

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undergoes negligible further changes as a function of potential (Figure S15a). Upon the addition of 1 ppm Fe(NO3)2 to the electrolyte solution, a slight increase (4%) in the mean height of the nanosheet was observed after 6 min at the OCV. This increase is attributed to the spontaneous initial incorporation of Fe into the Ni(OH)2 lattice (Figure S15b) and not deposition of FeOOH on the nanosheet surface. Major morphology changes were observed once a potential of 1.06 V vs RHE was applied, perhaps due to the onset of significant oxidation of Fe2+ from solution. Once the potential reaches 1.41 V vs RHE, corresponding to the oxidation wave of the Ni(OH)2, the mean height increased dramatically and continued to grow with time, suggesting rapid deposition and incorporation of Fe into/onto the film. We note also that the incorporation of Fe3+ into Ni(OH)2 formally requires charge compensation by anions. While the study here does not provide any insight into the anion processes that accompany the nanosheet dynamics and transformation, it is likely that carbonate anions, in addition to hydroxide, are involved in charge compensation, as has been shown recently by Hunter et al.20 In summary, we reported dynamic changes of SL-Ni(OH)2 using EC-AFM. From a fundamental perspective the results shed light on the dynamic nature of Ni(OH)2 based materials – nanoscale architectures17 that might be synthesized are not necessarily stable under reactive conditions or in the presence of Fe impurities as evidenced by the rapid transformation of the single layer nanosheet to porous loosely packed nanoparticles.24,25 Such materials have been previously shown to be dynamic in nature, for example porous Ni oxyhydroxide nanosheets can be formed from NiGa oxyhydroxide.26 This makes architecture design difficult for Ni-based electrochemically active materials at the nanometer or molecular scales. We further illustrated how Fe incorporates into the material. The rapid deposition of Fe with high anodic potentials appears to result in FeOxHy deposits on top of the NiOxHy nanosheet. Fe also incorporates into and expands the original NiOxHy even at the open circuit potential. The Fe incorporation is coincident with the anodic shifts in the observed Ni redox wave. These results illustrate the heterogeneity of Fe species in incorporated into materials from solution, and are thus important for mechanistic studies on Ni(Fe)OxHy OER catalysts.27 Notes


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The authors declare no competing financial interest. Acknowledgements This work was supported by the National Science Foundation Chemical Catalysis program under grant CHE-1566348. J.D. acknowledges support from Zhejiang University. Y.W. and J.D. acknowledge support from the National Key R&D Program of China (2016YFA0202900) and the National Natural Science Foundation of China (21622308). J.D. thanks Dr. Jingjing Qiu and Lisa Enman for fruitful discussions. Elizabeth Cochran is thanked for assistance with XRD measurements. The atomic force microscope was purchased using funds provided by the NSF Major Research Instrumentation Program, grant DMR-1532225. The project made use of CAMCOR shared facilities supported by grants from the W. M. Keck Foundation, the M. J. Murdock Charitable Trust, ONAMI, the Air Force Research Laboratory, the National Science Foundation, and the University of Oregon. Supporting Information Available: Detailed procedures for SL-Ni(OH)2 synthesis and in-situ EC-AFM. SEM images, XRD patterns, and XRF data. References (1) Chen, J.; Cheng, F., Acc. Chem. Res. 2009, 42, 713-723. (2) Wang, H.; Casalongue, H. S.; Liang, Y.; Dai, H., J. Am. Chem. Soc. 2010, 132, 7472-7477. (3) Su, Y.-Z.; Xiao, K.; Li, N.; Liu, Z. Q.; Qiao, S. Z., J. Mater. Chem. A 2014, 2, 13845-13853. (4) Kamath, P. V.; Dixit, M.; Indira, L.; Shukla, A.; Kumar, V. G.; Munichandraiah, N., J. Electrochem. Soc. 1994, 141, 2956-2959. (5) Ji, J.; Zhang, L. L.; Ji, H.; Li, Y.; Zhao, X.; Bai, X.; Fan, X.; Zhang, F.; Ruoff, R. S., ACS Nano 2013, 7, 62376243. (6) Li, B.; Cao, H.; Shao, J.; Zheng, H.; Lu, Y.; Yin, J.; Qu, M., Chem. Commun. 2011, 47, 3159-3161. (7) Corrigan, D. A., J. Electrochem. Soc. 1987, 134, 377-384. (8) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W., J. Am. Chem. Soc. 2014, 136, 6744-6753. (9) Friebel, D.; Louie, M. W.; Bajdich, M.; Sanwald, K. E.; Cai, Y.; Wise, A. M.; Cheng, M. J.; Sokaras, D.; Weng, T. C.; Alonso-Mori, R., J. Am. Chem. Soc. 2015, 137, 1305-1313. (10) Stevens, M. B.; Enman, L. J.; Batchellor, A. S.; Cosby, M. R.; Vise, A. E.; Trang, C. D. M.; Boettcher, S. W., Chem. Mater. 2017, 29, 120-140. (11) Dionigi, F.; Strasser, P., Adv. Energy Mater. 2016, 6, 1600621. (12) Görlin, M.; Chernev, P.; Ferreira de Araújo, J.; Reier, T.; Dresp, S. r.; Paul, B.; Krähnert, R.; Dau, H.; Strasser, P., J. Am. Chem. Soc. 2016, 138, 5603-5614. (13) Louie, M. W.; Bell, A. T., J. Am. Chem. Soc. 2013, 135, 12329-12337. (14) Chen, J. Y. C.; Dang, L.; Liang, H.; Bi, W.; Gerken, J. B.; Jin, S.; Alp, E. E.; Stahl, S. S., J. Am. Chem. Soc. 2015, 137, 15090-15093. (15) Ida, S.; Shiga, D.; Koinuma, M.; Matsumoto, Y., J. Am. Chem. Soc. 2008, 130, 14038-14039. (16) Zhu, Y.; Cao, C.; Tao, S.; Chu, W.; Wu, Z.; Li, Y., Sci. Rep. 2014, 4, 5787. 10 ACS Paragon Plus Environment

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(17) Hall, D. S.; Lockwood, D. J.; Bock, C.; MacDougall, B. R., Proc. R. Soc. A 2015, 471. (18) Klingan, K.; Ringleb, F.; Zaharieva, I.; Heidkamp, J.; Chernev, P.; Gonzalez-Flores, D.; Risch, M.; Fischer, A.; Dau, H., ChemSusChem 2014, 7, 1301-1310. (19) Nellist, M. R.; Laskowski, F. A. L.; Lin, F.; Mills, T. J.; Boettcher, S. W., Acc. Chem. Res. 2016, 49, 733740. (20) Hunter, B. M.; Hieringer, W.; Winkler, J. R.; Gray, H. B.; Muller, A. M., Energy Environ. Sci. 2016, 9, 1734-1743. (21) Lo, Y. L.; Hwang, B. J., Langmuir 1998, 14, 944-950. (22) Wehrens-Dijksma, M.; Notten, P. H. L., Electrochim. Acta 2006, 51, 3609-3621. (23) Nellist, M. R.; Chen, Y.; Mark, A.; Gödrich, S.; Stelling, C.; Jiang, J.; Poddar, R.; Li, C.; Kumar, R.; Papastavrou, G., Nanotechnology 2017, 28, 095711. (24) Kong, X.; Zhang, C.; Hwang, S. Y.; Chen, Q.; Peng, Z., Small 2017, 13, 1700334. (25) Gao, M.; Sheng, W.; Zhuang, Z.; Fang, Q.; Gu, S.; Jiang, J.; Yan, Y., J. Am. Chem. Soc. 2014, 136, 70777084. (26) Liang, H.; Li, L.; Meng, F.; Dang, L.; Zhuo, J.; Forticaux, A.; Wang, Z.; Jin, S., Chem. Mater. 2015, 27, 5702-5711. (27) Song, F.; Hu, X., Nat. Commun. 2014, 5.


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NiOOH Nanosheets

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