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Applied Catalysis B: Environmental

journal homepage: www.elsevier.com/locate/apcatb

An effective hybrid electrocatalyst for the alkaline HER: Highly dispersed Ptsites immobilized by a functionalized NiRu-hydroxide

Dan Lia, Xufang Chena, Yaozha Lva, Guoyang Zhanga, Yu Huanga, Wei Liua, Yang Lia,Rongsheng Chena, Colin Nuckollsb,*, Hongwei Nia,*a The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, ChinabDepartment of Chemistry, Columbia University, 3000 Broadway, Havemeyer Hall, MC3130, New York, NY 10027, USA

A R T I C L E I N F O

Keywords:NiRu-LDHPartially amorphousSingle Pt atomsHERElectrocatalysis

A B S T R A C T

Developing a highly active and robust catalyst for the hydrogen evolution reaction (HER) using the minimalamount of Pt is a continuing challenge. Herein, we demonstrate a functionalized NiRu-hydroxide (NiRu-OH)catalyst that acts as a capable candidate to immobilize highly dispersed single Pt atoms. We synthesize thiscatalyst by means of Ru leaching from a NiRu-layer double hydroxide (NiRu-LDH) under oxygen evolutionreaction (OER) conditions. The hybrid catalyst shows extremely high HER activity, with an overpotential of 38mV to drive a typical current density of 10 mA cmgeo

−2 in alkaline media. We ascribe the excellent catalyticactivity to the synergistic effect of the highly dispersed single Pt atoms and NiRu-OH in a dual-site alkaline HERmechanism. This work forms the basis for the exploration of matrices capable of trapping single atoms forvarious applications.

1. Introduction

In this study we describe a new catalyst for the hydrogen evolutionreaction (HER) that is effective because it has single sites of platinumatom functionalization in a NiRu-hydroxide matrix. Electrochemicalsplitting of water in an alkaline electrolyzer plays a crucial role in de-veloping energy systems, especially in the “hydrogen-based economy”[1–3]. Highly efficient and inexpensive electrocatalysts for oxygenevolution reactions (OER), such as Ni-based hydroxide used in elec-trolyzers, perform well in only alkaline or neutral media. In contrast,state-of-the-art electrocatalysts for HER, such as Pt, exhibit relativelysluggish kinetics in alkaline media, being at least two orders of mag-nitude slower than those in acidic media [4,5]. One widely acceptedhypothesis for the slow kinetics is that under alkaline conditions, thereaction pathway for the HER consists of the disassociation of water,production of hydrogen ad-atoms (Had) intermediates, and upon thedesorption of hydroxide (OH-) to refresh the catalyst surface, the re-combination of Had can generate H2 [6,7]. Pt is generally considered tohave an outstanding catalytic activity for H2 desorption but a weakactivity toward the dissociation of the HeOH bond. This creates a largekinetic energy barrier for the overall HER [5]. To overcome this, hybridcatalysts have been created with a combination of metal oxides or hy-droxides with Pt, promoting the dissociation of water and producing

Had that is subsequently adsorbed on the nearby Pt surfaces to form H2

[8,9]. These catalysts exhibit a synergistic boosting in the overall HERkinetics. However, the scarcity and high cost of Pt represent a primarybottleneck, hindering the large-scale implementation of electrolyzersfor the HER. Successful catalysts for HER will reduce the Pt loadingwhile maintaining high catalytic activity.

The strategy of isolating Pt atoms has been pursued recently becauseit uses less platinum and also exhibits high catalytic activity.Nevertheless, avoiding aggregation or detachment of Pt atoms duringoperation is an unmet challenge. To create highly active and stablesingle metal atom catalysts, one promising strategy is to functionalize aconductive matrix to trap single atoms through strong interactions. Forexample, O-vacancy defects on oxides, C-vacancy defects on graphene,N-vacancy defects on TiN and C3N4, as well as cation vacancies, such asMo-vacancy defects on MXene (Mo2TiC2Tx) all immobilize single Ptatoms successfully in recent years [10–16]. However, it is difficult fortransition metal hydroxides, which have sufficient catalytic capabilitytoward the cleavage of the HeOH bond during a HER in alkaline media,to form enough functional groups by traditional hydrothermal methods[13,16]. Recently, we found that the NiRu-layer double hydroxide(NiRu-LDH) is very unique. The sizes of the Ru and Ni hydroxides areconsiderably different, resulting in NiRu-LDH being unstable and easilyproducing defects [17,18]. Moreover, RuOx and Ru(OH)x are prone to

https://doi.org/10.1016/j.apcatb.2020.118824Received 9 December 2019; Received in revised form 25 January 2020; Accepted 28 February 2020

⁎ Corresponding authors.E-mail addresses: cn37@columbia.edu (C. Nuckolls), nihongwei@wust.edu.cn (H. Ni).

Applied Catalysis B: Environmental 269 (2020) 118824

Available online 29 February 20200926-3373/ © 2020 Elsevier B.V. All rights reserved.

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be oxidized to soluble RuO4 during OER catalysis [19,20]. The activeoxygen-coordinated Ru moieties with high valence states delaminatefrom the matrix causing degradation of the OER activity and the oxi-dative release of lattice oxygen during OER are both likely to createanchor sites for immobilization of single Pt atoms [21,22].

Herein, we designed a novel hybrid electrocatalyst consisting ofsingle Pt atoms immobilized on a partially amorphous NiRu-hydroxide(NiRu-OH) with abundant functional groups (termed Pt/NiRu-OH).Both the formation of the functionalized NiRu-OH and the im-mobilization of the Pt atoms occurred in an alkaline media three-elec-trode system where Pt acted as a counter electrode as the OER and HERwere run in series. The as-synthesized Pt/NiRu-OH exhibited high cat-alytic activity toward the HER under alkaline conditions with im-pressively low overpotentials (η) at 10mA cmgeo

−2 and a high amountof intrinsic catalytic activity. This study demonstrates that NiRu-LDHcan be converted into partial amorphous NiRu-OH during the OER,which produces abundant functional groups for immobilizing single Ptatoms. This hybrid catalyst with low Pt loading offers a promisingpathway toward highly efficient HER catalysis in alkaline media.

2. Results and discussion

We fabricate the NiRu-LDH supported on Ni foam via a hydro-thermal method (see details in the Supporting Information), and wecharacterize its morphology and structure by scanning electron micro-scopy (SEM) and transmission electron microscopy (TEM). The SEMimages in Fig. 1a and b indicate that the NiRu-LDH layer is uniformlygrown on the Ni foam. The magnified SEM image and the TEM imagesreveal the stacked nanoparticle structure of the NiRu-LDH layers. Thismorphology is beneficial to the solid-liquid contact and in turn to theelectrocatalytic reactions. The lattice spacing in the TEM image(Fig. 1c) is measured to be 0.594 nm and assigned to the (003) plane ofNiRu-LDH. Importantly, no defects can be observed in the high-re-solution images, and clear diffraction rings are exhibited in the se-lected-area electron diffraction (SAED) pattern from the low-resolutionimage (Fig. 1e), revealing good crystallinity in the NiRu-LDH.

We first investigate the HER performance by linear sweep voltam-metry (LSV) in a 1M KOH solution at a scan rate of 1mV s−1.Unfortunately, an overpotential of up to 166mV is needed to drive thetypical current density (j) of 10mA cmgeo

-2, similar to previously re-ported Ni-based hydroxide (Fig. 1f) [23–25]. Subsequently, we conductthe OER on the NiRu-LDH at 50mA cmgeo

-2 for 10 h to leach the Ruspecies and erode the crystal structure (Fig. S3). After the OER, we theninvestigate the HER performance of NiRu-OH. Note that after the OER,we carry out ∼20 cyclic voltammetry (CV) cycles until the HER curvebecame stable presumably from the metal oxyhydroxide sites that aregenerated during the OER that occlude the HER active sites. Specifi-cally, the NiRu-OH shows a similar overpotential at 10mA cmgeo

−2 tothat of the original NiRu-LDH, and the value increases to 191mV. Theslightly increased overpotentials are from the leaching of Ru, which iscrucial for the active site in the HER [7,26].

We utilize inductively coupled plasma-mass spectrometry (ICP-MS)to measure the samples and the corresponding electrolytes before andafter the OER. The amount of Ru detected confirms its leaching duringthe OER (Fig. 1g). Next, we perform CV on the NiRu-OH with a Pt wirecounter electrode and a scan rate of 20mV s−1. The HER currentdensity dramatically increases during the first 100 cycles and stabilizesafter approximately 500 cycles, and we observe significant positiveshifts in the onset potential (Fig. 1h). However, in contrast, we find aslow growth trend when a similar process was run for the original NiRu-LDH, NiFe-LDH and oxidized NiFe-LDH (after a 10 h OER). All thesesamples require at least 2000 cycles to stabilize (Fig. 1i, Fig. S4). Fur-thermore, we also conduct a long-term HER catalytic reaction at−0.235 V with a Pt wire counter electrode for both NiRu-OH and theoriginal NiRu-LDH. It is surprising that the current density of NiRu-OHincreases rapidly and tends to stabilize after approximately 40 h as the

HER proceeds (Fig. S5). Whereas the NiRu-LDH displays continuouslyslow growth with time. In addition, we performed CV on the NiRu-OHwith a graphite rod counter electrode for 500 cycles under similarconditions. As shown in Fig. S6, the HER activity of NiRu-OH after CVcycles changed slightly. Accordingly, we posit that during the initialperiod of the HER, the functionalized vacancies, especially Ru va-cancies, and a partially amorphous surface on NiRu-OH could effec-tively immobilize single Pt atoms (from the Pt wire) and prevent Ptatom aggregation. These highly dispersed Pt atoms then combine withwater disassociation sites from the hydroxide to effectively boost theoverall alkaline HER. Because of this interesting phenomenon and thepromising performance we systematically investigate the materials toconfirm the hypothesis for its activity.

We use X-ray diffraction (XRD) to detect changes in the crystaltexture. Fig. 2g shows the diffractograms of three samples. We indextwo major peaks to the Ni foam in all samples. The initial NiRu-LDHpeaks (red trace) are consistent with the Bragg reflections of the tran-sition metal LDH structure, especially the diagnostic reflection locatedat 2theta of 14.8°, which is indicative of interlayer galleries [17,18].Other reflections are all in agreement with the pattern for a NiRu-LDHpowder (Fig. S7). However, after the OER, the reflections from the in-terlayer galleries essentially disappear, and other new reflections in-dicate weak crystalline features, suggesting that the layered structure ofthe hydroxide is destroyed and that NiRu-LDH is now more amorphous.Thus, we refer to the NiRu-LDH after the OER as NiRu-OH (the−OH isconfirmed by X-ray photoelectron spectroscopy (XPS) as discussedbelow). Moreover, the Pt/NiRu-OH shows a similar pattern to the NiRu-OH and lack Pt peaks. The absence of Pt reflections is due to the lowcontent of Pt and its lack of crystallinity. The control sample, consistingof the original NiRu-LDH without the OER that acted as the workingelectrode for Pt deposition, is also characterized by XRD, and as ex-pected the LDH structure persisted (Fig. S7).

The surface morphology of the NiRu-OH and Pt/NiRu-OH is shownin Fig. 2a-d. Both NiRu-OH layers with similar nanoparticle-stackedstructures are uniformly deposited on the Ni foam, although cracks onsurfaces and edges are created because of the extended electrochemicalcorrosion and Ru leaching. The TEM images of the NiRu-OH layer inFig. 2e clearly illustrate the vacancies (circled in yellow), irregularcrystal lattice fringes and amorphous phase, revealing abundant defectsin the NiRu-OH. The low-magnification TEM-SAED pattern furthersuggests a partially amorphous structure (Fig. 2f). Apart from the de-fects in the Pt/NiRu-OH, we observe high-brightness spots with largesizes in the high-magnification high-angle annular dark field scanningtransmission electron microscopy (HAADF-STEM), confirming the im-mobilization of the highly dispersed single Pt atoms in both the crys-talline and amorphous regions in the NiRu-OH (Fig. 2h and i, S8). Thecorresponding energy dispersive X-ray spectroscopy (EDS) elementalmapping analysis showed in Fig. 2j further reveals the highly dispersedPt atoms on Pt/NiRu-OH.

The peaks of Ni, Ru, O and C are clearly presented in the surveyspectrum (Fig. S9), illustrating the purity of the as-synthesized mate-rials. Fig. 3a shows the Ni 2p spectrum of the three catalysts. Generally,the peaks apart from the satellite peaks (labeled Sat.) are the overlap ofthe doublets of Ni2+ and Ni3+ in Ni-based hydroxide [27,28]. We de-convolute the Ni 2p3/2 peaks into Ni2+ peaks at 855.10±0.05 eV andNi3+ peaks at 856.25±0.05 eV by fitting the experimental data in allthree catalysts [7,26]. We determine the ratio of Ni+3 to Ni2+ for NiRu-LDH to be approximately 1.1 by taking a ratio of the areas of the de-convoluted peaks. In stark contrast, the values are 4 times greater forboth NiRu-OH and Pt/NiRu-OH, revealing that the oxidation of Ni2+

during the OER is not reversed during the subsequent HER. It is alsolikely that the Ni3+ did not result in positive HER activity based on ourresults (Fig. 1f) even though it has been reported that Ni3+ is highlyefficient toward the cleavage of H2O [29]. The Ru 3d peaks and C 1speaks overlap directly in Fig. 3b; therefore, Ru 3d5/2 is mainly used as aguide for peaking fitting (we use the constraints that the binding energy

D. Li, et al. Applied Catalysis B: Environmental 269 (2020) 118824

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difference between 3d3/2 and 3d5/2 peaks is 4.40 eV). As expected, wecan index all of the peaks for Ru 3d5/2 at 281.80±0.05 eV to the ty-pical binding energy of Ru3+ [30,31]. We assign a small peak at280.20 eV in Pt/NiRu-OH to Ru°, which was converted from high va-lence state Ru species during the HER (this is consistent with our pre-vious observation [7]). The spectra also show CeC, CeO, CeO]O andC]O bonds presented from 284.60 eV to 289.60 eV from impuritiesfrom the ambient. The O 1s peaks of all catalysts show high peaks at abinding energy of 531.10 eV, which is indicative of the−OH, providingdirect evidence of the hydroxide structure in all three catalysts (Fig. 3c)[7,32]. Moreover, we also compare the Pt 4f peaks of Pt/NiRu-OH withthose of commercial Pt/C catalysts (Fig. 3d). The spectrum shown hereis the result after background correction since the binding energy of Ni3p is close to Pt 4f. We attribute two peaks of Pt 4f in Pt/C located at71.00 and 74.30 eV to the signal of Pt°, reflecting the active center inPt/C being metallic Pt [33,34]. In contrast, a weak and broad peaklocated between ∼ 71.70 and ∼ 74.40 eV is observed in Pt 4f7/2 of Pt/NiRu-OH. The valence states of Pt species in Pt/NiRu-OH fall in a range

between +2 and +4; thus, the Pt on NiRu-OH will bond with O or OH.We also use extended X-ray absorption fine structure (EXAFS) tech-nology to analyze the local coordination environment of Pt species inPt/NiRu-OH. Fig. 3f describes the Fourier transforms of the Pt L3-edgeEXAFS oscillations of the k2-weighted χ(k) signals (Fig. 3e), in whichthere is one prominent peak at ∼1.6 Å from the Pt–O contribution butno peak at ∼2.2 Å from the Pt–Pt contribution, strongly indicating thesole presence of single Pt atoms in the Pt/NiRu-OH. X-ray absorptionnear edge structure (XANES) measurements in Fig. 3g show the in-tensity of Pt/NiRu-OH is similar to that of PtO2. The signals of both Pt/NiRu-OH and PtO2 are much higher than that of Pt foil, further con-firming the electronic state of the Pt species in Pt/NiRu-OH are posi-tively charged.

The HER electrocatalytic activities of the Ni foam, NiRu-LDH, NiRu-OH, Pt/NiRu-OH and Pt/C are assessed in 1M KOH aqueous solution.Fig. 4a displays the 90 % iR corrected polarization of various samples.Among them, Pt/C undoubtedly exhibits the best HER activity of theaforementioned samples with an overpotential of 31mV at a current

Fig. 1. SEM images of (a, b) NiRu-LDH supported on Ni foam. (c) High-resolution transmission electron microscopy (HRTEM) image, (d) TEM image and (e)corresponding SAED patterns of yellow circled region in NiRu-LDH. (f) LSV curves of Ni foam, NiRu-LDH and NiRu-OH. (g) Metal content of electrolytes and samples.(h, i) Pt deposition on NiRu-OH and NiRu-LDH by CV method.

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density of 10mA cmgeo−2. The Pt/NiRu-OH shows a similar HER ac-

tivity with only ∼ 38mV to 10mA cmgeo−2, which outperforms the

other control samples of bare Ni foam (274mV), NiRu-LDH (165mV)and NiRu-OH (191mV) at the same current density. Such a value iseven less than those reported thus far for other alkaline compounds,such as Co0.85Se/NiFe-LDH hybrid (η10= 260mV) [35], NiFe-OH-F(η10= 206mV) [23], amorphous SrCo0.85Fe0.1P0.05O3-δ nanofilm(η10= 206mV) [36], Li+-Ni(OH)2-Pt (η10 ≈ 100mV) [8],Ni0.89Co0.11Se2 mesoporous nanosheet networks (η10= 85mV) [37]and edge-rich layered WS2-Pt (η10 ≈ 50mV) [34] (Table S2, SupportingInformation). The excellent HER activity of Pt/NiRu-OH is also con-firmed by its smaller Tafel slope (39mV decade-1), which is closed tothat of benchmark Pt/C (30mV decade-1) and lower than those of bareNi foam (168mV decade-1), NiRu-LDH (107mV decade-1) and NiRu-OH(106mV decade-1), verifying the fast reaction kinetics on Pt/NiRu-OH(Fig. 4b).

The Nyquist plot of various samples is investigated to provide in-sight into HER pathways. As displayed in Fig. 4c, all five samples ex-hibit almost the same ohmic resistance (RΩ) at high frequencies (∼1.7±0.1 Ω), revealing that the supported catalysts did not influencethe total resistances of the electrode and electrolyte. We observe twosemicircles, which are much smaller, on both Pt/NiRu-OH and Pt/C,suggesting a decreased charge transfer resistance (Rct) under that

overpotential during the HER. We determined the electrocatalyticdurability of Pt/NiRu-OH by measuring 5000 CV scans and a long-termtest under the potential of -0.075 V without iR compensation. As shownin Fig. 4d, the overpotential of Pt/NiRu-OH at 10 and 100mA cmgeo

−2

increases to only 8mV and 21mV, respectively, after 5000 CV cycles.Moreover, the Pt/NiRu-OH also exhibits a robust operation over aperiod of 30 h with negligible activity loss (insert in Fig. 4d). After 30 hoperation, the structural information of the Pt/NiR-OH is examinedusing SEM. As displayed in Fig. S11, the morphology of the Pt/NiRu-OHhave no obvious variations. These data demonstrate the excellent sta-bility during HER of our Pt/NiRu-OH catalyst, suggesting that the Ptatoms are firmly immobilized on NiRu-OH under alkaline and reductionpotential conditions.

The outstanding intrinsic activity of Pt/NiRu-OH toward HER isfurther evidenced by specific (evaluated by normalizing LSV curveswith electrochemically active surface area, ESCA) and mass activity.The ESCA is assessed by double-layer capacitance (Cdl), which is line-arly proportional to ESCA (see the calculation of Cdl in SupportingInformation). As shown in Fig. S2, the Pt/NiRu-OH demonstrates alarger ESCA than NiRu-OH, which is due to the surface effect of thesingle Pt atoms. Fig. 4e illustrates the normalizing LSV curves withobtained Cdl, explicitly revealing the excellent intrinsic activity on theactive site of Pt/NiRu-OH. Furthermore, it should be noted that the Pt

Fig. 2. SEM images of (a, b) NiRu-OH and (c, d) Pt/NiRu-OH supported on Ni foam. (e) HRTEM image and (f) TEM image and corresponding SAED patterns of theyellow circled region in Pt/NiRu-LDH. (g) XRD patterns of NiRu-LDH, NiRu-OH and Pt/NiRu-OH. Representative HAADF-STEM images of (h) crystalline and (i)amorphous region. (j) EDS elemental mapping analysis of the Pt/NiRu-OH catalyst. The yellow circles are drawn around single Pt atoms.

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loading of the Pt/NiRu-OH is determined to be ∼ 0.016mg (1.0wt%),which is ∼ 94 times lower than that of Pt/C (measured by ICP-MS,Table S1). Impressively, after normalizing the current density with Ptmass loading and eliminating the contributions of the NiRu-OH and Nifoam (see the calculation details in Supporting Information), the massactivities under an overpotential of 50mV are determined to be 1.01and 0.07 A mg−1 for Pt atoms on Pt/NiRu-OH and Pt/C, respectively.Moreover, as the applied overpotential reaches 77mV, the mass activ-ities are calculated to be 2.08 and 0.18 A mg−1 for Pt/NiRu-OH and Pt/C, respectively (Fig. 4f). These dramatic values strongly indicate theadvantages of Pt/NiRu-OH not only in improving HER activity of hy-droxide in alkaline media but also in efficiently decreasing Pt usage.

For comparison, 1 %, 5 % and 10 % Pt particles were coated ontoNiRu-OH (PtNP/NiRu-OH) to investigate the HER performance. Thecoating method are described in Supporting Information. The Pt par-ticles are obtained from commercial Pt/C powder and the size of Ptparticles falls in a range between 5–20 nm based on the TEM results(Fig. S12). As shown in Fig. S13, along with the increase of Pt loading,the HER activity enhances with the η decreasing from 151mV to 50mVgradually. The 1% PtNP/NiRu-OH, which was loaded identical Pt masswith Pt/NiRu-OH, exhibits tenuous HER activity, further confirming the

highly dispersed Pt atoms on NiRu-OH are more effective.Based on the data above, we propose a possible mechanism for the

electrochemical destruction of NiRu-LDH. Under the OER, the OH-ter-minated NiRu basal plane provides the active centers for oxygen evo-lution. On one hand, a high applied oxidizing voltage is conducive tofrequent adsorption/desorption of oxygenated species on Ru and Niatoms. The emergence of high valence states, such as Ru-O(H)x moietiesbreaks the lattice OH ligands due to a continuously strong nucleophilicattack by OH- ions, thereby facilitating delamination of Ru species fromthe grains. It is also possible for lattice O to participate in the formationof molecular oxygen. In this mechanism, both Ni and Ru species areinvolved in the frequent removal and insertion of lattice O as theoxygen evolves, leading to the surface self-reconstruction or partiallyamorphous state. Furthermore, with the leaching of the Ru species, OH-ions could traverse the Ru vacancies and attack the second layer.Therefore, the Ru leaching and self-reconstruction can occur in the nextlayer. In addition, the Pt immobilization and alkaline HER mechanismare discussed in Supporting Information.

Fig. 3. XPS spectra of (a) Ni 2p, (b) Ru 3d, (c) O 1s for NiRu-LDH, NiRu-OH and Pt/NiRu-OH. (d) XPS spectra of Pt 4f for Pt/NiRu-OH and Pt/C. (e) The k2-weightedEXAFS in Pt K-space for Pt foil, PtO2, and Pt/NiRu-OH. (f) The k2-weighted Fourier transform of EXAFS spectra derived from EXAFS of Pt foil, PtO2 and Pt/NiRu-OH(R is the Radial distance). (g) XANES spectra at the Pt L3-edge of Pt foil, PtO2 and Pt/NiRu-OH.

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3. Conclusion

In summary, our work demonstrates a novel strategy in which Ruleaching under OER conditions can convert crystalline NiRu-LDH intofunctionalized NiRu-OH. The resulting NiRu-OH acts as a capablecandidate to immobilize highly dispersed single Pt atoms and enhancethe catalytic activity. The hybrid catalyst, with NiRu-OH sites designedto accelerate water disassociation and single Pt atom sites to enhancehydrogen recombination, provides highly efficient HER catalysis in al-kaline media. Moreover, Pt/NiRu-OH with low Pt loading exhibitsmuch higher mass activity than commercial Pt/C and superior specificactivity to other Ni-based hydroxides. The simple and scalable strategydescribed in this work opens an alternative path for the synthesis ofother functionalized metal hydroxides and the development of highlyactive catalysts for application in water-splitting, fuel cells and metal-air batteries.

CRediT authorship contribution statement

Dan Li: Methodology, Formal analysis, Writing - original draft.Xufang Chen: Data curation, Methodology, Writing - review & editing.Yaozha Lv:Writing - review & editing. Guoyang Zhang: Methodology.Yu Huang: Methodology. Wei Liu: Methodology. Yang Li: Datacuration, Writing - review & editing. Rongsheng Chen: Formal ana-lysis, Writing - review & editing. Colin Nuckolls: Writing - review &editing. Hongwei Ni: Validation, Supervision, Writing - review &editing.

Declaration of Competing Interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

Acknowledgements

This research was funded by the National Natural ScienceFoundation of China (51471122 and 51604202) and the Key Programof Natural Science Foundation of Hubei Province of China(2015CFA128). The authors also would like to thank the Shiyanjia Lab(www.shiyanjia.com) for the TEM analysis.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in theonline version, at doi:https://doi.org/10.1016/j.apcatb.2020.118824.

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