Adsorption and On-site Transformation of Transition Metal Cations on

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Adsorption and On-site Transformation of Transition Metal Cations on Ni-doped AlOOH Nanoflowers for OER Electrocatalysis Yao Zhou, and Hua Chun Zeng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06020 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 16, 2019

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Adsorption and On-site Transformation of Transition Metal Cations on Ni-doped AlOOH Nanoflowers for OER Electrocatalysis Yao Zhou and Hua Chun Zeng* NUS Graduate School for Integrative Sciences and Engineering and Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 KEYWORDS: AlOOH, Transition metal cations, Adsorption, On-site transformation, OER catalysis *Email: [email protected] ABSTRACT: AlOOH has long been used as excellent adsorbent for removal of heavy metal cations from wastewater. Herein we report one-pot synthesis of carboxylic-functionalized Ni-doped AlOOH nanoflowers (AlOOH NFs) with high adsorptive capability toward various transition metal cations, and more importantly, the AlOOH NFs adsorbed with various transition metal cations were for the first time directly used as electrocatalyst for oxygen evolution reaction (OER). By simply manipulating the initial metal cation concentration and their molar ratio, the OER catalytic performance of the resulting catalysts could be modulated. The lowest overpotential at a current density of 10 mA cm-2 prepared from AlOOH NFs adsorbed with FeIII and NiII is 0.32 V in 0.1 M KOH and 0.275 V in 1.0 M KOH. Such AlOOH-supported electrocatalyst demonstrates remarkable stability, which shows no evident increase of the overpotential at 10 mA cm-2 after 2 h of steady electrolysis at an overpotential of 0.42 V. The excellent OER electrocatalytic activity originates from the on-site formation of ultrafine FeOOH and NiOOH nanoclusters with average sizes below 3 nm during the electrocatalytic process. As such, we demonstrate the workability of using functionalized AlOOH NFs as a bi-functional platform for adsorption of transition metal cations and easy preparation of efficient and cost-effective OER catalysts.

Introduction Adsorption of transition metal cations is one of the most important purification processes.1,2 Among various adsorbents, nanostructured AlOOH finds important applications for removal of transition metal cations from different water systems due to their low cost, large specific surface area and effectiveness in adsorption. Toward this end, various AlOOH-based composites have been devised and investigated,3,4 such as nanocomposites of AlOOH-reduced graphene oxide,5 Fe2O3-AlOOH fibers,6 -AlOOH@SiO2@Fe3O4 porous microspheres,7 and AlOOH superstructures.8 In addition to structural engineering and composite formulation, surface functionalization of AlOOH micro- or nanostructures with organic acids such as citrate9 or humus complex10 represents another strategy to modulate the interface interaction and to further improve their adsorptive capability. While their adsorption processes have been well studied, nevertheless, post treatment of the as-formed composites from the adsorption process has been rarely tapped in this type of studies. In some circumstances, AlOOH-based nanocomposites were employed for adsorption and preconcentration of metal cations in solution to construct

highly sensitive and selective electrochemical detection of various toxic metal cations.5,7 Meanwhile, the development of efficient oxygen evolution reaction (OER) electrocatalysts for water splitting is of significant importance toward clean and renewable hydrogen-based energy.11,12,13 Precious metals such as Ru, Ir and their metal oxides, though they are highly active,14,15 have limited applicability due to their high cost.16 In recent years, various solid state materials containing three transition metal cations, Fe, Co, and Ni, are found to demonstrate intrinsically excellent activity toward OER in alkaline conditions.13,17 In particular, Fe-Ni layered double hydroxides (LDHs) equipped with bimetal sites have been reported as the most promising OER catalysts so far.18,19 Toward the development of high-performance OER catalysts, various strategies based on structural engineering and/or chemical composition optimizations have been explored to modulate the local electronic property and enhance the conductivity and activity of these electrocatalysts.20-22 Recently, the relevant mechanistic study revealed that FeIII species adsorbed on the surface (most likely the edge and defect sites) of the Ni-based catalysts could promote the oxidation of NiII species into highly

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active NiIII/NiIV species, which are responsible for its enhanced OER catalytic activity. And during the OER electrocatalysis MOOH species (M = Fe, Co and Ni) are observed as the genuine active OER species in multiple systems and also explored as highly active OER electrocatalyts in last three years.23-25 For instance, a recent work revealed that ultrafine FeOOH nanoparticles could interact intimately with metal hydroxides through oxygen bridges and those LDH-supported ultrafine FeOOH nanoparticles demonstrate a strong size-dependent effect on OER electrocatalysis.26 It was also reported that one of the most abundant elements on earth, Al, might be able to promote the catalytic performance of Ni/Fe-based electrocatalysts toward OER, as the inverse spinel oxide such as NiFeAlO4 was identified as a novel OER electrocatalyst with low overpotential.27,28 Herein, we report a one-pot synthesis of Ni-doped, carboxylic acid-functionalized AlOOH nanoflowers (namely AlOOH NFs) which demonstrate a high adsorptive capacity toward various transition metal cations. Moreover, the AlOOH NFs have a large specific surface area and are able to adsorb various transition metal cations. For the first time, the resultant metal-adsorbed AlOOH NFs were explored directly as pre-catalysts for OER electrocatalysis. We also demonstrate that the metal cations adsorbed on the AlOOH NFs were transformed into ultrafine MOOH (M = Fe, Co, Ni) nanoclusters via an on-site manner with remarkable OER catalytic activity. As such, our Ni-doped AlOOH NFs serve as a bifunctional platform which enables effective adsorption of metal cations and facile preparation of cost-effective high-performance OER catalysts.

Experimental Section Materials and apparatuses. Polyvinylpyrrolidone (k30, PVP), FeCl36H2O (>98.0%), Co(NO3)26H2O (≥98%), Ce(NO3)36H2O (99%), Cr(NO3)39H2O (99%) and KOH (99.99%) were from Sigma Aldrich; Ni(NO3)26H2O (99.0%), Al(NO3)39H2O (≥95%), concentrated HNO3 (65%), ethanol (analytic grade), acetone (analytic grade) were from Merck; carbon black was from Nacalai Tesque Inc; perfluorosulfonic acid-PTFE copolymer (Nafion; 5% w/w) was from Alfa Aesar; deionized water was collected through the Elga MicroMeg purified water system. One-pot synthesis of Ni-doped AlOOH nanoflowers (AlOOH NFs). 260 mg of Al(NO3)3.9H2O, 260 mg of PVP was dissolved in the mixture of 10.0 mL of ethanol, 10.0 mL of H2O and 5.0 mL of acetone, followed by addition of 0.15 mL of Ni(NO3)2 (0.5 M). The mixture was transferred to a Teflon-lined stainless steel autoclave which was then kept at 180oC for 24 h. Thus-resulting precipitate was washed twice with ethanol and dried for further use. Adsorption of transition metal cations. The AlOOH NFs were well dispersed in the aqueous solution with different initial concentrations (C0) of different transition metal cations, which was then magnetically stirred over-

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night. Afterward, the mixture was centrifuged to obtain the supernatant solution from which the adsorbed metal cations could be detected using ICP-OES. The precipitates, i.e., the AlOOH NFs adsorbed with the relevant transition metal cations were dried in vacuum and used directly as electrocatalysts for OER. Note that the specific condition and different C0 of the transition metal cations in different cases are shown in detail in Supporting Information. OER catalyzed by AlOOH NFs adsorbed with transition metal cations. To prepare the working electrode, 3.0 mg of carbon black and 3.0−5.0 mg of the catalyst was dispersed in 0.5−1.0 mL of Nafion solution (which was prepared by mixing 5.0 mL of H2O, 5.0 mL of ethanol and 0.5 mL of 5% Nafion solution); the mixture was sonicated to yield a homogeneous thick ink. Then 2.0−4.0 μL of the prepared ink was drop-cast on the polished glass carbon working electrode (GCE), followed by natural drying in laboratory air. The catalyst loadings in each cases were also specified in Supporting Information in detail. Electrochemical measurements were recorded using a computer-controlled potentiostat (Autolab, PGSTAT 302N) with a standard-three-electrode configuration. The counter and reference electrodes were Pt gauze and Ag/AgCl with 3.0 M KCl, respectively. Unless otherwise specified, cyclic voltammograms (CVs) were obtained with a scan rate of 50 mV/s and linear sweep voltammograms (LSVs) of different catalyst-loaded GCE electrodes were obtained with a scan rate of 10 mV/s in a freshly prepared 0.10 M KOH solution. In these measurements, the working electrode was firstly cycled 10 times to obtain a stable response before experimental data were recorded. Unless otherwise specified, all of the potentials in this study were iR-compensated and referenced to the reversible hydrogen electrode (RHE). To evaluate the double layer capacitance (CDL) of the catalysts, cyclic voltammograms were measured in a non-Faradaic region with different scan rates, and then the cathodic charging currents at the open-circuit potential was plotted linearly versus the scan rate to yield the slope which is the relevant CDL. To test the catalyst stability for the OER, GCE was loaded with the as-prepared catalysts with a loading of 0.5 mg cm-2. These catalyst-modified GCE, after the 10 cycles of CV scan, was subjected to chronoamperometry analysis at the overpotential of 0.42 V in 0.10 M KOH for 2 h. CV curves and LSV curves were obtained and compared before and after the catalyst stability tests. Afterward, the spent catalyst was also collected and further characterized. Characterization methods. Morphologies of catalysts were observed by transmission electron microscopy (TEM, JEM-2010, 200 kV), high resolution TEM (HRTEM, JEM2100F, 200 kV), and field emission-scanning electron microscopy (FESEM, JSM-6700F). The structural information of the organic on the AlOOH NFs was also gathered by Fourier transformed infrared spectroscopy (FTIR, Bio-Rad FTS-3500ARX). Crystallographic information was


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obtained with powder X-ray diffraction (PXRD, D8 Advanced, Bruker, Cu Kα radiation at 1.5406 Å). Surface analysis with X-ray photoelectron spectroscopy (XPS, AXIS-HSi, Kratos Analytical) was conducted using a monochromatized Al Kα exciting radiation (hν = 1286.71 eV) with a constant analyzer-pass-energy of 40.0 eV. All binding energies (BEs) were referenced to C 1s peak (its BE was set at 284.5 eV) arising from C−C bonds. The compositional analysis was also performed with energy dispersive X-ray spectroscopy (EDX) and the elemental data was an average result from six different sampling areas. N2 adsorption−desorption experiments were performed using a surface area and pore size analyzer (NOVA 4200e, Quantachrome Instruments) at 77.3 K after overnight degassing in ﬂowing N2 at 150°C. Contents of metal cations were measured using inductively coupled plasma optical emission spectrometry (ICP-OES, Thermo Scientific). To evaluate the mass fraction of the organics, Thermogravimetric analysis (TGA, DTG-60AH, Shimadzu) for the studied samples was done in air with a gas flow rate of 50 mL/min and a heating rate of 10oC/min.

0.385 mL/g at P/P0 = 0.975. A weak hysteresis loop is found in the N2 adsorption-desorption isotherms (Figure 2c), and accordingly in the pore size distribution curve, a peak is found centered at 3.7 nm (Figure 2d), proving the existence of mesopores in the AlOOH NFs. The formation of AlOOH NFs is further supported by our XPS analysis. The signal of O 1s (Figure 2f) could be deconvoluted into three peaks; among them, the one at 530.0 eV could be assigned to −O− in AlOOH and the one at 531.4 eV is from −OH groups.29 Though with relatively weak intensity, the Ni 2p3/2 spectrum displays a well-defined peak at 856.0 eV, confirming the presence of NiII in the sample (Figure 2g).

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Results and Discussion Synthesis of Ni-doped AlOOH nanoflower. The formation of the Ni-doped AlOOH nanoflower (or namely, AlOOH NFs hereafter) was examined by FESEM and TEM methods in Figure 1. The average size of the NFs is ca. 600 nm. Such nanoflowers are constructed by aggregating numerous nanosheets into a radially projecting assemblage. Most of the nanosheets are assembled into such flowerlike morphology with high structural integrity, and there are only few less-aggregated nanosheets in the sample, as observed in Figure 1b,c. The individual nanosheets are very thin which have almost the same image contrast as the background at a high magnification (Figure S1). Besides, the nanosheets are found to be porous, as the heterogeneous contrast is observed across each nanosheet (Figure 1h and Figure S1). Due to the presence of NiII, the colloid of AlOOH NFs shows light green in color. Element mappings reveal homogeneous distribution of Al and O within the sample, whereas the signal from the element of Ni seems to be stronger in the interior than that at the edge within the nanoflower (Figure 2a), and the molar ratio of Ni to Al is 4.9%. Furthermore, as labeled in Figure 2b, the major peaks of its XRD pattern match well with that of the -AlOOH (PDF card #21-1307), except the peak at 18.5o which could be assigned to Al(OH)3 impurity (the purple curve). No signal of Ni-relevant crystals such as Ni(OH)2 or Ni(OOH)2 is found in the XRD pattern. The XRD pattern of the sample after calcination at 550 oC in air reveals the signals of -Al2O3 phase (Figure 2b, the green curve). Such results render direct evidence to the formation of Nidoped AlOOH crystals. The Brunauer-Emmett-Teller (BET) method was applied to the N2 adsorption isotherm of the AlOOH NFs (Figure 2c), which yields a specific surface area of 112 m2/g, with the pore volume obtained as

500 nm

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Figure 1. (a-d) FESEM and (e-h) TEM images of the Nidoped AlOOH nanoflowers (namely AlOOH NFs).

In the synthesis of such AlOOH NFs, the two metal precursors, Al(NO3)3 and Ni(NO3)2, hydrolyzed at the natural pH condition as no alkali was added. The solvent which was a mixture of ethanol, water, PVP and acetone played an important role in regulating the morphology of the



AlOOH NFs. A control experiment revealed that, such AlOOH NFs could not be formed in the absence of PVP (Figure S2a) or acetone (Figure S2b) or without sufficient ethanol (Figure S2c). It shall also be mentioned that the presence of NiII precursor was found to play an important role for the self-assembly of these nanosheets into a three dimensional flower. As displayed in Figure S2d,e,f, our control experiment revealed that without the addition of the NiII precursor, fusiform aggregates of AlOOH nanosheets were produced. Additionally, doping the boehmite or alumina with transition metal ions is believed to be an effective strategy to modify the catalytic activities of alumina.9,30 Therefore, our current one-pot hydrothermal process provides a facile route to prepare Ni-doped -Al2O3 heterogeneous nanocatalysts.

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PVP was slightly oxidized, generating carboxylic groups on the surface of the nanosheets.31 Accordingly, the XPS survey spectrum confirms the presence of C, N, O species in the sample (Figure S3). The deconvolution of highresolution C 1s XPS spectrum exhibited three components, among which the one at 286.1 eV originates from C-OH species and the one at 288.1 eV is assigned to C=O groups (Figure 2e). Consistently, a component at 532.6 eV could be identified for the O 1s XPS spectrum, which belongs to the C=O groups (Figure 2f). Shown in Figure 3a, for the as-formed AlOOH NFs, a strong band is present at 1712 cm-1 which is attributed to the C=O group in the carboxylic acid, and there is a shoulder peak at 1650 cm-1 which could be assigned to the C=O group in the amide. For comparison, the C=O bond in the amide groups of pure PVP yields a strong band centered at 1670 cm-1. The sharp peak at 1070 cm-1 is assigned to the −OH group in the AlOOH.5 In addition, the TGA measurement revealed that the organics in the sample decompose within 300−400oC and account for around 10% of the total weight (Figure 3b and Equation S1). Note that the samples for analysis were washed twice with ethanol. Adsorption of heavy metal cations using -AlOOH with different structures and/or surface properties have been well investigated.8 The carboxylic groups anchored on the AlOOH NFs could coordinate with the heavy metal cations, facilitating the adsorption process.9,10 Herein, the adsorption isotherms for four typical transition metal cations including CoII, NiII, CrIII and FeIII were obtained by measuring the amount of the metal cation adsorbed by per gram of the adsorbent (i.e., Cad. with a unit of g/g) and the cation concentration at equilibrium state (Ceq., with a unit of M) under different initial concentrations (C0) of metal cations, as shown in Figure 3c-f. Note that all the adsorption experiments were conducted without modifying the pH value of the solution. The adsorption isotherms for these metal cations were interpreted by different models. For both CoII and NiII cases, a linear trend is observed between their Cad. and Ceq., and the AlOOH NFs demonstrate stronger adsorption capacity toward NiII than CoII. The adsorption isotherm of FeIII is best correlated by Freundlich model, while that of CrIII is best fitted by Langmuir model.

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Figure 2. Characterizations of the AlOOH NFs: (a) Elemental mapping; (b) XRD spectra of the sample before and after calcination; (c) N2 adsorption-desorption isotherms and (d) Volumetric pore size distribution; and XPS spectra of (e) C 1s, (f) O 1s and (g) Ni 2p.

Adsorption of transition metal cations. The performance of our AlOOH NFs in the adsorption of metal cations was evaluated. Prior to this, their surface properties were further characterized. As they were produced with PVP under hydrothermal condition, the amide bond of


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report the application of M-AlOOH NFs (M = Fe, Co, and Ni) in the field of OER.

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Figure 3. (a) FTIR spectra of the as-prepared AlOOH NFs and the commercial PVP, (b) the TGA curve of the AlOOH NFs, the adsorption isotherms of different transition metal cations including (c) Co(II), (d) Ni(II), (e) Fe(III) and (f) Cr(III). The dots are the experimental data and the red lines are the calculated data. The fitting models used in each case are shown directly in each plot.

It should be noted that, in most of the aforementioned relevant works in literature, their initial concentrations of the metal cations were usually low, e.g., below 0.5 g/L, and their maximum adsorption capacities often fell within hundreds of milligram of adsorbate per gram of adsorbent,5,8,10,29 which is evidently lower than that presented by our AlOOH NFs. The excellent adsorption capacity of AlOOH NFs should be attributed to their relatively large specific surface area and the extensive presence of carboxylic acids on their surface, in addition to the inherently strong affinity between the boehmite and transition metal cations. With such high adsorption capacity, our AlOOH NFs can act as an excellent adsorbent for removal of heavy metal ions from water. As shown in Figure 3c-f, by manipulating the initial cation concentration C0, the amount of the corresponding metal cations adsorbed on the AlOOH NFs could be adjusted accordingly. Furthermore, multiple transition metal cations could be added into the matrix solution, which could be adsorbed on the AlOOH NFs simultaneously. In this regard, these transition-metal loaded AlOOH NFs can serve flexibly as solid precursors for making functional materials, because their composition can be easily tuned. In the following, we will

OER electrocatalysis by AlOOH NFs adsorbed with transition metal cations. Transition metal cations including FeIII, CoII, NiII have been intensively investigated for their low cost and excellent OER catalytic activity.11,32 It has been reported that only the FeIII species adsorbed on the surface of NiII-based OER catalysts (that is, not those in the bulk phase) are responsible for the enhanced OER catalytic activity. Through the above general adsorption process, we have demonstrated that the transition metal cations were in fact concentrated and immobilized on the surfaces of the nanosheets of those AlOOH NFs. On this basis, therefore, our AlOOH NFs were directly attempted for OER electrocatalysts in alkaline conditions. As detailed in Table S1, a series of electrocatalysts were prepared using matrix solutions with different initial concentrations (i.e., C0) of FeIII and NiII. Shown in Figure 4a, the bare AlOOH NFs themselves are not effective for OER catalysis, which requires very high overpotential to achieve a current density of 10 mA cm-2. The amount of the metal cations adsorbed on the AlOOH NFs, which depends on C0 of the relevant metal cations, significantly affects the resulting catalytic activity. Shown in Figure 4a, the overpotential to achieve 10 mA cm-2 for the catalyst prepared with low C0 which has a total mass fraction of FeIII and NiII as 18.1% is 60.0 mV larger than that with a total mass loading of 24.4% (Figure 4a,d). The comparison of their CV scans shows that the anodic and cathodic peaks of the case prepared with high C0 displayed much stronger current density (Figure 4b); it is also found to have a much larger double layer capacitance (and thus larger electrochemical active surface area, Figure 4c). Nevertheless, when the adsorption sites on the AlOOH are saturated, further increasing C0 of the metal cations in the matrix solution will not improve the OER catalytic activity of the resulting catalysts. The catalytic activity was found to be influenced by the molar ratio of FeIII to NiII. Shown in Figure 4d, for the afore-discussed two catalysts (i.e., the one prepared with low C0 and the other one prepared with high C0), the loading of FeIII is much lower than that of NiII, as they were prepared with an initial concentration of NiII (Co,Ni) which was much higher than that of FeIII (Co,Fe). The catalytic performance could be improved by optimizing the molar ratio of these two metal species. For instance, when prepared in a matrix solution with equal Co,Fe and Co,Ni, the loading of FeIII and NiII in the catalyst accounts for 14.4 wt% and 7.1 wt%, respectively, and the resulting overpotential were decreased slightly (Figure 4a,d). Nevertheless, further increasing the fraction of FeIII in the catalyst, e.g., the catalyst prepared with C0,Fe higher than C0,Ni, does not necessarily enhance its catalytic activity (Figure 4d).



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trocatalysts such as carbon-supported NiyFe1−yOx alloy na-

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noparticles with size less than 5 nm 33 or the hybrid between FeNi layered double hydroxides and carbon nanotubes (Table S2).18

The AlOOH NFs adsorbed with FeIII and NiII also demonstrated remarkable OER catalytic stability. Shown in Figure 4f, the GCE modified with the catalyst was subjected to chronoamperometry analysis for 2 h at an overpotential of 0.42 V. Throughout the analysis the current density was maintained largely at above 16 mA cm-2, and only a slight decrement in the current density was observed at the end (Figure 4f). The comparison of the LSV scans before and after the potentiostatic polarization revealed an ignorable increment of overpotential at 10 mA cm-2, though the anodic oxidation peak of the catalyst was changed (Figure 4g,h).

Figure 4. (a) LSVs, (b) CVs, (c) Calculations of double layer capacitance (CDL) from linear plot of the non-Faradic current (i) as a function of the scan rate (v), (d) Overpotentials at 10 mA cm-2 for OER catalysts with different mass loadings of Fe/Ni, (e) Effects of catalyst loading and KOH concentration, (f) Chronoamperometry analysis (without iR compensation) at an overpotential of 0.42 V in 0.1 M KOH, and comparison of (g) LSVs and (h) CVs for the catalyst before and after the chronoamperometry analysis. Further detailed conditions for these catalysts can be found in Table S1.

Specifically, with an electrocatalyst loading of 0.2 mg cm-2, the lowest overpotential for a current density of 10 mA cm-2 in 0.1 M KOH is obtained as 0.35 V when the catalyst was prepared with Co,Fe and Co,Ni equal to 0.12 M (Figure 4e, the blue curve). Increasing the catalyst loading to 0.5 mg cm-2 (which is translated into a total loading of 0.11 mg cm-2 for FeIII and NiII ions), the overpotential could be further brought down to 0.32 V in 0.1 M KOH (Figure 4e, the black curve), and the overpotential at 10 mA cm-2 was found to be 0.275 V when tested in 1.0 M KOH (Figure 5e, the red curve). Compared with those reported in the literature, the overpotential (0.275 V) in this work is in fact lower than many other transition metal oxides or hydroxides and even some noble metals oxides such as IrOx.13 Actually, it is comparable to many FeNi-based elec-

On-site transformation of transition metal cations on AlOOH NFs during OER electrocatalysis To understand their OER activity, the AlOOH NFs after being immobilized with metal cations through the adsorption process were characterized by various techniques. Shown in Figure 5a, the element mapping confirms affluent presence of Fe and Ni after the adsorption process. Our TEM observation reveals that the overall flower-shaped morphology of the AlOOH NFs is well maintained after the adsorption of transition metal cations (Figure 5b and Figure S4a,b), and at high resolution no evident change on each nanopetals of the AlOOH NFs is identifiable compared with the original AlOOH NFs depicted in Figure 1. XRD pattern of the AlOOH NFs adsorbed with FeIII and NiII was also obtained. No signal of FeIII- or NiII- relevant compounds could be found (Figure 5e, the green curve). In addition, as the OER catalysis was conducted in alkaline solution, the AlOOH NFs adsorbed with FeIII/NiII were dispersed in 0.1 M KOH solution for ten minutes at room temperature to understand the actual reactive components during the OER catalysis. In the thus-treated sample some flocculent-like substances are observed, which might be the relevant hydroxides (Figure 5d and Figure S5c,d,e,f); and XRD analysis of the KOH-treated sample reveals the presence of AlOOH, Al(OH)3 and Al2O3 phases only (Figure 5e, the purple curve). Such results indicate that, except the adsorption process, there was no other chemical reaction between the AlOOH and the adsorbed transition metal cations (that is, no generation of any new solid phase such as Al-Ni LDHs) under such an alkaline condition.


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a 100 nm

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ing and oxygen bridges, similar to the interfacial interaction between the FeOOH nanocrystals and the Ni-Fe LDH nanosheets reported recently which were prepared through hydrothermal synthesis.26 Hence, the straightforward conversion of Fe3+/Ni2+ into FeOOH or NiOOH on the AlOOH verified the intrinsic affinity between these metal (oxy)hydroxides, which offers a novel approach for preparation of transition metal oxyhydroxides. Also, both the single crystalline FeOOH and NiOOH and their composites could be catalytically active components responsible for the OER process. Importantly, consistent with those supported on the LDHs,26 the ultrafine size is an important reason to account for their high OER activity.

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Figure 5. (a) Element mappings and (b,c) TEM images of AlOOH NFs adsorbed with FeIII/NiII, (d) TEM image of AlOOH NFs adsorbed with FeIII/NiII after being treated with 0.1 M KOH, and (e) their XRD patterns of AlOOH NFs adsorbed with FeIII/NiII before and after KOH treatment.

The spent catalyst after 2 h of constant electrolysis was further characterized. Shown in Figure 6a and Figure S5, the overall morphology of the AlOOH NFs essentially remained the same, noting that the particulates on the nanopetals are the conductive carbon black. Our elemental mapping investigation confirms the presence of Fe, Ni, Al and O in the spent catalysts (Figure S6). More importantly, at a high resolution, numerous nanoclusters with average sizes of 2.8 nm are distributed homogeneously on the nanosheets of the AlOOH NFs (Figure 6b,c,d and Figure S7). Such ultrafine nanoclusters were formed on-site during the OER catalytic reaction. Based on their crystal lattice fringes, these nanoclusters could be single-, twinned- or poly-crystalline (Figure 6e and Figure S7). Fast Fourier Transform (FFT) analysis of multiple nanoclusters reveals that the nanocluster could be nanocrystals of either FeOOH or NiOOH, or composites of these two. For example, displayed in Figure 6f is the FFT pattern of a nanocluster framed in Figure 6e, from which two sets of inter-crystal-plane distances could seem to be identified. One includes 1.78 nm, 2.15 nm and 2.02 nm which likely correspond to the orthorhombic FeOOH (PDF card #26-0792), and the other includes 1.88 nm and 2.09 nm which could be attributed to the hexagonal NiOOH (PDF card #06-0075). The AlOOH nanosheets as the substrate were expected to play an important role in inducing the on-site formation and stabilization of such ultrafine FeOOH or NiOOH nanocrystals through extensive hydrogen bond-

10 nm

100 nm

c

d

AlOOH

20 nm

AlOOH

f

e

AlOOH

AlOOH

5 nm

1.78 nm 2.15 nm

5 nm

2.02 nm

1.88 nm 2.09 nm

Figure 6. (a,b) TEM and (c,d,e) HRTEM images of AlOOH NFs adsorbed with FeIII/NiII after constant electrolysis at η = 0.42 V for 2 h in 0.1 M KOH; (f) FFT pattern of the nanoparticle circled in (e).

Conclusions In summary, through an one-pot hydrothermal approach carboxylic-functionalized Ni-doped AlOOH nanoflowers (namely AlOOH NFs) are synthesized which possess high adsorptive capability toward various transition metal cations; moreover, for the first time the AlOOH NFs adsorbed with various transition metal cations were employed as electrocatalysts for oxygen evolution reaction (OER). The amount and the species of metal cations adsorbed on the AlOOH NFs could be controlled by



simply adjusting the initial concentration and the composition of the matrix solution used for adsorption, and their OER catalytic performances could vary accordingly. The lowest overpotential at the current density of 10 mA cm-2 achieved by AlOOH NFs adsorbed with FeIII and NiII ions was 0.32 V in 0.1 M KOH and 0.275 V in 1.0 M KOH; it also exhibited remarkable working stability. During the OER process, the adsorbed FeIII/NiII cations were transformed into ultrafine FeOOH or NiOOH nanoclusters with average of 2.8 nm, which are responsible for the high OER activity of the composite. As such, transition metal cations concentrated and supported on the AlOOH NFs via a simple adsorption process is proven in this work to be a facile and novel approach to construction of efficient and cost-effective OER catalysts. Along the same line, future investigations can be directed to addressing how to make use of heavy metal ions removed from wastewater treatment for the fabrication of OER catalysts in a sustainable manner. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACKNOWLEDGMENT Y.Z. would like to thank National University of Singapore (NUS) for providing her postgraduate scholarship. The authors gratefully acknowledge the financial support provided by the Ministry of Education, Singapore, NUS, and GSK Singapore. This project is also partially funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program.

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O2 O2

O2

adsorption

O2 O2

on-site conversion O2 O2

Mn+/AlOOH

MOOH/AlOOH

Removal of transition metal cations from water has been explored together with making efficient oxygen evolution reaction electrocatalysts for water splitting.


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Adsorption and On-site Transformation of Transition Metal Cations on

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