NiFeCr Hydroxide Holey Nanosheet as Advanced Electrocatalyst for

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NiFeCr Hydroxide Holey Nanosheet as Advanced Electrocatalyst for Water Oxidation Xin Bo, Yibing Li, Rosalie K. Hocking, and Chuan Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12629 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Applied Materials & Interfaces

NiFeCr Hydroxide Holey Nanosheet Electrocatalyst for Water Oxidation

as

Advanced

Xin Bo, † Yibing Li, † Rosalie K. Hocking ‡ and Chuan Zhao*, † †

School of Chemistry, The University of New South Wales, Sydney, NSW, 2052,

Australia ‡

Department of Chemistry and Biotechnology, Faculty of Science, Engineering &

Technology, Swinburne University of Technology, Hawthorn, Melbourne, VIC, 3122, Australia

Corresponding author email: [email protected]

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ABSTRACT: By introducing chromium into a nickel-iron layered double hydroxide (LDH), a nickel iron chromium hydroxide nanomesh catalyst has been achieved on nickel foam substrate via electrodeposition followed by partially etching of chromium. The electrodeposited chromium acts as sacrificial template to introduce holes in the LDH to increase the electrochemically active surface area and the remaining chromium synergistically modulates the electronic structure of the composite. The obtained electrode shows extraordinary performance for oxygen evolution reaction and excellent electrochemical stability. The onset potential of the as-prepared electrode in 1 M KOH is only 1.43 V vs. RHE and the overpotential to achieve a high current density of 100 mA·cm-2 is only 220 mV, outperforming benchmark non-precious NiFe hydroxide composite electrode in alkaline media.

KEYWORDS: oxygen evolution reaction, holey NiFeCr, synergistic effect, electronic structure, electrochemical active surface area

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INTRODUCTION One of the greatest challenges facing the society is to develop the clean energy carrier so as to replace the fossil fuels. Hydrogen gas (H2) is an ideal energy carrier which possesses large energy density (283 kJ·mol-1) and with H2O as the only combustion product. Electrocatalytic water splitting has been established in industry as a simple and efficient method to produce high purity H2, however, large-scale application of water splitting technology is impeded by the sluggish kinetics of oxygen evolution reaction (OER) and the high cost of noble metal based catalyst materials such as iridium and ruthenium oxide.1-3 Significant progress has been achieved in recent years for developing non-precious catalysts based on transition metals such as Ni, Fe, Cr, Mo, Co and their relevant oxides/hydroxides.4-6 Among these noble-metal-free catalysts, two-dimensional (2D) layered double hydroxide (LDH) exhibits advantageous properties for electrocatalysis due to its suitable kinetics and high surface area with a large number of exposed active sites. In particular, NiFe LDHs have attracted significant interest for OER, owing to their easy availability and outstanding OER performance in alkaline media and have been used as the benchmark non-precious OER catalyst.7-9 NiFe LDHs can be considered as Fe3+-incorporated Ni(OH)2 layers with anions intercalated in between. However, the close-packed basal planes can hinder the access to the active sites and subsequently electrochemical conversion to catalytically active high-valence phases, as well as fast mass transport of electrolyte and gaseous products, thus limiting the OER performance. In this regard, creating electrolyte/gas permeable holes in 2D nanostructures would provide an effective but challenging strategy for addressing this issue.10, 11 Other concepts have been also applied to improve the LDH OER catalysts by taking advantages of the synergistic interactions between transition metals. It has been reported that the presence of Cr can greatly improve the electronic conductivity and OER performance of nickel-based LDH catalysts.12 Dong et al have developed cobalt-chromium LDHs OER catalyst and suggested that Co2+ is the catalytic active site and Cr3+ is the charge transfer site to improve the poor electrical conductivity.12 Besides, the complex synergistic effect can also improve the performance. Li’ s group synthesized FeOOH/CeO2 LDH catalysts with a more complex hetero-layered morphology and open tube microstructure, deposited on nickel foam (NF) by using 3



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ZnO arrays as template. Density functional theory (DFT) calculations demonstrated that the strong electronic interactions between CeO2 and FeOOH can lower the energy barriers of the intermediates and products and thus promote the OER catalytic reactions. 13 Herein, we present a facile electrochemical deposition-etching approach to synthesize holely NiFeCr hydroxide composite on nickel foam (denoted as h-NiFeCr/NF). The new material significantly enhanced specific surface area and optimized electronic structure to improve OER performance. The electrodeposited chromium in LDH structure partially dissolves in KOH electrolyte in the following electrochemical etching process, acting as a soft template to create holes on NiFe LDHs nanostructures. Importantly, the remaining chromium in the NiFe LDH synergistically modulates the electronic structure of the NiFe composite, offering further enhancement to the OER activity. Thus, a nanoporous NiFeCr composite catalyst is achieved and outperforms significantly the benchmark NiFe composites as well as noble metal-based IrO2 catalysts. The mechanism of the enhanced OER activity was understood by using an array of techniques including electrochemical analysis, Raman spectroscopy, X-ray photoelectron spectroscopy and X-ray absorption spectroscopy.

RESULTS AND DISCUSSION To prepare the h-NiFeCr/NF composite, Cr was introduced into the LDH by electrodeposition and utilized as a soft-template and doping element. The electrodeposition was carried out under an applied potential of -1.0 V vs Ag/AgCl (1 M KCl) in the electrolytes containing NO3-, which could be reduced into NH4+ and OH- ions and then co-precipitate with the local Ni2+, Fe3+ and Cr3+ ions onto NF substrate (denoted as NiFeCr/NF). Afterwards, the obtained NiFeCr/NF electrode was subjected to electrochemical etching by multiple cyclic voltammetry (CV) scans in the potential range between -0.2 and 0.6 V vs Ag/AgCl (1 M KCl) in 1 M KOH. During this process, partial Cr3+ in LDHs dissolves into the strong alkaline electrolyte (Cr(OH)3 + OH- [Cr(OH)4]-), generating pores to the LDHs, resulting the holey NiFeCr hydroxide composites on NF (denoted as h-NiFeCr/NF). The morphology of the freshly prepared NiFeCr/NF before etching is studied by scanning electron microscope (SEM) and transmission electron microscopy (TEM) in 4


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Figure S1a-c, showing a nanosheet structure. High resolution TEM (HRTEM) (Figure 1a) shows there is no obvious lattice fringe in the nanosheet, indicating the amorphous nature. TEM-EDS in Figure 1b demonstrates the uniform dispersion of Ni, Fe, Cr and O respectively. The EDS mappings (Figure S2) of the composites also demonstrate that the O (17.40 %), Cr (0.61 %), Ni (51.50 %) and Fe (0.53 %) element are uniformly dispersed on NF substrate. After the etching process, it is found the surface of the h-NiFeCr/NF electrode was further roughened (Figure S1d, e). Many holes with diameter around 5 nm were formed on the nanosheet structure (Figure 1c, Figure S1f). The formation of nanopores is accompanied by a significant decrease in the EDS signal of chromium (Figure S3) from 0.61 % to 0.03 %, suggesting Cr was the sacrificial template for the generated pores. These newly formed pores offer enlarged specific surface area and easy access to the exposed active sites, as well as fast mass transport, leading to the improved OER activity. The pore-size distribution was evaluated by Brunauer-Emmett-Teller (BET) testing shown in Figure S4, indicating the formation of pores with a size distribution from ~2 nm to ~10 nm, which

is

close

to

the

pore

size

observed

from

TEM.

The

inserted

adsorption-desorption curve shows a typical II isotherm and the calculated specific surface area of the deposited catalyst on NF is 3.817 m2/g (by considering of mess distribution with NF substrate). TEM-EDS signal of h-NiFeCr/NF in Figure 1d also shows that trace amount of chromium is anchored in h-NiFeCr/NF after the etching process. To study the surface chemical composition and oxidation state of the NiFeCr LDHs before and after the electrochemical etching process, X-ray photoelectron spectroscopy (XPS) measurements were carried out in Figure 2. In O1s spectroscopy (Figure 2a), three simulated peaks appearing at ~532.6, ~531.8 and ~528.9 eV are identified as O in H2O(l), M-OH and M-O, respectively.14 After electrochemical etching, all the peaks shift negatively. Of note, by simulation the fitted peak area, the relative intensity ratio of peak H2O(l) to peak M-OH decreased from 0.316 (before) to 0.141 (after etching), indicating the H2O amount reduced after etching process. This state of H2O is considered to be stored between the layers of the α-Ni(OH)2, whose layer distance is ≥ 8 Å. As the material is activated in alkaline under CV cycling, α-Ni(OH)2 is aged into β-NiOOH, whose layer distance is further decreased to 4.8 Å, leading to the extrusion of H2O between the layers and thus less amount of H2O than 5



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that of the non-activated sample is detected by XPS.15 For the M-O simulation, the relative intensity ratio of fitted peak M-O to peak M-OH before (0.075) and after etching (0.076) maintains between the closed values. Ni2p XPS spectroscopy in Figure 2b shows characteristic peaks at ~856.6 eV with a satellite peak at ~862.4 eV of Ni2p3/2, indicating the oxidation state of nickel is Ni2+/3+.14 The weak signal at ~852.5 eV is assigned to Ni0, which is from NF substrate. It is interesting to notice that after CV etching the Ni2+/3+ peaks shift to lower binding energy, indicating the binding energy of Ni-O is weakened and oxidization valence of Ni is lowered.15 This is quite different from previously reported Ni-based OER catalyst, where high oxidation valence and the shift to higher binding energy are usually observed.16 However, the lower oxidation valence state of the Ni sites (

NiFeCr Hydroxide Holey Nanosheet as Advanced Electrocatalyst for

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