Selenide-Based Electrocatalysts and Scaffolds for Water Oxidation Applications

TLDR: Selenide-based electrocatalysts and scaffolds on carbon cloth are successfully fabricated and demonstrated for enhanced water oxidation applications, suggesting the potential of these materials to serve as advanced oxygen evolution reaction catalysts.

Abstract: Selenide-based electrocatalysts and scaffolds on carbon cloth are successfully fabricated and demonstrated for enhanced water oxidation applications. A max-imum current density of 97.5 mA cm(-2) at an overpotential of a mere 300 mV and a small Tafel slope of 77 mV dec(-1) are achieved, suggesting the potential of these materials to serve as advanced oxygen evolution reaction catalysts.

Figures

Figure 4. Microstructure characterization of (Ni, Co)0.85Se. (A-B) The typical TEM and highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images for as-prepared (Ni, Co)0.85Se nanotube arrays on CFC. (C-I) The aberration-corrected HRTEM images of as-obtained (Ni, Co)0.85Se nanotubes with the corresponding FFT images, which were obtained at or near the surface.

Figure 5. Detailed XPS analysis of as-prepared selenides. High resolution XPS spectra for (A) Ni 2p, (B) Co 2p and (C) Se 3d/Co 3p and peak fitting analysis of as-prepared (Ni, Co)0.85Se. The O 1s region of XPS spectra of as-obtained (D) (Ni, Co)0.85Se and (E) Co0.85Se. (F) Summary of the relative content from the O Ⅰ to O Ⅳ sub-bands in (D) and (E).

Figure 1. Schematic illustration of the design of (A, B) porous nickel cobalt selenide electrocatalyst on carbon cloth and (A, C) coaxial hybrid catalyst of (Ni, Co)0.85Se@hydroxides.

Figure 6. TEM images and electrochemical behavior of coaxial hybrid catalyst. (A-B) The typical SEM and TEM images of fresh (Ni, Co)0.85Se@NiCo-LDH coaxial nanotubes. (C) Comparison of the ECSA (Cdl) and corresponding Rf of all studied electrode. (C) Polarization curves of (Ni, Co)0.85Se and (Ni, Co)0.85Se@NiCo-LDH with corresponding (E) Tafel curves. (F) Chronopotentiometric measurements at J = 10 mA cm -2 for as-prepared selenides, along with that of CFC as control.

Figure 2. Structural characterization of as-obtained (Ni, Co)0.85Se. (A) The representative XRD pattern for as-prepared (Ni, Co)0.85Se nanotube arrays on CFC. (B) Temperature dependence of resistivity of the (Ni, Co)0.85Se and Co0.85Se pellet compressed from synthetic selenide nanopowders. (C-D) The typical SEM images for as-prepared (Ni, Co)0.85Se nanotube arrays on CFC.

Figure 3. OER activities and stabilities of selenide materials. Electrochemical properties of different selenides deposited directly on carbon fiber (CFC). (A) IR-corrected LSV curves of Co0.85Se, (Ni, Co)0.85Se and CFC substrate and (B) the corresponding Tafel plots. (C) Comparison of the double-layer capacitance (ECSA) and corresponding Rf of each electrode. (D) Long-term stability measurements at J = 10 mA cm -2 for all studied samples.

Full text

Selenide-Based Electrocatalysts and
Scaffolds for Water Oxidation Applications
Item Type Article
Authors Xia, Chuan; Jiang, Qiu; Zhao, Chao; Hedhili, Mohamed N.;
Alshareef, Husam N.
Citation Selenide-Based Electrocatalysts and Scaffolds for Water
Oxidation Applications 2015:n/a Advanced Materials
Eprint version Post-print
DOI 10.1002/adma.201503906
Publisher Wiley
Journal Advanced Materials
Rights This is the peer reviewed version of the following article: Xia,
C., Jiang, Q., Zhao, C., Hedhili, M. N. and Alshareef, H. N. (2015),
Selenide-Based Electrocatalysts and Scaffolds for Water
Oxidation Applications. Adv. Mater., which has been published in
final form at http://doi.wiley.com/10.1002/adma.201503906. This
article may be used for non-commercial purposes in accordance
With Wiley Terms and Conditions for self-archiving.
Download date 10/08/2022 00:56:05
Link to Item http://hdl.handle.net/10754/583824

1
DOI: 10.1002/
Article type: Communication
Selenide-Based Electrocatalysts and Scaffolds for Water Oxidation Applications
Chuan Xia, Qiu Jiang, Chao Zhao, Mohamed N. Hedhili, Husam N. Alshareef*
C. Xia, Q. Jiang, Dr. C. Zhao, Dr. M. N. Hedhili, Prof. H. N. Alshareef
Materials Science and Engineering, King Abdullah University of Science and Technology
(KAUST), Thuwal 23955-6900, Saudi Arabia
*E-mail: husam.alshareef@kaust.edu.sa
Keywords: selenides, oxygen evolution, metallic scaffolds, hybrid catalyst
Climate change and the expected shortage of fossil fuels have made it essential that
renewable energies such as solar and wind are developed.
[1]
Electrochemical water splitting
(2H
2
O→2H
2
+ O
2
) provides a promising option to convert solar energy into chemical fuels,
namely hydrogen and oxygen.
[2, 3]
Yet, the realization of efficient water oxidation reactions is
greatly hindered by the bottleneck oxygen evolution reaction (OER).
[4]
This is because OER
proceeds through a multistep proton-coupled electron transfer process that is kinetically
sluggish.
[5, 6]
A critical requirement for enabling the OER reaction to proceed efficiently is the
development of an appropriate electrocatalyst. While the state-of-the-art precious metal RuO
2
or
IrO
2
based catalysts are well developed and widely used, a substantial overpotential (η) is still
required to initiate the OER.
[2]
Over the last few decades, extensive efforts have been devoted to
designing and synthesizing efficient, durable, and low-cost alternatives based on earth-abundant

2
3d metals. In particular, currently, cobalt-based OER catalysts have already attracted
considerable attention, sparked by its intrinsic corrosion stability in alkali electrolyte, earth-
abundant nature and rich variable valence states. For instance, Hynn et al. reported that a
nanostructured Co(PO
3
)
2
powder catalyst can provide a catalytic onset overpotential of ~310 mV
vs. RHE and a per-metal turnover frequency of 0.10-0.21 s
-1
at η =440 mV, whose saturation
behavior was observed at a mass loading of >0.6 mg cm
-2
.
[7]
Zou et al. showed that Zn-Co
layered double hydroxide powder was a much more efficient and durable electrocatalyst in
alkaline medium compared with monometallic Co-OH, while the zinc dopant demonstrated as
OER inactive site.
[8]
Furthermore, Li et al. showed that nanostructured nickel substituted
cobaltite spinel (Ni
x
Co
3-x
O
4
) could deliver better OER performance compared to pristine
Co
3
O
4
.
[9]
Up to now, the efficiency of water oxidation is not satisfactory, and we believe that
there is still room to optimize the performance of nanostructured Co-based catalysts for the
following reasons: 1) For most material synthesis techniques, a binder and conductive agent are
usually required; however, inclusion of the binder and/or conductive agent have been
demonstrated to significantly increase the “dead volume” thereby reducing the active material
catalytic performance;
[10, 11]
2) many of the highly active Co-based catalysts including
hydroxides, oxides and chalcogenides are seriously poor conductors; 3) the powder catalyst
reaches its saturated performance at a very low geometrical mass loading, thereby resulting in a
relatively low catalytic current density and efficiency;
[7]
4) recent studies have shown that
integrated hybrid catalysts comprising several components can offer a strongly enhanced OER
catalytic performance.
[12]
To circumvent the above-mentioned disadvantages, several solutions are possible. For
example, one could utilize three-dimensional (3D) charge conducting nanostructures to scaffold

3
the nonconductive OER active materials and to serve as a self-standing current collector itself,
which minimizes the equivalent series resistance (ESR) of such an electrode. In contrast to their
bulk and solid counterparts, 3D nanostructures can significantly facilitate bubble convection
away from the electrode surface, particularly at high current densities. Such behavior prevents
the O
2
bubbles from accumulating and damaging the catalyst, resulting in improved
cyclability.
[13]
Although carbonaceous materials (CNT, graphene) and metals (Au, Ni) have been
investigated as backbone materials for a hybrid catalyst, the limited OER activity (in the case of
carbonaceous materials) or scarcity/cost (in the case of noble metals) make them impractical
choices.
[6, 10, 14]
CoSe
2
has recently been identified as a promising catalyst for water splitting due
to its intrinsic metallicity.
[15]
In this study we have investigated the potential of another intrinsically metallic material,
Co
0.85
Se, in pure and Ni-doped forms, to be used as OER catalyst. The pristine and Ni-doped
Co
0.85
Se attracted our attention for two reasons. First, the metallic nature of Co
0.85
Se, which is in
sharp contrast to the semiconducting nature of other cobalt selenides such as CoSe
[16]
and
Co
9
Se
8
[17]
, made it a potentially useful compound either as electroactive material, or as support
material to nonconductive OER compounds. Secondly, the nickel doping in these cobalt-based
materials resulted in non-stoichiometric compositions with unchanged matrix properties, leading
to higher electrical conductivity.
[18]
Moreover, the Ni doped compositions were found to exhibit
higher defect concentrations(dislocation, twin boundary and stepped surface) which serve as
active sites for the catalysis.
[19]
Therefore, we expected that nanostructured Ni doped cobalt
selenides ((Ni, Co)
0.85
Se, with their outstanding conductivity, corrosion resistance, and promising
OER activity could perform well as advanced OER catalysts, or to serve as excellent backbone
materials for docking insulating OER active materials in hybrid catalysts.

4
Herein, we reported a facile approach to synthesize metallic Co
0.85
Se and (Ni, Co)
0.85
Se
nanotube arrays on carbon fabric collector (CFC). The (Ni, Co)
0.85
Se nanoarrays exhibited higher
OER catalytic activity and better reusability than the undoped Co
0.85
Se. Furthermore, The (Ni,
Co)
0.85
Se nanoarrays performed better in alkaline medium than the industrial RuO
2
and IrO
2
catalyst, and exhibited superior properties compared to previously reported high-performance
Co-based OER catalysts. We show that this remarkable performance enhancement can be
attributed to the (1) unique structure and chemical composition, and (2) abnormally high
concentration of active defect sites in the (Ni, Co)
0.85
Se material system. The fundamental
reasons for this remarkable performance will be discussed in detail in the manuscript. We believe
that the present work provides a valuable route to achieve an inexpensive and efficient OER
electrocatalyst or hybrid catalyst (e.g., in combination with insulating layered double hydroxides).
Morphology- and phase-controllable (Ni, Co)
0.85
Se nanotube arrays were successfully
synthesized directly on carbon fabric collector (CFC) substrate by a simple hydrothermal process
(Figure 1A, B). Briefly, the self-sacrificial template of Ni-Co-precursor nanoarrays were
fabricated using a hydrothermal method. Afterwards, the Ni-Co-precursors were chemically
converted in situ into (Ni, Co)
0.85
Se by 1 M fresh NaHSe selenization, resulting in a uniform
distribution of Ni, Co and Se (supplementary note 6). The electrode fabrication method is
described in the experimental section in detail. The CFC was selected as a convenient, flexible,
low-cost, chemically inert, highly conductive support that has negligible OER activity. The
direct growth of cobalt selenides on CFC substrates gives a convenient, binder-free electrode
preparation technique which offers lower contact resistance between the catalyst and substrate,
hence minimizing the ohmic losses in the system.
[13]
X-ray diffraction (XRD) pattern (Figure 2A)
and inductively coupled plasma optical emission spectroscopy (ICP-OES) support the formation

Frequently asked questions

Q1. What is the effect of the high density of atomic steps at the surface?

In addition, it has been demonstrated that the high density of atomic steps at the surface could promote molecular adsorption due to reduced chemical reaction potential barriers, which is believed to give rise to better OER catalytic behavior. [30] 

Q2. What is the effect of nickel doping on the electrochemical activity of Co0.85Se?

the nickel doping in these cobalt-based materials resulted in non-stoichiometric compositions with unchanged matrix properties, leading to higher electrical conductivity. [18] 

Q3. What is the effect of the abundant vacancies on the surface of the electrode?

the abundant vacancies which result in the formation of extra dangling bonds were commonly recognized as promising feature to reduce the surface adsorption energy and further improve the overall electrocatalytic performance. [41] 

Q4. What is the role of ECSA and Rf in catalytic activity?

Electrochemical active surface area (ECSA) and corresponding roughness factor (Rf) are often primarily responsible for enhanced catalytic activity in nanostructured catalysts. 

Q5. What is the reason for the higher defects density in (Ni, Co)0.85S?

The higher defects density in (Ni, Co)0.85Se is presumably induced by the Ni doping process, a result which can be attributed to difference in physical and chemical properties between nickel and cobalt ions, a phenomenon that has been widely observed. [31] 

Q6. What is the role of the metallic (Ni, Co)0.85 Se nanoarray?

Se nanoarrays not only play a role as OERactive material, but also serve as an advanced 3D scaffold on which hybrid electrocatalysts can be fabricated. 

Q7. How many mV increase in the overpotential required to carry forward an efficient water ?

After a continuous 24 h electrolysis reaction, only a 24 mV increase in the overpotential required to carry forward an efficient water oxidation (at J =10 mA cm -2 ) was observed (from 216 mV to 240 mV). 

Q8. What is the critical requirement for enabling the OER reaction to proceed efficiently?

A critical requirement for enabling the OER reaction to proceed efficiently is the development of an appropriate electrocatalyst. 

Q9. What is the way to stabilize a high index facets?

While high index facets at the surface of crystals are generally expected to be unstable, due to their high surface energy, they can be strongly stabilized in the presence of a significant number of other surface atomic steps. [28, 29] 

Q10. What is the role of surface defects in the performance of the catalyst?

Previous investigations on catalyst systems have suggested that surface defects and lattice strain play a crucial role in the catalyst material performance. [19, 27, 28] 

Q11. How are the surface layers removed during the electrochemical preconditioning process?

these amorphous SeO2 surface layers are removed during the electrochemical preconditioning process the authors typically conduct before LSV measurements in 1 M KOH. 

Q12. What are the main reasons for the remarkable performance enhancement?

The authors show that this remarkable performance enhancement can be attributed to the (1) unique structure and chemical composition, and (2) abnormally high concentration of active defect sites in the (Ni, Co)0.85Se material system. 

Q13. Why did the authors study the surface oxygen state?

To gain more insights into the OER activity, the surface oxygen state was studied because they always function as active sites in the water oxidation reaction. 

Q14. What is the reason for the resistance of the Co0.85Se?

The exceptional resistance is ascribed to the unique active material/CFC electrode design, and to the incorporation of Ni in the hexagonal Co0.85Se lattice. [23] 

Q15. What is the main reason for the remarkable performance enhancement of the synthetic selenides?

The electrical transport properties of the synthetic selenides were evaluated experimentally from the temperature dependence of resistivity (see supporting information for details). 

Q16. What are the defects that can be found on the surface of (Ni, Co)0.8?

These defects include planar extended defects, stacking faults, and twin boundaries which run across the entire nanocrystal surfaces. 

Q17. What are the advantages of (Ni, Co)0.85Seex?

these features of (Ni, Co)0.85Seexhibit clear advantages over several material systems such as layered transition metal dichalcogenides and layered double hydroxide, which tend to be less conductive and more costly to produce. [13] 

Q18. What is the effect of the inverse current density on (Ni, Co)0.85?

If the applied potential increases to 1.59 V vs. RHE, the (Ni, Co)0.85Se shows a high current density of 122 mA cm -2while the pure Co0.85Se only shows a modest OERactivity (25 mA cm -2). 

Q19. What is the oxidation behavior of the CFC substrate?

while the CFC substrate shows a superhydrophobic nature with a contactangle of 161°, their as-prepared selenides catalysts, with or without Ni doping, both show superhydrophilic behavior (Figure S3 and supplementary videos). 

Q20. What is the performance of the (Ni, Co)0.85Se catalyst?

Se catalyst is superior to previously reported high-performance Co-based OER catalysts (Supplementary Table 1), as well as commercially used RuO2 and IrO2 catalysts.