Elsevier

Catalysis Today

Volume 347, 1 May 2020, Pages 63-69
Catalysis Today

Combined effect of nano-structured NiCo2S4 coated hematite photoanodes for efficient photoelectrochemical water oxidation

https://doi.org/10.1016/j.cattod.2018.05.045Get rights and content

Highlights

  • Thiospinel NiCo2S4 electrocatalyst coated on 1-D hematite nanorods for low bias PEC water oxidation.

  • The intrinsic effects of NiCo2S4 on PEC water oxidation is discussed in detail.

  • NiCo2S4 facilitated to reveal the intrinsic photovoltage of hematite, and hastened the water oxidation kinetics.

Abstract

A major challenge in photo electrochemical (PEC) water oxidation is the generation of hydrogen at a low external bias. Herein, thiospinel NiCo2S4 particles were grown on Sn-doped hematite 1D nanorods for low bias PEC water oxidation via a simple hydrothermal method. Upon incorporation of NiCo2S4 over hematite, the photocurrent increased from 0.85 to 1.51 mA cm–2 with a cathodic/negative shift in the onset potential of ∼330 mV. In addition, the intrinsic photovoltage of hematite increased from 180 to 290 mV upon illumination. These collective improvements originated from the inherent electrocatalytic activity of NiCo2S4 in the water oxidation reaction. The core level electrochemical analyses confirmed that the inclusion of NiCo2S4 hastened the charge injection at the electrode/electrolyte interface by suppressing the charge accumulation and recombination, eventually reducing the kinetic over potential.

Introduction

Natural fossil energy resources are depleting very rapidly and global energy requirements have doubled in the last decade. Thus, harvesting energy from various non-polluting and enduring resources such as wind, tides, and sunlight offers an encouraging solution for ample fuel production for the future [1,2]. In particular, the production of hydrogen, an ideal fuel, using artificial photosynthesis under solar irradiation, which is also known as photoelectrochemical (PEC) water oxidation, heralds a new revolution in energy technology [[3], [4], [5], [6]]. Over the past twenty years, semiconducting materials have been extensively investigated to meet the requirements for the bulk production of hydrogen using solar light [7,8]. Unfortunately, no material has been commercialized as yet because of several intrinsic problems associated with such materials and the complex fuel generation reaction. For example, the use of hematite (α-Fe2O3) is advantageous because it is a photoanode with an appropriate band gap for visible-light absorption (1.9–2.2 eV). It is also earth-abundant, non-toxic, stable, and inexpensive [9,10]. Nevertheless, it suffers from some critical disadvantages such as low conductivity (10–14 Ω–1 cm–1), low carrier mobility, short charge-carrier lifetime, and diffusion length, which restrict the solar-to-hydrogen efficiency to a certain limit that is far below the theoretical maximum [11,12].

Several approaches have been proposed to elevate the solar to hydrogen efficiency, including doping, the use of one-dimensional (1D) architectures, co-catalysts, surface passivation layers, and heterojunctions [13]. Notably, from previous reports, the use of 1D architectures like nanorods, nanotubes, and nanosheets is the best approach for fast carrier transport and efficient light trapping [14,15]. Recently, we demonstrated that the spinel nickel ferrite-coated 1D hematite nanorods could be utilized for efficient low bias solar water splitting [16]. Yin et al. recently used a TiO2 1D nanorod array with metal organic framework-derived Co3O4 catalyst for efficient water oxidation [17]. Hence, 1D structures with suitable light absorbing materials are essential for developing appropriate photoelectrodes for water oxidation. However, such architectures address only a few impediments associated with water oxidation; the other important concern with regard to efficient water oxidation is surface kinetics. For the evolution of one hydrogen molecule, four holes must be injected from the photoanode to the electrolyte, which is a thermodynamically uphill reaction [18]. A composite 1D architecture with thin co-catalytic layers is one of the effective ways to resolve this issue [7,19]. Electrocatalytic water oxidation is a well-developed process with several superior catalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) [20]. Combining these well-established electrocatalysts with photoelectrodes is a predominant strategy to reduce the potential barriers in surface kinetics [21]. It has been proven that electrocatalysts promote the water oxidation/reduction reaction by consuming the holes/electrons at the interface [22]. Usually, a very thin layer of catalyst over the photoanodes is preferred since they act as a tunneling point for holes from the space charge region to the electrolyte, which increases the charge injection and separation efficiency [23]. Noble metals and oxides such as Pt, IrO2, and RuO2 are the most efficient catalysts, but their usage is nearly impossible for large-scale production. Hence, transition metal oxides and sulfides are the preferred candidates for both electrocatalytic and photocatalytic water oxidation. Thiospinel NiCo2S4 (NCS), is one of the most famous electrocatalyst to have been adapted for energy storage and conversion applications [24,25]. Zhang et al. used NCS nanoneedles over reduced graphene oxide to demonstrate the overall electrocatalytic potential of NCS in alkaline electrolyte [26]. Shanmugam et al. recently demonstrated the long term stability of NCS in the OER and HER at low over potentials of 260 and 210 mV, respectively [27]. They used a 1D NCS nanowire array supported on nickel foam as an electrocatalyst in a strong alkaline electrolyte. It is evident from these results that the NCS thiospinel is very stable in alkaline conditions and exhibits high activity in the OER and HER.

Herein, we were able to successfully grow the NCS particles on 1D tin-doped hematite (Sn-Fe2O3) nanorods using a series of wet chemical methods. Unlike band edge aligned heterojunctions in PEC water splitting, the metallic NCS simply acted as a surface catalytic layer and did not contribute to light absorption. This is the first time that metallic NCS particles are combined with hematite nanostructures for PEC water oxidation. This structure exhibited a photocurrent density of 1.51 mA cm–2 at 1.23 VRHE in 1 M KOH electrolyte solution under AM 1.5 G illumination. Hereafter all potentials were mentioned with respect to reversible hydrogen electrode (RHE) unless notified. Additionally, a significant onset potential shift of 330 mV (100 μA photocurrent is considered as the onset) is evident when compared to bare hematite nanorods. Core level electrochemical analysis were conducted to explain the superior catalytic activity of NCS.

Section snippets

Preparation of photoanodes

In the first step, FeOOH nanorods were grown on fluorine-doped tin oxide coated glass (FTO) following a previously reported method [28]. Briefly, 1.8 g of FeCl3.6H2O and 3.4 g of NaNO3 were dissolved in 40 mL of distilled water and buffered to 1.5 pH using hydrochloric acid. The solution was then transferred to a Teflon-lined autoclave with a piece of cleaned FTO glass substrate and maintained at 100 °C for 6 h. The FeOOH thus formed was thoroughly cleaned with ethanol and deionized water

Physical and structural analyses

X-ray diffraction (XRD) analysis was carried out to confirm the phase purity of hematite and detect the presence of NCS, and the results are shown in Fig. S1 (Supporting Information). The three main peaks in the spectrum of bare Sn-Fe2O3 at angles 33.84, 35.75, and 49.61° corresponded to the crystal planes (104), (110), and (024) of the pure hematite phase (PDF 33-0664) [10]. The (110) orientation was dominant in the XRD spectra and it was evident that the nanorods were well aligned in one

Results and discussion

The J–V curves of the Sn-Fe2O3 and Sn-Fe2O3/NCS photoanodes are shown in Fig. 4(a). Initially, the singly doped (Sn4+) Fe2O3 showed a photocurrent of 0.85 mA cm–2 at 1.23 V, as reported previously [36]. An increased conductivity was observed because of the gradient doping of Sn4+ ions as well-aligned 1D architectures provided abridged pathways for the carriers [37,38]. The doping level of Sn into the hematite lattice is very crucial for improved PEC performance; an excess removal of the Sn

Conclusion

In summary, we attempted to fuse the well-known electrocatalyst NiCo2S4 on 1D Sn-doped hematite nanorods to accelerate the sluggish solar water oxidation reaction at a lower bias voltage. The photocurrent increased by two times to a value of 1.51 mA cm–2 at 1.23 Vwith the expected cathodic shift in onset potential of about 330 mV when compared to that of bare hematite. The combined effect of the 1D structure, gradient Sn doping, and fusion with NiCo2S4 remarkably enhanced the PEC performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgement

This research was supported by the National Research Foundation of Korea (NRF-2015M3A7B 4050 424). We also thank the Korea Basic Science Institute (KBSI) at the Gwangju Center for SEM and TEM analysis.

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