A highly efficient α-Fe2O3/NiFe(OH)x photoelectrode for photocathodic protection of 304 stainless steel under visible light

https://doi.org/10.1016/j.surfcoat.2020.126407Get rights and content

Highlights

  • The hematite electrode could provide photocathodic protection for 304 stainless steel under visible light illumination.

  • The α-Fe2O3/NiFe(OH)x photoelectrode exhibited better photocathodic protection performance than α-Fe2O3 electrode.

  • The NiFe(OH)x cocatalyst can accelerate the water oxidation kinetics.

  • No hole scavengers are needed during the photocathodic protection process.

Abstract

Developing highly stable and broad spectrum responsive semiconductor photoelectrode materials without the use of hole scavengers is challenging for photoelectrochemical protection application. In this study, α-Fe2O3/NiFe(OH)x electrodes were successfully prepared by combining hydrothermal method and simple drop-casting technique. The morphology, structure, composition and optical property of the obtained electrodes were well characterized by SEM, XRD, XPS and UV–vis DRS. α-Fe2O3/NiFe(OH)x electrode exhibited enhanced photocathodic protection performance as compared to hematite (α-Fe2O3) electrode, and greatly shift photo potential and self-corrosion potential of the coupled 304 stainless steel into a more negative region. Remarkably, under visible light illumination, the prepared electrodes can protect the 304 stainless steel without the use of hole scavengers. Meanwhile, the photoanodes can maintain a better photo-stability without losing its activity. The possible mechanism for the enhanced photocathodic protection performance is proposed, which is involved with the fast hole transfer from hematite to NiFe(OH)x cocatalyst and generates high valence state Ni species to drive the water oxidation reaction. The reduced charge recombination will result in the effective electron transfer from the α-Fe2O3/NiFe(OH)x electrode to the 304 stainless steel and realize the effective protection.

Introduction

Stainless steels are widely used in various corrosive environments due to their excellent corrosion resistance among numerous metallic materials. However, in a salt-rich marine environment, stainless steel would suffer from corrosion such as pitting corrosion, which is not beneficial for the long-term use of this material [1]. Thus, different methods have been developed to protect the stainless steels from corrosion, such as impressed current cathodic protection and sacrificial anode protection. Unfortunately, impressed current cathodic protection technique needs a high electric energy input, and the sacrificial anode protection will result in environmental pollution and material consumption [2]. In recent years, photoelectrochemical cathodic protection has attracted great attention since it can protect the metal materials by only using solar energy and water as the input sources. Under light illumination, the photoelectrode will generate photo-electrons after absorbing the photons with energy larger than the band gap, and then the excited electrons will transfer to the protected metal if the conduction band potential of the photoelectrode is higher than the Fermi level of the metal.

The development of semiconductor photoelectrodes with suitable band structure is the key to realize the wide application of photocathodic protection technique. Up to now, many semiconductors have been developed such as TiO2 [3], SnO2 [4], ZnO [5], α-Fe2O3/Fe3O4 [6], WO3 [7] and SrTiO3 [8]. However, it should be pointed out that most of these semiconductors have wide bandgap, which can only absorb UV-light accounting for 3–5% of the solar spectrum. To our knowledge, there are few narrow band gap semiconductors exhibiting photocathodic protection performance except for those such as C3N4, BiVO4 and α-Fe2O3. This is mainly ascribed to the many factors, including the positive conduction band position, small photovoltage and weak stability of many narrow band semiconductor. In addition, due to the quick charge recombination and slow water oxidation kinetics occurring on the electrode/electrolyte interface, most of photoelectrochemical protection systems have to use hole scavengers. Although the usage of hole scavengers could shift the OCP of the photoanode to lower levels, it's difficult to realize large-scale employment because the photoelectrode will gradually lose its protection ability as the scavengers are gradually consumed. To address these problems, it is necessary to develop semiconductor materials with visible-light response, high stability and meanwhile avoid the use of hole scavengers.

Recently, α-Fe2O3 has attracted much attention in the field of photo(electro)catalytic water splitting or organic degradation [[9], [10], [11]] due to its high solar-to-chemical energy conversion efficiency, suitable bandgap (2.1 eV) allowing for visible light absorption, high valence band potential for driving the water oxidation reaction, good photostability in alkaline solution and great elemental abundance. Moreover, the conduction band potential of hematite was estimated to be −0.62 V vs. SCE at pH 13.6 [12], which is more negative than the self-corrosion potential of 304 stainless steel (304SS). Therefore, hematite can be considered as a promising photoelectrode material for photocathodic protection of 304SS. However, limited by the short holes diffusion length of α-Fe2O3 and its poor conductivity, the photoelectrocatalytic water splitting activity is still very low and usually a higher overpotential is needed. Therefore, optimizing hematite photoelectrode for photocathodic protection is worthy of further exploration, especially for the case without the use of hole scavengers. To realize this, it is obvious that the water oxidation reaction needs to be accelerated. Currently, numerous strategies have been proposed to accelerate surface water oxidation reaction for hematite photoanode, such as doping with metal or non-metal element [13,14], morphology modulation [15], constructing heterojunction [16], passivation of surface states [17], or modification of cocatalyst [18,19]. Among these methods, the integration of electrocatalysts with hematite photoanode has been confirmed to be a very efficient method for the enhancement of water oxidation activity. For example, Wang and co-workers reported a dramatic enhancement of photocurrent and photovoltage by spin-coating NiFeOx onto hematite films, which is ascribed to the increase on the number of trapped holes at the electrode surface and the decrease of the surface recombination rate [20]. Li Yat's group found that the uniform deposition of Nisingle bondFe hydroxide overlayer on hematite nanowire photoanodes can significantly enhance the surface hole transfer kinetics and passivate the surface states, and thus exhibit the lowest onset potential among the Nisingle bondFe based electrocatalysts [21,22]. However, to our knowledge, the loading of Nisingle bondFe hydroxide on α-Fe2O3 for the photocathodic protection application has never been reported, and the underlying effect of Nisingle bondFe electrocatalysts on the photocathodic protection performance is also unclear. Therefore, attempts to develop α-Fe2O3/NiFe(OH)x photoanode for photoelectrochemical cathodic protection application in sacrificial-agent-free environment and understanding the role of NiFe(OH)x for the possible enhancement on the photocathodic protection performance is worth of in-depth study.

In this work, α-Fe2O3 was synthesized by hydrothermal method, followed by decoration of NiFe(OH)x overlayer by drop-casting technique. The prepared films were well characterized by X-ray diffraction, UV–vis diffuse reflectance spectroscopy, X-ray photo-electronic spectroscopy and scanning electron microscopy to reveal the crystal structure, light absorption, valence state and surface morphology. The photocathodic protection performance of 304SS coupled with α-Fe2O3 photoelectrode was evaluated by open circuit potential (OCP) technique and Tafel polarization curve. The effect of calcination temperature on the OCP variation of α-Fe2O3 was well analyzed. As the increase of calcination temperature, the OCP under visible light illumination was negatively shifted. Moreover, the effect of NiFe(OH)x deposition on the OCP, self-corrosion potential, water oxidation activity, charge transfer and energy band bending of α-Fe2O3 photoelectrode was systematically investigated. Finally, a possible mechanism for elucidating the enhanced photocathodic protection performance of α-Fe2O3/NiFe(OH)x is proposed.

Section snippets

Materials

FeCl3 (99%), Fe(NO3)3·9H2O, Ni(NO3)2·6H2O, KCl, NaOH and urea in AR grade were all purchased from Aladdin and used without further purification. Fluorine-doped tin oxide (FTO) glasses (14 Ω sq.−1, 2.2 mm thick) were obtained from Nippon Sheet Glass Co Ltd., Japan, and successively cleaned by deionized water, acetone, and ethanol for 15 min respectively, followed by drying under N2 stream at room temperature. The 304SS was machined into rectangular block with an area of 2.5 cm × 1 cm, thickness

Materials characterization

Fig. 1A shows the XRD pattern of α-Fe2O3 and α-Fe2O3/NiFe(OH)x film, where the α-Fe2O3 films were treated at 750 °C. The asterisks corresponding to the diffraction peaks at 2θ = 35.6°, 54.1°, and 64.0° were indexed to the α-Fe2O3 (JCPDS No.33-0664), whereas the masks of other triangle well matched to the FTO substrate (JCPDS No.46-1088). No diffraction peaks corresponding to NiFe(OH)x overlayer were observed with the α-Fe2O3/NiFe(OH)x films. It may be ascribed to the low loading or the

Conclusion

In summary, α-Fe2O3/NiFe(OH)x photoelectrodes were successfully synthesized by combining hydrothermal and drop casting technique. The prepared photoelectrodes could be used as a green, stable and visible light responsive material to prevent 304SS from corrosion. Meanwhile, no hole scavengers are needed in this system. The NiFe(OH)x decoration could greatly improve the water oxidation kinetics occurring at the electrode/electrolyte interface, and shift the photo potential and self-corrosion

CRediT authorship contribution statement

Fan Liya: Methodology, Writing-Original draft preparation.

Xiao Zhang: Investigation.

Chuanqun Zhang: Data collection and interpretation.

Jiangshan Li: Performance test.

Chenglin Wu: Supervision, Validation, Provide research funding.

Yuxiao Chu, Fangqi Ge, Yiyuan Liu: Films preparation and characterization.

Xianqiang Xiong: Guide the whole research idea, Supervision and Reviewing, Provide research funding.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by China Postdoctoral Science Foundation (2020M671692), National Students' Platform for Innovation and Entrepreneurship Training Program (201910350032, 201910350015), Chemical Engineering & Technology of Zhejiang Province First-Class Discipline (Taizhou University), Science and Technology Innovation Activity Plan of College Students in Zhejiang Province (2020R468010).

References (43)

  • M. Sun et al.

    Effect of ZnO on the corrosion of zinc, Q235 carbon steel and 304 stainless steel under white light illumination

    Corros. Sci.

    (2014)
  • X. Xiong et al.

    Online corrosion monitoring of photoelectrochemical cathodic protection of carbon steel using particle video microscope

    Optik

    (2020)
  • J.-G. Kim et al.

    Advanced Mg-Mn-Ca sacrificial anode materials for cathodic protection

    Mater. Corros.

    (2001)
  • J. Hu et al.

    SnO2 nanoparticle films prepared by pulse current deposition for photocathodic protection of stainless steel

    J. Electrochem. Soc.

    (2015)
  • Z. Wang et al.

    Understanding the roles of oxygen vacancies in hematite-based photoelectrochemical processes

    Angew. Chem. Int. Ed.

    (2018)
  • K. Sivula et al.

    Photoelectrochemical water splitting with mesoporous hematite prepared by a solution-based colloidal approach

    J. Am. Chem. Soc.

    (2010)
  • W. Wu et al.

    Single-crystalline α-Fe2O3 nanostructures: controlled synthesis and high-index plane-enhanced photodegradation by visible light

    J. Mater. Chem. A

    (2013)
  • J. Zhang et al.

    Understanding charge transfer, defects and surface states at hematite photoanodes

    Sustain. Energy Fuels

    (2019)
  • I. Cesar et al.

    Influence of feature size, film thickness, and silicon doping on the performance of nanostructured hematite photoanodes for solar water splitting

    J. Phys. Chem. C

    (2009)
  • Z. Luo et al.

    Dendritic hematite nanoarray photoanode modified with a conformal titanium dioxide interlayer for effective charge collection

    Angew. Chem. Int. Ed.

    (2017)
  • S.-S. Yi et al.

    Carbon quantum dot sensitized integrated Fe2O3@g-C3N4 core-shell nanoarray photoanode towards highly efficient water oxidation

    J. Mater. Chem. A

    (2018)
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