First demonstration of photoelectrochemical water splitting by commercial W–Cu powder metallurgy parts converted to highly porous 3D WO3/W skeletons
Graphical abstract
Introduction
Hydrogen as an environmental-friendly material with high energy density has a great potential for renewable energy production [1]. Hydrogen can be produced through different approaches such as hydrolysis [[2], [3], [4], [5]] and photocatalytic [[6], [7], [8]] methods. Hydrolysis of metals, metal hydrides or formic acid are among renowned hydrogen production strategies due to their high theoretical hydrogen yield [[2], [3], [4], [5]]. On the other hand, hydrogen production through photoelectrochemical (PEC) water splitting [[9], [10], [11]] has been found of great interest, particularly because of using clean renewable solar energy. So far, different groups of materials such as perovskites and metal oxides have been employed for water splitting [[12], [13], [14], [15]]. Inexpensive and non-toxic semiconducting metal oxides such as Fe2O3, TiO2 and WO3 are attractive candidates for PEC water splitting as photoanodes for oxygen evolution reaction (OER) [[15], [16], [17], [18], [19]]. TiO2 is a recognized semiconductor for OER, mainly because of its chemical stability. However, its bandgap is rather wide [16]. Hematite is known for its moderate bandgap and suitable valence band position, although its hole diffusion length is rather short [18]. Among these metal oxide photoanodes, WO3 as an n-type semiconductor (Eg~2.5–2.8 eV) with high photo-stability, environmental compatibility, and low cost has attracted significant attention [20,21]. Compared to TiO2, WO3 absorbs a broader range of solar spectrum while in comparison with Fe2O3, it has higher incident photon to current efficiency [22]. However, photoelectron conversion efficiency of WO3 is still limited due to poor oxygen evolution reaction kinetics and charge carrier recombination [21,23].
The PEC performance of WO3 is controlled by surface area, porosity, defects, and the nature of exposed surface planes [1]. Since nanostructuring could remarkably improve the photoelectrode performance [24], several studies have been performed on processing WO3 nanostructures with different morphologies through hydrothermal, sol-gel, sputtering, and chemical vapor deposition methods [[25], [26], [27], [28]]. In some methods, like hydrothermal, the products can be deposited onto a fluorine-doped tin oxide (FTO) glass to prepare the photoelectrode [[29], [30], [31]]. Due to higher series resistance of FTO glass, which decreases the charge collection efficiency in the external circuit [32], self-supported tungsten substrates in the form of foils or wire meshes [27,[33], [34], [35]] have been utilized. These substrates exhibit reasonable conductivity and dimensional stability after high temperature processing [32]. Because of higher surface area of porous substrates (compared to foils with flat surface), improved PEC performance could be expected. Moreover, scalability and low cost are crucial factors for commercialization a photoelectrode. Hou et al. [35] have shown that hydrothermally-treated tungsten wire mesh could yield a current density of 1.76 mA cm−2 at 1.6 V vs. reversible hydrogen electrode (RHE) in 0.01 M Na2SO4. A self-supported and hierarchical 3D WO3 micro-nano architecture has prepared by Cai et al. [36] through a combined laser processing route and thermal oxidation. The performance of the photoelectrode is shown to be 1.2 mA cm−2 at 1.23 V vs. RHE in 0.1 M H2SO4. The major advantage of the process is relied on its scalability for commercialization. A one-step, rapid and scalable flame synthesis of WO3 photoanodes for water splitting has been introduced by Chen et al. [37]. A current density of 0.91 mA cm−2 at 1.24 V vs. RHE in 0.1 M H2SO4 has been reported. Zhao et al. [38] have developed a scalable high temperature oxidation method to prepare 9 cm2 WO3 thin film, but the current density reached ~0.75 mA cm−2 at 1.2 V vs. RHE in 0.5 M H2SO4. Therefore, scalable processing of WO3 films on self-supporting substrates could be time consuming and costly which may be accompanied with an expense of performance loss.
Herein, we present, for the first time, a facile scalable and all-chemical procedure for the fabrication of self-supported WO3 nanostructured films. The process utilizes commercial W–Cu composite parts which are produced commercially by the powder metallurgy (P/M) technique. The copper network is first removed by electrochemical etching, and then WO3 nanoflakes are hydrothermally grown on the 3D porous W network. The photoanode exhibits a robust activity toward OER with the highest current density of 4.36 mA cm−2 at 1.23 V vs. RHE under 100 mW cm−2 simulated sunlight with 1.71 mA cm−2 photocurrent. The effect of initial copper concentration (the porosity of the W skeleton) on PEC performance is investigated. Electrochemical impedance spectroscopy and Mott-Schottky analysis demonstrate low charge transfer resistance and enhanced diffusion-controlled transport properties in the optimum 3D porous WO3/W electrode.
Section snippets
Materials
W–Cu composite parts were donated by ParsPaya welding industry and P/M laboratory of Amirkabir University of Technology. The composite parts were produced by P/M route and contained different wt% of copper (up to 30%). HNO3 (65%) and H2SO4 (95–97%) were supplied by Merck Inc., Germany. Acetone (99%), ethanol (96%) and deionized (DI) water were supplied from local providers.
Preparation of 3D nanostructured electrodes
A schematic procedure used for the fabrication of the electrodes is shown in Fig. 1. W–Cu composite parts were cut to small
Characterizations of porous W skeletons
To prepare 3D porous skeleton, electrochemical etching was performed. Representative FESEM images of W–Cu composites before and after copper etching is shown in Fig. 2a and b, respectively. Highly porous W structure after Cu etching in visible in Fig. 2b. The variation of current versus time during electrochemical etching is presented in Fig. 2c. Progressive electrochemical reaction of Cu with the electrolyte (HNO3) leaved open pores inside the W skeleton. EDS analysis indicated that almost all
Conclusions
In summary, we have successfully prepared highly porous 3D WO3/W skeletons from commercial P/M W–Cu parts by means of electrochemical Cu etching, followed by hydrothermal processing. PEC response of the electrodes depending on the porosity levels was studied. The main findings are summarized below.
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The fabricated electrodes composed of 3D W substrate with varying porosity levels (19–48%), which nanoflakes of WO3 with lateral dimensions of 200–300 nm were covered the internal surfaces. Removing
Acknowledgments
The authors acknowledge the funding support of Sharif University of Technology [Grant Program No. QA970816] and Iran National Science Foundation [INSF, Grant No. 96003521] and Iran Nanotechnology Innovation Council [Grant No. 126953]. The authors also acknowledge Dr. Nader Parvin (Amirkabir University of Technology) for P/M W–Cu part donation and Dr. Rahim Mohammadi for his help to interpret EIS data.
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