Transparent Nanoparticulate FeOOH Improves the ... - ACS Publications

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Transparent Nanoparticulate FeOOH Improves the Performance of a WO3 Photoanode in a Tandem Water-Splitting Device Wai Ling Kwong,† Cheng Choo Lee,‡ and Johannes Messinger*,† †

Department of Chemistry, Kemiskt Biologiskt Centrum (KBC) and ‡Umeå Core Facility for Electron Microscopy, Umeå University, S-90187 Umeå, Sweden S Supporting Information *

ABSTRACT: Oxygen evolution catalysts (OEC) are often employed on the surface of photoactive, semiconducting photoanodes to boost their kinetics and stability during photoelectrochemical water oxidation. However, the necessity of using optically transparent OEC to avoid parasitic light absorption by the OEC under front-side illumination is often neglected. Here, we show that furnishing the surface of a WO3 photoanode with suitable loading of FeOOH as a transparent OEC improved the photocurrent density by 300% at 1 V versus RHE and the initial photocurrent-to-O2 Faradaic efficiency from ∼70 to ∼100%. The data from the photovoltammetry, electrochemical impedance, and gas evolution measurements show that these improvements were a combined result of reduced hole-transfer resistance for water oxidation, minimized surface recombination of charge carriers, and improved stability against photocorrosion of WO3. We demonstrate the utility of transparent FeOOH-coated WO3 in a solar-powered, tandem water-splitting device by combining it with a double-junction Si solar cell and a Ni−Mo hydrogen evolution catalyst. This device performed at a solar-to-hydrogen conversion efficiency of 1.8% in near-neutral K2SO4 electrolyte. oxidation of WO3 itself to form tungsten peroxo species.10,11 The latter is known to cause photocatalytic deactivation and photocorrosion of WO3.12 These problems are likely caused by the low valence band maximum of WO3 (∼+3 V versus RHE). As a way forward, surface modifications of WO3 with oxygen evolution catalysts (OECs) are employed.6,13−18 Coupling suitable OECs to WO3 resulted in an improved Faradaic efficiency for water oxidation owing to the suppressed formation of tungsten peroxo species and an increased stability by shielding WO3 from corrosive electrolytes. Additionally, a cathodic shift of the water oxidation onset potential and a higher photocurrent density also were observed owing to the formation of a favorable junction between the OECs and WO3, which promotes the separation of photogenerated charge carriers.16,18 In contrast, when a photoelectrode is paired with an unsuitable OEC, its photoelectrochemical performance may be adversely affected.19 This has been demonstrated with BiVO4 photoanodes coupled with various OECs including RuOx, MnOx, NiOx, and CoOx.19−21 In addition to the favorable combination of OEC/WO3, the optical transparency of the OECs is also an important necessity because when a tandem water-splitting device is illuminated from the side of a large-band-gap WO3 (top light absorber), the OEC that is deposited on the surface of WO3 should not

1. INTRODUCTION Photoelectrolysis of water into molecular oxygen (O2) and hydrogen (H2) is one of the promising methods for harvesting and storing solar energy as fuel in a sustainable manner. For this method to be viable for large-scale implementation, a solar water-splitting device needs to meet several important criteria such as high efficiency, low material and manufacturing costs, and good stability.1 The design of a tandem water-splitting device that utilizes semiconducting metal oxide as the photoelectrode has been regarded as a promising strategy for a low-cost production of solar fuel owing to the ease of synthesis and the stability of the metal oxide in an aqueous environment.2,3 Here, the metal oxide is responsible for both light absorption and catalytic water splitting. Among the metal oxides, tungsten trioxide (WO3), an n-type semiconductor with an indirect band gap of ∼2.6 eV, has drawn great attention as one of the few non-noble oxides that decently perform as photoanodes for water oxidation in acidic solution.4−8 The performance of WO3 photoanodes, however, is limited by several factors, which include the water oxidation kinetics at its surface and the Faradaic efficiency for water oxidation. Water oxidation at the surface of a photoanode is often kinetically slow due to the high overpotential required for this four-electron, four-proton reaction.9 Furthermore, it is known that WO3 demonstrates lower-than-expected Faradaic efficiency for water oxidation.10,11 Depending on the choice of electrolyte, a portion or all of the photocurrent generated by WO3 is used for the oxidation of electrolyte anions and/or the © 2016 American Chemical Society

Received: March 8, 2016 Revised: May 4, 2016 Published: May 6, 2016 10941

DOI: 10.1021/acs.jpcc.6b02432 J. Phys. Chem. C 2016, 120, 10941−10950

Article

The Journal of Physical Chemistry C

three-electrode cell consisting of a platinum wire (counter electrode), Ag/AgCl/1 M KCl (reference electrode), and a FTO or WO3 thin film (working electrode) connected to a computer-controlled potentiostat. The electrodes were immersed in a 0.1 M FeSO4·7H2O (Scharlau; 99.5%) solution, which was purged with N2 for 1 h prior to the electrodeposition. During the electrodeposition process, the electrolyte was gently stirred and heated to maintain it at a temperature of 70 °C. The electrodeposition was carried out at 1.2 V versus Ag/AgCl for 10−60 s to obtain different FeOOH loadings. These decorated WO3 thin films were labeled as Fts/WO3, where t denotes the electrodeposition time, namely, 10, 20, 30, and 60 s. The electrodeposition of Ni−Mo was performed according to the procedures reported by Navarro-Flores et al. with a slight modification.32 In brief, the three-electrode cell described above was employed with FTO as the working electrode. The electrolyte solution contained 0.3 M NiSO4·6H2O, 0.2 M Na2MoO4·2H2O, and 0.3 M Na3C6H5O7·2H2O with a final pH adjusted to 10.5 by adding NH4OH. A fixed current density of −4.5 mA cm−2 was applied for 60 s. 2.3. Characterizations. Field-emission scanning electron microscopy (SEM, Carl Zeiss Merlin) was used to examine the surface morphology and thickness of the films. The optical absorption was measured using UV−visible spectrophometry (VWR UV-3100PC). X-ray diffraction (XRD, Siemens D5000; Cu Kα radiation; 40 kV; 30 mA) and Raman microscopy (Renishaw inVia; excitation at 633 nm using HeNe laser) were used to characterize the mineralogy of the films. Electrochemical measurements were performed using a three-electrode setup housed in a custom-built electrochemical cell with a quartz window. The Fts/WO3 films, a platinum coil, and Ag/AgCl/1 M KCl were used as the working, counter, and reference electrodes, respectively. These electrodes were connected to a computer-controlled potentiostat. All potentials reported were in reference to a reversible hydrogen electrode (RHE) via a conversion of VRHE = VAg/AgCl + 0.222 V + 0.059 × (electrolyte pH), unless specified otherwise. Illumination was provided by a solar simulator (Oriel Sol3A) with 1 sun intensity (100 mW cm−2; calibrated using a Si reference cell from Oriel model 91150 V) at the front side of the films. Front-side illumination and 0.5 M K2SO4 (Duchefa; > 99%; pH 6.6) as the electrolyte solution were used throughout this work unless stated otherwise. For sulfite oxidation, 0.1 M Na2SO3 (SigmaAldrich; ≥ 98%) was added to 0.5 M K2SO4, and the final pH was adjusted to 6.6 using H2SO4. Fresh electrolytes were used for each measurement. The linear sweep voltammograms (LSVs) were scanned at 10 mV s−1, unless stated otherwise. The electrochemical impedance spectra (EIS) were recorded at a DC potential of 0.82 V and an AC potential with a 20 mV amplitude and frequencies ranging between 100 kHz and 0.1 Hz. For Mott−Schottky plots, the capacitance was measured by applying AC potentials with an amplitude of 20 mV and frequencies of 1.5 and 0.5 kHz. The incident photon-to-current conversion efficiency (IPCE) was measured at 1.23 V using a tungsten−halogen lamp aligned with a monochromator as the light source. The intensity of the light was determined using an energy meter (Ophir Nova II) with a photodiode sensor (PD300-UV). The IPCE was calculated as follows22

interfere with the light absorption of WO3 and the underneath low-band-gap light absorber. A few examples of OECs that have been coupled to the surface of WO3 include IrO2,17 RuO2,18 PtOx,6 Co−Pi,13 Ag− Bi,14 Mn catalyst,15 B2O3−xNx,16 and FeOOH.22,23 Among them, FeOOH has attracted attention as an efficient earthabundant OEC.24−26 It appears to be the active catalytic form that best performs the oxygen evolution reaction. It is reported to form under electrochemical bias on the surface of both metallic iron and iron oxide.25,26 FeOOH can be easily synthesized using electrodeposition from a mildly acidic precursor solution at ambient temperatures.24−26 The uncomplicated synthesis conditions and the high efficiency for water oxidation make electrodeposited FeOOH an attractive candidate for surface modification of photoelectrodes. The pairing of FeOOH with metal oxides is relatively new and was introduced initially to the BiVO4 photoanode,19,27−29 Very recently, FeOOH-modified WO3 photoanodes were also investigated,22,23 but the necessity of controlled catalytic loading to achieve high optical transparency of FeOOH for application in a tandem water-splitting device was not considered. Here, we report the surface modification of WO3 thin films prepared by spray pyrolysis deposition with electrodeposited FeOOH. The goal was to improve the photoelectrochemical performance and the stability of WO3 photoanodes using this OEC for operation in near-neutral K2SO4 electrolyte. By controlling the electrodeposition time, the surface of the WO3 thin film was decorated with FeOOH in the form of individual nanoparticles or agglomerates to investigate the effect of morphology as well as the role of FeOOH on the photocatalysis, charge transport properties, selectivity for water oxidation, and stability of the WO3 photoanode. Compared to other synthesis methods including photodeposition,28 hydrothermal,30 and sol−gel,31 electrodeposition was used to prepare FeOOH because of the controllability in producing a thin layer of nanoparticles, which was shown in this study to be crucial in achieving optical transparency that rendered it suitable to be deposited on the surface of a WO3 photoanode to enhance the water oxidation kinetics. The utility of a transparent FeOOH-coated WO3 photoanode in a tandem water-splitting device was examined by connecting it with a double-junction Si solar cell and a Ni−Mo hydrogen evolution catalyst.

2. EXPERIMENTAL METHODS 2.1. Spray Pyrolysis Deposition of WO3 Thin Films. Solutions of 0.01 M WCl6 were prepared by dissolving WCl6 powder (Strem; 99.9%) in 1:1 (v/v) mixtures of ethanol and distilled water. The solutions were sprayed using a reagent sprayer (Camag) with compressed air (0.5 bar; 11.5 mL/min flow rate) onto heated (300 °C) fluorine-doped tin oxide coated glass (FTO; TEC15, Hartford) substrates. The nozzle of the atomizer was positioned at a distance of 45 cm and tilted at 45° from the substrate. To obtain a film thickness of ∼2.5 μm, multiple spray cycles were performed, where each cycle consisted of a continuous spray for 4 min and followed by a break of 1 min in order to restore the substrate temperature to 300 °C. The as-deposited films were subsequently annealed in air at 500 °C for 2 h. 2.2. Electrodeposition of FeOOH and Ni−Mo. The electrodeposition of FeOOH was based on the procedures reported by Kim et al.29 The procedure was carried out in a

IPCE(λ) = 10942

1240 × Jλ (λ) λ × Pλ(λ) DOI: 10.1021/acs.jpcc.6b02432 J. Phys. Chem. C 2016, 120, 10941−10950

Article

The Journal of Physical Chemistry C where Jλ (in mA cm−2) and Pλ (in mW cm−2) are the photocurrent density and the intensity of the light measured at wavelength λ (in nm). The solar photocurrent density (Jsolar in mA cm−2 nm−1) was calculated as follows22 Jsolar (λ) =

WO3 to form a tandem configuration, in which the FeOOHmodified WO3 side was pressed against an O-ring in a photoelectrochemical cell to allow an active area of 0.2 cm2 to be exposed to the electrolyte and the illumination. Ni−Mo was placed adjacent to FeOOH-modified WO3 in the electrolyte so that it did not block the incident illumination. A diagram of the assembled device is shown in Figure 10.

1 × IPCE(λ) × Esolar(λ) × λ 1240

where Esolar (in mW cm−2 nm−1) is the solar spectral irradiance at AM1.5G (ASTM G173-03). 2.4. Quantification of O2 and H2 Gases. The O2 evolved during photoelectrochemical operation was measured in a gastight two-compartment photoelectrochemical cell (Pine Research Instrumentation). The main compartment contained the bare or FeOOH-modified WO3 film as the working electrode and Ag/AgCl/1 M KCl as the reference electrode, while another compartment, which was separated from the main compartment by a glass frit, contained the platinum coil as the counter electrode. K2SO4 (0.5 M) was used as the electrolyte in both compartments. The cell was purged with N2 and tested for its gas-tightness for 1 h before starting the measurement. Photoelectrochemical operation was carried out by illuminating the front side of the films with a solar simulator (100 mW cm−2) and recording the photocurrent at a constant potential of 1.6 V for 2 h. Gas from the headspace of the main compartment was extracted every 20 min using a gastight syringe. The gas-phase aliquots were analyzed using a membrane-inlet mass spectrometer (MIMS; ThermoFinnigan Delta plus XP). The sensitivity of MIMS toward O2 was calibrated using known quantities of air. The setup above also was used for the measurement of H2 evolved electrocatalytically by Ni−Mo, which was placed in the main compartment of the cell as the working electrode. A constant current of −1.5 mA was applied for 2 h during which gas sampling was performed at 20 min intervals. Known amounts of H2 gas mixture (5% H2 in Ar) were used for calibration of the MIMS sensitivity toward H2. The amount of gas (O2 or H2) evolved was a total of the gas in the headspace of the main compartment and the dissolved gas in the electrolyte calculated using Henry’s law (76.9 MPa L mol−1 for O2; 126.6 MPa L mol−1 for H2).33 The Faradaic efficiency for O2 or H2 evolution was obtained as follows Faradaic efficiency =

3. RESULTS AND DISCUSSION 3.1. Morphological and Mineralogical Characterizations of the Bare and Surface-Modified WO3 Thin Films. The surface morphology of bare and FeOOH-decorated WO3 was examined using SEM. The bare WO3 thin film displayed the porous structure of interconnected grains of