Nanoscale

PAPER

Cite this: DOI: 10.1039/c7nr00216e

Received 10th January 2017,Accepted 3rd March 2017

DOI: 10.1039/c7nr00216e

rsc.li/nanoscale

Electrochemical synthesis of nanoporous tungstencarbide and its application as electrocatalysts forphotoelectrochemical cells†

Jin Soo Kang,‡a,b Jin Kim,‡a,b Myeong Jae Lee,‡a,b Yoon Jun Son,a,b Juwon Jeong,a,b

Dong Young Chung,a,b Ahyoun Lim,b,c Heeman Choe,d Hyun S. Park*c andYung-Eun Sung*a,b

Photoelectrochemical (PEC) cells are promising tools for renewable and sustainable solar energy conver-

sion. Currently, their inadequate performance and high cost of the noble metals used in the electro-

catalytic counter electrode have postponed the practical use of PEC cells. In this study, we report the

electrochemical synthesis of nanoporous tungsten carbide and its application as a reduction catalyst in

PEC cells, namely, dye-sensitized solar cells (DSCs) and PEC water splitting cells, for the first time. The

method employed in this study involves the anodization of tungsten foil followed by post heat treatment

in a CO atmosphere to produce highly crystalline tungsten carbide film with an interconnected nano-

structure. This exhibited high catalytic activity for the reduction of cobalt bipyridine species, which rep-

resent state-of-the-art redox couples for DSCs. The performance of tungsten carbide even surpassed

that of Pt, and a substantial increase (∼25%) in energy conversion efficiency was achieved when Pt was

substituted by tungsten carbide film as the counter electrode. In addition, tungsten carbide displayed

decent activity as a catalyst for the hydrogen evolution reaction, suggesting the high feasibility for its utili-

zation as a cathode material for PEC water splitting cells, which was also verified in a two-electrode water

photoelectrolyzer.

Introduction

Photoelectrochemical (PEC) cells, which are promising toolsfor the utilization of solar energy,1,2 can be categorized intothe following two types according to the functional role of theelectrolyte: (i) regenerative cells that employ a redox electrolytefor solar-to-electric energy conversion and (ii) photosyntheticcells, which are based on the consumption of the electrolytefor the production of solar fuels.1 Dye-sensitized solar cells(DSCs) and PEC water splitting cells are two typical examples

as they both deliver decent and reliable performance.3–10

However, the overall performance of PEC cells is limited bythe efficiency of light harvesting and charge collection at thesemiconductor photoelectrodes. Therefore, a variety of studieshave been conducted on the design and synthesis of photo-electrodes with advanced properties. Moreover, the importanceof the counter electrodes (CEs) of PEC cells, in which noblemetals are often used, has been underestimated. It is thusconsidered that to enable the practical use of PEC cells, it isessential to replace high-cost materials with economicalelectrocatalysts.11,12

N-type semiconductors are more frequently used in theabovementioned PEC cells than their p-type counterparts,which is mainly due to the greater variety of choices avail-able and their relatively superior performance.6 CEs in ann-type cells are generally responsible for the catalyticreduction of chemical species in the electrolyte, and in thecase of DSCs, in which iodide or cobalt redox couples areused, Pt is the best-performing electrocatalytic CEmaterial.13–18 In PEC water splitting, Pt is positioned at thetop of the activity scale for the hydrogen evolution reaction(HER).19,20 However, various efforts have been made in orderto replace Pt with low-cost and earth-abundant catalysts

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nr00216e‡These authors contributed equally to this work.

aCenter for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826,

Republic of Korea. E-mail: ysung@snu.ac.kr; Fax: +82-2-888-1604;

Tel: +82-2-880-1889bSchool of Chemical and Biological Engineering, Seoul National University,

Seoul 08826, Republic of KoreacFuel Cell Research Center, Korea Institute of Science and Technology (KIST),

Seoul 02792, Republic of Korea. E-mail: hspark@kist.re.kr; Fax: +82-2-958-5478;

Tel: +82-2-958-5250dSchool of Advanced Materials Engineering, Kookmin University, Seoul 02707,

Republic of Korea

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such as carbonaceous materials21–24 and conductingpolymers.25–29 Lately, inorganic transition metal compoundshave been found to exhibit excellent performance30–40 and,among these materials, tungsten carbides have exhibitednotably high electrocatalytic activity owing to their favorableelectronic structure, which is similar to that of Pt41,42 andoriginates from the modification of the density of states inan unfilled d-band induced by the formation of metal–carbon bonds.43

In 2010, Jang et al. initiated the utilization of tungstencarbide as CEs in DSCs; they synthesized WC using eitherhydrothermal or microwave methods to confirm the feasibilityof using WC in DSCs.44 A further enhancement in perform-ance was achieved by Ma et al., who introduced ordered meso-porous carbon (OMC) as a support for WC nanoparticles,which led to an increase in the number of active sites.45,46 Onthe basis of these pioneering studies, enhancements in thesynthesis procedures and morphological improvements weresubsequently achieved.47,48

The employment of tungsten carbide in the electro-catalytic HER can be found in studies spanning more thanfive decades. Since the pioneering works by Bianchi et al. andNikolov et al.,49–51 there have been efforts to achieveenhanced activity and stability in tungsten carbides.52–56

Recent investigations have mainly focused on the synthesesof nanostructured tungsten carbides for an efficient HER,which led to substantial improvements in performance.57–59

There were also a number of reports on the use oftungsten carbides as support materials for Pt or Pdcatalysts, and enhancements in activity and stability havebeen observed as a result of synergistic interactionsbetween the tungsten carbide support and the metallicnanocatalysts.60–62

In this study, as an extension of the development of cata-lytic tungsten carbide electrodes with nanoscale morphology,we prepared nanoporous tungsten carbide (np-WC) films bythe electrochemical anodization of tungsten foil followed bypost heat treatment. np-WC electrodes were directly utilizedas electrocatalytic cathodes for DSCs and PEC water splittingcells for the first time to the best of our knowledge. This pro-cedure enabled the template-free synthesis of 3-D nano-structured WC film owing to the self-ordering of amorphousoxides during the anodic oxidation.58,63–65 The oxides werecompletely converted into crystalline carbides by thermalannealing in the presence of CO, which was confirmed byvarious X-ray analyses and electron microscopy. In DSCsemploying a state-of-the-art cobalt redox electrolyte, theoverall catalytic activity of np-WC was found to be superior tothat of Pt, which led to a substantial increase (∼25%) inenergy conversion efficiency. In addition, np-WC exhibitednotably high activity in the HER, and the feasibility of usingWC in PEC water splitting for the sustainable and eco-friendly production of H2 was verified in a two-electrodesystem. In addition, the mechanism of the HER on the np-WC electrode was investigated in terms of its electrochemicalcharacteristics.

ExperimentalPreparation and physical characterization of nanoporous WO3

and WC electrodes

Unless stated otherwise, all of the reagents used in this studywere research-grade products obtained from Sigma-Aldrichand were used without further purification. Tungsten foil(99.95%, 0.1 mm thick, Alfa Aesar) was cut into pieces measur-ing 1.5 cm × 2.0 cm, and each piece was washed with acetone,ethanol, and deionized (DI) water for 10 min with the assis-tance of ultrasonication. The W metal foils were then anodizedat 40 V for 2 h using a 1 M aqueous solution of H2SO4 contain-ing 0.5 wt% NaF as the electrolyte. Pt mesh was employed asthe CE during anodic oxidation, and the distance between thetwo electrodes was 3 cm. After the anodization, the foils werewashed with ethanol and placed in a dry desiccator.Nanoporous WO3 (np-WO3) and np-WC films were prepared bythermal annealing of the anodized W foil at 450 °C for 4 h inair and at 800 °C for 4 h in a CO atmosphere, respectively. Themorphologies of the fabricated materials were characterizedusing a scanning electron microscope (SEM; Carl ZeissAURIGA) and a transmission electron microscope (TEM; JeolJEM-2100F) equipped with an energy-dispersive spectroscopy(EDS) facility. X-ray diffraction (XRD) patterns were obtainedwith a Rigaku D-MAX2500-PC diffractometer, and X-ray photo-electron spectroscopy (XPS) spectra were recorded using aThermo Sigma Probe spectrometer. Raman spectra wererecorded using a Horiba Jobin–Yvon LabRAM Aramis micro-scope with the 514 nm line of an Ar-ion laser as the excitationsource.

Fabrication of Pt reference counter electrodes and assembly ofdye-sensitized solar cell

Conventional Pt CEs were prepared by thermal decompo-sition.66 Isopropanol containing 50 mM H2PtCl6 was cast ontoFTO glass by spin-coating, and the FTO glass was thermallytreated at 400 °C for 20 min in air. For the fabrication of meso-porous TiO2 photoanodes, colloidal TiO2 paste (DSL 18NR-T,Dyesol) was doctor-bladed onto the FTO glass, which was pre-treated with 0.64 mM TiCl4 solution for the formation of aTiO2 blocking layer. The electrodes were then sintered at500 °C for 30 min in air to remove the solvent and organicbinders and also to enhance the connectivity between the TiO2

nanoparticles. In addition, post-treatment with TiCl4 was per-formed using a 0.25 mM aqueous solution of TiCl4 in order toincrease the dye loading amount and facilitate electron injec-tion from the dye molecules into the TiO2 conductionband.67,68 The electrodes were placed in a 0.1 mM solution ofY123 dye (DN-F05Y, Dyenamo),69 in which the solvent was amixture of acetonitrile and tert-butanol (1 : 1 vol. ratio), for48 h at 30 °C to allow the dye molecules to chemisorb onto theTiO2 surface. For the assembly of DSCs, thermoplastic sealantswith a thickness of 25 μm (Surlyn, DuPont) were used asspacers, and a [Co(bpy)3]

3+/2+ redox electrolyte with a compo-sition in accordance with a former report15 was injected

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through pre-drilled holes located on the side of thephotoanode.

Electrochemical and photoelectrochemical analysis

Cyclic voltammetry (CV) analysis was performed using a potentio-stat (Metrohm Autolab PGSTAT). During the CV measure-ments, Pt mesh and an Ag/AgCl (saturated KCl) electrode wereused as the CE and reference electrode, respectively, and thescan rate was 50 mV s−1. Evaluation of the performance of theDSCs was carried out under simulated AM 1.5G light (inten-sity: 100 mW cm−2) using a solar simulator (XIL model05A50KS source measure unit) and a potentiostat (Solartron1480 Multistat). The incident photon-to-current efficiencies(IPCEs) were measured with a McScience K3100 system, andelectrochemical impedance spectroscopy (EIS) analyses wereperformed using a Zahner Zennium electrochemical work-station with sinusoidal perturbations of 10 mV. Polarizationcurves for the hydrogen evolution reaction were obtainedusing a specially designed rotating disk electrode (RDE) at arotation speed of 2000 rpm. The geometric area of the home-built RDE tip was 0.196 cm2. The electrocatalyst films weredirectly mounted on the rotating tip using a home-built enclo-sure with an electrical contact for electrochemical measure-ments. A potentiostat (Metrohm Autolab PGSTAT) wasemployed to perform potential cycling within the range of−0.4 to 0.2 V vs. a reversible hydrogen electrode (RHE) in athree-electrode system, in which a glassy carbon rod and asaturated calomel electrode (SCE) were used as the CE andreference electrode, respectively. The scan rates were20 mV s−1, and a 0.5 M solution of H2SO4 saturated with H2

was used as the electrolyte. All the measurements wereperformed at 293 K. PEC water splitting measurements wereperformed in conditions of AM 1.5G illumination with theassistance of the solar simulator. Potential sweeps were carriedout within the range of 0.45 to 1.55 V (vs. RHE in a three-elec-trode system and vs. the CE potential in a two-electrodesystem) at a scan rate of 20 mV s−1 in a 0.5 M solution ofH2SO4 saturated with Ar. An Ag/AgCl reference electrode wasused for the three-electrode PEC measurements.

Results and discussionPhysical characterization of nanoporous tungsten oxide andcarbide films

For the physicochemical characterization of the anodized tung-sten foils before and after the heat treatments, X-ray analyseswere performed. Fig. 1 shows the XRD patterns of bare tung-sten and anodic tungsten oxide before and after thermalannealing in air or a CO atmosphere. In all cases, the presenceof tungsten metal beneath the layer formed by anodizationwas observed in the form of (110), (200), and (211) peaks,which were located at 2-theta positions of 40.3°, 58.3°, and73.2°, respectively (assigned according to JCPDS 04-0806).However, no characteristic peaks were observed for the as-anodized tungsten foil, except for the signals of the metal sub-

strate, which indicates (as reported in previous studies) thatthe anodic oxides formed on the surface were amorphous.61,62

After annealing at 450 °C for 4 h in air, peaks of monoclinicWO3 (JCPDS 43-1035) were apparent. In contrast, anodic tung-sten oxide after heat treatment at 800 °C for 4 h in a CO atmo-sphere clearly exhibited (001), (100), and (101) peaks of WC at2-theta positions of 31.5°, 35.7°, and 48.3°, respectively(assigned according to JCPDS 65-8828). This indicates that theamorphous anodic oxide had been successfully transformedinto crystalline carbide.

The chemical states of the tungsten oxide and tungstencarbide films were investigated by XPS analysis. Fig. 2 showsthe XPS spectra of tungsten foil and electrochemically pre-pared compound materials. The survey spectra in Fig. 2a showthat there were no elements other than C, O, and W in thetungsten foil, as-anodized tungsten foil, and np-WO3 and np-WC films. Fig. 2b shows the W 4f XPS spectra of bare tungstenfoil and the synthesized electrodes both before and after postheat treatment under various atmospheres. In the case of baretungsten foil, the peaks at 31.2 eV and 33.3 eV (correspondingto metallic W) and minor peaks at 35.7 eV and 37.8 eV(assigned to WO3 species) show the presence of a thin oxidelayer formed on the surface.59 In contrast, as-anodized tung-sten foil displayed only peaks of tungsten oxide, which indi-cated that the surface was completely covered with WO3.

One noteworthy observation was that the W 4f peaks wereslightly shifted to lower binding energies (BE) for the thermallyannealed tungsten oxide samples. From the color transition ofWO3 to dark blue after heat treatment in air (see the digitalphotograph displayed in Fig. S1, ESI†), the shifts in BE can beattributed to the electrochromic behavior of WO3.

70,71 WO3 iswell-known for its color transition induced by a decrease inthe valence of W, and this implies that partial reduction of

Fig. 1 XRD patterns of tungsten foil and anodized tungsten before andafter heat treatments in air or CO atmosphere. The bars under the XRDpatterns indicate the 2-theta positions of W (black color, JCPDS04-0806), WO3 (red color, JCPDS 43-1035), and WC (blue color, JCPDS65-8828).

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WO3 took place during the thermal annealing process, whichwas probably due to our experimental setup, which employeda box furnace. However, it was confirmed from the XRD pat-terns that the chemical identity of the heat-treated tungstenoxide sample is mainly WO3, and the chemical states of W inWO3 can be easily changed during electrochemical analysis bythe intercalation of cations such as H+ and Li+.71 Therefore,the shift in the W 4f peaks induced by heat treatment in airwas not considered to be an important factor in further investi-gations carried out in this study. In the case of np-WC, W 4fpeaks were observed at 31.8 eV and 34.0 eV, which effectively

matched the BE positions reported in a previous study ofWC.59 There is a large peak at a BE position of 284.6 eV for allthe samples in the C 1s spectra shown in Fig. 2c, but the inten-sity of this peak is by far the largest for np-WC, which indi-cates the presence of a much greater quantity of C atoms. Inaddition, a characteristic C 1s peak at 283.0 eV is also seen fornp-WC, which confirms the presence of W–C bonds.59

The morphologies of the anodized tungsten foils werecharacterized by SEM. Fig. 3a and b show SEM images of as-anodized tungsten foil, in which the nanoporous inter-connected structure is clearly observed. In general, the formationof a nanostructured metal oxide film in an anodizationprocess involves the electrochemical oxidation of a metal filmfollowed by chemical dissolution reactions. During the anodi-zation process, a local difference in the dissolution rate of theoxides, which depends on the electric field applied to the elec-trode and the F− concentration in the electrolyte, results in theformation of metal oxide films with nanopores. The nano-morphology of the tungsten oxide electrode obtained in thisstudy would also be formed by electrochemical anodization andchemical etching reactions in the NaF electrolyte.63–65 Afterthermal annealing at 450 °C for 4 h in air, the nano-oxides

Fig. 2 XPS (a) survey, (b) W 4f, and (c) C 1s spectra of tungsten foil andanodized tungsten with and without heat treatments in air or COatmosphere.

Fig. 3 SEM images of anodized tungsten (a and b) before and after heattreatment in (c and d) air and (e and f) CO atmosphere.

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were exfoliated, which led to decreases in pore sizes (Fig. 3cand d). Furthermore, when heat treatment at 800 °C in a COatmosphere was performed after the anodization process, thenanostructure shrank by a small amount, although the overallnanoporous interconnected structure was maintained, asshown in the SEM images displayed in Fig. 3e and f. The slightdifferences in morphology can be attributed to the chemicalcompositions of WO3 and WC, in which three O atoms andone C atom are present per W atom, respectively, and also theconsequent difference in the unit cell volume. However, in allcases the nanoporous interconnected structure of the frame-work was unchanged. In addition, the internal structures ofnp-WO3 and np-WC were characterized by SEM analysis withthe assistance of focused ion beam (FIB) milling. Fig. 4a and bshow cross-sectional SEM images and the correspondingelemental EDS maps of the anodic tungsten oxide and carbidefilms, respectively. The fabricated nanostructured tungstencompound layer had a thickness of about 200 nm in bothfilms, which indicates that the transformation from oxide tocarbide occurred without inducing changes in the originalnanostructure and thickness.

Additional morphological characterizations were performedusing TEM analyses. Fig. 5a shows the porous structure of np-WO3, and a lattice spacing of 0.31 nm was observed in the

high-resolution (HR)-TEM image shown in Fig. 5b, whichmatches the (112) plane of monoclinic WO3. Fig. 5c and dshow TEM images of np-WC, which also has a porous nano-structure. The lattice spacing of 0.25 nm shown in theHR-TEM image (Fig. 5d) corresponds to the (100) plane ofWC. However, unlike np-WO3, the presence of a few mono-layers of carbon shells was observed in np-WC, and the for-mation of these shells can be attributed to char from the pyro-lysis of CO gas.72,73 The presence of carbon shells on thesurface was also confirmed by Raman spectroscopy (Fig. S2,ESI†). There have been investigations into the effects of thethickness of carbon shells on the activity of electrocata-lysts,74,75 and Kimmel et al. confirmed that carbon shells com-posed of one or two monolayers do not impair electrocatalyticactivity but can help to enhance stability.76 As the observedcarbon shells had thicknesses within that range, their influ-ence was not taken into account in further investigations.Elemental analyses were also carried out by scanning trans-mission electron microscopy (STEM) and EDS (Fig. S3, ESI†),and we could clearly confirm that the nanostructured materialshown in the STEM images in Fig. S3a† (np-WO3) is composedof W and O, and that shown in Fig. S3b† (np-WC) is composedof W and C atoms.

Electrocatalytic activities and dye-sensitized solar cellapplications of nanoporous tungsten oxide and carbide films

The electrocatalytic activities of np-WO3 and np-WC wereinvestigated by CV analyses, and the results were comparedwith those for platinized FTO (Pt-FTO) electrodes. Fig. 6shows the CV curves for 10 cycles recorded using acetonitrilecontaining 10 mM Co(bpy)3(PF6)2, 1 mM Co(bpy)3(PF6)3, and0.1 M LiClO4 as the electrolyte. The currents measured by CV

Fig. 4 Cross-sectional SEM images and corresponding elemental EDSmaps of (a) tungsten oxide and (b) tungsten carbide prepared by FIBmilling.

Fig. 5 TEM images of tungsten oxide at (a) low and (b) high magnifi-cations and tungsten carbide at (c) low and (d) high magnifications.

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analyses were first normalized by the geometric areas of theelectrodes to enable practical comparisons between differentelectrodes. The current density normalized by the geometricarea is an effective and valid parameter for indicating theoverall catalytic performance as affected by factors includingthe catalytic kinetics and actual surface area of electrocata-lysts. Among the three electrodes, np-WO3 exhibited thelowest current density and the highest overpotential (η). Incontrast, both Pt-FTO and np-WC displayed higher electro-catalytic activity and stability. During the CV measurements,the anodic current (positive current in Fig. 6) was recordedwhen the potential was swept in the positive direction, andthe cathodic current (negative current in Fig. 6) wasmeasured during the backward sweep. The catalytic activitiesof the electrode materials could then be compared using theonset and peak-current potentials. Furthermore, the overallperformances of different catalysts were compared using thepeak current densities recorded in the CV measurements.The peak current is affected by the number of electrons thatparticipate in the charge transfer reactions at the electrodes,the kinetic activity of the catalysts, and the mass transport ofcobalt bipyridine species to the surface of the electrode forthe electrocatalytic reactions. In the anodic scan, the onset

and peak potentials appeared earlier for Pt-FTO in compari-son with np-WC, which indicates that Pt-FTO displayedsuperior electrocatalytic activity in comparison with np-WCfor the oxidation of the redox electrolyte, although the peakcurrent density was higher for np-WC. On the other hand, allthe parameters (onset potential, peak potential, and peakcurrent density) of np-WC were superior to those of Pt-FTOduring the cathodic sweep. Considering that the role of theCEs in DSCs consists of the reduction of [Co(bpy)3]

3+ to[Co(bpy)3]

2+, it was possible to conclude that the overall activityof np-WC was higher than that of Pt-FTO. In order to furtherunderstand the improved catalytic activity of Pt-FTO and np-WC, the actual surface areas of the electrodes were estimatedby measuring the electrochemical double-layer capacitance(EDLC). The EDLC was determined from the capacitive cur-rents at various scan rates in CV plots (see Fig. S4, ESI†). TheEDLCs of Pt-FTO and np-WC were 31.2 μF cm−2 and 225.3μF cm−2, respectively, which indicated that the effective area ofnp-WC was significantly larger in comparison with that of Pt-FTO. The larger surface area of np-WC, which originatedfrom its porous nanostructure, resulted in an increase in thecurrent density normalized by the geometric area in the CVmeasurements. In addition, CV plots for metallic tungsten

Fig. 6 (a) CV plots for platinum, tungsten oxide, and tungsten carbide in a [Co(bpy)3]3+/2+ redox electrolyte. The results from the initial scans are

plotted as dashed lines in (a), and the solid curves represent the stabilized currents after 10 cycles of scanning, which are displayed in (b), (c), and (d).

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and as-anodized tungsten were also obtained, and the inert-ness of these materials to the cobalt bipyridine redox coupleswas confirmed (Fig. S5, ESI†).

Then, we investigated the performances of DSCs thatemployed Pt-FTO, np-WO3, and np-WC as CEs. Fig. 7a showsthe photocurrent density ( J)–voltage (V) characteristics, and theopen-circuit voltage (Voc), short-circuit current density ( Jsc), fillfactor (FF), and energy conversion efficiency determined fromthe J–V curves are listed in Table 1. As expected from the CVresults, np-WO3 exhibited the lowest values of all these para-meters, which resulted in a low efficiency of 2.18%. On theother hand, for a conventional DSC with a Pt-FTO CE theefficiency was 5.68% with a Voc of 0.753 V and a Jsc of 11.3

mA cm−2. More importantly, for a DSC with an np-WC CE theenergy conversion efficiency increased to 7.08% owing to a dra-matic rise in the Jsc to 15.2 mA cm−2. The increase in the photo-current can be attributed to the following reasons: (i) the Pt-likeelectronic structure of tungsten carbide,41,42 (ii) its nanoporousmorphology with a large surface area, and (iii) its 3-D structure,which reduces the distance that bulky [Co(bpy)3]

3+/2+ redoxcouples must travel before they arrive at active sites on thesurface of np-WC.77 The energy conversion efficiency achievedin this work was one of the highest observed among DSCs thatemployed support-free tungsten carbide as the counter elec-trode, in comparison with the J–V characteristics reported inprevious publications that are summarized in Table S1.† IPCEspectra were obtained in the UV-visible light wavelength region,and these are shown in Fig. 7b. The trends in the Jsc wereclearly observable in the IPCE results.

To investigate the relative charge transfer kinetics at the CE/electrolyte interface, EIS analyses were performed. Fig. 8ashows Nyquist plots for DSCs with Pt-FTO, np-WO3, and np-WC CEs measured in dark conditions with a forward bias of0.7 V. In contrast to np-WO3, which displayed one large semi-circle, the DSCs that employed Pt-FTO and np-WC exhibitedthree semicircles, which corresponded to the kinetics at theCE/electrolyte interface (high-frequency region), electrolyte/photoanode interface (intermediate-frequency region), anddiffusion within the electrolyte (low-frequency region).78–80

The first x-axis intercept corresponds to the value of the seriesresistance (Rs), which indicates the general ohmic resistance ofthe cells.80 The Rs was found to be lower in the DSC with np-WC than in the DSC with Pt-FTO. In the Pt-FTO CE, FTO servesas the current collector, whereas the metallic tungsten foilbeneath the nanoporous carbide layer has the same role in thenp-WC electrode. Because metals generally have a significantlyhigher electrical conductivity than transparent conductingoxides, the use of different current collectors is expected to bethe most probable reason for the difference in the Rs, whichmay also have contributed to the higher photovoltaic perform-ance of the DSC that employed an np-WC CE. Moreover, therewas no significant difference in the size of the semicircles inthe intermediate-frequency region, which was mainly becauseidentical photoanodes were used for both DSCs. In addition,the semicircles found in the low-frequency region were alsosimilar, which indicated that the nanoporous structure of thenp-WC electrode,81 in which the pores are on average largerthan 50 nm (Fig. 3), did not impair the mass transport of thecobalt bipyridine redox species.

On the other hand, there was a notable difference in thesize of the semicircles in the high-frequency region, which isrelated to the charge transfer resistance at the CE/electrolyteinterface (Rce). A lower Rce means that the electrocatalyticactivity is higher, and the Rce is related to the exchange currentdensity ( J0) by the equation J0 = RT/nFRce.

82,83 From the EISresults, it was clear that the activity of np-WC was higher thanthat of Pt-FTO. In addition, for a more accurate and quantitat-ive comparison of the Rce of the electrodes, Rce values wereobtained by fitting high-frequency EIS results that were

Fig. 7 (a) J–V characteristics and (b) IPCE spectra of DSCs employingplatinum, tungsten oxide, and tungsten carbide counter electrodes.

Table 1 J–V characteristics of DSCs employing Pt-FTO, np-WO3, andnp-WC counter electrodes

Voc/V Jsc/mA cm−2 FF/% Efficiency/%

Platinum 0.776 11.3 64.8 5.68Tungsten oxide 0.605 8.3 43.2 2.18Tungsten carbide 0.753 15.2 61.9 7.08

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measured with forward biases of 0.6 V, 0.7 V, and 0.8 V usingthe equivalent circuit shown in Fig. S6 (ESI†) and ZView soft-ware. The fitting parameters are listed in Table 2 and Fig. 8b.At all these potentials, np-WC exhibited the lowest Rce values,

and therefore we could conclude that the high performance ofthe DSC that employed an np-WC CE originated from thesuperior electrocatalytic activity of np-WC for the reduction ofcobalt bipyridine species.

Electrocatalytic activity of nanoporous tungsten oxide andcarbide for the hydrogen evolution reaction andphotoelectrochemical water splitting

To evaluate the performance of np-WC as an electrocatalyticcathode for PEC water splitting, its activity in the HER wasfirst investigated. Fig. 9a shows iR-compensated HER polariz-ation curves for Pt foil, np-WO3, and np-WC electrodes, whichwere obtained by measurements with an RDE in 0.5 M H2SO4

solution at a rotation speed of 2000 rpm. The results show thatnp-WC exhibited acceptable activity in the HER, although itsperformance was inferior to that of Pt. From the EDLCs of Ptfoil and np-WC shown in Fig. S4 (ESI†), we could observe thatthe difference between the specific activities of Pt foil and np-WC was significantly greater than the differences observed inthe results normalized by the geometric area. However,because the effective performance of the counter electrode inPEC cells is dependent on the current normalized by the geo-metric area, evaluations of the catalysts were performed thatwere based on the HER currents divided by the geometric areaof the electrodes. For a quantitative comparison, the η valuesof Pt foil, np-WO3, and np-WC at 10 mA cm−2 were obtained,and these values were 11.5 mV, 270.0 mV, and 187.1 mV,respectively. The activity in the HER of np-WC was significantlyhigher in comparison with the value from the previous reportby Fei et al., in which surface-carburized anodic tungstenoxide exhibited an η value of greater than 250 mV at 10mA cm−2 using the same electrolyte.58 The superior performanceof np-WC is ascribable to the complete conversion of oxideinto carbide during the synthesis (as indicated in Fig. 1),which resulted in higher conductivity and a larger number ofactive sites within the nanostructured electrocatalysts.

In order to understand the mechanisms in the HER of np-WO3 and np-WC, we fitted the linear parts of the respectiveTafel plots using the Tafel equation (η = b log( j ) + a) and deter-mined the corresponding Tafel slopes (b; see Fig. 9b andTable 3 for the exact values). In general, there are three mecha-nisms involved in the HER in acidic media, as follows:

Hþ þ e� þ * ! Hads ðVolmerÞ

Hþ þ e� þHads ! H2 ðHeyrovsk�yÞHads þHads ! H2 ðTafelÞ

where * indicates an active site and Hads represents hydrogenadsorbed on the catalyst surface. Tafel slopes are known to bedependent on the rate-determining step (RDS), and values of120, 40, or 30 mV dec−1 indicate that the Volmer, Heyrovský,or Tafel reaction is the RDS for the overall HER, respectively.84

Pt foil exhibited a Tafel slope of 33.5 mV dec−1, which showsthat the RDS for the Pt catalyst was the reaction between twoHads atoms to form H2.

39 On the other hand, the Tafel slope

Fig. 8 (a) Nyquist plots for DSCs employing platinum, tungsten oxide,and tungsten carbide counter electrodes measured in dark conditionswith a forward bias of 0.7 V. (b) Charge transfer resistance at the counterelectrode/electrolyte interface in DSCs with forward biases of 0.6, 0.7,and 0.8 V.

Table 2 Charge transfer resistance at counter electrode/electrolyteinterface of DSCs under forward biases of 0.6, 0.7, and 0.8 V

Forwardbias/V

Rce/Ω cm2 @platinum

Rce/Ω cm2 @tungsten oxide

Rce/Ω cm2 @tungsten carbide

0.6 10.97 5859 7.820.7 9.11 5548 6.600.8 4.80 4834 3.97

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for np-WO3 was 132.6 mV dec−1, which indicated that theadsorption of H+ on the surface of WO3 limited the overallHER. As previously discussed, WO3 has electrochromic pro-perties, and there is a high possibility of the intercalation ofH+ into the lattice of WO3.

85,86 Therefore, the formation ofHads to produce H2 may have competed with the intercalationof H+, which seems to be the reason for the large Tafel slopesfor np-WO3. In the case of np-WC, the Tafel slope was between40 and 120 mV dec−1, which indicates that the RDS in the HERfor np-WC was the Volmer–Heyrovský reaction. According to areport by Voorhies et al., the surface of WC undergoes oxidationin acidic media, and WO3 forms on the surface in few-atomlayers.87 Given that the electronic structure of WC is similar tothat of Pt,41,42 the large Tafel slope for np-WC is also likely to bedue to the rapid intercalation of H+ into WO3 that had formedon the surface, which hindered the formation of Hads.

In general, the kinetics of oxygen evolution at the photo-anode of PEC water splitting cells is far slower than the HER(which takes place at the cathode) by a few orders of magni-tude.20 This implies that the overall water splitting ability of ann-type semiconductor has a greater dependence on the per-formance of the photoanode than on that of the H2-evolvingelectrode. In order to confirm the feasibility of using np-WC asan electrocatalytic cathode in PEC water splitting, PECmeasurements were also performed. To construct PEC cells,mesoporous WO3 photoanodes were prepared by casting andsintering commercial WO3 nanoparticles on an FTO glass sub-strate according to a method previously reported for TiO2.

88

The results of materials characterization of the WO3 photoelec-trodes are displayed in Fig. 10, which clearly shows that the

Fig. 9 (a) iR-corrected HER polarization curves and (b) Tafel slopes for platinum foil, tungsten oxide, and tungsten carbide electrodes.

Table 3 Overpotentials (η) at 10 mA cm−2 and Tafel slopes for Pt foil,np-WO3, and np-WC in the HER

η @ 10 mA cm−2/mV Tafel slope/mV dec−1

Platinum 11.5 33.5Tungsten oxide 270.0 132.6Tungsten carbide 187.1 87.8

Fig. 10 (a) An image, (b) cross-sectional SEM image, and (c, d) top-viewSEM images of mesoporous WO3 photoanode. (e) XRD pattern of meso-porous WO3 photoanode and reference 2-theta positions of FTO (blackcolored bars, JCPDS 41-1445) and WO3 (red colored bars, JCPDS 43-1035).

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WO3 layer was composed of highly crystalline nanoparticles ofa size of ∼100 nm. Fig. 11a shows the results of three-electrodePEC water splitting experiments performed in a 0.5 M H2SO4

electrolyte. The active areas of the WO3 photoanodes variedfrom 0.5 cm2 to 2.0 cm2 for the three-electrode PEC measure-ments, in which np-WC with a geometric area of 1.0 cm2 wasused as the counter electrode. The onset potential in PECwater splitting was ∼0.5 V vs. RHE, and this result effectivelymatched previous results obtained using WO3

photoelectrodes.89–92 The photocurrent increased as the areaof the photoanode was enlarged and reached ∼1.1 mA at2.0 cm2. After confirming the viability of np-WC as an electro-catalytic cathode in PEC water splitting, we performed two-electrode PEC water splitting to confirm that it is feasible touse the np-WC cathode in practical water photoelectrolyzers.Fig. 11b shows current (I)–V curves obtained using the sameWO3 photoanode with an active area of 2.0 cm2 and an np-WC

electrode with a size of 1.0 cm2. Although an increase in the η

value caused by the np-WC electrode was observable (whencompared with the results of the three-electrode measure-ments), the reliability of np-WC was confirmed by the fact thatthe limiting current was the same (∼1.1 mA). The perform-ances of Pt foil and np-WO3 were also evaluated using two-elec-trode PEC tests (see Fig. S7 for the results, ESI†), and weobserved that the trends in the η values for HER polarizationand PEC water splitting matched each other well.

Conclusions

In this study, np-WC films were synthesized by electrochemicalanodization and post heat treatment in a CO atmosphere andwere employed as electrocatalytic cathodes in PEC cells, e.g.DSCs and PEC water splitting cells. In DSCs that employed anelectrolyte based on cobalt redox complexes, np-WC displayedsuperior performance to that of state-of-the-art Pt-FTO whenused as a CE. In addition, np-WC exhibited substantial activityin the HER in PEC water splitting cells. Considering that WCis incomparably more economical in comparison to Pt andthat the electrochemical synthesis procedure is industriallyfavorable and reliable, the investigations performed in thisstudy are considered to be extremely practical. Moreover, thereis room for further optimization and improvement, such asstructural engineering and compositional modifications.Because the methods and investigations used in this study canbe applied to the design and synthesis of a number of othernanostructured electrocatalysts, as well as tungsten-basedmaterials, it is anticipated that this work will make a contri-bution to the practical utilization of PEC cells for renewableand sustainable solar energy conversion.

Acknowledgements

Y.-E. S. acknowledges that this work was supported by theInstitute for Basic Science (IBS) in the Republic of Korea(Project Code: IBS-R006-G1). H. S. P. acknowledges supportfrom the National Research Foundation of Korea (2016M3D1A1021142) and the Technology Development Program toSolve Climate Changes of NRF (2015 M1A2A2074688) fundedby the Ministry of Science, ICT & Future Planning ofKorea. H. C. acknowledges support from the Basic ScienceResearch Program (2014R1A2A1A11052513) through theNational Research Foundation (NRF) of Korea.

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Fig. 11 I–V characteristics of photoelectrochemical water splitting cellsemploying a mesoporous WO3 photoanode and an np-WC counterelectrode, which were measured in (a) three-electrode and (b) two-elec-trode configurations. The active areas of the photoelectrodes were 0.5,1.0, and 2.0 cm2 in the three-electrode measurements, and the two-electrode water splitting was performed with a WO3 photoanode withan active area of 2.0 cm2.

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