Turning commercial transition-metal oxides into efficient electrocatalysts via facile hydrogen treatment

Abstract

The electrocatalytic behavior of the Fe₂O₃, In₂O₃ and SnO₂ counter electrodes prepared in H₂ is enhanced significantly. This work demonstrated that hydrogen-treatment is a facile and universal method for the preparation of efficient electrocatalysts in dye-sensitized solar cells.

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Dye-sensitized solar cells (DSCs) consisting of a sensitizer dye adsorbed nanocrystalline titanium dioxide anode, an electrolyte containing the iodide/triiodide (I⁻/I₃⁻) redox couple and a counter electrode (CE) have been investigated extensively as a potential alternative to the conventional silicon solar cells due to their low-cost, easy fabrication and relatively high conversion efficiency.^1–4 The counter electrode, in which I₃⁻ is reduced to I⁻ to complete the cycle of electron transfer, plays an important role in DSCs. It is generally confirmed that platinum (Pt) is a superior CE material because of its high catalytic activity for triiodide reduction. However, the high cost and limited viability of Pt have severely restricted the large-scale application of the DSCs.^5,6 Thus, many low-cost CE materials have been screened and developed to overcome this problem, such as carbon materials,^7,8 conducting polymers^9,10 and inorganic semiconductor materials including metal sulfides,^11,12 metal selenides,¹³ metal nitrides,^14,15 metal oxides^16,17 etc. Clearly, the surface atomic geometrical and electronic structures of the CE materials can act as a pivotal role in enhancing the electrical conductivity and catalytic activity in DSCs.¹⁸

Recently, we have demonstrated that the low-cost commercial tungsten oxide (WO₃) with thermal treatment in hydrogen atmosphere presented an 800% enhancement of energy conversion efficiency of the DSCs.¹⁹ The oxygen vacancies introduced in material surface during the hydrogen treatment demonstrate its capacity for the improvement of electric conductivity and catalytic activity of the WO₃ as the CE material.²⁰ Therefore, this realistic approach is expected to be effective in increasing the energy conversion efficiency for various metal oxides with substoichiometric surface, as presented in the hydrogen-treated WO₃. Herein, in this work, we randomly choose three commercial transition-metal oxides (Fe₂O₃, In₂O₃ and SnO₂), and systematically analyze the electrocatalytic performance of the pristine commercial oxides (C–Fe₂O₃, C–In₂O₃ and C–SnO₂) and corresponding hydrogen-treated ones (H–Fe₂O₃, H–In₂O₃ and H–SnO₂). The energy conversion efficiency of the DSCs with different hydrogen-treated transition-metal oxides as CEs, predictably, would be much higher than the corresponding value of the commercial ones with stoichiometric surface.

Fig. 1 displays the photocurrent–voltage (J–V) curves for the DSCs with different CEs, and the detailed photovoltaic parameters are summarized in Table 1. The DSCs using commercial CEs showed very low energy conversion efficiencies of 1.13% for C–Fe₂O₃, 1.15% for C–In₂O₃ and 0.83% for C–SnO₂. In contrast, the DSCs using hydrogen-treated CEs exhibited high energy conversion efficiencies of 6.05% for H–Fe₂O₃, 5.84% for H–In₂O₃ and 3.05% for H–SnO₂, giving improvements of 535%, 509% and 476%, respectively. Aside from the energy conversion efficiency, the short-circuit photocurrent density (J_sc), open-circuit voltage (V_oc) and fill factor (FF) of DSCs are all increased remarkably, which means that the hydrogen-treated CEs have a higher electrical conductivity and a better electrocatalytic activity towards I₃⁻/I⁻ redox pairs. These interesting results may be attributed to the introduced oxygen vacancies during hydrogen treatment. Thus, it can be concluded that the hydrogen treatment is an efficient method for introducing oxygen vacancies to obtain highly active oxide CE materials.

Fig. 1 J–V curves of DSCs with different CEs: C–Fe₂O₃, H–Fe₂O₃, C–In₂O₃, H–In₂O₃, C–SnO₂ and H–SnO₂.

Table 1 Photovoltaic parameters of DSCs with different counter electrodes

Samples	Jsc (mA cm^−2)	Voc (mV)	FF (%)	η (%)
H–Fe2O3	17.92	670	50	6.05
C–Fe2O3	11.90	500	19	1.13
H–In2O3	14.97	710	55	5.84
C–In2O3	10.82	550	19	1.15
H–SnO2	16.32	725	26	3.05
C–SnO2	11.53	550	13	0.83

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To further confirm that the high catalytic activity of the hydrogen-treated CEs is caused by the introduction of oxygen vacancies, systematic characterizations were performed by using C–Fe₂O₃ and H–Fe₂O₃ as the examples. It is well known that the electrochemical impedance spectroscopy (EIS) is an effective and widely used method to examine the electrochemical characteristics and thereby to evaluate the catalytic activity of a catalyst. EIS measurement was carried out with symmetric cells fabricated with C–Fe₂O₃ and H–Fe₂O₃ as CEs, respectively. The results are shown in the form of a Nyquist plot (Fig. 2a). The equivalent circuit used to fit the experimental EIS data is shown in the inset of Fig. 2a. The high-frequency intercept on the real axis represents the series resistance (R_s), which is mainly composed of the bulk resistance of CEs materials, resistance of FTO glass substrate, contact resistance, etc.²¹ As expected, the R_s values of the H–Fe₂O₃ CE (16.0 Ω) is lower than that of the C–Fe₂O₃ CE (17.7 Ω), which indicated that H–Fe₂O₃ CE has a better electrical conductivity than C–Fe₂O₃ CE, and is also consistent with the tendency of the observed J_sc. The smaller R_s might be attributed to the positively charged oxygen vacancies which can greatly improve electron transfer by trapping electrons. The semicircle on the left side represents the R_ct which reflects the electrocatalytic activity of the CE materials for the reduction of triiodide ions.²² The R_ct value of H–Fe₂O₃ CE (3.25 Ω) is much smaller than that of C–Fe₂O₃ CEs (4.51 Ω), which means H–Fe₂O₃ has a better catalytic activity. These EIS results demonstrate that H–Fe₂O₃ behaves better than C–Fe₂O₃ as a CE catalyst for the triiodide reduction.

Fig. 2 EIS (a) and CV (b) of DSCs with C–Fe₂O₃ (black line) and H–Fe₂O₃ (red line) CEs, respectively.

To further evaluate the electrocatalytic properties of the as-prepared H–Fe₂O₃, cyclic voltammetry (CV) was carried out in a three-electrode system (see Fig. 2b). The CV curves of H–Fe₂O₃ show two well-defined reduction peaks and the corresponding oxidation peaks for each electrode. The pair of peaks at low potential correspond to the redox reaction shown in eqn (1), and the pair at high potential correspond to the redox reaction shown in eqn (2).²³

I₃⁻ + 2e = 3I⁻ (1)

3I₂ +2e = I₃⁻ (2)

As shown in Fig. 2b, compared to the CV curve for C–Fe₂O₃ electrode, H–Fe₂O₃ has relatively higher current densities, suggesting the redox reaction of I₃⁻/I⁻ has been greatly enhanced on the surface of H–Fe₂O₃ electrode, which produces favorable conditions for the regeneration of the sensitizer. The CV result confirms again that the H–Fe₂O₃ have a superior electrocatalytic activity in reducing triiodide, which is in accordance with the results of EIS analysis.

To check the morphologies and crystal structures of the C–Fe₂O₃ and H–Fe₂O₃, scanning electron microscopy (SEM) and X-ray powder diffraction (XRD) measurements were carried out. The SEM image (Fig. 3a) shows that H–Fe₂O₃ is formed in variably size blocks ranged from 100 nm to 500 mm and compared to the original C–Fe₂O₃ sample (ESI Fig. S1†). There is no obvious change observed in the size and morphology. As shown in Fig. 3b, the most intensive diffraction peaks of C–Fe₂O₃ match well with the typical hematite Fe₂O₃, which are shown at the bottom of the Fig. 3b. There is no obvious shift of the diffraction peaks accompanied with the emergence of some new peaks in the H–Fe₂O₃ sample. These new peaks (highlighted by * in Fig. 3b) may be indexed to iron and ferroferric oxide, confirming the successful reduction of Fe₂O₃. Compared to the C–Fe₂O₃, the diffraction peaks of H–Fe₂O₃ sample gradually disappear, while the (110) diffraction peak is more stronger. The (110) diffraction peak suggests that H–Fe₂O₃ have a preferred [110] direction on the substrate, which means that they were grown along the [110] axis. The conductivity of hematite along the [110] axis is four orders of magnitude higher than the [001] direction because of a hopping mechanism related to Fe²⁺/Fe³⁺ mixed valence states.²⁴

Fig. 3 SEM image (a) of H–Fe₂O₃ sample and XRD patterns (b) of H–Fe₂O₃ and C–Fe₂O₃ samples.

To further determine the effect of hydrogen treatment on the surface structure of Fe₂O₃, X-ray photoelectron spectroscopy (XPS) was performed to obtain Fe 2p core-level XPS spectra (Fig. 4). Typical Fe 2p peaks can be observed at 722.8 and 709.5 eV with a shake-up satellite line at 717.3 eV, which is generally assigned to Fe³⁺ in Fe₂O₃ and Fe₃O₄.²⁵ In addition, the H–Fe₂O₃ sample exhibits an obvious satellite peak around 713.1 eV corresponding to Fe²⁺,²⁶ implying the existence of Fe²⁺ (oxygen vacancies) sites which lead to a better catalytic activity. The oxygen vacancies can serve as shallow electron donor to greatly facilitate the electrons migration. Moreover, oxygen vacancies may act as the active sites for absorbing I₃⁻ and catalyzing its reduction for generating I⁻.

Fig. 4 Fe 2p XPS spectra of H–Fe₂O₃ sample.

Conclusions

In summary, the catalytic behavior of the Fe₂O₃, In₂O₃ and SnO₂ CEs prepared in H₂ is enhanced significantly. The enhancement in catalytic activity is because of the increased shallow electron donor and I₃⁻ absorption/reduction sites resulting from the formation of oxygen vacancies. The presence of hydrogen atmosphere is essential for the creation of oxygen vacancies. This work demonstrated a simple and universal method for the preparation of efficient electrocatalysts in dye-sensitized solar cells.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (21373083, 21203061), SRF for ROCS, SEM, SRFDP, Programme for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, Fundamental Research Funds for the Central Universities (WM1314018, WD1313009, WD1214036), and China Postdoctoral Science Foundation (2012M511056, 2013T60425).

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details, SEM image. See DOI: 10.1039/c3ra46109b

‡ These authors contributed equally to this work.

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