Hydrogen Production by Photoelectrochemically ... - Wiley Online Library

DOI: 10.1002/cssc.201100167

Hydrogen Production by Photoelectrochemically Splitting Solutions of Formic Acid Lei Li, Wenliang Guo, Yusong Zhu, and Yuping Wu*[a] A TiO2/FTO (FTO = fluorine-doped tin oxide) electrode was prepared by dip-coating FTO in a suspension of TiO2 prepared from a sol–gel method and was used as a photoanode to split an aqueous solution of formic acid to produce hydrogen. The surface of the TiO2/FTO film was covered with assemblies of TiO2 nanoparticles with a diameter of approximately 20 nm. Under irradiation by using a Xe lamp, splitting of formic acid was performed at different applied current densities. Compared to splitting water or utilizing FTO and Pt foil as the

anode, the splitting voltage is much lower and can be as low as 0.27 V. The results show that the splitting voltage is related to the concentration of free formate groups. The evolution rate of hydrogen measured by using gas chromatography is 130 mmol h1 at a current density of 20 mA cm2 and the energy-conversion efficiency can be 1.79 %. Photoelectrolysis of formic acid has the potential to be an efficient way to produce hydrogen with a high energy-conversion efficiency.

Introduction The energy crisis is one of the most urgent worldwide problems, because fossil fuels will last only a few more decades. There will be a big gap between demand and production of fossil fuels in the future.[1] A new substitute energy source is urgently required. Hydrogen is a promising renewable energy resource for future energy-consumption systems, because it is pollution free, abundant, and has a high energy density. Nowadays, hydrogen is mainly produced by thermal reforming of fossil fuels, for instance, natural gas.[2] There is an increasing demand for an alternative method to produce hydrogen in a sustainable manner.[3] Utilizing solar energy is a promising approach for hydrogen production in the format of heat, light, or electricity.[1, 4] By using sunlight irradiation, most research work on water splitting is focused on homogeneous reactions catalyzed by photocatalysts. Low separation efficiency of holes and electrons is still one of the great barriers for water splitting. It has been confirmed repeatedly that a photoelectrochemical (PEC) method may improve the separation efficiency of electrons and holes by utilizing a chemical or electrical bias to drive photogenerated electrons and holes in different directions and thus reduce their recombination probability.[5] However, most reported energy efficiencies are below 1 %. Formic acid (FA) is one of the major products formed in biomass processing that can be split into hydrogen and carbon dioxide.[6] In addition to hydrogen generation, a sustainable and reversible energy storage cycle can be envisioned by release of hydrogen stored in FA.[7] Furthermore, FA exists in some industrial waste waters, such as effluent from tanners, dye workshops, and printed-fabrics mills.[8] If FA can be used to produce hydrogen, it will not only protect the environment, but also provide clean energy. The overall reaction of water and FA splitting is given in Equations (1) and (2):

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H2 OðlÞ ! H2 ðgÞ þ 1=2 O2 ðgÞ; DG ¼ 237 kJ mol1

ð1Þ

HCO2 HðlÞ ! CO2 ðgÞ þ H2 ðgÞ; DG ¼ 48:38 kJ mol1

ð2Þ

The water-splitting reaction is an uphill reaction [Eq. (1)]. The lowest energy request for the absorbed photon is 1.23 eV for water splitting. However, for the splitting of FA [Eq. (2)], it is a downhill reaction that could be more easily conducted electrochemically. With the help of a photocatalyst, the electrochemical splitting of FA will be further facilitated under light irradiation. Since the pioneering work of Fujishima and Honda on water splitting by using a PEC method,[9] TiO2 has been extensively investigated for its photocatalytic activity and chemical stability in aqueous solutions and low cost for many applications, such as fuel cells,[10] photovoltaic devices,[11] and waste treatment.[12] Herein, a fluorine-doped tin oxide (FTO) electrode coated with a TiO2 film (TiO2/FTO) was prepared by using a dip-coating method. PEC splitting of FA in a PEC cell consisting of a TiO2/FTO photoanode and a Pt foil cathode was investigated, and an energy efficiency up to 1.79 % was achieved. The hydrogen-production rate was also analyzed by using gas chromatography (GC). [a] L. Li, W. L. Guo, Y. S. Zhu, Prof. Dr. Y. P. Wu New Energy and Materials Laboratory Department of Chemistry & Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials Fudan University, Shanghai 200433 (PR China) Fax: (+ 86) 21-5566 4223 E-mail: [email protected]

2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Results and Discussion Characterization of the synthesized TiO2 X-ray diffraction (XRD) patterns of the TiO2 powders annealed at different temperatures are shown in Figure 1. At low temperatures, the prepared TiO2 exists in the form of anatase (JCPDS No. 84-1286) with poor crystallinity. When the tempera-

mately 20 nm in diameter (Figure 2 b and inset). No obvious cracks can be observed. From the cross-section profiles of FTO and TiO2/FTO, the thicknesses of the FTO film and the coated TiO2 film after dip-coating four times are approximately 330 and 130 nm, respectively (Figure 2 c and the upper layer in Figure 2 d). The influence of the annealing temperature of the TiO2/FTO electrodes on PEC splitting of FA was tested in a solution of 2 m FA and 2 m FNa at different applied current densities and is shown in Figure 3. From the V–t curves (V = splitting voltage),

Figure 1. XRD patterns of TiO2 powders annealed at different temperatures.

ture is increased, the peaks corresponding to anatase become sharper, which indicates an increase in crystallinity. At 500 8C, a small peak at 2 q = 27.6 8 appears, which belongs to the rutile phase. This indicates that a mixture of anatase and rutile is formed. The ratio of anatase-to-rutile phases for the TiO2 powders from 500 and 600 8C was calculated by using the Spurr equation to be 86 and 58 %, respectively.[13] It was reported that mixed-phase TiO2 is more effective than the respective single phases because of improved electron–hole separation.[14] Scanning electron micrographs (SEM) of FTO and the TiO2/ FTO electrode are shown in Figure 2. Figure 2 a reveals that the rough surface of FTO consists of bulky particles. After the FTO is coated with a TiO2 film, the surface is uniform and compact and covered with assemblies of TiO2 nanoparticles of approxi-

Figure 3. V–t curves of TiO2/FTO electrodes annealed at a) different temperatures and b) different current densities.

it can be seen that the splitting voltages of the mixed-phase TiO2 treated at 500 and 600 8C are lower than that of the anatase single phase at 400 8C (Figure 3 a), which is consistent with a former report.[14] However, there is no clear difference between the splitting voltages of TiO2 powders from 500 and 600 8C, which have different ratios of anatase (Figure 3 b). As FTO glass is known to melt above 600 8C and by also considering an increased energy consumption at higher temperatures, 500 8C was selected as the annealing temperature to prepare TiO2/FTO electrodes.

Photoelectrochemical splitting of FA

Figure 2. SEM graphs of a, c) bare FTO and b, d) TiO2/FTO after heat treatment at 500 8C.

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The PEC splitting of FA performed in solutions of 2 m FA and 2 m FNa at different current densities is shown in Figure 4, and the results are summarized in Table 1.


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Photoelectrochemical Hydrogen Production

Figure 4. V–t curves of FA at different current densities with irradiation a) on and b) off.

Table 1. Splitting voltages at different current densities. J [mA1 cm2]

Von[a] [V]

Voff[a] [V]

0.2 2 10 16 20

0.27 0.01 0.28 0.50 0.66

1.97 2.44 2.64 2.83 3.35

[a] Mean value of three tests. Figure 5. V–t curves of solutions of a) NaOH with and without FA and of b) FTO in the presence of FA at a current density of 4 mA cm2.

When the irradiation is on, the splitting voltage increases with the current density. Even at 20 mA cm2, the voltage is only approximately 0.66 V, which is still much lower than the theoretical voltage for splitting water (1.23 V). When the irradiation is off, the change is clear. The voltage increases dramatically to approximately 3.3 V, which is attributable to polarization caused by absorbed intermediates and gases not reacting or diffusing away from the electrode promptly. These results clearly indicate the favorable photocatalytic action of TiO2. The V–t curves with and without FA are shown in Figure 5 a. Whether the irradiation was on or off did not have a significant effect, as only a small voltage gap could be seen in the solution without FA, and the voltage was much higher (> 2 V) than that of the reaction performed in a solution of FA with TiO2/ FTO as photoanode, which implies that no catalyzed reaction was detected. This clearly shows that FA, rather than H2O, was photoelectrochemically split. In Figure 5 b, a similar phenomenon occurred when FTO was used as the anode for comparison. The voltage was much higher (> 2 V) than that of the reaction performed with a TiO2 film coated onto the FTO electrode. This result again confirms the photocatalytic action of TiO2 in the PEC splitting of FA. It can be concluded that the photoelectrolysis of a solution of FA was successful, and that the reduction in the required voltage was extremely large, which might greatly enhance the energy-conversion efficiency. The splitting behavior in solutions of 4 m FA/1 m H2SO4, 4 m FA/2 m NaOH, and 2 m FA/4 m NaOH is shown in Figure 6. It is known that the splitting voltage can be used to evaluate the photocatalytic activity of TiO2/FTO on FA. A lower voltage signifies a higher photocatalytic activity. According to a previous ChemSusChem 2011, 4, 1475 – 1480

Figure 6. Splitting voltages of FA in different solutions at current densities ranging from 0.2 to 20 mA cm2.

work by Matsumura et al.,[15] photocatalytic splitting of FA will be favored in an acidic solution. The results in Figure 6 illustrate the same situation. However, the lowest splitting voltage was not obtained in the sulfuric acid solution, but in the solution of 4 m FA/2 m NaOH, which contains some free formate groups. Therefore, the splitting voltage may be related to the concentration of HCOONa (FNa). Further experiments were performed in solutions of 2 m FA with different concentrations of FNa, ranging from 0 to 3 m, and the results are shown in Figure 7. It can be observed that the voltage is much higher when there is no FNa in the solution. The splitting voltage increases along with the current


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Y. Wu et al. charged TiO2 and oxidized by photogenerated holes than FA in the molecular form. Absorbed formate groups are very reactive, and the polarization is small. At low concentrations of FNa, the amount of formate is not high. As a result, the polarization is large and the splitting voltage becomes high at large current density.

Hydrogen evolution After splitting FA, the resulting gas was analyzed when either TiO2/FTO of Pt foil was used as the anode. From the V–t curves of TiO2/FTO and Pt electrodes (Figure 8), it can be observed Figure 7. Splitting voltage in a solution of FA with various concentrations of HCOONa (FNa).

density, which is consistent with the above results (Figure 4). At a lower current density, for example, 0.2 mA cm2, the splitting voltage of FA almost increases with the concentration of FNa. At a higher current density, for example, 20 mA cm2, the voltage decreases with the concentration of FNa. The relationship between splitting voltage and concentration of FNa may be attributable to the interaction of vacant absorption sites and free formate groups. It is generally assumed that FA is oxidized to CO2 by a dual-pathway mechanism.[16] One is a dehydrogenation pathway through a reactive absorbed CCOOH group [Eqs. (3) and (4)], in which the vacant adsorption site on the electrode is indicated by * and (a) represents an absorbed state: HCOOH þ * Ð C COOHðaÞ þ Hþ þ e

ð3Þ

COOHðaÞ Ð CO2 þ Hþ þ e þ *

ð4Þ

The other is a dehydration pathway through absorbed carbon monoxide [Eqs. (5)–(7)]: HCOOH þ * Ð COðaÞ þ H2 O

ð5Þ

H2 O þ * Ð H2 OðaÞ

ð6Þ þ

COðaÞ þ H2 OðaÞ Ð CO2 þ 2 H þ 2 e þ 2 *

ð7Þ

Figure 8. V–t curves recorded by using a, b) TiO2/FTO and c, d) Pt as the anodes at current densities of 2 (a, c) and 20 mA cm2 (b, d).

that voltage oscillations occurred when the Pt electrode was used as the anode, even at relatively low current densities (Figure 8 c). This is caused by the formation of an absorbed poisonous intermediate, CO(a), during the dehydration pathway [Eqs. (5)–(7)]; this intermediate does not have any effect on the TiO2-catalyzed photoelectrochemical reaction.[18] The amount of hydrogen evolved was measured for 1 h at different current densities, as shown in Figure 9. At a current density of 20 mA cm2, the hydrogen production rate at TiO2/ FTO was 130 mmol h1, and the electrolysis yield was 70.0 %.

In a solution containing free formate groups, the dehydrogenation reaction [Eq. (3)] can be revised as shown in Equations (8) and (9): HCOOH Ð HCOO þ Hþ

ð8Þ

hv

ð9Þ

HCOO ðaÞ ! C COOHðaÞ þ e

It is suggested that the acidity of the solution can affect the surface properties of the TiO2/FTO electrode.[17] The solutions containing FA and FNa or NaOH may offer a buffer effect to change the pH of the solution. When the pH value is lower than the isoelectric point of TiO2, the TiO2 surface is positively charged. According to the dehydrogenation pathway [direct oxidation of FA, Eqs. (8) and (9)] mentioned above, free formate groups might be more easily absorbed by positively-

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Figure 9. H2-evolution rate and electrolysis yield at different current densities.


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Photoelectrochemical Hydrogen Production When the current density decreased to 2 mA cm2, the electrolysis yield increased to 95.4 %, which was higher than that achieved by using Pt foil (55.0 %). At 20 mA cm2, the energy-conversion efficiency was calculated to be 1.79 %, which was much higher than most reported values.[19] Since the TiO2 photocatalyst can only harvest UV light, the efficiency can be further improved in different ways, such as by using a visiblelight-driven photocatalyst,[20] reactor design,[21] or tandem cell configuration,[22] with the aim of a more efficient use of sunlight.

Conclusions

XL 30) and field emission SEM (FESEM, Hitachi S-4800). The X-ray diffraction (XRD) patterns were collected by using an XRD meter with monochromatized CuKa radiation (l = 1.54056 ; XRD Bruker D8 Advance). The ratio of the anatase phase was calculated from the resulting XRD patterns by using the Spurr equation [Eq. (10)], in which IA is the intensity of the (101) peak and IR is the intensity of the (110) peak.[13] %Anatase ¼

0:8½IA ð101Þ=IR ð110Þ 1 þ 0:8½IA ð101Þ=IR ð110Þ

ð10Þ

Photoelectrochemical cell

TiO2/FTO electrodes were prepared by dip-coating a suspension obtained by using a sol–gel method. The sample annealed at 500 8C was mixed-phase TiO2 with a composition of 86 % anatasee and 14 % rutile. Scanning electron micrographs show that the as-prepared TiO2 film was compact and uniform with nanoparticle assemblies of 20 nm in diameter. PEC splitting of an aqueous solution of FA to produce hydrogen was investigated in a PEC cell by using TiO2/FTO as the photoanode. Compared with splitting water or utilizing Pt foil as the anode, the splitting voltage was extremely low when the PEC reaction was performed in a solution of FA irradiated by using a Xe lamp. The PEC splitting reaction was favored in an acidic solution containing formate groups. An electrolysis yields of up to 95.4 % of FA and an energy-conversion efficiency of 1.79 % was achieved. Further improvement is underway in our laboratory. PEC splitting of solutions of FA may offer a new route for hydrogen generation and storage, an electricity storage option, or a solution for waste-water treatment.

According to Figure 10, the photoelectrochemical (PEC) set consisted of three parts. A square column vessel made of quartz glass with two electrodes was used as the reaction cell. The circulation channel with a pump and condenser was linked to the PEC cell. A pressure gauge was also contained in the channel to monitor the pressure of the system. A gas chromatograph (GC, Kexiao GC-1690) with a thermal conduction detector was connected to the channel. When it was time to analyze the gas, the lower part of the channel was closed and a new loop was formed at the upper part, at which the gas sample was collected by the gas chromatograph for analysis.

Experimental Section Samples and photoanode preparation All chemicals were obtained from commercial sources and used as received. Fluorine-doped tin oxide (FTO) was used as a transparent conductive substrate for the photoelectrode preparation. According to previous work,[23] TiO2 was prepared by a simple sol–gel method with an excess of water. Tetrabutyltitanate and ethanol were chosen as the Ti precursor and the solvent, respectively. First, solution A was prepared by adding tetrabutyltitanate (14.4 mL) to ethanol (24 mL) while stirring vigorously. Second, ethanol (12 mL) was added to deionized water (76 mL) and the pH was adjusted to 1 by adding HNO3 to form solution B. Then, solution A was added dropwise to solution B. After aging for a certain period of time, the TiO2 sol was obtained. The sol was poured into a large watch glass and dried under an infrared lamp to obtain TiO2 powders. The fluorine-doped tin oxide electrodes coated with a TiO2 film (TiO2/FTO) were prepared by using a dip-coating method. The coating suspension was prepared by adding the above TiO2 powders (0.36 g) and polyethylene glycol (PEG-1000, 0.54 g) to deionized water (9 mL). The rate of dipping was 1 cm min1. After dipcoating, the electrodes were treated at 400 8C for 30 min. This procedure was repeated four times and the electrodes were annealed at different temperatures. Morphologies of the TiO2 powders and TiO2/FTO electrodes were characterized by using scanning electron microscopy (SEM, Phillips ChemSusChem 2011, 4, 1475 – 1480

Figure 10. A diagram of the photoelectrochemical apparatus used.

Photoelectrochemical measurements were performed in a twoelectrode system with TiO2/FTO as the photoanode and Pt foil as the cathode with a potentiostat (CH Instrument, model 660C) and a battery testing system. Unless stated otherwise, the splitting solution was 4 m FA/2 m NaOH. A 500 W Xe lamp (Trusttech CHFXM500) equipped with a concentrator was used as the light source. Incident-light irradiance was tested with a probe of a power meter (Coherent, FieldMaxII-TO). In the PEC cell, hydrogen evolved at the Pt foil, circulated in the sealed system, and was analyzed by using GC periodically. The electrolysis efficiency was calculated from the ratio of the amount of evolved hydrogen to the


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Y. Wu et al. predicted amount on the basis of Faraday’s law. The energy-conversion efficiency was calculated from Equation (11), in which E corresponds to energy: h¼

E input

E output as H2 as incident light transmitted light þ electricity

[11]

ð11Þ

[12]

Acknowledgements Financial support from the National Basic Research Program of China (973 Program No. 2007CB209702) and STCSM (09QH1400400) is gratefully acknowledged. Keywords: electrochemistry · formic acid · hydrogen · photocatalysis · titania [1] D. A. J. Rand, R. M. Dell, Hydrogen Energy: Challenges and Prospects, Royal Society of Chemistry, Cambridge, 2008, p. 13. [2] J. D. Holladay, J. Hu, D. L. King, Y. Wang, Catal. Today 2009, 139, 244 – 260. [3] a) A. Turner, Science 2004, 305, 972 – 974; b) N. Armaroli, V. Balzani, Angew. Chem. 2007, 119, 52 – 67; Angew. Chem. Int. Ed. 2007, 46, 52 – 66. [4] J. Turner, G. Sverdrup, M. K. Mann, P. C. Maness, B. Kroposki, M. Ghirardi, R. J. Evans, D. Blake, Int. J. Energy Res. 2008, 32, 379 – 407. [5] a) A. J. Bard, J. Photochem. 1979, 10, 59 – 75; b) I. M. Butterfield, P. A. Christensen, A. Hamnett, K. E. Shaw, G. M. Walker, S. A. Walker, C. R. Howarth, J. Appl. Electrochem. 1997, 27, 385 – 395; c) J. M. Kesselman, N. S. Lewis, M. R. Hoffmann, Environ. Sci. Technol. 1997, 31, 2298 – 2302; d) K. Vinodgopal, S. Hotchandani, P. V. Kamat, J. Phys. Chem. 1993, 97, 9040 – 9044. [6] a) K. Yoshida, J. Kusaki, K. Ehara, S. Saka, Appl. Biochem. Biotechnol. 2005, 123, 795 – 806; b) I. Ntaikou, H. N. Gavala, G. Lyberatos, Int. J. Hydrogen Energy 2010, 35, 3423 – 3432. [7] a) S. Kakuta, T. Abe, ACS Appl. Mater. Interfaces 2009, 1, 2707 – 2710; b) A. Boddien, B. Loges, F. Gartner, C. Torborg, K. Fumino, H. Junge, R. Ludwig, M. Beller, J. Am. Chem. Soc. 2010, 132, 8924 – 8934; c) B. Loges, A. Boddien, F. Gartner, H. Junge, M. Beller, Top. Catal. 2010, 53, 902 – 914. [8] R. J. Candal, W. A. Zeltner, M. A. Anderson, J. Adv. Oxid. Technol. 1998, 3, 270 – 276. [9] A. Fujishima, K. Honda, Nature 1972, 238, 37 – 38. [10] a) Y. Qiao, S. J. Bao, C. M. Li, X. Q. Cui, Z. S. Lu, J. Guo, ACS Nano 2008, 2, 113 – 119; b) D. Chu, S. Wang, P. Zheng, J. Wang, L. Zha, Y. Hou, J. He, Y.

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Received: March 28, 2011 Published online on August 25, 2011


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