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Wireless Solar Water Splitting Device with Robust CobaltCatalyzed, Dual-Doped BiVO4 Photoanode and Perovskite Solar Cell in Tandem: A Dual Absorber Artificial Leaf Jin Hyun Kim, Yimhyun Jo, Ju Hun Kim, Ji Wook Jang, Hyun Joon Kang, Young Hye Lee, Dong Suk Kim, Yongseok Jun, and Jae Sung Lee ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b03859 • Publication Date (Web): 29 Oct 2015 Downloaded from http://pubs.acs.org on October 31, 2015
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Wireless solar water splitting device with robust cobalt-catalyzed, dual-doped BiVO4 photoanode and perovskite solar cell in tandem: A dual absorber artificial leaf Jin Hyun Kim1, Yimhyun Jo2, Ju Hun Kim1, Ji Wook Jang3, Hyun Jun Kang1, Young Hye Lee1, Dong Suk Kim2, Yongseok Jun4* and Jae Sung Lee3*
1
School of Environmental Science & Engineering, Department of Chemical Engineering
Pohang University of Science and Technology (POSTECH), Pohang, 790-784 South Korea 2
KIER-UNIST Advanced Center for Energy, Korea Institute of Energy Research, Ulsan, 689798 South Korea 3
School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798 South Korea E-mail:
[email protected] 4
Department of Materials Chemistry and Engineering, School of Energy and Chemical Engineering, Konkuk University, Seoul, 143-701 South Korea
1
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Keywords: Dual doping, cobalt carbonate, BiVO4, perovskite solar cell, tandem cell, artificial leaf
ABSTRACT Stand-alone, wireless solar water splitting device without external energy supply has been realized by combining in tandem CH3NH3PbI3 perovskite single junction solar cell with cobalt carbonate (Co-Ci)-catalyzed, extrinsic/intrinsic dual-doped BiVO4 (hydrogen-treated and 3 at% Mo-doped). The photoanode recorded one of the highest photoelectrochemical water oxidation activity (4.8 mA/cm2 at 1.23 VRHE) under simulated 1 sun illumination. The oxygen evolution Co-Ci co-catalyst showed similar performace to best known cobalt phosphate (Co-Pi) (5.0 mA/cm2 at 1.23 VRHE) on the same dual-doped BiVO4 photoanode, but significantly better stability. In a tandem artificial-leaf type device produced stoichiometric hydrogen and oxygen with an average solar-to-hydrogen efficiency of 4.3 % (wired), 3.0 % (wireless) under simulated
1
sun
illumination.
Hence,
our
device
based
on
D4
tandem
photoelectrochemical cell represents a meaningful advancement in performance and cost over the device based on a triple-junction solar cell-electrocatalyst combination.
Water splitting to H2 and O2 by using sun light has been attracting great interest as an ideal technology to harvest and store abundant solar energy as clean and easily transportable hydrogen
fuel.1-4
Among
various
approaches
to
the
solar
water
splitting,
photoelectrochemical (PEC) cell is the most promising device and a large number of research 2
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groups in the world strive to achieve the solar-to-hydrogen (STH) efficiency of >10 %, lifetime of >10 years and a competitive cost through this device.2
Researches on PEC water
splitting have been focused on development of efficient semiconductor photoelectrode materials in a typical three electrode PEC cell with a semiconductor as the working electrode, Pt as the counter electrode (CE), and a reference electrode. The cell requires a bias voltage supplied from an external energy source for efficient overall water splitting.1,2 However, the practical PEC water splitting cell should be a stand-alone device with no need of an external energy supply. Water splitting requires a Gibbs free energy of 1.23 eV per electron, but kinetic barriers (over-potential) involved in reduction and oxidation of water make the energy requirement significantly higher. In general, the small band gap of a visible light-active, single semiconductor photoelectrode cannot straddle both reduction (0.0 VRHE) and oxidation (1.23 VRHE) potentials of water. Hence, a tandem cell with another photoelectrode2,5 or a photovoltaic (PV) cell is commonly employed2,6,7 for the stand-alone devices. It should be noted that those systems of multiple light absorbers and multiple electron transfers are similar to the natural photosynthesis system. Recently, there have been a limited number of reports for stand-alone solar water splitting devices mostly employing well-known PV materials. For example, p-n GaAs/p-GaInP2 shows a high efficiency, but materials are rather expensive, toxic and unstable.2-4,8,9 More stable systems have also been demonstrated such as WO3/dyesensitized solar cell (DSC) and cobalt phosphate (Co-Pi)-W:BiVO4/double junction a-Si PV cell.1,10,11 However, wireless device has attracted more attention because of its mobility, flexible scale, and simple design. Thus, Nocera and coworkers5 reported a wireless solar hydrogen generator with a triple junction a-Si as the sole light absorber with H2 and O2 evolution electrocatalysts, 3
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which they called ‘artificial leaf’. The relatively low efficiency (2.5 % using 10 % efficiency triple junction a-Si cell) in spite of the expensive PV cell has been cited as the drawback of this artificial leaf. This single absorber system also has a lower theoretical efficiency (~12 %) than two tandem absorbers (~22%) or double parallel absorbers (~17%) for unbiased solar water splitting2. Here we report a wireless solar water splitting device depicted in Figure 1 (more detailed scheme in Figure S1 of Supporting Information, SI), which are made of lowcost, earth-abundant materials, yet highly efficient, durable, and thus suitable for applications to drive sustainable solar fuel production. An important feature of our devices is employment of metal halide (CH3NH3PbI3) perovskite single junction solar cell. This most-recently developed solar cell is known for its high efficiency (~18%),12 relatively low cost13 and viability for multiple modifications. Most interestingly, it has high photovoltage, large fill factor and abundant light adsorption (band gap = 1.5 eV, ~810 nm), which make it an ideal candidate for the 2nd absorber of the tandem PEC device. Recently, the double junction perovskite solar cell was used in a PV-electrolysis setup to obtain STH efficiency of 12.3 %.14 A single junction perovskite solar cell was also used very recently to fabricate a wired tandem cell with a Co-Pi/BiVO4 photoanode to give 2.5% STH efficiency and CoPi/Mn:Fe2O3 photoanode to give 2.4% STH efficiency.15,16 In addition to the relatively low efficiency, both systems suffer from instability. Thus, the Co-Pi/BiVO4-CH3NH3PbI3 tandem cell showed the photocurrent density starting from 2.0 mA/cm2 then decreasing to 1.5 mA/cm2 in just 5 min and a low faradaic efficiency of only ~50%.
This indicates that only
50% of the measured photocurrents are from water splitting and the other half probably comes from corrosion.16 The Co-Pi/Mn:Fe2O3-CH3NH3PbI3 cell showed 25% decay of photocurrent in 8 h due to the exposure of unencapsulated perovskite solar cell to humid 4
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atmosphere.15 Hence, fabrication of a stable solar water device with perovskite solar cell has yet to be demonstrated. Our stand-alone water splitting device employs a cobalt carbonate (Co-Ci)-catalyzed, extrinsic/intrinsic dual-doped BiVO4 (hydrogen-treated and 3 at% Mo-doped) photoanode and a CH3NH3PbI3 perovskite single junction solar cell, which is stable as well as more efficient than reported perovskite-based tandem cells. As the primary absorber, monoclinic scheelite BiVO4 is used with multiple modifications. The material is one of the most popular oxide photoanodes because of the direct band gap of 2.4 eV, wide pH stability, low cost, and nontoxicity.4,8,17 Recently, the performance of BiVO4 has been greatly improved by various modifications including doping, nano-structuring, heterojunction formation and surface modification by a co-catalyst.4 Thus the charge separation efficiencies for both bulk and surface of the BiVO4 photoanodes have reached ~90% by nano structuring and deposition of a co-catalyst, respectively.18,19 However, most of these modifications bring turbidity of the 1st absorber film and reduce the light intensity that reaches the 2nd absorber of the tandem cell.4,20-22 In the present case, we apply unique extrinsic/intrinsic dual doping techniques to increase bulk charge separation efficiency of BiVO4 film; hydrogen treatment at 300 oC to form intrinsic Vo.. defects23-25 and 3 at% Mo doping to form extrinsic Mov. defects in BiVO4 lattice. We chose Mo as an extrinsic dopant because our previous study showed that Mo performed better than the other common dopant (W) for modifying BiVO4.26 Then, its surface is decorated with Co-Ci as an oxygen evolution co-catalyst (OEC) to minimize surface recombination. The Co-Ci co-catalyst showed similar performace to best known cobalt phosphate (Co-Pi) co-catalyst on the same dual-doped BiVO4 photoanode, but significantly better stability. We were able to realize a highly stable and efficient unbiased 5
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water splitting device with perovskite solar cell for solar hydrogen production and demonstrated the first wireless artificial leaf with the photoanode-solar cell tandem (Supporting Movie 1).
RESULTS AND DISCUSSION Extrinsic/intrinsic dual-doped BiVO4 photoanodes The most important part of our artificial leaf device is the photoanode as the primary light absorber. In order to assess the effect of dual doping on bulk charge separation only, PEC measurements were performed first in a three electrode cell with an aqueous solution electrolyte containing a sacrificial reagent (sulfite) that does not involve surface charge recombination. Hydrogen treatment and Mo doping considerably increased photocurrent of BiVO4 photoanode (Figure 2a). The two modifications complement each other with H, 3% Mo:BiVO4 showing the much higher photocurrent than either H:BiVO4 or 3%Mo:BiVO4. In an extensive optimization study summarized in Table S1 and Figure S3-S6, the optimum temperature of hydrogen treatment and the optimum Mo-doping level giving the best performance were determined to be 300 oC and 3 at%, respectively. The performance of the optimized H, 3% Mo:BiVO4 is among the highest reported for BiVO4-based photoanodes – recording 5.1 mA/cm2 at 1.23 VRHE in sulfite electrolyte.3,4,8,9,11,18,21,27 In Figure 2b and Figure S7, the H, 3% Mo:BiVO4 photoanode records an incident photon-to-current conversion efficiency (IPCE) of ~80 % at 420 nm and a light harvesting threshold of ~510 nm. This IPCE value is one of the highest for metal oxide photoanodes.3,4,18,21,28
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The Mott-Schottky plots26 in Figure 1c and Figure S7 indicate that both hydrogen treatment 18 and Mo doping increase the charge carrier density (ND) significantly – BiVO4 (3.48×10 cm 3
) < H:BiVO4 (2.42×10
(4.69×10
20
19
cm-3) < 3% Mo:BiVO4 (1.18×1020 cm-3) < H, 3% Mo:BiVO4
cm-3). As mentioned already, this is due to the formation of intrinsic (Vo..) and
extrinsic (Mov.) defects as expressed in Kröger-Vink notation,23,29-31 Intrinsic defect: + → + .. + 2
Extrinsic defect: 2 + → 2 + . + 8 + + 2
The bulk separation efficiency ηbulk in Figure 2d was calculated according to the procedure described in Experimental Details S10 of SI. At 1.23 VRHE, ηbulk of BiVO4 increased from 12.8 % to 87.7 % upon dual doping. This value is similar to the highest reported values (~90 %) usually obtained by reducing the hole transport length by forming nanostructures.18,20,21,32 The doping also extended the hole diffusion length Lp estimated by the reported procedure33,34 described in S11 by using experimentally measured IPCE (Figure S7) and absorption coefficient (α) at λ = 400 nm. As summarized in Table S2, Lp increased in the order of BiVO4 (60 nm) < H:BiVO4 (100 nm) < 3% Mo:BiVO4 (420 nm)
