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Energy Harvesting from Atmospheric Humidity by a Hydrogel-Integrated Ferroelectric-Semiconductor System
Article
Energy Harvesting from Atmospheric Humidity by a Hydrogel-Integrated FerroelectricSemiconductor System Lin Yang, Dilip Krishna Nandakumar, Linqing Miao, ..., Jayraj V. Vaghasiya, Ki Chang Kwon, Swee Ching Tan [email protected]
HIGHLIGHTS BaTiO3@BiVO4 core-shell structure is designed for photoelectrochemical application Outward vector of built-in electric field is beneficial for driving holes to the surface Zn or Co super-hygroscopic hydrogel is used for humidity harvesting from atmosphere Photocurrent for series system is 0.4 mA/cm2, and the relative humidity reduced 12.0%
A ferroelectric-semiconductor hybrid is assembled, producing an outward vector of built-in electric field after positive polarization, while the Zn/Co superhygroscopic hydrogel is used for atmospheric humidity harvesting. The photoanode-hydrogel-solar cell series system can capture water from humid air and release the water splitting under indoor light, generating a photocurrent of 0.4 mA/cm2 and decreases the relative humidity by 12.0%. This device can serve as dehumidifier and power-generator for home painting, guiding the comfortable relative humidity and temperature level.
Yang et al., Joule 4, 1–13 January 15, 2020 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.joule.2019.10.008
Please cite this article in press as: Yang et al., Energy Harvesting from Atmospheric Humidity by a Hydrogel-Integrated Ferroelectric-Semiconductor System, Joule (2019), https://doi.org/10.1016/j.joule.2019.10.008
Article
Energy Harvesting from Atmospheric Humidity by a Hydrogel-Integrated Ferroelectric-Semiconductor System Lin Yang,1 Dilip Krishna Nandakumar,1 Linqing Miao,1 Lakshmi Suresh,1 Danwei Zhang,1 Ting Xiong,1 Jayraj V. Vaghasiya,1 Ki Chang Kwon,2,3 and Swee Ching Tan1,4,*
SUMMARY
Context & Scale
Direct utilization of abundantly available solar energy is a promising way to create a sustainable society. Here, we report a ferroelectric-semiconductor (BaTiO3@BiVO4) hybrid that uses an effective strategy to enhance charge separation and transfer during water oxidation. The tetragonal BaTiO3 can induce an outward vector of built-in electric field after positive polarization, which aids in increasing the photovoltage and accelerating holes’ transfer to BiVO4’s surface. A super-hygroscopic metal hydrogel serves as an atmospheric humidity harvester for continuous water supply to the hybrid, where water oxidation takes place. As the hydrogel absorbs moisture from ambient humid air, it functions as a dehumidifying agent and carries water to the photoanode for power generation, being connected in series with a solar cell, further boosting the carrier’s mobility. This photoanode-hydrogel and solar-cell intelligent assembly can generate a photocurrent of 0.4 mA/cm2 with a relative humidity reduction of 12.0% under an illumination of 10 mW/cm2.
The world’s total energy consumption is projected to increase from 30% to 50% in the next 20 years. However, solar water splitting provides a sustainable and environmentally benign route for the production of clean fuels to tackle the escalating energy needs. For traditional photoelectrochemical water splitting systems, there exists two great challenges: the water source and the extra electronic energy. Here, we developed a ferroelectric-semiconductor photoanode (BaTiO3@BiVO4) that can perform efficient water oxidation, and the versatile superhygroscopic hydrogel can directly harvest the humidity from the atmosphere. Combining the photoanode-hydrogel device with a solar cell, the relative humidity is decreased (12.0%) while the photocurrent is simultaneously generated (0.4 mA/cm2). That is, the functions of BiVO4, BaTiO3, hydrogel, and the solar cell are photo absorption and water oxidation, charge accelerator, humidity harvester, and extra electronic energy input.
INTRODUCTION In the search for alternatives to non-renewable energy, the abundant availability of sunlight has been the subject of intense research because of its potential as a clean energy source.1–3 Light-driven water splitting over a semiconductor surface in which photogenerated holes are used to oxidize the hydroxyl ion could serve as one pathway for sustainable energy production and reuse.4 The momentous challenge in scaling up the water splitting reaction is the need for large amounts of aqueous media. Atmospheric humidity holds a major share of water in vapor phase (around 5.3 g water per cubic meter with saturated relative humidity (RH) at 22.5 C in air), which could be used as a water source for energy generation. However, research on the realization of a single device that can effectively dehumidify air and produce electricity is sparse and the reaction mechanism of such devices are poorly understood.5 The limitations for such a system lies in the difficulty to (1) optimize photoelectrode structure for efficient spatial charge separation and transfer,6,7 (2) develop a versatile hygroscopic material for rapidly harnessing humidity from ambient environment, and (3) construct a single device that can be assembled by an electronic energy self-generation system (zero energy input). To comprehend this advanced apparatus, a photo-absorber and charge-manipulator, a humidity-absorber, and an energy source should be combined as an integrated system. In general, n-type semiconductors with higher Fermi level (closer to conduction band) are usually conducive for water oxidation, which is attributed to the high-activated energy of holes that could drive the sluggish four-electron transfer process.
Joule 4, 1–13, January 15, 2020 ª 2019 Elsevier Inc.
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Please cite this article in press as: Yang et al., Energy Harvesting from Atmospheric Humidity by a Hydrogel-Integrated Ferroelectric-Semiconductor System, Joule (2019), https://doi.org/10.1016/j.joule.2019.10.008
Bismuth vanadate (BiVO4) has recently emerged as one of the prospective photoanodes in photoelectrochemical (PEC) water splitting,8–10 which has many merits including broad visible light absorption (up to 520 nm) and adequate overpotential for oxygen evolution. Unfortunately, the fundamental deficiency of BiVO4 lies in its relatively weak charge separation and transfer.11–13 Nevertheless, embedding a ferroelectric material into a semiconductor is an effective method to overcome the dilatory charge migration, and this excellent structure could be benefited by the tiny displacement of atoms in an asymmetric ferroelectric crystal structure (charge redistribution creates a dipole).14–16 The formed built-in electric field by ferroelectric dipole could freely manipulate the photogenerated carriers’ transfer direction and speed, which is regarded as a driving force for electrons and holes.17–19 For example, it has been reported that SrTiO3-decorated TiO2 could enhance the photocurrent up to 1.43 mA/cm2, whereas the photocurrent of BaTiO3@TiO2 has been improved by about 2.6 times.20,21 Therefore, if the photoelectrode is synergistically embedded by the ferroelectric substance with well-tuned polarization, the mobility of charges will be reinforced, thereby leading to the higher efficiency in photon-to-electron conversion. Considering the basic drawbacks associated with conventional PEC systems for water splitting—the huge requirement of liquid—harnessing the moisture content in air is a viable alternative; as atmospheric humidity holds about 1.29 3 1016 kg of water on Earth. Atmospheric water is a ubiquitous ambient resource, which is not only redundant but also needs input of a large amount of energy to keep it within comfortable levels,22,23 especially in coastal areas with high RH. Recently, metalorganic frameworks, interpenetrating polymer network gel, and sulfur-rich MoSx have been utilized for harvesting moisture directly from the atmosphere.24–26 However, one of the critical limitations of these materials are the relatively slower humidity absorption rate and lower water uptake. To counter this problem, we have developed a super-hygroscopic hydrogel based on zinc/cobalt, which can harvest water more than 4 times its own weight.27 Here, we have demonstrated a ferroelectricsemiconductor hybrid along with the hydrogel and synergistically coupled them with a solar cell to concurrently dehumidify and generate energy from ambient environment. In a nutshell, we have proposed a strategy to suppress the recombination of photogenerated-electron-hole pairs, which is achieved by ferroelectric-semiconductor (BaTiO3@BiVO4) structure. The tetragonal ferroelectric-semiconductor will induce an outward vector of built-in electric field with the help of positive polarization, thus enhancing charge separation and transfer. When the hydrogel harvests humidity from the atmosphere, it acts as an electrolyte (water supplier) for the continuous PEC reactions. This investigation provides insight into atmospheric water splitting by an advanced PEC system, resulting in reduction of RH and temperature by 12.0% and 6.2 C and simultaneously generating a photocurrent of 0.4 mA/cm2. 1Department
RESULTS Structural Characteristics of a Semiconductor and Ferroelectrics All BiVO4 (BVO)-based photoanodes were synthesized through a three-step protocol, including electrodeposition, annealing, and alkaline solution washing.10 The schematic overview of the synthesis procedure is shown in Figure 1A. By this method, the ferroelectric BaTiO3 (BTO) nanoparticles could sink into the vertically intersected BiOI nanoflakes (crimson two-dimensional structure, Figure S1), guiding an in situ growth and forming the BTO@BVO hybrid. After washing by alkaline
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of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117574, Singapore
2Department
of Chemistry, National University of Singapore, 3 Science Drive, Singapore 117543, Singapore
3Centre
for Advanced 2D Materials (CA2DM), 6 Science Drive 2, Singapore 117546, Singapore
4Lead
Contact
*Correspondence: [email protected] https://doi.org/10.1016/j.joule.2019.10.008
Please cite this article in press as: Yang et al., Energy Harvesting from Atmospheric Humidity by a Hydrogel-Integrated Ferroelectric-Semiconductor System, Joule (2019), https://doi.org/10.1016/j.joule.2019.10.008
Figure 1. Ferroelectric-Semiconductor Synthesis and Structure: The Photoanode Synthesis Procedure and the Morphologies of Materials (A) Schematic diagram of the synthesis procedure for BTO@BVO hybrid on the FTO substrate, including electrodeposition and annealing. (B–D) The SEM images of (B) nanoporous BVO (inset figure is the cross-section view), (C) top view of BTO@BVO composite (inset is the large view of this structure), and (D) elemental mapping of BTO@BVO. The yellow arrows point to the BTO. (E) HRTEM lattice fringe with the interface of BTO and BVO, the inset is the structure of BVO surrounded by BTO nanoparticles.
solution, the yellow colored nanoporous BVO was obtained (composed by 100 nm sized nanoparticles), which is highly crystalline and interconnected (Scanning electron microscopy [SEM] image in Figure 1B). The as-prepared semiconductor was identified to have a monoclinic crystal structure, and the positions of binding energy correspond to the typically pure BVO (Figure S2). For fabrication of the ferroelectricsemiconductor, the BTO was annealed at 1,100 C for 30 min, resulting in the phase change of the crystal from cubic to tetragonal, as seen from the split double peaks, and leading to a larger c axis with asymmetrical structure (Figure S3).28 In addition, the X-ray photoelectron spectroscopy plots reflect the double peaks with Ba 3d at 781.1 and 796.1 eV,29 which suggests the classical Ba2+ in the annealed BTO (Figure S4). This BTO served as ferroelectric to construct the BTO@BVO hybrid, demonstrating a homogeneously inserted morphology after heat treatment (Figure 1C). The elemental mappings of Bi, Ba, and Ti indicate the color of Bi element is darker in the BTO region, while the colors of Ba and Ti elements are brighter in the corresponding position exactly, representing the proposed BTO@BVO structure (Figure 1D). The transmission electron microscopy (TEM) image depicts that the BTO nanoparticles are dispersed into BVO uniformly, without any obvious aggregation, and the size of BTO is about 10 nm in the hybrid structure (Figure 1E). The interface structure is conducted by the high resolution TEM (HRTEM), the inter layer distance (d-spacing) is calculated from the lattice fringes, and the values of 2.84 and 3.08 A˚ could be assigned to (101) and (112) crystal plane of BTO and BVO, respectively.30 These results clearly prove the sample is highly crystalline and BTO is evenly distributed on the nanoporous BVO (Figure S5). Furthermore, Kelvin probe force microscopy (KPFM) technique was used for understanding the charge distribution and surface potential on the nanometer scale under ambient conditions. Topography of the BTO@BVO portrays the nanoporous BVO surface covered by BTO nanoparticles (Figure 2A). For a semiconductor, the surface potential is always related to the surface energy (different from the bulk work function), which is ascribed to the formation of a space charge region near the
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Figure 2. Atomic and Kelvin Probe Force Microscopy Results and the Ferroelectric Loops for BaTiO3 (A) The height image (topography) of BTO@BVO by AFM technique at ambient environment. (B) KPFM surface potential map corresponding to the topography in Figure 2A, the prominent particle is BTO, while the others are BVO. (C) Surface potential shifts from BVO to BTO through the zone marked by orange arrow in Figure 2B. (D) Switching spectroscopy piezoresponse force microscopy of BTO nanoparticle with butterfly loopfor amplitude versus voltage and phase switching loop for phase versus voltage. (E) Polarization-electric field hysteresis loop for BTO film at ambient conditions, the hysteresis period is 20 ms (inset is the photograph of prepared BTO on Cu film). (F) Schematic diagram of built-in electric field formed by ferroelectric for enhancing charge separation and transfer, which assists the upward band structure bending by positive charge accumulation at BVO surface, the positive-negative charge pairs are accumulated at both sides of BTO to induce the built-in electric field (E in ).
interface.31,32 Therefore, surface potential mainly elucidates the properties of a thin surface layer, directly describing the work function and Fermi level of materials. A clear difference in the surface potential map is observed in BTO@BVO hybrid (Figure 2B), which could be attributed to the difference in the band structures of BTO and BVO. The brighter color of BTO implies that it has a higher surface potential when compared to the BVO (darker shade). To plainly contrast the surface potential changes within the ferroelectric-semiconductor structure, the surface potential curve is shown in Figure 2C, which is represented by the right orange direction (cross over in the BTO region). It is distinctly revealed that the surface potential signal in BTO@BVO delivers a 45 mV gap in space, illustrating that the Fermi level of BTO is higher than that of BVO, which is the result of a higher surface potential (due to the lower work function).32 Consequently, the calculated work function of BTO and BVO approaches to 4.90 and 4.95 eV, the lower work function of BTO (0.04 eV lower than BVO) was also confirmed by ultraviolet photoelectron spectrometer (UPS) measurement (Figure S6). To further investigate the properties of tetragonal phase BTO, switching spectroscopy piezoresponse force microscopy (SS-PFM) was performed to the ferroelectricity by local polarization switching behavior and piezoresponse. This method is an effective tool to check the statics and dynamics of ferroelectric domains at nanometer scale on a material’s surface.33 Figure 2D depicts the amplitude-voltage butterfly and phase-voltage loops of BTO, reflecting a typical ferroelectric behavior (has a representative polarization switchable behavior
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Figure 3. PEC Performances of As-Prepared Samples in Phosphate Buffer (pH 7.0) and Hydrogels Under LED Cool Daylight Illumination (10 mW/cm2) at 22.5 C (A) Standard physical setup for PEC water splitting test in 0.5 M phosphate buffer with/without 0.2 M Na 2 SO 3 , all illuminated area is fixed to 1 cm 2 (circular area with a radius of 0.57 cm). (B and C) The efficiencies of (B) charge separation and oxidation kinetics versus potential and (C) photovoltage changes for various photoanodes structure. (D) The photograph of hydrogel-integrated BTO@BVO device (the upper is primitive hydrogel [left], dry hydrogel [middle], and photoanode coated structure [right]); the lower is the twoelectrode test apparatus. (E and F) The water harvesting rate of (E) photoanode-hydrogel device when exposed to 70.0% relative humidity for 12 h under dark, and (F) current-voltage curves of this device with dark and light.
to form a stable built-in electric field).33,34 Polarization-electric field hysteresis loop measurement that implemented at 50 Hz also confirms the ferroelectricity of annealed BTO by the characteristic ferroelectric loop (Figure 2E). A remanent polarization of 0.33 mC/cm2 is observed while the downward trend at high electric field could be owed to the minor leakage of electricity. Moreover, the photoluminescence plots expound the charge separation feature with different structures, indicating that BTO@BVO has a lower charge recombination rate (higher charge separation) due to the existence of ferroelectric properties (Figure S7). For BTO@BVO hybrid, the BTO induces a built-in electric field (Ein) by nature or after polarization, which in turn affects the band structure of BVO (Figure 2F). The free electrons will flow from BVO to BTO due to the effect of ferroelectric dipole (rightward), leading to the electrons and holes getting accumulated pairwise at the BTO and BVO surfaces, thereby bending the band structure of BVO upward. The Ein will drive the photogenerated holes to the BTO surface where efficient water oxidation takes place, in other words, enhances the charge transfer process. Photoelectrochemical Water Oxidation in Neutral Solution To gain a better understanding of how the ferroelectric affects the activity of BVO in a PEC cell, typical current-potential (J-V) curves of all samples were acquired in 0.5 M phosphate buffer solution (pH 7.0) under light emitting diode (LED) cool daylight illumination (10 mW/cm2). The active area of the photoanode is maintained at 1 cm2 by a Teflon polymer gasket (physical setup is shown in Figure 3A), and the sample is illuminated from the front side for water oxidation. Different structures with varied BTO concentrations (from 1 to 9 mL) were conducted and the onset photo
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absorption edge was unchanged for all samples, and the band gap was determined to be 2.4 eV (Figure S8).10,13 The photocurrent responses clearly reveal the 5 mL BTO-decorated BVO has the highest photocurrent of 0.287 mA/cm2 at 1.23 V versus reversible hydrogen electrode (RHE), this phenomenon could be ascribed to the fact that less ferroelectric produces less Ein, but more ferroelectric will decrease the conductivity of photoanodes (Figure S9). In order to generate more directional Ein, the BTO@BVO was pretreated by applying a bias of G20 V for polarization.20 Thus, a positive polarized (positive voltage) photoanode generates a higher photocurrent than negative polarization. This is because, the positive polarization will induce an outward vector of Ein, which is beneficial for transfer of photogenerated holes to surface where water oxidation process occurs, whereas the negative polarization induces the inward vector of Ein that hinders the hole movement to the surface. Consequently, the 5 mL BTO-decorated BVO was chosen as the optimized photoanode with positive polarization for our further experiments. The composition percentage of BTO was verified by inductively coupled plasma (Table S1), indicating that the mass of BTO and BVO is 8.2 and 228.7 mg/cm2, respectively (the mass loading percentage of BTO is about 3.5%). For unmodified BVO, the photogenerated electronholes recombined easily at the surface. On the contrary, the BTO@BVO displays an enhanced photocurrent of 0.359 mA/cm2 at 1.23 V versus RHE with a photocurrent enhancement of 104% compared with BVO (Figure S10). The onset potentials of BVO and BTO@BVO are 0.225 and 0.153 V versus RHE, which depicts that the directed ferroelectric can effectively reduce the energy barrier for charge transfer (Ein could offer an extra dynamic for water oxidation).10 The negatively shifted onset potential of BTO@BVO is also corresponding well with the negative shift of flatband potential (77 mV) (Figure S11), manifesting the improved holes’ transfer kinetics. The positive quasilinear behavior of Mott-Schottky plot implies the n-type semiconductor, and the smaller slope means higher carrier concentration for [email protected] Additionally, Figure 3B represents the efficiency of charge separation (hsep) for BVO, and BTO@BVO is 67.8% and 81.8% at 1.23 V versus RHE, demonstrating that the directed ferroelectric could enhance the charge separation. The efficiency of oxidation kinetics (hox) is also improved from 34.2% for BVO to 56.1% for BTO@BVO at 1.23 V versus RHE, which shows a greater advantage for charge transfer with ferroelectric-semiconductor hybrid structure (attributed to the outward vector of Ein). These results elucidate that BTO@BVO is conducive to enhancing charge migration (holes will be pushed to surface for water oxidation, and electrons will be driven to fluorine-doped tin oxide [FTO] substrate) due to the upward band bending of BVO and polarized ferroelectric. To shed light on the photogenerated electrons’ transfer from the surface of photoanode to FTO substrate,35 the electron’s transport time (t) with different applied potential at 1 mW/cm2 (the wavelength of incident light is 365 nm) was calculated (Figure S12). The longer transport time of the generated electrons in BVO (0.92 ms) suggests a lower charge collection efficiency when compared with BTO@BVO (0.77 ms) at 1.23 V versus RHE,3 which is in close agreement with hox and their corresponding J-V curves. Also, the charge transfer impedance of BTO@BVO is lower at open circuit potential, which is described by the smaller semi-circle. When photogenerated carriers are separated spatially, charges will be redistributed because the electrons are excited from valence band to conduction band. Thus, the holes’ quasi-Fermi level (Ef,p) drops down from the electrons’ quasi-Fermi level (Ef,n) to create a photovoltage (Vph) at the photoanode and electrolyte interface,36 the value of Vph is calculated as |Vdark-Vlight| by measuring the open circuit potential in dark and illuminated state (Figure 3C). The BTO@BVO produces a higher Vph (0.459 V), reflecting a stronger charge transfer dynamics that is manipulated by the outward vector of Ein in ferroelectric-semiconductor hybrid. Moreover,
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the BTO@BVO exhibits nearly 24% enhancement of incident photon-to-electron conversion efficiency in the entire photo absorption region along with excellent stability (continuous water oxidation over 10 h) (Figure S13). Photoelectrochemical Water Oxidation in Hydrogels To estimate the performance of ferroelectric-semiconductor hybrid for simultaneous dehumidification and energy production, we have integrated the hybrid with a water source, which is capable of absorbing moisture from humid air at a rapid pace. Two super-hygroscopic hydrogels were developed, one Zn based, and another Co based, these hydrogels are extremely hygroscopic and colorful in ambient environment.27 The hydrogel portrays an interesting change in physical properties upon moisture sorption in dehydrated (DH) and hydrated (H) states (Figure S14). The SEM mapping identifies the Zn hydrogel contains Zn, C, O, and N elements in DH state, while Co hydrogel contains Co, C, O, N, and Cl elements (Figure S15). This result reveals that the hydrogels contain only a small fraction of metal (lower relative content of Zn or Co), and carbon is the major constituent in these hydrogels. Besides, TEM image of hydrogel indicates the particle-distributed morphology; the calculated d-spacing from the lattice is 0.29 and 0.51 nm for Zn and Co hydrogel, displaying weaker crystallinity (Figure S16). Thermogravimetric analysis indicated that the wet hydrogels have a total weight loss of 62.9% for Zn hydrogel and 38.5% for Co hydrogel around 100 C (the water absorption is 0.59 and 1.6 times in DH Zn and Co hydrogel).37 Subsequently, the super-hygroscopic hydrogel was used for harnessing ambient humidity and as the water source for photoanode with direct atmospheric water splitting (Figure S17). The physical setup of the device is composed of three components: a graphite sheet, as a counter electrode; hydrogel, for water harvesting; and BTO@BVO, for water oxidation (Figure 3D). The device was assembled and placed in an oven at 50 C for 20 min, causing the hydrogel to desorb all the water and reach DH state; then placed on a weighing balance at 70.0% RH; and the changes in weight with time was recorded. The Zn and Co hydrogels can absorb 10.1 and 21.0 mg of water, respectively, from ambient humid air within 12 h (Figure 3E). The J-V curves for the two-electrode system consisting of BTO@BVO and hydrogels were conducted, and a larger photocurrent of 0.658 mA/cm2 at 0.8 V was generated in the Co hydrogel system (the Zn hydrogel system only produces a photocurrent of 0.293 mA/cm2) (Figure 3F). This is a direct consequence of the two-fold water absorption capability of Co hydrogel leading to 2-fold higher photocurrent, thus, almost 5.3 mg water could be split within one day, theoretically. Furthermore, the onset potential for the Zn hydrogel system ( 0.49 V) is slightly lower than the Co hydrogel ( 0.46 V), indicating the energy barrier for charge transfer in Zn hydrogel is lower.9 Electrochemical impedance spectroscopy at open circuit potential was performed for interface charge migration kinetics under illumination (Figure S18). The small arc radius for ferroelectric-semiconductor hybrid suggests a faster charge transfer kinetics, while the resistance of the Co hydrogel (15.3 U) is lower than the Zn hydrogel (60.1 U) due to the rapid water harvesting by the Co hydrogel.8 Therefore, the photocurrent at higher applied bias is decided by the rate of moisture capture and the conductivity of hydrogels. Furthermore, the stability test of the device indicates that the consumed water could be replenished by hydrogels (no cocatalyst like CoOx was deposited on the surface of photoanode) and that the photocurrent just split 2.6 mg water while the hydrogel absorbed 21.0 mg humidity within 12 h (Figure S19). Solar Cell Combined with Device for Dehumidification To achieve a higher efficiency in atmospheric water splitting, an additional external bias is required. Here, we have assembled a commercially available polysilicon solar
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Figure 4. Solar Cell Driving the BaTiO3@BiVO4 Water Splitting under Hydrogel Systems (A) The circuit diagram of the solar cell, photoanode-hydrogel device, and ammeter in series connection. (B) Current-voltage curve and stability test for the polysilicon solar cell. (C) Current-time plots with chopped illumination for the hydrogel system by shading different components. (D) Water splitting in a sealed quartz tube by solar cell and device system, the diameter and length of this space is 6 and 100 cm, respectively. (E) The relative humidity changes by placing the system into the tube and the light illuminant is LED cool daylight (10 mW/cm 2 ), leading to the dry environment in tube. (F) The temperature changes in the tube space under tungsten lamp illumination with dry and wet environment.
cell in series with the ferroelectric-semiconductor hybrid and hydrogel device. The solar cell provides the required bias for efficient PEC atmospheric water splitting.38 The open circuit voltage for the solar cell is found to be around 1.15 V under 10 mW/cm2 illumination (Figure S20). Therefore, an integrated system was established, consisting of a solar cell, the photoanode-hydrogel, and an ammeter, for concurrent dehumidification and energy generation (Figure 4A). The purpose of the solar cell, hydrogel, photoanode, and ammeter are for providing bias, harvesting humidity, water oxidation, and photocurrent measurement, respectively. There is no extra electronic energy input, just stem from the weak abandoned light to the solar cell and photoanode. The J-V plot of the solar cell displays a stable short circuit current of 13.18 mA and an open circuit voltage of 1.15 V under continuous 10 h illumination (Figure 4B). This series connection was performed after the hydrogel was water saturated at 70.0% RH in air. The result clearly demonstrates that the photocurrent is switched off or on by dark or illumination, which is determined by blocking the solar cell or device (Figure 4C). A 0.4 mA/cm2 photocurrent (about 3.2 mg water will be split in one day) for the Co hydrogel system was generated only when the solar cell and the device were simultaneously illuminated (Video S1). Compared with the Co hydrogel system, the photocurrent is more stable in the Zn hydrogel system but merely reached 0.2 mA/cm2 , implying that the more the water absorbed and the more conductive the hydrogel is, the faster the water splitting will be and the larger the photocurrent will be. Moreover, we evaluated the photocurrent only by device and ammeter, in other words, it just depends on the energy band position and photovoltage of semiconductor for water splitting.32,36 A photocurrent of 0.064 and 0.138 mA/cm2 wass obtained for the Zn hydrogel and the Co hydrogel systems,
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respectively (Figure S21; Video S2), which was almost a 2-fold enhancement in photocurrent and coordinates well with the nearly 2-fold higher water absorption by the Co hydrogel. This system was placed into a sealed quartz tube for dehumidification and temperature estimation under LED cool daylight or tungsten lamp illumination at ambient environment (Figure 4D). Before the system was placed into the tube, it was placed at atmosphere for water absorption till the water is saturated, confirming the humidity changes observed in the tube were not due to additional hydrogel capture. The LED cool daylight does not emit any infrared radiations and hence there was no increase in temperature upon illumination (temperature remained at 22.5 C). After 24 h of illumination, the RH decreased to about 7.5% and 12.0% in Zn and Co hydrogel systems, respectively (Figure 4E). A higher RH reduction in the Co hydrogel system is attributed to its faster moisture capture ability, which aids in efficient dehumidification.4,39 In addition, a tungsten lamp was applied for detecting the increased temperature at different RH environment, the calorific lamp was simulated as the climate vicissitudes inside the tube. Figure 4F shows a dilatory temperature increasing in the dry tube space (low RH), while in the wet tube space (high RH) there was a rapid temperature increase under the tungsten lamp illumination. This apparently led to a 3.5 C and 6.2 C decrease in the Zn and Co hydrogel systems, respectively, suggesting minimal temperature changes at low RH state by using the intelligent system, which could be used for dehumidification and reduction in temperature.27 Device Conceptualization and Working Mechanism To demonstrate the practicality of an as-assembled device, we designed and fabricated a painting board, which is made of nanoporous yellow colored BTO@BVO on FTO substrate. The Zn and Co hydrogels have been used as paints to draw a jockey riding a horse (Figure 5A), the green-blue regions are covered by Co hydrogel, whereas the transparent regions (yellow regions) are covered with Zn hydrogel. This painting could potentially help dehumidify living spaces with zero energy input since it is self-sustaining and breaks down the absorbed moisture in the presence of ambient indoor light (Figure S22). There was an associated color change in the painting based on the amount of water absorbed by hydrogels and the level of RH. The Zn hydrogel turns from transparent to opaque upon moisture absorption, whereas the Co hydrogel turns pink when it absorbs moisture from the atmosphere. The reaction mechanism of the photoanode-hydrogel is like an orderly natural circulation; i.e., the hydrogel harvests moisture from the atmosphere, thereby leading to a dehumidifying effect and the transfer of the collected water to nanoporous photoanode for water oxidation (Figure 5B). The photoanode is composed of a ferroelectric component and a semiconductor. When BVO absorbs a photon, the electron will be excited to conduction band,40,41 and Vph is created immediately by the dropped Ef,p (Figure 5C). The larger Vph improves the water oxidation kinetics (lower energy barrier height for hole’s transfer), which could be attributed to the formed outward vector of Ein by positive polarization of the ferroelectric material. This outward vector of Ein can help the transfer of photogenerated holes to semiconductor’s surface, whereas the photogenerated electrons are driven to the bulk.42 This ferroelectricsemiconductor hybrid structure offers a more beneficial method to carry out atmospheric water splitting, with the higher charge transfer rate, while the hydrogel provides a continuous supply of water from the air for PEC reactions. To summarize, both moisture absorption and water splitting were achieved by using the photoanode-hydrogel device. The photoanode is composed of a ferroelectricsemiconductor hybrid; the ferroelectric component served as the charge separation and transfer accelerator while the semiconductor acted as the photo-absorber. After
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Figure 5. Painting and Water Splitting Model of the Intelligent Series System (A) Indoor painting with various hydrogels on the BTO@BVO surface for home decoration, the colorful painting can decorate our living spaces around windows and lamps. The yellow area is BTO@BVO, the green-blue and pink area is Co hydrogel, and the transparent and white area is Zn hydrogel (left, the dry hydrogel coated on photoanode; right, the painting is placed in 90.0% relative humidity for 5 h). (B) Water absorption by hydrogel from ambient environment, which serves as the source of water supply for photoanode with continuous conversion from humidity to O 2 . (C) The energy band diagram for the ferroelectric enhanced charge separation and transfer structure, the outward vector of built-in electric field (E in) in BTO, and the upward band bending of BVO are beneficial for photogenerated holes’ transfer to surface where efficient water oxidation takes place.
positive polarization of the hybrid, an outward vector of Ein was generated by the ferroelectric, which strengthens the holes’ transfer to the surface, where efficient water oxidation takes place. On the other hand, the super-hygroscopic hydrogel captures moisture from the atmosphere and serves as the water source to photoanode for water splitting. We have obtained the RH decrease of about 12.0% with a simultaneous photocurrent generation of 0.4 mA/cm2 by a solar cell-driven device. This integrated system opens new possibilities of concurrent dehumidification (at zero energy requirement) and power generation by harnessing both abundant light and ambient humidity, which are otherwise redundant resources.
EXPERIMENTAL PROCEDURES Preparation of BaTiO3@BiVO4 Hybrid BaTiO3@BiVO4 wass synthesized through three steps. First, a mixture of 0.08 M Bi(NO3)3 and 0.8 M KI solution was prepared in 50 mL deionized water, which was later mixed with 0.23 M p-benzoquinone in 20 mL absolute ethanol to form the [BiI4] species. A typical three-electrode system was used to electrodeposit BiOI, where FTO substrate acted as the working electrode, Ag/AgCl electrode and platinum foil (2 3 2 cm2) served as the reference electrode and the counter electrode, respectively. To obtain the BiOI nanoflakes, 0.1 V versus Ag/AgCl was applied on the FTO for 200 s, the BiOI sample was removed from the plating solution and
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washed by ethanol repeatedly and dried at 60 C overnight. Second, annealed 5 mL of 10 mg/mL BaTiO3 nanoparticle solution and 0.15 g vanadyl acetylacetonate (VO(acac)2) were all dissolved in 3 mL dimethyl sulfoxide as sources for BaTiO3 and vanadium. The obtained BiOI was soaked in 300 mL of this solution for 10 min and then annealed at 450 C in air for 2 h with a heating rate of 2 C/min to convert BiOI nanoflakes to nanoporous BaTiO3@BiVO4. Lastly, excess V2O5 on the surface of BaTiO3@BiVO4 was removed by soaking the sample in 1 M NaOH solution for 1.5 h under gentle stirring. Preparation Zn Hydrogel and Co Hydrogel For the fabrication of the Zn hydrogel, 0.7 M Zn(CH3COO)2 was dissolved in 2 mL 2-methoxyethanol and sonicated for 10 min; 84 mL ethanolamine was then rapidly added into the solution, and sonication continued for 20 min until all Zn(CH3COO)2 was dissolved to render a clear transparent liquid. For the preparation of the Co hydrogel, 0.7 M CoCl2 was dissolved in 2 mL 2-methoxyethanol and sonicated for 10 min; 84 mL ethanolamine was then rapidly added into the solution, and sonication was continued for 20 min until all CoCl2 was dissolved to render a blackish liquid. After that, 2 mL deionized water was mixed to the transparent or blackish liquid to get an opaque viscous liquid of milky white (Zn hydrogel) or pink color (Co hydrogel) precursor, followed by vigorously shaking for 1 min. At last, 0.5 mL of this precursor was coated on a graphite electrode and heated at 50 C for 20 min in oven, resulting in the formation of a thin, dry, transparent (Zn hydrogel) or green-blue (Co hydrogel) layer on the bottom of graphite. When these hydrogels were exposed to ambient humid air, they started to absorb water molecules from the atmospheric humidity.
SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.joule. 2019.10.008.
ACKNOWLEDGMENTS The authors acknowledge the financial support from Temasek Foundation Ecosperity (R-284-000-178-592) and Ministry of Education Academic Research Fund (R-284000-174-114).
AUTHOR CONTRIBUTIONS L.Y. and S.C.T. conceived the study and devised experiments. D.K.N. carried out the Kelvin probe force microscope measurements and revised the manuscript. L.M. conducted the transmission electron microscope tests. L.S. and J.V.V. participated in the discussion of the electrochemical results and revised the manuscript. D.Z. performed the annealing process and modified the manuscript. T.X. conducted the Raman and photoluminescence measurements. K.C.K. carried out and analyzed ferroelectric loop data. S.C.T. supervised the project.
DECLARATION OF INTERESTS The authors declare no competing interests Received: August 13, 2019 Revised: September 26, 2019 Accepted: October 21, 2019 Published: November 20, 2019
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Energy Harvesting from Atmospheric Humidity by a Hydrogel-Integrated FerroelectricSemiconductor System Lin Yang, Dilip Krishna Nandakumar, Linqing Miao, ..., Jayraj V. Vaghasiya, Ki Chang Kwon, Swee Ching Tan [email protected]
HIGHLIGHTS BaTiO3@BiVO4 core-shell structure is designed for photoelectrochemical application Outward vector of built-in electric field is beneficial for driving holes to the surface Zn or Co super-hygroscopic hydrogel is used for humidity harvesting from atmosphere Photocurrent for series system is 0.4 mA/cm2, and the relative humidity reduced 12.0%
A ferroelectric-semiconductor hybrid is assembled, producing an outward vector of built-in electric field after positive polarization, while the Zn/Co superhygroscopic hydrogel is used for atmospheric humidity harvesting. The photoanode-hydrogel-solar cell series system can capture water from humid air and release the water splitting under indoor light, generating a photocurrent of 0.4 mA/cm2 and decreases the relative humidity by 12.0%. This device can serve as dehumidifier and power-generator for home painting, guiding the comfortable relative humidity and temperature level.
Yang et al., Joule 4, 1–13 January 15, 2020 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.joule.2019.10.008
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Article
Energy Harvesting from Atmospheric Humidity by a Hydrogel-Integrated Ferroelectric-Semiconductor System Lin Yang,1 Dilip Krishna Nandakumar,1 Linqing Miao,1 Lakshmi Suresh,1 Danwei Zhang,1 Ting Xiong,1 Jayraj V. Vaghasiya,1 Ki Chang Kwon,2,3 and Swee Ching Tan1,4,*
SUMMARY
Context & Scale
Direct utilization of abundantly available solar energy is a promising way to create a sustainable society. Here, we report a ferroelectric-semiconductor (BaTiO3@BiVO4) hybrid that uses an effective strategy to enhance charge separation and transfer during water oxidation. The tetragonal BaTiO3 can induce an outward vector of built-in electric field after positive polarization, which aids in increasing the photovoltage and accelerating holes’ transfer to BiVO4’s surface. A super-hygroscopic metal hydrogel serves as an atmospheric humidity harvester for continuous water supply to the hybrid, where water oxidation takes place. As the hydrogel absorbs moisture from ambient humid air, it functions as a dehumidifying agent and carries water to the photoanode for power generation, being connected in series with a solar cell, further boosting the carrier’s mobility. This photoanode-hydrogel and solar-cell intelligent assembly can generate a photocurrent of 0.4 mA/cm2 with a relative humidity reduction of 12.0% under an illumination of 10 mW/cm2.
The world’s total energy consumption is projected to increase from 30% to 50% in the next 20 years. However, solar water splitting provides a sustainable and environmentally benign route for the production of clean fuels to tackle the escalating energy needs. For traditional photoelectrochemical water splitting systems, there exists two great challenges: the water source and the extra electronic energy. Here, we developed a ferroelectric-semiconductor photoanode (BaTiO3@BiVO4) that can perform efficient water oxidation, and the versatile superhygroscopic hydrogel can directly harvest the humidity from the atmosphere. Combining the photoanode-hydrogel device with a solar cell, the relative humidity is decreased (12.0%) while the photocurrent is simultaneously generated (0.4 mA/cm2). That is, the functions of BiVO4, BaTiO3, hydrogel, and the solar cell are photo absorption and water oxidation, charge accelerator, humidity harvester, and extra electronic energy input.
INTRODUCTION In the search for alternatives to non-renewable energy, the abundant availability of sunlight has been the subject of intense research because of its potential as a clean energy source.1–3 Light-driven water splitting over a semiconductor surface in which photogenerated holes are used to oxidize the hydroxyl ion could serve as one pathway for sustainable energy production and reuse.4 The momentous challenge in scaling up the water splitting reaction is the need for large amounts of aqueous media. Atmospheric humidity holds a major share of water in vapor phase (around 5.3 g water per cubic meter with saturated relative humidity (RH) at 22.5 C in air), which could be used as a water source for energy generation. However, research on the realization of a single device that can effectively dehumidify air and produce electricity is sparse and the reaction mechanism of such devices are poorly understood.5 The limitations for such a system lies in the difficulty to (1) optimize photoelectrode structure for efficient spatial charge separation and transfer,6,7 (2) develop a versatile hygroscopic material for rapidly harnessing humidity from ambient environment, and (3) construct a single device that can be assembled by an electronic energy self-generation system (zero energy input). To comprehend this advanced apparatus, a photo-absorber and charge-manipulator, a humidity-absorber, and an energy source should be combined as an integrated system. In general, n-type semiconductors with higher Fermi level (closer to conduction band) are usually conducive for water oxidation, which is attributed to the high-activated energy of holes that could drive the sluggish four-electron transfer process.
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1
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Bismuth vanadate (BiVO4) has recently emerged as one of the prospective photoanodes in photoelectrochemical (PEC) water splitting,8–10 which has many merits including broad visible light absorption (up to 520 nm) and adequate overpotential for oxygen evolution. Unfortunately, the fundamental deficiency of BiVO4 lies in its relatively weak charge separation and transfer.11–13 Nevertheless, embedding a ferroelectric material into a semiconductor is an effective method to overcome the dilatory charge migration, and this excellent structure could be benefited by the tiny displacement of atoms in an asymmetric ferroelectric crystal structure (charge redistribution creates a dipole).14–16 The formed built-in electric field by ferroelectric dipole could freely manipulate the photogenerated carriers’ transfer direction and speed, which is regarded as a driving force for electrons and holes.17–19 For example, it has been reported that SrTiO3-decorated TiO2 could enhance the photocurrent up to 1.43 mA/cm2, whereas the photocurrent of BaTiO3@TiO2 has been improved by about 2.6 times.20,21 Therefore, if the photoelectrode is synergistically embedded by the ferroelectric substance with well-tuned polarization, the mobility of charges will be reinforced, thereby leading to the higher efficiency in photon-to-electron conversion. Considering the basic drawbacks associated with conventional PEC systems for water splitting—the huge requirement of liquid—harnessing the moisture content in air is a viable alternative; as atmospheric humidity holds about 1.29 3 1016 kg of water on Earth. Atmospheric water is a ubiquitous ambient resource, which is not only redundant but also needs input of a large amount of energy to keep it within comfortable levels,22,23 especially in coastal areas with high RH. Recently, metalorganic frameworks, interpenetrating polymer network gel, and sulfur-rich MoSx have been utilized for harvesting moisture directly from the atmosphere.24–26 However, one of the critical limitations of these materials are the relatively slower humidity absorption rate and lower water uptake. To counter this problem, we have developed a super-hygroscopic hydrogel based on zinc/cobalt, which can harvest water more than 4 times its own weight.27 Here, we have demonstrated a ferroelectricsemiconductor hybrid along with the hydrogel and synergistically coupled them with a solar cell to concurrently dehumidify and generate energy from ambient environment. In a nutshell, we have proposed a strategy to suppress the recombination of photogenerated-electron-hole pairs, which is achieved by ferroelectric-semiconductor (BaTiO3@BiVO4) structure. The tetragonal ferroelectric-semiconductor will induce an outward vector of built-in electric field with the help of positive polarization, thus enhancing charge separation and transfer. When the hydrogel harvests humidity from the atmosphere, it acts as an electrolyte (water supplier) for the continuous PEC reactions. This investigation provides insight into atmospheric water splitting by an advanced PEC system, resulting in reduction of RH and temperature by 12.0% and 6.2 C and simultaneously generating a photocurrent of 0.4 mA/cm2. 1Department
RESULTS Structural Characteristics of a Semiconductor and Ferroelectrics All BiVO4 (BVO)-based photoanodes were synthesized through a three-step protocol, including electrodeposition, annealing, and alkaline solution washing.10 The schematic overview of the synthesis procedure is shown in Figure 1A. By this method, the ferroelectric BaTiO3 (BTO) nanoparticles could sink into the vertically intersected BiOI nanoflakes (crimson two-dimensional structure, Figure S1), guiding an in situ growth and forming the BTO@BVO hybrid. After washing by alkaline
2
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of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117574, Singapore
2Department
of Chemistry, National University of Singapore, 3 Science Drive, Singapore 117543, Singapore
3Centre
for Advanced 2D Materials (CA2DM), 6 Science Drive 2, Singapore 117546, Singapore
4Lead
Contact
*Correspondence: [email protected] https://doi.org/10.1016/j.joule.2019.10.008
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Figure 1. Ferroelectric-Semiconductor Synthesis and Structure: The Photoanode Synthesis Procedure and the Morphologies of Materials (A) Schematic diagram of the synthesis procedure for BTO@BVO hybrid on the FTO substrate, including electrodeposition and annealing. (B–D) The SEM images of (B) nanoporous BVO (inset figure is the cross-section view), (C) top view of BTO@BVO composite (inset is the large view of this structure), and (D) elemental mapping of BTO@BVO. The yellow arrows point to the BTO. (E) HRTEM lattice fringe with the interface of BTO and BVO, the inset is the structure of BVO surrounded by BTO nanoparticles.
solution, the yellow colored nanoporous BVO was obtained (composed by 100 nm sized nanoparticles), which is highly crystalline and interconnected (Scanning electron microscopy [SEM] image in Figure 1B). The as-prepared semiconductor was identified to have a monoclinic crystal structure, and the positions of binding energy correspond to the typically pure BVO (Figure S2). For fabrication of the ferroelectricsemiconductor, the BTO was annealed at 1,100 C for 30 min, resulting in the phase change of the crystal from cubic to tetragonal, as seen from the split double peaks, and leading to a larger c axis with asymmetrical structure (Figure S3).28 In addition, the X-ray photoelectron spectroscopy plots reflect the double peaks with Ba 3d at 781.1 and 796.1 eV,29 which suggests the classical Ba2+ in the annealed BTO (Figure S4). This BTO served as ferroelectric to construct the BTO@BVO hybrid, demonstrating a homogeneously inserted morphology after heat treatment (Figure 1C). The elemental mappings of Bi, Ba, and Ti indicate the color of Bi element is darker in the BTO region, while the colors of Ba and Ti elements are brighter in the corresponding position exactly, representing the proposed BTO@BVO structure (Figure 1D). The transmission electron microscopy (TEM) image depicts that the BTO nanoparticles are dispersed into BVO uniformly, without any obvious aggregation, and the size of BTO is about 10 nm in the hybrid structure (Figure 1E). The interface structure is conducted by the high resolution TEM (HRTEM), the inter layer distance (d-spacing) is calculated from the lattice fringes, and the values of 2.84 and 3.08 A˚ could be assigned to (101) and (112) crystal plane of BTO and BVO, respectively.30 These results clearly prove the sample is highly crystalline and BTO is evenly distributed on the nanoporous BVO (Figure S5). Furthermore, Kelvin probe force microscopy (KPFM) technique was used for understanding the charge distribution and surface potential on the nanometer scale under ambient conditions. Topography of the BTO@BVO portrays the nanoporous BVO surface covered by BTO nanoparticles (Figure 2A). For a semiconductor, the surface potential is always related to the surface energy (different from the bulk work function), which is ascribed to the formation of a space charge region near the
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Figure 2. Atomic and Kelvin Probe Force Microscopy Results and the Ferroelectric Loops for BaTiO3 (A) The height image (topography) of BTO@BVO by AFM technique at ambient environment. (B) KPFM surface potential map corresponding to the topography in Figure 2A, the prominent particle is BTO, while the others are BVO. (C) Surface potential shifts from BVO to BTO through the zone marked by orange arrow in Figure 2B. (D) Switching spectroscopy piezoresponse force microscopy of BTO nanoparticle with butterfly loopfor amplitude versus voltage and phase switching loop for phase versus voltage. (E) Polarization-electric field hysteresis loop for BTO film at ambient conditions, the hysteresis period is 20 ms (inset is the photograph of prepared BTO on Cu film). (F) Schematic diagram of built-in electric field formed by ferroelectric for enhancing charge separation and transfer, which assists the upward band structure bending by positive charge accumulation at BVO surface, the positive-negative charge pairs are accumulated at both sides of BTO to induce the built-in electric field (E in ).
interface.31,32 Therefore, surface potential mainly elucidates the properties of a thin surface layer, directly describing the work function and Fermi level of materials. A clear difference in the surface potential map is observed in BTO@BVO hybrid (Figure 2B), which could be attributed to the difference in the band structures of BTO and BVO. The brighter color of BTO implies that it has a higher surface potential when compared to the BVO (darker shade). To plainly contrast the surface potential changes within the ferroelectric-semiconductor structure, the surface potential curve is shown in Figure 2C, which is represented by the right orange direction (cross over in the BTO region). It is distinctly revealed that the surface potential signal in BTO@BVO delivers a 45 mV gap in space, illustrating that the Fermi level of BTO is higher than that of BVO, which is the result of a higher surface potential (due to the lower work function).32 Consequently, the calculated work function of BTO and BVO approaches to 4.90 and 4.95 eV, the lower work function of BTO (0.04 eV lower than BVO) was also confirmed by ultraviolet photoelectron spectrometer (UPS) measurement (Figure S6). To further investigate the properties of tetragonal phase BTO, switching spectroscopy piezoresponse force microscopy (SS-PFM) was performed to the ferroelectricity by local polarization switching behavior and piezoresponse. This method is an effective tool to check the statics and dynamics of ferroelectric domains at nanometer scale on a material’s surface.33 Figure 2D depicts the amplitude-voltage butterfly and phase-voltage loops of BTO, reflecting a typical ferroelectric behavior (has a representative polarization switchable behavior
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Figure 3. PEC Performances of As-Prepared Samples in Phosphate Buffer (pH 7.0) and Hydrogels Under LED Cool Daylight Illumination (10 mW/cm2) at 22.5 C (A) Standard physical setup for PEC water splitting test in 0.5 M phosphate buffer with/without 0.2 M Na 2 SO 3 , all illuminated area is fixed to 1 cm 2 (circular area with a radius of 0.57 cm). (B and C) The efficiencies of (B) charge separation and oxidation kinetics versus potential and (C) photovoltage changes for various photoanodes structure. (D) The photograph of hydrogel-integrated BTO@BVO device (the upper is primitive hydrogel [left], dry hydrogel [middle], and photoanode coated structure [right]); the lower is the twoelectrode test apparatus. (E and F) The water harvesting rate of (E) photoanode-hydrogel device when exposed to 70.0% relative humidity for 12 h under dark, and (F) current-voltage curves of this device with dark and light.
to form a stable built-in electric field).33,34 Polarization-electric field hysteresis loop measurement that implemented at 50 Hz also confirms the ferroelectricity of annealed BTO by the characteristic ferroelectric loop (Figure 2E). A remanent polarization of 0.33 mC/cm2 is observed while the downward trend at high electric field could be owed to the minor leakage of electricity. Moreover, the photoluminescence plots expound the charge separation feature with different structures, indicating that BTO@BVO has a lower charge recombination rate (higher charge separation) due to the existence of ferroelectric properties (Figure S7). For BTO@BVO hybrid, the BTO induces a built-in electric field (Ein) by nature or after polarization, which in turn affects the band structure of BVO (Figure 2F). The free electrons will flow from BVO to BTO due to the effect of ferroelectric dipole (rightward), leading to the electrons and holes getting accumulated pairwise at the BTO and BVO surfaces, thereby bending the band structure of BVO upward. The Ein will drive the photogenerated holes to the BTO surface where efficient water oxidation takes place, in other words, enhances the charge transfer process. Photoelectrochemical Water Oxidation in Neutral Solution To gain a better understanding of how the ferroelectric affects the activity of BVO in a PEC cell, typical current-potential (J-V) curves of all samples were acquired in 0.5 M phosphate buffer solution (pH 7.0) under light emitting diode (LED) cool daylight illumination (10 mW/cm2). The active area of the photoanode is maintained at 1 cm2 by a Teflon polymer gasket (physical setup is shown in Figure 3A), and the sample is illuminated from the front side for water oxidation. Different structures with varied BTO concentrations (from 1 to 9 mL) were conducted and the onset photo
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absorption edge was unchanged for all samples, and the band gap was determined to be 2.4 eV (Figure S8).10,13 The photocurrent responses clearly reveal the 5 mL BTO-decorated BVO has the highest photocurrent of 0.287 mA/cm2 at 1.23 V versus reversible hydrogen electrode (RHE), this phenomenon could be ascribed to the fact that less ferroelectric produces less Ein, but more ferroelectric will decrease the conductivity of photoanodes (Figure S9). In order to generate more directional Ein, the BTO@BVO was pretreated by applying a bias of G20 V for polarization.20 Thus, a positive polarized (positive voltage) photoanode generates a higher photocurrent than negative polarization. This is because, the positive polarization will induce an outward vector of Ein, which is beneficial for transfer of photogenerated holes to surface where water oxidation process occurs, whereas the negative polarization induces the inward vector of Ein that hinders the hole movement to the surface. Consequently, the 5 mL BTO-decorated BVO was chosen as the optimized photoanode with positive polarization for our further experiments. The composition percentage of BTO was verified by inductively coupled plasma (Table S1), indicating that the mass of BTO and BVO is 8.2 and 228.7 mg/cm2, respectively (the mass loading percentage of BTO is about 3.5%). For unmodified BVO, the photogenerated electronholes recombined easily at the surface. On the contrary, the BTO@BVO displays an enhanced photocurrent of 0.359 mA/cm2 at 1.23 V versus RHE with a photocurrent enhancement of 104% compared with BVO (Figure S10). The onset potentials of BVO and BTO@BVO are 0.225 and 0.153 V versus RHE, which depicts that the directed ferroelectric can effectively reduce the energy barrier for charge transfer (Ein could offer an extra dynamic for water oxidation).10 The negatively shifted onset potential of BTO@BVO is also corresponding well with the negative shift of flatband potential (77 mV) (Figure S11), manifesting the improved holes’ transfer kinetics. The positive quasilinear behavior of Mott-Schottky plot implies the n-type semiconductor, and the smaller slope means higher carrier concentration for [email protected] Additionally, Figure 3B represents the efficiency of charge separation (hsep) for BVO, and BTO@BVO is 67.8% and 81.8% at 1.23 V versus RHE, demonstrating that the directed ferroelectric could enhance the charge separation. The efficiency of oxidation kinetics (hox) is also improved from 34.2% for BVO to 56.1% for BTO@BVO at 1.23 V versus RHE, which shows a greater advantage for charge transfer with ferroelectric-semiconductor hybrid structure (attributed to the outward vector of Ein). These results elucidate that BTO@BVO is conducive to enhancing charge migration (holes will be pushed to surface for water oxidation, and electrons will be driven to fluorine-doped tin oxide [FTO] substrate) due to the upward band bending of BVO and polarized ferroelectric. To shed light on the photogenerated electrons’ transfer from the surface of photoanode to FTO substrate,35 the electron’s transport time (t) with different applied potential at 1 mW/cm2 (the wavelength of incident light is 365 nm) was calculated (Figure S12). The longer transport time of the generated electrons in BVO (0.92 ms) suggests a lower charge collection efficiency when compared with BTO@BVO (0.77 ms) at 1.23 V versus RHE,3 which is in close agreement with hox and their corresponding J-V curves. Also, the charge transfer impedance of BTO@BVO is lower at open circuit potential, which is described by the smaller semi-circle. When photogenerated carriers are separated spatially, charges will be redistributed because the electrons are excited from valence band to conduction band. Thus, the holes’ quasi-Fermi level (Ef,p) drops down from the electrons’ quasi-Fermi level (Ef,n) to create a photovoltage (Vph) at the photoanode and electrolyte interface,36 the value of Vph is calculated as |Vdark-Vlight| by measuring the open circuit potential in dark and illuminated state (Figure 3C). The BTO@BVO produces a higher Vph (0.459 V), reflecting a stronger charge transfer dynamics that is manipulated by the outward vector of Ein in ferroelectric-semiconductor hybrid. Moreover,
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the BTO@BVO exhibits nearly 24% enhancement of incident photon-to-electron conversion efficiency in the entire photo absorption region along with excellent stability (continuous water oxidation over 10 h) (Figure S13). Photoelectrochemical Water Oxidation in Hydrogels To estimate the performance of ferroelectric-semiconductor hybrid for simultaneous dehumidification and energy production, we have integrated the hybrid with a water source, which is capable of absorbing moisture from humid air at a rapid pace. Two super-hygroscopic hydrogels were developed, one Zn based, and another Co based, these hydrogels are extremely hygroscopic and colorful in ambient environment.27 The hydrogel portrays an interesting change in physical properties upon moisture sorption in dehydrated (DH) and hydrated (H) states (Figure S14). The SEM mapping identifies the Zn hydrogel contains Zn, C, O, and N elements in DH state, while Co hydrogel contains Co, C, O, N, and Cl elements (Figure S15). This result reveals that the hydrogels contain only a small fraction of metal (lower relative content of Zn or Co), and carbon is the major constituent in these hydrogels. Besides, TEM image of hydrogel indicates the particle-distributed morphology; the calculated d-spacing from the lattice is 0.29 and 0.51 nm for Zn and Co hydrogel, displaying weaker crystallinity (Figure S16). Thermogravimetric analysis indicated that the wet hydrogels have a total weight loss of 62.9% for Zn hydrogel and 38.5% for Co hydrogel around 100 C (the water absorption is 0.59 and 1.6 times in DH Zn and Co hydrogel).37 Subsequently, the super-hygroscopic hydrogel was used for harnessing ambient humidity and as the water source for photoanode with direct atmospheric water splitting (Figure S17). The physical setup of the device is composed of three components: a graphite sheet, as a counter electrode; hydrogel, for water harvesting; and BTO@BVO, for water oxidation (Figure 3D). The device was assembled and placed in an oven at 50 C for 20 min, causing the hydrogel to desorb all the water and reach DH state; then placed on a weighing balance at 70.0% RH; and the changes in weight with time was recorded. The Zn and Co hydrogels can absorb 10.1 and 21.0 mg of water, respectively, from ambient humid air within 12 h (Figure 3E). The J-V curves for the two-electrode system consisting of BTO@BVO and hydrogels were conducted, and a larger photocurrent of 0.658 mA/cm2 at 0.8 V was generated in the Co hydrogel system (the Zn hydrogel system only produces a photocurrent of 0.293 mA/cm2) (Figure 3F). This is a direct consequence of the two-fold water absorption capability of Co hydrogel leading to 2-fold higher photocurrent, thus, almost 5.3 mg water could be split within one day, theoretically. Furthermore, the onset potential for the Zn hydrogel system ( 0.49 V) is slightly lower than the Co hydrogel ( 0.46 V), indicating the energy barrier for charge transfer in Zn hydrogel is lower.9 Electrochemical impedance spectroscopy at open circuit potential was performed for interface charge migration kinetics under illumination (Figure S18). The small arc radius for ferroelectric-semiconductor hybrid suggests a faster charge transfer kinetics, while the resistance of the Co hydrogel (15.3 U) is lower than the Zn hydrogel (60.1 U) due to the rapid water harvesting by the Co hydrogel.8 Therefore, the photocurrent at higher applied bias is decided by the rate of moisture capture and the conductivity of hydrogels. Furthermore, the stability test of the device indicates that the consumed water could be replenished by hydrogels (no cocatalyst like CoOx was deposited on the surface of photoanode) and that the photocurrent just split 2.6 mg water while the hydrogel absorbed 21.0 mg humidity within 12 h (Figure S19). Solar Cell Combined with Device for Dehumidification To achieve a higher efficiency in atmospheric water splitting, an additional external bias is required. Here, we have assembled a commercially available polysilicon solar
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Figure 4. Solar Cell Driving the BaTiO3@BiVO4 Water Splitting under Hydrogel Systems (A) The circuit diagram of the solar cell, photoanode-hydrogel device, and ammeter in series connection. (B) Current-voltage curve and stability test for the polysilicon solar cell. (C) Current-time plots with chopped illumination for the hydrogel system by shading different components. (D) Water splitting in a sealed quartz tube by solar cell and device system, the diameter and length of this space is 6 and 100 cm, respectively. (E) The relative humidity changes by placing the system into the tube and the light illuminant is LED cool daylight (10 mW/cm 2 ), leading to the dry environment in tube. (F) The temperature changes in the tube space under tungsten lamp illumination with dry and wet environment.
cell in series with the ferroelectric-semiconductor hybrid and hydrogel device. The solar cell provides the required bias for efficient PEC atmospheric water splitting.38 The open circuit voltage for the solar cell is found to be around 1.15 V under 10 mW/cm2 illumination (Figure S20). Therefore, an integrated system was established, consisting of a solar cell, the photoanode-hydrogel, and an ammeter, for concurrent dehumidification and energy generation (Figure 4A). The purpose of the solar cell, hydrogel, photoanode, and ammeter are for providing bias, harvesting humidity, water oxidation, and photocurrent measurement, respectively. There is no extra electronic energy input, just stem from the weak abandoned light to the solar cell and photoanode. The J-V plot of the solar cell displays a stable short circuit current of 13.18 mA and an open circuit voltage of 1.15 V under continuous 10 h illumination (Figure 4B). This series connection was performed after the hydrogel was water saturated at 70.0% RH in air. The result clearly demonstrates that the photocurrent is switched off or on by dark or illumination, which is determined by blocking the solar cell or device (Figure 4C). A 0.4 mA/cm2 photocurrent (about 3.2 mg water will be split in one day) for the Co hydrogel system was generated only when the solar cell and the device were simultaneously illuminated (Video S1). Compared with the Co hydrogel system, the photocurrent is more stable in the Zn hydrogel system but merely reached 0.2 mA/cm2 , implying that the more the water absorbed and the more conductive the hydrogel is, the faster the water splitting will be and the larger the photocurrent will be. Moreover, we evaluated the photocurrent only by device and ammeter, in other words, it just depends on the energy band position and photovoltage of semiconductor for water splitting.32,36 A photocurrent of 0.064 and 0.138 mA/cm2 wass obtained for the Zn hydrogel and the Co hydrogel systems,
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respectively (Figure S21; Video S2), which was almost a 2-fold enhancement in photocurrent and coordinates well with the nearly 2-fold higher water absorption by the Co hydrogel. This system was placed into a sealed quartz tube for dehumidification and temperature estimation under LED cool daylight or tungsten lamp illumination at ambient environment (Figure 4D). Before the system was placed into the tube, it was placed at atmosphere for water absorption till the water is saturated, confirming the humidity changes observed in the tube were not due to additional hydrogel capture. The LED cool daylight does not emit any infrared radiations and hence there was no increase in temperature upon illumination (temperature remained at 22.5 C). After 24 h of illumination, the RH decreased to about 7.5% and 12.0% in Zn and Co hydrogel systems, respectively (Figure 4E). A higher RH reduction in the Co hydrogel system is attributed to its faster moisture capture ability, which aids in efficient dehumidification.4,39 In addition, a tungsten lamp was applied for detecting the increased temperature at different RH environment, the calorific lamp was simulated as the climate vicissitudes inside the tube. Figure 4F shows a dilatory temperature increasing in the dry tube space (low RH), while in the wet tube space (high RH) there was a rapid temperature increase under the tungsten lamp illumination. This apparently led to a 3.5 C and 6.2 C decrease in the Zn and Co hydrogel systems, respectively, suggesting minimal temperature changes at low RH state by using the intelligent system, which could be used for dehumidification and reduction in temperature.27 Device Conceptualization and Working Mechanism To demonstrate the practicality of an as-assembled device, we designed and fabricated a painting board, which is made of nanoporous yellow colored BTO@BVO on FTO substrate. The Zn and Co hydrogels have been used as paints to draw a jockey riding a horse (Figure 5A), the green-blue regions are covered by Co hydrogel, whereas the transparent regions (yellow regions) are covered with Zn hydrogel. This painting could potentially help dehumidify living spaces with zero energy input since it is self-sustaining and breaks down the absorbed moisture in the presence of ambient indoor light (Figure S22). There was an associated color change in the painting based on the amount of water absorbed by hydrogels and the level of RH. The Zn hydrogel turns from transparent to opaque upon moisture absorption, whereas the Co hydrogel turns pink when it absorbs moisture from the atmosphere. The reaction mechanism of the photoanode-hydrogel is like an orderly natural circulation; i.e., the hydrogel harvests moisture from the atmosphere, thereby leading to a dehumidifying effect and the transfer of the collected water to nanoporous photoanode for water oxidation (Figure 5B). The photoanode is composed of a ferroelectric component and a semiconductor. When BVO absorbs a photon, the electron will be excited to conduction band,40,41 and Vph is created immediately by the dropped Ef,p (Figure 5C). The larger Vph improves the water oxidation kinetics (lower energy barrier height for hole’s transfer), which could be attributed to the formed outward vector of Ein by positive polarization of the ferroelectric material. This outward vector of Ein can help the transfer of photogenerated holes to semiconductor’s surface, whereas the photogenerated electrons are driven to the bulk.42 This ferroelectricsemiconductor hybrid structure offers a more beneficial method to carry out atmospheric water splitting, with the higher charge transfer rate, while the hydrogel provides a continuous supply of water from the air for PEC reactions. To summarize, both moisture absorption and water splitting were achieved by using the photoanode-hydrogel device. The photoanode is composed of a ferroelectricsemiconductor hybrid; the ferroelectric component served as the charge separation and transfer accelerator while the semiconductor acted as the photo-absorber. After
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Figure 5. Painting and Water Splitting Model of the Intelligent Series System (A) Indoor painting with various hydrogels on the BTO@BVO surface for home decoration, the colorful painting can decorate our living spaces around windows and lamps. The yellow area is BTO@BVO, the green-blue and pink area is Co hydrogel, and the transparent and white area is Zn hydrogel (left, the dry hydrogel coated on photoanode; right, the painting is placed in 90.0% relative humidity for 5 h). (B) Water absorption by hydrogel from ambient environment, which serves as the source of water supply for photoanode with continuous conversion from humidity to O 2 . (C) The energy band diagram for the ferroelectric enhanced charge separation and transfer structure, the outward vector of built-in electric field (E in) in BTO, and the upward band bending of BVO are beneficial for photogenerated holes’ transfer to surface where efficient water oxidation takes place.
positive polarization of the hybrid, an outward vector of Ein was generated by the ferroelectric, which strengthens the holes’ transfer to the surface, where efficient water oxidation takes place. On the other hand, the super-hygroscopic hydrogel captures moisture from the atmosphere and serves as the water source to photoanode for water splitting. We have obtained the RH decrease of about 12.0% with a simultaneous photocurrent generation of 0.4 mA/cm2 by a solar cell-driven device. This integrated system opens new possibilities of concurrent dehumidification (at zero energy requirement) and power generation by harnessing both abundant light and ambient humidity, which are otherwise redundant resources.
EXPERIMENTAL PROCEDURES Preparation of BaTiO3@BiVO4 Hybrid BaTiO3@BiVO4 wass synthesized through three steps. First, a mixture of 0.08 M Bi(NO3)3 and 0.8 M KI solution was prepared in 50 mL deionized water, which was later mixed with 0.23 M p-benzoquinone in 20 mL absolute ethanol to form the [BiI4] species. A typical three-electrode system was used to electrodeposit BiOI, where FTO substrate acted as the working electrode, Ag/AgCl electrode and platinum foil (2 3 2 cm2) served as the reference electrode and the counter electrode, respectively. To obtain the BiOI nanoflakes, 0.1 V versus Ag/AgCl was applied on the FTO for 200 s, the BiOI sample was removed from the plating solution and
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washed by ethanol repeatedly and dried at 60 C overnight. Second, annealed 5 mL of 10 mg/mL BaTiO3 nanoparticle solution and 0.15 g vanadyl acetylacetonate (VO(acac)2) were all dissolved in 3 mL dimethyl sulfoxide as sources for BaTiO3 and vanadium. The obtained BiOI was soaked in 300 mL of this solution for 10 min and then annealed at 450 C in air for 2 h with a heating rate of 2 C/min to convert BiOI nanoflakes to nanoporous BaTiO3@BiVO4. Lastly, excess V2O5 on the surface of BaTiO3@BiVO4 was removed by soaking the sample in 1 M NaOH solution for 1.5 h under gentle stirring. Preparation Zn Hydrogel and Co Hydrogel For the fabrication of the Zn hydrogel, 0.7 M Zn(CH3COO)2 was dissolved in 2 mL 2-methoxyethanol and sonicated for 10 min; 84 mL ethanolamine was then rapidly added into the solution, and sonication continued for 20 min until all Zn(CH3COO)2 was dissolved to render a clear transparent liquid. For the preparation of the Co hydrogel, 0.7 M CoCl2 was dissolved in 2 mL 2-methoxyethanol and sonicated for 10 min; 84 mL ethanolamine was then rapidly added into the solution, and sonication was continued for 20 min until all CoCl2 was dissolved to render a blackish liquid. After that, 2 mL deionized water was mixed to the transparent or blackish liquid to get an opaque viscous liquid of milky white (Zn hydrogel) or pink color (Co hydrogel) precursor, followed by vigorously shaking for 1 min. At last, 0.5 mL of this precursor was coated on a graphite electrode and heated at 50 C for 20 min in oven, resulting in the formation of a thin, dry, transparent (Zn hydrogel) or green-blue (Co hydrogel) layer on the bottom of graphite. When these hydrogels were exposed to ambient humid air, they started to absorb water molecules from the atmospheric humidity.
SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.joule. 2019.10.008.
ACKNOWLEDGMENTS The authors acknowledge the financial support from Temasek Foundation Ecosperity (R-284-000-178-592) and Ministry of Education Academic Research Fund (R-284000-174-114).
AUTHOR CONTRIBUTIONS L.Y. and S.C.T. conceived the study and devised experiments. D.K.N. carried out the Kelvin probe force microscope measurements and revised the manuscript. L.M. conducted the transmission electron microscope tests. L.S. and J.V.V. participated in the discussion of the electrochemical results and revised the manuscript. D.Z. performed the annealing process and modified the manuscript. T.X. conducted the Raman and photoluminescence measurements. K.C.K. carried out and analyzed ferroelectric loop data. S.C.T. supervised the project.
DECLARATION OF INTERESTS The authors declare no competing interests Received: August 13, 2019 Revised: September 26, 2019 Accepted: October 21, 2019 Published: November 20, 2019
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