Pyroelectrocatalytic CO2 reduction for methanol driven by temperature-variation

Taking the advantages of pyroelectric nanostructured materials, we use the temperature-variation, a ubiquitous phenomenon in our daily life, to reduce carbon dioxide (CO2) for methanol through pyroelectrocatalytic process. Layered-perovskite bismuth tungstate nanoplates harvest heat energy from temperature-variation, driving pyroelectrocatalytic CO2 reduction for methanol at temperatures between 15 °C and 70 °C. The methanol yield can be as high as 55.0 µmol·g−1 after experiencing 20 cycles of temperature-variation. This ecient, cost-effective, and environmental-friendly pyroelectrocatalytic CO2 reduction route provides a new thought towards utilizing natural diurnal temperature-variation for future methanol economy.


Introduction
For hundreds of years, fossil fuels have always been the main energy source for human activities and industrial manufacture. As the demand of the energy resource increases, the availability of fossil fuels and more release of carbon dioxide (CO 2 ) in atmosphere have raised serious concerns for the society. For instance, energy crisis, greenhouse effect, and ocean acidi cation are some of the major challenges faced by humankind [1][2][3] . Converting CO 2 into hydrocarbon fuels is considered as one of the ideal solutions which can solve not only the environmental problems but also the high demand of energy consumption.
Various methods have been explored to convert CO 2 , such as photocatalytic reduction, electrocatalytic reduction, biological transformation, hydrogenation, and/or dry reforming [4][5][6][7] . Nevertheless, hydrogenation of CO 2 to form CH 3 OH process requires high operating temperatures (200-250 o C) and high pressures (5)(6)(7)(8)(9)(10), which limits the yield of methanol 8 . Photocatalytic reduction of CO 2 can be carried out in relatively mild temperature and pressure, but it always suffers insu cient light absorption and no response under dark condition 9 . Therefore, it is imperative to develop alternative, cost-effective, and environmentally friendly approach for CO 2 conversion.
Temperature variation is a recurrent phenomenon in our daily life 10 . The impact could be tremendous if we could harvest such abundant energy source during temperature variation. Such a motive is not unreasonable considering that pyroelectric materials can convert heat energy into electric energy via repeating cooling or heating process [11][12][13] . Similar to the photocatalysis, pyroelectrocatalytic process can produce positive and negative charges by using pyroelectric materials as the active media during temperature variation. The energy generated through such a process has been applied to some catalytic process such as dye decomposition [14][15][16] and water splitting 17,18 . Theoretical calculation shows that a pyroelectric engine in an ideal condition can reach an energy conversion e ciency as high as 84-92%, which is much higher than the photovoltaic energy conversion e ciency typically in the range of 20% 19,20 . Theoretically, Arvin Kakekhani et al. have approved the feasibility of pyroelectrocatalytic water splitting 21 . However, there is no report, based on our best knowledge, about collecting the energy using pyroelectric materials from temperature variation for CO 2 reduction.
The study of ferroelectrics' catalytic properties has been nearly seventy years. For example, the internal elds resulted from the polarization from the ferroelectrics can separate electrons and holes, thus enhance the catalytic e ciency 17 . Furthermore, ferroelectric polarization can affect molecular adsorption onto and desorption from the surface of the materials 21 . It is well known that all ferroelectric materials are pyroelectric materials. As the simplest member of bismuth layer-structured Aurivillius phase, bismuth tungstate (Bi 2 WO 6 ) exhibits excellent ferroelectric and pyroelectric properties. Meanwhile, Bi 2 WO 6 has some other interesting properties such as high ion conductivity, large spontaneous polarization (P ≅ 50 µC/cm 2 ), high Curie temperature (T C = 950 o C), and photocatalytic property 22,23 . As Bi 2 WO 6 is constructed by alternating (Bi 2 O 2 ) 2+ and (WO 4 ) 2layers, such a layered structure enables high thermal and chemical stabilities 24 . More importantly, the suitable energy band structure and surface properties of Bi 2 WO 6 allow it for CO 2 reduction into renewable hydrocarbon fuel 25,26 . In this work, we, for the rst time in the community, report the use of pyroelectrocatalytic Bi 2 WO 6 nanoplates to convert CO 2 into methanol by harvesting the energy from temperature variation below 100 o C. The e ciency has reached as high as 55.0 µmol·g −1 after experiencing 20 cycles between 15 o C and 70 o C. Our experimental work provides a new route and key step to CO 2 reduction for methanol through a pyroelectrocatalytic process which can be carried out near room temperature.

Results And Discussion
Characterization of Bi 2 WO 6 . Previous study shows that Bi 2 WO 6 is ferroelectric with an orthorhombic structure, an intermediate phase with orthorhombic B2cb structure above 670 o C, and a monoclinic structure above 950 o C 27 . We synthesized Bi 2 WO 6 nanoplates by a hydrothermal process (see the experimental details in the section of Materials and Methods). To con rm and identify the phase of the materials, X-ray diffraction (XRD) analysis was performed at room temperature. As shown in Fig. 1a, all the diffraction peaks can be assigned to Bi 2 WO 6 according to the standard JCPDS card No. 79-2381 (space group: Pca2 1 ; point group: mm2; orthorhombic crystal system).
The morphology of as synthesized Bi 2 WO 6 can be seen from the , which is perpendicular to b direction. Therefore, the STEM image re ects the layered structure of Bi 2 WO 6 , which is sandwiched by alternating perovskite-like (WO 4 ) 2and uorite-like (Bi 2 O 2 ) 2+ blocks. A comparison between the STEM image and the structure model is schematically illustrated in Fig. 1f. The left picture in Fig. 1f is the magni ed image of the area marked in red rectangle in Fig. 1e. The inset in Fig. 1e shows the simulated diffraction pattern in the [103] projection direction. Complete structure model is shown in the right side of Bi 2 WO 6 is paraelectric with a high-symmetry body-centered tetragonal structure (space group symmetry I4/mmm) at high temperature. When the temperature drops, symmetry of the crystal structure will be broken, the distortion of the symmetry tetragonal structure makes Bi 2 WO 6 to generate ferroelectric properties. This mainly includes two aspects. Firstly, the ions displace along the [110] axis of the tetragonal structure. Secondly, the WO 6 octahedra rotate around the a and c axes 28 . In order to characterize the ferroelectric properties of the as-synthesized Bi 2 WO 6 nanoplates, Ferroelectric domains of Bi 2 WO 6 nanoplates were observed using a piezoelectric force microscope (PFM) at a slow scanning frequency of 1 Hz with an area of 0.8×0.8 μm 2 . The nanoplates morphology of Bi 2 WO 6 shows in Fig. 2a is consistent with the result from TEM and SEM. Figs. 2b and 2c shows the vertical piezo-response amplitude and phase image, respectively. The distinct contrast in the images illustrates the different polarization in the Bi 2 WO 6 nanoplates. Figs. 2d and 2e display the local piezoelectric hysteresis loops of the Bi 2 WO 6 , including both "off" state (piezoelectric displacement contribution only) and "on" state (both the piezoelectric contribution and the displacement resulted from electrostatic interaction). The phase angles at "off" and "on" states change about 150° under 60 V DC bias eld, con rming the occurrence of a local polarization switching under an electric eld. The butter y-shaped hysteresis loop further con rms the local ferro-/piezoelectric response of Bi 2 WO 6 nanoplate.
It is noted that Bi 2 WO 6 also shows pyroelectric properties, where imbalanced polarization charges can generate electric eld when the material undergoes a temperature variation. The voltage produced by pyroelectric effect can be a driving force for electrochemical reaction. Fig. S1 shows the pyro-potential distribution across a Bi 2 WO 6 nanoplate tted by COMSOL nite element simulation, in which different colors represent different potentials. It can be seen potential difference occurs on the surfaces of the Bi 2 WO 6 nanoplates. In general, ferroelectric materials have greater pyroelectric and piezoelectric coe cients than non-ferroelectrics 28 . In order to con rm Bi 2 WO 6 can generation pyroelectric charges through temperature variation, pyro-current response of Bi 2 WO 6 nanoplates was measured. When the material is irradiated by the infrared light and accompanied with a rapid increase of temperature, the pyroelectric charges would be generated quickly due to the pyroelectric effect. The pyroelectric current can be expressed by the following Equation (1), where A is the area of electrode, p is the pyroelectric coe cient, and dT/dt is the temperature change rate. Therefore, the pyroelectric current is proportional to dT/dt, and any change in temperature can generate charges for pyroelectric materials. Fig. 2e shows the current dependent of the Bi 2 WO 6 nanoplates on the infrared light signal. Fig. 2f shows the detailed response of the material to the light illumination. When the light is on, a sharp increase of current density is induced by the pyroelectric effect due to the rapid increase of temperature within Bi 2 WO 6 nanoplates. The current density decays slowly due to the decrease of temperature, and maintains at a steady value under the equilibrium condition. A reversed current (negative current density shows in Fig. 2g) is generated due to the pyroelectric charges' re-distribution by instantaneous temperature decrease (dT/dt < 0) when the light is turned off. The output current returns to zero while there are no temperature change and light illumination. To con rm the temperature effect discussed above, we used xenon lamp (UV light), instead of the infrared light, to illuminate the sample.
Under such circumstances, there was no pyro-current was generated (see Fig. S2).
Pyroelectrocatalytic activity of Bi 2 WO 6 . To evaluate the pyroelectrocatalytic activity of Bi 2 WO 6 , we use the temperature variation via the pyroelectric effect and apply such a method for CO 2 catalytic reduction activity. The methanol yield increases with the temperature-variation cycles as shown Fig. 3a. The total methanol yield reaches 20.5 µmol·g −1 without adding any sacri cial agent after 20 temperature-variation cycles. The 1 H NMR in Fig. S3 also demonstrates that no other products can be detected in liquid phase.
Meanwhile the analyses of gaseous product in Fig. S4 show only a small amount of CH 4 and CO (0.11 µmol·g -1 and 0.20 µmol·g -1 , respectively), indicating high selectivity of Bi 2 WO 6 pyroelectrocatalytic CO 2 reduction to CH 3 OH. It has been similarly reported that the photocatalytic process can be reduced due to the recombination of electrons and holes. Such a process will signi cantly affect the catalytic e ciency 29,30 . To reduce the occurrence of electrons recombination with holes, sacri cial agents are usually added to the reaction system. Fig. 3b shows that more methanol can be generated by using Na 2 SO 3 as negative charge sacri cial agent. The methanol yield can be as high as 55.0 µmol·g −1 after 20 temperature-variation cycles, which is 2.5 times more than that without Na 2 SO 3 . It is also noted that the Bi 2 WO 6 nanoplates maintain their crystal structure and morphology after pyro-catalytic reduction as con rmed by the XRD analysis and SEM characterization (see Fig. S5). For further proof that the CO 2 reduction comes from pyroelectrocatalysis, the result in Fig. S6a shows that no methanol or other products can be detected under temperature variation when experiments was performed without Bi 2 WO 6 nanoplates. Furthermore, no methanol or other products can be detected in Fig. S6b  To have a better understanding of the pyroelectric catalysis, and ·OH detection were performed.
Pyroelectric charges can react with O 2 and OHin water to form superoxide anions ( ) and hydroxyl radicals (·OH). Such reactions can be expressed as the following equations, Experimentally, the ·OH can be detected by uorescence spectrometry using terephthalic acid as a photoluminescent ·OH trapping agent. The can be detected by UV-vis spectrophotometer since can react with nitro-blue tetrazolium (BNT) to produce diformazan and monoformazan. As shown in Fig.  3c, signi cant uorescence emission at ~425 nm which is associated with 2-hydroxyterephthalic acid is observed upon the temperature-variation cycles. The gradual increase of luminescence intensity with temperature-variation cycles indicates the formation of hydroxyl radicals. Fig. 3d presents the absorption spectra of diformazan and monoformazan, which are produced by BNT reacted with . The increase of peak absorption at ~630 nm and 720 nm with the temperature-variation cycles suggests the formation of superoxide anions 32 . The generation of both superoxide anions and hydroxyl radicals indicates the production of charges through pyroelectric catalytic reaction through temperature variation as illustrated by the reactions described by equations (2)-(4). Superoxide anions and hydroxyl radicals are considered to be the main active species in dye decomposition 33 . Except for the pyroelectrocatalysis of CO 2 reduction, we further performed RhB pyroelectrocatalytic decomposition experiment to fully demonstrate the pyroelectrocatalytic activities. In fact, RhB pyroelectrocatalytic decomposition is a visual evidence to prove the redox ability of pyroelectric generated charges. Accordingly, Rhodamine B (RhB) solution (5 mg L −1 ) is used to demonstrate the pyroelectrocatalytic dye decomposition of Bi 2 WO 6 in Fig. S7a and S7b. In order to prove that methanol is the product of CO 2 reduction, we did the isotopic labeling experiment using 13 CO 2 as feedstock. The 1 H NMR spectrum of the reaction solution (Fig. 3e) clearly shows the formation of methanol (δ=3.34 ppm) when the unlabeled CO 2 is used as feedstock. While using 13 CO 2 instead of CO 2 , the 1 H NMR spectrum of the reaction solution in Fig. 3f show doublet peaks between 3.7 and 3 ppm, which was attribute to the proton coupled to the 13 C of 13 CH 3 OH 34 . The small peak of CH 3 OH at 3.34 ppm possibly comes from the decomposed CO 2 of NaHCO 3 during the heating process, participating in CO 2 reduction 35 . The results indicate that CO 2 is the carbon source for the pyroelectrocatalytic CO 2 reduction into CH 3 OH.
Theoretical calculation of CO 2 reduction reaction path. To better understand the reaction mechanism for the CO 2 reduction, we carried out rst-principles calculations with SIESTA package which is based on density functional theory (DFT) 36 . The pseudopotentials were constructed by the Troullier-Martins scheme 37 .The Ceperley-Alder exchange-correlation functional as parameterized by Perdew and Zunger was employed for the local density approximation (LDA) 38,39 . In all calculations, the double-ζ plus polarization basis sets were chosen for all atoms. The atomic structures were fully relaxed using the conjugated gradient method until the Hellman-Feynman force on each atom is smaller than 0.02 eV/Å. Since Bi 2 WO 6 consists of alternative (WO 4 ) 2and (Bi 2 O 2 ) 2+ layers as discussed above, a slab model is constructed for the Bi 2 WO 6 (001) surface. The top of the slab is terminated by the WO square network, and the bottom of the slab is saturated by H atoms, as shown in Fig. S8. It is known that oxygen vacancies commonly exist in oxide semiconductors 40 . However, it is found that the formation energy of an oxygen vacancy in Bi 2 WO 6 (001) is as large as 3.2 eV, which indicates that the density of oxygen vacancies in Bi 2 WO 6 (001) is ignorable. To explore the possible process of the CO 2 reduction, it is necessary to gure out the ground-state adsorption con guration of the CO 2 molecule on Bi 2 WO 6 (001). Fig. 4a shows ve different adsorption con gurations for CO 2 . The lowest adsorption energy is −3.6 eV, which implies that the CO 2 molecule is strongly bound to Bi 2 WO 6 (001). In this case, the CO 2 molecule is bent with one C-W bond (2.00 Å) and two O-W bonds (2.09 and 2.26 Å) as shown in Fig. 4b (step "0"), which is different from previous report 9 . The C-O bond lengths are elongated by 0.1 Å, because of the interaction between CO 2 and Bi 2 WO 6 (001). The CO 2 reduction starts when the hydrogen ions in the solvent interact with the CO 2 molecule. Note that DFT calculations, the hydrogen atom as proton (H + ) and electron (e -) cannot be separated directly. To model the reaction between H and radical on Bi 2 WO 6 (001), a H atom is placed beside a certain site of the radical and carried out DFT calculations to optimize the interaction between them. Electron charge transfer happens between the H atom and radical, usually from H to the radical so that the H atom nally becomes H + , according to the chemical bonding between H and the radical (e.g. CO 2 molecule in this work). In other words, charge separation can be reached after self-consistent-eld iterations. In addition, the gas phase of H is assumed because the solvent does not involve in the reaction of the CO 2 reduction. The process of the CO 2 reduction is divided into a series of steps and the reaction energies are calculated step by step. All possible structural con gurations along with addition of one H ion are considered for each step simulation. For instance, from step "0" to step "1", the H ion may bind to the CO 2 molecule through C or O atom, or to the Bi 2 WO 6 (001) surface through W or O atoms, which results in different products. To determine the most possible reaction, the reaction energy of each product is estimated as: ΔE = E(n) − E(n − 1) − µ H , where E(n) is the total energy of a certain product at the nth step and µ H is the chemical potential of H. After optimizing all structural con gurations, we pick up the case with the lowest reaction energy at each step and plotted the optimized structural con guration in Fig. 4b. Obviously, the structural con gurations in Fig. 4b are the most possible products for each step. The reaction energy of the most possible product at each step is plotted in Fig. 4c. Here, the rst three H ions at the rst three steps prefer to bind to the C atom, and these reactions are exothermic due to the large negative reaction energies as seen in Fig. 4c. As a consequence, one C-O bond is broken and a CH 3 O * radical and a separate O ion are produced at step "3". Then the subsequent H ions will be attracted by the separated O ion until a H 2 O molecule forms (step "5").
However, the H 2 O molecule is not released from Bi 2 WO 6 (001), because it requires a large activation energy of about 1.7 eV. Finally, a methanol (CH 3 OH) molecule is produced after one more H ion attaches to the CH 3 O * radical (step "6"). As shown in Fig. 4c, the rst four reaction steps are exothermic while the last three are endothermic. In particular, an activation energy of 1.6 eV (equal to the reaction energy) is needed for the CH 3 OH molecule to be detached from the Bi 2 WO 6 (001) surface, i.e. from step "6" to step "7". Note that CH 3 OH may also be produced at steps "4" and "5" (short dashed lines in Fig. 4c), but the corresponding activation energies are respectively 1.77 and 1.87 eV, even larger than that for step "7". Therefore, the possibility to produce CH 3 OH at steps "4" and "5" is very small, because the major reactions at the rst two steps are exothermic. Nevertheless, the overall process of the CO 2 reduction is still exothermic as shown in Fig. 4c, that is, the activation energies in the last three reaction steps can be compensated by the energy released in the rst four steps. Hence the CO 2 reduction lasts spontaneously in principle. Nevertheless, given the energy loss in a solvent environment, energy supply is still needed but the energy demanding is not much. This is the reason that the temperature is not high in our experiments. It is worth pointing out that, when the CH 3 OH molecule is detached from the Bi 2 WO 6 (001) surface, the zero-point energy and enthalpy contribute to the free energy signi cantly 41 . Therefore, the reaction energies are also estimated through the free energies for CH 3 OH production indicated by the red lines in Fig. 4c. Interestingly, the activation energy of step "7" decreases to 0.95 eV, which implies that the reaction may happen at relatively high temperature, as found in our experiments.
On the basis of above analysis, a mechanism of pyroelectrocatalytic CO 2 reduction induced by pyroelectric Bi 2 WO 6 nanoplates is proposed in Fig. 5. When the catalyst temperature remains in a stable value, the internal spontaneous polarization balances with the external bound charges (Fig. 5a). It has been reported that the polarization density relies on the temperature of the pyroelectrocatalyst 42 . That is to say, the increase of temperature would lower the polarity of the pryoelectrocatalyst, so that the balance is broken and the free charges are generated. The free negative charges react with adsorbed CO 2 to form methanol and the free positive charges would be captured by Na 2 SO 3 to form Na 2 SO 4 (Fig. 5b). As a result, a new balance is established between polarization and bonding charges (Fig. 5c). On the other hand, the decrease of temperature caused the increase of spontaneous polarization, and the equilibrium will be broken again, thus leading to opposite charges transfer and new CO 2 reduction process (Fig. 5d).
The next balance is established to return the original state between polarization and bonding charges, so temperature variation will continuously produce the pyroelectrocatalytic behavior of CO 2 conversion into methanol.
In summary, a new approach for e cient CO 2 reduction is introduced by pyroelectrocatalytic Bi 2 WO 6 nanoplates during temperature variation. Experimental result shows that the yield of methanol generation from CO 2 can be as high as 55.0 µmol·g −1 after 20 cycles of temperature variation. This e cient and environment friendly process based on the pyroelectric nanomaterial Bi 2 WO 6 provides great potential for CO 2 reduction in utilizing environmental heat energy near room-temperature.
In a typical process, 485 mg of Bi(NO 3 ) 3 ·5H 2 O (1 mmol) and 165 mg of Na 2 WO 4 ·2H 2 O (0.5 mmol) were added into the mixed solution. White precipitate appeared immediately in the solution. After being washed for several times, the collected precipitate was added into a 50 mL Te on-lined autoclave and lled with deionized water up to 80% of the total volume. Then the autoclave was sealed into a stainless steel tank and kept at 433 K for 20 h. After reactions, the white as-prepared sample was centrifugated with a speed of 6000 rpm and washed three times with deionized water. Finally, the collected products were dried in vacuum at 333 K for 12 h for further use.
Characterization. The crystal structure was test by an X-ray diffractomer ( Philips PW3040/60, the Pyro-catalytic CO 2 reduction activity test. In the pyroelectrocatalytic CO 2 conversion process, Bi 2 WO 6 powder (40 mg) was suspended in 5 mL 0.2 M NaHCO 3 solution in a 35 mL borosilicate tube with the addition of 0.3 M Na 2 SO 3 as a sacri cial donor. High purity CO 2 gas was bubbled into the borosilicate tube for 10 min. Then the tube was immediately sealed with a rubber stopper. Then the tube was immediately sealed with a rubber stopper. The sample was suspended in the solution under stirring, being applied alternating temperature between 15 o C and 70 o C in water bath. The entire catalytic process is performed in dark. The detailed temperature pro le can be found from Fig. S9. To detect the formation of methanol, 1 mL solution was fetched out and analyzed by using a gas chromatograph (Persee G5) equipped with a KB-5 column connected to a ame ionization detector. For the nuclear magnetic resonance (NMR) test, 800 μL reaction solution, 100 μL D 2 O and 10 μL DMSO (0.1% vol aqueous solution) were taken into nuclear magnetic tubes, and detected with an NMR spectrometer with superconducting magnet (AVANCE NEO 400MHz, Switzerland).