Thermoelectricity Enhanced Electrocatalysis - Nano Letters (ACS

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Thermoelectricity enhanced electrocatalysis Tiva Sharifi, Xiang Zhang, Gelu Costin, Sadegh Yazdi, Cristiano F. Woellner, yang liu, ChandraSekhar Tiwary, and Pulickel M. Ajayan Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b04244 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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Thermoelectricity enhanced electrocatalysis Tiva Sharifi1,2*, Xiang Zhang1, Gelu Costin3, Sadegh Yazdi1, Cristiano F. Woellner4,1, Yang Liu1, Chandra Sekhar Tiwary1, Pulickel Ajayan1* 1

Department of Material science and Nanoengineering, Rice University, Houston, USA 2 3 4

Department of Physics, Umeå University, Umeå, Sweden

Department of Earth Science, Rice University, Houston, USA

Applied Physics Department, State University of Campinas, Campinas, Brazil

We show that thermoelectric materials can function as electrocatalysts and use thermoelectric voltage generated to initiate and boost electrocatalytic reactions. The electrocatalytic activity is promoted by the use of nanostructured thermoelectric materials in hydrogen evolution reaction (HER) by the thermoelectricity generated from induced temperature gradients. This phenomenon is demonstrated using two-dimensional layered thermoelectric materials Sb2Te3 and Bi0.5Sb1.5Te3 where a current density approaching ~50 mA/cm2 is produced at zeropotential for Bi0.5Sb1.5Te3 in the presence of a temperature gradient of 90 ˚C. In addition, turnover frequency reaches to 2.7 s-1 at 100 mV under this condition which was zero in the absence of temperature gradient. This result adds a new dimension to the properties of thermoelectric materials which has not been explored before and can be applied in the field of electrocatalysis and energy generation.

KEYWORDS:

Thermoelectrocatalysis,

Thermoelectric

materials,

Electrocatalysis,

Temperature gradient, Hydrogen evolution reaction. 1 ACS Paragon Plus Environment

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The concept and application of therrmoelectricity has been well explored in recent years due to increased global energy demand in parallel to global warming for which waste heat management is highly desired. The voltage generated in thermoelectric materials can supply energy to any energy demanding system when there is a chance of either existence of temperature gradient or the possibility to generate it if it does not cause any mis-functionality for the system. Electrocatalytic reactions are good example of such systems. Electrochemical reactions are non-spontaneous reactions which for them to occur, a proper catalyst and a sufficient external voltage are usually needed. Thermoelectric materials can act as mini voltage generators to boost electrochemical reaction and hence reduce/eliminate the external bias energy. In this case, thermoelectric material has a function similar to but conceptually different from the catalyst. As recently solar energy has been widely considered as a renewable energy resource to power up electrochemical systems such as water splitting reaction 1-2, temperature gradient could be naturally established and be utilized in the system. Here, we report the investigation of thermoelectric materials as a thermoelectrocatalyst to harvest thermal gradients in the system to facilitate the electrochemical reaction. We compare two well-known thermoelectric materials, antimony telluride (Sb2Te3) and bismuth antimony telluride alloy (Bi0.5Sb1.5Te3) in the form of nanosheets, which have different thermal conductivities and hence different figures of merit at the studied temperature. We studied the electrochemical performance of these materials in acidic medium for negative potentials. This is the region where HER (one of the water splitting half reactions) occurs and despite of small intrinsic catalytic activity, hydrogen evolution was observed from these materials. In the presence of the temperature gradient (TG), a significantly higher current density compared to the regular condition was measured from Bi0.5Sb1.5Te3 electrode while the applied TG was not enough to improve the measured current density from Sb2Te3. In the case of Bi0.5Sb1.5Te3, the current was generated in the system spontaneously when TG was induced and a current

2 ACS Paragon Plus Environment

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density of ~50 mA/cm2 was measured at zero potential. We calculated the turnover frequency (TOF) for Bi0.5Sb1.5Te3 to be 2.7 s-1 at 100 mV in the presence of TG which was close to zero in the absence of TG. Based on our results, we introduce the thermoelectrocatalytic property of thermoelectric materials which has not been explored before. The conversion efficiency in thermoelectric devices is usually shown by figure-of-merit (ZT) ( =

), where σ, S, T and are the electrical conductivity, the Seebeck

coefficient, the absolute temperature and the thermal conductivity. Seebeck coefficient is a measure of the produced voltage in the thermoelectric material as a response to the temperature gradient. Depending on the structure and properties of the material, ZT can have different values and exhibit its highest value at different temperatures. In order to be considered as a thermoelectrocatalyst, a thermoelectric material needs to meet specific requirements to be able to supply sufficient voltage for initiation of electrochemical reaction. Since thermoelectrocatalysis is to be investigated in aqueous electrolyte media, thermoelectric materials performing high ZT at low temperatures were desired in order to avoid applying temperatures much higher than 100 ˚C in the hot side and less than 0 ˚C in the cold side. Nanostructuring of thermoelectric materials is shown to improve their ZT

3-9

due to the size

confinement effect on increasing the electrical conductivity and reducing the thermal conductivity. The field of thermoelectricity has remarkably developed in recent years with the achievements in nonstructural engineering and synthesis of low dimensional thermoelectric materials 6, 10-14. 2D thermoelectric materials are good candidates which are also beneficial by offering high surface area to provide larger contact area with both the electrolyte and current collector and hence higher rate of reaction. In addition, their atomic scale thickness furnishes rapid charge transfer once the charge separation is stablished

15-16

. The lattice thermal

conductivity is reported to further reduce by introducing heavy guest elements such as Bi, Te and Pb into the structure of host material to form an alloy structure. These guest elements 3 ACS Paragon Plus Environment

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work as obstacle and slow down the thermal conduction in lattice. Based on these, among the known thermoelectric materials, Sb2Te3 and its alloy structure with bismuth in the form of nanosheets were chosen for this study. Sb2Te3 is a well-known p-type, topological insulator thermoelectric material with an insulating bulk band gap and a semi-metallic surface conductivity

17-18

. The thermoelectric

property of antimony telluride has been further improved by introducing bismuth, forming BixSb2-xTe3 alloy structure. BixSb2-xTe3 is a p-type room temperature thermoelectric material for which the ZT peak appears below 100 ˚C

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. Nanostructured Bi0.5Sb1.5Te3 is shown to

perform one of the highest ZT value at room temperature

20-21

. Both Sb2Te3 and BixSb2-xTe3

alloy structure are reported to have rhombohedral tetradymite-type crystal structure with five atoms in their unit cell. A remarkable anisotropy arises from the alternative Te(1)-Sb/BiTe(2)-Sb/Bi-Te(1) layered structure along the c-axis while the superscripts (1) and (2) denote two different chemical states for the tellurium anions in the structure. Sb2Te3 and BixSb2-xTe3 nanosheets were synthesized hydrothermally at 180 ˚C as described in detail in methods section. Scanning and transmission electron microscopy (SEM and TEM) of the as synthesized materials after washing and freeze drying are shown in Figure 1a-c. Both the materials were grown in the shape of hexagonal nanosheets with the slightly larger lateral dimension for Sb2Te3 than BixSb2-xTe3 which is in the order of ~ 1 µm. Atomic force microscopy (AFM) measurement shows the similar lateral dimension and a thickness of ~10 nm for most of BixSb2-xTe3 nanosheets as shown in Figure S1. Figures S2a-b show the corresponding selected area electron diffraction (SAED) pattern of Sb2Te3 and BixSb2-xTe3. The sharp and distinct diffraction spots reveals the perfectly crystalline structure of as-grown materials plus the crystalline structure has been altered slightly by introducing Bi in the structure of Sb2Te3. The well-ordered pattern of individual atoms in high angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) image of BixSb2-xTe3


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(Figure 1d), reveals that the crystalline structure of Sb2Te3 is well preserved by introducing Bi guest elements. The composition of the materials was studied by x-ray photoelectron spectroscopy (XPS), energy dispersive spectroscopy (EDS), wavelength dispersive spectroscopy (WDS) and x-ray diffraction analysis (XRD). Figure S3 shows the XPS spectra of the as synthesized materials with the assigned peaks confirming the presence of Sb, Bi and Te. The chemical composition of BixSb2-xTe3 was determined by EDS, confirming stoichiometry between the present elements to be Bi0.5Sb1.5Te3. Elemental WDS mapping of nanosheets by Electron Probe Micro-Analysis (EPMA) demonstrates a homogeneous distribution of all three elements (Bi, Sb, Te) throughout the nanosheets as shown in Figure 1e. XRD measurements confirmed the formation of Sb2Te3 and Bi0.5Sb1.5Te3 as well (figure S4) and no impurities of any other phases could be detected. All the diffraction peaks in both the structures could be indexed to the trigonal crystal structure belonging to space group R(3)m similar to the structure of tetdadymite (Bi2Te2S) as reported before 22. Cell parameters of Bi0.5Sb1.5Te3 is measured to be a=b=4.33 Å and c=30.49 Å which is well between the measured lattice parameters of Sb2Te3 (a=b=4.25 Å and c=30.45 Å) and reported lattice parameters of Bi2Te3 (a=b=4.38 Å and c=30.49 Å)

22

. In addition, Raman spectroscopy was

employed for structural investigation of the material. As shown in Figure S5, two distinct Raman features at 117 cm-1 and 136 cm-1 can be recognized for Sb2Te3 which are attributed to Eg and A2u vibrational modes of Sb2Te3

23-24

. For Bi0.5Sb1.5Te3, among the Raman active

modes, Eg which is responsible for the in-plane atomic vibration of atoms with respect to each other could be recognized at 130 cm-1 25-26.


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Figure 1. Scanning electron microscopy image of (a) Sb2Te3 and (b) Bi0.5Sb1.5Te3, (c) transmission electron microscopy of Bi0.5Sb1.5Te3 showing similar hexagonal nanosheet formation for both the structures, (d) HAADF-STEM image of Bi0.5Sb1.5Te3 and (e) Elemental WDS mapping of Bi0.5Sb1.5Te3 nanosheets for Te (red), Sb (green) and Bi (blue) by EPMA demonstrating a homogeneous distribution of all three elements throughout the nanosheets. The electrochemical behavior of the synthesized materials in response to temperature was investigated in a three electrode electrochemical cell. The cell was assembled as explained in 6 ACS Paragon Plus Environment

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methods section and schematically shown in Figure 2a. Working electrode was prepared by first transferring a multilayer graphene on top of a piece of silicon wafer. Then the synthesized thermoelectric (TE) materials (either Sb2Te3 or Bi0.5Sb1.5Te3) were dropped on the surface of graphene within a defined area from their respective dispersion. As shown in Figure S6, nanosheets are well distributed on the surface of graphene with a minimum overlapping of the nanosheets. The loading was kept similar at ~0.1 mg/cm2 for both the electrode materials. A copper strip was connected to the edge of graphene and then the contact and edges of graphene was covered carefully with epoxy resin, leaving only the TE loaded region of graphene to be exposed. To keep the electrolyte in place, an O-ring was mounted on top of epoxy and then 20 µl of 0.5 M H2SO4 was dropped on the exposed window. The counter and reference electrodes were inserted from top (see Figures 2a and S7). Graphene was used as the current collector in the assembly of working electrodes. However due to their high electrical conductivity, telluride nanosheets could play both the role of electrode material and current collector but to avoid any overlapping of nanosheets and hence reducing the measured current, we made use of graphene as current collector. In addition since graphene is electrochemically inactive, it only furnished the electrical connection between separated nanosheets and transferring charges to the copper contact and not contributing to any electrochemical reaction.


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Figure 2. (a) Schematic image of electrochemical cell placed on top of a TG system. TG system is made of a peltier cooler and a thin heater mounted on top of a heat sink. LSV curves in absence (black line) and presence of TG (red line) for (b) Sb2Te3 loaded working electrode showing similar behavior of the electrode material in both conditions and (c) Bi0.5Sb1.5Te3 loaded electrode demonstrating a significantly different behavior regarding the measured current density in the absence and presence of TG. The scan rate was 5 mV s

–1

and the

electrolyte was 0.5 M H2SO4 aqueous solution. The working electrode was then placed and fixed on top of a TG system which comprises a thin flexible heater and a peltier cooler both mounted on top of a heat sink (see methods section). Voltages on the heater and cooler were tuned to find the optimum combination in which the temperature of the electrolyte remains closest to room temperature (30 ˚C). Figures 2b and 2c show the linear sweep voltagrams (LSV) of Sb2Te3 and Bi0.5Sb1.5Te3 working electrodes in absence and presence of TG. The presented data is collected with the best 8 ACS Paragon Plus Environment

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combination of heater/cooler temperatures giving the highest thermoelectrocatalytic performance, which is ∆T~90 ˚C while the temperatures of hot and cold sides are 100 ˚C and 8.6 ˚C respectively. In the case of Sb2Te3 working electrode (figure 2b), there is no particular difference between the electrochemical performances of the material in these conditions (HER overpotential of ~400 mV), pointing towards insufficient TG and hence low thermoelectric activity of Sb2Te3 at the studied TG condition (∆T~90 ˚C). Since the electrolyte temperature is kept at room temperature, any significant effect originating from thermoelectric property of Sb2Te3 could not be recognized. For the other device which was assembled with Bi0.5Sb1.5Te3 working electrode (figure 2c), a regular LSV curve for HER is observed in the absence of TG (∆T = 0 ˚C). Bi0.5Sb1.5Te3 exhibits a poor catalytic activity for HER with a high HER onset potential of ~400 mV which is much higher than the well-known 2D HER catalysts such as transition metal dichalcogenides (TMDs) 27-28. However, in the presence of TG (∆T~90 ˚C) a remarkable improvement in the increasing rate of current density by voltage is observed. The current density at -0.6 V vs. RHE is 7 times higher than its value in the absence of TG. In the presence of TG, the measured current is so high that any onset potential for HER cannot be recognized and current is established once TG is introduced resulting in the appearance of a current density of 48 mA/cm2 at zero applied potential. We observed similar behavior despite of different current density in several working electrodes which was assembled in the same way (an example is shown in Figure S8). Current density could be different among the devices due to quality of the contacts, distribution of TE nanosheets and quality of graphene current collector. This electrochemical behavior is absolutely different from electrochemical behavior when temperature of electrolyte is higher than room temperature. To show this, instead of keeping the electrolyte temperature close to room temperature, we raised the temperature of hot side stepwise and recorded the electrochemical performance of the electrode at each condition. As can be seen in Figure S9, when we tried to increase the


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temperature of electrolyte instead of stabilizing the TG, a higher current density was measured only at higher voltages with a slight improvement in the onset potential. By increasing the electrolyte temperature, movement of ions across the electrolyte is accelerated which results in higher rate of electrochemical reaction and hence a higher current can be measured in unit time. However, since this improvement originates from an improvement in kinetics of electrochemical reaction, a dissimilar electrochemical behavior is observed compare to the behavior of the system in the presence of TG. In addition, other temperature gradients (different combinations of cold and hot sides) were also tested and for ∆T< 90 ˚C smaller thermoelectrocatalysis effect was observed as shown in figure S10. It is worth mentioning that for similar ∆T~90 ˚C but with temperatures on the hot side above 100 ˚C, we observed an increase in the temperature of the electrolyte above room temperature and hence we fixed the TG to be 90 ˚C with the aforementioned temperature combination on the cooler/heater. Platinum (Pt)-based catalysts are considered as the most effective electrocatalysts for the HER. PtC (carbon-supported Pt) is the best commercially available HER catalyst which drives the reaction at an overpotential of almost zero

29

. However, even for the state-of-the-art Pt-

based catalysts at least 100 mV is consumed to produce 50 mA/cm2 of current density

30-32

which is produced in our system by applying zero potential and just by utilization of the heat. Accomplishing zero-potential current significantly improve the efficiency of the full water splitting reaction. TOF is the best measure to represent the catalytic activity of a material as it takes to the account the number of evolved hydrogen molecules per mole of catalyst per unit time. We estimated TOF in 0.5 M H2SO4 aqueous solution in the presence of TG to be ~2.7 s1

at an overpotential of 100 mV whilst it is almost zero in the absence of TG pointing towards

the negligible intrinsic catalytic activity of Bi0.5Sb1.5Te3. TOF of Bi0.5Sb1.5Te3 in our studied condition (in the presence of TG) is 2-3 times higher than the best reported HER


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electrocatalyst such as PtC or MoS2 at similar potential

33

. It worth mentioning that similar

measurement was repeated for several devices which were loaded similarly with TE material. However, the measured current could be different amongst devices due to the difference between the quality of the copper contact, distribution of nanosheets and the quality of the underlayer graphene but all showed a similar pattern in the presence of TG. When temperature is maintained at the electrode surface, charge carriers having higher thermal velocities on the hot side of the TE material start to diffuse to the cold side of the material more quickly than in the opposite direction. A continuous maintenance of thermal gradient across the electrode surface would results in a planar charge separation across each individual TE nanosheet. HER is an electron demanding reaction in which for one hydrogen atom to be produced, two electrons are consumed. Manifestation of the charge separation in the electrode material affects the band structure and hence accomplishment of charge flow from one side of the nanosheets in a higher rate. The necessary voltage to run the electrochemical reaction is supplied by TE material even before the external bias is sufficient to start this reaction. This explains the higher rate of reaction when TG is applied to the working electrode and also appearance of current at zero applied potential since the potential that appears in I-V curve is the bias potential. The voltage generated in TE materials is defined by

= ∆ − Where is the voltage generated by TE material and , ∆, , are Seebeck coefficient, TG, measured current and resistance within the TE material respectively. As mentioned earlier, intrinsic properties of the TE materials such as thermal and electrical conductivities which are related to the morphology, crystallinity and elemental distribution affect the ZT and the power generated by TE material. Different Seebeck coefficients ranging from 100 ⁄ to 270 ⁄ are reported for nanostructured Bi0.5Sb1.5Te3 at room temperature

20, 34-35

.


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Considering a negligible resistance within the material itself (which can be an absolutely rough assumption), a voltage ranging from 36 mV to 96 mV can be produced by Bi0.5Sb1.5Te3 at ∆T~ 90 ˚C. For a noticeably similar morphology of Bi0.5Sb1.5Te3 as our studied structure, Seebeck coefficient is reported to be ~112 ⁄. By considering the same assumption, a voltage of 40 mV is generated in our material by introducing a TG of ∆T~ 90 ˚C. However, while the Seebeck coefficient is not significantly dependent on the thickness and size of Bi0.5Sb1.5Te3 platelets, electrical and thermal conductivities and hence the power generated by each nanosheet can be strongly influenced by the lateral and vertical sizes of nanosheets

36

.

The current generated by this extra potential is even more complicated to calculate due to different sources of thermal and electrical resistances in our working electrode as well. Additionally, the power generation and hence the current density achieved from TE material is also dependent on the number of TE units. Hence for an accurate estimation of the power generation, in addition to the intrinsic properties, number of the nanosheets involved in electrochemical reaction should be known as well. Putting these together, it is difficult to quantify the contribution of the thermoelectric voltage to the measured current from electrochemical reaction at this stage. It is worth mentioning that, the presented ∆T is the absolute temperature difference on the surfaces of the cooler and heater. The actual ∆T on the electrode surface should be lower due the temperature loses from cooler/heater to the electrode surface. It is evident that in such an ideal assembly of the working electrode the measured current can be really high due to the large contact area between the TE material and graphene current collector. The idea of thermoelectrocatalysis was put on test in a more applicable condition as well. In this case, working electrode was prepared by loading TE materials (0.06 mg/cm2) on carbon paper current collector (see methods section). We used Bi0.5Sb1.5Te3 as TE material since its superior electrochemical property over Sb2Te3 was already realized. SEM image of TE loaded


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carbon paper working electrode is shown in Figure S11. The electrode was then tested in a common three electrode electrochemical cell. TG condition was stablished in the electrolyte using a TG system while the temperature of the electrolyte was kept close to room temperature (see methods section). -0.5

-0.4

-0.3

-0.2

-0.1

0.0 0 -2 -4 -6 -8 -10 -12 -14 -16

TE/CP

-18

0

b Current density [mA/cm2]

a Current density [mA/cm2]

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25

50

75

100 125 150 175 200 225

10

10

0

0

-10

-10

-20

-20

-30

-30

Potential [V vs RHE]

Time [min]

Figure 3. LSV curves for (a) TE material loaded carbon paper working electrode (TE/CP), the initial performance in the absence of TG (black line) and in the presence of TG (red line). Blue line (∆Tf=0 ˚C) shows the performance of the same electrode right after removing TG, (b) Long-term current stability of the TE/CP working electrode at constant potential of -0.3 V in the presence of TG (∆T~90 ˚C) Bi0.5Sb1.5Te3 is used as TE material. The scan rate was 5 mV s –1 and the electrolyte was 0.5 M H2SO4 aqueous solution. LSV curve of Bi0.5Sb1.5Te3 loaded carbon paper (TE/CP) is shown in Figure 3a. The initial performance of the electrode in the absence of TG is marked with ∆Ti = 0 ˚C (black line). As it can be observed, by inducing a TG of ∆T = 90 ˚C, the LSV curve is identically shifted to generate higher current density (~ 2× times). In this case it usually takes few cycles for the electrode to maintain the TG and stabilize the current (usually between 7-8 cycles) and the shown curves are the 10th cycle. The same electrode was then removed from electrolyte and inserted in a fresh electrolyte where no TG in induced to the system. The LSV curve of the electrode in this condition is marked with ∆Tf = 0 ˚C (blue line) in Figure 3a. The electrode


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clearly preserves its original performance once the TG is removed which proves that the higher measured current density in TG condition is due to the contribution of TE material in electrochemical reaction. The smaller measured current in this case (compare to Figure 2c) is attributed to the arrangement of the nanosheets on the surface of carbon paper. As shown schematically in Figure S12, due to non-flat nature of carbon paper current collector, nanosheets are randomly oriented which results in a reduced contact area between nanosheets and the current collector. The temporal stability of achieved current during evolution of hydrogen in the presence of a TG of 90 ˚C was also measured for TE/CP electrode. As shown in figure 3b, a stable current can be measured on TE/CP over 4 h operation of the cell at a voltage of -0.3 V vs RHE. Interestingly, TE materials also have the capability to enhance (as a modifier) the catalytic activity of an intrinsic electrocatalyst in the presence of TG. Bi0.5Sb1.5Te3 nanosheets were simply added on top of a Pt coated carbon paper with a Pt:TE weight ratio of (1:16) without any further treatment. Electrochemical performance of TE/Pt in the absence and presence of TG is shown in Figure S13. As illustrated before, in the presence of TG, Bi0.5Sb1.5Te3 act as a thermoelectrocatalyst and therefore its contribution to the current adds up to the current from Pt electrocatalyst. In the absence of TE material, Pt-coated current collector (Pt/CP) show similar performance in the absence and presence of TG (see Figure S14). Considering Pt to be the catalyst and Bi0.5Sb1.5Te3 as the catalyst modifier, TOF is calculated to be 1.23 s-1 and 2.29 s-1 in the absence and presence of TG at an overpotential of 100 mV. In other words, by adding non-electrocatalyst Bi0.5Sb1.5Te3 to Pt, the level of catalytic activity per unit time is improved by almost two times in the presence of a sufficient TG. In summary, we have demonstrated the concept of thermoelectrocatalysis where electrocatalytic reactions are initiated and enhanced using thermoelectric voltage generated by catalysts. By inducing temperature gradient, we observed a rapid improvement in HER


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catalysis on Bi0.5Sb1.5Te3 working electrode. Current density was measured to be 48 mA/cm2 at zero applied potential and reached as high as 7 times its value in the absence of temperature gradient at an applied voltage of -0.6 V vs RHE. In addition, the thermoelectrocalysis performance of Bi0.5Sb1.5Te3 was tested as a modifying additive to Pt. In the presence of temperature gradient, the measured current density for HER was significantly improved and TOF was increased by almost two times at the overpotential of 100 mV. ASSOCIATED CONTENT

Supporting Information. Section I: materials and methods including material preparation and characterization. Section II: AFM image of Bi0.5Sb1.5Te3 nanosheet (Figure S1), SAED pattern (Figure S2), XPS spectra (Figure S3), XRD pattern (Figure S4) and Raman spectra (Figure S5) of Sb2Te3and Bi0.5Sb1.5Te3. SEM image of Bi0.5Sb1.5Te3 working electrode assembled on graphene current collector (Figure S6), Optical and schematic images of working electrode (Figure S7), LSV curves of Bi0.5Sb1.5Te3 working electrode prepared and tested in similar way as shown in Figure 2c (Figure S8), LSV curves of Bi0.5Sb1.5Te3 working electrode at different temperatures (Figure S9), LSV curves of Bi0.5Sb1.5Te3 working electrode at different temperature gradients (Figure S10), SEM image of Bi0.5Sb1.5Te3 loaded carbon paper working electrode (Figure S11), Schematic comparison of nanosheets arrangement on top of different current collectors (Figure S12), LSV curves of TE/Pt (Figure S13) and Pt/CP (Figure S14) working electrode in the absence and presence of TG. AUTHOR INFORMATION

Corresponding Author Prof. Pulickel Ajayan. Email: [email protected] Dr. Tiva Sharifi. Email: [email protected] 15 ACS Paragon Plus Environment

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Author Contributions T.S. performed most of the synthesis, electrochemical testing, electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, made figures and writing. X.Z performed synthesis and transfer of graphene as well as AFM measurement. G.C performed the EPMA measurements and contributed in related discussion and writing. S.Y performed STEM measurements and contributed to writing, C.F.W made the schematics and Y.L contributed in discussions. C.T contributed in discussions and writing. P.A supervised the project and contributed to the most of analysis, discussions and the writing. All authors took active part in scientific discussions and the finalization of the manuscript. ACKNOWLEDGMENT TS acknowledge vetenskaprådet (Grant No. 2015-06462), XZ acknowledge Air Force Office of Scientific Research (AFOSR-Grant No. FA9550-14-1-0268) and CFW thanks São Paulo Research Foundation (FAPESP-Grant No. 2016/12340-9) for their financial support. Computational and financial support from the Center for Computational Engineering and Sciences at Unicamp through the FAPESP/CEPID Grant No. 2013/08293-7, is acknowledged. EPMA facility at the Department of Earth Science, the Electron Microscopy Center (EMC) and shared equipment authority (SEA) at Rice University are acknowledged. Dr. Ashok Kumar and Hari kishan are kindly acknowledged for their inputs. REFERENCES 1. Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Pijpers, J. J. H.; Nocera, D. G., Science 2011, 334, 645-648. 2. Nocera, D. G., Acc. Chem. Res. 2012, 45, 767-776. 3. Hicks, L. D.; Dresselhaus, M. S., Phys. Rev. B 1993, 47, 12727-12731. 4. Hicks, L. D.; Dresselhaus, M. S., Phys. Rev. B 1993, 47, 16631-16634. 5. Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O'Quinn, B., Nature 2001, 413, 597-602. 6. Harman, T. C.; Taylor, P. J.; Walsh, M. P.; LaForge, B. E., Science 2002, 297, 2229-2232. 16 ACS Paragon Plus Environment

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