Washable and Flexible All Carbon Electrothermal Joule Heater for Electric Vehicles

Amid the rapid development of electric vehicles, a flexible and waterproof radiant heater that can withstand repeated bending and washing is highly desirable. Herein, a freestanding, ultra‐flexible, and washable joule heater is constructed using a biocompatible poly(styrene‐isoprene‐styrene) (SIBS) polymer as binder and carbon black (CB) as heating material. By controlling the amount of CB and the thickness of the film, a minimum resistivity, and conductivity of 26 mΩ cm and 7.4 S cm−1, respectively, is achieved. Remarkably, the 28% CB/SIBS film can reach a maximum temperature of 201 °C while maintaining a stable temperature at 130 °C for repeated ON/OFF cycles. Time‐of‐flight secondary ion mass spectrometry of post‐mortem material analysis shows that a 1 h stability test at 130 °C has no sign of degradation and the films remain extremely stable. The films also show exceptional electrothermal heater performance after carrying out mechanical property tests such as bending (over 30°), repetitive bending (1000 cycles), twisting (two turns), and washing (soaked in distilled water for over 12 h). These outstanding heater performances incorporate extreme chemical stability and mechanical flexibility proposing that the CB/SIBS‐based electrothermal elements hold great potential for numerous practical applications, such as heating systems in electric vehicles and wearable electronics.


Introduction
In winter, electric vehicles (EVs) cause a rapid mileage reduction due to heating power consumption. For instance, the driving run is decreased by 30.9% when the battery temperature drops from 20 to −10 °C. [1][2][3][4] EV battery life and power appear obliged as compared to conventional internal combustion motor vehicles. Therefore, increased winter mileage and high-efficiency vehicle interior heating capabilities are the core technological competitiveness of the EV industry. On the other hand, electrothermal heaters are picking up intrigue for a wide extent of applications including outdoor displays, vehicle window defrosters, heating retaining windows, and other heating systems. [5] Recently, great attention has been given to flexible heaters specifically due to the rise of next-generation devices which require irregular, moldable shapes as a major requirement specified in the automobile industry. [6,7] Therefore, developing new low-power heating systems for EVs is paramount. The proposed radiant heaters in this work will be beneficial to improve battery life and thus maintain the high mileage of the EVs.
Many materials fabricated for these purposes have drawbacks in terms of fabrication and material cost, high energy consumption, corrosion, and chemistry of ingredient materials in the device, and their health and safety impact on the human body. One interesting solution is the infrared radiation (IR) or thermal radiation system, which provides several benefits including low power consumption, lightweight, compatibility for different heated shapes, and so on. [2,8] Indeed, due to the high efficiency of these IR systems, the energy utilization of the vehicle's heating system can be reduced by 50%. [9] It also can enhance the battery life and mileage of EVs. [10] In general, thermal radiation materials include conductive infrared electrothermal materials (e.g., metal alloys, carbon-based materials, etc.) coated on robust polymer substrates forming nanocomposites, which can be used as Joule heaters. [11] Since their formation is flexible and thin, it is convenient to attach them to the surface of the vehicle's interior (e.g., headliner, door cover, rings, etc.). [12] Typically, metal alloys (e.g., nichrome, Kanthal) are used Amid the rapid development of electric vehicles, a flexible and waterproof radiant heater that can withstand repeated bending and washing is highly desirable. Herein, a freestanding, ultra-flexible, and washable joule heater is constructed using a biocompatible poly(styrene-isoprene-styrene) (SIBS) polymer as binder and carbon black (CB) as heating material. By controlling the amount of CB and the thickness of the film, a minimum resistivity, and conductivity of 26 mΩ cm and 7.4 S cm −1 , respectively, is achieved. Remarkably, the 28% CB/SIBS film can reach a maximum temperature of 201 °C while maintaining a stable temperature at 130 °C for repeated ON/OFF cycles. Time-of-flight secondary ion mass spectrometry of postmortem material analysis shows that a 1 h stability test at 130 °C has no sign of degradation and the films remain extremely stable. The films also show exceptional electrothermal heater performance after carrying out mechanical property tests such as bending (over 30°), repetitive bending (1000 cycles), twisting (two turns), and washing (soaked in distilled water for over 12 h). These outstanding heater performances incorporate extreme chemical stability and mechanical flexibility proposing that the CB/SIBS-based electrothermal elements hold great potential for numerous practical applications, such as heating systems in electric vehicles and wearable electronics.
as electrothermal materials. However, they exhibit major shortcomings such as high heavyweight, and low infrared radiation characteristics, [8] which restrict their potential applications.
Recently, several studies have explored carbon-based nanomaterials, such as carbon nanotubes, graphene, graphene-based materials, and their hybrid materials for electrothermal heaters due to their excellent mechanical properties, outstanding thermal conductivity, and strong chemical stability. [13,[14][15][16][17] Nevertheless, these reported nanocomposites suffer from high power consumption, instability, and so forth. [11] Certain drawbacks in electrothermal heater performances have been overcome by compositing the carbon source with additives such as silver particles, [18] silver nanowires, [19] SiO 2 particles, [20] and tourmaline. [21] In addition, in most of those studies, the carbon materials are coated on flexible substrates such as Polyethylene terephthalate (PET), which limits their performance. Basically, it is required that the heaters (the whole device along with the functional material) can work reliably under bending, stretching, folding, and even during washing. Thus, the development of a next-generation low-power, bendable, and washable radiant heater with superior performance is in high demand.
Herein, we construct freestanding, ultra-flexible, and washable radiant heaters using carbon black (CB) as heating material and poly(styrene-isoprene-styrene) (SIBS) as a binder. CB is a nonexclusive term for an imperative family of products utilized basically for the reinforcement of rubber, as a black pigment, and for their electrically conductive properties. Being a fluffy powder composed of elemental carbon, this material exhibits extreme fineness and high surface area. [22] There are two factors that determine the electrical conductivity of CB-filled polymer blends. The amount of CB presents in the filler-rich system and the degree of structural continuity of the system mainly governs electric conductivity properties. [23] In this work, SIBS plays dual roles as a polymer substrate and binder to disperse CB. In addition to excellent chemical stability and biocompatibility, SIBS offers among the least moisture permeability of any elastomer, due to the controlled dissemination of isoprene and butadiene monomer units in its mid-block. [24] By controlling the concentration of CB and the thickness of the film, minimum resistivity and square resistance were achieved. The thermal, electrical, and mechanical properties of the CB/SIBs films were investigated to confirm their flexibility and heating performance. Analytical techniques including the time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to uncover the degradation of CB/SIBs films.

Properties of CB/SIBS Films
The freestanding, flexible, and washable CB/SIBS films were synthesized using a simple casting and peeling-off method as shown in Figure 1a. A series of CB/SIBS films were prepared with CB contents of 16%, 22%, 28%, and 30% ( Figure S2, Supporting Information). Crack-free and uniform films were achieved at 28% CB in SIBS, and therefore all the samples were prepared with equal to and less than 28% CB content. The surface morphology and distribution of CB particles were studied using scan-ning electron microscopy. Figure 1b-d shows scanning electron microscopy (SEM) images of CB/SIBS films with 16%, 22%, and 28% CB content, respectively. The CB particles are homogeneously and densely dispersed in the polymer matrix. For comparison, SEM images of pure SIBS films and CB powder were captured (see Figure S3a,b and c,d, Supporting Information). CB exhibits a spherical particulate morphology while pure SIBS films are compact and smooth. From the SEM images, it was revealed that with an increase in the CB content, the polymer matrix density decreased. TEM images of the pure CB ( Figure  S3e,f, Supporting Information) show onion-like ring patterns, which are characteristic of amorphous nongraphitic carbons. [25] The composition of CB/SIBS films was studied by Fouriertransform infrared (FTIR) spectroscopy as shown in Figure 1e. Interestingly, no significant difference in the FTIR spectra of SIBS with and without CB was observed, suggesting the excellent chemical stability of the polymer. The characteristic vibration bands of SIBS are found to be at 3020 and 2957 cm −1 (assigned to CH stretching vibrations of alkene and alkanes, respectively [26] ). The peak located at 2912 cm −1 can be attributed to a methylene group CH stretching [27] while RCH 2 R alkane in phase stretching corresponds to the bond vibration appearing at 2843 cm −1 . [28] The peaks in the region of 1492-1376 cm −1 can be assigned to the stretching vibrations of the CC bond. [29] Meanwhile, the peaks appearing at 888 and 837 cm −1 correspond to bending vibrations of the CH and CC groups of isoprene. The absorption band at 749-757 cm −1 was attributed to a bending vibration of aromatic CH and CC groups of polystyrene. [29] Raman spectroscopy was also obtained for pure CB and the three CB/SIBS films. As shown in Figure 1f, pure CB has the D band at 1354 cm −1 and the G band at 1596 cm −1 . [30] There is no peak shift or intensity change for the D and G bands following the introduction of the SIBS polymer. This suggests the polymer does not destructively interact with CB nanocrystallines. XPS analysis was further performed to investigate the chemical composition of the CB/SIBS heating films. Figure 1h,i shows the C 1s and O 1s high-resolution XP spectra for the 28% CB/SIBS film. The component at 285.0 eV can be assigned to CC and CH bonds; the component at 286.0 eV can be assigned to COC and COH bonds and the component at 288.5 eV to RCO and OCO bonds. [31][32][33] In the practical application of radiant heaters, the core functional films are anticipated to be exceedingly electrically conductive to meet the requirements of safe operating voltage and rapid heating rate. As shown in Figure 1g, the proportion of CB has a great influence on the resistivity of CB/SIBS films. The resistivity of the CB/SIBS films drops from 94 to 26 Ω sq −1 with the increase of CB content from 16 to 28 wt%. The best electrical properties were recorded for the 28% CB/SIBS film with a conductivity of 7.4 S cm −1 , which was further used for other characterizations.

Electrothermal Heater Performance
The spatial distribution of temperature across the area of the heater was studied using IR imaging as illustrated in Figure 2a. Figure 2b-d shows IR images captured at 15 V for the CB/SIBS films with 16%, 22%, and 28% CB content, respectively. Figure 2f shows the temperature for the CB/SIBS films as they were supplied with increasing voltage from 1 to 15 V. For each voltage, the time was kept constant at 5 min ( Figure 2g) to stabilize the temperature. The temperature slowly increases for 16% and 22% CB/SIBS film after applying almost 10 V and eventually reaches 38 and 51 °C, respectively. Such a slow rate in the temperature increase might be due to the poor electrical conductivity of the film due to the lower CB loading. Conversely, for the 28% CB/SIBS film, the temperature quickly (in less than a minute) reaches 103 °C at 10 V input supply and attained a maximum temperature of 200 °C when the voltage was increased to 15 V. After this point, the CB/SIBS films started to crack. An exponential increase in temperature was observed from ≈5 V input voltage for the 28% CB/SIBS film. As given in Figure 2e, current starts to flow through the film above 3 V for all the films. A maximum current of around 245 mA flows through the 28% CB/SIBS film at an applied voltage of 20 V. This implies that increasing carbon ratio in the film directly influences the conductivity and that the 28% CB/SIBS film has the most suitable conductivity among the tested films.
It is well known that flexible and transparent polymers as substrates/binders tend to show poor stability due to their high thermal resistance. [34] Figure 3a shows the time versus temperature profiles of 28% CB/SIBS films at applied voltages of 11-13 V. The CB/SIBS heater has a heating rate of 5-20 °C min −1 and a cooling rate of 3-10 °C min −1 , implying that CB/SIBS films are capable of showing rapid temperature response to the input voltage. The relatively high heating/cooling rate of the CB/SIBS heater may be attributed to the small heat capacity of the heating material due to its low material mass. It is further interesting to note that the 28% CB/SIBS can sustain high temperatures such as 98-102, 125-129, and 130-133 °C for a prolonged time of 1 h for a given voltage of 11, 12, and 13 V, respectively, suggesting outstanding stability of the films. The infrared thermography images of the real heating of the films at an applied voltage of 11-13 V are displayed in Figure 3b. In general, the heaters have a relatively uniform temperature distribution, which is mainly due to the homogeneous distribution of CB in the SIBS polymer matrix. Furthermore, the stability and sensibility of the CB/SIBS radiant heater were examined by repetitive ON-OFF cycles with durations of ≈5 min at an applied voltage of 11 V. As seen in Figure 3c  a maximum temperature of 130 °C in 5 min and takes 10 min to return to RT. It shows stable heating performance for over 10 cycles. Thus, the results confirm that the CB/SIBS films demonstrate not only excellent heating performance but also outstanding stability at elevated temperatures.
To explore the IR radiation properties of CB/SIBS films emission spectra from a wavelength of 2.5 to 25 µm were collected. As shown in Figure S4 (Supporting Information), all the 03 samples' emission spectra were collected for 04 different temperatures (60, 85, 100, and 130 °C). As shown in the spectra for all 03 samples, negligible shifts in wavelength are observed. This might be due to the limited temperature range in the data recorded (temperature recording limited by films' stability). The emissivity of the 03 samples was calculated by using the Stefan-Boltzmann law [35] taking graphite (0.62) as a reference emissivity. [36] The calculated average emissivity values for 16% CB/SIBS, 22% CB/SIBS, and 28% CB/SIBS are 0.577, 0.58, and 0.61, respectively, which proves the films show properties of a grey-body radiation. [8]

A Post-Mortem Study of CB/SIBS Films
The structural, morphological, and compositional changes in the CB/SIBS films were investigated by characterizing them after stability testing at 130 °C for an hour. As seen in Figure S5 (Supporting Information), no significant change in surface morphology of the CB/SIBS (28%) was observed, which suggests that the high temperature and prolonged heating time did not disrupt the CB and SIBS microstructures. The compositional changes were studied by FTIR and Raman as shown in Figure 4a,b. The characteristic bands in FTIR and Raman spectra for CB and SIBS are visible and unchanged in their positions. Importantly, the intensity of the peak located at 692 cm −1 for CB/SIBS films after the heating test was reduced in comparison to that before the test. This may be due to the high temperature affecting the polymer composition. According to the literature, this peak can be assigned to the CH bond out-of-plane bends in aromatic compounds [37] and stretching vibrations of CH 3 groups. [38] Raman mapping is displayed in Figure 4c for a surface area of 200 × 200 µm2. The D and G bands were used to construct maps where the regional spectral variations can be recognized by the contrast of colors. The maximum D band intensity is displayed in red, and the lowest D intensity is in blue. As shown in Figure 4c,d, both before and after heating the films exhibit moderate D band intensity with no specific spatial variation. The maximum G band intensity is displayed in yellow, and the lowest G intensity is in blue. Similar to the D band, both before  and after heating the films show moderate G band intensity with no specific spatial variation. This implies that there is no chemical compositional change occurring in the CB component during the heating.
To get a deeper insight into the change in SIBS composition, ToF-SIMS analysis was performed. ToF-SIMS spectra were obtained in positive polarity and then calibrated for mass scale using peaks attributed to hydrocarbon ions (C 2 H 3 + , C 3 H 3 + , C 4 H 3 + , C 5 H 3 + , C 6 H 3 + ). The pressure in the analysis chamber was maintained at, or below, 1 × 10 −8 mbar during the data collection process. Spectra show a prominent C + peak at m/z 12 with metal ion species such as Na + , Si + , Adv. Mater. Technol. 2023, 8, 2201538  and K + . However, these metal ions were not detectable in XPS. Considering the SIBS molecule there are several possible hydrocarbon species that may be produced from the beam interaction as indicated in Scheme S2 (Supporting Information). All the peaks denoted in both spectra are for saturated aliphatic hydrocarbons (C n H 2n+1 ), unsaturated aliphatic hydrocarbons (C n H 2n−1 ), and aromatic hydrocarbons (C 6 H 5 (CH 2 ) m ). Out of these three types of hydrocarbon species, both spectra show a majority of aromatic hydrocarbon species as indicated in Figure 5. There are no specific peak intensity differences observed after heating the 28% CB/SIBS films. Figure 5 also shows selected hydrocarbon species from spectra, namely, C 6 H 7 + , C 7 H 10 + , C 8 H 7 + , and C 9 H 10 + which do not show any significant difference. According to the ToF-SIMS data, it can be concluded that heating the film at 130 °C for 1 h does not generate any chemical compositional changes to the CB/SIBS film.

Flexibility and Washability Tests
To realize practical application, radiant heaters are expected to withstand repetitive bending and twisting conditions. The twisting test of CB/SIBS film (28%) was performed by simple hand-twisting. Figure 6a shows the digital photographs of the CB/SIBS films up to two twists with their corresponding IR images. Notably, the average temperature of the film increases from 100 °C in planar configuration to 106 °C with a single twist, which further increases to 119 °C when the film is twisted twice. The film returns to its original temperature when untwisted and brought back to its planar configuration. When the number of twists increases, a temperature increase can be seen. This temperature increment is not due to the internal material heat resistivity as no current change was observed. Instead, this increment may be due to the heat transformation on the external surface of the film due to contact. Likewise, Figure 5. Images of selected hydrocarbon species distribution and ToF-SIMS spectra in positive polarity of 28% CB/SIBS film a) before heating and b) after heating. the bending test was performed by folding the CB/SIBS films at various angles. The photographs and thermal images at bending angles of 90°, 60°, 45°, and 30° are shown in Figure 6b. The films achieve an average temperature of 130° irrespective of the bending angle, indicating good mechanical stability and reliability of the films during bending. In addition, the bending stability of 22% CB/SIBS film was measured through a bending test (Figure 6c). The bending speed was 300 mm min −1 , and 1000 repetitions were performed. The result shows that there is no difference in the thermal properties of the film before and after 1000 bending cycles, demonstrating that our device has predominant flexibility and does not maintain mechanical damage.
In some circumstances, the film may be required to be waterproof while performing its activity in real-world applications. For instance, these materials can be used as personal heating devices in extremely cold weather, thus avoiding humid conditions as a portable and wearable heater. Subsequently, in these critical occasions, clothes with both heating and waterproofing work can play a critical part in lessening casualties due to hypothermia. A washability test was performed by holding CB/SIBS films in deionized (DI) water for various intervals of time from 5 min to 24 h. Heating performance was tested during the intervals and presented in Figure 6c. Impressively, the CB/SIBS film managed to achieve an average temperature of around 130 °C after each regular interval of time. The heater maintained its high performance even after dipping and holding the films for 12 h in DI water, confirming their water-resistant behavior.
A practical demonstration of the CB/SIBS film heater is provided by heating a glass vial with water (see Video S2, Supporting Information). The water in the vial starts to vaporize and then condense on the interior of the vial within 9 min at an applied voltage of 12 V. We have also synthesized largescale CB/SIBS films (10 cm × 10 cm) and applied them to different interior parts of a car, such as a door, steering wheel, and dashboard, as shown in Figure 7. IR images reveal that the temperature distribution is homogeneous when the heater is applied to various curved surfaces, implying the practical feasibility of the CB/SIBS films.

Conclusion
In conclusion, we have successfully built a freestanding, ultraflexible, and washable joule heater using SIBS polymer scaffold and CB as heating material, which can withstand harsh bending (over 30°), twisting (two turns), and washing (soaked in water for over 12 h) conditions. By controlling the amount of CB and the thickness of the film, minimum resistivity and conductivity were achieved. Remarkably, the films showed excellent stability at various temperatures and for repeated ON/OFF cycles. ToF-SIMS suggested that the CB/SIBS films are extremely stable with no sign of degradation after being held at a temperature of 130 °C for more than 1 h. These exceptional results, combined with the high chemical stability and mechanical flexibility, suggest that CB/SIBS-based radiant heaters hold great promise for many practical applications, such as heating systems in electric vehicles and wearable electronics.

Experimental Section
Preparation of Stretchable Heating Films: CB was purchased from TIMCAL. SIBS (styrene 22 wt%) and toluene were purchased from Sigma Aldrich. Initially, the SIBS polymer ( Figure S1, Supporting Information) solution was made by dissolving SIBS pellets in toluene. This solution was stirred overnight at room temperature to obtain a homogenous mixture. Radiant heater materials were prepared by mixing different ratios of CB and SIBS polymer mixer. The obtained ink mixtures were 16% CB/84% SIBS, 22% CB/78% SIBS, and 28% CB/72% SIBS. The ink material was coated on a glass substrate using a thickness gauge. The thickness of the final films was 50 µm. The film thickness was optimized to a value where it exhibits freestanding and flexible properties with maximum conductivity values. The surface areas of thin films used for these measurements were 2 cm × 2 cm with a 50 µm film thickness.
Materials Characterization: Morphological features of thin films were observed using a field emission scanning electron microscope (FESEM, Tescan Mira). The structural and compositional information of the standard CB and CB/SIBS films were analyzed using energy-dispersive X-ray spectroscopy in an FEI Quanta 200 scanning electron microscope. Film heaters' carbon and oxygen compositions were determined using the energy-dispersive X-ray spectroscopy attachment of an FEI Quanta 200 scanning electron microscope. FTIR spectroscopy was conducted to analyze the bonding characteristics of film samples. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were collected by a Nicolet-5700 spectrometer. The FTIR spectra were recorded in the range between 400 and 500 cm −1 wavenumber at a 4 cm −1 resolution. The chemical structural phase of the thin films was analyzed using a Renishaw in Via Raman microscope. Raman mapping was also conducted to investigate differences before and after heating. Thermal property characterization of films was acquired using a Teslong TDP100 Infrared Visible Light Camera. Resistivity, sheet resistance, and conductivity were measured using KSR-4 Four-Point Probe System. Films were connected to a benchtop power supply (Powertech MP -3092, DC Power Supply -30 V 10 A Adjustable Dual Digital Variable) to give input voltage. The copper tape was used as a connector to the electrothermal heater device. IONTOF M6 instrument (IONTOF GmbH, Münster, Germany) equipped with a reflection time-of-flight analyzer and Bi/Mn primary-ion source was used to obtain ToF-SIMS spectra. Bi 3 + cluster ions were selected from the pulsed primary ion beam for the analysis and "bunched" to attain optimal mass resolution. The ion dose was limited to 10 −11 ions cm −2 , to stay below the static limit. Spectra were acquired in positive polarity, and the mass scale was calibrated using peaks attributed to hydrocarbon ions (C 2 H 3 + , C 3 H 3 + , C 4 H 3 + , C 5 H 3 + , C 6 H 3 + ). During data acquisition, the pressure in the analysis chamber was maintained at, or below, 1 × 10 −8 mbar. Emission spectra of CB/SIBS samples and graphite reference samples were collected using a Nicolet iS50 FT-IR spectrometer with an emission accessory and a graphite furnace attached with a temperature range of 100-1000 °C. Repetitive bending tests were performed using a Tytron 250 Microforce Testing System (MTS).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.