Characterization of a Graphene-Based Thermoelectric Generator ...

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ScienceDirect Energy Procedia 75 (2015) 615 – 620

The 7th International Conference on Applied Energy – ICAE2015

Characterization of a Graphene-Based Thermoelectric Generator using a Cost-Effective Fabrication Process Lama Mahmouda, Mohammad Alhwaraia, Yarjan Abdul Samada, Baker Mohammada, Kin Laioa, Ismail Elnaggara Khalifa University, Abu Dhabi, United Arab Emirates, Masdar Institute, Abu Dhabi, United Arab Emirates

Abstract Thermoelectric materials which can generate electricity from heat-waste ambient sources could play an important role to unleash the next shift in ultra-low power portable electronic applications. Thermal Energy (TE) harvesting can be characterized using Seebeck coefficient which is greatly dependent on the materials properties. In this paper, a prototype of a graphene based thermoelectric generator (TEG) is being fabricated using a facile and cost effective fabrication process. Further, Seebeck coefficient and surface resistance responses are experimentally measured for a varying number of graphene layers ranging mostly from 50 to 1000 layers. The results show that a Seebeck coefficient with an average of 90μV/K is generated. In addition, the surface resistance is 10.3 and 0.03 kΩ/cm at the 50 and 1000 layers, respectively. As such, if graphene films are closely stacked together, they could potentially be used to generate enough power for running small ultra-low power operations with minimum cost in a wide variety of applications for both research and industry. © by Elsevier Ltd. This an open Ltd. access article under the CC BY-NC-ND license © 2015 2015Published The Authors. Published by isElsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of Applied Energy Innovation Institute

Energy Harvesting, Thermoelectric Generator, Graphene, Mica, self-powered Applications

1. Introduction Conventionally, most of the portable devices operate using batteries. However, battery-powered devices are limited to their voltage capacity, costly and bulk due to the additional size and weight of the battery. In addition, replacing or recharging batteries is sometimes impractical especially in hard to reach areas such as implantable biomedical devices. As such, there is a vital need for alternative energy solutions. Electrical energy harvesting provides an alternative solution as it is referred to as the process of capturing energy that would otherwise be lost such as thermal, solar and mechanical energy and converting it into a useful electrical energy [1]. The harvested energy could be used to operate small ultralow power sensors such as those developed using MEMS technology [2]. Thus, energy harvesting holds the promise of replacing batteries with green, cost-effective and virtually infinite power for wireless ultralow power devices.

1876-6102 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi:10.1016/j.egypro.2015.07.466

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Figure 1: Structure of a commercial Bi2Te3 TEG [3]

The basic TE energy conversion unit known as Thermo-Electric Generator (TEG) [3]. The building block of a conventional TEG unit is known as a thermocouple which consists of two n-type and p-type materials connected together. Consequently, as depicted in Fig. 1, a typical TEG is in the form of a module constructed from a number of these thermocouples which are electrically connected in series and thermally connected in parallel. When a temperature gradient is applied between the top and bottom sides of the module, electrons (in the n-type material) and holes (in the p-type material) diffuse away from the hot side towards the cold side. This results in a potential difference across the material which can be used to generate power [5]. The thermoelectric figure is often described as ZT – a unitless combination of two properties of a material: electric conductivity (σ) and thermal conductivity (κ) as well as Seebeck coefficient (S) and the temperature difference (∆T) [5]: ZT =

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Eq. 1

Seebeck coefficient is defined as the amount of generated potential when a temperature gradient is applied to the sides of a material and it is described as S= V/∆T [3]. The temperature gradient (∆T) can be attained between the ambient temperature (air) and any heat-waste source such as a hot exhaust engine [6]. Towards applications side, many researchers have applied TE energy harvesting in various instruments to provide electrical power. For instance, Featherston [6-8] utilized a commercially available TEG module of 30×30×4.1 mm3 to generate a power for wireless sensor systems in aircraft. The results indicated that an average output power ranging from 6.6 to 22mW was achieved during flight. In this case, the energy harvesting TEG was applied for application in wireless computing and mobile devices. Moreover, Settaluri [9] proposed a wearable TE system consisting of a TEG module, heat sink and DCDC convertor to harvest heat-waste energy from human body. The results showed that the fabricated TEG has an output voltage of 4.15 V with overall thickness below 5 mm. Since σ and к are material dependent, this means that the efficiency of TE systems is directly related to the properties of the TE materials [10]. The rest of the paper is organized as follows. We first investigate the current challenges (section 2) followed by an elaboration on the proposed experimental procedure steps (section 3). Section 4 explains the achieved results. Finally, a summary and future work in section 5 concludes this paper.

2. Thermal harvesting challenges 2.1 Commercial TEG Devices Facile and cost-effective fabrication process, high TE power efficiency, flexibility, small size, and light weight are the interrelated desired parameters to assure the feasibility of future portable TE-powered electronic applications [11]. In reality, most of the existing TEG devices are designed to be both, small and light, but with poor power efficiency and high cost. For example, commercial Bismuth Telluride (Bi2Te3) based TEGs, most of which have been in use for decades, have a maximum TE value near 1


(typically 0.8 in bulk TEGs) [12]. In such traditional bulk TEGs, the achieved Seebeck coefficient is no more than 25mV/k for a 10 cm2 device [13], with a limited TE efficiency up to 16% [14]. Such low efficiency is attributed to the relatively high effective heat flowing through the TE material used to fabricate the TEG due to its low thermal resistivity [7]. Taking the cost factor into consideration, according to V. Leonov, the maximal number of thermopiles used in the modeling does not exceed 32 in most of the industrial applications [14]. Previous studies reported several fabrication processes such as chemical vapor deposition (CVD) [15], gas flow induced voltage [16] and mechanical exfoliations on SiO2 substrates [17]. These mechanisms are not always advantageous since they are tied with the relatively complicated and thus, high cost fabrication process. Recently, some researchers reported a state-of-the-art flexible Bi2Te3 based TEGs deposited on polydimethylsiloxane (PDMS) thick film fabricated with holes by using a poly(methyl methacrylate) (PMMA) mould via dispenser printing technique, but, with a limited temperature range owing to the undesirable degradation in the polymer [18].

2.2 Experimental Challenges Ultimately, most of the available TE designs are inherently difficult to integrate into high-energydensity power profiles due to the following reasons. Firstly, it is difficult to obtain a large thermal gradient between the faces of the recovery system without the integration of heat sinks. Secondly, rare, costly and difficult to integrate materials need to be used for applications at the ambient temperature range. Finally, most of the heat sources in our environment are non-planar forms (i.e. pipes) and the creation of flexible recovery systems would be of a great value [19].

3. Design of the graphene-based thermoelectric generator (G-TEG): system components Graphene is a two-dimensional (2D) modern-day promising material with remarkable properties such as high electrical conductivity, stability at high temperatures (above 3400K), elasticity, stiffness and biocompatibility [20]. Hence, such properties make it a universally suitable material for some applications such as the TEG and a potential candidate for the post-Silicon electronics era [21]. There are several methods for fabricating graphene and chemically modified graphene from different carbon materials including carbide compounds and graphitic derivatives. Preparation of chemically modified graphene is usually referred to as reduced graphene oxide (rGO) or simply graphene oxide (GO). GO prepared by colloidal suspensions is low cost, flexible, scalable, and adaptable to a wide variety of applications [22]. In this paper, we propose a facile and cost-effective four stages process of testing the Seebeck coefficient response for a varying number of graphene layers ranging from 50 to 1000 layers. The method is schematically illustrated in Fig. 2 with four major steps named as layering, depositing GO, deoxidization and finally testing. The following sections elaborate on the experimental procedure steps .

Figure 2: A schematic diagram showing the fabrication steps of the graphene-based TEG (top) and experimental set-up for measuring the Seebeck coefficient and surface resistance of the Graphene-Based TEG – the testing stage (bottom)

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3.1 Fabrication Methodology The desired number of graphene layers is controlled by adjusting the density and controlling the volume of the GO. If needed, density of GO is reduced by adding a calculated volume of DI water. Starting from the simple equation; ϕ (density) is the ratio of the mass to volume, one can calculate the number of graphene layers per unit volume deposited on a given surface area and GO density. Similar process is repeated for the different number of tested graphene layers. Consequently, graphene films are initially prepared using a 2mg/ml solution of GO. Since a single layer of graphene is as thin as 1 nm, GO is carefully poured on the surface of a 2 inch diameter and 0.3 mm thick mica substrates and left for approximately 30 hours in order to get dried [23]. Mica is a flexible insulating Silicate material used in variety of applications due to its unique low electrical conductivity, low thermal conductivity and mechanical properties [24]. Finally, once fully dried, GO is then reduced to graphene using a solution of Hydrogen Iodide (HI) acid for 15 minutes. Then, the graphene/mica films are washed in DI water until a pH close to 7 is obtained. The number of desired graphene layers was controlled by adjusting the density of the GO by adding a calculated volume of distilled water.

3.2 Testing: Measuring Seebeck Coefficient along with Sheet Resistance The aim of this stage is study the Seebeck coefficient across the graphene layers. Therefore, a temperature difference is gradually applied at the opposite far sides of the graphene/mica substrates and subsequently; the resultant harvested voltage difference is recorded for each value of ∆T along with the graphene sheet resistance (see Fig. 2). The generated voltage is plotted against the temperature difference for the various number of graphene layers which are deposited on mica substrates. In addition, the graphene/mica surface sheet resistances are also measured. In the testing stage, a commercial planner 4x4 cm Bi2Te3 based TEG started heating up when connected to a voltage supply [25]. Thus, it could be used as a heat source to the graphene-based TEG. The heated TEG is applied to one side of the graphene/mica substrates as shown in Fig. 2, the other side is left at 25̊ C; the ambient temperature. If the applied voltage values on the TEG are ranging from 0 to 6V, then the resultant ∆T values between the heated TEG and the ambient room temperature are between 0 to 10̊C, respectively. The temperature difference between the two sides of the graphene/mica films are measured using two LM35 temperature sensors [26]. Then, the two LM35 temperature sensors are connected to an Arduino board and an LCD screen. With the aid of the Arduino board, the temperature difference is measured, displayed on the LCD screen (not shown in Fig. 2) and averaged for every 10 measured values of ∆T in order to maximize accuracy level. The waiting time till the temperature difference gets stabilized is at least 8 minutes.

4. Results Atomically, a single layer of graphene does not look perfectly homogenous and smooth on the surface area of the mica substrate (see Fig. 3). Instead, it could be envisaged as distributed islands of carbons laying above the base substrate (i.e. mica in our case) [27]. As a result, a single layer of graphene is undesired for some TE applications due to the discontinuity of the carbon islands, and thus, the interruption of the induced voltage across the single layer of graphene itself. However, as the number of graphene layers increases, Seebeck coefficient reaches a peak value and then starts to decline due to the augmented continuity of the carbon islands. Moreover, as the number of graphene layers increases, the resistance is expected to drop due to the formation of a continuous path for the electric current to pass.Experimentally, as figure 4 shows, Seebeck coefficient is almost fixed to 90 μV/K for the tested 50, 100, 200, 300, 400, 500 and 1000 number of graphene layers. The achieved Seebeck coefficient using the present experiment set-up is significantly higher than the described gas-flow-induced voltage method described in [16].


Fig. 3. AFM image showing the roughness of a single graphene layer deposited on a mica substrate As depicated in Figure 4, the resistance of the graphene samples is inversely proportional to the number of graphene layers with values of 10.3 and 0.03 kΩ/cm at 50 and 1000 layers, respectively. This result agrees well with the available data in the literature.

Seebeck Coefficient

Surface Resistance Ω

Fig. 4. (color online) Seebeck coefficient (black line) and surface resistance (red line) versus number of graphene layers. Connecting line only shows data trend.

5. Conclusion To sum up, Seebeck coefficient of graphene layers ranging from 50 to 1000 layers deposited on mica films remains almost fixed to 90 μV/K. Unlike the Seebeck coefficient, surface resistance decreases as number of graphene layers increases due to the formation of continuous carbon atoms. At 400 layers onwards, a significant decay in resistance is observed with a value of 30 Ω/cm at most. Moreover, the fabrication process presented in this paper is facile, scalable and cost-effective. Thus, it could potentially be used for a wide variety of applications in science and engineering.

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