**3. Thermal Energy Harvesting**

#### **3.1. Introduction to thermogalvanic cells**

Studies on the conversion of heat to electricity have been conducted as early as the 1960s [99]. Since then, several thermal converters have been developed: thermocouples, thermion‐ ic converters, thermally recharged cells, thermogalvanic cells, etc. [100]. The discussion in the subsequent sections will be limited to thermogalvanic cells, also known as thermocells. These are electrochemical systems that are able to directly transduce thermal energy to elec‐ trical energy [101]. The simple design of these systems allows them to function without the need for moving components. Their stability allows operation for extended periods without regular maintenance. Thermocells also have zero carbon emission hence it will not contrib‐ ute to the environmental impact of electrical power generation.

**Figure 12.** Ferri/Ferro Cyanide redox thermogalvanic cell [108]Reproduced with permission from Springer Science

Ferrocyanide is oxidized at the hot anode, the electron generated then travels through an ex‐ ternal circuit and returns to the cell via the cold cathode where it is consumed in the reduc‐ tion of ferricyanide[109]. The accumulation of reaction products at either half-cell is prevented by the diffusion and convection of the electrolyte that occurs naturally, thus elim‐

inating the need for moving mechanical components.

occurs and the thermal power is given by:

ternal loads are equal and is given by:

*3.1.2. Desirable material properties for thermal conversion cells*

*Power conversion efficiency and how it is affected by electrode material properties*

The power conversion efficieny (Φ) of a thermocell is defined as follows:

**<sup>Φ</sup>**<sup>=</sup> **Electrical output power**

The thermal power flowing through the cell is largely controlled by cell design and electro‐ lyte selection. When a reversible redox couple is used, no net consumption of the electrolyte

Where K is the thermal conductivity of the electrolyte, A is the electrode cross sectional area,

Qualitative behaviour of the current and voltage dependencies are shown in Figure 13; it de‐ picts that the maximum electrical output power (Pmax) is obtained when the external and in‐

**Thermal power flowing through cell**=**KA** <sup>∆</sup> **<sup>T</sup>**

ΔT is the thermal gradient and d is the distance between the two electrodes [110].

**Thermal power flowing through the cell** (1)

**Pmax** = 0.25**VOCISC** (3)

**<sup>d</sup>** (2)

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 513

+Business Media

#### *3.1.1. Low grade heat sources and conversion through thermogalvanic cells*

#### *Various unharnessed low grade heat sources*

The second law of thermodynamics dictates that a heat engine can never have perfect effi‐ ciency and will always produce surplus heat (usually around 100 o C). This waste heat (or low grade heat) is one of the world's most ubiquitous sources of untapped energy. (i.e. waste heat is produced by simply turning on an automobile). Roughly 70 % of the energy generated by an automobile motor is wasted; part of it ends up as a hot exhaust pipe and warm brakes. The Wartsila-Sulzer RTA96-C turbocharged two-stroke diesel engine, one of the most efficient engines in the world, is only able to convert around 50 % of the energy in the fuel to useful motion. The rest of that energy gets dissipated as waste heat [102]. Waste heat also exists in factories, particularly in the steel and glass production industry. Pipes that carry hot liquids are also low grade heat sources. Other scenarios wherein heat simply dissi‐ pates into the environment are power plants, household appliances, and various electronic gadgets. Research done to convert waste heat into electrical power by the use of ferromag‐ netic materials, thermocouples and thermionic converters has resulted in low efficiencies [103-105]. Advances in thermoelectric systems have been hampered by its high initial cost and material limitations; as these systems operate in the temperature range much higher than low grade heat [106].

#### *Description of how a thermogalvanic cell works*

A thermogalvanic cell, also known as a thermocell, is a thermal energy converter that uti‐ lises electrochemical reactions to attain conversion of low grade heat to electrical power. The two half cells of the system are held at different temperatures causing a difference in redox potentials of the mediator at the anode and cathode [107]. This reaction can drive electrons through an external circuit that allows generation of current and power. A schematic of a thermocell with a ferri/ferrocyanide redox couple is shown in Figure 12.

**Figure 12.** Ferri/Ferro Cyanide redox thermogalvanic cell [108]Reproduced with permission from Springer Science +Business Media

Ferrocyanide is oxidized at the hot anode, the electron generated then travels through an ex‐ ternal circuit and returns to the cell via the cold cathode where it is consumed in the reduc‐ tion of ferricyanide[109]. The accumulation of reaction products at either half-cell is prevented by the diffusion and convection of the electrolyte that occurs naturally, thus elim‐ inating the need for moving mechanical components.

#### *3.1.2. Desirable material properties for thermal conversion cells*

**3. Thermal Energy Harvesting**

**3.1. Introduction to thermogalvanic cells**

512 Syntheses and Applications of Carbon Nanotubes and Their Composites

*Various unharnessed low grade heat sources*

than low grade heat [106].

*Description of how a thermogalvanic cell works*

ute to the environmental impact of electrical power generation.

*3.1.1. Low grade heat sources and conversion through thermogalvanic cells*

ciency and will always produce surplus heat (usually around 100 o

Studies on the conversion of heat to electricity have been conducted as early as the 1960s [99]. Since then, several thermal converters have been developed: thermocouples, thermion‐ ic converters, thermally recharged cells, thermogalvanic cells, etc. [100]. The discussion in the subsequent sections will be limited to thermogalvanic cells, also known as thermocells. These are electrochemical systems that are able to directly transduce thermal energy to elec‐ trical energy [101]. The simple design of these systems allows them to function without the need for moving components. Their stability allows operation for extended periods without regular maintenance. Thermocells also have zero carbon emission hence it will not contrib‐

The second law of thermodynamics dictates that a heat engine can never have perfect effi‐

low grade heat) is one of the world's most ubiquitous sources of untapped energy. (i.e. waste heat is produced by simply turning on an automobile). Roughly 70 % of the energy generated by an automobile motor is wasted; part of it ends up as a hot exhaust pipe and warm brakes. The Wartsila-Sulzer RTA96-C turbocharged two-stroke diesel engine, one of the most efficient engines in the world, is only able to convert around 50 % of the energy in the fuel to useful motion. The rest of that energy gets dissipated as waste heat [102]. Waste heat also exists in factories, particularly in the steel and glass production industry. Pipes that carry hot liquids are also low grade heat sources. Other scenarios wherein heat simply dissi‐ pates into the environment are power plants, household appliances, and various electronic gadgets. Research done to convert waste heat into electrical power by the use of ferromag‐ netic materials, thermocouples and thermionic converters has resulted in low efficiencies [103-105]. Advances in thermoelectric systems have been hampered by its high initial cost and material limitations; as these systems operate in the temperature range much higher

A thermogalvanic cell, also known as a thermocell, is a thermal energy converter that uti‐ lises electrochemical reactions to attain conversion of low grade heat to electrical power. The two half cells of the system are held at different temperatures causing a difference in redox potentials of the mediator at the anode and cathode [107]. This reaction can drive electrons through an external circuit that allows generation of current and power. A schematic of a

thermocell with a ferri/ferrocyanide redox couple is shown in Figure 12.

C). This waste heat (or

*Power conversion efficiency and how it is affected by electrode material properties*

The power conversion efficieny (Φ) of a thermocell is defined as follows:

$$\mathbf{Q} = \frac{\text{Electrical output power}}{\text{Thermal power flowing through the cell}} \tag{1}$$

The thermal power flowing through the cell is largely controlled by cell design and electro‐ lyte selection. When a reversible redox couple is used, no net consumption of the electrolyte occurs and the thermal power is given by:

$$\text{Thermal power flowing through cell} = \text{KA} \frac{\Delta \text{T}}{\text{d}} \tag{2}$$

Where K is the thermal conductivity of the electrolyte, A is the electrode cross sectional area, ΔT is the thermal gradient and d is the distance between the two electrodes [110].

Qualitative behaviour of the current and voltage dependencies are shown in Figure 13; it de‐ picts that the maximum electrical output power (Pmax) is obtained when the external and in‐ ternal loads are equal and is given by:

$$\mathbf{P\_{max}} = 0.25 \mathbf{V\_{OC}} \mathbf{I\_{SC}} \tag{3}$$

Where Voc is the open circuit voltage and Isc is the short circuit current. Voc is highly depend‐ ent on the reaction entropy of the redox couple and the thermal gradient at which the elec‐ trodes are exposed to as shown in Equation 4:

$$\mathbf{V}\_{\rm oc} = \frac{\Delta \mathbf{S}\_{\rm B,A} \Delta \mathbf{T}}{\mathbf{nF}} \tag{4}$$

where ΔSB,A is the reaction entropy for a hypothetical redox couple A↔ ne- B, n is the num‐ ber of reactions involved in the redox reaction and F is Faradays constant [100].

Combining Equation 2 and Equation 3 allows the power conversion efficiency to be ex‐ pressed as:

$$\mathbf{Q} = \frac{0.25 \mathbf{V}\_{\text{oc}} \mathbf{I}\_{\text{sc}}}{\mathbf{KA} \left[ \begin{array}{c} \text{s} \\ \text{s} \end{array} \right]} \tag{5}$$

**Figure 13.** The typical dependencies of the current (*I*) on the effective voltage (*U*), internal resistance (*r*) and the useful

*Power conversion efficiency achieved by using flat electrodes and why recent developments in CNTs*

The chemical stability of platinum led to its extensive investigation in thermogalvanic cells. In fact, a study on the effects of platinum electrode cleaning was performed and it was de‐ duced that this affects the power delivery characteristics of thermogalvanic cells [116].

Comparison of thermoelectric converters operating at different conditions (i.e. thermal gra‐ dient, electrolyte, electrode separation, etc.) can be done by measuring their power conver‐

If power inputs are ignored, such as mechanical stirring, thermocells with platinum electro‐ des are able to attain power conversion efficiency relative to a Carnot engine of 1.2%. How‐

As mentioned previously, the discovery of CNTs led to widespread research on this materi‐ al to investigate its potential uses, one of them being electrochemical applications [39, 117-120]. CNT electrodes are known to exhibit Nernstian behaviour and more importantly, fast electron transfer kinetics with the redox couple ferri/ferrocyanide. Peak potential sepa‐ ration in cyclic voltammograms obtained using micron-sized MWNT electrodes and 5 mM potassium ferrocyanide is 59 mV, which is the expected theoretical value and implies that

(6)

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 515

sion efficiency relative to a Carnot engine operating at the same temperature (Φr).

ever, if power inputs are strictly excluded, the efficiency drops to 0.5% [100].

**<sup>Φ</sup>r**<sup>=</sup> **<sup>Φ</sup>thermogalvanic cell operating at** <sup>∆</sup>**<sup>T</sup> ΦCarnot engine operating at** <sup>∆</sup>**<sup>T</sup>**

power on the external resistance (*rext*) [111]Reprinted with permission from Elsevier.

*can augment thermogalvanic cell performance*

*3.1.3. CNTs vs flat electrodes - why CNTs can improve thermal harvesting*

Ohmic, mass transport and activation overpotentials are losses that need to be minimised in order to realise an improvement in thermocell conversion efficiency. At large electrode sepa‐ rations, ohmic overpotential is dictated by the electrolyte resistance; and mass transport overpotential is maximized. By decreasing the inter-electrode separation, an increase in gen‐ erated power will be observed as both ohmic and mass transport overpotentials will de‐ crease. However, the power conversion efficiency will be lowered as it will be harder to maintain the thermal gradient in the cell [112]. It has been shown that changes in electrolyte concentration affect its thermal conductivity [108]. Optimization of electrolyte concentration coupled with appropriate cell design is necessary to mitigate both overpotentials while maintaining large power conversion efficiency.

Activation overpotential is associated with the activation barrier needed to transfer an elec‐ trode to an analyte. For the same activation overpotential, larger current densities are real‐ ised when the exchange current density is increased. This increase is attained when the concentration of the redox couple in the electrolyte is maximised, the thermal gradient is in‐ creased and the number of possible reaction sites is augmented [113]. Porous electrodes have the advantage of increased electroactive surface area and will directly amplify the short circuit current density [114]. It must be noted that for porous electrodes, short circuit current density does not increase indefinitely with electrode thickness as mass transfer over‐ potential will become limiting. The reaction products formed within the pores of the anode will not be able to diffuse fast enough to the cathode and vice versa, generating concentra‐ tion gradients around both electrodes. Another way to decrease the activation overpotential in thermocells is by using catalytic electrodes attained by doping [115].

**Figure 13.** The typical dependencies of the current (*I*) on the effective voltage (*U*), internal resistance (*r*) and the useful power on the external resistance (*rext*) [111]Reprinted with permission from Elsevier.

#### *3.1.3. CNTs vs flat electrodes - why CNTs can improve thermal harvesting*

Where Voc is the open circuit voltage and Isc is the short circuit current. Voc is highly depend‐ ent on the reaction entropy of the redox couple and the thermal gradient at which the elec‐

Combining Equation 2 and Equation 3 allows the power conversion efficiency to be ex‐

Ohmic, mass transport and activation overpotentials are losses that need to be minimised in order to realise an improvement in thermocell conversion efficiency. At large electrode sepa‐ rations, ohmic overpotential is dictated by the electrolyte resistance; and mass transport overpotential is maximized. By decreasing the inter-electrode separation, an increase in gen‐ erated power will be observed as both ohmic and mass transport overpotentials will de‐ crease. However, the power conversion efficiency will be lowered as it will be harder to maintain the thermal gradient in the cell [112]. It has been shown that changes in electrolyte concentration affect its thermal conductivity [108]. Optimization of electrolyte concentration coupled with appropriate cell design is necessary to mitigate both overpotentials while

Activation overpotential is associated with the activation barrier needed to transfer an elec‐ trode to an analyte. For the same activation overpotential, larger current densities are real‐ ised when the exchange current density is increased. This increase is attained when the concentration of the redox couple in the electrolyte is maximised, the thermal gradient is in‐ creased and the number of possible reaction sites is augmented [113]. Porous electrodes have the advantage of increased electroactive surface area and will directly amplify the short circuit current density [114]. It must be noted that for porous electrodes, short circuit current density does not increase indefinitely with electrode thickness as mass transfer over‐ potential will become limiting. The reaction products formed within the pores of the anode will not be able to diffuse fast enough to the cathode and vice versa, generating concentra‐ tion gradients around both electrodes. Another way to decrease the activation overpotential

in thermocells is by using catalytic electrodes attained by doping [115].

**nF** (4)

B, n is the num‐

(5)

**Voc**<sup>=</sup> <sup>∆</sup> **SB**,**<sup>A</sup>** <sup>∆</sup> **<sup>T</sup>**

**<sup>Φ</sup>**<sup>=</sup> 0.25**VocIsc KA** <sup>∆</sup> **<sup>T</sup> d**

where ΔSB,A is the reaction entropy for a hypothetical redox couple A↔ ne-

ber of reactions involved in the redox reaction and F is Faradays constant [100].

trodes are exposed to as shown in Equation 4:

514 Syntheses and Applications of Carbon Nanotubes and Their Composites

maintaining large power conversion efficiency.

pressed as:

*Power conversion efficiency achieved by using flat electrodes and why recent developments in CNTs can augment thermogalvanic cell performance*

The chemical stability of platinum led to its extensive investigation in thermogalvanic cells. In fact, a study on the effects of platinum electrode cleaning was performed and it was de‐ duced that this affects the power delivery characteristics of thermogalvanic cells [116].

Comparison of thermoelectric converters operating at different conditions (i.e. thermal gra‐ dient, electrolyte, electrode separation, etc.) can be done by measuring their power conver‐ sion efficiency relative to a Carnot engine operating at the same temperature (Φr).

$$\mathbf{OP}\_{\mathbf{r}} = \frac{\boldsymbol{\Phi}\_{\text{thermal-angular cell operating at } \Delta \mathbf{T}}}{\boldsymbol{\Phi}\_{\text{Carnot engine operating at } \Delta \mathbf{T}}} \tag{6}$$

If power inputs are ignored, such as mechanical stirring, thermocells with platinum electro‐ des are able to attain power conversion efficiency relative to a Carnot engine of 1.2%. How‐ ever, if power inputs are strictly excluded, the efficiency drops to 0.5% [100].

As mentioned previously, the discovery of CNTs led to widespread research on this materi‐ al to investigate its potential uses, one of them being electrochemical applications [39, 117-120]. CNT electrodes are known to exhibit Nernstian behaviour and more importantly, fast electron transfer kinetics with the redox couple ferri/ferrocyanide. Peak potential sepa‐ ration in cyclic voltammograms obtained using micron-sized MWNT electrodes and 5 mM potassium ferrocyanide is 59 mV, which is the expected theoretical value and implies that the highest electron transfer rate was attained [121]. Incidentally, the ferri/ferrocyanide re‐ dox couple has been studied intensively in thermocell applications owing to the large volt‐ age that can be induced by a thermal gradient, also known as the Seebeck coefficient. The Seebeck coefficient 1.4 mV/K for the ferri/ferro cyanide redox couple implies that an open circuit potential of 84 mV is attainable at a thermal gradient of 60 o C (the usual limit for aqueous systems without significant cooling). The fast electron transfer of CNTs in ferri/ ferro cyanide primarily justifies its use as electrodes in thermocells.

power density improved by 32 %, generating 6.8 W/kg. Using the same test conditions, com‐ mercially available purified MWNT having 3-6 walls with a median diameter of 6.6 nm (SMW100, Southwest Nanotechnologies, Inc) and approximately 98 wt. % carbon yielded a specific power density of 6.13 W/kg. It must be noted that these tests were not done to maxi‐ mise the power generation capability of the thermocell but to gain further insight into CNT electrodes for thermal harvesting. Hence the small electrode separation, low electrolyte con‐

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 517

Electrical impedance spectroscopy (EIS) of the various carbon nanomaterials tested by Kang et al revealed that the P-SWNT electrode has a marginally lesser ohmicoverpotential (21 Ω) than the pristine SWNT (22 Ω). This finding explains the increased specific power density

**Figure 15.** Vertical "hot above cold" thermocell[113]Reprinted with permission from John Wiley & Sons, Inc.

It has been proven that the catalytic nature of MWNTs is due to the edges or sites where the tube terminates, regions that are more numerous in MWNTs than in SWNTs [124]. Due to this and the fact that P-MWNTs had lower ohmic resistance (18 Ω) compared to P-SWNTs, it

centration and small thermal gradient.

**Figure 14.** Schematic of the U-Cell used by Baughman et. al for thermal harvesting

generated when the P-SWNT electrodes are used.

The nanometre diameter of CNTs gives rise to large gravimetric and volumetric specific sur‐ face areas (SSA). Their unique aspect ratios allow porous electrodes to be fabricated by a va‐ riety of methods. Theoretically the SSA of CNTs can range from 50-1315 m2 /g, the value dictated by the number of walls [74]. Theoretical predictions are in good agreement with ex‐ perimental values obtained by the measurement of amount of gas (usually N2) adsorbed at 77 K and calculations using the Brunauer-Emmett-Teller (BET) isotherm. Kaneko et al. [122] have reported that MWNTs are mesoporous while Rao et al. [123] have shown that SWNTs are microporous. MWNT buckypapers of the same geometric area compared with platinum foil are known to have three times larger charging current density during cyclic voltamme‐ try in ferri/ferrocyanide aqueous electrolyte [114], evidence of the large accessible SSA of CNT electrodes.The large SSA of CNTs allows for a greater number of electroactive sites. When the CNT electrode porosity is controlled and the tortuousity is minimised in thermo‐ galvanic cells (so that mass transfer is not limited within the electrode), the short circuit cur‐ rent generated can be significantly augmented.

#### **3.2. Different types of CNTs investigated**

#### *3.2.1. SWNT and MWNT*

CNTs were first used as thermocell electrodes in 2009 [114]. Baughman et al. tested 0.5 cm2 MWNT buckypaper electrodes (with less than 1 % catalyst and with MWNT diameter of around 10 nm) in a U-Cell with electrode separation of 5 cm, a temperature gradient of 60 o C wherein Tcold= 5 o C. A schematic of the cell they used is shown in Figure 14. A specific power density of 1.36 W/m2 was obtained [114]. Platinum electrodes tested under the same condi‐ tions generated a specific power density of 1.02 W/m2 , proving that CNTs are viable materi‐ als for thermocell electrodes.

SWNT powders produced by arc discharge (ASA-100F, Hanwha Nanotech) with an average diameter of 1.3 nm, and composition of 20-30 wt. % CNTs, 40 wt. % carbon nanoparticles, 20 wt. % catalyst material, 10 wt% amorphous carbon and graphite, was tested by Kang et al. in thermal harvesting [113]. A vertical test cell with a "hot above cold" orientation (Figure 15), glass frit separator and electrode separation of 4 cm was employed with Thot= 46.4 o C and Tcold = 26.4 o C. SWNT electrodes with an area of 0.25 cm2 were immersed in 0.2M K3Fe(CN)6/K4Fe(CN)6 electrolyte. The specific power density obtained was around 5.15 W/kg. Commercially available purified SWNT powders (P-SWNT) sourced from Hanwha Nanotech (ASP-100F), refined by thermal and acid treatment (60-70 wt. % nanotubes, 10 wt. % catalyst material, 20 wt. % graphite impurities) was tested by the same group. The specific power density improved by 32 %, generating 6.8 W/kg. Using the same test conditions, com‐ mercially available purified MWNT having 3-6 walls with a median diameter of 6.6 nm (SMW100, Southwest Nanotechnologies, Inc) and approximately 98 wt. % carbon yielded a specific power density of 6.13 W/kg. It must be noted that these tests were not done to maxi‐ mise the power generation capability of the thermocell but to gain further insight into CNT electrodes for thermal harvesting. Hence the small electrode separation, low electrolyte con‐ centration and small thermal gradient.

**Figure 14.** Schematic of the U-Cell used by Baughman et. al for thermal harvesting

the highest electron transfer rate was attained [121]. Incidentally, the ferri/ferrocyanide re‐ dox couple has been studied intensively in thermocell applications owing to the large volt‐ age that can be induced by a thermal gradient, also known as the Seebeck coefficient. The Seebeck coefficient 1.4 mV/K for the ferri/ferro cyanide redox couple implies that an open

aqueous systems without significant cooling). The fast electron transfer of CNTs in ferri/

The nanometre diameter of CNTs gives rise to large gravimetric and volumetric specific sur‐ face areas (SSA). Their unique aspect ratios allow porous electrodes to be fabricated by a va‐

dictated by the number of walls [74]. Theoretical predictions are in good agreement with ex‐ perimental values obtained by the measurement of amount of gas (usually N2) adsorbed at 77 K and calculations using the Brunauer-Emmett-Teller (BET) isotherm. Kaneko et al. [122] have reported that MWNTs are mesoporous while Rao et al. [123] have shown that SWNTs are microporous. MWNT buckypapers of the same geometric area compared with platinum foil are known to have three times larger charging current density during cyclic voltamme‐ try in ferri/ferrocyanide aqueous electrolyte [114], evidence of the large accessible SSA of CNT electrodes.The large SSA of CNTs allows for a greater number of electroactive sites. When the CNT electrode porosity is controlled and the tortuousity is minimised in thermo‐ galvanic cells (so that mass transfer is not limited within the electrode), the short circuit cur‐

CNTs were first used as thermocell electrodes in 2009 [114]. Baughman et al. tested 0.5 cm2 MWNT buckypaper electrodes (with less than 1 % catalyst and with MWNT diameter of around 10 nm) in a U-Cell with electrode separation of 5 cm, a temperature gradient of 60 o

SWNT powders produced by arc discharge (ASA-100F, Hanwha Nanotech) with an average diameter of 1.3 nm, and composition of 20-30 wt. % CNTs, 40 wt. % carbon nanoparticles, 20 wt. % catalyst material, 10 wt% amorphous carbon and graphite, was tested by Kang et al. in thermal harvesting [113]. A vertical test cell with a "hot above cold" orientation (Figure 15), glass frit separator and electrode separation of 4 cm was employed with Thot= 46.4 o

K3Fe(CN)6/K4Fe(CN)6 electrolyte. The specific power density obtained was around 5.15 W/kg. Commercially available purified SWNT powders (P-SWNT) sourced from Hanwha Nanotech (ASP-100F), refined by thermal and acid treatment (60-70 wt. % nanotubes, 10 wt. % catalyst material, 20 wt. % graphite impurities) was tested by the same group. The specific

C. A schematic of the cell they used is shown in Figure 14. A specific power

C. SWNT electrodes with an area of 0.25 cm2 were immersed in 0.2M

was obtained [114]. Platinum electrodes tested under the same condi‐

, proving that CNTs are viable materi‐

riety of methods. Theoretically the SSA of CNTs can range from 50-1315 m2

C (the usual limit for

/g, the value

C

C and

circuit potential of 84 mV is attainable at a thermal gradient of 60 o

ferro cyanide primarily justifies its use as electrodes in thermocells.

516 Syntheses and Applications of Carbon Nanotubes and Their Composites

rent generated can be significantly augmented.

tions generated a specific power density of 1.02 W/m2

**3.2. Different types of CNTs investigated**

*3.2.1. SWNT and MWNT*

wherein Tcold= 5 o

Tcold = 26.4 o

density of 1.36 W/m2

als for thermocell electrodes.

Electrical impedance spectroscopy (EIS) of the various carbon nanomaterials tested by Kang et al revealed that the P-SWNT electrode has a marginally lesser ohmicoverpotential (21 Ω) than the pristine SWNT (22 Ω). This finding explains the increased specific power density generated when the P-SWNT electrodes are used.

**Figure 15.** Vertical "hot above cold" thermocell[113]Reprinted with permission from John Wiley & Sons, Inc.

It has been proven that the catalytic nature of MWNTs is due to the edges or sites where the tube terminates, regions that are more numerous in MWNTs than in SWNTs [124]. Due to this and the fact that P-MWNTs had lower ohmic resistance (18 Ω) compared to P-SWNTs, it was expected that P-MWNTs would perform better in thermal harvesting. The authors at‐ tributed the enhanced performance of P-SWNT to the larger specific surface area, which compensated for the decreased electroactive sites and higher ohmic resistance.

#### *3.2.2. Functionalized CNTs*

Functionalising or doping (using Nitrogen or Boron atoms) may be used to fine tune the physical and chemical properties of CNTs [125-127]. With advances in technology, CNT functionalization is a reasonably simple process [128]. Dai et. al have shown that nitrogendoped carbon nanotubes (NCNTs) have high electrocatalytic activity in oxygen reduction reactions as compared to undoped CNTs [129]. The increase in performance was brought about by a four electron pathway for oxygen reduction reactions that was attained by aligning the nanotubes and integrating nitrogen into the carbon lattice. The additional electrons contribut‐ ed by nitrogen atoms can enhance electronic conductivity by providing electron carriers for the conduction band [130]. The many active defects and hydrophilic properties of NCNTs allow for enhanced electrolyte interaction in aqueous solutions [131]. Boron doped CNTs (BCNTs) are also attractive for electrochemical applications owing to the increased edge plane sites on the CNT surface; proven to be the predominant region for electron transfer [132]. Examples of the electrocatalytic performance of BCNTs are the improved detection of Lcysteine, enhanced electroanalysis of NADH and enhancement of field emission [126, 127,133].

**Figure 16.** a) Thermal energy conversion response of various electrodes used by Cola et. Al b.Cyclicvoltammograms of various electrodes in 0.1M K3Fe(CN)6/K4Fe(CN)6 at 100 mV/s vs Ag/AgCl reference electrode [115]. Reproduced with

Using BCNT and NCNT in an asymmetric configurations resulted in increased currents at small thermal gradients as compared to the symmetric arrangements. This current then de‐ creased non-linearly as the temperature difference was increased. At small thermal gradi‐ ents, with the BCNT at the cold side of the cell, the slower kinetics induced an accumulation of reactants at its surface. The faster kinetics at the NCNT, brought about by the increased temperature, kept the electrolyte concentration in its vicinity low. Both factors allow the re‐ dox reactions to occur rapidly until a threshold thermal gradient is reached. At this point, the ion concentration in the vicinity of the NCNT is sufficiently large to slow the kinetics and reduce the currents generated. The threshold temperature gradient is attained at lower temperatures when the NCNT is kept at the cold side of the cell because the slower kinetics at this temperature promotes accumulation of K+ ions on the NCNT surface and leads to the

The recent discovery of graphene through micromechanical exfoliation has sparked a flur‐ ry of research into its possible applications [135]. Graphene consists of a single layer of carbon atoms bonded in a hexagonal lattice. Like CNTs, its remarkable properties (charge

al candidate for electrochemical applications [136, 137]. In order to scale up graphene pro‐ duction, graphite is normally exfoliated in the liquid state through surfactant/solvent stablilization [138] or chemical conversion resulting in a graphene like structure known as reduced graphene oxide (RGO), shown in Figure 17 [139]. Being of the same composition as CNTs, investigation of the possibility of synthesizing composites of these two carbon materials and exploring their performance as electrode materials has been done by sever‐

/V-s and specific surface area of 2630 m2

/g) make it an ide‐

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 519

permission from J. Electrochem. Soc.

"blocking" effect discussed previously.

carrier mobility of 200000 cm2

al research groups [83, 91, 140].

*3.2.3. Composites*

The possibility of using nitrogen-doped CNT and boron-doped CNT electrodes in thermo‐ cells was investigated by Cola et.al [115]. Doping was attained by using a plasma-enhanced chemical vapour deposition process. Tests were run using a U-cell configuration (Figure 14), Tcold = 20 o C, thermal gradient of up to 40 o C, and electrolyte concentration of 0.1 M potassi‐ um ferri/ferrocyanide. The electrodes were sized to 0.178 cm2 and were set up in a symmet‐ ric and asymmetric (N(hot)-B(cold) and B(hot)-N(cold)) fashion.

Results (Figure 16a) indicate inferior thermocell performance for both NCNT and BCNT as compared to Pt and pristine CNTs. The poor performance of the doped CNTs was brought about by the sluggish kinetics, evidenced by the large peak separations in the cyclic voltam‐ mograms taken at a scan rate of 100 mV/s (Figure 16b). It was theorized that the slow-mov‐ ing kinetics for the doped electrodes was caused by the electrostatic effects at the electrodeelectrolyte interface [134]. The positively charged BCNTs repulsed the similarly charged potassium (K+ ) counter ion which decreased the electrolyte concentration in the vicinity of the electrode. The negatively charged NCNTs led to a strong electrostatic attraction with K+ , an effect which at low concentrations can improve electron transfer kinetics. However, the large bulk concentration (needed to achieve significant short circuit currents in thermocells) led to a high density of the K+ ions in the vicinity of the electrode. This effectively acted as a barrier to the redox reactions that were supposed to occur at the electrode.

**Figure 16.** a) Thermal energy conversion response of various electrodes used by Cola et. Al b.Cyclicvoltammograms of various electrodes in 0.1M K3Fe(CN)6/K4Fe(CN)6 at 100 mV/s vs Ag/AgCl reference electrode [115]. Reproduced with permission from J. Electrochem. Soc.

Using BCNT and NCNT in an asymmetric configurations resulted in increased currents at small thermal gradients as compared to the symmetric arrangements. This current then de‐ creased non-linearly as the temperature difference was increased. At small thermal gradi‐ ents, with the BCNT at the cold side of the cell, the slower kinetics induced an accumulation of reactants at its surface. The faster kinetics at the NCNT, brought about by the increased temperature, kept the electrolyte concentration in its vicinity low. Both factors allow the re‐ dox reactions to occur rapidly until a threshold thermal gradient is reached. At this point, the ion concentration in the vicinity of the NCNT is sufficiently large to slow the kinetics and reduce the currents generated. The threshold temperature gradient is attained at lower temperatures when the NCNT is kept at the cold side of the cell because the slower kinetics at this temperature promotes accumulation of K+ ions on the NCNT surface and leads to the "blocking" effect discussed previously.

#### *3.2.3. Composites*

was expected that P-MWNTs would perform better in thermal harvesting. The authors at‐ tributed the enhanced performance of P-SWNT to the larger specific surface area, which

Functionalising or doping (using Nitrogen or Boron atoms) may be used to fine tune the physical and chemical properties of CNTs [125-127]. With advances in technology, CNT functionalization is a reasonably simple process [128]. Dai et. al have shown that nitrogendoped carbon nanotubes (NCNTs) have high electrocatalytic activity in oxygen reduction reactions as compared to undoped CNTs [129]. The increase in performance was brought about by a four electron pathway for oxygen reduction reactions that was attained by aligning the nanotubes and integrating nitrogen into the carbon lattice. The additional electrons contribut‐ ed by nitrogen atoms can enhance electronic conductivity by providing electron carriers for the conduction band [130]. The many active defects and hydrophilic properties of NCNTs allow for enhanced electrolyte interaction in aqueous solutions [131]. Boron doped CNTs (BCNTs) are also attractive for electrochemical applications owing to the increased edge plane sites on the CNT surface; proven to be the predominant region for electron transfer [132]. Examples of the electrocatalytic performance of BCNTs are the improved detection of Lcysteine, enhanced electroanalysis of NADH and enhancement of field emission [126, 127,133].

The possibility of using nitrogen-doped CNT and boron-doped CNT electrodes in thermo‐ cells was investigated by Cola et.al [115]. Doping was attained by using a plasma-enhanced chemical vapour deposition process. Tests were run using a U-cell configuration (Figure 14),

Results (Figure 16a) indicate inferior thermocell performance for both NCNT and BCNT as compared to Pt and pristine CNTs. The poor performance of the doped CNTs was brought about by the sluggish kinetics, evidenced by the large peak separations in the cyclic voltam‐ mograms taken at a scan rate of 100 mV/s (Figure 16b). It was theorized that the slow-mov‐ ing kinetics for the doped electrodes was caused by the electrostatic effects at the electrodeelectrolyte interface [134]. The positively charged BCNTs repulsed the similarly charged

the electrode. The negatively charged NCNTs led to a strong electrostatic attraction with K+

an effect which at low concentrations can improve electron transfer kinetics. However, the large bulk concentration (needed to achieve significant short circuit currents in thermocells) led to a high density of the K+ ions in the vicinity of the electrode. This effectively acted as a

barrier to the redox reactions that were supposed to occur at the electrode.

) counter ion which decreased the electrolyte concentration in the vicinity of

C, and electrolyte concentration of 0.1 M potassi‐

and were set up in a symmet‐

,

compensated for the decreased electroactive sites and higher ohmic resistance.

518 Syntheses and Applications of Carbon Nanotubes and Their Composites

*3.2.2. Functionalized CNTs*

Tcold = 20 o

potassium (K+

C, thermal gradient of up to 40 o

um ferri/ferrocyanide. The electrodes were sized to 0.178 cm2

ric and asymmetric (N(hot)-B(cold) and B(hot)-N(cold)) fashion.

The recent discovery of graphene through micromechanical exfoliation has sparked a flur‐ ry of research into its possible applications [135]. Graphene consists of a single layer of carbon atoms bonded in a hexagonal lattice. Like CNTs, its remarkable properties (charge carrier mobility of 200000 cm2 /V-s and specific surface area of 2630 m2 /g) make it an ide‐ al candidate for electrochemical applications [136, 137]. In order to scale up graphene pro‐ duction, graphite is normally exfoliated in the liquid state through surfactant/solvent stablilization [138] or chemical conversion resulting in a graphene like structure known as reduced graphene oxide (RGO), shown in Figure 17 [139]. Being of the same composition as CNTs, investigation of the possibility of synthesizing composites of these two carbon materials and exploring their performance as electrode materials has been done by sever‐ al research groups [83, 91, 140].

**Figure 18.** SEM images of reduced graphene oxide-SWNT composites

*Solvent/surfactant exfoliation*

solvent volume (Δ *H*

dispersion.


*mix*) [138]:

∆*H* -

bility theory states that a negative free energy of mixing ( ∆*G*

∆*G* -

*mix* <sup>=</sup> <sup>2</sup>

Where *Rbun* is the radius of the dispersed nanotube bundles, *δNT* and *δsol*

*3.3.1. Current processing techniques for CNTs relevant to thermocells*

results also becomes an issue when CNTs are not dispersed adequately.

**3.3. Developments in processing and fabrication of CNTs for better cell design**

One of the major obstacles to research on CNT characterisation and application is their spontaneous aggregation brought about by attractive van der Waals interactions in both aqueous and organic solutions. The resulting aggregates or "bundles" can reach lengths of several microns and diameters of tens of nanometers. Debundling of these aggregates is es‐ sential as they have mediocre properties as compared to individual tubes. Reproducibility of

Liquid phase separation is one of the simplest methods wherein stable CNT dispersions are attainable. Stable well-exfoliated CNT dispersions is achieved by appropriate selection of the solvent as forced dispersion via ultrasonication will result in agglomeration of the CNTs in a very short span of time. Selection of solvents can be based on the enthalpy of mixing per

the nanotube and solvent surface energies and *∅* is the nanotube volume fraction. The solu‐


*mix* = ∆*Hmix* - *T* ∆*S*

*Rbun* (*δNT* - *<sup>δ</sup>sol*)2*<sup>∅</sup>* (7)


are square roots of

*mix* ) is indicative of a stable

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 521

*mix* (8)

**Figure 17.** SEM image of the cross section of a reduced graphene oxide film

Kang et al. have shown that when composites composed of 1:1 weight RGO and P-SWNT are used in thermocells, the specific power generated (5.3 W/kg) is comparable to that of SWNTs (the experimental conditions used were discussed in section 3.2.1) [113]. It must be noted that when RGO alone is used, the specific power generated is 3.87 W/kg. However, the composite electrode produces a specific power that is only 78 % of the P-SWNT. This decrease in performance can be attributed to the large ohmic resistance observed in the RGO electrode (35.6Ω) that is 55 % higher than the P-SWNT electrode. The chemical conversion of graphite to RGO involves oxidising graphite, exfoliation and then a subsequent reduction. It is surmised that the incomplete removal of the oxygen containing functional groups is the cause of the pronounced ohmic resistance. Another reason for the poor performance of the composite electrodes is the restacking of the RGO sheets during electrode preparation, which impedes electrolyte diffusion and results in sluggish kinetics.

Optimisation of the RGO-SWNT composition for thermocell electrodes was done by Chen et. al [141]. The amount of RGO added ranged from 1 to 20 % by weight. A U-cell was used with 0.75 cm2 electrodes separated by 10 cm, a thermal gradient of 60 o C with Tcold= 20 o C and 0.4 M ferri/ferrocyanide electrolyte. The optimised composite 99 % SWNT-1 % RGO generated a specific current density of 26.78 W/kg. By using large amounts of SWNTs the RGO sheets were prevented from restacking, which resulted in the appropriate nanoporosi‐ ty that promoted redox mediator diffusion. The sheet like structure of the RGO provided in‐ creased pathways for electrons in the composite thus contributing to its enhanced performance. The interaction between SWNTs and RGO is clearly seen in Figure 18.

**Figure 18.** SEM images of reduced graphene oxide-SWNT composites

#### **3.3. Developments in processing and fabrication of CNTs for better cell design**

#### *3.3.1. Current processing techniques for CNTs relevant to thermocells*

#### *Solvent/surfactant exfoliation*

**Figure 17.** SEM image of the cross section of a reduced graphene oxide film

520 Syntheses and Applications of Carbon Nanotubes and Their Composites

which impedes electrolyte diffusion and results in sluggish kinetics.

with 0.75 cm2

Kang et al. have shown that when composites composed of 1:1 weight RGO and P-SWNT are used in thermocells, the specific power generated (5.3 W/kg) is comparable to that of SWNTs (the experimental conditions used were discussed in section 3.2.1) [113]. It must be noted that when RGO alone is used, the specific power generated is 3.87 W/kg. However, the composite electrode produces a specific power that is only 78 % of the P-SWNT. This decrease in performance can be attributed to the large ohmic resistance observed in the RGO electrode (35.6Ω) that is 55 % higher than the P-SWNT electrode. The chemical conversion of graphite to RGO involves oxidising graphite, exfoliation and then a subsequent reduction. It is surmised that the incomplete removal of the oxygen containing functional groups is the cause of the pronounced ohmic resistance. Another reason for the poor performance of the composite electrodes is the restacking of the RGO sheets during electrode preparation,

Optimisation of the RGO-SWNT composition for thermocell electrodes was done by Chen et. al [141]. The amount of RGO added ranged from 1 to 20 % by weight. A U-cell was used

and 0.4 M ferri/ferrocyanide electrolyte. The optimised composite 99 % SWNT-1 % RGO generated a specific current density of 26.78 W/kg. By using large amounts of SWNTs the RGO sheets were prevented from restacking, which resulted in the appropriate nanoporosi‐ ty that promoted redox mediator diffusion. The sheet like structure of the RGO provided in‐ creased pathways for electrons in the composite thus contributing to its enhanced

C with Tcold= 20 o

C

electrodes separated by 10 cm, a thermal gradient of 60 o

performance. The interaction between SWNTs and RGO is clearly seen in Figure 18.

One of the major obstacles to research on CNT characterisation and application is their spontaneous aggregation brought about by attractive van der Waals interactions in both aqueous and organic solutions. The resulting aggregates or "bundles" can reach lengths of several microns and diameters of tens of nanometers. Debundling of these aggregates is es‐ sential as they have mediocre properties as compared to individual tubes. Reproducibility of results also becomes an issue when CNTs are not dispersed adequately.

Liquid phase separation is one of the simplest methods wherein stable CNT dispersions are attainable. Stable well-exfoliated CNT dispersions is achieved by appropriate selection of the solvent as forced dispersion via ultrasonication will result in agglomeration of the CNTs in a very short span of time. Selection of solvents can be based on the enthalpy of mixing per

solvent volume (Δ *H mix*) [138]:

$$
\Delta H\_{\text{mix}} = \frac{2}{R\_{\text{bw}}} (\delta\_{\text{NT}} - \delta\_{\text{sol}})^2 \mathcal{Q} \tag{7}
$$

Where *Rbun* is the radius of the dispersed nanotube bundles, *δNT* and *δsol* are square roots of the nanotube and solvent surface energies and *∅* is the nanotube volume fraction. The solu‐

bility theory states that a negative free energy of mixing ( ∆*G mix* ) is indicative of a stable dispersion.

$$
\stackrel{\cdot}{\Delta G\_{mix}} = \Delta H\_{mix} - T \stackrel{\cdot}{\Delta S\_{mix}} \tag{8}
$$

The entropy of mixing per unit volume ( ∆*S mix* ) of nanotubes is generally small owing to inducing coordinated growth by holding the tubes together [151]. The ability to tailor the growth of CNTs in three dimensional configurations is highly advantageous in thermocell applications. This configuration promotes enhanced ion accessibility with the CNT matrix allowing larger current to be generated. The alignment of the tubes also minimises the tor‐ tuosity of the CNT electrodes which decreases the probability of forming concentration gra‐ dients within the electrode itself, leading to a decrease in the mass transfer overpotential. The wide range of substrates that can be used for CNT synthesis (metallic, carbon, etc) via CVD allows the fabrication of electrodes with a high degree of flexibility; materials that are

The development of flexible electrodes for electrochemical applications has paved the way for innovative cell designs for thermal energy conversion. CNT forests grown directly on thermocell casings, scroll electrodes, and thermocells that can be wrapped around cooling/

When inter electrode separation is decreased, larger specific power is generated as mass transfer is enhanced over shorter distances. However, this results in decreased power con‐ version efficiency as larger thermal energy is required to maintain a similar thermal gradient [112]. Scroll electrodes (Figure 21) can be employed to mitigate this problem. Using scrolled MWNT buck papers, each with a diameter of 0.3 cm and mass of 0.5 mg, aligned along their rolling axis inside a glass tube containing 0.4 M ferri/ferrocyanide, an electrode separation of

obtained. The power conversions efficiency of 0.24 % is an order of magnitude higher than thermocells using Pt electrodes tested under similar conditions [114]. The relative efficiency of the Mark II thermocell is 17 % higher than that obtained when using platinum electrodes,

C a specific power density of 1.8 W/m2 was

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 523

heating pipes have been attained through flexible CNT electrodes [114].

C and thermal gradient of 60 o

highly desirable in thermocells [152].

**Figure 20.** Aligned CNTs produced by CVD process

*Mark II thermocell*

5 cm, Tcold of 5 o

giving Φr of 1.4 %.

*3.3.2. Cell design breakthroughs attained using CNTs*

their size and rigidity [142]. In order to realize a minimisation of ∆*G mix* then solvents which result in small values of ∆*Hmix* are necessary. Based on Equation 7, the most effective sol‐ vents at dispersing CNTs would be those that have a surface energy close to the nanotube surface energy (~70 mJ/m2 ); i.e. solvents with surface tension around 40 mJ/m2 [143].

Another method to attain stable dispersions of CNTs is through the use of surfactants. Its inherent advantage over solvent dispersion was the fact that it was carried out in aqueous media, lessening its hazards and environmental impact. It relies on the principle wherein colloids are stabilized by surface charges [144]; i.e. Coulomb repulsion. Adsorption of the amphiphilic surfactant molecules onto CNTs is attained through their hydrophobic tails. This introduces a removable surface charge that creates an electric double layer around the nanotube; of which the magnitude and sign is proportional to its zeta potential [145]. This double layer provides repulsive forces that counteract the attractive van der Waals forces [146]. Selection of surfactants for CNTs dispersion depends on the size of their molecules. Low molecular weight surfactants will be able to pack tightly around the nanotube surface resulting in better stabilization [147].

**Figure 19.** CNT bucky paper

Filtration of CNT dispersions results in a planar mat of randomly arranged tubes [148] that can be up to several hundred microns thick. These mats or "bucky papers" (Figure 19) have been instrumental in CNT evaluation (owing to their simple processing) as electrode materi‐ als not only for thermocells but for other electrochemical applications as well. Post-treat‐ ment of buckypapers via annealing or acid wash is essential to ensure complete removal of the solvent or surfactant used to attain the CNT dispersion [149].

#### *Chemical vapour deposition*

Chemical vapour deposition (CVD), a process that involves deposition of solids from a gas phase, has proven to be a viable method for attaining highly oriented CNTs on planar sub‐ strates (Figure 20) [150]. One of the theories behind the large degree of alignment is the re‐ duction in free energy brought about by the van der waals interactions along the tube length inducing coordinated growth by holding the tubes together [151]. The ability to tailor the growth of CNTs in three dimensional configurations is highly advantageous in thermocell applications. This configuration promotes enhanced ion accessibility with the CNT matrix allowing larger current to be generated. The alignment of the tubes also minimises the tor‐ tuosity of the CNT electrodes which decreases the probability of forming concentration gra‐ dients within the electrode itself, leading to a decrease in the mass transfer overpotential. The wide range of substrates that can be used for CNT synthesis (metallic, carbon, etc) via CVD allows the fabrication of electrodes with a high degree of flexibility; materials that are highly desirable in thermocells [152].

**Figure 20.** Aligned CNTs produced by CVD process

#### *3.3.2. Cell design breakthroughs attained using CNTs*

The development of flexible electrodes for electrochemical applications has paved the way for innovative cell designs for thermal energy conversion. CNT forests grown directly on thermocell casings, scroll electrodes, and thermocells that can be wrapped around cooling/ heating pipes have been attained through flexible CNT electrodes [114].

#### *Mark II thermocell*

The entropy of mixing per unit volume ( ∆*S*

522 Syntheses and Applications of Carbon Nanotubes and Their Composites

surface energy (~70 mJ/m2

resulting in better stabilization [147].

**Figure 19.** CNT bucky paper

*Chemical vapour deposition*


result in small values of ∆*Hmix* are necessary. Based on Equation 7, the most effective sol‐ vents at dispersing CNTs would be those that have a surface energy close to the nanotube

Another method to attain stable dispersions of CNTs is through the use of surfactants. Its inherent advantage over solvent dispersion was the fact that it was carried out in aqueous media, lessening its hazards and environmental impact. It relies on the principle wherein colloids are stabilized by surface charges [144]; i.e. Coulomb repulsion. Adsorption of the amphiphilic surfactant molecules onto CNTs is attained through their hydrophobic tails. This introduces a removable surface charge that creates an electric double layer around the nanotube; of which the magnitude and sign is proportional to its zeta potential [145]. This double layer provides repulsive forces that counteract the attractive van der Waals forces [146]. Selection of surfactants for CNTs dispersion depends on the size of their molecules. Low molecular weight surfactants will be able to pack tightly around the nanotube surface

Filtration of CNT dispersions results in a planar mat of randomly arranged tubes [148] that can be up to several hundred microns thick. These mats or "bucky papers" (Figure 19) have been instrumental in CNT evaluation (owing to their simple processing) as electrode materi‐ als not only for thermocells but for other electrochemical applications as well. Post-treat‐ ment of buckypapers via annealing or acid wash is essential to ensure complete removal of

Chemical vapour deposition (CVD), a process that involves deposition of solids from a gas phase, has proven to be a viable method for attaining highly oriented CNTs on planar sub‐ strates (Figure 20) [150]. One of the theories behind the large degree of alignment is the re‐ duction in free energy brought about by the van der waals interactions along the tube length

the solvent or surfactant used to attain the CNT dispersion [149].

); i.e. solvents with surface tension around 40 mJ/m2

their size and rigidity [142]. In order to realize a minimisation of ∆*G*

*mix* ) of nanotubes is generally small owing to

*mix* then solvents which

[143].


When inter electrode separation is decreased, larger specific power is generated as mass transfer is enhanced over shorter distances. However, this results in decreased power con‐ version efficiency as larger thermal energy is required to maintain a similar thermal gradient [112]. Scroll electrodes (Figure 21) can be employed to mitigate this problem. Using scrolled MWNT buck papers, each with a diameter of 0.3 cm and mass of 0.5 mg, aligned along their rolling axis inside a glass tube containing 0.4 M ferri/ferrocyanide, an electrode separation of 5 cm, Tcold of 5 o C and thermal gradient of 60 o C a specific power density of 1.8 W/m2 was obtained. The power conversions efficiency of 0.24 % is an order of magnitude higher than thermocells using Pt electrodes tested under similar conditions [114]. The relative efficiency of the Mark II thermocell is 17 % higher than that obtained when using platinum electrodes, giving Φr of 1.4 %.

*Flexible thermocell*

dient of 15o

**3.4. Conclusion**

one day be realised.

One of the main applications of thermocells is to harvest thermal energy from automobile exhaust pipes and cooling or heating lines in industrial facilities. Flexible thermocells can be wrapped around these pipes and convert them to sources of electrical power. A flexible thermocells consisting of two MWNTbucky paper electrodes kept apart by 2 layers of No‐ mexHT 4848 impregnated with 0.4M ferri/ferrocyanide and wrapped in a stainless steel sheetis shown in Figure 23. The cell was wrapped around a cooling pipe and a thermal gra‐

**Figure 23.** a) schematic b) photo of a flexible thermocell that can be wrapped around cooling/heating pipes [114]

Research on CNTs as electrode materials for thermogalvanic cells is still in its early stages. However, these initial results indicate that these nanocarbons are capable of generating sig‐ nificant amounts of power; much larger than when conventional electrodes are used. With‐ out excluding the energy input from mechanical stirring, thermocells with platinum electrodes are able to attain a power conversion efficiency relative to a Carnot engine of 1.2% [100]. This value was surpassed with the use of CNT electrodes, reaching Φ<sup>r</sup> = 1.4%, in a thermocell that did not utilise any mechanical stirring and relied only on convection and diffusion to cause mass transfer of reaction products. The most important breakthrough is in the area of cell design as the robustness of CNT electrodes allows them to be conformed into a variety of shapes in order to mitigate heat flow from the hot to cold side of the thermocell. Flexible thermocells are also possible; devices that can harvest heat from heating or cooling pipes. Optimisation of the porosity of these CNT electrodes is essential in order to minimise the tortuosity and to reduce the mass transport overpotential in these systems. Doping CNT electrodes can alter their electroactive surface area by up to 4-fold; this feature can be ex‐ ploited by selecting the right electrolyte. The use of CNT-RGO composites has demonstrated the synergistic effect of these two materials, augmenting the power conversion efficieny of thermocells. Further developments in the field of CNT synthesis and processing will de‐ crease the cost of these materials such that commercialisation of thermogalvanic cells may

ed proving that flexible thermocells are now a possibility [114].

Reprinted with permission from American Chemical Society

C was applied using a resistive heater. A specific power 0.39 W/m2 was generat‐

Carbon Nanotubes for Energy Applications http://dx.doi.org/10.5772/51784 525

**Figure 21.** Mark II thermocell[114] Reprinted with permission from American Chemical Society

#### *Coin cell*

Thin coin type thermocells which could be powered by extremely low thermal gradients were developed using MWNTs and 0.4M ferri/ferrocyanide as the electrolyte (Figure 22). Coin cells fabricated using MWNT bucky paper electrodes and exposed to a thermal gradi‐ ent of 45o C generated a specific power of 0.389 W/m2 (equivalent to a normalised power density Pmax/ΔT<sup>2</sup> of 1.92 x 10-4 W/m2 K). Coin cells with electrodes made of MWNT forests around 100 µm tall, grown directly on the internal stainless steel surface of the packaging substrate using a trilayer catalyst (30 nm Ti, 10 nm Al, 2 nm Fe) plasma enhanced CVD method was also developed. The specific power generated at a thermal gradient of 60o C was 0.980 W/m2 ,giving Pmax/ΔT<sup>2</sup> =2.72 x 10-4 W/m2 K. The larger normalised power density of the coin cell with MWNT forest electrodes is due to its nanotube alignment, which promotes electrolyte diffusion, and the lower thermal (0.01 cm2 K/W) and electrical resistance at the MWNT forest/substrate junction [153]. The thermal resistance for bucky papers is around 0.05 cm2 K/W which leads to larger loss of thermal energy at the electrode/substrate junction and 30% less power conversion efficiency [154].

**Figure 22.** Coin cell for thermal energy conversion [114] Reprinted with permission from American Chemical Society

#### *Flexible thermocell*

One of the main applications of thermocells is to harvest thermal energy from automobile exhaust pipes and cooling or heating lines in industrial facilities. Flexible thermocells can be wrapped around these pipes and convert them to sources of electrical power. A flexible thermocells consisting of two MWNTbucky paper electrodes kept apart by 2 layers of No‐ mexHT 4848 impregnated with 0.4M ferri/ferrocyanide and wrapped in a stainless steel sheetis shown in Figure 23. The cell was wrapped around a cooling pipe and a thermal gra‐ dient of 15o C was applied using a resistive heater. A specific power 0.39 W/m2 was generat‐ ed proving that flexible thermocells are now a possibility [114].

**Figure 23.** a) schematic b) photo of a flexible thermocell that can be wrapped around cooling/heating pipes [114] Reprinted with permission from American Chemical Society

#### **3.4. Conclusion**

**Figure 21.** Mark II thermocell[114] Reprinted with permission from American Chemical Society

C generated a specific power of 0.389 W/m2

=2.72 x 10-4 W/m2

of 1.92 x 10-4 W/m2

524 Syntheses and Applications of Carbon Nanotubes and Their Composites

electrolyte diffusion, and the lower thermal (0.01 cm2

and 30% less power conversion efficiency [154].

,giving Pmax/ΔT<sup>2</sup>

Thin coin type thermocells which could be powered by extremely low thermal gradients were developed using MWNTs and 0.4M ferri/ferrocyanide as the electrolyte (Figure 22). Coin cells fabricated using MWNT bucky paper electrodes and exposed to a thermal gradi‐

around 100 µm tall, grown directly on the internal stainless steel surface of the packaging substrate using a trilayer catalyst (30 nm Ti, 10 nm Al, 2 nm Fe) plasma enhanced CVD method was also developed. The specific power generated at a thermal gradient of 60o

coin cell with MWNT forest electrodes is due to its nanotube alignment, which promotes

MWNT forest/substrate junction [153]. The thermal resistance for bucky papers is around 0.05 cm2 K/W which leads to larger loss of thermal energy at the electrode/substrate junction

**Figure 22.** Coin cell for thermal energy conversion [114] Reprinted with permission from American Chemical Society

(equivalent to a normalised power

K/W) and electrical resistance at the

C was

K). Coin cells with electrodes made of MWNT forests

K. The larger normalised power density of the

*Coin cell*

ent of 45o

0.980 W/m2

density Pmax/ΔT<sup>2</sup>

Research on CNTs as electrode materials for thermogalvanic cells is still in its early stages. However, these initial results indicate that these nanocarbons are capable of generating sig‐ nificant amounts of power; much larger than when conventional electrodes are used. With‐ out excluding the energy input from mechanical stirring, thermocells with platinum electrodes are able to attain a power conversion efficiency relative to a Carnot engine of 1.2% [100]. This value was surpassed with the use of CNT electrodes, reaching Φ<sup>r</sup> = 1.4%, in a thermocell that did not utilise any mechanical stirring and relied only on convection and diffusion to cause mass transfer of reaction products. The most important breakthrough is in the area of cell design as the robustness of CNT electrodes allows them to be conformed into a variety of shapes in order to mitigate heat flow from the hot to cold side of the thermocell. Flexible thermocells are also possible; devices that can harvest heat from heating or cooling pipes. Optimisation of the porosity of these CNT electrodes is essential in order to minimise the tortuosity and to reduce the mass transport overpotential in these systems. Doping CNT electrodes can alter their electroactive surface area by up to 4-fold; this feature can be ex‐ ploited by selecting the right electrolyte. The use of CNT-RGO composites has demonstrated the synergistic effect of these two materials, augmenting the power conversion efficieny of thermocells. Further developments in the field of CNT synthesis and processing will de‐ crease the cost of these materials such that commercialisation of thermogalvanic cells may one day be realised.
