2.1 Alkaline fuel cells
Alkaline fuel cells are regarded as desirable clean energy alternatives to traditional fuels, and they may serve as solutions to ameliorate worsening greenhouse effect [8,9]. The alkaline fuel cell is the first successfully developed fuel cells, whose use date back to 1960s in Apollo space shuttle. The structure of an alkaline fuel cell is shown in Fig. 2.1 [10]. The electrolytes used are alkaline solutions such as KOH or NaOH. Pure hydrogen as the fuel oxidizes at the anode, while air or pure oxygen as the oxidant reduces at the cathode. The fuel cell operates through a redox reaction between hydrogen and oxygen following the equations:
Anode：H2+2OH- →2H2O+O2 Eo= -0.83 V (2-1)
Cathode：1/2O2+H2O+2e- →2OH- Eo= 0.40 V (2-2)
Overall :1/2O2+H2 → H2O Eo= -1.23 V (2-3)
The energy conversion efficiency of alkaline fuel cell is high, up to 70%, which is mainly because the oxygen reduction reaction rate is faster in the alkaline electrolyte than that in acidic electrolyte. Besides, the alkaline fuel has another advantage that non-platinum-group catalysts can be used in the system. However, the air is widely used as the oxidant in alkaline fuel cells, but aqueous alkaline solutions do not reject CO2. The fuel cells could become “poisoned” through the conversion of KOH to K2CO3, resulting in reduced lifetime and efficiency. There are several ways
6
to conquer CO2 poisoning problems such as adding filters before inputting air to the electrode, applying the electrochemical methods to eliminate CO2, cycle purification electrolyte, and using pure oxygen as the oxidant.
Figure 2.1 The structure of an alkaline fuel cell [10].
2.2 Perovskite structure
The perovskite structure is a compound with an ABO3 formula as shown in Fig.
2.2 [11]. In the structure, the oxygen ion and cation A form a close packed structure, while the cation B with a smaller size occupies the body-center position. Perovskites have many derivatives by substituting different atoms into A and B sites. For example, the A site can be alkaline earth metals such as La, Ca, and Sr. On the other hand, at the
7
B site transition metals such as Mn, Co, Ir, and Cu can be used. Therefore, Ca and Sr can be incorporated into perovskite occupying the A site forming La1-xCaxCoO3 and La1-xSrxCoO3 perovskite structures.
Figure 2.2 The structure of perovskite with a chemical formula ABO3 [11].
Nowadays, the Ca doped LaCoO3 perovskite has been exploited as the electrocatalyst in many researches. The La0.6Ca0.4CoO3 has also been exhibited as a bi-functional electrocatalyst with good performances. A wide variety of transition metal ions have been explored for partial replacements at the Co3+ sites to improve the oxygen reduction reaction (ORR) kinetics [12,13]. Recently, we synthesized the La0.6Ca0.4CoIr0.25O3.5-δ and La0.6Ca0.4Co0.8Ir0.2O3, which demonstrate improved performances for both the ORR and oxygen evolution reaction over those of La0.6Ca0.4CoO3 [14,15]. We realize that the successful incorporation of Ir4+ at the Co3+
8
sites is responsible for the catalytic enhancements. Because metallic Ru is known as an ORR electrocatalyst, a similar strategy can be employed to introduce Ru3+ at the Co3+ sites for possible catalytic reactions [16,17]. The perovskite lattice is expected to provide a stable platform for hosting Ru3+. As a result, a reduced amount of Ru is used as opposed to the metallic form.
2.3 Oxygen reduction reaction
ORR is a critical step in fuel cell electrochemistry because a significant overpotential is often required to activate the relatively stable oxygen molecules [18].
Conventional fuel cells involve an acidic electrolyte, noble metals and alloys such as Pt and Pt3Ni are selected for their chemical inertness [19,20]. However, to reduce the system cost it is necessary to adopt less-expensive materials with comparable performances [21,22]. The ORR in an alkaline electrolyte reveals less polarization loss, leading to a fuel cell with better efficiency [23]. In an alkaline solution, the ORR occurs via direct four-electron pathway or two electron peroxide route listed below [24];
The straightforward route is known as the four-electron pathway in which the
9
oxygen molecule reacts with water and four electrons in a single step to form hydroxyl ions directly (2-4). An alternative one is the peroxide pathway where the oxygen molecule accepts two electrons and water in the first step to convert to peroxide ions (2-5). Subsequently, the peroxide ions react with two more electrons and water leading to the formation of hydroxyl ions (2-6), or decomposition to form hydroxyl ion and oxygen (2-7). In addition, the electrocatalysts that reduce oxygen through the four electron pathway are preferred because less electrode polarization is expected. Many materials have been investigated as an electrocatalyst to enhance the ORR. They include metals (Pt, Ag) [25-30], metal oxides (RuO2, MnO2, CoO) [31-33], perovskites (LaCoO3, La0.6Ca0.4CoO3) [34-38], spinels (NixAl1-xMn2O4, Ni2Co2O4, Mn3xCo3-3xO4) [39-41], as well as pyrolyzed N-4 chelate compounds (CoTMPP) [42]. A thorough review was provided recently by Wang, discussing available non-platinum electrocatalysts [43]. Because perovskite is established as the two-electron catalyst, the decomposition of H2O2 can be selected as a litmus test for quick catalytic evaluations [44]. For example, Jiang et al. studied the ORR ability for cobalt oxide/graphite air electrodes by determining their homogeneous and heterogeneous rate constants for the H2O2 decomposition [45]. Because the transition metal ions are the active species for the ORR, Falcón et al. adopted a similar technique to correlate the oxidation states of Ni and Fe in LaFexNi1-xO3+δ [46]. Thus, the novel perovskites were chosen in this research work in alkaline fuel cells.
2.4 Bi-functional electrocacatlysts
Oxygen reduction and evolution are critical electrochemical reactions in many industrial applications [47,48]. For example, in solid oxide fuel cells cathode oxygen
10
reduction is the most energy-consuming step and responsible for the largest polarization loss [49]. Likewise, oxygen evolution poses serious challenges in chlor-alkali productions and water electrolysis cells [48,50]. To accelerate electrochemical reactions, electrocatalysts are used and they are typically designed and synthesized for single purpose only. However, in particular systems such as rechargeable metal-air, air-metal hydride, and regenerative fuel cell, bi-functional electrocatalysts are required [51-53]. An excellent review was provided recently by Jörissen detailing materials selection and construction principle of bi-functional oxygen-air electrodes [54]. Conventional fabrication methods for bi-functional gas diffusion electrodes (GDE) entail lamination of current collector, carbonaceous materials impregnated with suitable electrocatalysts, and polytetrafluorethylene (PTFE) resin.
Oxides including perovskites (ABO3), spinels (AB2O4), and pyrochlores (A2B2O7) have been investigated extensively for their bi-functional catalytic abilities in alkaline electrolyte [55,56]. Among them, the perovskites have received considerable attention because of reasonable electrical conductivity and corrosion resistance. Previously, Bockris and Otagawa had conducted comprehensive analysis on electrocatalysis in perovskites [38]. Among the plausible candidate materials, the lanthanum cobaltate (LaCoO3) was documented widely in literatures for its simple synthesis and incorporation of various dopants in a wide variety of compositions [57-60]. Tiwari et al. suggested that B site cation (i.e., Co3+) in LaCoO3 directly contributes to the catalytic performances [61]. As a result, many efforts including introduction of additional cations, as well as controlling the degree of oxygen vacancies were engaged to manipulate oxidation states of Co. To date, La0.6Ca0.4CoO3
11
has demonstrated impressive characteristics and thus can be found in many cell samples [62,63].
Iridium oxide (IrO2) is used as the dimensionally stable anode in chlor alkali cells [55]. It processes the rutile structure where the Ir4+ is coordinated by six neighboring oxygen atoms forming an octahedron. Surprisingly, the IrO2 exhibits excellent electrical conductivity and corrosion resistance, that are desirable for many catalytic reactions. For example, De Pauli and Trasatti prepared mixed oxides of IrO2 and SnO2 and reported impressive behaviors of oxygen evolution in acid electrolyte [64]. Unfortunately, implementation of IrO2 for oxygen evolution in alkaline electrolyte is less studied due to the concerns on possible dissolution [65]. Because of similar octahedral coordination of Ir4+ in IrO2 and Co3+ in LaCoO3, it would be interesting to explore possible replacements of Ir4+ at Co3+ sites in perovskite matrix.
In this way, we expect the perovskite structure provides the necessary chemical stability in alkaline electrolyte while the Ir4+ contributes to the oxygen evolution.
2.5 Gas diffusion electrodes and electrocatalyst support materials
A gas diffusion electrodes (GDE) is required for fuel cells and metal air batteries.
The GDE is a porous platform incorporating current collector, hydrophobic polytetrafluoroethylene (PTFE) resin and carbonaceous material impregnated with catalytic nanoparticles [66,67]. The porosity, pore structure, and hydrophobicity of the GDE must be carefully designed to allow an extensive reaction interface between the gaseous oxygen and liquid electrolyte in the vicinity of the electrocatalyst. To date, it
12
is recognized that the GDE is the crucial component enabling successful implementation of fuel cells.
Many carbonaceous materials have been investigated as possible electrocatalyst supports [68-70]. They range from conventional carbon powders such as Vulcan XC72, Black Pearls, Shawiningan Blacks and active carbons, to less familiar ones including glassy carbons, carbon fibers and hard carbon spherules [71-73]. A detailed discussion was recently provided by Dicks [74]. With recent progress in the synthesis of nanostructured materials, exotic forms of carbon such as carbon nanotubes (CNTs) and carbon nanocapsules (CNCs) have been reported [75,76]. The CNTs exhibit extraordinary physical properties such as mechanical strength, excellent electronic and thermal conductivity, as well as chemical stability and high surface area. These are desirable physical properties for electrocatalyst supports. Hence, the CNTs’
applicability as an electrocatalyst support has received much attention recently. For example, Kongkanand et al. observed significant enhancements in the electrocatalytic activities of single-walled CNTs decorated with Pt nanoparticles [77]. A similar result was reported by Che et al. and they attributed the enhanced capabilities to the accessible inner surface of CNTs [78]. In addition, the strategy of hybrid carbonaceous materials has been explored. Huang et al. mixed active carbons with the CNTs and observed substantial improvements in electrocatalytic performance once Pt was loaded [79].
2.6 Supercapacitors
Supercapacitors have attracted considerable attention recently because they
13
possess the advantages in power output, energy density, and cycle life compared with conventional rechargeable batteries, for the applications in electric vehicles, power tools and uninterrupted power systems [80-91]. From the standpoint of operation mechanism, there are two types of supercapacitors; EDLCs and pseudocapacitors. The EDLCs store charges via ions adsorption/desorption at the interface between the electrode and electrolyte [84-88, 92,93]. In contrast, the pseudocapacitors entail facile faradaic reactions occurring on the electrode material to store charges in different oxidation states such as hydrous RuO2, V2O5, NiO, and MnO2 [94-106]. For example the hydrous RuO2 exhibits the best performance resulting from its distinctive characteristics of highly reversible and fast faradaic reaction mechanism, high specific capacitance, and very good conductivity.
Among them, the pseudocapacitors are able to deliver larger capacitances but with cost premium and relatively short cycle life. Therefore, there have been considerable interests in exploring alternative electrode materials. Carbonaceous materials are the electrode materials used in the majority of commercially available EDLCs, possessing the advantages of low cost and long cycle life. Besides, specific capacitor of EDLC can be increased by using organic electrolytes [107-109]. Since the charge/discharge reaction on the electrode surface provides a shorter ions moving path without chemical reaction, the EDLC exhibits a low internal resistance through interactive-free electrodes. As a result, EDLC conducts non-faradaic process, which is completely reversible under a higher power density. Besides, material properties including surface area, pore sizes, pore size distribution, surface functional group and electrolyte window can be controlled to improve EDLC [110,111]. For the EDLCs, the capacitive responses are proportional to the effective surface area available for
14
ions adsorption/desorption so an excessive surface area with a desirable porosity for the electrode material is necessary. To date, considerable efforts have devoted to the carbon-based materials since their rich varieties exhibit a wide range of pore sizes, surface areas, electric conductivities, and surface properties. In literature, carbonaceous materials including active carbons, carbon blacks, glassy carbons, and nanostructured carbons (carbon nanotubes, nanocapsules, nanofibers, gels) have been investigated [112-119]. Among them, the carbon xerogels and aerogels possess unique properties of extremely low density and high porosity [120,121].
2.7 Resorcinol-formaldehyde carbon gels
The carbon xerogels and aerogels are synthesized via a sol-gel process in which precursors in liquid states are properly mixed, and condense to form continuous colloidal networks, followed by solvent removal and pyrolysis to obtain a porous carbon structure with interconnected channels. One of the sol-gel approaches that has been studied extensively is the polycondensation of resorcinol ® and formaldehyde (F) [120-122]. According to Elkhatat and Al-Muhtaseb [123], the concentrations of the precursors, catalysts, solvents, and pH value play important roles in determining the resulted morphologies and porosities of the gel structures. In addition, the solvent removal step for the wet gels is critical because the drag of surface tension induces contraction of the colloidal networks resulting in substantial reduction in the pore size.
So far, many R-F derived porous carbons have been prepared and evaluated for possible applications in EDLCs, catalysis (as a catalyst support), filtration, gas separation, and adsorption [121, 123,124].
15
Previously, many RF-derived xerogels and aerogels have been synthesized and evaluated for capacitive behaviors [125,126]. The preparation of xerogels involves direct solvent evaporation that engenders notable structural contraction and possible collapse at the extreme case. As a result, after pyrolysis the carbon xerogels typically contain 25% porosity with reveal a moderate surface area (150-900 m2g-1) and small pores (1-10 nm) [127]. In contrast, the carbon aerogels experience a supercritical drying step that sublimates the solvents with negligible shrinkage of carbon networks.
After pyrolysis, the carbon aerogels demonstrate a substantially larger surface area and pore volume, and consequently, a larger capacitance [127-129]. Despite of those merits, the carbon aerogels are of little commercial interest because the supercritical drying is energy-consuming and improper for production in large quantity. On the other hand, the carbon xerogels suffer from unnecessary structure alteration and hence, their pore size and pore volume are not adequate for EDLC applications. An alternative preparation route to minimize structure contraction during sol-gel transformation is via repeated solvent exchanges that reduce the surface tension of the solvent sequentially in the wet gels [126]. In this way, the solvent with reduced surface tension is able to evaporate slowly rendering a dried gel known as “ambient gel” whose structure is closely resemble to that of aerogel. This enables a large number of mesopores and macropores in the carbon skeleton. This ambient gel could be of potential interest because its desirable surface area and porosity, as well as the simple drying process.
16