Pt and Pd-Based Electrocatalysts for Ethanol and Ethylene Glycol Fuel Cells

Abstract

Direct Oxidation Fuel Cells (DOFCs) are power systems that can replace H₂/O₂ fuel cells in different applications where the use of hydrogen is a major problem. The use of liquid fuels can be of great advantage due to the easiness of their transport and handling. A considerable number of small organic molecules have been considered as fuels in DOFCs. Methanol is the most studied liquid fuel, but the main problem of this molecule is its high toxicity score. Some alternative liquid fuels are taking an important role and are being considered as replacements of methanol, mainly because they are prone to be electro-oxidized at low temperatures at suitable electrocatalysts. C₂-fuels such as ethanol (EtOH, C₂H₅OH) and ethylene glycol (EG, C₂H₆O₂) are some of the most interesting molecules for DOFCs, because of their high energy density and due to the fact that only one C–C bond scission occurs during the dissociative adsorption of the molecule to form CO₂. In this chapter we present a description of the Direct Ethanol Fuel Cells (DEFC) and the Direct Ethylene Glycol Fuel Cell (DEGFC). We describe the reaction mechanism of the electro-oxidation of these fuels, the problems related to their crossover and the development of EtOH and EG tolerant cathodes.

3.1 Introduction

Polymer Electrolyte Membrane Fuel Cells (PEMFCs) have a number of advantages over other types of fuel cells. For example, due to their configuration, large power densities can be obtained in stacks of compact size. Moreover, PEMFCs can be fueled with a variety of fuels [1]. Besides the use of H₂, it has been demonstrated that liquid fuels such as methanol, ethanol and ethylene glycol can work as reliable source of chemical energy to react with O₂, to have a Direct Oxidation Fuel Cell (DOFC) configuration. Methanol has been largely studied and the complex oxidation mechanism of this alcohol on Pt-alloys anodes has been proposed [2–5]. It has been demonstrated that higher catalytic activities for the methanol oxidation reaction can be obtained by using Pt-Ru nanostructures over a wide range of operating temperatures [1].

Unfortunately, methanol is a toxic substance. Among the organic molecules used in fuel cell applications, it has one of the highest ecotoxicity scores, only behind hydrazine [6]. Therefore, alternative organic substances like C₂H₅O and C₂H₆O₂ are being tested in DOFCs. Due to the fact that the oxidation of these C₂-fuels form reaction intermediates such as CO or oxalic acid, Pt-alone anodes electrocatalysts become rapidly depolarized. Electrocatalysts with alternative chemical compositions are needed in order to sustain high current densities, maintaining an excellent stability. Pt-alloys have demonstrated a high catalytic activity for the oxidation of EtOH and EG [7–10]. The high performance of this type of materials relies on the bifunctional mechanism and the ligand effect. The first one describes the synergetic effect of adding an alloying metal M into the electrocatalyst’s structure, which actively participates in the water discharge process to form (–OH) intermediates [11]:

$$ {H}_2{O}_{ads(M)}\to O{H}_{ads(M)}+{H}^{+}+{e}^{-} $$

(3.1)

The (–OH) molecules take part in the oxidation of CO_ads in the neighboring Pt sites:

$$ C{O}_{ads(Pt)}+O{H}_{ads(M)}\to COO{H}_{ads(Pt)}\to C{O}_2+{H}^{+}+{e}^{-} $$

(3.2)

Reactions 3.1 and 3.2 take place at lower potentials (200–300 mV) on Pt-alloys compared to Pt-alone anodes [11–12]. The ligand effect has been related to an increase in the Pt d-band vacancies, modifying the adsorption energy of alcoholic intermediates on Pt, thus, the reaction rate is affected by electronic effects due to the interaction between Pt and the alloying element [1].

3.2 Direct Ethanol Fuel Cells (DEFC)

In a DEFC the anodic, cathodic and overall reactions are:

$$ \mathrm{Anode}:{C}_2{H}_5 OH+3{H}_2O\to 2C{O}_2+12{H}^{+}+12{e}^{-} $$

(3.3)

$$ \mathrm{Cathode}:3{O}_2+12\;{H}^{+}+12\;{e}^{-}\to 6\;{H}_2O $$

(3.4)

$$ \mathrm{Overall}\ \mathrm{reaction}:{C}_2{H}_5 OH+3{O}_2\to 2C{O}_2+3{H}_2O $$

(3.5)

In several reports, Lamy et al. have developed in detail the different relationships that allow us to calculate several important thermodynamic parameters of DEFCs [13, 14]. The Gibbs energy change of ethanol (under standard conditions) is −1,326.7 kJ/mol. Therefore, the electromotive force (EMF) of a DEFC is:

$$ {E}_{FC}^{\mathrm{o}}=1.145\;V $$

(3.6)

The thermodynamic efficiency of the cell, considering a value of ΔH ^o = −1,367.9 kJ/mol for ethanol, is:

$$ {\varphi}_{rev}=0.97 $$

(3.7)

The energy density in the case of C₂H₅OH is:

$$ {W}_e=8.01\frac{ kWh}{ kg} $$

(3.8)

3.2.1 Ethanol Oxidation Reaction (EOR) on Pt and Pt-Alloys

The understanding of the EOR mechanism on Pt-based electrocatalysts at low temperatures is relevant because of the importance of the DEFC technology. Several studies indicate that the electrocatalysts with higher catalytic activity for the EOR are Pt-Sn/C alloys [1, 15–19]. Even though, recently novel anode chemical compositions have been proposed. For example, the modification of Pt with rare earth oxides seems to improve the catalytic activity for this reaction. Cerium oxide is an interesting material for this kind of applications because it has been reported that can be used as co-catalyst or as co-support [20]. CeO₂ has the purpose of acting as a buffer for intermediates that serve to oxidize CO species into CO₂. Anodes of composition Pt-CeO₂/C have demonstrated an enhanced catalytic activity for the EOR, in some cases higher than that of Pt-Ru/C materials [21]. Also, CeO₂ has been used as co-support along with Vulcan. The addition of cerium oxide to disperse Pt-Sn lead to higher catalytic activity of a Pt-Sn/CeO₂-C anode, related to a Pt-Sn/C material [22]. At the same time, tri-metallic electrocatalyst have shown a high catalytic activity for the EOR. Basu et al. demonstrated that the Pt-Ir-Sn/C (20:5:15) anode increased the power density obtained from a DEFC [23].

The EOR mechanism on Pt-based anodes has been studied by different methods. The proposed path in acid media involves the first steps of ethanol adsorption via an O-adsorption or a C-adsorption. From these dissociative reactions, acetaldehyde (AAL) is formed [17, 24–26]:

$$ Pt+C{H}_3C{H}_2 OH\to Pt-{\left( OC{H}_2C{H}_3\right)}_{ads}+{e}^{-}+{H}^{+} $$

(3.9)

$$ Pt+C{H}_3C{H}_2 OH\to Pt-{\left( CHOHC{H}_3\right)}_{ads}+{e}^{-}+{H}^{+} $$

(3.10)

The formation of AAL at potentials lower than 0.6 V vs. RHE has been reported [17]. Acetic acid also forms as intermediate of this reaction, so the following mechanisms can be proposed [17]:

$$ E<0.6\ \mathrm{V}\ vs\ \mathrm{RHE} $$

$$ \mathrm{Pt}\hbox{--} {\left({\mathrm{OCH}}_2{\mathrm{CH}}_3\right)}_{\mathrm{ads}}\to \mathrm{Pt}+{\mathrm{CH}\mathrm{OCH}}_3+{\mathrm{e}}^{-}+{\mathrm{H}}^{+} $$

(3.11)

$$ \mathrm{Pt}\hbox{--} {\left({\mathrm{CHOHCH}}_3\right)}_{\mathrm{ads}}\to \mathrm{Pt}+{\mathrm{CHOCH}}_3+{\mathrm{e}}^{-}+{\mathrm{H}}^{+} $$

(3.12)

The next step is the formation of Pt-COCH₃ species according to the reaction [17]:

$$ \mathrm{E}<0.4\ \mathrm{V}/\mathrm{RHE} $$

$$ \mathrm{Pt}+{\mathrm{CHOCH}}_3\to \mathrm{Pt}-{\left({\mathrm{COCH}}_3\right)}_{\mathrm{ads}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} $$

(3.13)

Studies carried out by Lamy et al. show that CO species may adsorb on Pt at potentials as low as 0.3 V vs. RHE [17]. Meanwhile, Iwasita et al. found CH₄ traces at potentials lower than 0.4 V vs. RHE [24].

$$ \mathrm{E}>0.3\mathrm{V}/\mathrm{RHE} $$

$$ \mathrm{Pt}-{\left({\mathrm{COCH}}_3\right)}_{\mathrm{ads}}+\mathrm{Pt}\to \mathrm{Pt}-{\left(\mathrm{CO}\right)}_{\mathrm{ads}}+\mathrm{Pt}-\left({\mathrm{CH}}_3\right) $$

(3.14)

$$ \mathrm{E}<0.4\ \mathrm{V}/\mathrm{RHE} $$

$$ \mathrm{Pt}-{\left({\mathrm{CH}}_3\right)}_{\mathrm{ads}}+\mathrm{Pt}-{\left(\mathrm{H}\right)}_{\mathrm{ads}}\to 2\mathrm{Pt}+{\mathrm{CH}}_4 $$

(3.15)

Moreover, Lamy et al. reported that at relatively high potentials (0.6 V vs. RHE) the dissociative adsorption of water molecules occurs on Pt, forming –OH species (Reaction 3.16) that further oxidize the alcoholic residues [17]. In this step, several reactions take place, including the oxidation of CO species (Reaction 3.17) and AAL (Reaction 3.18):

$$ \mathrm{E}>0.6\ \mathrm{V}/\mathrm{RHE} $$

$$ \mathrm{Pt}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Pt}-{\left(\mathrm{OH}\right)}_{\mathrm{ads}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} $$

(3.16)

$$ \mathrm{Pt}-{\left(\mathrm{CO}\right)}_{\mathrm{ads}}+\mathrm{Pt}-{\left(\mathrm{OH}\right)}_{\mathrm{ads}}\to 2\mathrm{Pt}+{\mathrm{CO}}_2+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} $$

(3.17)

$$ \mathrm{Pt}-{\left({\mathrm{CH}\mathrm{OCH}}_3\right)}_{\mathrm{ads}}+\mathrm{Pt}-{\left(\mathrm{OH}\right)}_{\mathrm{ads}}\to 2\mathrm{Pt}+{\mathrm{CH}}_3\mathrm{COOH}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} $$

(3.18)

The dissociative adsorption of ethanol occurs at lower potentials on Pt-Sn/C electrocatalysts than on Pt/C anodes. OH species are formed on Sn sites at low potentials, leading to the oxidation of (CO)_ads into CO₂, in agreement with the bifunctional mechanism.

3.2.2 DEFC Performances

Power densities obtained from DEFCs have been increased by using Pt-alloys anodes. Testing bi-metallic and tri-metallic alloys, Xin et al. showed that the catalytic activity decreases in the order Pt-Sn/C>Pt-Ru/C>Pt-W/C>Pt-Pd/C>Pt/C [16]. In a single cell, the Pt₁Ru₁Sn₁/C anode showed a better performance than the Pt₁Ru₁/C electrocatalyst. However, the performance of the Pt₁Sn₁/C anode remained higher than that of Pt₁Ru₁Sn₁/C [16]. Studies with different Pt:Sn ratios carried out by the same group indicate that the best performance is reached by using the Pt₂Sn₁/C composition as anode in a single DEFC [15].

Figure 3.1 shows the performance of a DEFC equipped with Pt₃Sn₁/C (triangles) and Pt₁Ru₁/C (squares) anodes, and Pt/C cathodes in both cases. The fuel cell operated at 80 °C and 90 °C. Under these experimental conditions, a comparison of the polarization curves at both temperatures shows that higher performances in current density and open circuit potential can be obtained with the PtSn/C alloy. For instance, there is a twofold increase in current density at 0.4 V when the PtSn–Pt MEA is used in the fuel cell, compared to the current density obtained at the same voltage with the PtRu–Pt based MEA, at any of the two operating temperatures (Fig. 3.1).

Fig. 3.1

Polarization curves of a DEFC at 80 °C (open symbols) and 90 °C (closed symbols). Anodes: PtSn/C (triangles) and PtRu/C (squares). Cathodes: Pt/C in both cases. Catalyst loading on all electrodes: 1 mg/cm². Membrane: Nafion® 117. Ethanol concentration and flow rate: 1 M and 3 mL/min. Oxygen pressure and flow rate: 1 atm and 0.4 L/min

Figure 3.2 shows the power density attained by the DEFC of Fig. 3.1, this time with four different anode-cathode configurations (shown in the Fig. 3.2). The higher power density (27 mW/cm²) is reached by a PtSn/C-Pt/C MEA configuration, followed by the PtRu/C-Pt/C MEA. Interestingly, the MEA composed of PtSn/C-Ru/C (anode-cathode) attains a power density similar to that of the PtRu/C-Pt/C MEA, demonstrating the high contribution of the PtSn/C anode to the current density – voltage characteristics of a DEFC integrated by a low-performance cathode (Ru/C).

Fig. 3.2

Power density curves of a DEFC at operating at 90 °C with four different anode-cathode MEA configurations (shown in the figure). Catalyst loading on all electrodes: 1 mg/cm². membrane: Nafion® 117. Ethanol concentration and flow rate: 1 M and 3 mL/min. Oxygen pressure and flow rate: 1 atm and 0.4 L/min

3.3 Direct Ethylene Glycol Fuel Cells (DEGFC)

DEGFCs are being considered as an alternative to DMFCs. Peled et al. are among the workers that have extensively investigated the use of EG in fuel cells [10, 27–29]. Ethylene glycol (EG) is the simplest aliphatic diol. It has a high solubility in aqueous solutions [9].

The anode, cathode and overall reactions in a DEGFC are [30]:

$$ \mathrm{Anode}:{C}_2{H}_6{O}_2+2{H}_2O\to 2C{O}_2+10{H}^{+}+10{e}^{-} $$

(3.19)

$$ \mathrm{Cathode}:5/2{O}_2+10{H}^{+}+10{e}^{-}\to 5{H}_2O $$

(3.20)

$$ \mathrm{Overall}\ \mathrm{reaction}:{C}_2H{}_6{O}_2+5/2{O}_2\to 2C{O}_2+3{H}_2O $$

(3.21)

Under standard conditions, the Gibbs energy change of EG is −1,181 kJ/mol, therefore the EMF of a DEGFC is:

$$ {\mathrm{E}}_{\mathrm{FC}}^{\circ }=1.22\mathrm{V} $$

(3.22)

The thermodynamic efficiency of a DEGFC, considering a value of ΔH° = −1,181 kJ/mol for ethylene glycol, is:

$$ {\varphi}_{rev}=0.99 $$

(3.23)

The energy density of EG is:

$$ {W}_e=5.29\frac{ kWh}{ kg} $$

(3.24)

3.3.1 Electro-oxidation of Ethylene Glycol on Pt-Based Anodes

The study of the direct electro-oxidation of EG is a very active area of research. The development of anode electrocatalysts is of interest in order to find a suitable material for this reaction. In his work, Peled’s group found that the intermediates formed during the electro-oxidation of EG include mainly oxalic acid and glycolic acid [29]. Therefore, it is necessary to use Pt-alloys or Pt-composite electrodes that may sustain a high catalytic activity in long-term tests. Peled et al. studied the performance of a DEGFC with Pt-Ru/C anodes and Pt/C cathodes [27–31]. They developed for the first time a high performance 10 cm² DEGFC stack based on Pt-Ru/C anodes [10]. At the same time, the studies by Selvaraj et al. on the electrocatalytic oxidation of ethylene glycol showed that Pt-Ru/C anodes have a higher catalytic activity than Pt/C anodes for this reaction [9]. A comparison of the performance of Pt-Ru/Vulcan versus Pt-Ru/CNTs showed that the use carbon nanotubes produced larger anodic current densities [9]. Spinacé et al. compared the performance of Pt-Sn/C and Pt-Sn-Ni/C anodes for the electro-oxidation of EG. They reported higher current densities for the trimetallic Pt-Sn-Ni/C (with 50:40:10 composition) related to the Pt-Ru/C anode [32]. Chetty and Scott showed a superior performance of the ternary PtRuW/Ti anode electrocatalyst related to PtRu/Ti, PtRuNi/Ti and PtRuPd/Ti anodes (where Ti was a titanium mesh) [33].

The EG oxidation reaction mechanism on bimetallic anodes (Pt-Ru) has been proposed as [9, 34]:

$$ {\left(C{H}_2 OH\right)}_2\to {\left(C{H}_2 OH\right)}_{2(ads)} $$

(3.25)

$$ {\left(C{H}_2 OH\right)}_{2(ads)}\to {\left(: COH\right)}_2 $$

(3.26)

$$ \left(: COH\right)+2{H}_2O\to {(HCOOH)}_{ads}+2{H}^{+}+2{e}^{-} $$

(3.27)

$$ Pt{(HCOOH)}_{ads}\to Pt{(CO)}_{ads}+{H}_2O $$

(3.28)

$$ Pt+{H}_2O\to Pt{(OH)}_{ads} $$

(3.29)

$$ Pt{(OH)}_{ads}+ Pt{(CO)}_{ads}\to Pt+C{O}_2+{H}^{+} $$

(3.30)

$$ Ru+{H}_2O\to Ru{(OH)}_{ads}+{H}^{+}+{e}^{-} $$

(3.31)

$$ Ru{(OH)}_{ads}+ Pt{(CO)}_{ads}\to Pt+ Ru+C{O}_2+{H}^{+}+{e}^{-} $$

(3.32)

Equations 3.31 and 3.32 describe the capability of Ru atoms for the water discharge process at lower potentials (0.35 V) than Pt (0.6 V) [34].

3.3.2 DEGFC Performances

Peled’s group demonstrated the capacity of C₂H₆O₂ as a real fuel, using it to fuel single cells [27, 28, 31]. The DEGFC delivered power densities as high as 320 mW/cm² at 130 °C with a nanoporous proton-conducting membrane (NP-PCM) [27].

Figure 3.3 shows the polarization curve of a DEGFC with a 20 % Pt-Sn/C anode and a 20 % Pt/C cathode obtained by Rodríguez Varela and Savadogo. The operating temperature is 80 °C with a Nafion 117 membrane. The maximum power density of the cell is 17.5 mW/cm². The same authors tested a DEGFC cell equipped with a Pt-Sn/C anode and a Pd/C cathode, which delivered a maximum power density of 13.4 mW/cm² [35].

Fig. 3.3

Polarization curves of a DEGFC. Cell temperature: 80 °C. Anode: 20 % PtSn/C, cathode 20 % Pt/C. Catalyst loading in both electrodes: 2 mg/cm². Membrane: Nafion® 117. EG concentration and flow rate: 1 M and 2 mL/min. Oxygen flow rate: 0.5 L/min without backpressure

3.4 ORR at Ethanol Tolerant Electrocatalysts

The DOFC offers several advantages in comparison to hydrogen fed polymer electrolyte membrane PEM fuel cells, because the alcohol based aqueous solution can be processes inside and stored easier than gases. However, it should be acknowledged that the DOFC shows significantly lower power density than PEMFC due to slow oxidation kinetics and mass transport problem, i.e., transport of carbon dioxide out of the system and alcohol crossover through the membrane to the cathode side, where the alcohol oxidation on the cathodic compartment leads to a significant performance loss and lowering of the efficiency of the DOFC, situation which could be alleviated by use of alcohol tolerant electrocatalysts.

Electrocatalysts used for low temperature fuel cells have been extensively studied aiming the improvement of the catalytic activity, selectivity and greater stability at low cost. Beside intense studies of the ethanol and ethylene glycol penetration due to crossover effect into the cathode space of a fuel cell operation, there are some problems related to the cathodic reaction. The oxygen reduction reaction (ORR) is a multi-electron process which occurs at the fuel cell cathode side and has been the focus of attention because of the sluggish kinetics which limits the energy conversion efficiency of PEM fuel cells. The significant overpotential for the oxygen reduction reaction and the alcohol tolerance electrocatalyst required to overcome this electrochemical process, which could be attained by modification of the electronic structure of the electrode surface, reducing the tendency to bond to oxygen-containing species and hence increasing the number of available sites for oxygen adsorption and reduction processes. Although significant progress has been made in understanding how the ORR occurs, the exact reaction mechanism is still the subject of extensive discussions and remains indefinable. Recent publications give a good overview of the current state of the art in the development of ethanol [36] and ethylene glycol fuel cells [34], where a description mechanism of the ethylene glycol electrooxidation in acid media on Pt and different Pt-based alloys are analyzed.

The oxygen reduction reaction is of great importance in the development of novel cathode electrodes in PEM fuel cells. It is a complex process and includes several individual reactions where a desired discrete step involves the water formation through a fourth-electron pathway and the other is production of hydrogen peroxide as intermediate. Recent detailed review articles devoted to the present state and advances made in recent year on the ORR appear reported in literature [37–39]. In these articles authors emphasize that one of the target on cathode electrodes is to reduce the platinum content or the most expensive metallic content in electrocatalysts. Ruthenium-based chalcogenides have been considered as an alternative to Pt for the ORR because with the incorporation of Se the activity and effective selectivity improves significantly, modifying the electronic superficial structure of the catalyst weakening the O–O bond at the interface, making the catalyst more stable in the fuel cell operation. Furthermore, the action of adding a second metal to ruthenium chalcogenides have also been investigated as thin film electrodes in acid media and membrane-electrode assemblies, conducting to alcohol tolerance oxygen reduction electrocatalysts [39–42]. Lately, investigations on Pd-based alloys as cathode catalysts have been provided better understanding of the relationship between the size, shape, nanostructure, composition, and activity for the oxygen reduction reaction. The performances exhibited by these catalysts are equivalent to that reported for Pt and the enhanced activity of the alloy surface is assigned to bi-functional effects in which the unique catalytic properties of each element in the alloy combine in synergetic manner to yield a surface which improve the stability and enhance the catalytic activity compared to each element alone.

Most of the recent theoretical studies on ORR are based on Density Functional Theory (DFT) calculations, relating on the correlation of the electronic structure of the outermost layer of the surface composition and the catalytic activities of Pt- and Pd-based alloys [43–45]. It was found that there is a strong linear correlation between the adsorption energy of O and OH which are governing the rate of ORR on metallic catalyst surfaces. The ORR involves the sequential addition of protons and electrons to adsorbed oxygen. An approach to understand the electrochemistry of the ORR taking into account three possible mechanisms (oxygen dissociation, peroxyl dissociation and hydrogen peroxide dissociation) have been proposed by Nilekar and Mavrikakis [46], to describe how an oxygen molecule is reduced to form water molecule involving different intermediate species. The ORR reactivity of different surfaces is dictated by the strength of the oxygen adsorption, with the OH removal via hydrogenation and O–O bond scission. The detailed series of protonation elementary steps of the three ORR mechanisms is shown in Table 3.1, where intermediate species such as atomic hydrogen (H), atomic oxygen (O), hydroxyl (OH), peroxyl (OOH), and hydrogen peroxide (H₂O₂) are being the rate-determining step with surfaces with stronger and weaker oxygen binding.

Table 3.1 Three possible mechanisms for oxygen reduction reaction [43]

Since carbon supported nanoparticles of Pt is the model of fuel cell electrocatalyst, almost all theoretical and experimental publications are referred to Pt and Pt-based alloys that exhibit significant ORR activity, even in presence of methanol [47, 48]. Experiments have shown that Pt monolayer deposited on a number of metals (Au, Rh, Pd, Ru and Ir) has significant changes in ORR electrocatalytic activity compared to pure Pt [49] and the authors reported a Volcano-type dependence of monolayer catalytic activity on the substrate was novel in the field of electrocatalysis and catalysis in general. For pure Pt (111) surface the ORR has been reported to follow a peroxyl dissociation mechanism [45] for its rate-determining O protonation reaction and on Pt (111) modified by a subsurface of transition metal containing Ni, Co and Fe, the ORR adopt a hydrogen peroxide dissociation mechanism for their rate-determining O₂ protonation reaction. The theoretical study reported by Wang et al. [45] reveals also how subsurface transition metals would modify the electronic structure and the catalytic activity of the outermost Pt monolayer surface.

Some alloys of carbon supported nanoparticles of Pt-Pd catalyst and Pt-based cathode catalysts modified with S, P and Bi have been reported as highly tolerant to ethanol for direct ethanol fuel cells (DEFC) [41, 50], observing a decreasing in the overpotential of the ORR on the bimetallic alloy in relation to that observed on pure Pt, ascribed to a reduced ethanol adsorption on Pt-Pd. Recently, Savadogo and Rodríguez Varela, have published that Pd and Pd-Co cathode catalysts exhibit high degree of tolerance to ethanol during the ORR in acid medium, observing a selective cathodic process attributed to a slow rate of adsorption of the ethanol and the reaction intermediates on the catalytic surface [51, 52]. Also, a high catalytic activity is observed originated from the synergistic effect between the supports and the bimetallic catalysts, making those material promising Pt-free alternative for cathodic electrocatalysts in direct alcohol fuel cells (DAFC). The enhanced catalytic activity of bimetallic surfaces in comparison with pure metal surfaces is usually ascribed to two effects: the bifunctional effect in which the unique catalytic properties of each of the elements in the compound are combined to yield a more active surface than each of the elements alone, and the electronic effect in which one of the elements alters the electronic properties of the other [53, 54].

Figure 3.4 shows the polarization curves of the ORR at the 10 % Pt₁Co₁/C cathode. The scans were performed in a non-stirred 0.5 M H₂SO₄ solution with and without 0.5 M EtOH at 2 mV/s. The purpose of such test is to evaluate the detrimental effect of C₂H₅OH in the catalytic activity for the ORR of the Pt-alloy. The most important parameter for these evaluations is the shift of the onset potential for the ORR when ethanol is present in the solution and therefore, the increase in cathode overpotential [19]. The Pt₁Co₁/C cathode clearly shows a high selectivity for the ORR. The characteristics of the curves indicate the high insensibility of the PtCo material to the presence of the organic molecule. The Pt₁Co₁/C cathode also showed a higher mass activity for the ORR when compared to a Pt₃Cr₁/C cathode [19].

Fig. 3.4

Polarization curves of the ORR at Pt₁Co₁/C without and with 0.5 M C₂H₅OH. Support electrolyte: 0.5 M H₂SO₄. Scan rate: 2 mV/s

The performance characteristic of a DEFC employing 1 mg/cm² 20 % Pt₃Cr₁/C ethanol tolerant ORR catalyst at the cathode and 20 % PtSn/C at the anode is shown in Fig. 3.5. Non-preheated ethanol was pumped to the anode side without backpressure at a flow rate of 2 ml/min. The performance curves at 60 °C and 90 °C are presented. The expected enhancement in cell voltage vs. current density performances is observed with the increased temperature in Ref. [19].

Fig. 3.5

Polarization curves of the DEFC based on a 20 % Pt₃Cr₁/C cathode at 60 °C and 90 °C, with a 20 % PtSn/C anode. Catalyst loading: 1 mg/cm². Membrane: Nafion® 117. Ethanol concentration: 1 M

3.5 ORR at Ethylene Glycol Tolerant Electrocatalysts

Active cathode catalysts are required to eliminate the adverse effect brought along by the crossover of EG. The electrocatalytic activity of the catalysts toward the ORR, both in presence and absence of C₂H₆O₂ is evaluated for application in DEGFCs. Carbon supported Pd electrocatalysts synthesized by the formic acid method, and Pt₄₀Pd₆₀/MWCNT produced by reduction with NaBH₄, have been recently reported by Rodríguez Varela et al. as active and EG tolerant cathode electrocatalyst [35, 55]. Mixed-potential due to cathodic ethylene glycol oxidation is detrimental to the performance of the cathode catalyst in DEGFCs and this effect was observed when Pt was used as cathode catalyst. However, good performances were observed when PtCoSn/C and Pd/C catalyst were used as cathodes in DEGFCs [29, 56]. Elsewhere, a 30 % Pt₇₀-Co₃₀/MWCNT material was evaluated as cathode in 0.5 M H₂SO₄. The alloy showed a good behavior as ORR selective electrocatalyst and a high tolerance to EG [47]. The EIS results of the ORR at Pt-Co/MWCNT in the absence and presence of the liquid fuel showed one-arc spectra, indicating that the ORR mechanism at the cathode may not change when that substance was in the solution. The behavior of Pt-Co/MWCNT for the ORR with EG, EtOH or 2-Prop was similar. Thus, the ORR mechanism with the three organic molecules was proposed to be analogous [47].

Figure 3.6a depicts the linear scan voltammograms (LSVs) of the ORR on 20 % Pd/C without ethylene glycol at ω = 400, 800, 1,200 and 1,600 rpm. The LSVs show a clear dependence of the ORR current densities with potential and rotation rate. The ORR on the Pd electrocatalyst seems to be under kinetic and mixed control in the potential scanned, not reaching a well defined limiting current. Levich-Koutecky plots 1/j vs. 1/ω^1/2 at different potentials corresponding to the experimental values are shown in the insert in Fig. 3.6a. Clearly, these plots show linearity and parallelism, indicating first order kinetics with respect to molecular oxygen.

Fig. 3.6

(a) LSVs of the ORR at different rotation rates without EG. (b) LSVs of the ORR in the presence of increasing EG concentrations (indicated). The polarization curve of the ORR without EG at 1,600 rpm (from Fig. 3.6a) is also shown in Fig. 3.6b. Electrocatalyst: 20 % Pd/C. Electrolyte: O₂-saturated 0.5 M H₂SO₄ at 25 °C. Scan rate: 10 mV/s [56] (Reproduced with permission of the publisher)

Figure 3.6b shows the LSVs of the ORR on the Pd/C electrocatalyst in 0.5 M H₂SO₄ solution containing 0.125, 0.25, 0.5 or 1 M EG. The polarization curve of the ORR without EG at 1,600 rpm (from Fig. 3.6a) is shown as well in Fig. 3.6b for comparison purposes. The high selectivity for the ORR and tolerance of Pd/C to the presence of C₂H₆O₂ concentrations as high as 1 M, can be clearly seen in Fig. 3.6b. The values with EG remain practically unchanged with respect to the value without organic molecule.

No organic molecule oxidation peak, normally observed at Pt/C, appear in the case of Pd/C, making the negative effect of the liquid fuel on the cathode overpotential and in the onset potential for the ORR exceptionally limited.

The ORR was also analyzed on perovskite-type oxide La_1 − xSr_x MnO₃ in direct ethylene glycol alkaline fuel cells (DEGAFC) observing a high tolerance to EG since the cathodic polarization curves were not affected by the concentration of EG supplied to the anodic side [57].

3.6 Conclusions

The use of C₂-fuels in DOFCs has proven to be advantageous from a lower toxicity point of view, related to methanol. Moreover, high energy densities have been attained from ethanol and ethylene glycol fuel cells. Due to the fact that only one C–C scission occurs during the dissociative adsorption of these fuels, their oxidation at low temperatures (i.e., 80 °C, the operating temperature of DOFCs) at Pt-based electrocatalysts to form CO₂ is possible. Although the reaction mechanism of EtOH and EG can be complex, most of the reaction intermediates have been identified. In the case of C₂H₅OH, acetic acid, acetaldehyde and CO are the main intermediates. Pt-Sn/C alloys have shown the best performance for this reaction. On the other hand, during the oxidation of C₂H₆O₂, oxalic and glycolic acid are the most important intermediates. The definition of the most active catalysts for the EG oxidation reaction is still an on-going line of research. The studies indicate that Pt-Ru/C and Pt-Sn/C materials have a high catalytic activity of this anodic reaction, although the most active chemical composition is yet to be clearly defined. Novel anode compositions, such as ceria-modified Pt (Pt-CeO₂/C), are attractive alternatives for this reaction. A combination of theoretical and experimental studies have been performed to achieve a desirable design of stable nanocatalysts that have higher activity and alcohol tolerance ORR, with lower Pt loading and increased resistance to OH and diverse intermediate poisoning.