New Trends in Direct Ethanol Fuel Cells

C H A P T E R

15

New Trends in Direct Ethanol Fuel Cells Thiago dos Santos Almeida and Adalgisa Rodrigues De Andrade Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Av. Bandeirantes 3900, 14040-901 Ribeirão Preto, SP, Brazil

15.1  ETHANOL ELECTRO-OXIDATION IN ACID MEDIUM The principle of PEM-DEFC (proton exchange membrane direct ethanol fuel cell) operation is illustrated in Figure 15.1. The anode consists of an ethanol solution, while the cathode is composed by humidified air or oxygen, so that good conductivity is maintained in the  PEM. Ethanol is oxidized to carbon dioxide, and protons and electrons are produced on the anode side. Protons then transfer through the polymeric electrolyte to the cathode side, where they react with oxygen and electrons to produce water (Eqs. (15.1)–(15.3)).

Anode : CH3 CH2 OH + 3H2 O → 2CO2 + 12H+ + 12e− , Cathode : 3O2 + 12H+ + 12e− → 6H2 O,

E0 = 0.084 V,

E0 = 1.223 V,

Global Equation : CH3 CH2 OH + 3O2 → 2CO2 + 3H2 O,

E0 = 1.145 V.

(15.1) (15.2) (15.3)

From a technological viewpoint, the great interest in using this process is to find a way to achieve the maximum theoretical density energy for the complete oxidation of ethanol to CO2, which releases 12 electrons. The major difficulty in implementing this technology is related to the slow kinetics of oxygen reduction at the temperature in which the membrane is

Batteries, Hydrogen Storage and Fuel Cells http://dx.doi.org/10.1016/B978-0-444-53880-2.00020-X

429

© 2013 Elsevier B.V. All rights reserved.

430

15.  New Trends in Direct Ethanol Fuel Cells

FIGURE 15.1  Schematic diagram of a DEFC.

chemically stable (below 120 °C) and the strong adsorption of the intermediate of ethanol oxidation onto the active sites of the Pt catalyst, which leads to a less efficient energy in the real system [1,2]. If one wants to prepare devices to work at milder temperatures (T < 120– 150 °C), great attention must be given to the development of effective catalysts for application in fuel cells with direct feed. The interest in ethanol electro-oxidation for application in direct ethanol fuel cells was recognized a long time ago [3,4], which accounts for the significant level of comprehension of this electrochemical process as well as for the mechanism proposed in the literature [5,6]. A brief description of the mechanism of ethanol electro-oxidation and fuel cell operation will be presented, in order to contextualize what has been made to improve and enable the application of this type of electronic device. Several studies on the electro-oxidation of ethanol have been mainly devoted to the identification of the adsorbed intermediates onto the electrode and to the elucidation of the reaction mechanism by means of various techniques, such as differential electrochemical mass spectrometry (DEMS) [7–12] and in situ Fourier transform infrared spectroscopy (FTIR) [13–15]. On the basis of the foregoing work, the global mechanism of ethanol oxidation in acidic solution may be summarized according to the following scheme of parallel reactions [2]:

CH3 CH2 OH → [CH3 CH2 OH]ads → CO2 (total oxidation), CH3 CH2 OH → [CH3 CH2 OH]ads → CH3 CHO → CH3 COOH (partial oxidation).

(15.4) (15.5)



431

15.1  Ethanol Electro-Oxidation in Acid Medium

FIGURE 15.2  General scheme of ethanol oxidation on Pt–Sn electrocatalysts (reproduced from Ref. [19]).

From this general equation, highlights of the detailed mechanisms have been proposed by different groups [7,16–18]. On the basis of previous findings by Simoes et al. [19], the following steps have been proposed (Figure 15.2): The first hypothesis suggests that the ethanol molecule is “doubly” adsorbed onto the surface of the nanocatalyst through decoupling of the CH bond in both carbon atoms (step 7). Thereafter, the CC bond would be cleaved (step 8), generating the CO and CHx species. The second hypothesis implies the rupture of the CH bond of the resulting intermediate after adsorption of acetaldehyde, followed by a CC bond cleavage (step 10), with the consequent formation of the same intermediate (step 8). Regardless of the steps involved in CO generation, these species can react with OHads adsorbed species, which culminates in the CO2 production through the bifunctional mechanism. The following route for ethanol oxidation to acetic acid has been described [20]:

Pt + CH3 CH2 OH → Pt − (OCH2 CH3 )ads + e− + H+ ,

(15.6)

Pt − (OCH2 CH3 )ads → Pt + CH3 CHO + e− + H+ ,

(15.7)

Pt + CH3 CH2 OH → Pt − (CHOH − CH3 )ads + e− + H+ ,

(15.8)

Pt − (CHOH − CH3 )ads → Pt + CH3 CHO + e− + H+ ,

(15.9)

or,

432

15.  New Trends in Direct Ethanol Fuel Cells

Pt + CH3 CHO → Pt − (CO − CH3 )ads + e− + H+ ,

(15.10)

Pt + H2 O → Pt − OHads + e− + H+ ,

(15.11)

Pt − (CO − CH3 )ads + Pt − OHads → 2Pt + CH3 COOH.

(15.12)

For the complete oxidation of ethanol to CO2, two additional steps have been proposed [21]:

Pt + CH3 CH2 OH → Pt − COads + Pt − CHx ,

(15.13)

Pt − COads + Pt − OHads → CO2 + e− + H+ .

(15.14)

On the basis of the products determined by HPLC analysis, we have suggested the mechanism shown in Figure 15.3 [22].

15.1.1  Catalysts for Ethanol Electro-Oxidation in Acidic Medium The development of an efficient material for the electro-oxidation of ethanol is a challenge for many research groups [1,20,23–27]. In acidic medium, Pt-based catalysts are still considered as the main material for this purpose. However, pure Pt does not display good catalytic activity due to poisoning of its active sites with intermediates generated from reactions occurring in parallel with the oxidation of ethanol, which ends up requiring higher overpotential for the oxidation of the target fuel [24,28,29]. Thus, one of the main goals of researchers in the area of electrocatalysis is the development of effective electrocatalysts that could perform ethanol oxidation at lower potentials [30–32]. Currently, the development of efficient catalysts involves the incorporation of metals into platinum, so as to increase the performance of the electrocatalysts with respect to the alcohol [22,33–36].

FIGURE 15.3  Proposed mechanism for ethanol oxidation (reproduced from Ref. [22]).



15.1  Ethanol Electro-Oxidation in Acid Medium

433

The proposal of a new material for the electro-oxidation of ethanol is based on two different effects that the incorporation of a second or third metal can cause directly on Pt or on its neighborhood. The bifunctional effect described by Watanabe and Motoo [37,38] occurs when elements that are less noble than Pt and which display high affinity for water molecules can easily generate oxygen or hydrated oxide species next to a Pt site onto which the alcohol and its intermediates are adsorbed. The electronic effect is based on the charge transfer between the metals or groups with different electronegativity [39] and can be verified when one metal present in the alloy changes the chemical properties of the Pt atoms present on the surface of the catalysts, thereby lowering the electronic density at the Fermi level. Alternatively, the metal can partially withdraw electron of the Pt d-orbital, with consequent reduction in the chemisorption energy of the intermediates. The better performance of the catalysts that act by this effect is attributed to the weakening of the binding energy of the Pt–C bond prompted by the electron donation, which favors the oxidation of organic byproducts. As a consequence, the number of Pt sites that is free for the adsorption and oxidation of molecules is increased [40,41]. The simultaneous occurrence of both mechanisms cannot be ruled out, and this normally explains the synergistic effect obtained when materials are modeled with multimetal catalysts.

15.1.1.1  Efficient Second/Third Metals on Pt-Based Catalysts Metals such as Sn, Ni, Rh, Ru, Pd, and W are frequently employed as co-catalysts in Pt-based catalysts, in order to minimize the effect of Pt poisoning with CO and other byproducts, thereby improving the catalytic activity [14,21,22,24,27,28,36,42–47]. Many investigations have reported that the Pt–Sn composition displays excellent activity for ethanol oxidation. An excellent review of this topic has been outlined by Antolini [2]. More recently, Purgato et al. [20,48] have investigated the introduction of different Sn ratios into Pt catalysts prepared by the Pechini method, and Sn enhances the catalytic activity of Pt. Oliveira Neto et al. [28] have shown that the addition of Sn contributes to the formation of a more selective catalyst for ethanol oxidation. Using gas chromatography analysis, Song et al. [49] have also verified that the PtSn catalyst presents greater selectivity for the oxidation of this alcohol compared with Pt alone. Nevertheless, most of the products formed in the presence of PtSn contain CC bonds. Therefore, it is mandatory that the activity of the PtSn/C catalyst is enhanced by addition of a third element, so that the dehydrogenation reaction can be improved and the CC bond can be cleaved during ethanol oxidation. The addition of Ni as the second or third element is claimed to increase the activity of Pt electrocatalysts. The main advantage of the introduction of this metal is the reduction of the ethanol oxidation potential, coupled with the rise in current density. A literature search reveals that Sn and Ni can introduce electronic modifications into Pt [50,51], in other words, these metals decrease the energy of the chemisorption of ethanol and its oxidation intermediates, such as CO. The participation of the Sn site in a bifunctional mechanism is also claimed, in order to explain the enhanced activity of the Pt–Sn/C catalyst [50,52]. The effect that Ni introduction has on Sn-sites must also be considered. The addition of a third metal boosts the oxophilic character of the surface, thus raising the

434

15.  New Trends in Direct Ethanol Fuel Cells

strength of the SnO bond and contributing to increased acidity of the surface of the SnOH sites. This, in turn, can accentuate the bifunctional character of ethanol electrooxidation [53]. Ribadeneira and Hoyos [54] have evaluated the catalytic effect of Ni on binary PtSn/C and PtRu/C catalysts. To this end, four different compositions containing nickel were prepared, namely PtRuNi 75:15:10 and 75:10:15; and PtRuNi 75:10:15 and 75:15:10. The addition of Ni to the PtRu/C and PtSn/C catalysts significantly improves their catalytic activities with respect to ethanol electro-oxidation. Bonesi et al. [55] have studied PtSn/C, PtSnNi/C, and PtRuNi/C catalysts prepared by the PVP-Polyol route. The electrochemical characterization of these compositions by chronoamperometry revealed a higher current density for the composition PtSnNi/C, again demonstrating the beneficial effect of the addition of Ni. The role of Ru in the mechanism of ethanol electro-oxidation is similar to that of Sn. The adsorption and decomposition of ethanol and its intermediate reaction products happen on Pt active sites, while the dissociative adsorption of water occurs over Sn or Ru sites, to form oxygen-containing surface species. Antolini et al. [24] have shown that the Ru addition of Ru to PtSn catalysts can enhance the catalytic activity of a certain composition. However, this enhancement is related to the Ru/Sn ratio that is present in the alloy, as well as to the synergetic effect of Ru and Sn oxides. Cunha et al. [56] have investigated the effect of adding Ru to PtSn/C catalysts. The PtSnRu atomic ratios 80:10:10 and 90:05:05 yielded good performance in the electrochemical oxidation of ethanol. The onset of ethanol oxidation occurs near 0.2 V vs. RHE, but product analysis revealed that acetaldehyde was the major product. Pt-based materials containing W have furnished interesting results for alcohol oxidation. Ribeiro et al. [21] have studied the electro-catalytic oxidation of ethanol on PtSn/C, PtW/C, and PtSnW/C catalysts containing 40 wt.% metal loading. High catalytic activity was achieved in the case of the materials containing W. The composition PtSnW/C (85:8:7) was more active than the other investigated compositions. This activity was ascribed to the presence of Sn and W, which decrease the overpotential for the removal of the intermediate species generated during ethanol oxidation. In addition, Maillard et al. [57] have carried out a study of the tolerance of the Pt-WOx composite supported on carbon to CO. The characterization by X-ray absorption demonstrated that WOx promotes slight modification of the electronic surface of platinum, whereas the electrochemical characterization showed that species such as WOx act as Brønsted acids in the process of dehydrogenation and in the dissociation of water molecules, which assists the electro-oxidation process. The addition of Rh to Pt-based catalysts has also gained much attention from research groups seeking to improve ethanol oxidation. Sen Gupta and Datta [58] have reported that the activity of PtRh (90:10, 70:30, and 35:65) catalysts prepared on graphite strongly depends on Rh concentration. The presence of this metal at loadings above 30 wt.% positively contributes to ethanol oxidation, because Rh aids the cleavage of the CC bond. However, this metal in excess inhibits the active sites of platinum, because Rh strongly adsorbs oxygen species. Very small amounts of Rh, on the other hand, do not provide sufficient sites for CC bond cleavage. Using DEMS and in-situ FTIR studies, de Souza et al. [59] have monitored the catalytic activity of Pt, Rh, and PtRh prepared on gold. These authors showed that the Pt–Rh combination



435

15.1  Ethanol Electro-Oxidation in Acid Medium

significantly reduces the concentration of acetaldehyde and augments the concentration CO2 during ethanol electro-oxidation as compared to the activity of pure platinum. The researchers attributed this behavior to dehydrogenation, CC bond cleavage, and formation of Rh–O species, which act as an oxygen reservoir and promote the formation of the COO bond, with consequent CO2 release. Lima and Gonzalez [42] have investigated Pt–Ru, Pt–Rh, and Pt–Ru–Rh on carbon with high surface area, as prepared by the impregnation method. XANES characterization evidenced pronounced electronic modification of the Pt 5d-band in the presence of Rh and Ru. This elicited a lowering in the strong adsorption of the intermediate species generated during ethanol oxidation on Pt, thereby decreasing the onset of ethanol oxidation and increasing the oxidation reaction rate, mainly to CO. The presence of Rh prompted the formation of a larger quantity of acetic acid and CO2. Ethanol electro-oxidation by PtSn/C, PtRh/C, and PtSnRh/C catalysts containing 20 wt.% metal loading and prepared by the alcohol reduction method has been studied by Spinacé et al. [60]. These authors observed that the catalytic activity of the compositions PtRh/C 50:50 and 90:10 is lower than the activity of the catalyst PtSn/C with a composition of 50:50, and that the variation in the amount of Rh in the binary catalyst does not bring about a significant change in activity. However, when Rh is combined with the Sn in a ternary catalyst PtSnRh/C 50:40:10, there is an elevation in the catalytic activity, showing that the use of Rh as a third element improves the catalytic activity of ethanol oxidation. Electrochemical studies have been performed in acidic medium by Kadirgan et al. [61], who have evaluated the activity of Pt–Pd/C nanoparticles in the electro-oxidation of methanol and ethanol. Chronoamperometry investigation conducted at 0.45 V vs. ERH showed that Pt and Pd have a synergistic effect, which increases the catalytic activity of the catalyst two or threefold as compared to that of pure Pt (Pt/C Etek). This effect was attributed to a change in the electron density of platinum, responsible for the weakening of the PtCO bond and the consequent decrease in its poisoning rate. TABLE 15.1 Summary of PEM-DEFC Performance Catalyst

Method of Synthesis

Operating Conditions

Power (mW cm−2) Reference

Pt75Sn25

Formic acid

Po2 = 3 bar; [EtOH] = 1.0 mol L−1; T = 90 °C

20

[62]

Pt50Sn50

Polyol

Po2 = 2 bar; [EtOH] = 1.0 mol L−1; T = 90 °C

55

[43]

Pt80Sn20

Pechini

Po2 = 3 bar; [EtOH] = 2.0 mol L ; T = 90 °C

37

[48]

Pt50Ru50

Polyol

Po2 = 2 bar; [EtOH] = 1.0 mol L ; T = 90 °C

45

[43]

Pt52Ru48

Colloid

Po2 = 1 bar; [EtOH] = 2.0 mol L ; T = 80 °C

60

[53]

Pt50Ru50

Citric acid reduction

Po2 = 2 bar; [EtOH] = 2.0 mol L ; T = 90 °C

20

[63]

Pt85Sn8W7

Pechini

Po2 = 3 bar; [EtOH] = 2.0 mol L ; T = 90 °C

33

[21]

Pt86Sn10Ru4

Bönnemann

Po2 = 3 bar; [EtOH] = 2.0 mol L ; T = 80 °C

50

[25]

Po2 = 2 bar; [EtOH] = 2.0 mol L ; T = 100 °C

42

[60]

Po2 = 0.2 MPa; [EtOH] = 1.0 mol L−1; T = 90 °C 38

[64]

Pt50Sn40Rh10 Alcohol reduction Pt1Ru1W1

Ethylene glycol reduction

−1 −1 −1 −1 −1 −1 −1

436

15.  New Trends in Direct Ethanol Fuel Cells

The majority of the elements in the periodic table have been assessed as possible metals for an ideal nanoparticle distribution that could modify the structure of Pt. However, the metals described above exert a more significant effect when it comes to ethanol electrooxidation. Table 15.1 summarizes some results from PEM-DEFC.

15.2  ETHANOL ELECTRO-OXIDATION IN ALKALINE MEDIUM The operation principle of an anion exchange membrane direct ethanol fuel cell (AEM-DEFC) is illustrated in Figure 15.4. In the anode side a mixture of ethanol and NaOH aqueous solution is inserted as fuel, in the cathode, the inlet of humidified oxygen closes the system. Ethanol is oxidized to carbon dioxide and electrons are produced on the anode, while oxygen accepts electrons and is reduced on the cathode, to produce hydroxide ions. The latter are then transferred to the anode through the electrolyte. The equations are shown below [65]:

Anode : CH3 CH2 OH + 12OH− → 2CO2 + 9H2 O + 12e− , Cathode : 3O2 + 6H2 O + 12e− → 12OH− ,

E0 = 0.40 V,

Global equation : CH3 CH2 OH + 3O2 → 2CO2 + 3H2 O,

FIGURE 15.4  Schematic diagram of alkaline direct ethanol fuel cell.

E0 = −0.74 V,

E0 = 1.24 V.

(15.15) (15.16) (15.17)



15.2  Ethanol Electro-Oxidation in Alkaline Medium

437

From a thermodynamic viewpoint this system furnishes a gain of 100 mV compared with the PEM-DEFC operated in acidic medium. Electrocatalytic reactions carried out in alkaline medium are more efficient than those accomplished in acidic media [66–68]. However, only this advantage does not justify the immediate applicability of this system in fuel cell technology. Despite the high catalytic efficiency of ethanol oxidation in alkaline medium, carbonation of the electrolyte takes place in this environment, i.e., CO2 formed during the electro-oxidation reaction reacts with OH− present in the electrolyte support, generating CO32−. This species can precipitate carbonate salts on the electrode, thereby blocking the catalytic sites and decreasing the pH of the solution, which in turn deactivates the electrode over time [69]. Consequently, the initial high reactivity of the system is reduced as the fuel system is operated [70]. The stability of the exchange anionic membrane is another important issue. Many studies have been carried out on this topic, to find an efficient and stable membrane for use in direct alkaline ethanol fuel cell (DAEFC) [71]. The existing membranes are not stable during the long time of fuel cell operation. On an anion exchange membrane (AEM), the OH− is responsible for the ionic conduction in the system. The advantage is that the ionic flow is in the reverse direction, thereby inhibiting the ethanol crossover [72]. An Na+-modified Nafion® exchange membrane can also be useful in direct alkaline ethanol fuel cell (DAEFC) [73]. Two important features can be obtained with the use of this membrane. One is that hydroxide ions produced by oxygen reduction react rapidly with Na+ ions, leading to the formation of sodium hydroxide in the cathode side of the cell. The other is that Na+ ion transport through Nafion® offers much greater stability than anion ion conducting membranes. Nevertheless, the main disadvantage of the modified Na+-Nafion® membrane is its low ionic conductivity as compared to the unmodified or the anion conducting membranes [68]. Whereas there is a significant understanding of the phenomena taking place in the interface electrode surface/solution and of the reaction mechanism in the case of ethanol electro-oxidation in acid medium, the ethanol electro-oxidation in alkaline medium has been much less investigated due to some difficulties inherent to the utilization of alkaline fuel cells [74]. Investigation of the ethanol electro-oxidation mechanism in alkaline medium and its application in a direct ethanol fuel cell has gained significant attention recently. However, the reaction mechanism of ethanol electro-oxidation in alkaline medium is not clear yet. In the literature it is possible to find a mechanism proposal based on the products from the oxidation reaction [23,74,75]. Tremiliosi-Filho et al. [76] have investigated ethanol oxidation on gold electrodes in alkaline medium. By means of FTIR experiments, these researchers observed that acetic acid or acetate was the only product of the reaction. Some carbonates species were also detected, but at low concentrations. Yi et al. [77], Liang et al. [78], and Nguyen et al. [79] have studied ethanol electro-oxidation on palladium catalysts, and they have proposed a mechanism in accordance with the one previously put forward by Tremiliosi-Filho et al. [76] and Jiang et al. [23]. The proposed mechanism can be written as follows for platinum, gold, and palladium electrodes:

M + OH− → M − OHads + e− ,

(15.18)

M + CH3 CH2 OH → M − [CH3 CH2 OH]ads ,

(15.19)

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15.  New Trends in Direct Ethanol Fuel Cells

M − [CH3 CH2 OH]ads + 3OH− → M − [CH2 CO]ads + 3H2 O + 3e− ,

(15.20)

M − [CH2 CO]ads + M − OHads → M − CH3 COOH + M,

(15.21)

M − CH3 COOH + OH− → M + CH3 COO− + H2 O,

(15.22)

M = Pt, Pd, or Au. The rate-determining step is step 21, in which the adsorbed ethoxy intermediate is removed by the adsorbed hydroxyl ions, to form acetate [79]. Liang et al. [78] have also studied the influence of ethanol and KOH concentration on the catalytic behavior of the Pt catalyst for ethanol concentrations ranging from 0.25 to 4.0 mol L−1. The oxidation current increased with rising ethanol concentrations at potentials below −0.2 V vs. Hg/HgO. However, at higher potentials (e.g., −0.1 V vs. Hg/HgO), the first peak current decreased when the concentration of ethanol exceeded 3.0 mol L−1. The elevation in current with ethanol concentration below −0.2 V vs. Hg/HgO can be explained by observing Eqs. (15.19) and (15.20), which suggest that adsorption of ethoxy onto the Pd electrode can be accelerated with an elevation in ethanol concentration, which culminates in larger coverage of adsorbed ethoxy (θCH3COads). This finding was confirmed by the authors by means of FTIR data. Fang et al. have investigated the influence of pH on the mechanism of ethanol electrooxidation on a palladium electrode [80]. Cyclic voltammetry and in situ Fourier Transform Infrared (FTIR) spectroelectrochemistry were used for identification of the oxidation products at different NaOH concentrations. The activity for ethanol oxidation on Pd was largely affected by pH as well as by the resulting product. Sodium acetate was the main product for NaOH concentrations higher than 0.5 M. Nevertheless, CO2 was identified as the pH was lowered (below 13).

15.2.1  Catalysts for Ethanol Oxidation in Alkaline Medium An interesting advantage to using alkaline electrolytes is that the oxidation of alcohol in this medium does not significantly depend on the structure of the catalyst [81], which opens up the opportunity of using non-precious metals, such as Pd, Ag, Ni, and perovskite-type oxides [47,79,82–84]. The latter are significantly cheaper than Pt-based catalysts employed in acid fuel cells. However, when it comes to alkaline media, the use of non-precious metals becomes reality. All the Pt-based catalysts described in acidic medium can be applied to ethanol electrooxidation in alkaline medium, too. Nevertheless, Pd is a metal that has displayed good performance in basic medium, mainly due to its low tolerance to CO poisoning in the Pdsites, thereby favoring the rapid oxidation of this molecule to CO2. Because the properties of Pd are similar to those of Pt, some literature works have been devoted to the substitution of Pt with Pd during the preparation of electrocatalysts. Shen and Xu [45–47] have studied the oxidation of alcohols such as methanol, ethanol, glycerol, and ethylene glycol on



439

15.2  Ethanol Electro-Oxidation in Alkaline Medium

Pd-based catalysts containing Ce, Ni, Mn, or Co oxides. Results showed an excellent activity for the electro-oxidation of different alcohols whose oxidation started close to −0.6 V vs. Hg/HgO. The Pd–NiO/C catalyst gave the highest catalytic activity and the lowest rate of poisoning by intermediates. These same authors also compared the activity of metal oxides supported on Pt and Pd in alkaline medium, and the activity was higher in the case of oxides-Pd/C. Xu et al. [46] have investigated Pd-based catalysts as a replacement for Pt-based catalysts for ethanol electro-oxidation in alkaline medium. The results showed that Pd/C has higher catalytic activity and better steady-state behavior for ethanol oxidation than Pt/C. These authors also evaluated the effect of adding CeO2 and NiO to the Pt/C and Pd/C electrocatalysts, and the addition of the oxides significantly improved the catalytic activity of these materials for ethanol electro-oxidation due to the more facile formation of OHads species on the oxide surface. The generation of OHads species at lower potential can transform COlike poisoning species on Pt and Pd to CO2 or other products, which could be dissolved in water, thereby setting the active sites on Pt and Pd for further electrochemical reaction. Other works using Pd-based catalysts can be found in the literature [85–88], and all of them describe the same result: Pd-based catalysts are more active than Pt-based catalysts in alkaline medium. Hassan and Hamid [89] have studied ethanol electro-oxidation on electrodeposited Ni–Cr2O3 nanocomposites. These nanocomposites presented larger surface area, higher catalytic activity, and higher stability regarding the electrochemical ethanol oxidation. The use of non-noble metal catalysts such as Ru–Ni–Ti for ethanol oxidation in alkaline medium has been examined by Kim and Park [90,91]. Anodic peak currents for ethanol oxidation increased with rising ethanol concentration during the forward potential sweeps, whereas the cathodic peak currents reduced at these electrodes. This suggests that these TABLE 15.2 Summary of AEM-DEFC Performance Catalyst

Method of Synthesis Operating Conditions 3

Power (mW cm−2)

Reference

PtRu

–

Jo2 = 100 cm  min ; EtOH/NaOH = 1.0/ 0.5 mol L−1; T = Room temperature

58

[93]

Pd

NaBH4 reduction

Jo2 = 100 cm3 min−1; EtOH/NaOH = 3.0/5.0 67 mol L−1; T = 60 °C

[94]

Pd3Ni2

NaBH4 reduction

Jo2 = 100 cm3 min−1; EtOH/ NaOH = 3.0/5.0 mol L−1; T = 60 °C

90

[94]

Pd

THF/H2

Jo2 = 200 cm3 min−1; EtOH/NaOH = 10% wt./2.0 mol L−1; T = 80 °C

∼70

[95]

Pd-(Ni-Zn)

Spontaneous deposition

Jo2 = 200 cm3 min−1; EtOH/NaOH = 10% wt./2.0 mol L−1; T = 20 °C

55

[96]

Pd-(Ni-Zn)

Spontaneous deposition

Jo2 = 200 cm3 min−1; EtOH/NaOH = 10% wt./2.0 mol L−1; T = 80 °C

170

[96]

RuV

H2 reduction 430 °C

Jo2 = 200 cm3 min−1; EtOH/ NaOH = 2.0/3.0 mol L−1; T = 80 °C

125

[65]

−1

440

15.  New Trends in Direct Ethanol Fuel Cells

electrodes act as effective electron transfer mediators. The presence of ruthenium redox couples such as perruthenate (Ru(VII)/ruthenate (Ru(VI)) render excellent reversibility and stability, and these couples act as heterogeneous electron transfer mediators [92]. The most efficient oxidation was observed at the RuO2-modified Ni electrode. Nickel was found to facilitate ethanol electro-oxidation at the thermally prepared RuO2-modified Ni electrode in alkaline medium. Table 15.2 summarizes the latest data obtained for AEM-DEFC. As stated in Table 15.2, the better efficiency of the catalyst in basic medium reflects higher power density as compared to DEFC operating in acidic medium.

15.3  ALKALINE-ACID DIRECT ETHANOL FUEL CELL As described before in this chapter, conventional DEFCs can be divided into two types as a function of the employed membrane, namely proton exchange membrane DEFCs (PEMDEFCs) and anion exchange membrane DEFCs (AEM-DEFCs), used in acidic and alkaline medium, respectively. As previously reported, Pt-based catalysts undergo rapid poisoning of the catalytic sites, which compromises cell performance. On the other hand, the kinetics of both ethanol oxidation (OER) and the oxygen reduction reaction (ORR) in alkaline medium are much faster than the corresponding kinetics in acidic medium, which substantially improves cell performance. The main limitation to the cell performance in AEM-DEFCs is the physical and chemical stability of the AEM [71]. Another problem encountered with the AEM is that its ionic conductivity is about one order of magnitude lower than that of Nafion membranes. Regarding these operating problems, An et al. [97,98] have recently reported a new approach for the development of direct alcohol fuel cells. Initially, the operating problems in PEM-DEFC and AEM-DEFC were not only related to the limitation of this kind of cell, but also to the thermodynamic theoretical voltage of oxygen reduction. To improve the theoretical voltage of the cell, these researchers made use of a new tendency in terms of oxygen source supply: the use of hydrogen peroxide (H2O2). This is advantageous because there is allowance for the operation of the fuel cell in the absence of oxygen, not to mention the low activation loss of the reduction reaction due to two-electron transfer and the absence of issues regarding water flooding [99]. The new type of direct ethanol fuel cell described by An et al. [97] is composed of an alkaline anode and an acid cathode separated by a conducting charger (see Figure 15.5). The theoretical voltage of this system can be evaluated by means of the equations described in the paragraphs below. In basic medium (NaOH), considering that total oxidation takes place, ethanol is firstly oxidized to CO2 in the anode compartment, as shown in Eq. (15.23):

CH3 CH2 OH + 12OH− → 2CO2 + 9H2 O + 12e− ,

E0 = −0.74 V.

(15.23)

On the cathode, hydrogen peroxide (H2O2) is decomposed into water in the presence of H2SO4, according to Eq. (15.24).

6H2 O2 + 12H+ + 12e− → 12H2 O,

E0 = 1.78 V.

(15.24)



15.4 Search for Efficient Catalysts

441

FIGURE 15.5  Schematic diagram of alkaline-acid direct ethanol alkaline fuel cell.

As a result, the overall reaction of this new cell can be obtained by combining Eqs. (15.23) and (15.24), i.e.,

CH3 CH2 OH + 6H2 O2 → 2CO2 + 9H2 O,

E0 = 2.52 V.

(15.25)

The total voltage of this type of cell can reach a value of 2.52 V. Compared with 1.14 V and 1.24 V obtained for the conventional PEM-DEFC and the AEM-DEFC, respectively, the theoretical voltage increases significantly. The authors claimed that a simple modification to the Nafion® 117 membrane must be made in other to stabilize the membrane in both supports [98]. The resulting power density is much higher, i.e., 240 mW cm−2 at 60 °C, as compared to conventional PEM-DEFCs. The authors also highlighted the stability under constant current density during the 5 h of continuous operation [97]. Nevertheless, this approach needs to be further improved, so that this method can be more widely employed. Its main features are: improvement in the actual OCV (1.60 V), which is still much lower than the theoretical value (2.52 V); diminished ethanol crossover; and inhibition of hydrogen peroxide decomposition in the Pt catalyst site. This innovation must also overcome the drawback of replacing an easily accessible and inexpensive oxygen source such as atmospheric oxygen.

15.4  SEARCH FOR EFFICIENT CATALYSTS Researchers have looked not only for novel catalysts with different chemical compositions but also for new preparation routes. The use of different methodologies for the synthesis of metal nanoparticles supported on high surface area carbon or other conducting supports plays a crucial role in the development of fuel cell technology [26,55,100]. Many research groups have described that the activity of these materials is highly dependent on their composition, morphology, and size [101,102]. Thus, several methods for the synthesis of nanostructured

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15.  New Trends in Direct Ethanol Fuel Cells

catalysts have been developed, aiming to obtain materials with homogeneous metallic distribution, small particle size, and high catalytic activity [62,100,103,104]. The use of a reducing agent such as sodium borohydride is convenient and, in combination with other strategies, allows for good control of nanoparticle size and composition. Dong-Ha Lim et al. [100] have synthesized PtSn/C catalysts by this methodology, and they obtained particles with sizes ranging from 1.0 to 4.0 nm, as well as catalytic activity in acidic medium of 14–56 mA mgPt−1 at 0.6 V vs. NHE for ethanol oxidation. Godoi et al. [44] have prepared PtSn/C catalysts by the microemulsion method using sodium borohydride to reduce the nanoparticles. These authors claimed that the reduction of metals trapped in the micelles enabled the achievement of well-distributed particles with sizes around 2.5 nm. However, their catalytic activity for ethanol electro-oxidation in aqueous medium (0.5 mol L−1) was less than 0.5 mA cm−2. The alcohol-reduction method, also known as the polyol method, has been developed by Toshima and Yonezawa [105] with a view to obtaining colloidal dispersions of nanoparticles with uniform particle size and homogeneous distribution. In this method, the reflux of an alcohol solution, usually ethylene glycol, containing the metallic ion and a stabilizing agent, normally a polymer, provides homogeneous colloidal dispersions of the corresponding metal nanoparticles. The alcohol simultaneously acts as a solvent and as a reducing agent [62]. This method offers some advantages such as good reproducibility, satisfactory distribution, and small particle size. Moreover, the polyol method can be easily modified by addition of other reagents such as steric or electrostatic stabilizers, thus enabling better control of nanoparticle size and distribution on the carbon support. Oliveira Neto et al. [28] have investigated a series of PtRu/C, PtSn/C, and PtSnRu/C catalysts prepared by the alcohol-reduction method using an ethylene glycol/water solution. Particle sizes in the order of 2.7 nm were achieved for the PtSn/C composition, which displayed catalytic activity close to 8.0 A gPt−1. Spinacé et al. [60] have used the same method for the production of PtSn/C, PtRh, and PtSnRh/C catalysts. Their results were similar to those described in [28], i.e., small particle size (2.0 nm), and uniform particle distribution on the carbon support, which culminated in significant catalytic activity for ethanol electro-oxidation. Another frequently employed route is the reduction of formic acid developed with the purpose of obtaining dispersed platinum catalysts supported on carbon (Pt/C) for PEMFC fuel cells [106]. In this methodology, catalysts are formed by addition of high surface area carbon support (Vulcan XC-72, Cabot) to a solution of formic acid, used as reducing agent. The resulting mixture is heated to 80 °C, followed by addition of aliquots of a solution containing platinum and other metal salts to the reaction vessel. After complete metal reduction, the nanocatalyst is filtered, dried, and ground. Colmati and co-workers [106] have examined how experimental conditions affect Pt75Sn25/C catalysts prepared by the formic acid method. Prior to heat treatment, particle sizes of 4.5 nm and experimental compositions close to the nominal one were achieved, but these particles presented low power density, i.e., 20 mW cm−2, for ethanol oxidation in a PEMDEFC. Antolini et al. [24] have looked into the effect of introducing ruthenium into PtSn/C catalysts. Again, the desired catalytic composition and homogeneously distribution small particles (3.5 nm) on the carbon support were obtained, but the power density reported for DEFC was relatively low (28 mW cm−2) for a PEM-DEFC. An important method that has been extensively investigated is the Thermal Decomposition of Polymeric Precursors (DPP) [20–22,48], which is based on the Pechini method [107].



15.4 Search for Efficient Catalysts

443

DPP was initially developed for the preparation of thin films and was later adapted for the preparation of materials in the powder form. DPP consists of dissolving metal salts in the presence of a hydroxyl acid (citric acid) and a polyhydroxylic alcohol (ethylene glycol), to obtain a solution with good metal distribution consisting of a resin of the precursor metal. To obtain the catalyst, each metal is stoichiometrically mixed with carbon Vulcan XC-72 or any other support and fired at high temperatures; e.g., 300 °C–400 °C. The major advantage of this route is the attainment of robust catalysts with experimental composition close to the nominal one. This method has been employed in the synthesis of a series of PtSn/C materials [19–21,27]. Nevertheless, proper control of particle morphology and metal distribution must be improved, in order to obtain better catalytic activity for ethanol electro-oxidation, which −1 is currently under 1.0 A gpt . Another method that has gained importance due to its operational simplicity, efficiency, and reduced preparation time is the microwave-assisted heating method (MW) [61,104,108]. Regarding the formation of nanoparticles, heating rate, and uniformity, this method promotes rapid reduction of the metallic precursors and is responsible for the achievement of nanometric particle size [109]. However, for this route to be applied to the synthesis of nanomaterials some criteria must be met: the employed reducing agents/solvent must exhibit high reduction rate, high capacity for conversion of electromagnetic energy into thermal energy, and a suitable temperature profile for reduction of all the metal ions. The efficiency of reducing agents in stabilizing the nanoparticles and preventing the formation of metal clusters is related to the oxidizing species that is generated during the heating process [104]. The oxidation of small-chain alcohols occurs through interaction between the metallic ions and the hydroxyl group(s) present in these molecules. There is subsequent reduction to metal particles and formation of carbonyl and/or carboxyl species, which adsorb onto the metal particle surface. As a result, the metal surface is stabilized and agglomeration is prevented, which culminates in the production of small particles [110].

15.4.1  Combinatorial Optimization As discussed before, there is a large number of materials and preparation methods that have already been studied for ethanol electro-oxidation since the beginning of the development of fuel cells. It is possible to find a large library of materials with different compositions and metal ratios in the literature. With this library and the understanding about the electronic and bifunctional effects, a new direction for the development of new active materials for ethanol electro-oxidation is being currently proposed: the combinatorial optimization of the composition of Pt-based catalysts. Due to the growing demand for functional materials that can be employed in modern science and technology, the combinatorial method has received growing attention since the late 1990s [111,112]. Combinatorial screening allows for the rapid screening of electrocatalysts, particularly in the case of materials that can be used as anodic or cathodic catalysts of low-temperature polymer electrolyte membrane fuel cells (PEM-FC). The simultaneous synthesis and screening of a large number of potentially active catalysts is a very attractive approach for the attainment of an ideal composition corresponding to the best catalyst to be employed in the PEM-DEFC. The combinatorial synthetic methods enable analysis of large number of substances at a more rapid pace and in a cheaper way as compared to traditional synthetic chemistry.

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15.  New Trends in Direct Ethanol Fuel Cells

A variety of instruments and methods have been proposed for the generation and screening of these libraries of materials, including fluorescence spectroscopy [113–115], mass spectrometry [116], high performance liquid chromatography HPLC/mass spectrometry [117], and Fourier transform infrared (FTIR) spectrometry [118]. However, in situ fluorescence has been highlighted in the context of the mapping of these materials, due to their easy handling in aqueous medium and in the presence of the target fuel. This technique is based on the sensitivity of a proton-sensitive dye to the local pH, since the dye fluoresces upon pH changes in a certain range [119]. According to Eq. (15.1), when ethanol is oxidized in the interface catalyst/solution, in acidic medium, there is a drop in the local pH. This results in an increase in the local concentration of protons (H+) which react with the proton-sensitive dye (quinine, for example), making it fluoresce. As a consequence, regions on the surface of the library with higher catalytic activity than other regions can be distinguished (Figure 15.6). After the scanning, the components of the hotspots can be characterized by X-ray diffraction (XRD), photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM), thus providing precise information on the composition, structure, and morphology of the highlighted compositions. Guerin et al. [119] have stated that the principal limitation of the fluorescence method is the relative low sensitivity (signal-to-noise) of the optical response as compared to the direct measurement of the electrode current. The coupling between electrochemical measurement and different techniques that allows for superficial screening, the so-called electrochemical combinatorial method, has played an important role in the search for new catalysts with application in fuel cells [120–126].

FIGURE 15.6  Schematic catalyst library screening by fluorescence. (a) Ethanol electro-oxidation reaction. (b) pH-dependent Quinine fluorescence. (c) Screening of catalyst library film during ethanol oxidation in ethanol/ quinine solution monitored by a UV Vis lamp.



15.4 Search for Efficient Catalysts

445

15.4.1.1  Synthesis of Catalyst Library As discussed before, there are many ways of producing the catalysts that are to be investigated in the combinatorial synthesis. Although this subject has been presented by many researchers, a brief discussion on the most employed ones will be provided here. The aqueous combustion synthesis (CS) is an attractive technique for the production of different oxides [127]. CS involves a self-sustained reaction between an oxidizer (e.g., metal nitrate) and a fuel (e.g., glycine, hydrazine). The nanoparticles are generally achieved by using fast heating (up to 104 Cs−1) to values as high as 1500 °C. The main disadvantage of this method is that, the different compositions have to be prepared one by one. Only when all compositions are ready they are placed onto a conductive support in different spots, and screening is conducted. Many reduction methods can be employed for production of the desired library. For instance, metal reduction by H2 atmosphere is largely utilized for the synthesis of catalysts and can be applied in the preparation of the catalyst library. An example of this approach can be obtained elsewhere in the literature [114]. Briefly, small amounts of the metal precursor solution are deposited at defined stoichiometric proportions onto a sheet of conductive support (carbon paper, carbon cloth, etc.) (Figure 15.7) and are reduced to the corresponding metal under hydrogen atmosphere at high temperature. The arrays can be prepared manually, by using micropipette [114] or an ink jet printer [115]. Some authors have used the chemical reduction with NaBH4 [115,128]. The main difference is that in this technique aqueous solutions of metal salts are dispersed in different regions onto a Teflon-coated Toray carbon sheet or any other conductive support, using manually prepared solution mixtures or an ink jet printer. The completed array is reduced with sodium borohydride. Finally, the reduced array is thoroughly washed with pure water. Another way to achieve a wide variety of the metallic compositions is sputtering deposition, where a set of ion beams hits a target surface. Each sputter gun holds targets of a specific element of interest. This process, called sputter deposition, is the primary method for the production of thin films [129]. Details of the preparation can be obtained elsewhere [130].

FIGURE 15.7  Schematic catalyst library array prepared by reduction with NaBH4.

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15.  New Trends in Direct Ethanol Fuel Cells

FIGURE 15.8  Active area/compositions for electro-oxidation of ethanol.

The literature contains many works related to combinatorial investigation for methanol electro-oxidation; however, there is lack of investigation involving ethanol oxidation. For instance, one of the first applications of the combinatorial method was proposed for methanol electrooxidation in 1998 by Reddington et al. [115]. These authors studied methanol electro-oxidation on 645-membered array electrodes containing five metals, namely platinum, ruthenium, osmium, iridium, and rhodium, as well as 80 binary, 280 ternary, and 280 quaternary combinations. Recently, Chu and Shul [128] have applied combinatorial chemistry to the screening of 66 PtRuSn-anode arrays for investigation of methanol, ethanol, and 2-propanol oxidation. The screening was performed by employing quinine as indicator of the catalytic activity, which allowed for selection of the optimum composition of electrocatalysts for DAFCs (Direct Alcohol Fuel Cells). PtRuSn (80:20:0), PtRuSn (50:0:50), and PtRuSn (50:30:20) furnished the lowest onset potential for methanol, ethanol, and 2-propanol electro-oxidation according to the CV results, respectively. The active area/composition for ethanol electro-oxidation is represented in Figure 15.8 as adapted from Ref. [128].

15.4.2  Ethanol Oxidation on Amorphous Alloys Amorphous alloys, or metallic glasses, are stronger, harder, and more corrosion-resistant than conventional metal alloys. They are highly versatile and can be tailored for the formation of nanocomposite materials with superior hardness and toughness. Certain amorphous alloys with special composition and structure possess very high catalytic or electrocatalytic activity, which is superior to that of their crystalline counterparts or other conventional catalysts [131]. These materials have attracted attention due to their application as catalysts for fuel cells [132]. In general, the amorphous state is more active than the crystalline one, due to the presence of a denser active surface area, due to its homogeneity. The necessary amount of platinum in the amorphous state is reduced, so the overall cost of the catalysts is lowered. The main advantage of this kind of material is related to the possibility of changing the nature of the electrode by alloying different catalytic elements that will decorate a conducting matrix [65].



15.4 Search for Efficient Catalysts

447

Initial studies on electrocatalysis for fuel cell application using amorphous alloys were related to the oxygen evolution reaction (OER) [131,132]. Ni–based amorphous alloys have a significant contribution to the development of materials for application in fuel cells due to their high stability in alkaline medium, relative low overpotential, low cost, and high corrosion resistance. The synthesis of the alloys is relatively simple. The catalyst can be obtained via the mechanical alloy technique, where the metallic elements are placed in a planetary ball mill for several hours, at the desired ratio. An important observation is that the as-synthesized catalyst does not display significant catalytic activity as compared to the crystalline material. An acid treatment with HF or HF-HNO3 is required for activation of the material surface [133,134]. Although ethanol electro-oxidation has been studied in the past [135,136], only recently has this subject gained the attention of researchers, particularly of a specific research group in Spain that has been conducting studies on ethanol electro-oxidation on NiNb-based amorphous alloys [137]. Blanco et al. [138] have studied the catalytic activities of the Ni59Nb40Pt0.6Pd0.4 and Ni59Nb40Pt0.6Rh0.4 bi-catalysts and of the Ni59Nb40Pt0.6Rh0.2Ru0.2 tri-catalyst alloys in the electrooxidation of ethanol and CO via cyclic voltammetry, CO striping, and chronoamperometry in acidic media. The results showed that the amorphous alloy was obtained after 40 h in a planetary ball mill, and that the material was stable up to 662 °C. Above this temperature, the material started to crystallize. The bi-catalysts displayed similar behavior for ethanol electro-oxidation and were more effective as compared to the tri-metallic alloy. All the compositions gave acetaldehyde and acetic acid peaks in the cyclic voltammogram. The current density toward ethanol oxidation decreased with the presence of Ru, although the tri-metallic electrode was the most tolerant to CO, with lower surface coverage as compared to the other materials that were studied. The co-catalytic effect of Ru and Rh on NbNi-based catalysts with respect to the electrooxidation of ethanol and CO has also been investigated [137]. The lower amount of Pt in the compositions aims to reduce the cost of the catalyst. The Ni59Nb40Pt0.6Rh0.4 and Ni59Nb40Pt0.6 Rh0.2Ru0.2 alloys were obtained with a planetary ball mill, as described before [137], and these amorphous alloys contained only 0.6% at. platinum. The electrochemical results showed that the addition of the co-catalysts reduced the amount of platinum in the electrode and improved the tolerance with respect to CO electro-oxidation. Ni59Nb40Pt0.6Rh0.4 gave the best catalytic activity for use in DEFCs, since this material afforded greater electrical efficiency for ethanol electro-oxidation as well as higher current density values. However, the Ni59Nb40Pt0.6Rh0.2Ru0.2 catalyst provided increased CO tolerance, due to the bifunctional effect of ruthenium. As a result, the current densities toward ethanol oxidation were significantly lower as compared to the other studied catalysts. Barroso et al. [139] have examined the activity of NiNb-based materials with respect to the acetic acid decarboxylation reaction. They showed that these materials can be applied in direct ethanol fuel cells. Ni59Nb40Pt0.6Cu0.4 and Ni59Nb40Pt0.6Ru0.4 were synthesized by the same method described above. In situ FTIR reflectance was used for monitoring of the products formed during ethanol oxidation and for understanding the oxidation mechanism in amorphous alloys. Electrochemical results showed that the addition of co-catalysts decreased the amount of platinum in the electrocatalyts and improved the reaction toward the electrooxidation of ethanol and CO. Ni59Nb40Pt0.6Cu0.4 afforded the best current densities, probably because of its irregular electronic configuration (3d104s1). The FTIR data aided understanding

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15.  New Trends in Direct Ethanol Fuel Cells

and enabled the proposal of a possible mechanism for ethanol oxidation on NiNb-based amorphous alloys as described in Figure 15.9. The conclusions from the FTIR studies were that CO2 production did not come only from the CO oxidation path. The FTIR experiments led to the identification of CO2 from acetic acid decarboxylation. The obtained spectra revealed that the Ni59Nb40Pt0.6Cu0.4 alloy favored the acetic acid oxidation at lower potentials than the alloy containing Ru.

FIGURE 15.9  General scheme for ethanol electro-oxidation on amorphous alloys (reproduced from Ref. [139]).



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Wang et al. [140] have studied amorphous CoSn/C alloys decorated with Pt as highly efficient electrocatalysts for ethanol oxidation. The catalysts were prepared using a two-stage chemical synthesis (sol-gel preparation and Steady-state replacement method). XRD results evidenced that the CoSn-base was in the amorphous state, but the characteristic peaks of the Pt fcc crystalline structure appeared after Pt deposition. The TEM images confirmed that Pt was deposited onto CoSn/C. The electrochemical measurements showed that the mass activity of the Pt–CoSn/C catalyst was 454.6 mA mgPt−1, which was about 1.71 and 1.74 times those of Pt/C and PtSn/C. The authors attributed this behavior to the possible modification in the Pt electronic structure elicited by the amorphous CoSn alloy. Materials based on amorphous alloys can open new perspectives regarding ethanol electro-oxidation.

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