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Investigation of electrochemical oxidation of methanol in a cyclone flow cell
Electrochimica Acta 49 (2004) 2179–2187
Investigation of electrochemical oxidation of methanol in a cyclone flow cell Tanja Vidakovi´c a,1 , Mihai Christov a,2 , Kai Sundmacher a,b,∗ b
a Max-Planck-Institut für Dynamik komplexer technischer Systeme, Sandtorstraße 1, D-39106 Magdeburg, Germany Process Systems Engineering, Otto-von-Guericke University Magdeburg, Universitätsplatz 2, D-39106 Magdeburg, Germany
Received 13 October 2003; received in revised form 22 December 2003; accepted 30 December 2003
Abstract Electrochemical oxidation of methanol on carbon supported Pt/Ru gas diffusion electrodes was investigated in a cyclone flow cell at room temperature using chronoamperometry, cyclic voltammetry and electrochemical impedance spectroscopy. The influence of the flow rate was checked. It was proved that the cyclone cell is suitable for the investigation of methanol electrooxidation and provides additional information on the mass transfer limitations in the electrode assembly. Chronoamperometric measurements showed slow, but constant current decay at all investigated potentials. Impedance measurements in water and methanol containing solutions were performed and the experimental data were fitted to an appropriate equivalent circuit. © 2004 Elsevier Ltd. All rights reserved. Keywords: Methanol oxidation; Cyclone flow cell; Membrane electrode assembly; Carbon supported Pt/Ru catalyst; Electrochemical impedance spectroscopy
1. Introduction In order to increase the efficiency of the direct methanol fuel cell (DMFC) for its practical application it is necessary to improve the performance of the anode side of the DMFC, i.e. the methanol oxidation reaction. Several reasons can be responsible for the poor anode performance in DMFC: first, the slow methanol oxidation kinetics (the thermodynamic equilibrium potential for methanol oxidation is close to the reversible potential of hydrogen oxidation, but the kinetics in the former case is the limiting factor [1]), second the internal Ohmic resistance within the electrode structure, and finally the membrane resistance as well as the interfacial resistance at the contact area between the polymer membrane (protonic conductor) and the catalyst layer. The first reason is in the domain of electrocatalysis and the current state of
∗ Corresponding author. Tel.: +49-391-6110-350; fax: +49-391-6110-353. E-mail address: [email protected] (K. Sundmacher). 1 On leave from Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia and Montenegro. 2 Permanent address: Department of Physical Chemistry, University of Chemical Technology and Metallurgy, 1756 Sofia, Bulgaria.
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.12.047
the art is based on the Pt/Ru system, an electrocatalyst first suggested three decades ago [2]. Pt/Ru can be either carbon supported or unsupported where—according to the theory of bifunctional catalysis—Pt is responsible for methanol adsorption and Ru for the donation of OH species [3]. The internal Ohmic resistance is a combination of the electrolyte resistance within the electrode, the electronic resistance of the electrode material and the contact resistance between the electrode’s components [4]. The study of the proton conductivity of Nafion 117 has shown that its impedance is constant over the frequency range from 1 Hz to 10 kHz and hence has behaviour similar to conventional liquid electrolytes [5]. The resistance at the contact layer between the catalyst particles and the polymer membrane can be influenced by the preparation procedure (pressure, temperature, amount of added Nafion). A more intimate contact between the catalyst particles with both, the back diffusion layer (it serves as well as a current collector) and the polymer membrane (as a proton conductance medium) will decrease the influence of these resistances on the performance of the overall membrane electrode assembly (MEA) [6]. A special type of electrochemical cell—a cyclone flow cell was designed in order to study the kinetics of electrochemical reactions at MEAs [7]. It gives the possibility to investigate the MEA under technically relevant conditions:
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the reactants are supplied through the back diffusion layer, like in a fuel cell, and in contrast to standard half-cell measurements the reaction occurs at the contact layer between the catalyst particles and the polymer membrane (Nafion) without the influence of anion adsorption from the supporting electrolyte; it enables potentiostatic control during the measurements (half-cell measurements using a reference electrode with a stable and well defined potential, in contrast to typical fuel cell set-up, where the influence of the cathode cannot be excluded) and, last but not least, the hydrodynamic conditions in the liquid phase are well defined by a vortex flow with the thickness of the external diffusion layer being similar to the diffusion layer at a rotating disk electrode (RDE) [7]. The aim of this work was to test if the cyclone flow cell is a suitable tool for the investigation of methanol electrooxidation under fuel cell relevant conditions, by means of cyclic voltammetry (electrochemical characterisation of catalyst and reaction), chronoamperometry (investigation of long term stability of the catalyst) and especially by electrochemical impedance spectroscopy (suitable for the determination of Ohmic and charge transfer resistances [4,8,9]).
2. Experimental 2.1. Preparation of MEA All measurements were performed in a cyclone flow cell as depicted in Fig. 1. The working electrode compartment (1) was supplied with a methanol/water mixture (1.0 M methanol), while the counter electrode (2) and the reference electrode compartments with 0.1 M sulphuric acid solution (in some experiments, 0.5 or 1.0 M sulphuric acid solution). The catalyst was 20 wt.% Pt, 10 wt.% Ru supported on Vulcan XC-72 with a Pt:Ru atomic ratio of 1:1 and a total metal loading of 1.0 mg cm−2 . The back diffusion layer was a Teflonised carbon cloth. The gas diffusion electrodes (catalyst and back diffusion layer) were obtained from ElectroChem, Inc., Woburn, MA. The MEA was prepared by hot pressing of the gas diffusion electrode at 130 ◦ C at a pressure of 10 MPa during 3 min onto a Nafion 105 membrane (details about membrane pre-treatment and MEA preparation are given in [10]). Before pressing, the catalyst layer was sprayed with a proper amount of Nafion solution in 2-propanol so that the total amount of Nafion on the catalyst layer was 15 wt.% of the total metal loading on the surface. The geometric area of the MEA was 2.0 cm2 . Both methanol/water and sulphuric acid containers were deaerated with nitrogen and the deaerated solutions were circulated through the cell. Electrolyte solutions were prepared from sulphuric acid (Merck, extra pure), methanol (Merck, extra pure) and ultrapure water (Millipore, 18 M cm). All potentials were measured and reported versus a saturated silver/silver chloride reference electrode. All experiments were performed at room temperature 22 ± 0.5 ◦ C.
Fig. 1. Schematic representation of (a) experimental set-up: (1) working electrode compartment, (2) counter electrode compartment; (b) membrane electrode assembly.
2.2. Electrochemical measurements All electrochemical measurements were carried out with a Zahner impedance measurement unit (IM6e). Preconditioning of the electrode was done by cyclic voltammetry in the potential range from −0.2 to 0.5 V versus Ag/AgCl at a sweep rate of 20 mV s−1 . Five cycles were enough to obtain reproducible electrode behaviour. Normally, only the first cycle differs from the subsequent cycles. This procedure was applied in all steady state, quasi steady state and chronoamperometry experiments. Steady state experiments were performed potentiostatically with a fixed delay of 5 min at each potential, in the potential range from 0 to 0.5 V versus Ag/AgCl. The potential step was 25 mV. Quasi steady state measurements were performed in the same manner as the steady state measurements but at the sweep rate of 1 mV s−1 . Chronoamperometry. After the preconditioning procedure described above the potential was stopped at the desired set point and the current was recorded over the time (1 h for measurements in water and 2 h in methanol containing solution). Ohmic drop compensation was done during all these measurements using Ohmic resistance values estimated from prior impedance measurements.
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Impedance measurements were performed immediately after the chronoamperometric measurement at the same dc potential, over a frequency range between 1.12 mHz and 870 kHz, moving from high to low frequencies and, immediately after that, in the opposite direction. The amplitude of the sinusoidal signal was 5 mV (from base to peak).
3. Results and discussion 3.1. Cyclic voltammetry Fig. 2 shows cyclic voltammograms obtained at a MEA with 30% PtRu on Vulcan XC-72 in the absence of methanol in the working electrode compartment at the sweep rate of 20 mV s−1 in quiet electrolyte and at different flow rates. The anodic limit is chosen in order to prevent Ru dissolution [11,12]. The hydrogen adsorption desorption region on the Pt/Ru MEA is not very pronounced, which is well known from the literature for this kind of catalyst [13]. The general shape of the cyclic voltammogram resembles the cyclic voltammetry behaviour known for this kind of catalyst from the literature [3,12,13]. Some small influence of the flow rate is observed only in the hydrogen adsorption/desorption region, but almost within the experimental error. 3.2. Steady state and quasi-steady state measurements Tafel plots for methanol oxidation in quiet electrolyte and at a flow rate of 30 l h−1 are shown in Fig. 3. These data were collected from steady-state experiments. It should be noted that real steady state conditions could not be reached in reasonable time periods. This is discussed in more detail later on. The correction for the background current, recorded under the same conditions in the absence of methanol, is done, but the correction was not significant. In the potential range from 0.05 to 0.3 V versus Ag/AgCl a well defined straight
Fig. 2. Cyclic voltammograms at 30% Pt/Ru on Vulcan XC-72 in quiet electrolyte and at different flow rates. Sweep rate 20 mV s−1 . Pure water used in the working compartment.
Fig. 3. Tafel plots for methanol oxidation at 30% Pt/Ru on Vulcan XC-72 in quiet electrolyte at a flow rate of 30 l h−1 . Data collected in steady state experiment. Inset: Polarisation curves for methanol oxidation on 30% Pt/Ru on Vulcan XC-72 in quiet electrolyte and at different flow rates. Data collected at a sweep rate of 1 mV s−1 in 1.0 M methanol solution in working compartment.
line, with a slope of approximately 125 mV per decay is obtained. Tafel slopes in the range from 105 to 130 mV per decay were obtained in a thin film study of methanol oxidation at similar Pt/Ru carbon supported catalysts [12] and in a DMFC study at an unsupported Pt/Ru catalyst [3]. This value of the Tafel slope is usually ascribed to the oxidative removal of adsorbed species from the surface as the rate-determining step (rds) [3]. The same result is obtained with a flow rate of 30 l h−1 , which is an indication for the kinetic control of the overall reaction. In the potential region from 0.3 to 0.5 V versus Ag/AgCl, there is an increase of the slope to approximately 200 mV per decay and then the system approaches its limiting current region. Under the conditions of the enhanced flow rate (30 l h−1 ) an increase of the current is obtained, indicating that mass transport limitations appear, but this influence is not very pronounced (the current increases by 20–30% on flow rate). Since the limiting current cannot be directly observed in the experiments, it was estimated from an equation proposed in [14]: 1/m 1 1 1/m I (1−1/m) = + (1) I Ikin IL where I is the measured current, Ikin the kinetic current, IL the limiting current and m the apparent reaction order. Ikin for the mixed activation diffusion region (from 0.3 to 0.5 V versus Ag/AgCl) is calculated by the extrapolation of the linear Tafel region (full line in Fig. 3). Following this procedure, the limiting current is calculated to be between 21 and 30 mA, depending on the choice of the reaction order. Lowest standard deviations are obtained for m = 1.5 and IL = 25.6 ± 0.6 mA. From the latter value one can estimate the overall mass transfer resistance at the MEA.
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The limiting current is influenced by the hydrodynamic conditions in the cyclone flow cell (mass transfer in the liquid phase) and by the diffusion through the back diffusion layer and the catalyst layer as well. The relation between these quantities can be approximated in the following manner: 1 = IL
1
1 + I I bdl cl
internal mass transfer
+
1 Idif
(2)
external mass transfer
where Ibdl is the diffusion current through the back diffusion layer, Icl is the diffusion current through the catalyst layer and Idif is the diffusion current defined by the hydrodynamic conditions. Similar equations were used for the study of oxygen reduction on ink-type electrodes [15] and for oxygen reduction and hydrogen oxidation on a Nafion coated electrode [16]. An estimation of Idif (only external mass transfer) is possible due to defined hydrodynamic conditions in the cyclone flow cell. Idif is defined as follows: Idif = km ne FcCH3 OH
(3)
where km is the mass transfer coefficient, ne is the number of transferred electrons, F Faraday’s constant and cCH3 OH is the concentration of methanol in the bulk solution. The mass transfer coefficient can be calculated from the following correlation equation, which was determined for the cyclone flow cell [7]: Sh = 0.0136Re2/3 Sc1/3
(4)
Eq. (4) is valid for Re > 103 and Sc ≥ 1. The Reynolds number is defined as [7]: Re =
ωR2 ρ µ
µ the viscosity in where ρ is fluid density in R the electrode radius and ω can be expressed as
Pa s−1 ,
1/2
4Rin
2 πR3/2 din
DaII =
d CL i/6F CL cCH3 OH DCH 3 OH
∼
resistance of diffusion in CL resistance of electrochemical reaction
V
(6)
In Eq. (6), Rin is the cyclone radius at the middle of the inlet tube (here Rin = 29.7 mm), R the electrode radius (here R = 8.0 mm), din the diameter of the inlet tube (here din = 4.0 mm) and V the volumetric flow rate. For flow rate 30 l h−1 Re is about 11,000. The Schmidt number in the Eq. (4) is defined as µ (7) Sc = ρD where D stands for diffusion coefficient of methanol in water. Using the value for D = 1.24 × 10−9 m2 s−1 [17], the mass transfer coefficient at a flow rate of 30 l h−1 is calculated to be km = 9.9 × 10−6 m s−1 and the corresponding diffusion current is IL = 1.1 A (number of transferred electrons is ne = 6, i.e. complete oxidation of methanol
(8)
where dCL is thickness of catalyst layer (ca. 10 m), i the current density applied (magnitude: 10 mA cm−2 ), cCH3 OH the methanol bulk concentration (1.0 M) and DCL is the effective methanol diffusion coefficient in CL (magnitude: 10−9 m2 s−1 ). The given data result in a Damköhler number of DaII ≈ 0.02. This value is clearly less below one, which indicates that the rate limiting process in the catalyst layer is the electrochemical reaction, and not the catalyst layer diffusion. This is true for low current densities applied in this work. The relevance of the mass transport resistance in the back diffusion layer (BDL) can be estimated by means of the following Biot number (Bi): BDL/CL
Bim
=
(5) kg m−3 ,
ω=
is assumed). The obtained value is more than one order of magnitude higher than the limiting current determined from the experiment. This explains the minor influence of varying flow rates in the quasi-steady state experiment (inset Fig. 3) and reveals the importance of the diffusional resistances inside the back diffusion layer and the catalyst layer as more important factors for the overall limiting current. The relevance of the mass transfer resistance within the catalyst layer (CL) can be evaluated by means of the Damköhler number (Da) of second kind:
DCL /d CL resistance of diffusion in BDL ∼ BDL BDL D /d resistance of diffusion in CL (9)
where DCL is effective methanol diffusion coefficient in CL (ca. 10−9 m2 s−1 ), dCL the thickness of catalyst layer (ca. 10 m), DBDL the effective methanol diffusion coefficient in BDL (ca. 10−9 m2 s−1 ) and dBDL is the thickness of back diffusion layer (ca. 100 m). Assuming that the diffusion coefficients in the two layers are of similar magnitude, one can see that the diffusion layer resistance is of higher importance due to the fact that the BDL is about ten times thicker BDL/CL than the CL, i.e. Bim ≈ 10. In a similar manner, the relevance of the external film resistance can be estimated in relation to the resistance exerted by the back diffusion layer. For this purpose, the following Biot number can be used: L/BDL
Bim
=
DBDL /d BDL km resistance of external film diffusion ∼ resistance of diffusion in BDL
(10)
where km is the film mass transport coefficient (ca. 10−5 m s−1 ). Using the values given above, one gets a Biot L/BDL ≈ 1. value of Bim
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Multiplying DaII with the Biot numbers defined above yields the reaction rate in relation to the mass transfer rates: BDL/CL
DaII × Bim ≈ 0.2 resistance of diffusion in BDL ∼ resistance of electrochemical reaction BDL/CL
(11)
L/BDL
DaII × Bim × Bim ≈ 0.2 resistance of external film diffusion ∼ resistance of electrochemical reaction
(12)
In conclusion, the experiments presented in this work were carried out in the reaction kinetic regime because, as shown be the estimated dimensionless parameter groups, all external and internal mass transport resistances are about five to ten times smaller than the reaction resistance. Obviously, the chosen experimental set-up and operating conditions are suitable to identify the reaction processes in the catalyst layer. The current increase of about 20% on stirring (Fig. 3) is consistent with the estimations, presented above. In the quasi-steady state experiments with varying flow rates (10, 20 and 30 l h−1 ) (inset Fig. 3), the same behaviour as in the steady state experiments is observed. Here, the currents are somewhat higher, which once again emphasises that the real steady state is not achieved. 3.3. Chronoamperometry Potentiostatic current versus time curves, obtained in methanol containing solutions at different potentials on a Pt/Ru MEA, are shown in Fig. 4. During the prolonged polarisation at constant potential the i(t) curve shows a pronounced decay in the beginning, followed by a less steeper decay during the whole time of the measurements. This decay was observed in both quiet and agitated solution. Even if the currents at 0.3 V and flow rate of 20 l h−1 are higher, the current decay persists. So, the reason for this cannot be
Fig. 4. Current vs. time curves at different potential values for Pt/Ru on Vulcan XC-72/Nafion in quite electrolyte and at a flow rate of 20 l h−1 ; 1.0 M methanol in water in working compartment.
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semi-infinite diffusion, as at these conditions stationarity in the mass transfer should be achieved due to the forced convection. A slow but constant current decay is observed in the literature for smooth Pt/Ru alloys with different Pt/Ru compositions, at different temperatures and methanol concentrations [18–20]. Several reasons have been discussed as a possible explanation for such behaviour. Gasteiger et al. [18] pointed out the effect of surface-active impurities in the electrolyte and found that the level of deactivation is consistent with the surface blockage via diffusion-limited adsorption of the impurities. On the contrary, Hoster et al. [19] found out that the steep and fast decay at the beginning is caused by the formation of higher Ru-oxide forms (like RuO2 and RuO3 ), while the following slow decay is due to the poisoning of the surface via adsorbed species other than CO (it was suggested that the poisoning effect is produced by the slow adsorption of hydrocarbon-like species formed from methanol adsorbates in a second-order surface reaction). It is stated that smooth electrodes always show a current decay. Freshly prepared rough Pt/Ru electrodeposit does not exhibit any kind of current decay [19] and the same is claimed for fuel cell catalysts at elevated temperature [11,13]. This was explained by the higher activity of the rough surface for the oxidation of hydrogenated residues to CO2 as well as by the presence of a large number of surface defects (steps, kinks, etc.), which can exhibit enhanced catalytic activity for oxidation of organic residues [19]. But, depending on the experimental conditions during the preparation of technical Pt/Ru electrodes some small loss of activity can be expected ([19] and references therein and [20]). Our catalyst has shown deactivation at all investigated potentials. 3.4. Impedance measurements 3.4.1. Pure water Impedance patterns for a Pt/Ru MEA in water, in direction from high to low frequency and vice versa, at different potentials are shown in Fig. 5. The impedance patterns in both directions almost coincide. There is no strong dependence on the potential. The impedance plots are characterised by one semicircle accompanied by an inductive loop in the high frequency region (1–870 kHz). It is the result of the resistance and the inductance of the connecting cables and of the electrolyte solution on the counter electrode’s side of the MEA. Then a second semicircle in the medium frequency region (1 Hz to 1 kHz) appears, overlapped with a very large arc in the low frequency region. The first two semicircles are not altered by the potential, while the third one is affected to a very small extent revealing a process with slow rate. The potentially dependent low frequency arc can be the consequence of the water discharge reaction, i.e. the formation of different kinds of oxide species on both Ru and Pt (only at higher potentials). Processes at the high frequency end of the impedance spectra are usually attributed to the Ohmic resistance. To
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Fig. 6. Impedance plots in water at 0.3 V vs. Ag/AgCl for Pt/Ru on Vulcan XC-72/Nafion, Pt/Ru on Vulcan XC-72/without Nafion membrane and Vulcan XC-72/Nafion electrodes (without Pt/Ru).
influence this part of the impedance spectrum. The high frequency region in the impedance plot coincides for the MEA, with and without Nafion membrane as well as for the electrode without metal loading (Fig. 6). The influence of the sulphuric acid concentration on the high frequency end of the spectra is more significant (Fig. 7). With the increase of the sulphuric acid concentration the semicircle in the frequency region from 1 to 870 kHz disappears and almost pure inductance of the cables is recorded. It was discussed that the “electrolyte resistance” behaves as a real resistance only if the capacitance of the fictitious capacitor formed by the equipotential plane at the location of the reference electrode
Fig. 5. Impedance plots for Pt/Ru on Vulcan XC-72/Nafion in water at different potentials: (a) whole spectrum, (b) high and medium frequency region; f, forward (from high to low frequencies); b, backward (from low to high frequencies).
the Ohmic resistance contribute the resistance of the electrolyte (sulphuric acid and Nafion membrane in our case), the resistance of the catalyst layer and the resistance of the back diffusion layer. Each process is characterised by impedance in a certain typical frequency range. These processes can be overlapped and some of them can have minor influence on the resulting impedance spectra. The influence of the electrode composition (Pt/Ru MEA with and without Nafion membrane, MEA without metal loading), sulphuric acid concentration in the counter electrode compartment, as well as composition of the solution in the working electrode compartment (water or methanol in water) on the high frequency end of the spectra was investigated. The presence of methanol in the working electrode compartment does not
Fig. 7. Influence of the change of the sulphuric acid concentration in the counter electrode compartment on the high frequency end of the spectra. MEA: PtRu Vulcan XC-72/Nafion. Water in the working compartment.
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and the “end” of the double layer at electrolyte side can be neglected [21]. Otherwise, the “electrolyte resistance” can contain a conductive part and appears as a semicircle in the impedance plot. It can be concluded that the high frequency end of the spectra can be attributed to the Ohmic resistance of the electrolyte (sulphuric acid) and to the inductance of the connecting cables. The semicircle in the frequency region from 1 Hz to 1 kHz is observed in systems with Nafion membrane for both Pt/Ru/Nafion and Vulcan XC-72/Nafion, with methanol (Fig. 8) and without methanol (Fig. 6), but with different diameters depending whether Pt/Ru is in the system or not (for the Pt/Ru MEA without Nafion membrane this semicircle does not appear at all; Fig. 6). The increase of
Fig. 8. Impedance plots for Pt/Ru on Vulcan XC-72/Nafion in 1.0 M methanol solution at different potentials: (a) whole spectra, (b) enlarged part of the spectra; f, forward (from high to low frequencies); b, backward (from low to high frequencies).
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the sulphuric acid concentration decreases the diameter of the semicircle (Fig. 7). This semicircle is observed only on the commercial MEA, used in this study, but not for home-made Pt/Ru carbon supported or unsupported MEA and Pt/Ru fuel cell anodes [8] as well, and can be due to different manufacturing procedures, rather than to the catalytic process itself. It is obvious that the manufacture procedure of the electrodes is very important, because additional resistances can be introduced which cannot be easily explained and detected by other electrochemical methods.
4. Methanol/water-solutions Impedance plots at different potentials for Pt/Ru MEA in methanol containing solutions in direction from high to low frequency values and immediately after that, are shown in Fig. 8. Again the impedance plot can be divided into three parts. High and medium frequency semicircles are not dependent on the potential (Fig. 8b), like in water (Fig. 5b), and impedance patterns in both directions coincide well within this frequency region (1 Hz to 870 kHz). The impedance spectra in water and methanol differ significantly in the low frequency region. Instead of the very large arc in water, almost potentially independent, in methanol containing solution this large arc appears only at the lowest potential value (0.0 V versus Ag/AgCl) and then turns into a semicircle at all other potential values. Increasing the potential, the diameter of the semicircle decreases which indicates that the charge transfer resistance for methanol oxidation becomes smaller. The semicircle in the low frequency region is accompanied by a pseudo-inductive loop, present at all potential values higher than 0.0 V versus Ag/AgCl. Pseudo-inductive behaviour in the low frequency region has been reported in the literature for methanol oxidation on smooth polycrystalline platinum [22], on carbon-supported Pt-nanoparticles [23] and Pt/Ru fuel cell anodes [8]. Such pseudo-inductive patterns are known to characterise systems with adsorbed intermediates or with a transition between a passive and an active state [24]. It is explained that an initially adsorbed CO layer generated from methanol dehydrogenation covers the reaction sites. When some of the weakly adsorbed CO begins to be oxidised, adsorption and subsequent methanol oxidation can take place on the adsorption sites, which are set free. Thus, pseudo-inductive patterns could diagnose the process of CO removal from the surface. According to the literature Pt/Ru is less poisoned with residues from methanol oxidation than Pt but some traces of CO adsorbed at the surface still have been detected. Methanol concentration [25,26], temperature and time can influence the nature of adsorbed species. Fan et al. have shown in a Fourier transform infrared diffuse reflection spectroscopy study of the direct methanol anode under similar conditions (1.0 M methanol concentration, room temperature) the existence of a CO adsorption peak for a Pt/Ru catalyst (1:1 atom ratio) [27].
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Table 1 Values of elements of the equivalent circuit in Fig. 9
Pure water Methanol/water
R1 ()
R2 ()
L1 (mH)
C (F)
R3 ()
CPA1 ( s−n )
n (CPA1 )
R4 ()
CPA2 ( s−n )
n (CPA2 )
R5 ()
R6 ()
L2 (kH)
9. 8 9. 9
0. 30 0. 30
0. 56 0. 68
0. 10 0. 11
8. 3 10. 7
35. 0 199. 6
0. 61 0. 87
35. 41 16. 14
12. 53 24. 02
0. 83 0. 90
446,000 47. 9
∞ 155
∞ 7. 51
Hysteresis between the impedance patterns in forward and backward directions in the low frequency region is observed. The resistance in backward direction is higher than in forward direction, i.e. it increases with time. In the discussion about the current decay during prolonged polarisation at constant potentials, several reasons for the observed phenomena were discussed. These reasons will be also responsible for the hysteresis phenomena observed in the impedance spectra, namely impurity effects, slow adsorption of hydrocarbons like species formed from methanol adsorbate in a second-order surface reaction, formation of higher Ru-oxide species, and mass transfer limitations due to the removal of the reaction products such as CO2 [18,19]. The equivalent circuit shown in Fig. 9 can be used to simulate the impedance spectra. The corresponding calculated curves as well as the experimental curves for pure water and methanol/water-solutions at 0.2 V versus Ag/AgCl are also shown in Fig. 10. The parameter values are listed in Table 1. To find them, the mean values for the impedance in both directions were used. In the case of pure water, the difference is very small, but in the case of methanol/water-solutions this is not the case because the process does not reach a steady state, as discussed in Section 3.3. Therefore, it is difficult to describe the two spectra in forward and backward directions separately. The values of the elements R1 , R2 , L1 , C and R3 , which describe the high frequency circles with the inductive loop, are almost the same for pure water and aqueous methanol solution and they are assigned to the “Ohmic resistance”. R6 and L2 represent the low frequency inductive loop, discussed above. These elements are practically absent in the absence of methanol and therefore, they are marked as infinitely high in Table 1. Overall, the calculated spectra reasonably fit the experimental points. The equivalent circuit needed to de-
Fig. 9. Proposed equivalent circuit.
Fig. 10. Impedance plots (experimental and simulated curves) for P/Ru on Vulcan XC-72/Nafion in water and 1.0 M methanol and at 0.2 V vs. Ag/AgCl; f, forward (from high to low frequencies); b, backward (from low to high frequencies).
scribe the observed impedance spectra is quite complicated having many elements. Most of the elements represent influencing factors, which are not due to the electrochemical reaction itself. The elements, which describe the low frequency part of the impedance spectra and are connected with the electrochemical processes on the anode, are found with less accuracy because of the mentioned non-stationarity of the process. This is the reason why only a qualitative interpretation is presented.
5. Conclusions The cyclone flow cell has some advantages for the investigation of the methanol electrooxidation in comparison to both conventional three-compartment electrochemical cells (allows the use of MEA like in a fuel cell) and fuel cells consisting of anode and cathode (the influence of the cathode is eliminated and potential control versus a reference electrode of stable potential is possible). Furthermore, well-defined hydrodynamic conditions in the cyclone cell allow a better determination of diffusion controlled processes. The Tafel slope of 125 mV per decay determined in the steady state experiments and the low frequency pseudoinductive loop in the impedance spectra of methanol/watersolutions confirm that the reaction between adsorbed carbon containing species and adsorbed oxygen containing species
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is the rate determining step in the potential range from 0.05 to 0.3 V versus Ag/AgCl. At higher potentials, diffusion processes begin to play a significant role, but without reaching pure mass transfer controlled conditions. The chronoamperometric investigations and the observed hysteresis of the impedance spectra showed a decrease of the electrode’s performance with time, what confirms similar results reported in the literature for other types of electrodes as well. The very complicated impedance spectra in both water and water methanol solution in the frequency region from 1 Hz to 870 kHz can be explained by the influence of the MEA manufacturing procedure and by the influence of the experimental set-up. Therefore, it is very important to investigate these features in more detail in order to avoid additional resistances, which can increase the overall electrode resistance and thus lower the fuel cell performance. The impedance plots in water did not show any pronounced influence on potential, while in methanol/watersolutions the total impedance decreases with potential.
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[7] K. Sundmacher, J. Appl. Electrochem. 29 (1999) 919. [8] J. Müller, P. Urban, W. Hölderich, J. Power Sources 84 (1999) 157. [9] J. Amphlett, B. Peppley, E. Halliop, A. Sadiq, J. Power Sources 96 (2001) 204. [10] L. Jörissen, V. Gogel, J. Kerres, J. Garche, J. Power Sources 105 (2002) 267. [11] D. Chu, S. Gilman, J. Electrochem. Soc. 143 (1996) 1685. [12] S. Gojkovi´c, T. Vidakovi´c, D. –Durovi´c, Electrochim. Acta 48 (2003) 3607. [13] W. Lizcano-Valbuena, V. Paganin, E. Gonzalez, Electrochim. Acta 47 (2002) 3715. [14] S. Zeˇcevi´c, S. Gojkovi´c, B. Nikoli´c, Elektrohemijsko inženjerstvo, Tehnološko-metalurški fakultet, Beograd, 2001. [15] S.Lj. Gojkovi´c, S.K. Zeˇcevi´c, R.F. Savinell, J. Electrochem. Soc. 145 (1998) 3683. [16] M. Watanabe, H. Igarashi, K. Yosioka, Electrochim. Acta 40 (1995) 329. [17] D.R. Lide (Ed.), CRC Handbook for Chemistry and Physics, 82nd ed., CRC Press, New Jersey, 2001. [18] H. Gasteiger, N. Markovi´c, P. Ross, E. Cairns, J. Electrochem. Soc. 141 (1994) 1795. [19] H. Hoster, T. Iwasita, H. Baumgartner, W. Vielstich, J. Electrochem. Soc. 148 (5) (2001) A496. [20] A. Hamnett, S. Weeks, B. Kennedy, G. Troughton, P. Christensen, Ber. Bunsenges. Phys. Chem. 94 (1990) 1014. [21] C.A. Schiller, Zahner Messsysteme—Basics and Applications, Erlangen. [22] R. Melnick, T. Palmore, J. Phys. Chem. B 105 (2001) 1012. [23] I.-M. Hsing, X. Wang, Y.-J. Leng, J. Electrochem. Soc. 149 (5) (2002) A615. [24] M. Ciureanu, H. Wang, Z. Qi, J. Phys. Chem. B 103 (1999) 9645. [25] W.F. Lin, J.T. Wang, R.F. Savinell, J. Electrochem. Soc. 144 (1997) 1917. [26] N. Markovic, H. Gasteiger, P. Ross Jr., X. Jiang, I. Villegas, M. Weaver, Electrochim. Acta 40 (1995) 91. [27] Q. Fan, C. Pu, E. Smotkin, J. Electrochem. Soc. 143 (1996) 3053.
Investigation of electrochemical oxidation of methanol in a cyclone flow cell Tanja Vidakovi´c a,1 , Mihai Christov a,2 , Kai Sundmacher a,b,∗ b
a Max-Planck-Institut für Dynamik komplexer technischer Systeme, Sandtorstraße 1, D-39106 Magdeburg, Germany Process Systems Engineering, Otto-von-Guericke University Magdeburg, Universitätsplatz 2, D-39106 Magdeburg, Germany
Received 13 October 2003; received in revised form 22 December 2003; accepted 30 December 2003
Abstract Electrochemical oxidation of methanol on carbon supported Pt/Ru gas diffusion electrodes was investigated in a cyclone flow cell at room temperature using chronoamperometry, cyclic voltammetry and electrochemical impedance spectroscopy. The influence of the flow rate was checked. It was proved that the cyclone cell is suitable for the investigation of methanol electrooxidation and provides additional information on the mass transfer limitations in the electrode assembly. Chronoamperometric measurements showed slow, but constant current decay at all investigated potentials. Impedance measurements in water and methanol containing solutions were performed and the experimental data were fitted to an appropriate equivalent circuit. © 2004 Elsevier Ltd. All rights reserved. Keywords: Methanol oxidation; Cyclone flow cell; Membrane electrode assembly; Carbon supported Pt/Ru catalyst; Electrochemical impedance spectroscopy
1. Introduction In order to increase the efficiency of the direct methanol fuel cell (DMFC) for its practical application it is necessary to improve the performance of the anode side of the DMFC, i.e. the methanol oxidation reaction. Several reasons can be responsible for the poor anode performance in DMFC: first, the slow methanol oxidation kinetics (the thermodynamic equilibrium potential for methanol oxidation is close to the reversible potential of hydrogen oxidation, but the kinetics in the former case is the limiting factor [1]), second the internal Ohmic resistance within the electrode structure, and finally the membrane resistance as well as the interfacial resistance at the contact area between the polymer membrane (protonic conductor) and the catalyst layer. The first reason is in the domain of electrocatalysis and the current state of
∗ Corresponding author. Tel.: +49-391-6110-350; fax: +49-391-6110-353. E-mail address: [email protected] (K. Sundmacher). 1 On leave from Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, Serbia and Montenegro. 2 Permanent address: Department of Physical Chemistry, University of Chemical Technology and Metallurgy, 1756 Sofia, Bulgaria.
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2003.12.047
the art is based on the Pt/Ru system, an electrocatalyst first suggested three decades ago [2]. Pt/Ru can be either carbon supported or unsupported where—according to the theory of bifunctional catalysis—Pt is responsible for methanol adsorption and Ru for the donation of OH species [3]. The internal Ohmic resistance is a combination of the electrolyte resistance within the electrode, the electronic resistance of the electrode material and the contact resistance between the electrode’s components [4]. The study of the proton conductivity of Nafion 117 has shown that its impedance is constant over the frequency range from 1 Hz to 10 kHz and hence has behaviour similar to conventional liquid electrolytes [5]. The resistance at the contact layer between the catalyst particles and the polymer membrane can be influenced by the preparation procedure (pressure, temperature, amount of added Nafion). A more intimate contact between the catalyst particles with both, the back diffusion layer (it serves as well as a current collector) and the polymer membrane (as a proton conductance medium) will decrease the influence of these resistances on the performance of the overall membrane electrode assembly (MEA) [6]. A special type of electrochemical cell—a cyclone flow cell was designed in order to study the kinetics of electrochemical reactions at MEAs [7]. It gives the possibility to investigate the MEA under technically relevant conditions:
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the reactants are supplied through the back diffusion layer, like in a fuel cell, and in contrast to standard half-cell measurements the reaction occurs at the contact layer between the catalyst particles and the polymer membrane (Nafion) without the influence of anion adsorption from the supporting electrolyte; it enables potentiostatic control during the measurements (half-cell measurements using a reference electrode with a stable and well defined potential, in contrast to typical fuel cell set-up, where the influence of the cathode cannot be excluded) and, last but not least, the hydrodynamic conditions in the liquid phase are well defined by a vortex flow with the thickness of the external diffusion layer being similar to the diffusion layer at a rotating disk electrode (RDE) [7]. The aim of this work was to test if the cyclone flow cell is a suitable tool for the investigation of methanol electrooxidation under fuel cell relevant conditions, by means of cyclic voltammetry (electrochemical characterisation of catalyst and reaction), chronoamperometry (investigation of long term stability of the catalyst) and especially by electrochemical impedance spectroscopy (suitable for the determination of Ohmic and charge transfer resistances [4,8,9]).
2. Experimental 2.1. Preparation of MEA All measurements were performed in a cyclone flow cell as depicted in Fig. 1. The working electrode compartment (1) was supplied with a methanol/water mixture (1.0 M methanol), while the counter electrode (2) and the reference electrode compartments with 0.1 M sulphuric acid solution (in some experiments, 0.5 or 1.0 M sulphuric acid solution). The catalyst was 20 wt.% Pt, 10 wt.% Ru supported on Vulcan XC-72 with a Pt:Ru atomic ratio of 1:1 and a total metal loading of 1.0 mg cm−2 . The back diffusion layer was a Teflonised carbon cloth. The gas diffusion electrodes (catalyst and back diffusion layer) were obtained from ElectroChem, Inc., Woburn, MA. The MEA was prepared by hot pressing of the gas diffusion electrode at 130 ◦ C at a pressure of 10 MPa during 3 min onto a Nafion 105 membrane (details about membrane pre-treatment and MEA preparation are given in [10]). Before pressing, the catalyst layer was sprayed with a proper amount of Nafion solution in 2-propanol so that the total amount of Nafion on the catalyst layer was 15 wt.% of the total metal loading on the surface. The geometric area of the MEA was 2.0 cm2 . Both methanol/water and sulphuric acid containers were deaerated with nitrogen and the deaerated solutions were circulated through the cell. Electrolyte solutions were prepared from sulphuric acid (Merck, extra pure), methanol (Merck, extra pure) and ultrapure water (Millipore, 18 M cm). All potentials were measured and reported versus a saturated silver/silver chloride reference electrode. All experiments were performed at room temperature 22 ± 0.5 ◦ C.
Fig. 1. Schematic representation of (a) experimental set-up: (1) working electrode compartment, (2) counter electrode compartment; (b) membrane electrode assembly.
2.2. Electrochemical measurements All electrochemical measurements were carried out with a Zahner impedance measurement unit (IM6e). Preconditioning of the electrode was done by cyclic voltammetry in the potential range from −0.2 to 0.5 V versus Ag/AgCl at a sweep rate of 20 mV s−1 . Five cycles were enough to obtain reproducible electrode behaviour. Normally, only the first cycle differs from the subsequent cycles. This procedure was applied in all steady state, quasi steady state and chronoamperometry experiments. Steady state experiments were performed potentiostatically with a fixed delay of 5 min at each potential, in the potential range from 0 to 0.5 V versus Ag/AgCl. The potential step was 25 mV. Quasi steady state measurements were performed in the same manner as the steady state measurements but at the sweep rate of 1 mV s−1 . Chronoamperometry. After the preconditioning procedure described above the potential was stopped at the desired set point and the current was recorded over the time (1 h for measurements in water and 2 h in methanol containing solution). Ohmic drop compensation was done during all these measurements using Ohmic resistance values estimated from prior impedance measurements.
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Impedance measurements were performed immediately after the chronoamperometric measurement at the same dc potential, over a frequency range between 1.12 mHz and 870 kHz, moving from high to low frequencies and, immediately after that, in the opposite direction. The amplitude of the sinusoidal signal was 5 mV (from base to peak).
3. Results and discussion 3.1. Cyclic voltammetry Fig. 2 shows cyclic voltammograms obtained at a MEA with 30% PtRu on Vulcan XC-72 in the absence of methanol in the working electrode compartment at the sweep rate of 20 mV s−1 in quiet electrolyte and at different flow rates. The anodic limit is chosen in order to prevent Ru dissolution [11,12]. The hydrogen adsorption desorption region on the Pt/Ru MEA is not very pronounced, which is well known from the literature for this kind of catalyst [13]. The general shape of the cyclic voltammogram resembles the cyclic voltammetry behaviour known for this kind of catalyst from the literature [3,12,13]. Some small influence of the flow rate is observed only in the hydrogen adsorption/desorption region, but almost within the experimental error. 3.2. Steady state and quasi-steady state measurements Tafel plots for methanol oxidation in quiet electrolyte and at a flow rate of 30 l h−1 are shown in Fig. 3. These data were collected from steady-state experiments. It should be noted that real steady state conditions could not be reached in reasonable time periods. This is discussed in more detail later on. The correction for the background current, recorded under the same conditions in the absence of methanol, is done, but the correction was not significant. In the potential range from 0.05 to 0.3 V versus Ag/AgCl a well defined straight
Fig. 2. Cyclic voltammograms at 30% Pt/Ru on Vulcan XC-72 in quiet electrolyte and at different flow rates. Sweep rate 20 mV s−1 . Pure water used in the working compartment.
Fig. 3. Tafel plots for methanol oxidation at 30% Pt/Ru on Vulcan XC-72 in quiet electrolyte at a flow rate of 30 l h−1 . Data collected in steady state experiment. Inset: Polarisation curves for methanol oxidation on 30% Pt/Ru on Vulcan XC-72 in quiet electrolyte and at different flow rates. Data collected at a sweep rate of 1 mV s−1 in 1.0 M methanol solution in working compartment.
line, with a slope of approximately 125 mV per decay is obtained. Tafel slopes in the range from 105 to 130 mV per decay were obtained in a thin film study of methanol oxidation at similar Pt/Ru carbon supported catalysts [12] and in a DMFC study at an unsupported Pt/Ru catalyst [3]. This value of the Tafel slope is usually ascribed to the oxidative removal of adsorbed species from the surface as the rate-determining step (rds) [3]. The same result is obtained with a flow rate of 30 l h−1 , which is an indication for the kinetic control of the overall reaction. In the potential region from 0.3 to 0.5 V versus Ag/AgCl, there is an increase of the slope to approximately 200 mV per decay and then the system approaches its limiting current region. Under the conditions of the enhanced flow rate (30 l h−1 ) an increase of the current is obtained, indicating that mass transport limitations appear, but this influence is not very pronounced (the current increases by 20–30% on flow rate). Since the limiting current cannot be directly observed in the experiments, it was estimated from an equation proposed in [14]: 1/m 1 1 1/m I (1−1/m) = + (1) I Ikin IL where I is the measured current, Ikin the kinetic current, IL the limiting current and m the apparent reaction order. Ikin for the mixed activation diffusion region (from 0.3 to 0.5 V versus Ag/AgCl) is calculated by the extrapolation of the linear Tafel region (full line in Fig. 3). Following this procedure, the limiting current is calculated to be between 21 and 30 mA, depending on the choice of the reaction order. Lowest standard deviations are obtained for m = 1.5 and IL = 25.6 ± 0.6 mA. From the latter value one can estimate the overall mass transfer resistance at the MEA.
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The limiting current is influenced by the hydrodynamic conditions in the cyclone flow cell (mass transfer in the liquid phase) and by the diffusion through the back diffusion layer and the catalyst layer as well. The relation between these quantities can be approximated in the following manner: 1 = IL
1
1 + I I bdl cl
internal mass transfer
+
1 Idif
(2)
external mass transfer
where Ibdl is the diffusion current through the back diffusion layer, Icl is the diffusion current through the catalyst layer and Idif is the diffusion current defined by the hydrodynamic conditions. Similar equations were used for the study of oxygen reduction on ink-type electrodes [15] and for oxygen reduction and hydrogen oxidation on a Nafion coated electrode [16]. An estimation of Idif (only external mass transfer) is possible due to defined hydrodynamic conditions in the cyclone flow cell. Idif is defined as follows: Idif = km ne FcCH3 OH
(3)
where km is the mass transfer coefficient, ne is the number of transferred electrons, F Faraday’s constant and cCH3 OH is the concentration of methanol in the bulk solution. The mass transfer coefficient can be calculated from the following correlation equation, which was determined for the cyclone flow cell [7]: Sh = 0.0136Re2/3 Sc1/3
(4)
Eq. (4) is valid for Re > 103 and Sc ≥ 1. The Reynolds number is defined as [7]: Re =
ωR2 ρ µ
µ the viscosity in where ρ is fluid density in R the electrode radius and ω can be expressed as
Pa s−1 ,
1/2
4Rin
2 πR3/2 din
DaII =
d CL i/6F CL cCH3 OH DCH 3 OH
∼
resistance of diffusion in CL resistance of electrochemical reaction
V
(6)
In Eq. (6), Rin is the cyclone radius at the middle of the inlet tube (here Rin = 29.7 mm), R the electrode radius (here R = 8.0 mm), din the diameter of the inlet tube (here din = 4.0 mm) and V the volumetric flow rate. For flow rate 30 l h−1 Re is about 11,000. The Schmidt number in the Eq. (4) is defined as µ (7) Sc = ρD where D stands for diffusion coefficient of methanol in water. Using the value for D = 1.24 × 10−9 m2 s−1 [17], the mass transfer coefficient at a flow rate of 30 l h−1 is calculated to be km = 9.9 × 10−6 m s−1 and the corresponding diffusion current is IL = 1.1 A (number of transferred electrons is ne = 6, i.e. complete oxidation of methanol
(8)
where dCL is thickness of catalyst layer (ca. 10 m), i the current density applied (magnitude: 10 mA cm−2 ), cCH3 OH the methanol bulk concentration (1.0 M) and DCL is the effective methanol diffusion coefficient in CL (magnitude: 10−9 m2 s−1 ). The given data result in a Damköhler number of DaII ≈ 0.02. This value is clearly less below one, which indicates that the rate limiting process in the catalyst layer is the electrochemical reaction, and not the catalyst layer diffusion. This is true for low current densities applied in this work. The relevance of the mass transport resistance in the back diffusion layer (BDL) can be estimated by means of the following Biot number (Bi): BDL/CL
Bim
=
(5) kg m−3 ,
ω=
is assumed). The obtained value is more than one order of magnitude higher than the limiting current determined from the experiment. This explains the minor influence of varying flow rates in the quasi-steady state experiment (inset Fig. 3) and reveals the importance of the diffusional resistances inside the back diffusion layer and the catalyst layer as more important factors for the overall limiting current. The relevance of the mass transfer resistance within the catalyst layer (CL) can be evaluated by means of the Damköhler number (Da) of second kind:
DCL /d CL resistance of diffusion in BDL ∼ BDL BDL D /d resistance of diffusion in CL (9)
where DCL is effective methanol diffusion coefficient in CL (ca. 10−9 m2 s−1 ), dCL the thickness of catalyst layer (ca. 10 m), DBDL the effective methanol diffusion coefficient in BDL (ca. 10−9 m2 s−1 ) and dBDL is the thickness of back diffusion layer (ca. 100 m). Assuming that the diffusion coefficients in the two layers are of similar magnitude, one can see that the diffusion layer resistance is of higher importance due to the fact that the BDL is about ten times thicker BDL/CL than the CL, i.e. Bim ≈ 10. In a similar manner, the relevance of the external film resistance can be estimated in relation to the resistance exerted by the back diffusion layer. For this purpose, the following Biot number can be used: L/BDL
Bim
=
DBDL /d BDL km resistance of external film diffusion ∼ resistance of diffusion in BDL
(10)
where km is the film mass transport coefficient (ca. 10−5 m s−1 ). Using the values given above, one gets a Biot L/BDL ≈ 1. value of Bim
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Multiplying DaII with the Biot numbers defined above yields the reaction rate in relation to the mass transfer rates: BDL/CL
DaII × Bim ≈ 0.2 resistance of diffusion in BDL ∼ resistance of electrochemical reaction BDL/CL
(11)
L/BDL
DaII × Bim × Bim ≈ 0.2 resistance of external film diffusion ∼ resistance of electrochemical reaction
(12)
In conclusion, the experiments presented in this work were carried out in the reaction kinetic regime because, as shown be the estimated dimensionless parameter groups, all external and internal mass transport resistances are about five to ten times smaller than the reaction resistance. Obviously, the chosen experimental set-up and operating conditions are suitable to identify the reaction processes in the catalyst layer. The current increase of about 20% on stirring (Fig. 3) is consistent with the estimations, presented above. In the quasi-steady state experiments with varying flow rates (10, 20 and 30 l h−1 ) (inset Fig. 3), the same behaviour as in the steady state experiments is observed. Here, the currents are somewhat higher, which once again emphasises that the real steady state is not achieved. 3.3. Chronoamperometry Potentiostatic current versus time curves, obtained in methanol containing solutions at different potentials on a Pt/Ru MEA, are shown in Fig. 4. During the prolonged polarisation at constant potential the i(t) curve shows a pronounced decay in the beginning, followed by a less steeper decay during the whole time of the measurements. This decay was observed in both quiet and agitated solution. Even if the currents at 0.3 V and flow rate of 20 l h−1 are higher, the current decay persists. So, the reason for this cannot be
Fig. 4. Current vs. time curves at different potential values for Pt/Ru on Vulcan XC-72/Nafion in quite electrolyte and at a flow rate of 20 l h−1 ; 1.0 M methanol in water in working compartment.
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semi-infinite diffusion, as at these conditions stationarity in the mass transfer should be achieved due to the forced convection. A slow but constant current decay is observed in the literature for smooth Pt/Ru alloys with different Pt/Ru compositions, at different temperatures and methanol concentrations [18–20]. Several reasons have been discussed as a possible explanation for such behaviour. Gasteiger et al. [18] pointed out the effect of surface-active impurities in the electrolyte and found that the level of deactivation is consistent with the surface blockage via diffusion-limited adsorption of the impurities. On the contrary, Hoster et al. [19] found out that the steep and fast decay at the beginning is caused by the formation of higher Ru-oxide forms (like RuO2 and RuO3 ), while the following slow decay is due to the poisoning of the surface via adsorbed species other than CO (it was suggested that the poisoning effect is produced by the slow adsorption of hydrocarbon-like species formed from methanol adsorbates in a second-order surface reaction). It is stated that smooth electrodes always show a current decay. Freshly prepared rough Pt/Ru electrodeposit does not exhibit any kind of current decay [19] and the same is claimed for fuel cell catalysts at elevated temperature [11,13]. This was explained by the higher activity of the rough surface for the oxidation of hydrogenated residues to CO2 as well as by the presence of a large number of surface defects (steps, kinks, etc.), which can exhibit enhanced catalytic activity for oxidation of organic residues [19]. But, depending on the experimental conditions during the preparation of technical Pt/Ru electrodes some small loss of activity can be expected ([19] and references therein and [20]). Our catalyst has shown deactivation at all investigated potentials. 3.4. Impedance measurements 3.4.1. Pure water Impedance patterns for a Pt/Ru MEA in water, in direction from high to low frequency and vice versa, at different potentials are shown in Fig. 5. The impedance patterns in both directions almost coincide. There is no strong dependence on the potential. The impedance plots are characterised by one semicircle accompanied by an inductive loop in the high frequency region (1–870 kHz). It is the result of the resistance and the inductance of the connecting cables and of the electrolyte solution on the counter electrode’s side of the MEA. Then a second semicircle in the medium frequency region (1 Hz to 1 kHz) appears, overlapped with a very large arc in the low frequency region. The first two semicircles are not altered by the potential, while the third one is affected to a very small extent revealing a process with slow rate. The potentially dependent low frequency arc can be the consequence of the water discharge reaction, i.e. the formation of different kinds of oxide species on both Ru and Pt (only at higher potentials). Processes at the high frequency end of the impedance spectra are usually attributed to the Ohmic resistance. To
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Fig. 6. Impedance plots in water at 0.3 V vs. Ag/AgCl for Pt/Ru on Vulcan XC-72/Nafion, Pt/Ru on Vulcan XC-72/without Nafion membrane and Vulcan XC-72/Nafion electrodes (without Pt/Ru).
influence this part of the impedance spectrum. The high frequency region in the impedance plot coincides for the MEA, with and without Nafion membrane as well as for the electrode without metal loading (Fig. 6). The influence of the sulphuric acid concentration on the high frequency end of the spectra is more significant (Fig. 7). With the increase of the sulphuric acid concentration the semicircle in the frequency region from 1 to 870 kHz disappears and almost pure inductance of the cables is recorded. It was discussed that the “electrolyte resistance” behaves as a real resistance only if the capacitance of the fictitious capacitor formed by the equipotential plane at the location of the reference electrode
Fig. 5. Impedance plots for Pt/Ru on Vulcan XC-72/Nafion in water at different potentials: (a) whole spectrum, (b) high and medium frequency region; f, forward (from high to low frequencies); b, backward (from low to high frequencies).
the Ohmic resistance contribute the resistance of the electrolyte (sulphuric acid and Nafion membrane in our case), the resistance of the catalyst layer and the resistance of the back diffusion layer. Each process is characterised by impedance in a certain typical frequency range. These processes can be overlapped and some of them can have minor influence on the resulting impedance spectra. The influence of the electrode composition (Pt/Ru MEA with and without Nafion membrane, MEA without metal loading), sulphuric acid concentration in the counter electrode compartment, as well as composition of the solution in the working electrode compartment (water or methanol in water) on the high frequency end of the spectra was investigated. The presence of methanol in the working electrode compartment does not
Fig. 7. Influence of the change of the sulphuric acid concentration in the counter electrode compartment on the high frequency end of the spectra. MEA: PtRu Vulcan XC-72/Nafion. Water in the working compartment.
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and the “end” of the double layer at electrolyte side can be neglected [21]. Otherwise, the “electrolyte resistance” can contain a conductive part and appears as a semicircle in the impedance plot. It can be concluded that the high frequency end of the spectra can be attributed to the Ohmic resistance of the electrolyte (sulphuric acid) and to the inductance of the connecting cables. The semicircle in the frequency region from 1 Hz to 1 kHz is observed in systems with Nafion membrane for both Pt/Ru/Nafion and Vulcan XC-72/Nafion, with methanol (Fig. 8) and without methanol (Fig. 6), but with different diameters depending whether Pt/Ru is in the system or not (for the Pt/Ru MEA without Nafion membrane this semicircle does not appear at all; Fig. 6). The increase of
Fig. 8. Impedance plots for Pt/Ru on Vulcan XC-72/Nafion in 1.0 M methanol solution at different potentials: (a) whole spectra, (b) enlarged part of the spectra; f, forward (from high to low frequencies); b, backward (from low to high frequencies).
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the sulphuric acid concentration decreases the diameter of the semicircle (Fig. 7). This semicircle is observed only on the commercial MEA, used in this study, but not for home-made Pt/Ru carbon supported or unsupported MEA and Pt/Ru fuel cell anodes [8] as well, and can be due to different manufacturing procedures, rather than to the catalytic process itself. It is obvious that the manufacture procedure of the electrodes is very important, because additional resistances can be introduced which cannot be easily explained and detected by other electrochemical methods.
4. Methanol/water-solutions Impedance plots at different potentials for Pt/Ru MEA in methanol containing solutions in direction from high to low frequency values and immediately after that, are shown in Fig. 8. Again the impedance plot can be divided into three parts. High and medium frequency semicircles are not dependent on the potential (Fig. 8b), like in water (Fig. 5b), and impedance patterns in both directions coincide well within this frequency region (1 Hz to 870 kHz). The impedance spectra in water and methanol differ significantly in the low frequency region. Instead of the very large arc in water, almost potentially independent, in methanol containing solution this large arc appears only at the lowest potential value (0.0 V versus Ag/AgCl) and then turns into a semicircle at all other potential values. Increasing the potential, the diameter of the semicircle decreases which indicates that the charge transfer resistance for methanol oxidation becomes smaller. The semicircle in the low frequency region is accompanied by a pseudo-inductive loop, present at all potential values higher than 0.0 V versus Ag/AgCl. Pseudo-inductive behaviour in the low frequency region has been reported in the literature for methanol oxidation on smooth polycrystalline platinum [22], on carbon-supported Pt-nanoparticles [23] and Pt/Ru fuel cell anodes [8]. Such pseudo-inductive patterns are known to characterise systems with adsorbed intermediates or with a transition between a passive and an active state [24]. It is explained that an initially adsorbed CO layer generated from methanol dehydrogenation covers the reaction sites. When some of the weakly adsorbed CO begins to be oxidised, adsorption and subsequent methanol oxidation can take place on the adsorption sites, which are set free. Thus, pseudo-inductive patterns could diagnose the process of CO removal from the surface. According to the literature Pt/Ru is less poisoned with residues from methanol oxidation than Pt but some traces of CO adsorbed at the surface still have been detected. Methanol concentration [25,26], temperature and time can influence the nature of adsorbed species. Fan et al. have shown in a Fourier transform infrared diffuse reflection spectroscopy study of the direct methanol anode under similar conditions (1.0 M methanol concentration, room temperature) the existence of a CO adsorption peak for a Pt/Ru catalyst (1:1 atom ratio) [27].
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Table 1 Values of elements of the equivalent circuit in Fig. 9
Pure water Methanol/water
R1 ()
R2 ()
L1 (mH)
C (F)
R3 ()
CPA1 ( s−n )
n (CPA1 )
R4 ()
CPA2 ( s−n )
n (CPA2 )
R5 ()
R6 ()
L2 (kH)
9. 8 9. 9
0. 30 0. 30
0. 56 0. 68
0. 10 0. 11
8. 3 10. 7
35. 0 199. 6
0. 61 0. 87
35. 41 16. 14
12. 53 24. 02
0. 83 0. 90
446,000 47. 9
∞ 155
∞ 7. 51
Hysteresis between the impedance patterns in forward and backward directions in the low frequency region is observed. The resistance in backward direction is higher than in forward direction, i.e. it increases with time. In the discussion about the current decay during prolonged polarisation at constant potentials, several reasons for the observed phenomena were discussed. These reasons will be also responsible for the hysteresis phenomena observed in the impedance spectra, namely impurity effects, slow adsorption of hydrocarbons like species formed from methanol adsorbate in a second-order surface reaction, formation of higher Ru-oxide species, and mass transfer limitations due to the removal of the reaction products such as CO2 [18,19]. The equivalent circuit shown in Fig. 9 can be used to simulate the impedance spectra. The corresponding calculated curves as well as the experimental curves for pure water and methanol/water-solutions at 0.2 V versus Ag/AgCl are also shown in Fig. 10. The parameter values are listed in Table 1. To find them, the mean values for the impedance in both directions were used. In the case of pure water, the difference is very small, but in the case of methanol/water-solutions this is not the case because the process does not reach a steady state, as discussed in Section 3.3. Therefore, it is difficult to describe the two spectra in forward and backward directions separately. The values of the elements R1 , R2 , L1 , C and R3 , which describe the high frequency circles with the inductive loop, are almost the same for pure water and aqueous methanol solution and they are assigned to the “Ohmic resistance”. R6 and L2 represent the low frequency inductive loop, discussed above. These elements are practically absent in the absence of methanol and therefore, they are marked as infinitely high in Table 1. Overall, the calculated spectra reasonably fit the experimental points. The equivalent circuit needed to de-
Fig. 9. Proposed equivalent circuit.
Fig. 10. Impedance plots (experimental and simulated curves) for P/Ru on Vulcan XC-72/Nafion in water and 1.0 M methanol and at 0.2 V vs. Ag/AgCl; f, forward (from high to low frequencies); b, backward (from low to high frequencies).
scribe the observed impedance spectra is quite complicated having many elements. Most of the elements represent influencing factors, which are not due to the electrochemical reaction itself. The elements, which describe the low frequency part of the impedance spectra and are connected with the electrochemical processes on the anode, are found with less accuracy because of the mentioned non-stationarity of the process. This is the reason why only a qualitative interpretation is presented.
5. Conclusions The cyclone flow cell has some advantages for the investigation of the methanol electrooxidation in comparison to both conventional three-compartment electrochemical cells (allows the use of MEA like in a fuel cell) and fuel cells consisting of anode and cathode (the influence of the cathode is eliminated and potential control versus a reference electrode of stable potential is possible). Furthermore, well-defined hydrodynamic conditions in the cyclone cell allow a better determination of diffusion controlled processes. The Tafel slope of 125 mV per decay determined in the steady state experiments and the low frequency pseudoinductive loop in the impedance spectra of methanol/watersolutions confirm that the reaction between adsorbed carbon containing species and adsorbed oxygen containing species
T. Vidakovi´c et al. / Electrochimica Acta 49 (2004) 2179–2187
is the rate determining step in the potential range from 0.05 to 0.3 V versus Ag/AgCl. At higher potentials, diffusion processes begin to play a significant role, but without reaching pure mass transfer controlled conditions. The chronoamperometric investigations and the observed hysteresis of the impedance spectra showed a decrease of the electrode’s performance with time, what confirms similar results reported in the literature for other types of electrodes as well. The very complicated impedance spectra in both water and water methanol solution in the frequency region from 1 Hz to 870 kHz can be explained by the influence of the MEA manufacturing procedure and by the influence of the experimental set-up. Therefore, it is very important to investigate these features in more detail in order to avoid additional resistances, which can increase the overall electrode resistance and thus lower the fuel cell performance. The impedance plots in water did not show any pronounced influence on potential, while in methanol/watersolutions the total impedance decreases with potential.
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