"F.Saeed and S.Basu"
Keywords: Corrosion, Pt-Ru/C Electrode Catalyst ,Direct Ethanol Proton Exchange Membrane Fuel Cell (DEPEMFC), Potentiodynamic Polarization.
Abstract:
The corrosion behaviour of nano grade Pt-Ru (40%:20% by wt.)/C catalyst used in Direct Ethanol Proton Exchange Fuel Cells (DEPEMFC) at a loading of 1 mg/cm2 was studied in different ethanol concentrations e.g.,1M and 5M by varying the speed of agitation (250, 750 and 1500 RPM ) at a temp. of 80oC . The results reveal that variation in speed of agitation and ethanol concentrations plays an important role in corrosion rate. The corrosion rate is increased with increasing the concentration of ethanol from 1M to 5M at different RPM e.g., 250, 750 and 1500 RPM. The results also showed the possibility of the initiation of pitting corrosion on the Pt, Ru and C catalyst surface due to the effect of galvanic coupling generated among these metals .
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ISSN 1466-8858 Volume 12, Preprint 28 submitted 20 June 2009 Studying the Corrosion Behavior of Nanograde Pt-Ru/C Catalyst Used In Direct Ethanol Proton Exchange Membrane Fuel Cells (DEPEMFC) 1-F.Saeed 2- S.Basu (1)Corrosion Engineering Division, Mechanical Design and Technology Center Royal Scientific Society Amman-Jordan farqad@rss.gov.jo (2)Chemical Engineering Dept.,Indian Institute of Technology (IIT) New Delhi – India sbasu@chemical.iitd.ac.in Abstract The corrosion behaviour of nano grade Pt-Ru (40%:20% by wt.)/C catalyst used in Direct Ethanol Proton Exchange Fuel Cells (DEPEMFC) at a loading of 1 mg/cm2 was studied in different ethanol concentrations e.g.,1M and 5M by varying the speed of agitation (250, 750 and 1500 RPM ) at a temp. of 80oC . The results reveal that variation in speed of agitation and ethanol concentrations plays an important role in corrosion rate. The corrosion rate is increased with increasing the concentration of ethanol from 1M to 5M at different RPM e.g., 250, 750 and 1500 RPM. The results also showed the possibility of the initiation of pitting corrosion on the Pt, Ru and C catalyst surface due to the effect of galvanic coupling generated among these metals . Keywords : Corrosion, Pt-Ru/C Electrode Catalyst ,Direct Ethanol Proton Exchange Membrane Fuel Cell (DEPEMFC), Potentiodynamic Polarization. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 28 submitted 20 June 2009 List of Figures and Tables Fig.1: Front view of the Perspex electrode holder setup with bolts and nuts made of Plastic material used to tighten the two circular parts of the electrode holder. Fig.2: Top view of the Perspex electrode holder setup with bolts and nuts made of Plastic material used to tighten the two circular parts of the electrode holder. Fig.3 : Sketch of the experimental corrosion cell setup Fig.4: Potentiodynamic polarization curves of Pt-Ru ( 40% : 20% by wt.)/C in 1 M ethanol, for 1 hour at 80 oC , different RPM e.g., 250, 750 and 1500 Fig.5: Potentiodynamic polarization curves of Pt-Ru ( 40% : 20% by wt.)/C in 5 M ethanol, for 1 hour at 80 oC , different RPM e.g., 250, 750 and 1500 Fig.6: Potentiodynamic polarization curves of Pt-Ru ( 40% : 20% by wt.)/C in1 M and 5M ethanol, for 1 hour at 80 oC , different RPM e.g., 250, 750 and 1500 Fig.(7) (a) Pt-Ru /C layer before exposure to the corrosive environment (b) Pt-Ru /C layer after exposure to the corrosive environment 1 M Ethanol after 1 hour at 80 oC , speed of agitation equal to 250 RPM . Fig.(8) (a) Pt-Ru /C layer before exposure to the corrosive environment (b) Pt-Ru /C layer after exposure to the corrosive environment 1 M Ethanol after 1 hour at 80 oC , speed of agitation equal to 1500 RPM . Table 1 : Corrosion rate of Pt-Ru/C catalyst in 1 M Ethanol, different RPM e.g., 250, 750 and 1500 . Table 2 : Corrosion rate of Pt-Ru/C catalyst in 5 M Ethanol, different RPM e.g., 250, 750 and 1500 . © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 28 submitted 20 June 2009 Introduction Direct Ethanol Proton Exchange Membrane Fuel Cells (DEPEMFC) deliver high power density, which offers low weight, cost and volume. It operates at low temperature ( i.e. 80 oC), allowing for faster startups and immediate response to changes in the demand for power 'Alkire[1]'. Ethanol can be produced in large quantities by fermentation of sugar containing bio mass resources and is thus renewable in nature .The energy density of ethanol (8030 Wh kg -1 ) and Ethanol is less toxic than other fuels such as Methanol .Therefore efforts to use of the so-called bio-ethanol for operating PEMFC to be installed as a power source to run mobile phones, portable PCs, vehicle engines are obvious in considerable manner ' Basu[2], Fujiwara[3], Basu[4]' .On the other hand, bio-ethanol used for the above mentioned operations contains water molecules which was found surprisingly aggressive toward metallic materials and more detailed electrochemical kinetics of metal electrode in this medium are yet unknown ' Souza[5] '. Therefore understanding the root causes for the corrosion failures in DEPEMFC is an ultimate goal of this investigation because electrochemical reactions occur within the electrode assembly such as the anodic reaction, and thus, the latter is a key component of a fuel cell. In other words the above mentioned understandings will be reflected directly on reducing the direct and indirect economic losses ( i.e. replacement of corroded parts, shut down of DEPEM Fuel Cell), because the durability of DEPEMFC is affected by both its physical properties and the fuel cell operating conditions. The state of the art fuel cell electrode is normally composed of carbon-supported noble metal catalyst such as Pt-Ru. The carbon support provides several desired functions. First, it enables the uniform dispersion of Pt/Ru particles. Second, it retards the sintering or agglomeration of Pt/Ru particles. Third, it provides electronic continuity. Carbon as a support has good chemical and electrochemical stabilities, and these properties make it popular fuel cell catalyst support. The aim of the present investigation is to study the corrosion behavior of Pt-Ru/C with at a loading of 1 mg/cm2 used as catalyst on a treated carbon sheet covered with a layer of carbon black to be kept inside a special holder made of Perspex material with an opening of 4 cm2 to expose the Pt,Ru/C layer to the corrosive environment. Different ethanol concentrations were used during the investigation e.g.,1M and 5M and different RPM e.g., 250, 750 and 1500 RPM. at a temp. of 80oC.The potentiostat was used to accelerate the corrosion reaction by applying potential across the working electrode and the counter electrode, then to measure the half cell potential using Ag/AgCl reference electrode in order to draw the potentidynamic polarization curve and as result conducting © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 28 submitted 20 June 2009 Tafel extrapolation to calculate the corrosion rate at each operating condition in order to simulate the actual corrosion behavior of the anodic portion used in DEPEMFC according to the following oxidation reaction : C2H5OH + 3H2O 2CO2 + 12 H+ + 12 e- (1) Experimental Materials : The catalyst used to prepare the electrode was Nano grade Pt-Ru ( 40% : 20% by wt.)/C from Johnson Matthey Inc.,UK . Carbon black , Vulcan XC 72 R, GP-3915 obtained from CABOT Co. which was used as a primary substrate layer. Carbon paper ( Lydall 484C-1,USA) used as a substrate for the catalyst powder . A mixture of Nafion ® ( SE-5112,Dupont USA ) and 1-Propanol (MERK) was used as a binder. Ethanol ( MERK ) and distilled water was used as an electrolyte. Preparation of the working electrode The working electrode was prepared by cutting a 3 x 3 cm. from the Carbon sheet. Preparing a slurry of Carbon black powder dispersed in 1-Propanol using an ultrasonic water bath for 30 min. . The Carbon Black slurry was uniformly spread on an area of 2 x 2 cm. in the middle of the above mentioned 3 x 3 cm. Carbon sheet. It was then dried in an oven for 1 hr. at 100 oC. The weight of dried electrode was measured using a digital balance and kept in a special plastic bag . Weighing 4 mg of the Nanograde Pt-Ru ( 40% : 20% by wt.)/C powder using a digital balance and then dispersing the required quantity of the catalyst powder in 4 drops of Nafion solutions and 5 drops of 1-propanol then keeping the mixture in an ultrasonic water bath for 30 min. The catalyst slurry was uniformly spread in the form of continuous wet film using a brush over the above mentioned 2 x 2 cm. Carbon Black layer on the surface of the carbon sheet . Then the working electrode was dried in an oven for 1hr. at 100 oC. . The weight of the dried electrode was measured using a digital balance to ensure that 4 mg. of the catalyst spread on the surface of the electrode and kept in a special plastic bag to keep it dry . © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 28 submitted 20 June 2009 Experimental setup and method Electrode holder setup A circular shaped working electrode holder setup was designed and fabricated from Perspex sheet to hold the working electrode in a square having a slot of 0.1 mm in depth and 3 x 3 cm. in area and then to be covered from the top with circular shaped cover made of Perspex having an opening of 2 x 2 cm. in order to keep an area of 4 cm2 of the working electrode to be exposed to the corrosive environment . Plastic bolts and nuts were designed and fabricated in order to attach tightly the cover to the working electrode holder. A special tunnel was drilled in the circular electrode holder in order to enable the insertion of an electric wire used as a conductive path between the bottom of the working electrode surface and the potentiostat keeping in mind that the bottom surface is not exposed to the mentioned corrosive environment. Teflon tape was used to prevent leakage of the electrolyte to the undesired parts of the working electrode. Figs.(1-2), show the above mentioned circular shaped electrode holder setup . Methodology Methodology Before starting the experiment , the desired electrolyte was kept in 2 lit. glass beaker then to be inserted into the water bath in order to increase the temp. of the electrolyte to 80oC. Then the Perspex electrode holder setup was inserted inside the electrolyte at the desired temp. and connected to the potentiostat .Platinum electrode was used as a counter electrode while Ag/AgCl was used as a reference electrode and all were connected to the potentiostat .A glass stirrer connected to an automatic agitator was used to vary the speed of agitation as shown in fig.(3). The potentiostat was kept on the run mode by clicking the start button in the operating software package in order to draw the potentiodynamic polarization curve at scan rate of 0.05 V/Sec. with step potential of 25 mV for 1 hr. .The corrosion rate was calculated at the end of the each experimental run using the same software package to conduct Tafel extrapolation at each operating condition. Each condition was repeated for two times and in some cases for three times in order to check the degree of reproducibility among the results. Before conducting the experiments , the free corrosion potential of the working electrode was also measured using the voltmeter and the reference electrode. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 28 submitted 20 June 2009 Results and discussion Effect of Ethanol concentration and speed of agitation on corrosion rate of PtPt-Ru/C catalyst layer It is obvious from the corrosion rates given in tables (1-2) and figs.(4-6), that the corrosion rate is increased with increasing the concentration of ethanol from 1M to 5M at different RPM e.g., 250, 750 and 1500 RPM .This behaviour indicates that the electrochemical reaction is under the effect of concentration polarization following equ.(2) ' Fontana [6], Uhlig [7], Revie [8], Bard [9], Kane [10] ' : η = ± β log ( 1- i/iL ) Where (2) η = overpotential, β = Tafel constant = 2.3 RT/ α n F. On the other hand iL is the limiting diffusion current = nFkCb, k= mass transfer coefficient and Cb is the bulk concentration . Effect of Galvanic Coupling The galvanic coupling among dissimilar metals can be treated by application of Mixed Potential theory ' Fontana [6], Uhlig [7], Revie [8], Bard [9], Saeed [11] '.Therefore , in the case of Pt-Ru ( 40% : 20% by wt.)/C three metals are connected to each other forming a galvanic active combination ' Saeed [11], David [12], West [13],Lee[14] '.The response of this combination to the corrosive environment differs from the behavior of these metals when they are not connected to each other , because each metal has its own exchange current density (io) and Equilibrium Potential ( Eeq) and as a result the corrosion current (icorr.) will deviate in its position towards the coupling potential (Ecoupling) rather than the corrosion potential of the metal when it is alone. It is obvious that Ecoupling at any time represents the result of the galvanic interaction and electromotive force between different metals. Furthermore ,the summation of galvanic currents on the three metals is equal to zero , Σ I g = I g Pt + I g Ru + I g C = 0.0 ( i.e. Σ I a = Σ I c ) ' Saeed [11], West [13] '. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 28 submitted 20 June 2009 As a result of this assumption the metals in the combination might show an increase in the current to less negative values and other might show a decrease in the current to less positive values . In other words, the behaviour of these metals in the given environment might respond in different manners so the more negative ( active ) metal in the combination might corrode sacrificially and get depleted to protect the other metals of less negativity ( i.e. More Noble ). This act is also affected with the surface area of each metal in the couple as an important variable .In our case Ru % is less than Pt% and C % and as a result there will be a difference in the area ratio of cathodic elements in the combination to anodic elements in same combination (i.e Ac/Aa ) raising the act of the sacrificial depletion of the most active metal in the combination to protect the other elements (i.e. more noble ) by generating a very small ( i.e tiny currents ) negative and positive so that the summation of the galvanic anodic currents generated by the three metals (i.e. Σ I g ,a) will be gathered by the most active metal in the combination while the summation of the galvanic cathodic currents (i.e. Σ I g ,c) will be gathered by the most noble metals in the combination' Saeed [11], West [13],Lee[14] '. As a result galvanic corrosion will occur which will encourage the formation of pitting corrosion. The stereomicroscopic analysis for the surface of selected samples as shown in figs(7 and 8 ) in which the red circles refer to the severely corroded areas due to pitting corrosion as a result of galvanic corrosion . Conclusions Conclusions Studying the corrosion behavior of Pt-Ru (40%:20% by wt.)/C catalyst layer which is used in DEPEMFC in two different ethanol concentrations and various speed of agitation at 80 oC showed that the electrochemical oxidation reaction of Ethanol given in equ.(1) : C2H5OH + 3H2O On 2CO2 + 12 H+ + 12 e- (1) Pt-Ru (40%:20% by wt.)/C catalyst layer is under the control of concentration polarization because the corrosion rate is increased with increasing the concentration of ethanol from 1M to 5M at different RPM e.g., 250, 750 and 1500 RPM . On the other hand ,Pt-Ru ( 40% : 20% by wt.)/C catalyst layer represents the case of three metals in connection to each other forming a galvanic active combination. Therefore the possibility of the initiation of pitting corrosion on the Pt, Ru and C catalyst surface due to the effect of galvanic coupling was obvious from the results . © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 28 submitted 20 June 2009 Acknowledgements This study was sponsored by INSA-JRD-TATA and hosted by the Fuel Cell Lab./Chemical Engineering Department / IIT-Delhi as a part of a joint research fellowship.The authors are grateful to Dr.Ganga Radhakrishnan /Honorary Director / CCSTDS/INSA for her efforts in coordinating and finalizing the sponsoring issues of this study. The authors also thank Prof. B. N. Jain/Deputy Director (Faculty)/IIT-Delhi and Prof. S.K. Gupta/Head of the Chemical Engineering Dept. for their efforts in hosting this study at the Fuel Cell Lab./ Chemical Engineering Department / IIT-Delhi. Table (1) : Corrosion rate of Pt-Ru/C catalyst in 1 M Ethanol Speed of agitation Corrosion Rate , ,RPM µm/y 250 21.8 750 17.48 1500 31.7 Table (2) : Corrosion rate of Pt-Ru/C catalyst in 5 M Ethanol Speed of agitation Corrosion Rate , ,RPM µm/y 250 27 750 51 1500 36 © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 28 submitted 20 June 2009 Wire Connection to be used to connect the working electrode (i.e.Pt-Ru/C) to the potentiostat Fig.(1) Front view of the Perspex electrode holder setup with bolts and nuts made of Plastic material used to tighten the two circular parts of the electrode holder. Fig.(2) Top view of the Perspex electrode holder setup with bolts and nuts made of Plastic material used to tighten the two circular parts of the electrode holder. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 28 submitted 20 June 2009 Agitator with speed controller Reference electrode container with 1mm opening to be kept R.E. C.E. W.E. Glass Beaker of 2 lit. 1mm in distance from the W.E. Fig.(3) Sketch of the experimental corrosion cell setup Potential Vs.Ag/AgCl (V) 0.6 0.4 0.2 0 250 RPM -0.2 750 RPM -0.4 1500 RPM -0.6 -0.8 -1 1.00E-07 1.00E-05 1.00E-03 1.00E-01 1.00E+01 Log i (A) Fig.(4) Potentiodynamic polarization curves of Pt-Ru ( 40% : 20% by wt.)/C in 1 M ethanol, for 1 hour at 80 oC , different RPM e.g., 250, 750 and 1500 © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 28 submitted 20 June 2009 Potential vs.Ag/AgCl (V) 0.6 0.4 0.2 0 250 RPM -0.2 750 RPM -0.4 1500 RPM -0.6 -0.8 -1 1.00E-07 1.00E-05 1.00E-03 1.00E-01 1.00E+01 Log i (A) Fig.(5) Potentiodynamic polarization curves of Pt-Ru ( 40% : 20% by wt.)/C in 5 M ethanol, for 1 hour at 80 oC , different RPM e.g., 250, 750 and 1500 . Potential vs. Ag/AgCl (V) 0.6 0.4 0.2 250 RPM , 1M Ethanol 0 750 RPM, 1M Ethanol 1500 RPM, 1M Ethanol 250 RPM, 5M Ethanol -0.2 -0.4 750 RPM, 5M Ethanol -0.6 1500 RPM, 5M Ethanol -0.8 -1 1.00E-07 1.00E-05 1.00E-03 1.00E-01 1.00E+01 Log i (A) Fig.(6) Potentiodynamic polarization curves of Pt-Ru ( 40% : 20% by wt.)/C in 1 M and 5M ethanol, for 1 hour at 80 oC , different RPM e.g., 250, 750 and 1500 . © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 28 (a) submitted 20 June 2009 (b) Fig.(7) (a) Pt-Ru /C layer before exposure to the corrosive environment (b) Pt-Ru /C layer after exposure to the corrosive environment 1 M Ethanol after 1 hour at 80 oC , speed of agitation equal to 250 RPM .The white circle in the stereomicroscopic image (b) without magnification show clearly the degree of depletion of Pt-Ru /C layer by forming a group of pits due to galvanic corrosion. (a) (b) Fig.(8) (a) Pt-Ru /C layer before exposure to the corrosive environment (b) Pt-Ru /C layer after exposure to the corrosive environment 1 M Ethanol after 1 hour at 80 oC , speed of agitation equal to 1500 RPM . The white circle in the stereomicroscopic image (b) without magnification show clearly the degree of depletion of Pt-Ru /C layer by forming a group of pits due to galvanic corrosion. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 28 submitted 20 June 2009 References [1] R.C. Alkire, D.M. Kolb, Advances in Electrochemical Science and Engineering, first edition, John Wiely & Sons,1998 . [2] S.Basu, Recent Trends in Fuel Cell Science and Technology, first edition, Springer, 2007. [3] N.Fujiwara, Z.Siroma, S.Yamazaki, T.Ioroi, H.Senoh, K.Yasuda, Journal of Power Sources, 185 ,pp621-626, 2008. [4] S.Basu, A.Agrwal, H.Paramanik, Journal of Power Sources,10, pp1245-1257,2008. [5] J.P. Souza ,Journal of Corrosion Science, 27,pp1351-1364,1987. [6] M.G. Fontana, N.D. Greene, Corrosion Engineering, third edition, McGraw-Hill (1985). [7] H.H. Uhlig, Corrosion and Corrosion Control, third edition, John Wiely & Sons (1985). [8] R.W.Revie, Uhlig's Corrosion Handbook, third edition, John Wiely & Sons (2000). [9] A.J. Bard, L.R.Faulkner, Electrochemical methods:Fundementals and Applications, second edition, John Wiely & Sons,2001. [10] R.D. Kane, Corrosion2004, paper no. 04543, NACE publications, 2004. [11] F.Saeed ,Corrosion2008, paper no.2374, NACE publications, 2008. [12] H.David, P.Jaeger, Corrosion2000, paper no.00712, NACE publications, 2000. [13] M.West, Electrodeposition and corrosion processes, VNR, second edition,1970. [14] W.J. Lee, H.Park, Journal of Applied Surface Science, 228, pp 410–417, 2004. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work.