Mass-Action Kinetics Approach to Concurrent H2 and CO Electrochemistry in a Patterned SOFC Anode

Journal ofThe Electrochemical Society, 163 (13) F1399-F1411 (2016) F1399 0013-465l/2016/l63(l3)/F1399/13/$33.00 © The Electrochemical Society Mass-Action Kinetics Approach to Concurrent H2 and CO Electrochemistry in a Patterned SOFC Anode Nicolas Valle Marchante“qq” and Jack Brouwer“q’ “National Fuel Cell Research Center (NFCRC), University of California, Irvine, California 92697-3550, USA b Department of Mechanical and Aerospace Engineering, University of California, Irvine, California 92697-2 700, USA SOFC is a fuel ﬂexible technology suitable for producing clean energy. Understanding of the multi-reaction mechanism that a complex H2-CO fuel presents to the electrochemical kinetics is numerically approached in this work. By using existing fundamental reaction mechanisms and kinetic parameters, elementary reactions involved in an SOFC anode have been assembled, modeled and analyzed. This involves both homogeneous and heterogeneous chemistry, electrochemistry and surface diffusion. The use of the patterned anode approach removes the mass transport complications and allows comparison with pre-existing experimental data. The model provides both polarization curves and surface coverage distribution, among other results, providing a high level of detail and understanding of the physical phenomena involved. In particular, analysis is focused upon understanding how the competitive H2 and CO reactions behave.The presence of CO was found to stabilize OCV response to temperature and while it occupied most of the active sites it did not penalize overall performance except when signiﬁcant product species were present. © 2016 The Electrochemical Society. [DOI: 10.1 149/2.0861613 jes] All rights reserved. Manuscript submitted July 18, 2016; revised manuscript received September 22, 2016. Published October 4, 2016. The operation of SOFC with both gaseous“ and even liquid hydrocarbons‘ has been widely discussed, analyzed, and reported in the literature taking both experimental and analytic approaches. Particular attention has been put in fuel cell and reformer integra- tion. By analyzing several degrees of internal and external reforming, it has been pointed out by Margalef et al. that the use of Direct Internal Reforming (DIR) in High Temperature Fuel Cells (HTFC) presents an efﬁciency advantage.“ Because of the presence of carbon in hy- drocarbon molecules, analysis of carbon deposition and coking, a key issue for fuel cell durability and, ultimately, its commercialization, has been and discussed by Klein et al.3 Following Mizusaki’s pioneering work,7 assuming that H2 is the only electro-active species in a H2-CO mixture is common in the literature.3’8’° However, Sukeshini et al. results suggest that CO can be incorporated to the fuel stream and provide similar performance to that of pure H2, even at Hz concentrations as low as 25%.”) These results open a new research paradigm, as interest in concur- rent electrochemical reactions is renewed. For doing so, an elementary approach to the system of equations is required. Unfortunately, the el- ementary electrochemical mechanisms and kinetic constants are still among the less understood phenomena of the fuel cell community.“ For this reason, researchers have been widely using a simpliﬁed ap- proach, the Butler-Volmer equation. The Butler-Volmer equation has the advantage of simple solution, but, it presents several disadvantages when simulating a competitive reacting scenario. Even though this approach has been used previously for concurrent reactions,” it presents several drawbacks, as it assumes that a) charge—transfer occurs in a single step, b) composition is at equilibrium, c) there is a known current exchange density, which most of the times is a value that needs to be ﬁtted; ﬁnally, and most importantly d) it does not account for the competition of active sites that multiple reaction systems will present. These disadvantages can be addressed by taking a fundamental approach to the competing rates of elementary electrochemical re- actions. Surface diffusion has to be taken into account as well. This approach requires the characterization of all the kinetic parameters of all chemical and electrochemical steps involved in the mechanism. This approach was ﬁrst stated by Bessler et al.”&#39;14 and has gained popularity in recent years. Hecht et al.” and Janardhanan and Deutschmann” proposed a detailed mechanism for the reforming of methane in an SOFC anode and its kinetic characterization, including homogeneous and hetero- geneous chemistry. By ﬁtting the electrochemical parameters to the experimental re- sults, new mechanisms for H2 and CO electrochemistry have been “E-mail: nvm @ uci.edu proposed, characterized and discussed. Other authors, such as Moyer et al.” have applied this approach for modeling conventional tubular solid oxide cells. In order to reduce the uncertainty arising from the porous media characterization, a simpliﬁed geometry is commonly used in these analysis. Most importantly, TPB density should be known and per- fectly determined. This approach has been used in experiments per- formed by Mizusaki et al.,”’’19 Sukeshini et al.“) and Habibzadeh et al.,2°‘21 among others. The modeling of these experiments has been successfully imple- mented by Goodwin et al.,“ Yurkiv et al.23*24 and Hanna et al.,” among others. Although there is still some debate among the scientiﬁc commu- nity about the chemical and electrochemical details of the mechanisms involved, it seems clear that single channel hydrogen spillover is the rate-controlling reaction for H2, as stated by Vogler et al. in Ref. 27, while the reaction mechanism for CO electrochemistry is still unclear.” Model Formulation D0main.—Figure 1 presents a drawing of a patterned anode cell. Ni strips act as the anode. Ni provides good catalytic activity and is a common material used in commercial fuel cells. The width of the strips depends upon the experimental setup, but normally it is be- tween 50 and 100 um. The spacing between them is normally the same as the width. As a reference, Mizusaki et al. experiments were per- formed with an 8 mm diameter, 1 mm thick YSZ substrate over which 0.5-1 um thick Ni strips were deposited. More details can be found in Ref. 7. A conventional yttria—stabilized zirconia (YSZ) pellet acts as the electrolyte. YSZ is assumed to be homogeneous and mono—crystalline, and so the ﬂux of ions ﬂowing to the anode strips is considered homogeneous.” Anode active area is the contact line between the Ni strip and the YSZ pellet. It is well-deﬁned and known. The cathode is formed by Pt paste that is screen printed on the YSZ pellet. Because of the exceptional catalytic activity of Pt and the huge amount of active sites that a porous structure provides compared to the patterned one (i.e., the anode side) thermodynamic equilibrium is as- sumed at the cathode. This approach is supported by the experimental work performed by Mizusaki et al.7 Note that the porosity of the cathode provides way more active sites than the patterned anode. For this reason, the current density is different at both electrodes. The cell is enclosed in a furnace to control its temperature. Due to the small geometry of the cell and the large thermal mass of the Downloaded on 2016-10-18 to IP 128.195.207.247 address. Redistribution subject to ECS terms of use (see ecsd|.orglsiteIterms_use) unless CC License in place (see abstract).