Modeling and Simulating Electrochemical Reduction of CO 2 in a Microfluidic Cell

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1 Mrio R. Eden, John D. Siirol nd Gvin P. Towler (Editors) Proceedings of the 8 th Interntionl Conference on Foundtions of Computer-Aided Process Design FOCAPD 214 July 13-17, 214, Cle Elum, Wshington, USA 214 Elsevier B.V. All rights reserved. Modeling nd Simulting Electrochemicl Reduction of in Microfluidic Cell Kunn Wu,, Erik Birgersson, Pul J.A. Kenis nd Iftekhr A. Krimi * Deprtment of Chemicl nd Biomoleculr Engineering, Ntionl University of Singpore, Singpore, Deprtment of Chemicl nd Biomoleculr Engineering, University of Illinois r Urn-Chmpign, Urn, Illinois 61891, United Sttes cheik@nue.edu.sg Astrct A stedy-stte, isotherml model hs een presented for the electrochemicl reduction of to CO in microfluidic cell. The model integrtes the trnsports of chrge, mss, nd momentum with electrochemistry. After vlidtion using experimentl polriztion curves, extensive simultions revel trde-off etween the two performnce mesures: current density nd conversion. A more negtive overpotentil t the cthode increses the prtil current density for reduction, ut decreses the Frdic efficiency. As feed concentrtion or flow rte increse, the current density nd frdic efficiency increse, ut conversion decreses slightly. A longer chnnel improves conversion, ut t the cost of Frdic efficiency nd current density. Keywords: Cron dioxide; Electrochemicl reduction; Microfluidics; Modeling. 1. Introduction Diminishing supplies of conventionl energy sources nd growing concern over greenhouse gs emissions present significnt chllenges to the world s rpidly incresing demnd for energy. The electrochemicl reduction of cron dioxide hs the potentil to store power from intermittent energy sources such s wind, solr, nd hydroelectricity in chemicl form vi chemicl fuels nd feedstocks such s formic cid nd CO (Jhong, M, & Kenis, 213). When comined with renewle source, this technology lso provides mens to reduce emissions. The mjority of existing studies on electrochemicl conversion of re experimentl in nture, focusing on the possile mechnisms for the mny products of electroreduction, or exploring different types of electrodes nd ctlysts to improve performnce (Hori, 21; Jhong et l., 213). First-principles modeling of electrochemicl microrectors cn complement the current experimentl work y elucidting the complex trnsport nd electrochemistry prticulrly in porous electrodes, nd help in designing nd optimizing such rectors. Li nd Olomn (27) first presented crude cthode model for the electroreduction of to potssium formte in continuous trickle-ed rector. Delcourt nd Newmn (21) proposed detiled model for reduction to CO in cell similr to protonexchnge-memrne fuel cell ut with n dditionl queous uffer lyer. Ni (212) nd Xie & Xue (212) lso modeled electroreduction in solid oxide electrolysis cell. However, the present literture is lcking detiled mthemticl model of

2 64 K. Wu et l. microfluidic electrochemicl cell for the queous electrochemicl reduction of, which hs een demonstrted to e n effective rector nd verstile nlyticl tool (Whipple, Finke, & Kenis, 21). Though Wng nd his coworkers (213) hve developed model for electroreduction to formte in microfluidic cell, their computtionl pproch for simulting the electrode-electrolyte interfce is rther oscure, nd potentil distriution nd the definition of overpotentil re oversimplified. We propose stedy-stte isotherml model for n electrochemicl microfluidic cell to reduce to CO. The model is clirted nd vlidted using experimentl dt, nd the sensitivity of severl design nd operting vriles is nlysed vi simultions. 2. Mthemticl Formultion Consider the microfluidic electrochemicl cell of Figure 1, which is equipped with prllel, rectngulr, multi-lyered chnnels nd opertes in co-flow mode. A mixture of N 2 nd enters the cthode gs chnnel, while the node is open to the tmosphere. An queous KCl electrolyte flows etween two gs diffusion electrodes (GDEs). The cthode is coted with Ag ctlyst, while the node is coted with Pt lnk t the electrode-electrolyte interfce. A grphite current collector cks ech GDE t the other side. As we supply electricity to this microfluidic cell, reduces to CO nd wter reduces to H 2 vi rections (1) nd (2) t the cthode-electrolyte interfce. The hydroxide ions produced from wter splitting migrte to the electrolyte-node interfce, nd get oxidised to O 2 vi rection (3). CO H O 2 CO 2OH (1) 2H O 2 H 2OH (2) 2OH 1 2O H O 2e (3) 2 2 e 2 e We mke the following ssumptions nd simplifictions. 1) The system is isotherml nd t stedy-stte. This is resonle ssumption for n electrochemicl cell with flowing electrolyte. 2) The electrolyte is n incompressile Newtonin fluid nd the flow is lminr. 3) Gs is wekly compressile nd the flow in the gs chnnel is lminr. 4) The side wlls of the cell re impermele nd the slip is zero. 5) Concentrtions do not vry long the cell cross section, s we enforce slip nd zero species flux t the left nd right wlls of the cell. CO2 N2 CO2 N2 CO H2 K +, Cl - H +, OH - K +, Cl - H +, OH - Figure 1. Schemtics of () the front nd () the side view of the microfluidic electrochemicl cell N2, O2

3 Modeling nd Simulting Electrochemicl Reduction of in Microfluidic Cell 641 We consider trnsport in the gs chnnels, porous GDEs, nd the electrolyte; mteril lnce in the gs phse (cthode:, N 2, CO nd H 2 ; node: O 2 nd N 2 ) nd electrolyte phse (H +, OH -, K + nd Cl - ); the electronic nd ionic chrge lnce; nd chrge trnsfer kinetics. This gives us the following conservtion equtions (4) (5) (6) (7) We ccount for diffusion nd convection in the cthode, ut only diffusion in the node due to the open oundry. In the electrolyte, we consider convection, diffusion, nd the migrtion of chrge species. Then, the molr fluxes of the vrious species re s follows, (8) We define the current densities t the electrodes nd in the electrolyte s: (9) The electrokinetics ssocited with rections (1) depends on concentrtion while tht for rection (2) nd (3) re ssumed e mss-trnsport-independent. (1) (11) (12) The overpotentils of vrious species t the triple-phse-oundries re given y the difference etween the driving potentil difference nd the reversile potentil of the hlf-cell (13)

4 642 K. Wu et l. For oundry conditions, we enforce constnt compositions nd flow rtes t the inlets; constnt reference pressure nd no diffusive species fluxes t outlets; no slip nd no flux t the cthode gs chnnel wlls; chrge insultion nd zero slip t the verticl wlls of the GDEs; constnt potentils t the electrode/gs chnnel interfce; nd molr fluxes nd current densities t the electrode/electrolyte interfces. We implemented the ove model in COMSOL, with se cse prmeter vlues shown in Tle 1, nd solved using the finite element method. We used coordinte serch method to minimize the sum of squred difference etween simulted nd experimentl prtil current densities to find the est-fit kinetic prmeter vlues. 3. Clirtion nd Vlidtion The electrokinetic prmeters in Eq. (1) to Eq. (12) re system specific, so we first fit these prmeters using experimentl polriztion curve. The fitted kinetic prmeter vlues re reported in Tle 1. Figure 2 compres the experimentlly mesured current densities with the model predictions. The computed prtil current densities gree well with the experimentl results. Tle 1. Key prmeters used in se cse simultion Prmeter Vlue Operting temperture, T (K) 298 Operting pressure, p (tm) 1. Chnnel length, L (cm) 5.5 Chnnel width, W (cm) 2.5 Gs chnnel/electrode/electrolyte thickness, H g / H gde / H elec (cm) 1./.3/.15 Electrode porosity, 63 Electrode permeility, (m 2 ) 2.49E-12 Inlet / N 2 flow rte, Q CO2 / Q N2 (cm 3 /min).7/6.7 Inlet electrolyte flow rte (cm 3 /min).4 Cthode/node pplied potentil, V cth / V node (V) -1.72/1.28 Fitted kinetic prmeters for CO formtion, k CO (m/s) / CO 5.233E-8/.17 Fitted kinetic prmeters for H 2 formtion, (A/m 2 ) / H E-7/.2 Fitted kinetic prmeters for O 2 formtion, (A/m 2 ) / O E-7/.456 Men Current Density (A/m 2 ) Prtil Current Density (CO) Experimentl Simulted Men Current Density (A/m 2 ) Prtil Current Density (H 2 ) Experimentl Simulted Applied Cthodic Potentil (V) Applied Cthode Potentil (V) Figure 2. Comprison of men prtil current densities corresponding to () CO formtion nd () H 2 formtion from numericl results nd experimentl dt. -1.8

5 Modeling nd Simulting Electrochemicl Reduction of in Microfluidic Cell Results nd Discussion Figure 3() presents the simulted current-potentil. An increse in the negtive potentil t the cthode increses the current densities for oth CO nd H 2 formtions. The side rection of H 2 formtion does not show significnt effect on the totl current density, until the pplied potentil exceeds -.7 V (vs. AgCl) t ph 7. At high negtive potentils, H 2 production rte increses much fster thn CO, leding to decrese in the Frdic efficiency. Thus, if CO is the only desired product, then n intermedite potentil should e pplied. However, for syngs production, suitle potentil should e chosen to otin the required CO to H 2 rtio. Figure 3() shows the effects of concentrtion, gs feed rte, nd chnnel length. As feed concentrtion increses, the concentrtion t the rection interfce lso increses. This leds to n increse in current density nd Frdic efficiency. This confirms tht the rection kinetics for reduction depends on concentrtion t the triple-phse oundry. However, it is noticed tht the overll conversion decreses with incresing feed concentrtion. In the other words, GDEs tht llow fster trnsport of to the recting interfce is very criticl in improving current density, Frdic efficiency nd conversion. Current Density (A/m 2 ) c Current Density nd Plot 8 1 Totl Current Density Prtil Current Density (CO) Prtil Current Density (H 2 ) Applied Cthodic Potentil (V) Performnce Plot (Inlet :1%) 3.25 Current Density (CO) (A/m 2 ) d Performnce Plot (Inlet Flow Rte: 1sccm) % t the inlet Performnce Plot (Inlet :1%).9 1 Conversion Conversion Conversion x Current Density (A/m 2 ) Current Density (A/m 2 ) Inlet Volumetric Flow Rte x Chnnel Length (m) Figure 3. Effects of () pplied cthode potentil, () concentrtion in the feed, (c) inlet gs flow rte nd (d) chnnel length on cell performnce

6 644 K. Wu et l. Initil chnge in the inlet gs flow rte hs similr effect s increse in feed concentrtion. As shown in Figure 3(c), with fster flow rte, more ccumulting on the rection interfce nd thus fster rte of rection nd lrger current density. The increse in gs flow rte results in shorter residence time in the rector, nd therefore decrese in conversion. However, when the volumetric flow rte exceeds.23 cm 3 /s with 1% in the feed, further increse in flow rte hs negligile effect. This my e ecuse ccumulted t the recting interfce hs een in excess for rection, nd rection kinetics for reduction no longer depends on surfce concentrtion of. Figure 3(d) shows tht longer chnnel length increses conversion, ut decreses Frdic efficiency nd current density. A longer chnnel llows more residence time, thus improves conversion. However, the lower verge concentrtion in the cell reduces the current density. If conversion is specified, we cn use this model to determine the optiml chnnel length. 5. Conclusions We presented first-principles electrochemicl model for reduction to CO in microfluidic cell. It ccounts for ll the significnt physics nd electrochemistry in the cell such s the trnsport of species nd chrges, momentum nd mss conservtions, nd electrochemicl rections. At present, model solution requires out 3 s using 1 GB of virtul memory. Simultion results successfully predict the experimentl dt fter the fitting of unknown prmeters. It lso revels the importnce of improving trnsport in the GDEs, the limiting effect of feed concentrtion, feed flow rte nd chnnel length. Our model provides sis for design nd optimiztion, nd cn e suitly extended for stck design nd optimiztion. References Delcourt, Chrles, & Newmn, John. (21). Mthemticl Modeling of CO2 Reduction to CO in Aqueous Electrolytes II. Journl of The Electrochemicl Society, 157(12), B1911. Hori, Y. (21). - reduction, ctlyzed y metl electrodes Hndook of Fuel Cells: John Wileys & Sons. Jhong, Huei-Ru Molly, M, Sicho, & Kenis, Pul J. A. (213). Electrochemicl conversion of CO2 to useful chemicls: current sttus, remining chllenges, nd future opportunities. Current Opinion in Chemicl Engineering, 2(2), doi: 1.116/j.coche Li, Hui, & Olomn, Colin. (27). Development of continuous rector for the electro-reduction of cron dioxide to formte Prt 2: Scle-up. Journl of Applied Electrochemistry, 37(1), doi: 1.7/s Ni, Meng. (212). An electrochemicl model for syngs production y co-electrolysis of H2O nd CO2. Journl of Power Sources, 22, doi: 1.116/j.jpowsour Wng, Huizhi, Leung, Dennis Y. C., & Xun, Jin. (213). Modeling of microfluidic electrochemicl cell for CO2 utiliztion nd fuel production. Applied Energy, 12, Whipple, Devin T., Finke, Eryn C., & Kenis, Pul J. A. (21). Microfluidic Rector for the Electrochemicl Reduction of Cron Dioxide: The Effect of ph. Electrochemicl nd Solid- Stte Letters, 13(9), B19. doi: / Xie, Yunyun, & Xue, Xingjin. (212). Modeling of solid oxide electrolysis cell for syngs genertion with detiled surfce chemistry. Solid Stte Ionics, 224,