Predicting Shunt Currents in Stacks of Bipolar Plate Cells with Conducting Manifolds

A m e t h o d is p resen ted for predic t ing shun t currents in s tacks of bipolar plate cells wi th conduc t ing manifolds . The m e t h o d is based on the r e q u i r e m e n t that the potent ia l drop th rough the solut ion in a mani fo ld be large enough to force cu r ren t to leave the solut ion and to enter the conduc t ing manifold . The cur ren t that leaves the solut ion in the mani fo ld enters the conduc t ing mani fo ld at the anode end of the s tack and re turns to the solut ion at the ca thode end. This could cause ca tas t rophic fai lure of a manifold. S h u n t cur rents occur in s tacks of b ipolar plate electrolyzers because of the c o m m o n electrolyt ic solutions. Previous w o r k (1, 2) has shown h o w to de t e rmine shun t currents for e lect rolyzers that have n o n c o n d u c t i n g manifolds . The pu rpose of this paper is to p resen t a m e t h o d for de te rmining shun t cur rents in e lectrolyzers wi th conduc t ing manifolds. It is impor t an t to account for shun t currents in large s tacks of b ipolar plate cells wi th conduc t ing manifo lds because of the possibi l i ty of p roduc ing u n w a n t e d species in the mani fo lds (hydrogen gas in the chlor ine gas/anolyte man i fo lds of chlor-alkali cells, e.g.) and the m u c h worse poss ib i l i ty o f fai lure of the mani fo lds due to dissolut ion. C o n d u c t i n g (metal) mani fo lds are often se lec ted over o ther p ip ing mater ia ls for e conomic reasons. For example , in the case of the m e m b r a n e chlor-alkali process, t i t an ium and n icke l are h ighly corros ion res is tant to the anolyte and catholyte , respect ively . Plast ic (i.e., PTFE) l ined steel p ipe is of ten used for the p ip ing of this process. Unfor tuna te ly , a typical mani fo ld does no t have s tandard l ined p ipe dimens ions ; consequen t ly , fabr icat ion of a mani fo ld wi th a plast ic l iner wou ld be difficult. F igures 1 and 2 show pictorial ly and schematical ly , respect ively , h o w shunt cur ren t can en ter a conduc to r in an e lec t ro ly t ic solution. As shown in the figures, cur ren t enters a br ine solut ion by ox ida t ion of chlor ide ions to form pr imar i ly chlor ine gas, t ravels in the solut ion to a p la t inum foil, en ters the foil by reduc t ion of wa te r to form hyd rogen gas, and leaves the foil by ox ida t ion of chlor ide ions. Finally, t he cur ren t leaves the cell by reduc t ion of water. I f the P t foil in Fig. 2 were rep laced wi th a commerc i a l l y p u r e Ti foil, anod ic cur ren t wou ld cause dissolut ion of the t i tan ium, wh ich wou ld occur at the end of the foil neares t t he ca thode. I f the solut ion in Fig. 2. were changed to a sod i u m h y d r o x i d e solut ion and the foil changed to nickel, shun t cur ren t m igh t leave the foil by ox ida t ion of hydroxy l ions to fo rm o x y g e n which wou ld be an impur i ty gas in the h y d r o g e n of a chlor-alkali cell. Consequent ly , i t is desirable to be able to predic t the locat ion and ex t en t to wh ich shun t cur rents en ter and leave conduc t ing mani fo lds in electrolyzers . This can be done in a wors t case sense by a s imple ex tens ion of a p rev ious ly pub l i shed m o d e l (1, 2). * Electrochemical Society Active Member . Model Figu re 3 is a c i rcui t analog mode l for shun t currents in a s tack of b ipolar cells wi th conduc t ing manifolds . The circui t inc ludes cur ren t paths th rough the wall of the manifold (I4j) and in the meta l mani fo ld (/53). Values for these cur ren ts can be obta ined by ex t end ing the set of equa t ions p re sen t ed earl ier (1, 2), as shown in Par t I o f the Append ix , and solving these equa t ions after va lues have been set for the inpu t variables: N, IT, Yo, R1, R2, R3, Rm, and Rwj. Values for N, IT, Vo, R1, R2, and R3 are set in the same way as before (1, 2), and a va lue for the res is tance of a s e g m e n t of manifold (Rm) can be obta ined in the usual m a n n e r by us ing the specific res is tance of the meta l (p), the l eng th of the current pa th in the mani fo ld s egmen t (l), and the cross-sect ional area of the cur ren t pa th (A) pl R,~ = [1] A Values for the res is tances Rwj are de t e rmined in an iterat ive process that yields va lues for the b ranch currents and, consequen t ly , the shun t currents . The first step in the process is to ignore the top half of the circui t in Fig. 3 and solve for I u, I2j and I3j, as before (1, 2). The b ranch currents in the solut ion phase o f the mani fo ld (I3~) can then be used to p red ic t the potent ia l drop f rom po in t a to po in t b in Fig. 3 N V~ = ~ R313j first step in i tera t ion process [2] j=l I f V~ is less than the decompos i t i on potent ial , Vd, for the e l ec t rochemica l react ions associa ted wi th the shun t curren t en te r ing and leaving the conduc t ing mani fo ld (Vd Nerns t potent ia l p lus overpotent ials) , no cur ren t will en ter the wal l of the manifold. I f V~ is larger than Vd, cur ren t wil l en ter the meta l wall of the mani fo ld because it wou ld t h e n offer the pa th of least resistance. For a wors t case basis, the decompos i t i on potent ia l can be set equa l to the Nerns t potent ia l (i.e., ignore the overpotent ia ls) and the wal l res is tance set equa l to zero. In a more realistic case for t i t an ium and o ther va lve metals , it m a y be desi rable to inc lude the pi t t ing potent ia l (4) in the decompos i t i on potential. I t is impor t an t to note that i f the n u m b e r of cells in the