Why do fuel cells use thin GDLs

Why do fuel cells use thin GDLs?

In our pre­vi­ous blog arti­cle, we high­light­ed that gas dif­fu­sion lay­ers (GDLs) are com­mon­ly avail­able in var­i­ous thick­ness­es. Inter­est­ing­ly, elec­trolyz­ers tend to uti­lize thick­er GDLs, while fuel cells typ­i­cal­ly require thin­ner ones. Case in point, cus­tomers build­ing fuel cells pre­fer the 180-um thick LINQCELL GDP 180, where­as those man­u­fac­tur­ing elec­trol­y­sis stacks choose the thick­er LINQCELL GDL 1500 (thick­ness = 1.5 mm). This brings us to the ques­tion, “Does the GDL thick­ness matter?”

Appar­ent­ly, it does.

Thick­ness, which can affect the over­all per­for­mance of the cell stack, is anoth­er crit­i­cal para­me­ter to con­sid­er in GDL selec­tion and design. While thick­ness does not affect some of the struc­tur­al prop­er­ties of GDLs, includ­ing pore size dis­tri­b­u­tion, it influ­ences some trans­port and elec­tri­cal properties.

First, increas­ing the thick­ness increas­es the break­through pres­sure of the GDLs. Break­through pres­sure is the max­i­mum pres­sure required so that a flu­id from a reser­voir can pass through the pores of the GDL and start flow­ing out on the oth­er side. Sim­ply put, if the pres­sure from the reser­voir is high enough to reach the break­through pres­sure, the flu­id will flow through the GDL. 

Breakthrough pressure as a function of GDLs with different PTFE concentrations

Sim­i­lar to break­through pres­sure, water sat­u­ra­tion, which is the degree at which the GDL is soaked or sat­u­rat­ed with water, also increas­es with increas­ing thick­ness. Aside from the thick­ness, hydropho­biza­tion treat­ments also affect the water sat­u­ra­tion of GDLs. As dis­cussed in our pre­vi­ous arti­cle about the man­u­fac­tur­ing process for GDLs, PTFE coat­ings and/or micro­p­orous lay­ers are applied on top of the graphi­tized car­bon paper sub­strate to improve its wet­ta­bil­i­ty and make it more hydropho­bic. Hydropho­biza­tion treat­ments on thin GDLs low­er the over­all sat­u­ra­tion. How­ev­er, the effect of the hydropho­biza­tion treat­ment becomes insignif­i­cant as the thick­ness increas­es, which could have impli­ca­tions in the water man­age­ment per­for­mance of the GDL. 

Gas per­me­abil­i­ty, through-plane trans­port of water, and through-plane elec­tri­cal con­duc­tiv­i­ty all decrease with increas­ing GDL thick­ness. This is par­tic­u­lar­ly detri­men­tal for high cur­rent den­si­ty and wet oper­a­tions, which require effi­cient elec­tri­cal con­duc­tance and water removal from the system.

Contact Angle vs. GDL Thickness at different Placements

Increas­ing the thick­ness result­ed to a dras­tic drop in the con­tact angle of PTFE-treat­ed GDLs (5 wt%), imply­ing that the effect of the hydrophibiza­tion treat­ment became less significant.

Gas permeability vs. GDL Thickness and PTFE Content

The gas per­me­abil­i­ty, char­ac­ter­ized through the Dar­cy coef­fi­cient, decreased at increas­ing thick­ness. This could have severe impli­ca­tions to reac­tant deliv­ery dur­ing fuel cell operation.

Hav­ing all these dis­cussed, the ques­tion, “Why do fuel cells use thin GDLs and elec­trolyz­ers use thick­er ones?” remains.

To answer this ques­tion, we have to under­stand the fun­da­men­tal reac­tions involved in fuel cells and electrolyzers. 

Fuel cells pro­duce water and gen­er­ate elec­tric­i­ty using input H2 and O2 gas. Con­verse­ly, water elec­trolyz­ers split water into H2 and O2 gas using elec­tric­i­ty. These def­i­n­i­tions may sound sim­ple. How­ev­er, in real­i­ty, much more com­plex process­es are at play. Car­bon-based GDLs are com­mon­ly used in the cath­ode side of fuel cells and water elec­trolyz­ers. In a poly­mer elec­trolyte mem­brane fuel cell, the oxy­gen reduc­tion reac­tion (ORR) occurs (Equa­tion 2) at the cath­ode in which oxy­gen gas reacts with hydro­gen ions and elec­trons to form water. ORR occurs at elec­trode regions in which three dif­fer­ent phas­es, name­ly, the sol­id cat­a­lyst at the cat­a­lyst lay­er, gaseous reac­tants, and ion-con­duct­ing phase of the elec­trolyte, meet. The inter­face between these phas­es are com­mon­ly known as the triple-phase bound­ary (TPB). How does this relate to the GDL thickness? 

As pre­vi­ous­ly dis­cussed, as the GDL thick­ness increas­es, the effect of the hydropho­biza­tion treat­ment becomes insignif­i­cant, and the gas per­me­abil­i­ty and through-plane trans­port of water decreas­es. All of these things can affect the reac­tant deliv­ery and water man­age­ment in the stack. Water man­age­ment is very cru­cial in fuel cell oper­a­tions because water is pro­duced dur­ing the cathod­ic reac­tion. Accu­mu­la­tion of water caus­es flood­ing, which decreas­es the avail­able gas dif­fu­sion path­ways, the con­cen­tra­tion of TPBs at which the reac­tion can occur, and oxy­gen con­cen­tra­tion on the cat­a­lyst lay­er. Poor water man­age­ment decreas­es the effi­cien­cy of the fuel cell. As such, thin­ner GDLs with high­er through-plane water trans­port, gas per­me­abil­i­ty, and hydropho­bic­i­ty are pre­ferred in fuel cells.

Triple phase boundary at the cathode of a PEMFC stack

In con­trast, water elec­trolyz­er oper­a­tion (Equa­tion 3 and 4) only involves two phas­es: the sol­id cat­a­lyst and the liq­uid phase reac­tants and elec­trolyte ions. Look­ing at cath­ode in which the GDL is used, hydro­gen gas is pro­duced through the hydro­gen evo­lu­tion reac­tion dur­ing elec­trol­y­sis (Equa­tion 3). The absence of a gaseous reac­tant, which is used in fuel cells, sim­pli­fies the water man­age­ment aspect in water elec­trolyz­ers to a cer­tain extent. There­fore, com­pared to fuel cells, water elec­trolyz­ers can use thick­er GDLs, which have good mechan­i­cal prop­er­ties, uni­form cur­rent dis­tri­b­u­tion, and low pres­sure drop across the cell

Ulti­mate­ly, the opti­mal GDL thick­ness required to strike a bal­ance between water man­age­ment, elec­tri­cal con­duc­tance, and reac­tant deliv­ery depends on the oper­at­ing con­di­tions and cell design. Con­tact us, and our appli­ca­tion engi­neers and in-house water elec­trolyz­er and fuel cell experts will assist you in select­ing the most suit­able prod­ucts tai­lored to your application.

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