Effect of Graphitization: Mechanical, Chemical & Electrical

How does graphitization affect the structure and properties of carbon fiber papers?

The man­u­fac­tur­ing process of our LINQCELLTM car­bon-based gas dif­fu­sion (or porous trans­port) lay­ers involves three major steps: (1) paper wet lay­ing, (2) car­boniza­tion, and (3) graphitization. 

Manufacturing process of carbon-based gas diffusion layers

Dur­ing the wet paper lay­ing step, the car­bon fibers pro­duced from car­bon sources, com­mon­ly poly­acry­loni­trile (PAN), are impreg­nat­ed with car­boniz­able resins, which serve as binders for the indi­vid­ual fiber strands. Thin lay­ers of impreg­nat­ed car­bon fibers are formed using a paper mak­ing machine. Once the desired thick­ness or num­ber of stacked lay­ers is reached, the mate­r­i­al is com­pressed and heat-rolled to fuse the car­bon fibers and solid­i­fy the binder resin. Then, cur­ing through car­boniza­tion is per­formed, fol­lowed by graphitization.

Carbonization vs. Graphitization

Car­boniza­tion and graphi­ti­za­tion are both heat treat­ments. How do these two process­es dif­fer from each oth­er? Car­boniza­tion and graphi­ti­za­tion are process­es that trans­form organ­ic (car­bon-based) mate­ri­als; how­ev­er, they yield dif­fer­ent forms of car­bon with dif­fer­ent prop­er­ties. Car­boniza­tion is typ­i­cal­ly per­formed at 600 to 1200 °C under inert con­di­tions (i. e., in the absence of air nor oxy­gen). Car­boniza­tion removes volatile mate­ri­als, such as water, gas­es, and some light organ­ic com­pounds, from the car­bon fiber papers, there­by pro­duc­ing a car­bon-rich mate­r­i­al. After car­boniza­tion, the car­bon fiber paper pre­sum­ably retains its ini­tial struc­ture and properties.

On the oth­er hand, graphi­ti­za­tion is per­formed at high­er tem­per­a­tures than those employed in car­boniza­tion. Graphi­ti­za­tion is the heat treat­ment of car­bon-based mate­ri­als at tem­per­a­tures typ­i­cal­ly above 1000 °C under con­trolled con­di­tions. As you would have expect­ed, such high-tem­per­a­ture heat­ing also changes the com­po­si­tion of the car­bon fiber paper. Sim­i­lar to car­boniza­tion, graphi­ti­za­tion pro­duces car­bon-rich mate­ri­als, which have an even high­er car­bon con­cen­tra­tion than those that have only under­gone carbonization. 

In fact, as shown in Fig­ure 1, the car­bon con­tent of PAN-derived car­bon fibers increased from 93.85% to 99.87% as the graphi­ti­za­tion tem­per­a­ture was increased from 1300 to 2700 °C. Aside from that, graphi­tized car­bon-based mate­ri­als under­went dehy­dro­gena­tion and den­i­tro­gena­tion. Case in point, hydro­gen and nitro­gen con­cen­tra­tions were both low­er than 0.05% after graphi­ti­za­tion at 2700 °C.

Elemental Composition of PAN-derived Carbon Fibers at different Graphitization Temperatures

These com­po­si­tion­al changes observed dur­ing graphi­ti­za­tion are often accom­pa­nied by struc­tur­al changes. Please stay with me as we dig deep­er into these changes.

Structural changes due to graphitization

Amorphous, Crystalline, and Graphitic Structure

What do I exact­ly mean with “struc­tur­al change” or at least, with the word “struc­ture?” Struc­ture describes how atoms are arranged in a mate­r­i­al. To iden­ti­fy the struc­tur­al changes, we have to look into the arrange­ment of car­bon atoms before and after graphitization.

Ini­tial­ly, car­bon fibers are amor­phous mate­ri­als, mean­ing, their atom­ic arrange­ments do not exhib­it long-range order. To put it sim­ply, the car­bon fibers are com­posed of ran­dom­ly-ori­ent­ed car­bon atoms at first. Dur­ing the ini­tial stages of heat treat­ment, carbon–carbon bonds are bro­ken down, and volatile com­po­nents like hydro­gen, nitro­gen, and oth­ers evap­o­rate, leav­ing behind a car­bon-rich struc­ture. Even­tu­al­ly, pro­longed heat­ing at such high tem­per­a­tures gives the car­bon atoms suf­fi­cient ener­gy for them to rearrange them­selves and achieve a more-ordered struc­ture. Dur­ing graphi­ti­za­tion, car­bon atoms tend to form hexag­o­nal rings and then stack up into par­al­lel lay­ers, form­ing a struc­ture that is sim­i­lar to that of graphite. Sim­ply put, graphi­ti­za­tion makes the car­bon atoms in the car­bon fiber paper assume a more crys­talline struc­ture, resem­bling that of graphite. How­ev­er, graphi­ti­za­tion does not trans­form the entire car­bon net­work struc­ture into graphite. For this rea­son, the degree of graphi­ti­za­tion is used to quan­ti­fy the extent at which the car­bon struc­ture is trans­formed into lay­ers of hexag­o­nal­ly bond­ed atoms. More for­mal­ly, DOG is a mea­sure of the sp3-to-sp2 hybridiza­tion tran­si­tion in the car­bon mate­r­i­al. In gen­er­al, high DOG implies that there are more graphite-like struc­tures in the mate­r­i­al. As such, the mate­r­i­al pos­si­bly pos­sess­es a high­er degree of crys­tallini­ty (i. e., high­er degree of struc­tur­al order). 

Changes in Carbon Structure during Graphitization

Our LINQCELLTM graphi­tized car­bon-based gas dif­fu­sion lay­ers have been graphi­tized at 1600 and 2000 °C. Now, how does graphi­ti­za­tion tem­per­a­ture affect the result­ing struc­ture of the graphi­tized car­bon fiber papers in terms of the degrees of crys­tallini­ty and graphi­ti­za­tion? Raman spec­troscopy and X‑ray dif­frac­tion mea­sure­ments demon­strate that high­er graphi­ti­za­tion tem­per­a­tures yield high­er graphitic order­ing, mean­ing car­bon-based mate­ri­als sub­ject­ed to high­er tem­per­a­tures exhib­it high­er degrees of crys­tallini­ty and graphi­ti­za­tion. High­er graphi­ti­za­tion tem­per­a­tures pro­vide the car­bon atoms more ener­gy, mak­ing them more mobile and facil­i­tat­ing the for­ma­tion of the pre­ferred graphitic structure. 

Property changes due to graphitization

Imag­ine atoms as LEGO blocks form­ing your build­ings. Arrang­ing LEGO blocks in dif­fer­ent ways cre­ates dif­fer­ent struc­tures, which in turn affect how strong, flex­i­ble, and sta­ble your cre­ation is. This briefly explains why prob­ing struc­tur­al changes in car­bon fibers after graphi­ti­za­tion is impor­tant. Prop­er­ty is a func­tion of struc­ture. Ergo, iden­ti­fy­ing struc­tur­al changes helps us iden­ti­fy mate­r­i­al prop­er­ty changes after a treatment.

Since graphi­ti­za­tion trans­forms the orig­i­nal­ly amor­phous struc­ture of the car­bon fiber paper into a graphitic one, we can expect that the prop­er­ties of the graphi­tized car­bon fiber papers will become sim­i­lar to those of graphite. Graphite has been used in a wide array of appli­ca­tions, par­tic­u­lar­ly in elec­tro­chem­i­cal ener­gy stor­age devices as elec­trode mate­ri­als or sub­strates for super­ca­pac­i­tors, bat­ter­ies, fuel cells, and water elec­trolyz­ers, owing to its excel­lent mechan­i­cal prop­er­ties, good chem­i­cal sta­bil­i­ty, and high ther­mal and elec­tri­cal con­duc­tiv­i­ty. As such, the fol­low­ing prop­er­ty changes are expect­ed after graphitization. 

Mechanical property changes

Graphi­ti­za­tion is known to increase a mate­ri­al’s Young’s mod­u­lus. Young’s mod­u­lus (also known as the mod­u­lus of elas­tic­i­ty) is a deter­mi­nant of a mate­ri­al’s stiff­ness or resis­tance to (recov­er­able) defor­ma­tion under an applied load. High Young’s mod­u­lus means that the mate­r­i­al is stiff and is unlike­ly to deform under an applied force. Increas­ing the Young’s mod­u­lus is ben­e­fi­cial for graphi­tized car­bon papers used as elec­trode mate­ri­als in elec­tro­chem­i­cal ener­gy stor­age devices (like fuel cells and water elec­trolyz­ers). At the core of these devices is the mem­brane elec­trode assem­bly, which is fab­ri­cat­ed by apply­ing high com­pres­sive forces. Since graphi­tized car­bon fiber papers exhib­it improved (high­er) Young’s mod­u­lus, they can with­stand high com­pres­sive stress­es with­out under­go­ing exces­sive defor­ma­tion. This is real­ly cru­cial in main­tain­ing its poros­i­ty and desir­able elec­tro­chem­i­cal prop­er­ties in gen­er­al. CAPLIN­Q’s LINQCELL GDL1500B (improved ver­sion of GDL1500) is a graphi­tized car­bon plate that has been well used as gas dif­fu­sion lay­er or porous trans­port lay­er of fuel cells and elec­trolyz­ers. Aside from its excel­lent elec­tri­cal prop­er­ties, GDL1500B also exhibits opti­mum com­press­ibil­i­ty. From its orig­i­nal thick­ness of 1.5 mm, GDL1500B can be com­pressed to approx­i­mate­ly 1.3 mm at 2 MPa. This facil­i­tates seam­less con­tact with the cat­a­lyst lay­er and there­by enhances the over­all per­for­mance of the device.

Electrical and thermal conductivity changes

Graphi­ti­za­tion increas­es the mate­ri­al’s elec­tri­cal con­duc­tiv­i­ty. As dis­cussed above, graphi­ti­za­tion trans­forms the struc­ture of amor­phous graphi­tiz­ing car­bon-based mate­ri­als into lay­ers of hexag­o­nal car­bon rings. The car­bon atoms in each ring are cova­lent­ly bond­ed, where­as the lay­ers or stacks are kept togeth­er by a weak van der Waals force. This struc­tur­al re-arrange­ment forms elec­tri­cal path­ways at which elec­trons can move through the mate­r­i­al, which con­sid­er­ably min­i­mizes the resis­tance and improves the elec­tri­cal con­duc­tiv­i­ty of the graphi­tized car­bon fiber papers. 

On a sim­i­lar note, graphi­ti­za­tion increas­es the mate­ri­al’s ther­mal con­duc­tiv­i­ty. Heat can trav­el faster in the graphitic struc­ture because the carbon–carbon bonds form a sys­tem that allows the vibra­tions caused by the tem­per­a­ture dif­fer­ences to prop­a­gate rapid­ly in between the lay­ers of the stacked structure. 

Chemical stability

Chem­i­cal sta­bil­i­ty refers to the abil­i­ty of a mate­r­i­al to retain its prop­er­ties and struc­ture under expo­sure to dif­fer­ent chem­i­cal envi­ron­ments. Graphi­ti­za­tion increas­es the resis­tance to oxi­da­tion, reduces the reac­tiv­i­ty, and sta­bi­lizes the ther­mal behav­ior of car­bon fiber papers. These have pos­i­tive impli­ca­tions in the per­for­mance of graphi­tized car­bon fiber papers over sig­nif­i­cant peri­ods of usage or application.

TLDR: Graphi­ti­za­tion improves the mechan­i­cal (i.e., increas­es the Young’s mod­u­lus), elec­tri­cal and heat trans­port capa­bil­i­ties (i.e., increas­es the elec­tri­cal and ther­mal con­duc­tiv­i­ties), and chem­i­cal sta­bil­i­ty of car­bon-based mate­ri­als.

Now, what hap­pens to these mate­r­i­al prop­er­ties with increas­ing graphi­ti­za­tion tem­per­a­ture?

Advantages of higher graphitization temperature

As a gen­er­al rule of thumb, high­er graphi­ti­za­tion tem­per­a­tures yield high­er degree of graphi­ti­za­tion. In oth­er words, car­bon-based mate­r­i­al become more graphite-like with increas­ing graphi­ti­za­tion tem­per­a­ture. From this, mate­ri­als graphi­tized at high­er tem­per­a­tures are expect­ed to exhib­it more (improved) graphite-like prop­er­ties. That is, at increas­ing graphi­ti­za­tion tem­per­a­tures, high­er Young’s mod­u­lus (low­er com­press­ibil­i­ty), high­er elec­tri­cal and ther­mal con­duc­tiv­i­ties, and bet­ter chem­i­cal sta­bil­i­ties are to be expected.

Giv­en these con­sid­er­able struc­tur­al and prop­er­ty improve­ments after graphi­ti­za­tion, you may find our graphi­tized car­bon fiber papers fit for your appli­ca­tions, may it be in fuel cells, water elec­trolyz­ers, super­ca­pac­i­tors, and bat­ter­ies. Con­tact us, and our appli­ca­tion engi­neers will assist you in select­ing the most suit­able prod­ucts tai­lored to your application.

About Rose Anne Acedera

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