# Carbon Fiber Papers Prepared by Wet-Laid Technique Using PVB/PF Composite Fibers as the Binders
## 1 Introduction  
Carbon fiber paper (CFP), with the properties of high porosity, good electrical conductivity, excellent mechanical strength, outstanding corrosion resistance, etc. [1], has become the ideal material as a gas diffusion layer (GDL) of proton exchange membrane fuel cells (PEMFCs) and provides reactant and product permeability, transport of electrons, and heat and support of membrane electrode assembly (MEA) [2; 3; 4]. The first commercially available CFP was produced by the wet-laid technique, and the process normally consisted of four steps: papermaking, impregnation of phenol-formaldehyde resin (PF), compression molding, and heat treatment. In the first step, binders like polyvinyl alcohol (PVA) and polyvinyl butyl (PVB) are dispersed in water with the chopped carbon fibers (CFs). These binders are also required to absorb more PF during the impregnation process to increase the residual carbon content after carbonization and finally improve the CFP's tensile strength, electrical properties, etc. Typically, the mass ratio of PVA fibers and CFs in the dispersion is fixed to 1-1.5:10 [5; 6], for a low addition of PVA may not play the role of binding CFs and absorbing much PF, while too much PVA will lead to floccules. However, these binders should be dissolved in some solvents to obtain dispersion with chopped CFs for papermaking, which causes severe environmental problems. PVA fibers are in a half-dissolved state in water, so the addition of PVA in the mats after filtration is relatively low and uncontrollable. Meantime, the residual PVA in the water causes serious water pollution and huge water-treatment cost. The dispersion then goes through a wire screen to remove the water and the resultant mats are dried to absorb more PF. The subsequent PF impregnation is usually processed in a PF/ethanolsolution and always leads to an uncontrollable distribution of resin and uniformity of mats due to the gravity of fluid during the drying process. After impregnation with PF, the mats are heated to approximately 150 \({}^{\circ}\)C for solvent evaporation. The solvents are difficult to recycle, resulting in an unfriendly operating environment. For further absorption of PF to achieve better strength of CFP, the mats are performed with impregnation and compression molding process several times, which costs a lot of time and energy [7]. In recent years, many efforts have focused on improving the CFP's properties rather than the green and low-cost manufacturing techniques, including product permeability, mechanical properties, electrical conductivity, etc. [8; 9; 10; 11; 12; 13]. Functional components like graphite [6; 14], carbon nanotubes (CNTs) [15; 16; 17; 18; 19; 20; 21], and carbon black (CB) [22] were introduced into the CFP in the process of PF impregnation. However, the increase of one property always accompanies sacrificing others in these methods, and these processes are much more complicated. Considering the wet-laid technique is still the most mainstream method, green manufacturing approaches, for example, to reduce water pollution and simplify the preparation process to lower production costs, are urgently needed. In order to address this issue, some bio-materials were developed as substitutes for CFs and traditional binders. For example, [PERSON] et al. used natural wood as the carbon substrate precursor. The unique, accessible perpendicular channels and the presence of a microporous layer of this carbon substrate after heat treatment provided a possibility for practical application [23]. Similarly, inexpensive, natural fiber-based papers and fabric selected as the carbon substrates by [PERSON] et al. also showed the potential to lower the cost of CFP making [24]. As for binders, cellulose fibers [25; 26], the most abundant renewable material, as a substitution for conventional binders, were reported without using polluting, volatile organic compounds.