**3. Dopamine-induced carbon@silica hybrid as fuel cell anode catalyst**

Aside from high proton conductivity and self-humidifying properties, principles of practical design for potential materials for polymer electrolyte membrane fuel cell (PEMFC) demand a fabrication method that is non-toxic and requires no complex apparatus for easy and largescale production. Hybrid catalysts with carbon material and inorganic material have received attention from many researchers for PEMFC [54,55].

ZnMn2O4 catalyst (Figure 14). This catalyst has the ion pair Zn2+/Mn3+ in its crystal lattice compared to Co2+/Mn3+ and Zn2+/Co2+ in the other mixed spinels. The high activity of the catalyst was attributed to the nature of the manganese ions combined with structural effects in the

**Figure 14.** Comparison of BHET yields among the pure and mixed oxide catalysts [13].

For all the glycolysis reactions above using various catalysts, the recycled BHET was recovered via simple recrystallization. Several characterization methods have been employed to verify the structure and purity of the recovered monomer. The FT-IR spectra of the recycled BHET matched that of the standard sample [10], without extra peaks characteristic of contamination. As it is also possible that dimers and oligomers are not effectively separated, thermal and structure analyses have been performed using thermogravimetric analysis (TGA) and nuclear magnetic resonance spectroscopy (NMR). The proton and carbon NMR spectra identified peaks corresponding to distinct groups in the monomer and dimer backbones [13]. Along with TGA thermogram profiles, the structure characterization verified the good separation of the

**3. Dopamine-induced carbon@silica hybrid as fuel cell anode catalyst**

Aside from high proton conductivity and self-humidifying properties, principles of practical design for potential materials for polymer electrolyte membrane fuel cell (PEMFC) demand a

**2.5. Purity of recovered BHET monomer**

BHET from its oligomers.

spinel crystal [13].

156 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

In hybrid material synthesis, a variety of methods are used to combine organic and inorganic materials. However, these fabrication methods require expensive equipment and complex processes, thereby limiting mass production [56–58]. To resolve this problem, our group has developed a simple and green fabrication of a hybrid catalyst for PEMFC. The hybrid compo‐ site consists of Pt nanoparticle, conductive dopamine coating layer, and silica support. Pt nanoparticle has been used as a typical catalyst for electrode catalyst for energy storage application. We also used Pt nanoparticle as the anode catalyst for PEMFC. To deposit Pt nanoparticles on the surface of silica, we applied dopamine coating as an adhesive layer [9]. The overall synthesis procedure for the metal and dopamine-induced carbon@silica nano‐ composite is illustrated in Figure 15.

**Figure 15.** Overall procedure for synthesis of metal (M)- and nitrogen (N)-doped carbon@silica nanocomposite.

For the preparation of a hybrid anode catalyst, silica was fabricated by water-in-oil microe‐ mulsion method that produces a narrow size distribution. Silica is generally applied as support material because of its mechanical, chemical, and thermal stability. Its size and surface property can also be easily modified. Additionally, it has hygroscopic property that maintains the humidity needed for the proton conductivity in PEMFC. The humidity dependence of the membrane's conductivity typically required humidity control using an external equipment. However, the application of silica as support for the anode catalyst could provide both humidity and conductivity to the proton conductive membrane [8].

To deposit Pt nanoparticles on the surface of silica, we applied dopamine coating as an adhesive layer and self-reducing agent. Dopamine is a mussel-inspired adhesive based on a protein located at the end of the byssus of mussels. The protein allows mussels to strongly attach on rock surfaces or support in sea. The adhesion property of the protein helps mussels hold onto substrates in wet condition. Mussel-inspired dopamine is similar in chemical structure, with catechol and amine functional group as the protein [59]. Attempting to mimic the adhesive functionality, many researchers coated dopamine on various types of substrates. After dopamine coating, the substrate achieved a more hydrophilic surface and can be well dispersed in water. Dopamine coating also acts as a metal adhesive layer through the selfreducing ability of catechol group in the dopamine chemical structure [60]. Moreover, the coating is a simple and controllable dip-coating process in alkaline water solution (pH 8.5). The coating thickness can be tailored in nanoscale by adjusting the temperature, pH, time, and concentration of dopamine. Because of these advantages, dopamine coating has been applied to a variety of fields, such as surface modification, heavy metal removal, carbon material functionalization, and fabrication of conductive N-doped carbon sources [56–58,61].

We prepared Pt and N-doped carbon@silica with dopamine coating via metal decoration and carbonization steps. The dopamine coating procedure was performed on a Tris buffer solution at pH 8.5. Silica particles were dispersed in the solution. Then, 10 mM dopamine was added and mixed mildly for 6 h. After the coating step, the particle was washed several times with DI water. A thin layer of dopamine was observed to be coated well on silica with a thickness of 11.5 nm as shown in Figures 16c and 16d. The prepared dopamine-coated silica was redispersed in DI water. The dopamine-coated silica was then used to serve as catalyst support with metal adhesion properties. The Pt nanoparticles on the coated silica were deposited through the self-reducing ability of catechol groups in dopamine, as shown in Figure 17. This phenomenon releases electrons via the transformation of the R-OH group to a C=O group, which reduce Pt ions into Pt nanoparticles.

The dopamine coating layer naturally prohibits electron transfer (low conductivity). To enhance catalyst conductivity, we conducted carbonization of the Pt and dopamine-coated silica in an inert atmosphere at 800 °C. The carbonized dopamine layer contained nitrogen (N) and had other functional groups removed. N-doped carbon layer from dopamine coating enhanced both mechanical stability and conductivity of the material [57].

We applied Pt and dopamine-induced carbon@silica hybrid composite for polymer membrane electrolyte fuel cell as anode catalyst. The composite was evaluated and compared with commercial Pt/C catalyst under zero relative humidity. The carbonized dopamine coating and silica could be useful in PEMFC under non-humidified conditions. Pt and N-doped car‐ bon@silica-based composites and commercial Pt/C were evaluated by galvanostatic polariza‐ tion and power density curves as functions of current density for a single PEMFC at zero relative humidity (Figure 18). The Pt and dopamine-induced carbon@silica composite exhib‐ ited a maximum power density of 0.55 W cm-2, which exceeds that of commercial Pt/C (0.45

To deposit Pt nanoparticles on the surface of silica, we applied dopamine coating as an adhesive layer and self-reducing agent. Dopamine is a mussel-inspired adhesive based on a protein located at the end of the byssus of mussels. The protein allows mussels to strongly attach on rock surfaces or support in sea. The adhesion property of the protein helps mussels hold onto substrates in wet condition. Mussel-inspired dopamine is similar in chemical structure, with catechol and amine functional group as the protein [59]. Attempting to mimic the adhesive functionality, many researchers coated dopamine on various types of substrates. After dopamine coating, the substrate achieved a more hydrophilic surface and can be well dispersed in water. Dopamine coating also acts as a metal adhesive layer through the selfreducing ability of catechol group in the dopamine chemical structure [60]. Moreover, the coating is a simple and controllable dip-coating process in alkaline water solution (pH 8.5). The coating thickness can be tailored in nanoscale by adjusting the temperature, pH, time, and concentration of dopamine. Because of these advantages, dopamine coating has been applied to a variety of fields, such as surface modification, heavy metal removal, carbon material

functionalization, and fabrication of conductive N-doped carbon sources [56–58,61].

which reduce Pt ions into Pt nanoparticles.

158 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

We prepared Pt and N-doped carbon@silica with dopamine coating via metal decoration and carbonization steps. The dopamine coating procedure was performed on a Tris buffer solution at pH 8.5. Silica particles were dispersed in the solution. Then, 10 mM dopamine was added and mixed mildly for 6 h. After the coating step, the particle was washed several times with DI water. A thin layer of dopamine was observed to be coated well on silica with a thickness of 11.5 nm as shown in Figures 16c and 16d. The prepared dopamine-coated silica was redispersed in DI water. The dopamine-coated silica was then used to serve as catalyst support with metal adhesion properties. The Pt nanoparticles on the coated silica were deposited through the self-reducing ability of catechol groups in dopamine, as shown in Figure 17. This phenomenon releases electrons via the transformation of the R-OH group to a C=O group,

The dopamine coating layer naturally prohibits electron transfer (low conductivity). To enhance catalyst conductivity, we conducted carbonization of the Pt and dopamine-coated silica in an inert atmosphere at 800 °C. The carbonized dopamine layer contained nitrogen (N) and had other functional groups removed. N-doped carbon layer from dopamine coating

We applied Pt and dopamine-induced carbon@silica hybrid composite for polymer membrane electrolyte fuel cell as anode catalyst. The composite was evaluated and compared with commercial Pt/C catalyst under zero relative humidity. The carbonized dopamine coating and silica could be useful in PEMFC under non-humidified conditions. Pt and N-doped car‐ bon@silica-based composites and commercial Pt/C were evaluated by galvanostatic polariza‐ tion and power density curves as functions of current density for a single PEMFC at zero relative humidity (Figure 18). The Pt and dopamine-induced carbon@silica composite exhib‐ ited a maximum power density of 0.55 W cm-2, which exceeds that of commercial Pt/C (0.45

enhanced both mechanical stability and conductivity of the material [57].

**Figure 16.** SEM and TEM images of (a and b) silica, (c and d) dopamine-coated silica, and (e and f) Pt and dopamineinduced carbon@silica. Scale bars are 100 nm.

**Figure 17.** Reduction pathway from Pt ion into Pt nanoparticle using self-reducing ability of dopamine coating layer.

W cm-2). The improved performance of the composite is attributed to the hybrid synergy effect of silica hygroscopic property and N-doped dopamine-induced carbon layer. Additionally, the N-doped carbon obtained after the carbonization step could make the composite more conductive with the enhancement of charge transfer at interfaces. As a result, these advantages of the Pt and dopamine-induced carbon@silica composite could contribute to enhance performance under low humidity conditions compared to the commercial Pt/C catalyst.

**Figure 18.** I-V polarization (left axis) and power density (right axis) curve for PEMFC single cell (H2/O2) with Pt and Ndoped carbon@silica nanocomposites (red triangle) and commercial Pt/carbon (black circle) used as an anode catalyst under relative humidity at 0%.
