Introduction

Proton-exchange membrane fuel cells (PEMFC) create power by changing over chemical energy (hydrogen and oxygen gas) to electrical power. Because of the high cost of PEMFC components, the usage of this cell is limited1. The bipolar plates are the one of the main components in fuel cell. It works as conductor for electrical current from cell to cell2. The graphite is the foremost commonly utilized material for bipolar plates manufacture3. It has many advantages such as the great corrosion resistance. On another hand, there are many problems facing the use of graphite as the bipolar plates such as its brittle texture and high gas permeability4. The use of metallic materials for constructing bipolar plates has been highly welcomed in scientific circles5,6,7. They characterize by high electrical conductivity and low cost. Most commercialized bipolar plates made nowadays are stainless steel8. Metal corrosion is a huge problem, particularly in bipolar plates9. The presence of corrosion products and passive layer on the bipolar plate’s surface decrease the performance of PEMFC. To address this problem, many scientists have developed different conductive coatings to screen the metallic bipolar plates10,11,12. These coatings prevent the bipolar plate’s corrosion and consequently improve the PEMFC performance. Researchers have taken a major step towards protect the bipolar plates using conductive polymer coatings such as polyaniline (PANI)13. This kind of polymers is characterized by good conductivity and high thermal stability14. To maximize the efficiency of PANI, the combination of carbon nanotubes (CNT) with PANI was developed by many researchers15. Ramezanzadeh16 proposed polyaniline modified GO nanosheets coatings to improve the performance of stainless steel bipolar plate. Sharma and his colleagues17 improved the corrosion Resistance stainless steel bipolar plates using composite PANI and titanium nitride nanoparticle. Jiang et al.18 investigated the graphene oxide incorporated polypyrrole(PPY) matrix. The results showed that PPY-GO composite coatings work as good anti-corrosion coatings for stainless steel bipolar plates in the aggressive solutions. Gao et al.19 reported phosphomolybdic acid doped PANI coating for corrosion protection of 303SS. Show and his colleagues20 used CNT/PTFE composite coating for stainless steel bipolar plate. This coating decreased the contact resistance and increased the output power of the fuel cell. Here, we prepared a new composite coating for stainless steel bipolar plate. The base of this composite coating is polyaniline polymer (PANI) with Zn-Porphyrin (Zn-Pr). The main functions of new composite are the increasing the corrosion resistance of stainless steel bipolar plate and the enhancing output power of the fuel cell. The porphyrin molecules have very attractive properties. Its structure defined as a group of heterocyclic macrocycle organic compounds21. The insert of metals like Zn, Ni, and Co into porphyrin macrocycle structure influence on the optical absorption spectrum and the electrical and magnetic properties21. To our knowledge, this is the first study to use PANI/Zn-Pr composites coatings for PEMFC.

Experimental

Materials

Grade 303 stainless steel (303SS) (composition %: 0.15 C; 2.0 Mn; 1.0 Si; 0.2 P; 0.15 S; 17 Cr; 8 Ni; balance Fe) was used as the bipolar plate. The 303SS was cut into rectangle shape specimens with total surface area 1.12 cm2. These specimens were cleaned according to standard methods ASTM G1–03.

Polyaniline polymer and sulfuric acid (98%) were supplied from SigmaeAldrich Co. Xylene was supplied from PRABHAT CHEMICALS Co.

Zn-Porphyrin (Fig. 1) was synthesized according to the reported procedure22.

Figure 1
figure 1

Zn-Porphyrin structure.

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PANI/Zn-Pr composites preparation and application of coating

The xylene (10 ml) and Zn-Pr powder (0.5, 0.8 and 1.0 gm) were mixed using mechanical stirrer (part 1). The xylene and PANI (1:1 ratio) (90 ml) were mixed using a high speed mechanical stirrer (part 2). The final composite was obtained by mixing part 1 and part 2 using mechanical stirrer followed by ultrasonication (3.0 h) and then ground for 1.0 h to obtain the desired fineness. PANI/Zn-Pr composites coatings were applied on the whole surface of clean 303SS using spray gun (Walther PILOT). The coated 303SS samples were cured at 343 K for 1.0 h.

The dry thickness of the PANI and PANI/Zn-Pr composites films was in the range 53 ± 5 μm using Elcometer 456 gauge (Elcometer Co).

Permeability Testing Cups (BYK Instruments) offer a simple method to check the permeability of PANI/Zn-Pr composites coatings within a 24 h period.

Electrochemical measurements

The anti-corrosion of new PANI/Zn-Pr composite coating was evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques.

The polarization curves were recorded using recommended scan rate (1.25 mV s−1) in the potential range ± 250 mV with regard to the open circuit potential.

EIS plots were recoded in the the frequency range 0.01 Hz–100 kHz at the open circuit potential. The AC voltage amplitude was 10 mV. Electrochemical data were collected using Potentiostat/Galvanostat (model Gill AC, 947, ACM).

The electrochemical experiments were repeated at least 3 times to ensure accuracy. All values are presented in the form of mean values and standard deviation.

PEMFC performance measurements

For the PEMFC performance test, the laboratory single cell fuel cell testing stations from Fuel Cell Store was used. In this cell, H2 and O2 gas were flow with rate 150 ml/min. Bare 303SS and coated 303SS were used as bipolar plates. Nafion 117 was used as the proton electrolyte membrane. Platinum particles on acetylene black powder (as support) were used as the catalyst on the surface of anode and cathode electrodes.

Results and Discussion

Anti-corrosion performance of PANI/Zn-Pr composites

We first recoded the polarization responses of uncoated stainless steel bipolar plate 303SS in 1.0 M H2SO4 solution. The same experiments were applied for coated 303SS by neat PANI and PANI/Zn-Pr composites. All these curves are collected in Fig. 2.

Figure 2
figure 2

Polarization curves of uncoated and coated 303SS by neat PANI and PANI/Zn-Pr composites in 1.0 M H2SO4 solution at 298 K.

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The corrosion potential (Ecorr) and corrosion current density (jcorr) were extracted form Tafel curves23 (see Table 1) to assess anti-corrosion performance of PANI/Zn-Pr composites.

Table 1 Electrochemical parameters and the corresponding corrosion inhibition efficiency for uncoated and coated 303 SS in 1.0 M H2SO4 solution at 298 K.
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The jcorr values of coated 303SS by neat PANI and PANI/Zn-Pr composites were much lowers than that of the control 303SS. The incorporation of Zn-Pr molecules into PANI leads to increase its anti-corrosion performance. This is clearly shown through the considerable decrease of jcorr values form 5.90 μA cm−2 to 0.15 μA cm−2. It was noted also there is a significant shift in Ecorr to more noble direction in the cases of PANI/Zn-Pr composites, indicating the protective effect of new composites24. Here, the composites layer covered both cathodic and anodic sites on the surface of 303 SS and caused the shifting in the Ecorr values.

We calculated anti-corrosion efficiencies (ηj%) of coatings from the Eq. (1)25.

$${{\eta }}_{j} \% =\frac{{{j}}_{{\rm{corr}}}^{o}-{{j}}_{{\rm{corr}}}^{c}}{{{j}}_{{\rm{corr}}}^{o}}\times 100$$
(1)

where jocorr and jccorr are the corrosion rates for uncoated and coated 303 SS, respectively.

In the case of coated samples with neat PANI, the ηj% was 77.02% (see Table 1). The use of PANI/Zn-Pr composites leads to enhance the anti-corrosion efficiency of PANI. The maximum efficiency was obtained at 1.0% of Zn-Pr (Table 1).

The permeability of composites coatings is an important property to measure the electrolyte transfer through a coatings film. The presence of pores across the PANI coating is responsible for allowing corrosive cell electrolyte permeation. Therefore, blocking these pores can result in effectively hindering the corrosion of bipolar plates. In this respect, the dispersion of Zn-Pr particles in the texture of PANI coating leads to low electrolyte permeability of the PANI/Zn-Pr composites compared to neat PANI coating (see Table 2).

Table 2 Water permeability of PANI coating in the absence and presence of Zn-Pr at 298 K.
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Electrochemical impedance spectroscopy

To exclude the effect of anti-corrosion and conductivity activities caused by the addition of Zn-Pr, we measured the electrochemical impedance responses (Nyquist plot) of bare 303SS, neat PANI and PANI/Zn-Pr composites in 1.0 M H2SO4 solution. All measurements were achieved after 7 days.

We found that the Nyquist plots for bare 303SS comprise of two capacitive loops (see Fig. 3). The best an equivalent circuit described this case is given in Fig. 4.

Figure 3
figure 3

Nyquist plot of bare 303 SS in 1.0 M H2SO4 solution at 298 K.

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Figure 4
figure 4

Equivalent circuit to fit the impedance bare 303 SS in 1.0 M H2SO4 solution.

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The electrolyte resistance was represented by Rs. The passive film resistance and capacitance were represented by Rf and Cf, respectively. Rct and Cdl are charge transfer resistance and double layer capacitor related to 303SS/H2SO4 solution interface26. We noted that the Nyquist plots for coated samples by the neat PANI and PANI/Zn-Pr composites were changed to one capacitive loop (at high frequencies) and straight line (at low frequencies) (Fig. 5)27. This line is due to diffusion impedance of the coating barrier layer (Zd)28. The equivalent circuit described, the PANI and PANI/Zn-Pr composites (0.5% Zn-Pr) is shown in Fig. 6. In this case, Cc and Rc represent the capacitance and resistance of the coating layer, respectively.

Figure 5
figure 5

Nyquist plots for coated 303 SS by the neat PANI and PANI/Zn-Pr composites in 1.0 M H2SO4 solution at 298 K.

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Figure 6
figure 6

Equivalent circuit to fit the impedance coated 303 SS by the neat PANI and PANI/Zn-Pr composite (0.5% Zn-Pr) in 1.0 M H2SO4 solution.

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By incorporation 1.0% of Zn-Pr into PANI coating, the Zd part for diffusion process was disappeared and the equivalent circuit was represented only by Cc and Rc elements (see Fig. 7).

Figure 7
figure 7

Equivalent circuit to fit the impedance coated 303 SS by PANI/Zn-Pr composite (1.0% Zn-Pr) in 1.0 M H2SO4 solution.

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All the equivalent circuit elements for uncoated and coated samples are listed in Tables 3 and 4.

Table 3 Fitted electrochemical parameters of 303 SS in 1.0 M H2SO4 solution at 298 K.
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Table 4 Fitted electrochemical parameters of coated 303 SS in 1.0 M H2SO4 solution at 298 K.
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Initially the Rc value of PANI coatings is comparatively low than for passive film Rf. This implies that the PANI coatings have considerable conductivity if it compares with the passive layer formed on the bare 303SS18,29. By incorporation Zn-Pr into PANI coating, the coating resistance Rc increased and the coating capacitance CC decreased.

Here, Zn-Pr particles repair the PANI coating flaws and block the passage of corrosive solution towards 303 SS substrate and caused the changing in the Rc and CC values.

This indicates that Zn-Pr plays a vital role in enhancing the anti-corrosion property of PANI coatings. Furthermore, at 1.0% of Zn-Pr, the PANI coating become a very good barrier for restriction the diffusion of corrosive solution.

PEMFC performance

To assess the effect of the PANI/Zn-Pr composites coating on the PEMFC Performance, we explored the reliance of the current density on the output cell voltage and the power density for single PEMFC cell using uncoated or coated 303SS bipolar plates. This data was showed in Fig. 8(a,b).

Figure 8
figure 8

Output cell voltage (a) and power density (b) for single PEMFC cell using uncoated and coated 303 SS bipolar plates.

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The open voltage for all samples was 0.805 V. Generally, the out voltage cell decreased with current density increasing due to IR drop30. The out voltage in the case of coated 303SS bipolar plates by PANI/Zn-Pr composites was significantly decreased comparing with bare bipolar plates and neat PANI bipolar plates (see Fig. 8(a)).

The supreme power density for single PEMFC cell using uncoated 303SS bipolar plates is 435 mW cm−2 (Fig. 8(b)). This value was increased to 720 and 1353 mW cm−2 in the case of neat PANI and PANI/Zn-Pr composite, respectively. This confirms that the using of PANI-Zn-Pr composite leads to increase the performance of PEMFC cell.

Mechanism and explanation

The above results reveal the strong effect of Zn-Pr on the anti-corrosion effect of PANI coating for 303SS bipolar plates, leading to an increased PEMFC performance.

The 303SS bipolar plates suffered from the corrosion and dissolution during the immersion in cell electrolyte. In addition, the passive film formed during the corrosion process, led to the increase in the contact resistance and the membrane electrodes contamination31,32.

The PANI coating plays as a barrier for 303SS bipolar plates. Where this coating decreases the contact between the 303SS surface and corrosive electrolyte. But the performance of PANI coating is not high enough to prevent the bipolar plate’s corrosion due to the presence of pores in the texture of PANI polymer33. These pores allow the passage of corrosive solution to contact with the bipolar plates34. The corrosive ions enhance the active dissolution of 303SS35,36. These ions may lead to pitting corrosion and induce metal failure37.

The incorporation of Zn-Pr with PANI polymer to form PANI/Zn-Pr composite, leads to efficiency coating barrier. The main role of Zn-Pr is the reducing the pores in the PANI polymer texture10,38. This will significantly affect on the degree of contact between the 303SS surface and corrosive electrolyte.

Additionally, the presence of Zn-Pr molecules act as a conductive path though PANI polymer, leading to the increase in the output power of the PEMFC cell39,40. The conducting characterization of Zn-Pr molecules is due to the aromaticity of the macrocycle, leading to the electrons mobility though the PANI/Zn-Pr composite41,42.

Conclusions

PANI/Zn-Pr composites coatings have been developed, which is effective to significantly increase the output power of the PEMFC cell and decrease the degree of contact between the 303SS bipolar plates and corrosive electrolyte. Anti-corrosion properties of PANI/Zn-Pr composites were confirmed using polarization experiments to define their anti-corrosion efficiency. The highest anti-corrosion activity of the PANI/Zn-Pr composite (i.e. 99.41%) was obtained at 1.0% of Zn-Pr. EIS data also further confirmed that the Zn-Pr plays a significant role in enhancing the coating resistance of PANI. The use of coated 303SS bipolar plates by PANI/Zn-Pr composites leads to increase the output density for single PEMFC cell. Therefore, the new PANI/Zn-Pr composites are considered effective coatings for bipolar plates in future PEMFC technologies.