**2.1. Materials**

it has been shown that heteroatom doping can induce redistribution of graft load. In this regard, heteroatom doped graphene such as N, S, P, B have recently demonstrated that they can effectively improve the ORR catalytic activity [26–30]. It has been suggested that the dopant (whether its electronegativity toward carbon is greater (such as N, S) or lower (such as

and the cost of electrocatalysts in fuel cells, non-Pt catalysts supported on graphene have also been developed in recent years. For N-doped graphene, the ORR catalytic activity is strongly dependent on the nitrogen types and the doping concentration. Nitrogen-doped graphene nanocomposites, have demonstrated an improved electrocatalytic activity of ORR, due to an interpenetration network formed between N and graphene, which can efficiently accelerate the reaction, the transport of ions and electrons and therefore synergistically improves the catalytic activity of ORR. The interaction between graphene support and composites may also affect the stability of electrocatalysts. For example, the strong link between N-doped sites of

The main drawbacks of the mentioned catalysts are the preparation methods with multiple operating activities as well as the sophisticated equipment, making the processes less attractive to be transposed on a larger production scale. Other disadvantages are reaction conditions involving

Calculations using functional density theory (DFT) showed that the electrocatalytic activity of the heteroatom doped graphene is strongly dependent on the electronic spin density and the distribution of the electric charge density on the atoms. Catalytic active sites of doped graphene are typically high density spinning carbon atoms. N, P or B doped graphene introduces unpaired electrons and determines a local high density resulting in a high electrocatalytic ORR performance. Halogens are other important elements that offer new properties for alternative energy devices and technologies due to the effect of the difference in electronegativity between halogen atoms (x = 2.66–3.98) and C atoms (x = 2.55). They have a different electron loss capac-

ized bonds to promote the transfer of the burden due to its large atomic size (the largest in the halogen group) [31–35]. In addition, it is known that the sides of the halogenated graphite have

Previous studies have shown that the single layered basal structure of graphene can guarantee its electrochemical durability. In fact, the carbon corrosion starts from graphite defects, and carbonaceous materials with several graphite layers usually exhibit fewer structural defects. Therefore, the intrinsic grafting capacity of the graphene could improve the durability of graphene composite materials. More efforts need to be devoted to scalable and reproducible synthesis, with a compositional and morphological control, as well as investigations into properties and catalytic mechanism. In this area, major efforts have been made to refine these properties, including the adsorption of halogen molecules on the surface of the monolayer graphene as a promising approach due to the diversity of halogen properties and the variety of formed structures. The adsorption of atoms and halogen molecules on the graphene layer has been studied both theoretically and experimentally to adjust the electronic structure of the graphene layer.

thus facilitating efficient conversion into water after reduction and protonation.

(x = 3.44). It is important to note that iodine halide may form partially ion-

and to weaken the O─O bond from the adsorbed oxygen,

high temperatures for thermal decomposition, high vacuum or supercritical conditions.

adsorption. To reduce Pt loading

B) could create electronically loaded sites favorable for O<sup>2</sup>

82 Advances In Hydrogen Generation Technologies

graphene may result in increased resistance of hybrid catalysts [25].

ity compared to O2

−

sufficient possibilities to attract O<sup>2</sup>

Graphite powder, K2 S2 O8 , P2 O5 , conc. H2 SO<sup>4</sup> , KMnO4 , HI were purchased from Sigma-Aldrich., H2 O2 and HCl were obtained from Oltchim SA Romania. Carbon paper gas diffusion layer (GDL, SGL), membrane (Nafion-212), ionomer solution (5 wt.% Nafion) were purchased from Ion Power, USA. Commercial catalyst (HISPEC 4000 Pt/C 40 wt.%) was purchased from Alfa Aesar. The purity of reactants (H2 and O2 ) was 99.999%.

mixing water, 5 wt.% Nafion solution (DuPont), water and isopropyl alcohol (IPA) (Aldrich), ionomer/water/isopropyl alcohol = 6/14/80 (volume), with the Pt/C catalyst (Hispec 4000 Alfa Aesar). The prepared catalyst ink was mixed in an ultrasonic bath (30°C, 2 h) and sprayed using Sono-Tek ultrasonic coating equipment (Exacta Coat, Sono-Tek Corporation, USA), at a flow rate of 0.5 ml min−1, onto the both side of pretreated Nafion 212 membrane in order to fabricate the catalyst coated membrane (CCM). The catalyst loadings were 0.2 mg cm−2 at the anode and cathode respectively. Two procedures of operation were used: (i) ultrasonic-spray procedure of catalytic ink containing Pt/C in order to obtain 0.2 mg cm−2 Pt at each side and (ii) I-doped graphene 0.2 mg cm−2 was integrated into cathode electrode by ultrasonic-spraying on gas diffusion layer GDL (carbon paper Toray TGP-H-120). Thus, the cathode catalyst layer was modified by taking into account the deposition of I-doped graphene supplementary sprayed on GDL (EC-TP1-090T, carbon cloth Toray, USA). The as prepared electrodes were hot-pressed on GDL on each side, for 2 min at 300 kgf and 120°C. Then, the obtained membrane electrode assembly (MEA), together with silicon type gaskets were introduced in a

Iodine Doped Graphene for Enhanced Electrocatalytic Oxygen Reduction Reaction in PEM Fuel…

The electrochemical evaluation was performed in a single fuel cell system PEMFC with an

a configured workstation (PARSTAT 2273), fuel cell (ElectroChem, USA), DS electronic load (AMETEK Sorensen SLH 60 V/120 A 600 W), bubble-type humidifier (ARBIN DPHS 10, USA). The PEMFC was operated at 0.3 V for 0.5 h and at 0.5 V for MEA conditioning, until the stable voltage was maintained uniformly. After steady state operating conditions were maintained, the fuel cell polarization plots and ORR performances were recorded. The flow rates of reac-

lers (Alicat Scientific, USA) calibrated before experiments. The cell temperature and pressure were set at 60–65°C and 1 bar pressure. The relative humidity for the anode and the cathode was set to 80 and 90%, respectively. The polarization curves were taken with a scan rate of 1 mV s−1. Polarization curves were taken in a galvanostatic mode with a hold time of 5 min per point. The negative and the positive sweeps were performed and the average values were

N2 flow was kept to almost zero, from 1.2 to 0.05 V at a scan rate of 0.05 V/s. The developed control system based on NI c-RIO hardware was used to control the PEMFC system. The electrochemically active Pt surface area (ECSA) was derived by the integration of the inferior hydrogen adsorption peaks. The software used to control and operate the test station with the

The characterization of the prepared materials was firstly performed in order to validate the

(ElectroChem, USA). The *home-made* electrochemical test station includes

(ElectroChem, USA).

http://dx.doi.org/10.5772/intechopen.76495

85

and air) were adjusted using flow control-

/N<sup>2</sup>

mode cell, while the

single PEM fuel cell system with an active area of 5 cm2

tants (100 ml min−1 and 250 ml min−1) gases (H2

presented. Cyclic voltammetry measurements were performed in a H2

**3.1. Morphological and structural properties of I-doped graphene**

structural quality and to confirm the iodine presence.

acquiring of experimental data has been developed in LabVIEW® environment.

*2.4.2. Electrochemical measurements*

**3. Results and discussions**

active area of 25 cm2

### **2.2. Catalysts preparation**

I-doped graphene electrocatalyst was synthesized via a facile process described in detail elsewhere, through nucleophilic substitution of graphene oxide (GrO) by reduction with hydroiodic acid (HI) catalyzed by AlI3 [24, 25]. Briefly, the graphite oxide (GO) was prepared starting from graphite powder by a modified Hummers method including specific steps, as follows. The pre-oxidation was used to prepare the preoxidized GO, namely the graphite (7.5 g), K2 S2 O8 (6 g), and P2 O5 (6 g) were introduced into conc. H2 SO<sup>4</sup> (50 ml) and P2 O5 (50 g), under continuously stirring at 80°C. The product was washed, filtrated, dried at 60°C. The as pre-oxidized GO was mixed into conc. H2 SO<sup>4</sup> , and then KMnO4 (45 g) was slowly added during stirring and cooling in water-ice bath. The suspension was stirred at 40°C until it became brown, and then was diluted using de-ionized water. H2 O2 30 wt.% (50 ml,) solution was slowly introduced. The yellow mixture was centrifuged, washed with a 1:10 HCl aqueous solution in order to remove residual metal ions. The obtained GO solution was dispersed by stirring using an IKA Ultraturrax T 25 (2 h), and ultrasonic bath (ELMA T 490DH model) at 110 W/40 kHz and 35°C (4 h). Graphene oxide (GrO) 4 g L−1 was obtained. Taking out a sharepart of as-prepared GrO dispersion, the hydroiodic acid HI 55 wt.% (170 g) was added (in 4 h) at 80°C, as reduction agent and precursor iodine dopant. The obtained mixture was washed using de-ionized water for several times, dried to constant weight at 50°C (more than 8 h), and grated to powder. The final step was the elemental iodine removal by repeated extraction in acetone using a Soxhlet extractor.

### **2.3. Catalysts characterization**

The microstructure and morphology of prepared samples were evaluated by using the following equipment: field emission scanning electron microscope (FESEM SU 5000 Hitachi) equipped with EDS-energy dispersive X-ray spectroscopy and WDS-wavelength dispersive; X-ray photoelectron spectroscope (XPS, Quantera SXM equipment), with a base pressure in the analysis chamber of 10−9 Torr and X-ray source Al Kα radiation (1486.6 eV, monochromatized) and the overall energy resolution estimated at 0.65 eV by the full width at half maximum (FWHM) of the Au4f7/2 line; Fourier transform infrared (FTIR) spectrometer (Nicolet Impact 410, Thermo Fisher, USA); Autosorb IQ (Quantachrome, USA) with adsorption and desorption experiments performed at 77 K after initial pre-treatment of the samples by degassing at 115°C for 4 h; X-ray fluorescence spectrometer (XRF, Rigaku ZSX Primus II), equipped with an X-ray tube with Rh anode, 4.0 kW power, with front Be window of 30 μm thickness.

#### **2.4. Electrode preparation and electrochemical measurements**

#### *2.4.1. Electrode preparation and MEA assembling*

A detailed description of the electrodes and membrane electrode assembly fabrication procedure was reported in our previous studies. Summarizing, a catalyst ink was prepared by mixing water, 5 wt.% Nafion solution (DuPont), water and isopropyl alcohol (IPA) (Aldrich), ionomer/water/isopropyl alcohol = 6/14/80 (volume), with the Pt/C catalyst (Hispec 4000 Alfa Aesar). The prepared catalyst ink was mixed in an ultrasonic bath (30°C, 2 h) and sprayed using Sono-Tek ultrasonic coating equipment (Exacta Coat, Sono-Tek Corporation, USA), at a flow rate of 0.5 ml min−1, onto the both side of pretreated Nafion 212 membrane in order to fabricate the catalyst coated membrane (CCM). The catalyst loadings were 0.2 mg cm−2 at the anode and cathode respectively. Two procedures of operation were used: (i) ultrasonic-spray procedure of catalytic ink containing Pt/C in order to obtain 0.2 mg cm−2 Pt at each side and (ii) I-doped graphene 0.2 mg cm−2 was integrated into cathode electrode by ultrasonic-spraying on gas diffusion layer GDL (carbon paper Toray TGP-H-120). Thus, the cathode catalyst layer was modified by taking into account the deposition of I-doped graphene supplementary sprayed on GDL (EC-TP1-090T, carbon cloth Toray, USA). The as prepared electrodes were hot-pressed on GDL on each side, for 2 min at 300 kgf and 120°C. Then, the obtained membrane electrode assembly (MEA), together with silicon type gaskets were introduced in a single PEM fuel cell system with an active area of 5 cm2 (ElectroChem, USA).

#### *2.4.2. Electrochemical measurements*

(GDL, SGL), membrane (Nafion-212), ionomer solution (5 wt.% Nafion) were purchased from Ion Power, USA. Commercial catalyst (HISPEC 4000 Pt/C 40 wt.%) was purchased from Alfa

I-doped graphene electrocatalyst was synthesized via a facile process described in detail elsewhere, through nucleophilic substitution of graphene oxide (GrO) by reduction with

starting from graphite powder by a modified Hummers method including specific steps, as follows. The pre-oxidation was used to prepare the preoxidized GO, namely the graphite

(6 g) were introduced into conc. H2

under continuously stirring at 80°C. The product was washed, filtrated, dried at 60°C. The as

ing stirring and cooling in water-ice bath. The suspension was stirred at 40°C until it became

slowly introduced. The yellow mixture was centrifuged, washed with a 1:10 HCl aqueous solution in order to remove residual metal ions. The obtained GO solution was dispersed by stirring using an IKA Ultraturrax T 25 (2 h), and ultrasonic bath (ELMA T 490DH model) at 110 W/40 kHz and 35°C (4 h). Graphene oxide (GrO) 4 g L−1 was obtained. Taking out a sharepart of as-prepared GrO dispersion, the hydroiodic acid HI 55 wt.% (170 g) was added (in 4 h) at 80°C, as reduction agent and precursor iodine dopant. The obtained mixture was washed using de-ionized water for several times, dried to constant weight at 50°C (more than 8 h), and grated to powder. The final step was the elemental iodine removal by repeated extraction in

The microstructure and morphology of prepared samples were evaluated by using the following equipment: field emission scanning electron microscope (FESEM SU 5000 Hitachi) equipped with EDS-energy dispersive X-ray spectroscopy and WDS-wavelength dispersive; X-ray photoelectron spectroscope (XPS, Quantera SXM equipment), with a base pressure in the analysis chamber of 10−9 Torr and X-ray source Al Kα radiation (1486.6 eV, monochromatized) and the overall energy resolution estimated at 0.65 eV by the full width at half maximum (FWHM) of the Au4f7/2 line; Fourier transform infrared (FTIR) spectrometer (Nicolet Impact 410, Thermo Fisher, USA); Autosorb IQ (Quantachrome, USA) with adsorption and desorption experiments performed at 77 K after initial pre-treatment of the samples by degassing at 115°C for 4 h; X-ray fluorescence spectrometer (XRF, Rigaku ZSX Primus II), equipped with an X-ray tube with Rh anode, 4.0 kW power, with front Be window of 30 μm thickness.

A detailed description of the electrodes and membrane electrode assembly fabrication procedure was reported in our previous studies. Summarizing, a catalyst ink was prepared by

, and then KMnO4

O2

SO<sup>4</sup>

[24, 25]. Briefly, the graphite oxide (GO) was prepared

SO<sup>4</sup>

(50 ml) and P2

(45 g) was slowly added dur-

30 wt.% (50 ml,) solution was

O5

(50 g),

) was 99.999%.

and O2

Aesar. The purity of reactants (H2

84 Advances In Hydrogen Generation Technologies

hydroiodic acid (HI) catalyzed by AlI3

(6 g), and P2

pre-oxidized GO was mixed into conc. H2

acetone using a Soxhlet extractor.

**2.3. Catalysts characterization**

O5

brown, and then was diluted using de-ionized water. H2

**2.4. Electrode preparation and electrochemical measurements**

*2.4.1. Electrode preparation and MEA assembling*

**2.2. Catalysts preparation**

(7.5 g), K2

S2 O8

> The electrochemical evaluation was performed in a single fuel cell system PEMFC with an active area of 25 cm2 (ElectroChem, USA). The *home-made* electrochemical test station includes a configured workstation (PARSTAT 2273), fuel cell (ElectroChem, USA), DS electronic load (AMETEK Sorensen SLH 60 V/120 A 600 W), bubble-type humidifier (ARBIN DPHS 10, USA). The PEMFC was operated at 0.3 V for 0.5 h and at 0.5 V for MEA conditioning, until the stable voltage was maintained uniformly. After steady state operating conditions were maintained, the fuel cell polarization plots and ORR performances were recorded. The flow rates of reactants (100 ml min−1 and 250 ml min−1) gases (H2 and air) were adjusted using flow controllers (Alicat Scientific, USA) calibrated before experiments. The cell temperature and pressure were set at 60–65°C and 1 bar pressure. The relative humidity for the anode and the cathode was set to 80 and 90%, respectively. The polarization curves were taken with a scan rate of 1 mV s−1. Polarization curves were taken in a galvanostatic mode with a hold time of 5 min per point. The negative and the positive sweeps were performed and the average values were presented. Cyclic voltammetry measurements were performed in a H2 /N<sup>2</sup> mode cell, while the N2 flow was kept to almost zero, from 1.2 to 0.05 V at a scan rate of 0.05 V/s. The developed control system based on NI c-RIO hardware was used to control the PEMFC system. The electrochemically active Pt surface area (ECSA) was derived by the integration of the inferior hydrogen adsorption peaks. The software used to control and operate the test station with the acquiring of experimental data has been developed in LabVIEW® environment.
