**3. Results and discussions**

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

The characterization of the prepared materials was firstly performed in order to validate the structural quality and to confirm the iodine presence.

The morphology and microstructure of as-prepared samples were observed using scanning electron microscopy (SEM). I-doped graphene materials have a typical wrinkled microstructure, fluffy and almost transparent, overlapping layers, forming a cluster composed of many tiny sheets with corrugation and scrolling, building a good porosity (**Figure 1**). These features are highly valuable for PEMFC electrode applications, providing a high surface area and ensuring efficient mass transport and good catalytic sites accessibility. The energy-dispersive X-ray spectroscopy (EDX) used in addition to SEM was used as microanalysis technique for a rough appraisal of chemical composition. The EDX spectrum revealed the microanalysis of doped graphene and confirmed the iodine presence. The spectrum is dominated by peak of C, but additional elements were identified (associated **Table 1**).

> The chemical composition of the I-doped graphene was investigated using X-ray photoelectron spectroscopy (XPS). Surface XPS analysis is an extremely sensitive technique (<10 nm) and was mainly focused on carbon chemistry as well as detection of iodine incorporated in the matrix at the sensitivity limit of the equipment XPS (~ 0.1% atoms). XPS spectra were collected using a monochromatic X-ray (AlK) radiation source (1486.6 eV) after the samples were placed in a 10 to 10 Torr vacuum chamber. Neutralization of the charge loading of the samples was done in dual mode using an electron gun and another argon ion. Thus, the energy calibration with the C1s line (284.8 eV) was made with great accuracy and allowed to be compared

**Element Line type Apparent concentration k ratio wt.% wt.% sigma** C K series 105.92 1.05920 84.39 0.15 O K series 2.68 0.02346 13.48 0.15 Si K series 0.30 0.00278 0.13 0.01 S K series 2.00 0.02121 0.03 0.02 I L series 4.06 0.05058 2.01 0.04

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Total 100.00

**Table 1.** Microanalysis of I-doped graphene.

**Figure 2.** C1s deconvoluted spectra for C1s in I-doped graphene.

The introduction of chemical bonds was confirmed and high resolution spectra were collected for prominent transitions. The XPS spectra of the most prominent transitions of the chemical elements were obtained: C1s, O1s, I3d5/2, which indicated that iodine had been successfully binded on the surfaces of graphene. As anticipated, on the basis of the composition, in the survey spectra the sample show the predominant presence of carbon (1s, 284.5 eV) along with heteroatoms O (1s, 532 eV) and I (3d, 619.8 eV). High resolution C1s signals from XPS analysis are provided in **Figure 2**. High-resolution XPS spectra were further collected to quantify the doping amounts of the heteroatoms in the carbon framework. The detailed surface compositions of the prepared materials associated numerical values (chemical states rel. conc. wt.%) are systematized in **Table 2**. The C─C peak can be deconvoluted with 2

with the data from the established databases as well as from the literature [29, 30].

**Figure 1.** The SEM micrographs and EDX spectrum of prepared I-doped graphene.


**Table 1.** Microanalysis of I-doped graphene.

The morphology and microstructure of as-prepared samples were observed using scanning electron microscopy (SEM). I-doped graphene materials have a typical wrinkled microstructure, fluffy and almost transparent, overlapping layers, forming a cluster composed of many tiny sheets with corrugation and scrolling, building a good porosity (**Figure 1**). These features are highly valuable for PEMFC electrode applications, providing a high surface area and ensuring efficient mass transport and good catalytic sites accessibility. The energy-dispersive X-ray spectroscopy (EDX) used in addition to SEM was used as microanalysis technique for a rough appraisal of chemical composition. The EDX spectrum revealed the microanalysis of doped graphene and confirmed the iodine presence. The spectrum is dominated by peak of C, but additional elements were identified (associated

**Figure 1.** The SEM micrographs and EDX spectrum of prepared I-doped graphene.

**Table 1**).

86 Advances In Hydrogen Generation Technologies

The chemical composition of the I-doped graphene was investigated using X-ray photoelectron spectroscopy (XPS). Surface XPS analysis is an extremely sensitive technique (<10 nm) and was mainly focused on carbon chemistry as well as detection of iodine incorporated in the matrix at the sensitivity limit of the equipment XPS (~ 0.1% atoms). XPS spectra were collected using a monochromatic X-ray (AlK) radiation source (1486.6 eV) after the samples were placed in a 10 to 10 Torr vacuum chamber. Neutralization of the charge loading of the samples was done in dual mode using an electron gun and another argon ion. Thus, the energy calibration with the C1s line (284.8 eV) was made with great accuracy and allowed to be compared with the data from the established databases as well as from the literature [29, 30].

The introduction of chemical bonds was confirmed and high resolution spectra were collected for prominent transitions. The XPS spectra of the most prominent transitions of the chemical elements were obtained: C1s, O1s, I3d5/2, which indicated that iodine had been successfully binded on the surfaces of graphene. As anticipated, on the basis of the composition, in the survey spectra the sample show the predominant presence of carbon (1s, 284.5 eV) along with heteroatoms O (1s, 532 eV) and I (3d, 619.8 eV). High resolution C1s signals from XPS analysis are provided in **Figure 2**. High-resolution XPS spectra were further collected to quantify the doping amounts of the heteroatoms in the carbon framework. The detailed surface compositions of the prepared materials associated numerical values (chemical states rel. conc. wt.%) are systematized in **Table 2**. The C─C peak can be deconvoluted with 2

**Figure 2.** C1s deconvoluted spectra for C1s in I-doped graphene.


**Table 2.** XPS data: quantification assessment, element relative concentrations (wt.%) and the chemical state relative concentrations (%).

(Gaussian-Lorentzian) curves at 284.5 eV (sp2) and 285.2 eV (sp3) binding energies (BEs). From analysis of the profile, a clear suppression of the C─O species peaks are observed, confirming the successful reduction of oxygen containing species during doping and reduction processes. Moreover, the sp2 bonded carbon peak is sharp, suggesting an increase in the graphitic carbon configuration. I3d signal was detected as doublet 3d5/2, 3d3/2 with BEs at 619.8 eV and 624 eV.

After examination of **Table 3** it follows:


The doping of iodine heteroatom into graphene is expected to produce some changes in surface area and pore size of prepared material. Due to the porous appearance of the materials confirmed by SEM analysis and of the need to obtain a high surface area for efficient ORR performance, we subsequently analyzed the surface area. In order to investigate the structure and to characterize the porosity of prepared iodine doped graphene, the nitrogen adsorption–desorption isotherms were studied using Brunauer-Emmett-Teller (BET) and are provided in **Figure 3**. The surface area was determined to be 480 m2 g−1. The hysteresis study also revealed that hysteresis loops showed parallel adsorption and desorption branches. Such behavior was regarded as Type H1 among the IUPAC classification. This observation allows a better understanding of the porous features of the prepared samples, in the sense to be open at end, but unconnected to each other. As shown, the isotherm curves of adsorption/desorption performance of samples were compatible with isotherm Type IV, with an abrupt increase at high relative pressure, with respect to IUPAC classification. Based on the mentioned classification, a mainly mesoporous structure was estimated for the electrocatalyst samples.

Barrett-Joyner-Halenda (BJH) method was utilized to measure the porous texture of prepared iodine doped graphene. A few ultramicropores in the doped graphene material are present, based on the fact that I-doped graphene possess a vertical uptake under P/P0 = 0.04. A hysteresis loop type H1, from P/P0 = 0.4 to P/P0 = 1.0 due to the co-existence of both micropores, mesopores and some macroporous could be observed, the latter possessing a slit-like structure and most likely attributed to the pores between individual graphene sheets. This observation suggests there are good transport properties among the microspores, mesoporous and mac-

**Table 3.** XPS experimental data for chemical stability: the studied samples and the atomic concentrations (%) and mass

roporous channels in I-doped graphene.

**Figure 3.** BET isotherm corresponding to I-doped graphene.

(mass%) at all samples, all temperatures and time intervals.

**Sample/treatment conditions Chemical element**

I-doped graphene fresh material 90.04/86.02 9.81/12.48 0.15/1.50 I-doped grapheme/T = 60°C; t = 8 h 91.56/87.89 8.29/11.69 0.15/1.52 I-doped graphene/T = 60°C; t = 24 h 91.35/87.62 8.50/10.86 0.15/1.52 I-doped graphene/T = 60°C; t = 4 h 90.69/86.87 9.17/11.69 0.14/1.44 I-doped graphene/T = 60°C; t = 28 h 91.44/88.01 8.44/10.82 0.12/1.17 I-doped graphene/T = 60°C; t = 48 h 87.59/83.13 12.27/15.51 0.14/1.36 I-doped graphene/T = 80°C; t = 4 h 90.74/87.10 9.14/11.69 0.12/1.21 I-doped graphene/T = 80°C; t = 8 h 88.22/84.04 11.67/14.81 0.11/1.16 I-doped graphene/T = 80°C; t = 24 h 88.05/83.90 11.85/15.04 0.11/1.06 I-doped graphene/T = 80°C; t = 28 h 90.05/86.14 9.82/12.51 0.13/1.34 I-doped graphene/T = 80°C; t = 48 h 81.28/75.79 18.60/23.11 0.11/1.10

**C1s O1s I3d5/2**

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**Table 3.** XPS experimental data for chemical stability: the studied samples and the atomic concentrations (%) and mass (mass%) at all samples, all temperatures and time intervals.

**Figure 3.** BET isotherm corresponding to I-doped graphene.

(Gaussian-Lorentzian) curves at 284.5 eV (sp2) and 285.2 eV (sp3) binding energies (BEs). From analysis of the profile, a clear suppression of the C─O species peaks are observed, confirming the successful reduction of oxygen containing species during doping and reduc-

**Table 2.** XPS data: quantification assessment, element relative concentrations (wt.%) and the chemical state relative

**Sample C O I C**─**C (sp2) C**═**C (sp3) C**─**O C**═**O C**─**O**═**O OH**─**C**═**O** I-doped graphene 86.1 12.4 1.5 71.0 — 16.1 — 12.9 —

graphitic carbon configuration. I3d signal was detected as doublet 3d5/2, 3d3/2 with BEs at

• Both the standard and *all* samples at studied temperatures (60 and 80°C) and different

• The amount of iodine is at the XPS detection limit (~ 0.1 atom%, ~ 1 wt.%) but still can be

• The samples treated at temperature of 60°C show a higher iodine concentration (~ 0.15

• Both samples at 28 h (both 60 and 80°C) show a particularity: the 60°C sample shows a diffusion tendency of iodine from the surface in volume, while sample 80°C segregates iodine

• Samples 48 h (both 60 and 80°C) show the highest relative oxygen concentration accompanied by the lowest C concentration. This phenomenon is due to oxygen segregation in

The doping of iodine heteroatom into graphene is expected to produce some changes in surface area and pore size of prepared material. Due to the porous appearance of the materials confirmed by SEM analysis and of the need to obtain a high surface area for efficient ORR performance, we subsequently analyzed the surface area. In order to investigate the structure and to characterize the porosity of prepared iodine doped graphene, the nitrogen adsorption–desorption isotherms were studied using Brunauer-Emmett-Teller (BET) and are provided in **Figure 3**. The surface area was determined to be 480 m2 g−1. The hysteresis study also revealed that hysteresis loops showed parallel adsorption and desorption branches. Such behavior was regarded as Type H1 among the IUPAC classification. This observation allows a better understanding of the porous features of the prepared samples, in the sense to be open at end, but unconnected to each other. As shown, the isotherm curves of adsorption/desorption performance of samples were compatible with isotherm Type IV, with an abrupt increase at high relative pressure, with respect to IUPAC classification. Based on the mentioned classification, a mainly mesoporous structure was estimated for the elec-

atom%, ~ 1.5 wt.%) than those treated at 80°C (~ 0.11 atom%, ~ 1.15 wt.%).

bonded carbon peak is sharp, suggesting an increase in the

tion processes. Moreover, the sp2

88 Advances In Hydrogen Generation Technologies

detected and quantified

trocatalyst samples.

from the volume to the surface.

volume at the surface of the samples.

After examination of **Table 3** it follows:

times (4, 8, 24, 28 and 48 h) show *iodine* in their matrix.

619.8 eV and 624 eV.

concentrations (%).

Barrett-Joyner-Halenda (BJH) method was utilized to measure the porous texture of prepared iodine doped graphene. A few ultramicropores in the doped graphene material are present, based on the fact that I-doped graphene possess a vertical uptake under P/P0 = 0.04. A hysteresis loop type H1, from P/P0 = 0.4 to P/P0 = 1.0 due to the co-existence of both micropores, mesopores and some macroporous could be observed, the latter possessing a slit-like structure and most likely attributed to the pores between individual graphene sheets. This observation suggests there are good transport properties among the microspores, mesoporous and macroporous channels in I-doped graphene.

A briefly analysis on pore size distribution provided by the desorption branch using the BJH calculation approach was carried out. The pore size distribution curves presented in inserted plot, estimated according to BJH method, confirm that the greater part of the pores in the synthesized materials have size below 4.5 nm. The related curve of pore sizes highlighted a specific mesoporous structure, composed by primary and secondary pores. It is well known that the pores in an ORR catalyst layers must act two complementary roles, namely the primary pores, up to 0.04 μm, work as reaction volume, while secondary pores, from 0.04 to 0.1 μm, play the gas channel role in the porous structure [36]. As pore size distribution for I-doped graphene appears, although the samples possess limited primary structure, a structure of secondary pores is well confirmed, indicating that the transport properties in catalyst volume could be successfully obtained also after doping process.

One fuel cell with standard cathode configuration based on a commercial Pt catalyst and one fuel cell with an I-doped graphene layer placed between cathode GDL and CL have been

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The experimental data are represented as polarization and power density curves, while the numerical simulations are given as points on the curves. The numerical results for the cases investigated are in good agreement with experimental data, imparting the confidence that the computational fluid dynamics (CFD) models can be used as design tool for future improvements, and also to analyze how the fuel cell performs locally. Insightful information on the distribution and uniformity of phenomena that are taking place inside a fuel cell can be

In general, the CFD simulation of FC is based on the following phases: development of the geometry; meshing the geometry (splitting into finite volumes) to obtain the computational grid; setting the mathematical model for domains and boundaries (domains for solid and gas-liquid phases, inlet and outlet gas supply system and gas channels, walls, layers combinations and electrode contacts); setting the boundary conditions; running the case for calculat-

In order to compare the performances of the developed cathode versus commercial Pt catalyst with same loading, each FC performance reflected in its polarization curve was carried out. Analyzing the results, it can be seen that I-doped graphene coated GDL can lead to significant improvement in the performance of the fuel cell, up to an average of 20% in terms of both current and power density. The high electrical conductivity of the I-doped graphene layer enhanced the electrical contact between the GDL and CL, leading to an improved performance for all regions of the polarization curve. The microstructure optimization of this microporous layer based on I-doped graphene (high specific surface area, excellent mechanical properties, high thermal and electrical conductivity) promotes the rapid transport of electrons between active sites and the electrode and increases the electroactive catalyst surface,

In addition to the global performance curves, it is important to analyze how the fuel cell performs locally. Namely, the profiles of the key variables within the components of the fuel cell could give information on the uniformity of distributions for these variables. Less uniform distribution of some key variables, e.g., the current density, can lead to the presence of some undesirable phenomena such as hotspots or flooding, thus negatively affecting the lifetime and durability of the fuel cell. In direct connection with the uniformity of key variables it is the microstructure of the fuel cell layers. This complex microstructure (gas pores, carbon particles, Pt catalyst particles, ionomer network) and the possible interactions between flu-

developed to investigate the PEM fuel cells performance. The mass transport resistance due to the catalyst microstructure (resistance due to an ionomer film or due to a liquid water film surrounding particles), the liquid water transport through hydrophobic porous layers and the two-phase flow (liquid saturation) in the gas channels are taken into account in our model

and all water phases) are taken into consideration in our study by CFD models

ing parameters of interest; visualization of calculated results.

**3.3. CFD models developed to investigate the PEMFC performance**

developed *based on ANSYS Multiphysics software and PEM fuel cell Module.*

considered.

obtained by CFD modeling.

hence the increase of the performance.

ids (H2

, O2

#### **3.2. Electrochemical characterization of I-doped graphene**

Based on the structural information presented above, the prepared iodine doped graphene was evaluated as ORR cathode under practical FC operation conditions. It is easy to anticipate a better performance of cathode including the catalytic system Pt/C and I-doped graphene, mainly based on our recently results [34–36], in comparison to commercial Pt/C. This improvement in performance is likely due to higher electrical conductivity and the durability parameters, essential for the PEMFC commercialization. The fuel cell performance was characterized by measuring current-voltage polarization curves under various experimental conditions.

The testing cell is based on a 5 cm2 single cell system using 0.2 mg Pt cm−2 loading as anode catalyst and the developed cathode (0.2 mg Pt cm−2 loading sprayed on the membrane and 0.2 mg I-doped graphene cm−2 loading sprayed on the GDL acting the MPL role).

The sensitivity of the fuel cell performance to the cathode side microstructure is analyzed by experimental investigations and numerical simulations, the results being reported in **Figure 4**.

**Figure 4.** Polarization curves/power density plots (line) and model results (dots) for the PEMFC with cathode: commercial Pt/C (blue), and I-doped graphene (red). Cell runs with H2 /air, 1 bar anode and cathode back pressure; temperature 65C; flow rates H<sup>2</sup> /air: 100 ml min−1/250 ml min−1.

One fuel cell with standard cathode configuration based on a commercial Pt catalyst and one fuel cell with an I-doped graphene layer placed between cathode GDL and CL have been considered.

A briefly analysis on pore size distribution provided by the desorption branch using the BJH calculation approach was carried out. The pore size distribution curves presented in inserted plot, estimated according to BJH method, confirm that the greater part of the pores in the synthesized materials have size below 4.5 nm. The related curve of pore sizes highlighted a specific mesoporous structure, composed by primary and secondary pores. It is well known that the pores in an ORR catalyst layers must act two complementary roles, namely the primary pores, up to 0.04 μm, work as reaction volume, while secondary pores, from 0.04 to 0.1 μm, play the gas channel role in the porous structure [36]. As pore size distribution for I-doped graphene appears, although the samples possess limited primary structure, a structure of secondary pores is well confirmed, indicating that the transport properties in catalyst volume

Based on the structural information presented above, the prepared iodine doped graphene was evaluated as ORR cathode under practical FC operation conditions. It is easy to anticipate a better performance of cathode including the catalytic system Pt/C and I-doped graphene, mainly based on our recently results [34–36], in comparison to commercial Pt/C. This improvement in performance is likely due to higher electrical conductivity and the durability parameters, essential for the PEMFC commercialization. The fuel cell performance was characterized by measuring current-voltage polarization curves under various experimental conditions.

catalyst and the developed cathode (0.2 mg Pt cm−2 loading sprayed on the membrane and

The sensitivity of the fuel cell performance to the cathode side microstructure is analyzed by experimental investigations and numerical simulations, the results being reported in **Figure 4**.

**Figure 4.** Polarization curves/power density plots (line) and model results (dots) for the PEMFC with cathode:

commercial Pt/C (blue), and I-doped graphene (red). Cell runs with H2

/air: 100 ml min−1/250 ml min−1.

temperature 65C; flow rates H<sup>2</sup>

0.2 mg I-doped graphene cm−2 loading sprayed on the GDL acting the MPL role).

single cell system using 0.2 mg Pt cm−2 loading as anode

/air, 1 bar anode and cathode back pressure;

could be successfully obtained also after doping process.

**3.2. Electrochemical characterization of I-doped graphene**

The testing cell is based on a 5 cm2

90 Advances In Hydrogen Generation Technologies

The experimental data are represented as polarization and power density curves, while the numerical simulations are given as points on the curves. The numerical results for the cases investigated are in good agreement with experimental data, imparting the confidence that the computational fluid dynamics (CFD) models can be used as design tool for future improvements, and also to analyze how the fuel cell performs locally. Insightful information on the distribution and uniformity of phenomena that are taking place inside a fuel cell can be obtained by CFD modeling.

In general, the CFD simulation of FC is based on the following phases: development of the geometry; meshing the geometry (splitting into finite volumes) to obtain the computational grid; setting the mathematical model for domains and boundaries (domains for solid and gas-liquid phases, inlet and outlet gas supply system and gas channels, walls, layers combinations and electrode contacts); setting the boundary conditions; running the case for calculating parameters of interest; visualization of calculated results.

In order to compare the performances of the developed cathode versus commercial Pt catalyst with same loading, each FC performance reflected in its polarization curve was carried out. Analyzing the results, it can be seen that I-doped graphene coated GDL can lead to significant improvement in the performance of the fuel cell, up to an average of 20% in terms of both current and power density. The high electrical conductivity of the I-doped graphene layer enhanced the electrical contact between the GDL and CL, leading to an improved performance for all regions of the polarization curve. The microstructure optimization of this microporous layer based on I-doped graphene (high specific surface area, excellent mechanical properties, high thermal and electrical conductivity) promotes the rapid transport of electrons between active sites and the electrode and increases the electroactive catalyst surface, hence the increase of the performance.

### **3.3. CFD models developed to investigate the PEMFC performance**

In addition to the global performance curves, it is important to analyze how the fuel cell performs locally. Namely, the profiles of the key variables within the components of the fuel cell could give information on the uniformity of distributions for these variables. Less uniform distribution of some key variables, e.g., the current density, can lead to the presence of some undesirable phenomena such as hotspots or flooding, thus negatively affecting the lifetime and durability of the fuel cell. In direct connection with the uniformity of key variables it is the microstructure of the fuel cell layers. This complex microstructure (gas pores, carbon particles, Pt catalyst particles, ionomer network) and the possible interactions between fluids (H2 , O2 and all water phases) are taken into consideration in our study by CFD models developed to investigate the PEM fuel cells performance. The mass transport resistance due to the catalyst microstructure (resistance due to an ionomer film or due to a liquid water film surrounding particles), the liquid water transport through hydrophobic porous layers and the two-phase flow (liquid saturation) in the gas channels are taken into account in our model developed *based on ANSYS Multiphysics software and PEM fuel cell Module.*

**Figure 5** shows the distribution of the current density for 0.5 V potential differences between anode and cathode external walls. The plane displayed is at the interface between the cathode catalyst layer and the MPL for both cases investigated. The cathode having I-doped graphene and Pt/C exhibited 85 m2 Pt g−1, which is about 2.5 times higher than that of commercial Pt/C electrode at the same Pt loading (38 m2 Pt g−1). These values were taken into consideration in the simulations and the results show clearly the increase of current density value for the FC with higher active area. An average current density of 1.135 A/cm2 (**Figure 5** left) was obtained for the commercial Pt/C electrode, compared to 1.36 A/cm2 for the I-doped graphene case (**Figure 5** right). The current density profile is directly linked to the distribution of the water mass fraction at the interface between the cathode catalyst layer and the adjacent MPL, as can be seen in **Figure 6**. A proper water management and a uniform distribution within the fuel cell assist in humidifying the membrane and increasing the performance. It can be concluded that the higher mesoporous and macro porous nanostructures of the I-doped graphene coated

GDL facilitate the mass transport of oxygen into the catalyst layer, the removal of water molecules from the electrolyte, ensure a conductive paths for the ions and electrons during the global transport process, resulting in a better performance in ohmic and mass transport

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

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In order to examine the electrochemical stability of I-doped graphene cathode in comparison with Pt/C cathode, the electrodes were tested in the *in-situ* experiment by cyclic voltammetry (CV) measurements, elaborated in Experimental Section. The cyclic voltammetry results are presented in **Figure 7**. The CV curves show three characteristic potential regions: the hydrogen adsorption/ desorption region (0.05–0.4 V), double layer plateau region and the formation and reduction of surface Pt oxide. All curves present a well-defined hydrogen adsorption/desorption region.

The electrochemical active area of the electrocatalysts were evaluated from the integrated area

This higher active area suggests more platinum sites available for the ORR which leads to the improved FC performances seen in the polarization curves. Generally, the utilization of Pt dispersed on the catalyst support is proportional to the surface area of Pt nanoparticles in contact with the electrolyte, so the ECSA results indicate that interface between catalyst and

Moreover, the open pores and vacancies could serve as active intercalation sites, contributing to the high charge transfer based on the conductive nanosheets with large surface area and a continuous electronic pathway, providing a high electrode-electrolyte contact interface. The mentioned synergistic effects could not only improve the ions and electrons transportation with nanometer-scale diffusion but also limits the ohmic resistance losses with big contribu-

**Figure 7.** Cyclic voltammetry performed on *in situ* FC measurements in following operation conditions: temperature—

; potential scan rate—50 mV/s.

adsorp-

93

Pt g−1).

Pt g−1, which is

under the adsorption peaks from CV, representing the total charge associated with H+

about 2.5 times higher than that of commercial Pt/C electrode at the same Pt loading (38 m2

tion on metal. Thus, the cathode having Pt/C + I-doped graphene exhibited 85 m2

ionomer increases as effect of I-graphene added into the catalyst layer [29, 33].

regions of the polarization curves.

Operating conditions used are from experiments.

tion to electrode stability in PEMFC device.

60°C; the air in the original FC cathode was replaced with N<sup>2</sup>

**Figure 5.** Current density profile for 0.5 V at the interface between the cathode catalyst layer and the MPL for the PEMFC with: commercial Pt/C (left), and I-doped graphene (right).

**Figure 6.** Water mass fraction profile for 0.5 V at the interface between the cathode catalyst layer and the MPL for the PEMFC with: commercial Pt/C (left), and I-doped graphene (right).

GDL facilitate the mass transport of oxygen into the catalyst layer, the removal of water molecules from the electrolyte, ensure a conductive paths for the ions and electrons during the global transport process, resulting in a better performance in ohmic and mass transport regions of the polarization curves.

In order to examine the electrochemical stability of I-doped graphene cathode in comparison with Pt/C cathode, the electrodes were tested in the *in-situ* experiment by cyclic voltammetry (CV) measurements, elaborated in Experimental Section. The cyclic voltammetry results are presented in **Figure 7**. The CV curves show three characteristic potential regions: the hydrogen adsorption/ desorption region (0.05–0.4 V), double layer plateau region and the formation and reduction of surface Pt oxide. All curves present a well-defined hydrogen adsorption/desorption region.

Operating conditions used are from experiments.

**Figure 5** shows the distribution of the current density for 0.5 V potential differences between anode and cathode external walls. The plane displayed is at the interface between the cathode catalyst layer and the MPL for both cases investigated. The cathode having I-doped graphene

the simulations and the results show clearly the increase of current density value for the FC

(**Figure 5** right). The current density profile is directly linked to the distribution of the water mass fraction at the interface between the cathode catalyst layer and the adjacent MPL, as can be seen in **Figure 6**. A proper water management and a uniform distribution within the fuel cell assist in humidifying the membrane and increasing the performance. It can be concluded that the higher mesoporous and macro porous nanostructures of the I-doped graphene coated

**Figure 6.** Water mass fraction profile for 0.5 V at the interface between the cathode catalyst layer and the MPL for the

**Figure 5.** Current density profile for 0.5 V at the interface between the cathode catalyst layer and the MPL for the PEMFC

PEMFC with: commercial Pt/C (left), and I-doped graphene (right).

with: commercial Pt/C (left), and I-doped graphene (right).

Pt g−1, which is about 2.5 times higher than that of commercial Pt/C

Pt g−1). These values were taken into consideration in

(**Figure 5** left) was obtained

for the I-doped graphene case

and Pt/C exhibited 85 m2

electrode at the same Pt loading (38 m2

92 Advances In Hydrogen Generation Technologies

with higher active area. An average current density of 1.135 A/cm2

for the commercial Pt/C electrode, compared to 1.36 A/cm2

The electrochemical active area of the electrocatalysts were evaluated from the integrated area under the adsorption peaks from CV, representing the total charge associated with H+ adsorption on metal. Thus, the cathode having Pt/C + I-doped graphene exhibited 85 m2 Pt g−1, which is about 2.5 times higher than that of commercial Pt/C electrode at the same Pt loading (38 m2 Pt g−1). This higher active area suggests more platinum sites available for the ORR which leads to the improved FC performances seen in the polarization curves. Generally, the utilization of Pt dispersed on the catalyst support is proportional to the surface area of Pt nanoparticles in contact with the electrolyte, so the ECSA results indicate that interface between catalyst and ionomer increases as effect of I-graphene added into the catalyst layer [29, 33].

Moreover, the open pores and vacancies could serve as active intercalation sites, contributing to the high charge transfer based on the conductive nanosheets with large surface area and a continuous electronic pathway, providing a high electrode-electrolyte contact interface. The mentioned synergistic effects could not only improve the ions and electrons transportation with nanometer-scale diffusion but also limits the ohmic resistance losses with big contribution to electrode stability in PEMFC device.

**Figure 7.** Cyclic voltammetry performed on *in situ* FC measurements in following operation conditions: temperature— 60°C; the air in the original FC cathode was replaced with N<sup>2</sup> ; potential scan rate—50 mV/s.
