Characterization and optimization of graphite felt/BP2000 composite electrode for the H₂/Br₂ fuel cell

Abstract

A promising graphite felt/BP2000 composite electrode is fabricated and investigated as a cathode for the hydrogen bromine (H₂/Br₂) fuel cell, which significantly improves the fuel cell performance. The structure, composition, electrochemical properties and performance of the composite electrodes are optimized and characterized using X-ray diffraction, scanning electron microscopy, mercury intrusion, electrochemistry impedance spectroscopy and single cell polarization curves. The effects of binder, Nafion content, catalyst loading and operating temperature are examined. Maximum power densities of 0.91, 1.13 and 1.28 W cm⁻² have been achieved at 30, 50 and 70 °C, respectively, with 30% Nafion content and 2.5 mg cm⁻² catalyst loading in the composite electrode. The performance improvement may be attributed to the increase in the electrochemical catalytic activity and mass transport of the composite electrode, for the loading of high specific surface area BP2000 catalyst and Nafion binder.

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1. Introduction

With the rapid development of renewable energy, low cost and high efficiency of large scale energy storage systems are needed to accommodate the intermittent nature of energy supply from solar and wind resources to integrate renewable energy with the current electricity grid.^1,2 Hydrogen bromine (H₂/Br₂) fuel cells are of increasing interest for applications in large scale energy storage due to their lower-cost reactants, high energy conversion efficiency and flexible operation.^3,4 The anodic and cathodic reactions associated with the hydrogen bromine fuel cell are shown below.

Anode: H₂(g) → 2H⁺(aq) + 2e⁻ E^θ = 0.00 V (1)

Cathode: Br₂(aq) + 2e⁻ → 2Br⁻(aq) E^θ = 1.09 V (2)

Overall: Br₂(aq) + H₂(g) → 2HBr(aq) E^θ = 1.09 V (3)

During discharge, hydrogen is oxidized to protons at the anode and bromine is reduced to bromide ion at the cathode. Meanwhile, the protons pass through a proton-conducting membrane and combine with bromide ions to produce HBr at the cathode.

In marked contrast with the hydrogen oxygen fuel cell, in which sluggish oxygen reduction kinetics lead to substantial activation overpotential, the H₂/Br₂ fuel cells are capable of operation with little activation loss associated due to the relatively fast and reversible bromine redox kinetics.^5,6 Recent studies showed that the H₂/Br₂ system is a suitable option for energy storage because of its high energy efficiency and power density capability.^7,8

Catalyst and electrode materials play an important role in H₂/Br₂ fuel cells. With Pt/C sprayed on Toray paper as bromine electrode, the maximum power density of 1.5 W cm⁻² at 80 °C (0.9 M Br₂ in 1 M HBr) had been reported.⁹ However, noble metals are not practical for their high cost, low specific surface area and dissolution in HBr solution.^10,11 Recently, porous carbon materials were investigated as bromine electrode materials by Cho et al.⁴ Several investigations have identified that the low electrochemical activity and major transport limiting processes of bromine electrode limit the H₂/Br₂ fuel cell performance.^12,13 A previous modeling study conducted on the H₂/Br₂ fuel cell system suggested that increasing the active surface area by either employing multiple plain carbon electrodes or increasing the electrode thickness is evident for improving the fuel cell performance.¹⁴ To increase the activity and surface area, a stack of acid-pretreated 3 SGL 10AA carbon papers was used as the bromine electrode. A peak power density of 1.4 W cm⁻² at 55 °C (0.9 M Br₂ in 1 M HBr), the ambient peak power and limiting current of 0.85 W cm⁻² and 1.6 A cm⁻² were obtained, respectively.⁴ Nevertheless, multiple electrodes increase the diffusion pathway for reactants, which would lead to mass transport limited performance at higher current densities. Electrode architecture investigation showed that addition of a carbon black catalyst layer to the positive side of the membrane decreased the limiting current density.¹⁵ The hydrophobic property and lower porosity of carbon catalyst layer may impede the transport of aqueous bromine to the reaction sites, thus resulting in lower cell performance.

Levelized cost analysis indicates that improving the electrocatalysts activity, increasing the mass transport and current density of the H₂/Br₂ fuel cell can make the system more cost-effective.¹⁶ Since the low porosity of carbon paper, mass transport was restricted and ambient limiting current density was lower than 1.2 A cm⁻² (0.9 M Br₂/1 M HBr).^7,12,15,17 Our previous work showed that hydrogen bromine fuel cell performance was improved by surface modified graphite felt due to its high porosity and the increase in oxygen-containing functional groups. The peak power density of 0.69 W cm⁻² and limiting current above 1.5 A cm⁻² were obtained at 30 °C (0.6 M Br₂/1 M HBr).¹⁸ However, there was still voltage drop in the kinetic region due to the low specific surface area of graphite felt. On the other hand, coating a high specific surface area catalyst on the graphite felt is a more suitable option, because the electrode surface area can be improved without affecting the electrode porosity and thickness. Since the bromine redox kinetics is sufficient on carbon, the catalytic activity of carbon blacks with 20 times more surface area per cm² is comparable to that of Pt catalyst.⁴ Black Pearls 2000 (BP2000) carbon black was used as the catalyst carrier in proton exchange membrane fuel cells. Compared with other carbon black, such as XC-72 (250 m² g⁻¹) and EC300J (800 m² g⁻¹), BP2000 possesses higher specific surface area of 1500 m² g⁻¹.¹⁹ Furthermore, the surface oxides on BP2000 surface could improve bromine redox activity.^18,20 And therefore BP2000 is chose as the catalyst.

In this work, we focus on improving cell performance by optimizing the composite bromine electrode. High porosity graphite felt was used as the matrix scaffold of bromine electrode to improve the convective transport of reactants to the active sites. And BP2000 carbon black was used as the catalyst to increase the specific surface area and active sites of bromine electrode. Binders were used to improve the hydrophility of catalyst layer and enforce the cohesion between catalyst layer and graphite felt. The composite electrodes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), mercury intrusion and contact angle. The effect of pore structure, hydrophobicity, binder and catalyst loading on cell performances was investigated.

2. Experimental

2.1. Materials

The graphite felt with a thickness of 3 mm was purchased from Hunan Jiuhua Carbon Hi-Tech Co., Ltd., China. BP2000 carbon black was obtained from Cabot Corp. Anhydrous ethanol was purchased from Tianjin Kermel Chemical Reagent Co., Ltd. 5 wt% Nafion solution and 10 wt% PTFE emulsion were supplied by Dupont Corp. Hydrobromic acid and bromine liquid from Alfa Aesar were used to prepare the electrolyte solutions.

2.2. Preparation of GF/BP2000 composite electrode

The porous graphite felt made of polyacrylonitrile-based carbon fibers was used as matrix scaffold of bromine electrode. The graphite felt electrode (GF) with the size of 2.0 × 2.0 cm was sonicated in anhydrous ethanol for 30 min, rinsed with deionized water for 30 min and then dried in oven at 100 °C for 5 h.

A homogeneous ink consisted of BP2000, anhydrous ethanol solution and 5 wt% Nafion or PTFE binder was coating on the GF and dried in air to prepare GF/BP2000 composite electrodes. This procedure was repeated several times until the desired BP2000 loading was achieved. Finally, the electrodes were dried in oven at 100 °C for 5 h. BP2000/binder weight ratios were 85/15, 70/30, and 50/50, respectively. The resulting BP2000 loadings were 1.0, 2.0, 2.5 and 3.0 mg cm⁻² with BP2000/binder weight ratio 70/30. The composite electrodes with Nafion and PTFE binder were named CE-Nafion and CE-PTFE, respectively.

2.3. Physical characterizations

The GF and GF/BP2000 composite electrodes were employed to evaluate their physical characterization. X-ray diffraction (XRD) patterns were obtained by a PANalytical diffractometer (X'Pert PRO) with a Cu Kα radiation source (λ = 1.54056 Å). Scanning electron microscopy (SEM) (JEOL, JSM-6360LV, Japan) was used to characterize the surface morphology of electrodes. The pore size distributions of electrodes were measured by the mercury intrusion method with a Poremaster GT60 (Quantachrome). Contact angles were measured by KRUSS DSA100 Drop Shape Analysis System. 3 µL of water droplet was dropped on the surface of the electrodes, and then the contact angles were calculated by fitting a tangent to the three-phase point using a drop shape analyzer.

2.4. Performance of hydrogen bromine fuel cell

To validate H₂/Br₂ fuel cell performance, an in-house single cell was used, which consisted of two graphite end-plates with parallel flow fields were engraved, a Teflon gasket (2 mm), a bromine electrode, a hydrogen electrode and a proton exchange membrane.¹⁸ GF and GF/BP2000 composite electrodes were employed as the bromine electrodes (4 cm²). The hydrogen electrode which had a catalyst loading of 0.6 mg cm⁻² Pt/C (70 wt%, Johnson Matthey), was attached to the electrolyte membrane Nafion212 by hot pressing at 140 °C and 1 MPa. Bromine electrode and Nafion212 with hydrogen electrode were positioned between two graphite plates. Teflon gasket was applied to seal the bromine electrode. 0.5 L electrolyte (0.6 M Br₂/1 M HBr) was circulated over the cathode, and hydrogen (0.05 MPa) without humidification was fed to the hydrogen electrode. PARSTAT 2273 (EG&G Instruments) was used to obtain the electrochemistry impedance spectroscopy (EIS), which were simulated with ZSimpWin software. The frequency varied from 10 kHz to 100 mHz with 10 mV amplitude of sinusoidal potential perturbation.

3. Results and discussion

3.1. Physical characterization

XRD patterns of GF and GF/BP2000 composite electrodes are shown in Fig. 1. According to literatures, the diffraction peaks located at 26.4°, 43.5° and 54.3° are corresponded to the (002), (100) and (004) planes, respectively.^21,22 Compared to the GF electrode, it can be found that the positions of diffraction peaks of GF/BP2000 composite electrodes hardly change but a dramatic decrease in diffraction intensity. These results demonstrate that more structure defects are introduced on the GF/BP2000 composite electrodes.²¹ The weakened intensity of diffraction peaks are attributed to the adhesion of the small particle BP2000, which is amorphous carbon.¹⁹ This can be confirmed by the observation with SEM imagines in Fig. 2.

Fig. 1 XRD patterns of the graphite felt electrode and GF/BP2000 composite electrode.

Fig. 2 SEM images of (a) the GF electrode, (b) CE-PTFE, (c and d) CE-Nafion.

Fig. 2 shows the surface morphology changes introduced by coating BP2000. As shown in Fig. 2a, the surface of GF is smooth. Fig. 2b–d show the SEM imagines of GF/BP2000 composite electrodes. Obvious morphology changes can be induced by coating BP2000 and the surface roughness is enhanced. As shown in Fig. 2b, the dispersion of catalyst is poor and aggregations can be observed on GF of CE-PTFE electrode. On the contrary, it can be seen that BP2000 particles are uniformly dispersed on the surface of GF (Fig. 2c). Moreover, Fig. 2d shows the composite electrode morphology, it can be seen that the carbon black is uniformly dispersed in the CE-Nafion electrode. Since BP2000 catalyst possesses high specific surface area of 1500 m² g⁻¹,¹⁹ the adhesion of catalyst layer will increase the surface area of composite electrode. Accordingly, it is reasonable to infer that the electrochemical activity of composite electrode will be improved.

Nafion and PTFE play a role of the binders between BP2000 powders and improve the adhesion between catalyst layer and GF. Pore structure of electrode is an important factor in mass-transport process.^23,24 To investigate the effect of binders on the electrode pore size distributions and porosities, mercury intrusion porosimetry was conducted. As the pressure increases, larger pores are firstly intruded with mercury and then smaller pores are gradually intruded. So the pore size distribution is obtained basing on the relationship between pore size and intruded mercury amount. Fig. 3 shows the pore size distributions of GF and composite electrodes. It can be seen that the pore size is mainly from 50 to 200 µm for the GF and CE-Nafion electrodes. However, the CE-PTFE electrode shows an obvious decrease in pore size distribution, from 20 to 100 µm. Obviously, the pore volume of nano-scale pore is negligible compared with that of 20 to 200 µm pore, which may be due to the aggregation of BP2000 spherical particles and the coverage of catalyst particles by binder solid.²⁵Table 1 shows the corresponding pore structure parameters of the three electrodes. Since reactants first flow through the big pore, large mean pore diameter is beneficial to mass transfer of reactants. It indicated that the influence of Nafion binder on pore-size distribution and porosity is negligible. It may be attributed to the solution state of Nafion ionomer in ethanol. It is well known that the Nafion solution forms a solution in the solvents with ε > 10.²⁶ When ethanol (ε = 24.5) was used as a solvent, the Nafion ionomer totally dissolves to form a solution state. Therefore, the distribution of Nafion in the catalyst layer is essentially even and the catalyst layer is uniformly coated on GF (see Fig. 2c), which induces little effect on the porosity and pore-size distributions of CE-Nafion electrode. However, PTFE binder results in smaller porosity and pore-size distribution. The mean pore diameter decreases from 86.51 µm for GF to 52.47 µm, and the specific pore volume decreases to 5.67 mL g⁻¹, which is 60% lower than that of GF. The emulsion form of PTFE induced catalyst agglomerates and formed large clumps in big pores,²⁵ which results in smaller porosity and pore size distributions of CE-PTFE electrode. Accordingly, the mass transfer resistance will be enlarged.

Fig. 3 Pore-size distribution curves of composite electrodes with different binders from mercury intrusion porosimetry measurement.

Table 1 The pore structure parameters of GF/BP2000 composite electrodes with different binders

Electrode	Mean pore diameter (µm)	Specific pore volume (mL g^−1)	Porosity (%)
GF	86.51	9.58	94.56
CE-PTFE	52.47	5.67	91.15
CE-Nafion	86.28	9.75	94.68

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Liquid water transport through electrodes relies strongly on not only pore structure and porosity but also degree of hydrophobicity.²⁷ The contact angles of the GF and composite electrodes surface were measured to analyze the hydrophobicity. The images of the contact angle measurements are shown in Fig. 4, the corresponding contact angles of GF, CE-PTFE and CE-Nafion are 127.9°, 138.5° and 119.4°, respectively. Obviously, the addition of PTFE binder improves the hydrophobicity of the composite electrode surface. On the contrary, Nafion binder decreases the hydrophobicity.

Fig. 4 The contact angles of (a) GF, (b) CE-PTFE and (c) CE-Nafion.

According to Young–Laplace equation:

where ΔP is the capillary pressure difference, σ is the surface tension, θ is the contact angle and r is the pore diameter. The CE-PTFE electrode possesses small pore diameter and big contact angle, hence ΔP is bigger than the others. That is to say, the mass transfer of reactants is restricted in the CE-PTFE electrode. On the contrary, Nafion binder decreases the mass transfer resistance in the CE-Nafion electrode, which is beneficial for improving limiting current density and cell performance.¹⁷

3.2. Effect of binders on hydrogen bromine fuel cell performance

A hydrogen bromine single fuel cell was used to examine the performance of the composite electrodes. Fig. 5a shows the effect of binders on the cell performance. For the case of the GF electrode, there is dramatic performance drop in the low current density region (i.e. less than 0.2 A cm⁻²) indicating huge activation loss (i.e. kinetic loss) due to the low kinetics of bromine on GF.⁴ However, there is significant enhancement at the low current density region (i.e. kinetic-dominant region) for the composite electrodes, indicating that the reaction kinetics is improved dramatically by coating BP2000 catalyst.²⁸ It is noteworthy that the larger surface area of composite electrode would be favorable to enhance the reaction rate of bromine for more reaction sites and function groups are provided.^14,18 It is clear that the CE-Nafion electrode yields the best performance in all regions of the polarization curve. The maximum power densities are 0.51 W cm⁻², 0.67 W cm⁻² and 0.85 W cm⁻² for GF, CE-PTFE and CE-Nafion electrodes at 30 °C with 0.6 M Br₂ in 1 M HBr, respectively. Although the low current density performances are comparable for composite electrodes, the high current density region (i.e. bigger than 1.0 A cm⁻²) is significantly affected by mass-transfer losses for CE-PTFE electrode due to its pore structure and high hydrophobility.

Fig. 5 (a) Effect of binders on H₂/Br₂ fuel cell performance at 30 °C, (b) EIS diagrams of the H₂/Br₂ fuel cell at 30 °C.

As shown in Fig. 5a, the limiting current densities are 1.45, 1.20 and 1.55 A cm⁻² for GF, CE-PTFE and CE-Nafion electrodes, respectively. Obviously, the limiting current densities are decreased by PTFE binder and increased by Nafion binder. According to Faraday's law, the amount of reactant consumption of the electrode reaction is given in eqn (5).

where m is the amount of reactant consumption, I is the current, n is the number of electron, and F is the Faraday constant.

Consumption rate of reactant can be written as follows:

Flux of reactant at the interface between electrode and electrolyte can be expressed by eqn (7).

where k_m is the mass transfer coefficient, C⁰ is the reactant concentration of solution, C^s is the concentration of reactant on the electrode surface and A is the electrode surface area.

Assuming the electrode reaction is controlled by mass transfer, therefore, the relationship between k_m and limiting current density (j_lim) can be given by eqn (8).

It indicates that j_lim increases with increasing k_m. According to the results of cell performance, it is known that k_m is reduced by PTFE binder, which enlarges the mass transfer resistance and leads to severe concentration polarization at high current density regions. On the contrary, Nafion binder can enhance k_m and alleviate concentration polarization to some extent at high current density regions.

The electrochemical impedance test was carried out at open circuit voltage (OCV) and 30 °C. Fig. 5b shows the comparison of Nyquist plots of the three electrodes. Compared to GF electrode, it shows a dramatic decrease in intercept and diameter of the circle for the composite electrodes. Furthermore, two obvious arcs, accounting for the charge transfer at high frequencies and mass transfer processes at low frequencies, indicate the existence of concentration polarization for three electrodes,²⁸ which is certainly consistent with the fuel cell performance in Fig. 5a.

The impedance parameters are listed in Table 2, which are obtained from fitting the experimental data with the equivalent circuit of LR_Ω(R₁Q₁)(R_ctQ_dl).^18,21,29 It can be seen in Table 2, R_Ω of composite electrodes are lower than that of GF electrode and CE-Nafion electrode possesses the smallest R_Ω. It may be attributed to the homodisperse of catalyst on GF (as shown in Fig. 2c). Furthermore, lower R_Ω is helpful to decrease ohmic polarization.

Table 2 Impedance parameters obtained through fitting the experimental data to the LR_Ω(R₁Q₁)(R_ctQ_ct) circuit for GF/BP2000 composite electrodes

Electrode	R Ω/Ω cm^−2	R ct/Ω cm^−2	R 1/Ω cm^−2
GF	0.234	0.813	0.352
CE-PTFE	0.220	0.042	0.121
CE-Nafion	0.205	0.043	0.052

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As shown in Table 2, the charge transfer resistance R_ct values decrease obviously after coating catalyst. The CE-Nafion electrode has the smallest R_ct value, which further suggests that the composite electrode yields higher electrochemical catalytic activity.³⁰ Smaller R_ct means faster catalytic reaction rate which contributes to decreasing electrochemical polarization. These results are certainly consistent with the fuel cell performance at low current density regions in Fig. 5a. It indicates that BP2000 which possesses high specific surface area can effectively lower the charge transfer resistance and ohmic resistance, and improve the electrochemical catalytic activity of composite electrode and cell performance.

3.3. Effect of Nafion content on hydrogen bromine fuel cell performance

The Nafion ionomer is applied in the catalyst layers and investigated extensively in proton exchange membrane fuel cell and solid polymer electrolyte water electrolyser. Nafion content plays an important role in regulating pore structure, electrochemical surface area, ohmic resistance and mass transfer property.^31–34 Hence, an optimum Nafion content in the catalyst layer is necessary for good performance.

Nafion loading in the electrode is expressed as wt% of Nafion (dry weight of Nafion ionomer divided by the total weight of BP2000 catalyst and Nafion ionomer, multiplied by 100). Single cell test was conducted at 30 °C with cathode electrolyte of 0.6 M Br₂/1.0 M HBr. The influences of the Nafion ionomer content on the performances of H₂/Br₂ fuel cell were shown in Fig. 6. Four composite electrodes were tested, of which some other parameters (BP2000 loading, temperature, hydrogen electrode, etc.) were the same except the ionomer content in cathode. There was no visible performance difference at low and medium current density regions (lower than 1.0 A cm⁻²). It indicates that the electrochemical activity of composite electrodes with identical BP2000 catalyst loading is not influenced by changing Nafion content. However, in the mass-transport polarization region (current density higher than 1.0 A cm⁻²), the cell performance increased with the Nafion content. The electrode with 50% Nafion ionomer content exhibited the best performance. Since the reactant bromine is dissolved in water solution, the hydrophilicity of composite electrode is very important for the bromine electrode reaction. Due to the improvement in hydrophilic and mass transfer property of composite electrodes, the maximum power density increased from 0.79 to 0.89 W cm⁻² and the limiting current density increased from 1.4 to 1.8 A cm⁻² when Nafion content increased from 0 to 50%.

Fig. 6 Effect of Nafion loading on H₂/Br₂ fuel cell performance.

Although electrode hydrophilic was enhanced by further increasing Nafion content, the ohmic resistance will increase as Nafion is an electron insulator. And the electrochemical surface area will decrease due to the coverage of catalyst particles by Nafion solid,²⁵ which results in cell performance deterioration. Moreover, Nafion is expensive. Considering the balance of cell performance and cost, 30% Nafion content is suggested.

3.4. Effect of BP2000 loading on hydrogen bromine fuel cell performance

To evaluate the influences of BP2000 catalyst loading on the cell performance, CE-Nafion electrodes with various catalyst contents were investigated by single cell test. As shown in Fig. 7, the optimum loading is 2.5 mg cm⁻² and the maximum power density is 0.91 W cm⁻². Compare to GF electrode, the maximum power density is enhanced by 78%. It is seen from the figure that the cell performance was not obviously affected by BP2000 loading at low and intermediate current density. It indicates that high electrode performance can be obtained even with low BP2000 loading because of its high specific surface area and catalytic activity.⁴ Apparently, at high current density regions, the cell performances first increased and then decreased with increasing BP2000 loading. Particularly, the loading effect was prominent at high current density region. Since the amount of Nafion was increased with increasing BP2000 loading, the hydrophility of composite electrodes could be improved, which results in higher cell performances. However, the cell performance deteriorated when the loading increased to 3.0 mg cm⁻². It may be attributed to the increased mass transfer resistance and ohmic resistance because of the thicker coating or the clumps of catalyst in big pores.^23,35

Fig. 7 Effect of BP2000 loading on H₂/Br₂ fuel cell performance.

Cell performance is expected to increase due to enhancement in the conductivity, transport properties and electrochemical kinetics.^4,12,13,36 Further, the composite electrode with BP2000 loading 2.5 mg cm⁻² and 30% Nafion was used to investigate the effect of temperature on cell performance. As presented in Fig. 8, the OCV decreased from 1.06 V to 1.01 V in going from 30 °C to 70 °C due to bromine crossover to the anode and its reduction by hydrogen.⁹ The maximum power density increased from 0.91 W cm⁻² at 30 °C to 1.13 W cm⁻² at 50 °C, and to 1.28 W cm⁻² at 70 °C, and the limiting current density increased to 2.6 A cm⁻² at 70 °C. The cell performance is better than the reported value (0.7 W cm⁻² at 20 °C, 1.1 W cm⁻² at 50 °C), which was obtained with Pt or Pt–Ru noble metal catalyst in bromine electrode and a high concentration electrolyte of 0.9 M Br₂/1 M HBr.^4,9 Moreover, in consideration of its high concentration (0.9 M Br₂/1 M HBr), the cell performance at 50 °C is comparable to the literature (1.13 W cm⁻² at 40 °C).⁴ The excellent performance of the composite electrode demonstrates its potential application in H₂/Br₂ fuel cell. R_Ω, which was obtained from fitting the equivalent circuit, was 0.21, 0.18 and 0.16 Ω cm⁻² at 30, 50 and 70 °C, respectively. Livshits reported that the cell resistant was only 0.12 Ω cm⁻² at 50 °C with nanoporous proton conducting membrane (NP-PCM).⁹ The cell with Nafion membrane and composite electrode shows high ohmic resistance. It was probably caused by the dehydration of Nafion membrane in HBr solution and larger contact resistance between electrode and membrane.^4,7,36 Consequently, the cell performance can be further improved by enhancing the conductivity of membrane and composite electrode.

Fig. 8 Polarization curves measured on the H₂/Br₂ fuel cell with CE-Nafion at different temperatures.

4. Conclusions

In this paper, the characterization and optimization of GF/BP2000 composite electrode for hydrogen bromine fuel cell was investigated. It was shown that the importance of BP2000 catalyst with high specific area (1500 m² g⁻¹) and high conductivity for improving the electrochemical activity and decreasing the ohmic resistance and charge transfer resistance. The cell performance at high current density region was increased due to the use of Nafion binder, which improves the hydrophility and mass transfer of composite electrode. Maximum power densities of 0.91 W cm⁻², 1.13 W cm⁻² and 1.28 W cm⁻² have been achieved at 30, 50 and 70 °C, respectively. Analysis of the discharge polarization curves indicates that further improvement can be obtained by minimizing the ohmic losses associated with the cell and optimizing bromine transport into the composite electrode.

Acknowledgements

We thank the National Basic Research Program of China (973 Program, Grant No. 2012CB215500) and Science and Technology Research Projects in Hebei Universities (QN2014142 and ZD20131032), Hebei Science and Technology program (15214408) and Youth Science and Technology Talents funding of Xingtai (2015ZZ051) for financial support.

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