A study of the performance of SPEEK electrolyte of a novel PEMFC

Analysis of performance of fuel cell is carried out by considering non-fluorinated electrolyte and variety of potential electrodes. The study spans low temperature regions and carried out MD investigation of transport of ions in a novel PEM fuel cell (PEMFC), which is composed of electrodes and SPEEK as a non-fluorinated electrolyte. Typical values of ion conductivity σ near room temperature are σ = 8.5 × 10−2 S cm−1 when Pt is used as electrode, σ = 26 × 10−2 S cm−1 when Pd3Ag is used as electrode, and σ = 0.34 × 10−2 S cm−1 when Pd is used as electrode. Thus, this study reveals a twofold implications. Firstly, the choice of electrolyte affects whether the traditional Pt electrode remains to be superior for the overall fuel cell performance. Secondly, using SPEEK electrolyte has a potential to yield a closely matching ion conductivity with the one obtained from the costly Nafion electrolyte.


Introduction
While pursuing an investigation of efficient components of fuel cells, studying the properties of different electrodes and electrolytes is an expected activity.Since a couple of decades ago, platinum has been considered as a natural electrode for fuel cells [1], whereas Nafion polymer has been considered as a preferred electrolyte [2].However, alongside, a research activity have been going-on to investigate efficiency of alternative electrolytes such as Polyvinyl alcohol (PVA) [3], Polybenzimidazole (PBI) [4], and Sulfonated Poly Ether Ether Ketone (SPEEK) [5].Because of a need to make resource conservation, interests to find alternative electrode materials has also been rising.Furthermore, in order to investigate accurate efficiency of fuel cell components, we believe that one needs to include the effects of electrodes and electrolytes on each other performance.In our previous work, we have reported performance character of novel fuel cell design by considering various potential electrodes and a Nafion polymer as an electrolyte [6].In this work, we reveal performance properties of a novel fuel cell by considering various electrodes and a SPEEK polymer as an electrolyte.By investigating interplay between electrolytes and electrodes, it is more likely that we determine the utmost real properties of candidate fuel cells.There is a common agreement in literature that proton ions be modelled by hydronium ions, and thus, Molecular Dynamics (MD) is applied in this work to study the diffusion dynamics of hydronium ions in the fuel cells.Among target parameters to be used in the analysis of the performance properties is the ion conductivity, which is obtained from the output of the MD simulations applied on the Pt/Pd 3 Ag/Pd/SPEEK systems.Thus, we believe that this research has a lot to contribute to the sector of fuel cell technologies by providing valuable insights into the interactions between different materials and their impacts on ion conductivity ability.

Methodology
A computational approach, within modelling and simulation of the problems, is used as a method of investigation.A single formula unit of SPEEK is modelled with chemical formula of C 38 H 20 O 6 -6SO - 3 , as shown in figure 1.The PEMFC is modelled by the simulation box shown in figure 2. The simulation box contains electrodes, three formula units of SPEEK and eighteen hydronium ions (H 3 O + ).Once the simulation box was constructed, a minimization procedure was carried out by using COMPASS atomic force fields, which is generated according to a literature [7], and as implemented in the Biovia Materials Studio software.The geometry optimization is done using the so-called smart method, with a convergence criteria for the forces being 1.0 × 10 −3 kcal/mol.Å.The MD is run for a time of 100 ps, during which equilibrium is achieved.In the NPT ensemble, at a fixed temperatures and a pressure of 1 bar, the equilibration process was conducted according to a literature [8] and has resulted in the densities of the box, when Pt, Pd 3 Ag, and Pd are used as an electrode, to be 1.42 g cm −3 , 1.28 g cm −3 , and 1.00 g cm −3 , respectively.Subsequently, the membranes were subjected to an MD equilibration run using annealing and temperature cycling methods.The annealing process involved heating the membranes from 150 K to 600 K in increments of 150 K, followed by cooling the membranes using the same temperature intervals.
Finally, after achieving the equilibrium density, a further equilibration run is conducted for 100 ps at NVT ensemble condition, then followed by a production run for 100 ps at NVE ensemble condition.Upon the simulating process, temperature is regulated using the Nosé-Hoover approach, and a time step of 1 fs is used.Ewald summation as well as a cut-off radius of 12.5 Å is employed upon treating van der Waals and Coulomb interactions [9,10].An atomic trajectory, used for analyzing properties, is captured for every 1 ps.Figures 3-6 show the variation of energy and temperature during both the NVT and NVE runs.
In the next section (section 3), analysis of some of the dynamics properties such as diffusion coefficient (D), mean squared displacement (MSD), radial distribution function (RDF), and ion conductivity (σ) are given, as well as discussed.

Results and discussion
Table 1 reveals the computed outcomes from a novel PEMFC by considering Platinum, Pd 3 Ag, and Palladium as electrodes while SPEEK is the polymer electrolyte.The MSD values, among others, describe the dynamics of the protonated (hydronium) ions.These values of D, MSD, RDF, and σ are calculated according to the concepts described elsewhere [6].The considered bond pair, denoted by S-O * , represents a bond between S atom of SO -

System I
This considers Platinum as an electrode, i.e. anode and cathode, and SPEEK as electrolyte.As shown in figure 7, MSD values show increases over increasing simulation time as well as over increasing temperature.The time derivative of the MSD gives diffusion coefficients, which have estimated values of 0.56 × 10 −4 cm 2 s −1 at T = 150 K, 0.81 × 10 −4 cm 2 s −1 at T = 300 K, 0.90 × 10 −4 cm 2 s −1 at T = 450 K and 4.04 × 10 −4 cm 2 s −1 at T = 600 K.The corresponding ion conductivity have an estimated values of 12 × 10 −2 S cm −1 at 150 K, 8.5 × 10 −2 S cm −1 at 300 K, 6.3 × 10 −2 S cm −1 at 450 K, and 21 × 10 −2 S cm −1 at 600 K. Some of these values seem to be lower than the counterparts when Nafion polymer is used as an electrolyte [6] (see table 1), particularly, at temperatures below room temperature, while some of the values are slightly higher than the counterparts under the Nafion polymer, particularly, at temperatures above 450 K.However, all the   [11][12][13].Furthermore, a literature [14] has suggested conductivity from Nafion polymer to be as high as 10 × 10 −2 S/cm.The causes for some of the disparities between our calculated values and the values in the literature can be attributed to the incorporation of the interaction effects from the electrodes.It looks that, in this work, due to addition of interaction potential from electrode, the resulting average driving force acting on the ions has resulted in an increased conductivity.At the highest peak of the RDF of S-O * , i.e. the first coordination shell, the bond distance is approximately 3.0 Å.The corresponding CN values are 3.64 at T = 150 K, 0.31 at T = 300 K, 0.10 at T = 450 K, and 0.33 at T = 600 K. Details of approaches employed for estimating CN values from an RDF data is given elsewhere, in literature [6].

System II
This considers Pd 3 Ag as an electrode, i.e. anode and cathode, and SPEEK as electrolyte.By utilizing the raw data underlying figure 8, we have calculated the self-diffusion coefficients for the protonated ions to be 0.42 × 10 −4 cm 2 s −1 , 1.59 × 10 −4 cm 2 s −1 , 1.98 × 10 −4 cm 2 s −1 , and 0.0069 × 10 −4 cm 2 s −1 , respectively, at T = 150 K, T = 300 K, T = 450 K, and T = 600 K.These results are closely aligned with research reports by scholars [11].In all plots, the MSD values increase as the simulation time increases.In addition, MSD values increase with increases in temperature, in the range [150, 450] K.The corresponding value of conductivity are estimated to be 14 × 10 −2 S cm −1 , 26 × 10 −2 S cm −1 , 21 × 10 −2 S cm −1 , and 0.60 × 10 −2 S cm −1 .It seems that the novel material in System II shows a more preferable  promising results, in terms of its conductivity, when compared to the benchmark (System I).Furthermore, our simulation results show that System II exhibits a conductivity which is more closely comparable to that of a costly Pt based traditional PEMFC [6], where Nafion polymer is the electrolyte.Relative to System I, the CN in System II have higher values of 4.25, 4.24, 3.92, and 2.46, respectively, at T = 150 K, T = 300 K, T = 450 K, and T = 600 K.The higher CN values seem to have resulted in higher conductivities, which are, in turn, closely compared to those reported in literature [12,13,15].The raw RDF data plotted in figure 9 is the base for the estimated CN values (given above).

System III
This considers Palladium as an electrode, i.e. anode and cathode, and SPEEK as electrolyte.By using the raw data underlying figure 10, we have estimated self-diffusion coefficients for the ions to be 0.002 × 10 −4 cm 2 s −1 , 0.038 × 10 −4 cm 2 s −1 , 0.026 × 10 −4 cm 2 s −1 , and 0.0061 × 10 −4 cm 2 s −1 , respectively, at T = 150 K, T = 300 K, T = 450 K, and T = 600 K.These outcomes are slightly smaller compared to the investigation for Systems I & II.However, the results are still reasonably comparable to the reported values in previous scholarly works [11].The MSD values increase with time for all temperatures considered.In addition, the MSD values increase with increases in temperature, in the range [150, 450] K.The values of conductivity is estimated to be 0.04 × 10 −2 S cm −1 at T = 150 K, 0.34 × 10 −2 S cm −1 at T = 300 K, 0.15 × 10 −2 S cm −1 at T = 450 K, and 0.027 × 10 −2 S cm −1 at T = 600 K.The corresponding CN values are estimated to be 8.80, 2.57, 0.14, and 0.15, respectively.In figure 11, the diffusion coefficients and conductivity, which are presented in table 1, are visualized in a graphic plot.As can be seen from the figures, it looks that the novel System II gives a closely comparable performance to the Pt electrode combined with Nafion polymer (for instance, see our previous  work [6]), especially over a temperature ranges of near room temperature.At the same time, Pd 3 Ag combined with SPEEK polymer (System II) outperforms Pt combined with SPEEK polymer (System I), particularly, at near room temperatures.The results of conductivities near room temperature (i.e.300 K-450 K) are particularly worth emphasized in this discussion, in part due to a claim of proven stability of the SPEEK polymer in such temperature ranges [15].

Conclusion
This study provides important information on the self diffusion coefficients, and conductivity of novel PEMFCs for different temperature, where the electrodes are made of Pt, Pd 3 Ag, and Pd, while SPEEK polymer is used as the electrolyte.The proton transport takes place by the S-O * bond-formation and bond-breaking.The proton conductivity is relatively higher in System II, compared to the values in Systems I & III, at around room temperatures.This is somewhat different from the sequence of performance property investigated when Nafion polymer is used as electrolyte.The force field modelling takes into account of the interactions of the hydronium ions with the electrolyte as well as electrodes, and thus, has resulted in an improved conductivity values when compared to the case where the interaction potential from the electrode is not taken into account.The values of conductivity in System I ranges from 6.3 × 10 −2 S cm −1 to 21 × 10 −2 S cm −1 for temperatures between 150 K and 600 K; while for System II, it ranges from 0.06 × 10 −2 S cm −1 to 26 × 10 −2 S cm −1 ; and in System III, it ranges from 0.027 × 10 −2 S cm −1 to 0.34 × 10 −2 S cm −1 .Thus, at near room temperatures, it seems that Pd 3 Ag electrode combined with SPEEK electrolyte performs better than the costly-Pt electrode combined with SPEEK electrolyte.In addition, the investigated ion conductivity performances with SPEEK electrolyte is closely comparable to the counterpart performances under Nafion polymer.

Figure 2 .
Figure 2. Side view of PEMFC model.The SPEEK electrolyte and hydronium ions, in the middle, are sandwiched between electrodes on the edges.

Figure 3 .
Figure 3. Equilibration of temperature of the system during NVT run.

Figure 4 .
Figure 4. Equilibration of energy of the system during NVT run.

Figure 5 .
Figure 5. Production of temperature of the system during NVE run.

3
and O atom of H 3 O + .It is believed that the ion transport in the electrolyte takes place by bond formation and breaking of the S-O * .The Coordination Number (CN) denotes the number of such nearest bond pairs.

Figure 6 .
Figure 6.Production of energy of the system during NVE run.

Figure 7 .
Figure 7. MSD graphs of the protonated ion under consideration of novel PEMFC: System I, at temperatures as indicated in the plot legend.

Figure 8 .
Figure 8. MSD graphs of the protonated ion under consideration of novel PEMFC: System II, at temperatures as indicated in the plot legend.

Figure 9 .
Figure 9.The radial distribution function (RDF) of S-O * bond pair.(a) Left side, with System I, (b) Right side, with System II.

Figure 10 .
Figure 10.MSD graphs of the protonated ion under consideration of PEMFC: System III, at temperatures as indicated in the plot legend.

Table 1 .
Diffusion coefficient (D) [×10 −4 cm 2 s −1 ] and conductivity (σ) in [S/cm] as calculated at the given temperature (T) in[K].CN indicates the coordination number in this work, while σ nafion indicates the conductivity in [S/cm] when Nafion is used as electrolyte.
conductivity values are closely comparable with those reported in literature