Toward Improved Alkaline Membrane Fuel Cell Performance Using

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Toward Improved Alkaline Membrane Fuel Cell Performance using Quaternized Aryl-Ether Free Polyaromatics Sandip Maurya, Cy H. Fujimoto, Michael R. Hibbs, Claudia Narvaez Villarrubia, and Yu Seung Kim Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00358 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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Chemistry of Materials

Toward Improved Alkaline Membrane Fuel Cell Performance using Quaternized Aryl-Ether Free Polyaromatics Sandip Mauryaa, Cy H. Fujimotob, Michael R. Hibbsb, Claudia Narvaez Villarrubiaa, and Yu Seung Kima,* MPA-11: Materials Synthesis & Integrated Devices, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA a

b

Materials Science and Engineering Center, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA

ABSTRACT: Aryl-ether free polyaromatics are attractive candidates for polymer electrolytes for alkaline membrane fuel cells (AMFCs) due to their excellent high pH stability. However, high-performance of AMFCs employing aryl-ether free polyaromatic ionomers has not yet been realized. Our recent small molecule study suggested that this may be due to the undesirable adsorption of phenyl groups of the polyaromatic electrolytes onto electrocatalysts, which inhibits the hydrogen oxidation reaction at the fuel cell anode. Here, we perform an illustrative case study to demonstrate the adverse impact of phenyl adsorption in AMFCs using aryl-ether free Diels-Alder poly(phenylene) ionomers. Implementing several mitigating strategies for phenyl adsorption and increasing H2 transport significantly improves the H2/O2 AMFC performance, ca. the peak power density from 220 to 880 mW cm-2 at 80 °C. This study gives insights on designing advanced polyaromatic ionomers for high-performing AMFCs.

Alkaline membrane fuel cells (AMFCs) offer an attractive alternative to acidic polymer electrolyte membrane fuel cells (PEMFCs) because inexpensive, non-precious metal catalysts can potentially be employed.1 Quaternized polyaromatics are one of the promising polymer electrolytes for AMFCs as these materials have good hydroxide conductivity, mechanical properties, and processibility. In addition, good thermo-oxidative stability of quaternized polyaromatics enables AMFCs to operate at > 60 °C in which greater catalytic activity and lower cell resistance can be acquired in the combination with less carbonation problems. Typical polyaromatic electrolytes are quaternized poly(aryl ether sulfone)s prepared via the polycondensation reaction between aromatic dihalides and dihydroxy monomers followed by quaternization.2-7 With this synthetic route, the formation of aryl-ether linkages (C-O-C bonds) is unavoidable. The aryl-ether group in the quaternized polyaromatics was found to be chemically unstable under high pH conditions.8-12 In order to avoid such degradation, aryl-ether free quaternized polyaromatics have been prepared via Diels-Alder reaction13, acid-catalyzed Friedel-Craft polycondensation14, 15, metal-catalyzed coupling reactions16-19 and cyclo-polycondensation20. These aryl-ether free quaternized polyaromatics exhibited excellent alkaline stability. AMFC performance employing polyaromatic ionomers is, however, relatively poor. While the best performing H2/O2 AMFC employing aryl-ether containing polyaromatic ionomers exhibited the peak power density of 1,000

mW cm-2,21 more recent AMFCs employing other aryl-ether containing polyaromatics exhibited the peak power density of 300 – 380 mW cm-2.22-25 The AMFC performance using aryl-ether free polyaromatics was even lower, ca. the peak power density of < 210 mW cm-2.8, 15, 26 In contrast, AMFCs employing poly(vinylbenzyl ammonium)s (PVBAs) exhibited much higher performance, ca. the peak power density of 800 – 1,500 mW cm-2.27-29 One possible reason that may explain the low performance of AMFCs employing polyaromatic ionomers is undesirable adsorption of the phenyl group in polyaromatic ionomers on catalyst surface which has an adverse impact on the hydrogen oxidation reaction (HOR).30 In this communication, we investigate this possible performance-limiting factor and implement several mitigating strategies to improve the performance of AMFCs. For this purpose, we first investigate the impact of phenyl group adsorption on hydrogen oxidation activity of Pt by measuring HOR voltammograms of Pt in contact with various alkaline ionomers. Secondly, the H2/O2 AMFC performance of membrane electrode assemblies (MEAs) employing Diels-Alder poly(phenylene) ionomers. Commercial Pt/C and Pt-Ru/C alloy catalysts that have different phenyl adsorbing characteristics are used. Thirdly, improved H2/O2 fuel cell performance of MEAs using less-phenyl group containing ionomers is demonstrated. Lastly, the H2 mass transport issue with the polyaromatic electrolytes is briefly discussed. Table S1 displays the physico-electrochemical properties and phenyl group contents of the ionomers used in this study.

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PF-PMG

1

OH

PF-PMG

CF2 CF2

CF CF2

N

5.6

PVBA

N O CF2 CF O CF2 CF2 N

N

C

OH N

CF3

PP-BTMA OH N

OH2

1

1

PP-BTMA

N OH

N

Alkaline organic solution

Alkaline polymer electrolyte

NH

OH

PVBA

F

N OH

Figure 1. HOR voltammograms of (a) Pt/C in 0.1 M TMAOH and BTMAOH measured from rotating disk electrode (RDE) at 25 -1 °C, rotating speed: 900 rpm, scan rate: 5 mV s .30 (b) HOR voltammograms of Pt with three polymer electrolytes from microe-1

lectrode mini-cells (Fig. S1) at 25 °C, 100% RH, scan rate: 5 mV s , the current density was normalized by the limiting current density.

Figure 1a compares the HOR voltammograms of a Pt/C in 0.1 M tetramethylammonium hydroxide (TMAOH) and benzyltrimethylammonium hydroxide (BTMAOH). The HOR current density of Pt/C in 0.1 M BTMAOH drops as the electrode potential increases from 0.0 V to 0.06 V vs. RHE (reversible hydrogen electrode), then gradually recovers as the cell potential further increases to ~0.3 V. This is due to the phenyl group adsorption in the orientation where the phenyl group is parallel to the Pt catalyst surface.30 Figure 1b compares the HOR voltammograms of a Pt microelectrode in contact with three alkaline ionomers, perfluorinated phenylpentamethyl guanidinium (PFPMG)31, PVBA32 and benzyltrimethyl ammonium functionalized poly(phenylene) (PP-BTMA). Lower HOR current densities of Pt were obtained with more phenyl group containing ionomers. The potential range of the suppressed HOR current density for PP-BTMA is 0.0 V – 0.6 V which is greater than that observed with Pt/C in 0.1 M BTMAOH (0.0 V – 0.3 V). This is attributed to the adsorbed phenyl groups in the BTMAOH solution which starts to desorb at > 0.06 V as the molecular configuration is rearranged by cationic group adsorption which does not occur with nonionic group substituted phenyl groups in PP-BTMA. The phenyl group adsorption and its adverse impact on HOR activity can be reduced with Pt-Ru alloy catalysts. Figure 2a compares the HOR voltammograms of commercial Pt/C and Pt-Ru/C catalysts in 0.1 M BTMAOH. As shown, the suppression of HOR current observed with the Pt/C catalyst between 0.0 V and 0.3 V disappeared with the Pt-Ru/C, suggesting that the fewer phenyl groups may have adsorbed on Pt-Ru/C. Density functional theory calculation suggested that the adsorption energy of the phenyl group in parallel to the Pt surface is much greater than that to the Pt-Ru alloy (−2.30 eV for Pt vs. −1.32 eV for PtRu).30 Figure 2a also shows that the intrinsic HOR activity between the Pt/C and Pt-Ru/C catalysts is almost same, as the HOR exchange current density for both electrodes is similar ca. 1.94 mA cm-2 for Pt-Ru/C and 1.76 mA cm-2 for

Pt/C,30 indicating that the significantly improved HOR activity with the Pt-Ru catalyst is due to the lower phenyl group absorbed characteristics rather than intrinsically better HOR activity of the Pt-Ru/C catalyst. Figure 2b shows HOR voltammograms of Pt/C and PtRu/C microelectrodes in contact with PP-BTMA. It is clear that the suppression of HOR current density between 0.0 V to 0.6 V is much less with Pt-Ru/C, although the phenyl group adsorption is not completely removed even with the Pt-Ru/C catalyst. Figure 2c compares the iR-corrected polarization curves of the Pt/C or Pt-Ru/C anode catalyzed MEAs using the PP-BTMA ionomer. At very high potential, ca. > 0.92 V, a similar performance between the Pt-Ru/C and Pt/C anode catalyzed MEAs was observed. This is consistent with the RDE results (Fig. 2a), indicating that the intrinsic activity of both catalysts is similar. As the cell voltage decreases, however, the performance of the Pt-Ru/C anode catalyzed MEA becomes greater, confirming the fact that the adverse effect derived from the phenyl group adsorption is less with the Pt-Ru/C catalyst. The improved AMFC performance obtained with the PPHTMA ionomer and Pt-Ru/C anode catalyst is compared with the performance of PEMFC MEAs. Figure 2d shows the PEMFC performance of MEAs employing perfluorosulfonic acid (Nafion) and sulfonated Diels-Alder poly(phenylene) (PP-SA). The iR-corrected performance of the Nafion-based MEAs using Pt/C and Pt-Ru/C anode catalysts are similar, indicating that the intrinsic HOR activity of the Pt/C and Pt-Ru/C is similar. However, the iR-corrected performance of PP-SA-based MEA using Pt-Ru/C anode catalyst is notably better than that of the MEA using Pt/C anode catalyst. This suggests that the phenyl group adsorption may also limit the PEMFC performance, although the impact of the phenyl group adsorption under the acidic environment is relatively small. Comparing the kinetic region between the acidic and alkaline systems, the alkaline PP-BTMA-based MEA exhibits a comparable performance with the acidic Nafion-based MEA, ca. ~13 mA


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Chemistry of Materials

OH2 N OH

OH N

N OH

CF CF2 O CF2

1

CF2

CF3

O 3S H

CF2

5.6

CF O CF2 CF2 SO3 H

Figure 2. (a) HOR voltammograms of Pt/C and Pt-Ru/C in 0.1 M BTMAOH -1 30 from RDE at 25 °C, rotating speed: 900 rpm, scan rate: 5 mV s . (b) Comparison of HOR voltammograms of Pt/C and Pt-Ru/C in contact with PP-BTMA -1 from microelectrode mini cells at 25 °C, 100% RH. Scan rate: 5 mV s , normalized the current density by the limiting current. (c) iR-corrected polarization curves of the MEAs employing PP-BTMA; membrane: PP-HTMA, anode: Pt/C -2 -2 -2 (0.6 mgPt cm ) or Pt-Ru/C (0.5 mgPt cm ), cathode: Pt/C (0.6 mgPt cm ); measured at 80 °C, Pabs = 41 kPa and 100% RH. (d) iR-corrected polarization

O 3S H

curves of the MEAs employing acidic Nafion or PP-SA ionomer; membrane: -2 -2 Nafion 212 or PP-SA, anode: Pt/C (0.6 mgPt cm ) or Pt-Ru/C (0.5 mgPt cm ), -2

cathode: Pt/C (0.6 mgPt cm ); Pabs = 41 kPa and 100% RH.

cm-2 at 0.9 V. However, the PP-SA-based MEA exhibits inferior performance, ca. 5 mA cm-2 at 0.9 V. This suggests that the intrinsically lower HOR kinetics under alkaline environment is not a critical performance-limiting factor of AMFCs. The impact of the molecular structure of Diels-Alder poly(phenylene) ionomer was investigated. Figure 3a shows the iR-corrected polarization curve of an MEA using the hexamethylene-trimethyl-ammonium functionalized poly(phenylene) (PP-HTMA) in which the ammonium group is separated from poly(phenylene) backbone by hexamethylene side chain while maintaining the same number of phenyl groups in the ionomer backbone (Table S1). At very high cell voltage, ca. > ~0.88 V, the AMFC performance of MEA employing PP-HTMA was similar to that of MEA using PP-BTMA. However, as the cell voltage decreases, the AMFC performance employing PP-HTMA becomes lower. The inferior performance at the lower cell potentials is probably due to the less efficient desorption of the phenyl group connected to the alkylammonium side chain. It has previously been shown that directly attaching ammonium group to phenyl group (BTMA) helps desorbing the phenyl group from the catalyst surface30 and this effect is likely lessened when the cation is tethered by a flexible spacer (HTMA). Further structural change of PP-HTMA was made with the hexamethylene-trimethyl-ammonium functionalized poly(phenylene alkylene) (PPA-HTMA) which has less

phenyl groups in the polymer backbone and side chain. Figure 3a shows that the performance of the MEA employing PPA-HTMA is superior to that of the other two MEAs. The kinetic performance of the MEA employing PPAHTMA is even greater than that of the acidic Nafion-based MEA, ca. 24 mA cm-2 vs. 13 mA cm-2 at 0.9 V, suggesting that kinetic overpotential in the AMFC employing PPAHTMA was less than that in the Nafion-based PEMFC. The microelectrode experiment also shows that the HOR activity of Pt-Ru/C in contact with PPA-HTMA is superior to that of Pt-Ru/C in contact with PP-BTMA at the potential range of 0.0 V – 0.4 V vs. RHE (Fig. 3b). This confirms that the improved the AMFC performance of MEA using PPAHTMA in Fig. 3a is due to the better HOR activity of PtRu/C with the PPA-HTMA ionomer which has fewer -phenyl groups in the ionomer backbone. Figure 3c exhibits the progressive AMFC performance improvement of MEAs employing quaternized Diels-Alder poly(phenylene)s by implementing mitigation strategies adopted in this study. The peak power density of the MEA employing PP-BTMA and Pt/C anode catalyst is 220 mW cm-2, which is comparable to the reported AMFC performance employing other aryl-ether free polyaromatics8, 15, 26. Using the less-phenyl group adsorbed Pt-Ru/C anode catalyst, the peak power density of the MEA jumped to 460 mW cm-2, and further improvement to the peak power density of 650 mW cm-2 was achieved by employing PPAHTMA and the Pt-Ru/C anode catalyst.



N

Page 4 of 6

OH

N OH

N

OH

N OH

Figure 3. (a) iR-corrected polarization curves of the MEAs employing PP-HTMA, PPA-HTMA; membrane: PP-HTMA, anode: -2 -2 Pt-Ru/C (0.5 mgPt cm ), cathode: Pt/C (0.6 mgPt cm ); Pabs = 41 kPa; (b) HOR voltammograms of Pt-Ru/C in contact with PP -1

A-HTMA measured at 25 °C, 100% RH. Scan rate: 5 mV s , normalized the current density by the limiting current. (c) H2/O2 A -2

MFC performance improvement of MEAs; Pabs of 285 kPa; membrane: PP-HTMA, anode: Pt/C (0.6 mgPt cm ) or Pt-Ru/C (0.5 -2

-2

mgPt cm ), cathode: Pt/C (0.6 mgPt cm ). Flow rate: 500 sccm for H2 and 300 sccm for O2 for all MEAs except the high flow o ne (2000 sccm from H2 and 1000 sccm for O2).

When compared with the performance of acidic Nafionbased MEA which exhibited 1,600 mW cm-2 peak power density (Fig. S2), the AMFC performance of the MEA employing PPA-HTMA and Pt-Ru anode catalyst is still significantly lower. Considering that the cell resistance difference between the AMFC and PEMFC MEAs was small (< 10 mΩ cm2, see Table S2), the lower performance at the high current density region may be related with the limited H2 mass transport at the AMFC anode. Figure 3c demonstrates that increasing anode flow rate may partly resolve this issue; the peak power density of the AMFC employing PPAHTMA further increased to 880 mW cm-2 as the anode flow rate increased from 500 sccm to 2000 sccm. The improved performance with increased anode flow rate was also observed with the MEA employing PP-HTMA which has much lower water uptake (Fig. S3). This suggests that the origin of the limited H2 mass transport for polyaromaticbased MEAs may be cation-hydroxide-water co-adsorption which limits the H2 access to the anode catalyst surface33 rather than water flooding34. More efforts to understand the H2 transport issue for the AMFC MEAs employing polyaromatic ionomers may be required. In conclusion, we demonstrate that phenyl group adsorption is the major limiting factor for aryl-ether free polyaromatic ionomers used in AMFCs. Mitigating strategies such as using less phenyl group adsorbed Pt-Ru catalyst and ionomers with fewer phenyl moieties help to improve the AMFC performance. Excellent kinetic performance of the AMFCs employing PPA-HTMA is a great promise for the alkaline stable aryl-ether free polyaromatic ionomers in use of AMFCs. H2 mass transport issue in the high current density region remains to be resolved for further improvement of AMFC performance and durability.

ASSOCIATED CONTENT Supporting Information. Synthesis and characterization of polymers and microelectrode data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

ACKNOWLEDGMENT We cordially thank Dr. K. S. Lee and Dr. E. J. Park for supplying PVBAs for this study. This work was supported by the US DOE, Office of Energy Efficiency and Renewable Energy (EERE), Fuel Cell Technologies Office (FCTO) under contract no. DE-AC52-06NA25396 (Los Alamos National Laboratory) and DE-AC04-94AL85000 (Sandia National Laboratories).

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Toward Improved Alkaline Membrane Fuel Cell Performance Using

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