Micro Galvanic Cell To Generate PtO and Extend the Triple-Phase

Oct 25, 2017 - Herein, we demonstrate that a micro galvanic cell, where Pt and oxygen functional groups (OFGs) on the surface of carbon black are elec...
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Letter Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX

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Micro Galvanic Cell To Generate PtO and Extend the Triple-Phase Boundary during Self-Assembly of Pt/C and Nafion for Catalyst Layers of PEMFC Zhi Long,†,‡ Liqin Gao,‡,§ Yankai Li,† Baotao Kang,† Jin Yong Lee,# Junjie Ge,‡,∥ Changpeng Liu,‡,∥ Shuhua Ma,*,† Zhao Jin,*,‡,∥ and Hongqi Ai*,† †

Shandong Provincial Key Laboratory of Fluorine Chemical Materials, School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, Shandong, China ‡ State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China § Graduate School of the Chinese Academy of Sciences, Beijing 100039, China ∥ Laboratory of Advanced Power Sources, Changchun Institute of Applied Chemistry, 5625 Renmin Street, Changchun 130022, Jilin, China # Department of Chemistry, Sungkyunkwan University, Suwon440-746, R. Korea S Supporting Information *

ABSTRACT: The self-assembly powder (SAP) with varying Nafion content was synthesized and characterized by XRD, XPS, HRTEM, and mapping. It is observed that the oxygen from oxygen functional groups transfers to the surface of Pt and generate PtO during the process of self-assembly with the mechanism of micro galvanic cell, where Pt, carbon black, and Nafion act as the anode, cathode and electrolyte, respectively. The appearance of PtO on the surface of Pt leads to a turnover of Nafion structure, and therefore more hydrophilic sulfonic groups directly contact with Pt, and thus the triple-phase boundary (TPB) has been expanded.

KEYWORDS: PEMFCs, catalyst layers, triple-phase boundary, Pt/C, Nafion

P

TPB.10 But the self-assembly of catalyst layer consisted of Pt/C and Nafion is yet to be understood, which plays a vital role to further significantly improve the power density of PEMFC. In addition, understanding the self-assembly and building the model are important to shed light on proton transfer, electrochemical mechanisms in a polymer electrolyte and the design of novel catalysts and ionomers instead of expensively commercial Pt/C and Nafion, respectively. Herein, we demonstrate that a micro galvanic cell, where Pt and oxygen functional groups (OFGs) on the surface of carbon black are electrodes and Nafion is the electrolyte to transfer proton, can form during the self-assembly of Pt/C and Nafion in CLs, which brings out an oxidation of Pt to PtO. The appearance of PtO would cause that more −SO3H in Nafion directly contact with Pt and TPB is extended.

roton exchange membrane fuel cells (PEMFCs) have been attracting a lot of attention as promising energy converters, especially in automobile applications, because of their high efficiency, high energy density, high power density, zero emissions, and rapid cold start-up.1−3 Membrane electrode assembly (MEA) as the most important part is the area of electrochemical reaction, especially catalyst layers (CLs) consisting of Pt/C as a catalyst and an electronic conductor, and Nafion as a proton transferred channel to extend the triplephase boundary (TPB).4,5 Many interfaces are formed during the self-assembly of Pt/C and Nafion due to some physical effects, such as the adsorption between platinum and Nafion, chelation between carbon black and platinum, the interaction between oxygenic groups on the surface of carbon black and Nafion and changes of Nafion structure in the electrocatalytic reaction.6−9 These interfaces are also active sites of electrochemical reaction, where catalysts, electron pathway, and proton pathway are simultaneously contained to satisfy the special needs to make the electrochemical reaction. Cheng et al. had confirmed that Nafion is able to promote the activity of platinum due to effective proton transfer and improvement of © XXXX American Chemical Society

Received: August 8, 2017 Accepted: October 25, 2017 Published: October 25, 2017 A

DOI: 10.1021/acsami.7b11852 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

between Pt/C and Nafion only occur on the surface of platinum so as to make no effect on the inner of platinum (Table S1). In addition, the average size of the Pt crystallites, obtained by Debye−Scherrer equation and fwhm’s (Table S2), increases 3−5 Å on the basis of Pt(111), Pt(200), Pt(200), and Pt(311) after adding Nafion (Table S3), suggesting that the self-assembly could facilitate Pt nano particles growing. Furthermore, Pt 4f spectra (Figure 1d) from SAP of various Nafion contents show that the peak of Pt(0) 4f7/2 positively shifts from 70.2 to about 71.3 eV after adding Nafion because sulfonic acid groups or fluorine from Nafion adsorbs on the subsurface of Pt to reduce the electron density. Pt 4f spectrum were fitting (Figure 1d−f and Figure S2) and total contents of Pt(II) become obviously more after adding Nafion in comparison with Pt/C (17.32%), such as 62.05% of Pt(II) for N25 (Table S4), which further ensures the generation of PtO. In addition, TEM and mapping were used to obtain the relationship between the distribution of Pt and that of oxygen for SAP with Nafion content of 25 wt %, shown in Figure 2. For

The self-assembly of Pt/C and Nafion was made by preparing a CL ink with a general method,11 where Pt/C, Nafion isopropanol and deionized water were mixed and dispersed by the ultrasound. The detailed preparation was shown in the Supporting Information. Therefore, the selfassembly powder (SAP) with various Nafion contents from 15 wt % (N15) to 45 wt % (N45), was synthesized. As can be seen from Figure 1a, Pt/C exhibits the characteristics of Pt shown as

Figure 1. (a) XRD patterns for 60% Pt/C, CLs with various Nafion contents and Nafion 115; peak profile analyses of XRD patterns, (b) 60% Pt/C, (c) N25; (d) Pt 4f spectrum from CLs of various Nafion contents and Pt/C; Pt 4f spectrum from (e) 60% Pt/C, (f) N25.

Figure 2. (a) TEM image of SAP. (b) HRTEM image of SAP, where the (111) lattice of Pt can be observed, and the amorphous PtO is visible and (c−e) elemental mappings of SAP.

SAP, Pt nanoparticles are uniformly dispersed and little aggregation of Pt nanoparticles is observed (Figure 2a). The (111) lattice of Pt can be obviously observed and the amorphous PtO on the surface of Pt nanoparticles is also visible (Figure 2b). For Pt/C without Nafion, no amorphous phase is found and the (111) lattice of Pt extends to the periphery (Figure S7). OFGs on the surface of carbon should be equably distributed; however, the density of O is improved in the area of higher density of Pt for SAP (Figure 2c−e). But it is unable to guarantee the oxygen location to Pt surfaces, and therefore the source of PtO was analyzed in the following paragraph. The reason generating PtO is analyzed from the perspective of the source of oxygen. In our condition, the oxygen source of PtO possibly comes from oxygen functional groups (OFGs) on the surface of carbon and oxygen in air or from the solution. Therefore, several contrast experiments have been performed to confirm the oxygen source. First, to obviate disturbance of O2 in air, we degassed Pt/C at vacuum and 50 °C for 90 min in inert gas condition, then deoxygenated deionized water, 5 wt % Nafion solution and isopropanol were added in order and

the face centered cubic (fcc) crystalline structure and has major peaks at around 2θ = 39.7° (111), 46.2° (200), 67.4° (220), 81.2° (311), and 85.7° (222) and the characteristics of graphite structure at around 2θ = 25° (002).12,13 Nafion 115 displays a XRD pattern that has two broad diffraction features, one at 2θ = 10−25° resulting from a convolution of amorphous (2θ = 16.41°) and crystalline (2θ = 17.51°) scattering of the polyfluorocarbon chains of Nafion, and another one at 2θ = 30−45° attributed to the amorphous region of Nafion.14,15 SAP with varying Nafion contents shows the characteristics of Pt/C as well as Nafion; however, it is interesting that the peaks at about 28 and 31°, which are attributed to PtO(100) and PtO(002), are obviously observed, indicating that the chemical reaction occurs and the microstructure of Pt/C is changed after adding Nafion to Pt/C. To realize microstructure changes, we analyzed peak profiles of every XRD pattern (Figure 1b, c and Figure S1). The difference of platinum crystalline interplanar spacing for various SAP is lower than 0.1 Å, therefore platinum crystalline structure is not changed by addition of Nafion and the interactions B

DOI: 10.1021/acsami.7b11852 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

bonded on the surface of carbon. Meanwhile, electron transfers via the carbon black. Then OFGs obtains the proton and electron to be reduced to generate H2O and Pt−OH resolves Pt, PtO and H2O. The mechanism of micro galvanic is able to effectively reduce the activation energy so that platinum can easily participate in the reaction and is oxidized. To tell the model precisely, we used quantum chemical calculation to verify the feasible reaction pathways and stable intermediates. For the model designed, we used water molecules as the proton-transfer channel to simplify the model instead of Nafion, Pt is near the OFGs on the surface of carbon so as to be a capable proton pathway for one molecule of water, and graphene is used as a kind of typical model compound of carbon black. The results of the energy profile are shown in Figure 3. It is difficult to transfer oxygen of OFGs to Pt and generate PtO, where higher energy barriers (67.33 kcal mol−1 from R to TS1 and 131.03 kcal mol−1 from PC1 to TS2) are necessary, as a result, it is unable to occur under a general condition, for Pt/C without proton transferred channel. However, the energy barriers are dramatically reduced for one molecular of water as proton transferred channel between Pt and OFGs by 15.48 times from R to TS1 (4.35 kcal mol−1) and 46.63 times from PC1 to TS2 (2.81 kcal mol−1), therefore the reaction readily occurs at the room temperature. The results of quantum chemical calculation well confirm the model of micro galvanic cell for the self-assembly behaviors of Pt/C and Nafion. We then further investigate the effect of the appearance of PtO on the electrochemical reaction. Keffer et al.8,9 reported that the presence of a PtO nanoparticle created a strong attraction not only to water molecules but also to sulfonate groups and hydronium ions because of the charge distribution on the PtO surface by MD simulations. Wood et al.7 confirmed that a hydrophobic Nafion region was formed adjacent to a Pt film, however, when Nafion was in contact with PtO surface, the Nafion on the PtO surface became hydrophilic by neutron reflectometry. Therefore, the formation of trace amounts of PtO on the surface of Pt maybe lead turnover of Nafion and more −SO3H could directly contact with Pt, thus extending the triple-phase boundary shown in Scheme S1. To certify the above, we used a micro-MEA16 to measure the change of electrochemical active surface areas (ESAs) caused by further increasing PtO through electrooxidation with chronoamperometry at 0.8 V. If the ESA is increased in comparison with original one after electrooxidation, it suggests that the generation of PtO is beneficial for extending TPB and improvement of Pt utilization; otherwise, it suppresses Pt activity because more active sites of Pt are blocked. The result is shown in Figure S8a. Areas of cyclic voltammetry curves from 0.05 to 0.5 V are shown in Figure S8b. The area increases from 577.3 to 683.1 μC after electrooxidation for 10 min. Therefore, the formation of PtO really leads to a change in electrochemical active surface areas and it increases with electrooxidation time because PtO leads to turnover of Nafion structure to extend the triple-phase boundary. However, the area decreases to 569.7 μC after electrooxidation for 30 min because more PtO covering Pt surface and PtO does not show electrochemical active surface areas. In conclusion, the SAP with varying Nafion content was synthesized. It is proved that the oxygen from oxygen functional groups transfer to the surface of Pt and generate PtO during the process of self-assembly by the reaction mechanism of the micro galvanic cell, where Pt and carbon

disperse them with ultrasound. Although the peak of Pt 4f negatively shift (Figure S4) and the content of Pt(II) is lower (45.22%), comparing with in air (62.06%) shown in Table S5, there are obvious diffraction peaks of PtO for SAP without disturbance of O2 (Figure S5), suggesting that Pt is not oxidized by oxygen in air. The contrast experiment with Pt black instead of Pt/C has also been performed. As no obvious PtO diffraction peaks was founded, we supposed that oxygen in solution is not the main oxygen source either, but OFGs on the surface of carbon should be response for in situ generation of PtO (Figure S3). In addition, solution composition of N25 has been analyzed by GC-MS. As shown in Figure S6. There are only the peaks of air, isopropanol, and n-propanol from 5% Nafion solution but no reduction production from isopropanol, suggests that organic alcohol is not involved in the generation of PtO. Besides that, as we known, water in mixture solvent is not able to oxidize platinum because of the lower reduction potential (H2O → H2, E = 0 V). Therefore, platinum can not be oxidized by H2O either. Therefore, according to the results above, OFGs on the surface of carbon is proved to be the main source to generate PtO and the summary is shown in Table 1. Table 1. Contrast Experiment to Guarantee the Oxygen Source of PtO and Mechanism catalyst

solution

atmosphere

Pt/C Pt/C Pt Black Pt/C

H2O/IPA H2O/IPA H2O/IPA H2O/IPA

air N2 air air

a

Nafiona PtOa √ √ √ ×

√ √ × ×

conclusion standard O2 is unnecessary carbon is necessary Nafion is necessary

√ shows the existence, whereas × shows the inexistence.

According to the above, it is known that all of Pt, carbon, and Nafion are indispensable for the appearance of PtO, especially Nafion, which is well-known as a good proton conductor and the electrolyte for PEMFC. Therefore, it is speculated that PtO is generated by the mechanism of micro galvanic cell (Scheme 1), where Pt and carbon black are acted as the anode and Scheme 1. Schematic Diagram of Micro Galvanic Cell during Self-Assembly of CLs with Ultrasound Treatment

cathode, respectively, Nafion is used as electrolyte to form proton pathway and carbon black constitutes the electron transferred channel. Therefore, a complete structure is built for the micro galvanic cell. The detailed reaction is shown as follows. Hydroxyl from H2O is first adsorbed Pt to generate proton and electron. The proton transfers to OFGs via Nafion C

DOI: 10.1021/acsami.7b11852 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 3. (a) Energy profile for the generation of PtO without the proton transferred channel (kcal mol−1); (b) energy profile for the generation of PtO with the proton transferred channel (kcal mol−1). Inset: TS represents the transition state, PC1 represents there is one hydrogen to transfer to OFG and PC1 represents there is another hydrogen to transfer to OFG and generate H2O. The H, C, O, and Pt atoms are shown in white, gray, red, and blue, respectively.

Notes

black are used as the anode and cathode, and Nafion is used as electrolyte to form proton pathway. The energy barrier of reaction is sharply reduced by 15.48 times during the process from R to TS1 and 46.63 times during the process from PC1 to TS2, respectively, after building proton transferred channel between Pt and oxygen functional groups. The important discovery of micro galvanic cell deepens the understanding of the interaction between electrocatalysts and polymer electrolyte, which provides a novel idea to design new catalysts, support with moderate OFGs and electrolytes to improve the utilization of platinum as well as reveals the MEA activation mechanism with discharge at low current density (high potential), i.e. increasing PtO on the surface of Pt to make a turnover of Nafion structure, thus more hydrophilic sulfonic groups directly contact with Pt, the triple-phase boundary has been expanded and the utilization of platinum is improved.

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The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

We acknowledge financial support from the National Natural Science Foundation of China (21673221, 21633008, 21603216), the Strategic priority research program of CAS (XDA09030104), Jilin Province Science and Technology Development Program (20150101066JC, 20160622037JC), the Hundred Talents Program of Chinese Academy of Sciences and the Recruitment Program of Foreign Experts (WQ20122200077).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b11852. Detailed experimental procedure for preparation of SAP, prepreparation of Nafion 115 membrane, quantum chemical calculation and electrochemical characterization and characterization for Pt/C, Nafion, SAP with various Nafion content and Pt black (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.M.). *E-mail: [email protected] (Z.J.). *E-mail: [email protected] (H.A.). ORCID

Baotao Kang: 0000-0001-6946-2279 Jin Yong Lee: 0000-0003-0360-5059 Shuhua Ma: 0000-0001-7549-7435 Hongqi Ai: 0000-0002-9933-390X D

DOI: 10.1021/acsami.7b11852 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces (10) Cheng, K.; Liu, X.; Li, W.; Kou, Z.; Mu, S. Enhancing the Specific Activity of Metal Catalysts Toward Oxygen Reduction by Introducing Proton Conductor. Nano 2016, 11, 1650055. (11) Long, Z.; Deng, G.; Liu, C.; Ge, J.; Xing, W.; Ma, S. Cathode Catalytic Dependency Behavior on Ionomer Content in Direct Methanol Fuel Cells. Chin. J. Catal. 2016, 37, 988−993. (12) Andersen, S. M.; Skou, E. Electrochemical Performance and Durability of Carbon Supported Pt Catalyst in Contact with Aqueous and Polymeric Proton Conductors. ACS Appl. Mater. Interfaces 2014, 6, 16565−16576. (13) Chabi, S.; Kheirmand, M. Electrocatalysis of Oxygen Reduction Reaction on Nafion/Platinum/Gas Diffusion Layer Electrode for PEM Fuel Cell. Appl. Surf. Sci. 2011, 257, 10408−10413. (14) Ludvigsson, M.; Lindgren, J.; Tegenfeldt, J. Crystallinity in Cast Nafion. J. Electrochem. Soc. 2000, 147, 1303−1305. (15) Mauritz, K. A.; Moore, R. B. State of Understanding of Nafion. Chem. Rev. 2004, 104, 4535−4586. (16) Long, Z.; Li, Y.; Deng, G.; Liu, C.; Ge, J.; Ma, S.; Xing, W. Micro-Membrane Electrode Assembly Design to Precisely Measure the in Situ Activity of Oxygen Reduction Reaction Electrocatalysts for PEMFC. Anal. Chem. 2017, 89, 6309−6313.

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DOI: 10.1021/acsami.7b11852 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX