Highly Durable Supportless Pt Hollow Spheres Designed for

Sep 26, 2016 - Highly Durable Supportless Pt Hollow Spheres Designed for Enhanced Oxygen ... *E-mail: [email protected]. ... Large Pt hollow spheres ...
0 downloads 0 Views 7MB Size
Download PDF
Research Article www.acsami.org

Highly Durable Supportless Pt Hollow Spheres Designed for Enhanced Oxygen Transport in Cathode Catalyst Layers of Proton Exchange Membrane Fuel Cells Didem C. Dogan,†,‡ Seonghun Cho,†,‡ Sun-Mi Hwang,† Young-Min Kim,§,∥ Hwanuk Guim,§ Tae-Hyun Yang,† Seok-Hee Park,† Gu-Gon Park,†,‡ and Sung-Dae Yim*,†,‡ †

Fuel Cell Laboratory, Korea Institute of Energy Research (KIER), Daejeon, 305-343, Republic of Korea University of Science and Technology (UST), Daejeon, 305-350, Republic of Korea § Korea Basic Science Institute (KBSI), Daejeon, 305-806, Republic of Korea ∥ Department of Energy Science, Sungkyunkwan University (SKKU), Suwon, 440-746, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Supportless Pt catalysts have several advantages over conventional carbon-supported Pt catalysts in that they are not susceptible to carbon corrosion. However, the need for high Pt loadings in membrane electrode assemblies (MEAs) to achieve state-of-the-art fuel cell performance has limited their application in proton exchange membrane fuel cells. Herein, we report a new approach to the design of a supportless Pt catalyst in terms of catalyst layer architecture, which is crucial for fuel cell performance as it affects water management and oxygen transport in the catalyst layers. Large Pt hollow spheres (PtHSs) 100 nm in size were designed and prepared using a carbon template method. Despite their large size, the unique structure of the PtHSs, which are composed of a thin-layered shell of Pt nanoparticles (ca. 7 nm thick), exhibited a high surface area comparable to that of commercial Pt black (PtB). The PtHS structure also exhibited twice the durability of PtB after 2000 potential cycles (0−1.3 V, 50 mV/s). A MEA fabricated with PtHSs showed significant improvement in fuel cell performance compared to PtB-based MEAs at high current densities (>800 mA/cm2). This was mainly due to the 2.7 times lower mass transport resistance in the PtHSbased catalyst layers compared to that in PtB, owing to the formation of macropores between the PtHSs and high porosity (90%) in the PtHS catalyst layers. The present study demonstrates a successful example of catalyst design in terms of catalyst layer architecture, which may be applied to a real fuel cell system. KEYWORDS: supportless Pt, PEMFC, catalyst layer, oxygen transport, durability, MEA

■

INTRODUCTION Energy concerns owing to the limited supply of fossil fuels and global warming have brought alternative energy research, particularly that concerning fuel cells, into prominence in the last few decades. In fuel cell research, proton exchange membrane fuel cells (PEMFCs), which exhibit high efficiency and low emissions, have attracted much attention as power sources for electric vehicles.1 Pt is considered the most reliable material for PEMFCs, as it is durable in acidic media under high potentials as well as active in the oxygen reduction reaction (ORR). Thus, Pt nanoparticles supported on carbon black particles (Pt/C) are the most widely applied electrocatalyst in © XXXX American Chemical Society

PEMFCs. The use of carbon supports for Pt catalysts helps maintain the Pt nanoparticle size below 4 nm, thus increasing the surface area of the Pt and allowing reduced Pt loadings in fuel cell electrodes.2 However, durability issues associated with the electrochemical corrosion of carbon supports, particularly at the high potentials formed during startup/shutdown or hydrogen starvation in fuel cell systems, have limited the wide application of Pt/C to commercial fuel cell systems.3−7 In Received: July 28, 2016 Accepted: September 26, 2016

A

DOI: 10.1021/acsami.6b08177 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

catalysts, including PtNTs, Pd−Pt nanodendrites, and dendritic PtNTs, have rarely been applied in single fuel cells. It is wellknown that the formation of a triple phase boundary (TPB) for facile transport of electrons, protons, and oxygen in cathode catalyst layers is critical for high fuel cell performance. For effective TPB formation in catalyst layers, different catalyst layer architectures, such as vertically aligned carbon nanotubes (VACNTs),9 vertically aligned nanostructured thin films (NSTFs),30,31 and 3D-ordered macroporous (3DOM)32 materials, have been successfully applied. The design of these catalyst layer architectures is mainly focused on building ordered structures containing macropores that allow easy access of oxygen to the Pt nanoparticles, thus promoting enhanced fuel cell performance, particularly under high-current conditions. In this respect, conventional supportless Pt catalysts such as PtB are unsuitable for making ideal catalyst layer structures because the irregular and small PtB aggregates usually form dense and less porous catalyst layers, leading to poor single-cell performance. This is also the reason that PtBbased membrane electrode assemblies (MEAs) require high Pt loading to achieve acceptable performance. These previous findings indicate that, in the development of supportless Pt catalysts for application in PEMFCs, the catalyst should be designed from the perspective of catalyst layer architecture, facilitating the efficient transfer of the catalyst activity to a real fuel cell. Here, we report the design and synthesis of a supportless Pt catalyst from the perspective of catalyst layer architecture. Three-dimensional supportless Pt hollow spheres (PtHSs) 100 nm in size, prepared by a simple carbon template method, were designed for the formation of ordered and macroporous catalyst layers. The PtHSs are composed of an interconnected Pt shell comprising Pt nanoparticles and a hollow center. The design rationale for the PtHSs is as follows: (i) Large and regular PtHSs with an average diameter of 100 nm can form a larger mean pore size and a larger pore volume when fabricated into catalyst layers than is possible with conventional PtB (Figure 1), allowing improved oxygen transport; (ii) the porous and hollow structure of the PtHSs could allow them to function as liquid water reservoirs, making more catalyst sites accessible to gaseous oxygen and facilitating oxygen transport in the catalyst layers, particularly under high currents where water flooding easily occurs; (iii) although the PtHSs are large, they exhibit sufficient surface area owing to the thinness of their Pt nanaoparticle shells, leading to adequate Pt utilization; (iv) the surface area can be controlled by modifying the Pt nanoparticle size and the morphology of the PtHS shells; and (v) the PtHS shell structure generated by the assembled Pt nanoparticles will increase the catalyst durability by preventing the aggregation and sintering of Pt nanoparticles, as already observed in PtNTs,20 thus promoting ultrastable behavior in the PtHS catalysts during the fuel cell reaction. The improved durability of the PtHSs, and the enhanced oxygen transport of the PtHSbased catalyst layer architecture, is fully demonstrated in a halfcell and a single PEMFC, respectively. Recently, supportless Pt−Fe catalyst with hollow capsule structure which is similar to our catalyst design has been prepared and tested in a single cell after being fabricated into MEA. By optimizing catalyst ink preparation through ionomer morphology control, they showed high MEA performance at low Pt loading, giving us an insight that the PtHS structure provides efficient catalyst layer architecture, as we expected.33 We believe that the present approach with supportless PtHS

order to address carbon-corrosion-induced fuel cell degradation, three categories of materials have been investigated as alternative electrocatalyst supports/active metals in PEMFCs: (i) highly durable carbon materials, such as carbon nanotubes (CNT),8,9 carbon nanofibers (CNF),10 and graphenes;11,12 (ii) noncarbon materials, such as Ti4O7,13,14 Nb-doped SnOx,15 and WO3;16,17 and (iii) supportless Pt catalysts.18 Of these, the use of supportless Pt nanoparticles may be the best strategy, as it eliminates the problem of support corrosion. However, the absence of a support for Pt catalysts leads to difficulties in controlling the Pt particle size, resulting in larger Pt particles than those found in typical Pt/C [ca. 7 nm in unsupported Pt black (PtB) vs ca. 4 nm in Pt/C]. The smaller specific surface area resulting from the larger particle size of supportless Pt catalysts leads to lower catalytic mass activity, necessitating higher Pt loadings to achieve comparable fuel cell performance to that of Pt/C catalysts. This limits the general application of supportless Pt catalysts, as was observed in the early stages of PEMFC development.2,19 In order to overcome the limitations of supportless Pt catalysts, extensive effort has recently been devoted to increasing their intrinsic catalytic activity by introducing alloy materials and modifying the morphology of the catalysts using advanced nanotechnology. These new supportless Pt catalysts exhibit improved ORR activity and improved durability compared to conventional PtB catalysts.20−27 For example, supportless Pt−Pd alloy nanotubes (PtPdNTs) with anisotropic morphology (50 nm in diameter, 5−20 μm in length, and 4−7 nm wall thickness) have been reported to exhibit 2.1 times higher mass activity and 2.7 times higher specific activity than PtB.20 The Pt nanotubes (PtNTs) also exhibited high durability after 1000 potential cycles [0−1.3 V vs a reversible hydrogen electrode (RHE)], showing only 20% electrochemically active surface area (ECSA) loss, compared to 51% ECSA loss for PtB.20 Another supportless Pt alloy nanostructure comprising Pd−Pt bimetallic nanodendrites (23.5 nm in size consisting of 3-nm-thick Pt branches on a Pd core) has been reported to exhibit 5 times higher Pt mass activity than PtB owing to its higher surface area and the increased number of ORR-active facets on the Pt branches.21 However, these Pd−Pt nanodendrites showed only a slight improvement in durability.21 Dendritic Pt nanotubes (46 nm in branch diameter and 2.5 nm in wall thickness of the Pt nanotube branches) composed of numerous assembled Pt nanoparticles are also effective supportless catalysts, showing 5.25 times higher mass activity and 3.68 times higher specific activity than PtB owing to the unique surface morphology of the dendritic Pt nanotubes.24 In terms of durability, the dendritic Pt nanotubes exhibited 4.8 times more stability than PtB after 4000 potential cycles (0.6−1.2 V vs RHE).24 Supportless Pt−Co and Pt−Fe catalysts with hollow sphere structure have also demonstrated enhanced electrocatalytic activity of methanol oxidation and ORR, respectively.28,29 The supportless Pt−Fe catalyst revealed 9 times higher specific activity than commercial Pt/C, showing very stable behavior during the start−stop and potential cycle durability tests.28 These studies indicate that morphology control of supportless Pt nanostructures is a promising approach to the improvement of their catalytic ORR activity and durability. Although morphology control is an effective method for improving the catalytic activity of supportless Pt catalysts, realization of their intrinsic activity in a fuel cell is another technical barrier to overcome. Highly active supportless Pt B

DOI: 10.1021/acsami.6b08177 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

The Pt loading was 50 wt %, as confirmed by thermogravimetric analysis (TGA). Heat treatment of the Pt/CSs was performed at 350 °C under air for 4 h to remove the CS template. The optimal temperature for the complete oxidation of the CSs was determined on the basis of the TGA data [Figure S1, Supporting Information (SI)]. Temperatures above 350 °C led to larger and distorted PtHSs, attributed to the aggregation and sintering of Pt nanoparticles. Characterization of PtHSs. The morphology of the PtHSs was studied by high-resolution transmission electron microscopy (HRTEM) (FEI Tecnai F20 and Zeiss Libra 200MC), at an acceleration voltage of 200 kV, and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) (Hitachi HD-2, 700 C, 300 kV). The images of the cathode catalyst layers in MEAs were obtained by scanning electron microscopy (SEM) (S-4800, Hitachi) at an acceleration voltage of 5.0 kV. The surface area of the CSs was determined by N2 adsorption−desorption isotherms at 77 K using the Brunauer−Emmet−Teller (BET) method (ASAP 2020 Micromeritics). Half-Cell Test. The kinetics of the ORR over Pt catalysts was examined by a rotating disk electrode (RDE) method in a conventional three-electrode system. A hydrogen electrode (Hydroflex, Gaskatel GmbH) and a Pt wire were used as the reference and counter electrodes, respectively. For convenience, all potentials quoted in this study are referenced to an RHE. A glassy carbon disk electrode (projected area 0.196 cm2, Pine Instrument Co.) coated with a catalyst slurry was used as the working electrode. The catalyst slurry was prepared by dispersing the Pt catalysts in an aqueous solution containing DI water, isopropyl alcohol, and 5 wt % Nafion solution. The catalyst suspension was cast onto a precleaned glassy carbon RDE and allowed to dry in air. After solvent evaporation, a thin catalyst layer remained on the glassy carbon surface with a Pt loading of 17 μgPt. Electrochemical measurements were performed in N2-saturated 0.1 M HClO4 solutions. Prior to measurement, the electrode was cycled between 0.02 and 1.20 V at 100 mV/s for 50 cycles to produce a clean electrode surface. The cyclic voltammograms (CVs) were recorded using a Solartron 1285 potentiostat/galvanostat (Solartron Analytical) in a potential range between 0.05 and 1.20 V with a 20 mV/s sweep rate. The ECSAs of the catalysts were estimated by the integration of HUPD (under potentially deposited hydrogen) charge between 0.05 and 0.40 V, with a calculated value of 0.21 mC/cm2 for polycrystalline Pt electrodes. Linear sweep voltammetry (LSV) measurements were conducted to obtain oxygen reduction polarization curves in an O2saturated 0.1 M HClO4 solution, with an RDE rotating rate of 1600 rpm and a sweep rate of 10 mV/s. Accelerated stress tests (ASTs) were performed by potential cycling between 0 and 1.3 V at 50 mV/s up to 2000 cycles in N2-saturated 0.1 M HClO4 at room temperature. The full-scale voltammograms between 0.05 and 1.20 V, as well as the polarization curves, were recorded periodically to track the ECSA degradation of the Pt catalysts. Fuel Cell Test. Preparation of MEAs. In order to measure the fuel cell performance of the PtHSs and PtB, the catalysts were incorporated into MEAs by a modified decal process. Catalyst inks were prepared by mixing the respective catalyst powder (PtB, HiSPEC 1000, Johnson Matthey Fuel Cells; PtHSs, prepared in this study) and a Nafion dispersion (5 wt %, Dupont Fuel Cells) in a mixture of water and isopropyl alcohol. The catalyst ink (95 wt % catalyst and 5 wt % Nafion binder on the basis of solid content) was coated onto a decal substrate by spraying, and the catalyst-coated decal was fully dried at room temperature. The MEA was prepared by transferring the catalyst layer from the decal substrate to a Nafion 212 membrane by a hotpressing process at 20 MPa and 120 °C. The Pt content of the MEA was controlled at 0.55 ± 0.02 and 0.22 ± 0.01 mg/cm2 for the cathode and anode, respectively, calculated by the weight change of the decal before and after catalyst transfer. Fuel Cell Test. I−V characteristics of the prepared MEAs were measured in a single fuel cell consisting of a MEA with an active area of 10 cm2, flow field plates made of graphite, gas diffusion layers (GDL, Sigracet 10BC), gaskets, and current collectors derived from gold-coated copper blocks. The cell temperature was set to 80 °C, and

Figure 1. Schematics of catalyst layer architectures fabricated with (a) PtB and (b) PtHSs.

catalysts is promising due to their high durability as well as potential performance, and our study is expected to provide useful insight for developing advanced supportless Pt catalysts and their MEAs.

■

EXPERIMENTAL SECTION

Preparation of PtHSs. PtHSs were prepared by a hard-templateassisted method, which provides flexibility in controlling the size and shape of hollow spheres.32 The following three steps were involved (Figure 2a): The preparation of the carbon spheres (CSs), the loading of Pt nanoparticles onto the CSs, and removal of the CS templates to form the PtHSs. CSs were chosen as the hard template for Pt loading due to their widespread availability and the ease of the process. CSs were prepared from glucose under hydrothermal conditions at temperatures above 140 °C, which led to aromatization and carbonization.34 A 0.5 M glucose solution [D-(+)-glucose, ≥99.5%, Sigma-Aldrich] was prepared using DI water, and the clear solution was placed in a 2 L stainless steel autoclave. The autoclave was maintained at 180 °C for 3 h with continuous stirring, generating 1013 kPa of pressure. Upon completion, the reactor was left to cool to room temperature (25 °C). The products were isolated by centrifugation and cleaned three times by redispersion in water and ethanol, centrifugation, and washing. The final products were dried in an oven at 70 °C for 4 h. Pt nanoparticles were dispersed onto the CSs via a conventional wet impregnation method.34 PtCl4 solution (5 g, 10 wt %, RTX) was dissolved in 250 mL of ethylene glycol (extra pure, OCI Co.) under vigorous stirring for 30 min, followed by the addition of 0.5 g of CSs. The resulting solution was stirred at 160 °C under reflux for 3 h. After cooling to room temperature, 0.1 M H2SO4 was added to adjust the pH of the solution to below 3, and the solution was left to stir continuously for 12 h to ensure a complete loading of Pt onto the CSs. The Pt/CS powder in the solution was then filtered and thoroughly washed with water. The resulting Pt/CSs were dried at 70 °C for 6 h. C

DOI: 10.1021/acsami.6b08177 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 2. Characterization of CSs, Pt/CSs, and PtHSs. (a) Schematic of the preparation of PtHSs using a CS template. TEM images of (b, c) CSs, (d, e) Pt/CSs, and (f, g) PtHSs at different magnifications. (h) HAADF STEM image of PtHSs and (i) the HR-TEM image and (j) the corresponding fast Fourier transform (FFT) pattern of a (110)-oriented PtHS nanocrystal. fully humidified H2 and air (or O2) were fed into the cell under ambient pressure. Polarization curves were measured by increasing the current density from 0 to 1200 mA/cm2. The H2 flow rate was set at 196 sccm in all tests. However, different air flow rates of 366, 610, and 1219 sccm, which corresponded to stoichiometric ratios of 1.5, 2.5, and 5.0, respectively, based on 1.4 A/cm2, were applied during each test to observe the effect of the cathode flow rate. After measuring the I−V curve, electrochemical impedance spectroscopy (EIS) analyses were performed using an EIS potentiostat (BioLogic Science Instruments, HCP-803) to obtain high frequency resistance (HFR) values. The EIS spectra were measured using sweeping frequencies over the range from 30 Hz to 200 kHz at six points/decade. After EIS measurement, CV was performed to measure the ECSAs of the MEAs using an EIS potentiostat. Before undergoing CV measurement, the cathode was purged with N2 [1500 sccm, 100% relative humidity (RH)], while the anode was fed with H2 (350 sccm, 100% RH) until the open circuit voltage (OCV) dropped below 0.1 V. The cathode voltammograms were measured under the N2 gas flow within a scan range of 0.01−1.20 V at a scan rate of 10 mV/s. The ECSA of Pt in the MEAs was determined from the H-adsorption charge between 0.085 and 0.40 V, and an assumption of 210 μC/cm2 Pt was used to calculate the Pt surface area.

a typical amorphous carbon structure with a surface area of 32 m2/g, as determined by BET measurement. After Pt loading, Pt nanoparticles of ca. 7 nm diameter are uniformly distributed on the surface of the CSs (Figure 2d,e). A 50 wt % Pt loading on the CSs is adequate for monolayer dispersion of the Pt particles. The removal of CSs at 350 °C under oxidizing conditions results in the hollow Pt shell, which is composed of aggregated Pt nanoparticles that maintain their initial spherical framework, as shown in the TEM image (Figure 2f). The average diameter of the PtHSs is ca. 100 nm, showing a 23% decrease in size compared to that of the Pt/CSs (ca. 130 nm). This indicates that the Pt−Pt particle distance is reduced during the removal of the CSs, leading to a decreased PtHS size. The HR-TEM image (Figure 2g) clearly shows that the Pt−Pt particle distance is shortened and that the PtHS shell is composed of aggregates of Pt nanocrystals with an average size of ca. 7 nm, which is consistent with the XRD data (Figure S2, SI). The TEM image in Figure 2g shows that the thickness of the PtHS shell is ca. 10 nm and that the shell has a porous structure that is permeable to reactants and products during electrochemical reactions. The HAADF-STEM image (Figure 2h) also confirms that the PtHSs have a hollow structure, as the intensive, bright contrast shown at the edges of the structure demonstrates the interaction of the incident electron beam predominantly with the condensed Pt atoms in the shell. From the enlarged HR-TEM image of the Pt nanocrystals (Figure 2i),

■

RESULTS AND DISCUSSION Morphology changes in the CSs, Pt/CSs, and PtHSs during the preparation of the PtHSs were observed by TEM, as shown in Figure 2b−g. The CSs exhibit perfect spherical morphology with a size distribution of 130 ± 20 nm (Figure 2b). The HRTEM image of an individual CS (Figure 2c) indicates that it has D

DOI: 10.1021/acsami.6b08177 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

adsorbed hydroxyl species (OHad) layer beyond 0.6 V. The ECSA also shows very similar values of 10.8 and 9.2 m2/gPt for PtHSs and PtBs, respectively. The lower ECSA values of PtHSs and PtBs compared to the geometrical surface area of those catalysts (∼30 m2/g), which was calculated on the basis of a Pt particle size of 7 nm, are mainly due to the inhibition effect of Nafion ionomer added to catalyst ink on ECSA, as already reported by Papandrew et al.27 The ORR polarization curves (Figure 3b) show very similar onset potentials of ca. 1.0 V and a slightly higher half-wave potential (E1/2) with the PtHSs (0.894 V) than with PtB (0.886 V). The mass and specific activities, which were calculated at 0.9 V on the basis of the ORR polarization curves, Pt loadings, and ECSA values, are 0.028 A/ mgPt and 2.38 mA/cm2Pt, respectively, for the PtHSs, and 0.025 A/mgPt and 2.19 mA/cm2Pt, respectively, for PtB. The electrochemical data obtained with the PtHSs and PtB show that both catalysts have very similar electrochemical activities. This similar electrochemical behavior is quite reasonable considering that PtHSs have a typical bulk Pt structure. The similarity in ECSA values is likely due to the comparable Pt nanoparticle sizes of the catalysts, and their analogous specific activities are also possibly a result of the similar surface structure of the two Pt catalysts, as indicated by the TEM results. Further modification of the PtHSs to improve their ECSA and ORR activity is possible by introducing smaller Pt nanoparticles (Figure S3, SI) and/or Pt alloys35 to the existing PtHSs. However, the present study focuses on the catalyst layer architecture for a PtHS MEA. Thus, a separate study on the modification of PtHS catalysts to enhance their ORR activity is currently underway. The improved durability of the PtHSs is one of the main motivations for our study. In order to evaluate the durability of PtHSs, ASTs were performed by applying linear potential sweeps between 0 and 1.3 V at 50 mV/s in N2-saturated 0.1 M HClO4 solution at room temperature. Wide-range potential sweeping was performed to simulate all possible degradation mechanisms, including Pt dissolution (Oswald ripening), Pt aggregation (coarsening), and Pt detachment from the carbon supports.7 The ASTs were also performed on commercial PtB catalysts for benchmarking purposes. After 2000 cycles, the CV results clearly show that the PtHSs are twice as durable as the commercial PtB, with 37% and 18% ECSA losses (and consequent 50 mV and 12 mV half-wave potential losses) being observed for PtB and the PtHSs, respectively (Figure 4). Considering that the PtHSs and PtB have similar particle sizes and facets, the catalysts should in principle have comparable durability against Pt dissolution. Thus, the superior durability of the PtHSs compared to PtB could be due to reduced Pt coarsening in the PtHSs during the AST, resulting from the unique structure of the chainlike aggregates of Pt nanoparticles in the PtHSs. The PtB would be more vulnerable to the coarsening of Pt nanoparticles due to its random distribution of smaller aggregates, as observed in the SEM images of the PtB catalyst layers (Figure 5). PtNTs have also demonstrated high durability during potential cycles owing to the unique morphology of their Pt aggregates, showing 2.5 times lower ECSA loss than for PtB under similar experimental conditions.20 It is also noteworthy that the PtHSs do not exhibit any structural changes during the ASTs. The single-cell study was performed with MEAs fabricated from PtHSs or PtB using a decal process. A cross-sectional view of the prepared MEAs was obtained by SEM and reveals the catalyst layer architecture of the PtHSs and PtB, as shown in

we can see a typical (111) planar spacing of 0.227 nm predicted for the face-centered cubic (fcc) structure of the bulk Pt crystal. The corresponding fast Fourier transform (FFT) pattern (Figure 2j) of the image that is equivalent to electron diffraction decisively confirms that the Pt nanoparticles consisting of PtHSs exhibit a typical fcc structure. The PtHS XRD pattern (Figure S2, SI) supports the HR-TEM results. On the basis of the characterization results for the PtHSs prepared in this study, it can be concluded that the PtHSs are porous and have 100-nm-sized three-dimensional hollow sphere morphology consisting of Pt nanoparticle aggregates. The electrocatalytic activities of the PtHSs were measured by CV (Figure 3a) and ORR polarization curves (Figure 3b), and

Figure 3. Electrochemical evaluation of the PtHSs and PtB. (a) CV curves of the PtHSs and PtB in N2-purged 0.1 M HClO4 solution at room temperature with a scan rate of 5 mV/s. (b) ORR polarization curves for the PtHSs and PtB in O2-saturated 0.1 M HClO4 at room temperature with a rotation rate of 1600 rpm and a sweep rate of 5 mV/s. Inset: Comparison of specific and mass activities for the PtHSs and PtB at 0.9 V.

the results were compared with those of a commercial PtB. The electrochemical measurements were performed in a typical three-electrode system containing a 0.1 M HClO4 electrolyte solution at room temperature. The CVs for both the PtHSs and PtB exhibited very similar curve shapes, including a well-defined HUPD adsorption/desorption process (2H2O ↔ OHad + H3O+ + e−) in the range of 0 < E < 0.4 V and the formation of an E

DOI: 10.1021/acsami.6b08177 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 4. Electrochemical durability evaluation of the PtHSs and PtB. (a) CV curves before and after AST cycles. (b) Oxygen reduction activity before and after AST cycles. (c) ECSA change with respect to the number AST cycles. AST was performed by potential cycling between 0 and 1.3 V at 50 mV/s in N2-saturated 0.1 M HClO4 at room temperature.

the PtHSs have enough mechanical strength to withstand the MEA preparation process. The fuel cell performance of the prepared MEAs, using a Pt/ C catalyst layer for the anode and PtHS or PtB catalyst layers for the cathode, was evaluated in a single cell with H2/air at atmospheric pressure, as shown in Figure 6. For both MEAs, the HFR values (Figure 6a) determined from EIS are slightly higher (81 mΩ cm2) for the PtHSs than for PtB (66 mΩ cm2), probably due to the difference in catalyst layer thickness (2.9 ± 0.3 vs 0.7 ± 0.1 μm). The IR-corrected I−V curves (Figure 6a) which were determined by HFR show slightly lower performance in the PtHS MEA at low-current regions, while the performance is reversed in the high-current regions (above 800 mA/cm2), where the superior performance of PtHS to PtB becomes evident, with the performance gap becoming more significant with increasing current density. The slightly lower performance of the PtHSs than that of PtB in low-current regions is associated with the lower ECSA value for PtHSs than for PtB, as determined from the CVs in Figure 6b (PtHSs, 2.2 m2/gPt; PtB, 5.4 m2/gPt). The ECSA values obtained in MEAs are smaller than those obtained in RDE (PtHSs, 10.8 m2/gPt; PtB, 9.2 m2/gPt), which is possibly due to the nonuniform and

Figure 5. The PtB nanoparticles form irregular agglomerates with an average size of 26.2 ± 3.9 nm in the catalyst layers, and the catalyst layer thickness is ca. 0.7 ± 0.1 μm with a 0.57 mg/ cm2 Pt content. The PtHS catalyst layers, however, contain regular PtHS particles with an average diameter of 112.8 ± 13.9 nm, leading to a ca. 2.9 ± 0.3 μm catalyst layer thickness with a 0.55 mg/cm2 Pt content, which is 4 times thicker than the PtB catalyst layer. The thicker catalyst layer observed with the PtHSs is mainly attributed to the large three-dimensional spherical shape of the nanoparticles, resulting in the porosity of the PtHS catalyst layer being twice that of the PtB catalyst layer (90.0% vs 45.5%). The porosity was calculated using the catalyst layer thickness and Pt content values, as well as the inner volume of PtHSs. The higher porosity of the PtHS layer mainly derives from their inner volume. The more porous structure of the PtHS layers with bigger macropores can also be observed in the SEM images shown in Figure 5. It is also noteworthy that the PtHS particles keep their initial shape without experiencing any damage to the hollow structure during MEA fabrication, which involved a hot-pressing procedure under 20 MPa pressure at 120 °C, indicating that F

DOI: 10.1021/acsami.6b08177 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 5. SEM images of a cross-section of the MEAs at different magnifications: (a−c) PtB catalyst layer and (d−f) PtHS catalyst layer.

cell performance data obtained at different air flow rates, the potential differences resulting from the air flow rate change, such as HF−LF (the potential difference between HF and LF), HF−MF (the potential difference between HF and MF), and MF−LF (the potential difference between MF and LF), were calculated at 200, 400, 600, and 800 mA/cm2, respectively (Figure 7b). The results show that the potential differences, which are dependent on the air feed rate, are more sensitive in PtB than in the PtHSs and that the sensitivity is more significant at high currents. The HF−LF potential differences at 600 and 800 mA/cm2 are 105 and 191 mV, respectively, in PtB, and 28 and 55 mV, respectively, in the PtHSs. The degree of fuel cell performance change with respect to air feed rate has been used as a general indicator of the mass transport resistance in a cell. Air-flow-rate-sensitive behavior in a cell indicates higher oxygen transport resistance mainly originating from water management. Therefore, the less-sensitive behavior of the PtHSs indicates better water management and thus a lower mass transport resistance in the PtHS catalyst layer than in the PtB layer. The quantitative analysis of mass transport overpotentials for both MEAs was conducted using the following equation:

partial coating of Nafion ionomer film on the surface of the supportless Pt catalysts in MEA.33 In addition, the difference in ECSA values between the two MEAs in a single cell is inconsistent with the ECSA values obtained in a half-cell (PtHSs, 10.8 m2/gPt; PtB, 9.2 m2/gPt). This is probably due to the limited availability of Nafion ionomers to the inner surface of the PtHSs, thus inhibiting the formation of a TPB at the inner surface of the PtHSs. Nafion ionomers have been reported to form large solvated particles more than 200 nm in diameter in water−IPA solvents,36 and thus, the access of large Nafion particles to the hollow core of the PtHSs is severely limited. Therefore, further optimization of the catalyst slurries, such as the introduction of size-controlled Nafion ionomers, would be necessary to improve the ECSA values of the PtHSs. Conversely, the superior performance of the PtHSs in highcurrent regions suggests that mass-transport loss is lower in PtHS catalyst layers than in PtB layers, and this difference in oxygen transport resistance becomes more significant with increasing current. In order to examine the mass-transport behavior of the catalyst layers in the MEAs in more detail, the effect of gas flow rate on the fuel cell performance was observed, as shown in Figures 7a and S4 (SI). As the air flow increases from 366 sccm (low flow rate, λ = 1.5, LF) to 610 sccm (medium flow rate, λ = 2.5, MF) and further to 1219 sccm (high flow rate, λ = 5.0, HF) at a fixed H2 feed rate (196 sccm), the fuel cell performance increases steadily, particularly in high-current regions. This is typical behavior for a PEM fuel cell because higher gas flow rates in the cathode enhance the discharge of liquid water in the catalyst layers due to the increased pressure difference and facilitate the access of oxygen into the Pt nanoparticles, leading to improved fuel cell performance.37,38 On the basis of the fuel

ηtx,cath = Erev(PO2, P H2, T ) − ηORR − i(HFR + R H+,cath) − Vcell

(1)

where ηtx,cath is the mass transport overpotential in the cathode, Erev(PO2,PH2,T) is the thermodynamic equilibrium potential as a function of temperature and the partial pressure of the gas, ηORR is the cathode kinetic overpotential, i is the current density, HFR is the cell resistance (including membrane resistance and G

DOI: 10.1021/acsami.6b08177 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

from the I−V results is described in our previous study.3 Figure 7c shows that the mass transport overpotentials at 900 mA/cm2 are 353 and 133 mV in PtB and PtHS, respectively, indicating 2.7 times lower oxygen transport resistance in PtHS catalyst layers. Figure 8 shows the MEA performance with respect to the current density normalized to the Pt loading (a, mass activity)

Figure 8. Comparison of (a) mass activity and (b) specific activity between PtB and PtHS MEAs at atmospheric pressure. Cell temperature, 80 °C; gas flow rate, H2/air = 196/366 sccm, 100% RH; Pt content, 0.57 and 0.55 mgPt/cm2 for PtB and PtHS, respectively. Figure 6. Single-cell performance behavior of PtB and PtHS MEAs with H2/air at atmospheric pressure. (a) IR-corrected I−V curves (top) and HFR values (bottom) for PtB and PtHS MEAs (cell temperature, 80 °C; gas flow rate, H2/air = 196/366 sccm with 100% RH; Pt content, 0.57 and 0.55 mgPt/cm2 for PtB and the PtHSs, respectively). (b) CVs of PtB and PtHS MEAs (cell temperature, 80 °C; gas flow rate, H2/N2 = 400/400 sccm with 100% RH; ionomer content, 5 wt %; Pt content, 0.55 and 0.57 mgPt/cm2 for PtB and the PtHSs, respectively).

and electrochemically active Pt surface area (b, specific activity). In both cases, the MEA performance of the PtHSs is higher than that of PtB, particularly at high currents. The performance difference in specific activity is more dominant, indicating that the TPBs formed in the PtHSs are more active than those in PtB. This is also associated with the better oxygen transport ability in the PtHSs, leading to the formation of more active TPBs in the PtHS catalyst layers. From the results obtained in a single cell, including the degree of fuel cell performance change with respect to air flow rate and the quantified mass transport resistance, it is clear that the superior fuel cell performance of the PtHS MEA relative to the PtB MEA, particularly at high currents, is mainly due to

the contact and bulk electronic resistances), and RH+,cath is the proton resistances in the cathode. The results of the calculations are shown in Figure 7c. The detailed method to calculate the mass transport overpotential

Figure 7. Effect of air flow rate on fuel cell performance for PtB and PtHS MEAs at atmospheric pressure. (a) Comparison of cell performance as a function of stoichiometry. (b) Dependency of cell performance on air flow rate at different current densities. (c) Comparison of mass transport overpotentials at different current densities. Cell temperature, 80 °C; gas pressure, 101.3 kPa (abs); gas flow rate, H2 = 196 sccm, air = 1219 sccm (high flow rate, HF), 610 sccm (medium flow rate, MF), 366 sccm (low flow rate, LF), 100% RH; Pt content, 0.57 mgPt/cm2 (PtB) and 0.55 mgPt/ cm2 (PtHS); ionomer content, 5 wt %; catalyst layer thickness, 0.7 ± 0.1 μm (PtB) and 2.9 ± 0.3 μm (PtHS). H

DOI: 10.1021/acsami.6b08177 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

ACS Applied Materials & Interfaces

■

more facile oxygen transport in the PtHS cathode catalyst layers. This is consistent with our design strategy for the PtHS catalyst and the catalyst layers, which contain larger macropores and higher porosity constructed from the three-dimensional and large spherical hollow structure of the PtHSs. Considering that the PtHS catalyst layer is 5 times thicker than the PtB layer, the greater oxygen transport ability in the PtHSs is closely associated with improved water management in the catalyst layers. The thick PtHS catalyst layer has 90% porosity, which is beneficial for liquid water storing and thus facilitates oxygen transport into the catalyst layers. The ultrathin PtB catalyst layers, however, show higher mass-transport resistance, probably due to water flooding, which results from the low porosity and small pore volume of the ultrathin catalyst layers. Easier MEA fabrication with the PtHSs due to the thick layer characteristics, even at low Pt contents of 0.2−0.4 mg/cm2, is another merit of the PtHSs. Conventional PtB catalysts usually form a catalyst layer thinner than 1 μm at low Pt contents, and this can lead to difficulty in choosing a suitable thin-layer coating process, as well as in controlling the uniform catalyst layers in a MEA. The present study indicates that the catalyst should be designed in terms of the catalyst layer architecture as well as the intrinsic activity of the catalyst in order to realize its catalytic activity in a real fuel cell system. Our study also indicates that oxygen transport in the cathode catalyst layer is a critical issue for high performance in current fuel cell electrodes.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions from all the authors. All authors have given approval to the final version of the manuscript. D.C.D. and S.C. contributed equally to this work. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Research Technology Evaluation and Planning (KETEP) funded by the Ministry of Trade, Industry & Energy (MTIE), Republic of Korea (20113020030020), and the International Collaborative Energy Technology R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (20138520030780). This research was also supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (2015M1A2A2056550). H.G. also gratefully acknowledges grant T34619 from Korea Basic Science Institute.

■

■

CONCLUSIONS We have designed a PtHS material in order to form a suitable architecture in cathode catalyst layers for high fuel cell performance. Large hollow Pt spheres ca. 100 nm in size were synthesized using a carbon template method. The unique structure of PtHSs composed of Pt nanoparticles ca. 7 nm in diameter exhibited a high surface area comparable to that of commercial PtB and 2 times greater durability than PtB under wide-range potential cyclings (0−1.3 V, 50 mV/s). When the PtHSs were used to fabricate a MEA using a decal process, the catalyst layers were 2 times more porous and 4 times thicker than the commercial PtB-based catalyst layers, due to the unique morphology of the PtHSs. The PtHS-based catalyst layers were found to be beneficial for water management, resulting in lower mass transport resistance by a factor of 2.7 compared to that of conventional PtB, which led to a significantly better fuel cell performance in high-current regions. This study demonstrates that the catalyst should be designed in terms of catalyst layer architecture to realize its catalytic activity in real fuel cell systems. PtHS catalysts can be applied to fuel cell systems where high reliability is needed, such as space and submarine applications, and to other electrochemical systems that are operated at high potentials where a highly corrosive environment is formed, such as water electrolyzers and regenerative fuel cell systems.

■

Research Article

REFERENCES

(1) Chu, S.; Majumdar, A. Opportunities and Challenges for a Sustainable Energy Future. Nature 2012, 488 (7411), 294−303. (2) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity Benchmarks and Requirements for Pt, Pt-alloy, and Non-Pt oxygen Reduction Catalysts for PEMFCs. Appl. Catal., B 2005, 56 (1−2), 9− 35. (3) Park, J. H.; Yim, S. D.; Kim, T.; Park, S. H.; Yoon, Y. G.; Park, G. G.; Yang, T. H.; Park, E. D. Understanding the Mechanism of Membrane Electrode Assembly Degradation by Carbon Corrosion by Analyzing the Microstructural Changes in the Cathode Catalyst Layers and Polarization Losses in Proton Exchange Membrane Fuel cell. Electrochim. Acta 2012, 83, 294−304. (4) Liu, Z. Y.; Zhang, J. L.; Yu, P. T.; Zhang, J. X.; Makharia, R.; More, K. L.; Stach, E. A. Transmission Electron Microscopy Observation of Corrosion Behaviors of Platinized Carbon Blacks under Thermal and Electrochemical Conditions. J. Electrochem. Soc. 2010, 157 (6), B906−B913. (5) Meier, J. C.; Galeano, C.; Katsounaros, I.; Witte, J.; Bongard, H. J.; Topalov, A. A.; Baldizzone, C.; Mezzavilla, S.; Schüth, F.; Mayrhofer, K. J. J. Design Criteria for Stable Pt/C Fuel Cell Catalysts. Beilstein J. Nanotechnol. 2014, 5, 44−67. (6) Ishigami, Y.; Takada, K.; Yano, H.; Inukai, J.; Uchida, M.; Nagumo, Y.; Hyakutake, T.; Nishide, H.; Watanabe, M. Corrosion of Carbon Supports at Cathode during Hydrogen/Air Replacement at Anode Studied by Visualization of Oxygen Partial Pressures in a PEFCStart-up/Shut-down Simulation. J. Power Sources 2011, 196 (6), 3003−3008. (7) Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D.; Wilson, M.; Garzon, F.; Wood, D.; Zelenay, P.; More, K.; Stroh, K.; Zawodzinski, T.; Boncella, J.; McGrath, J. E.; Inaba, M.; Miyatake, K.; Hori, M.; Ota, K.; Ogumi, Z.; Miyata, S.; Nishikata, A.; Siroma, Z.; Uchimoto, Y.; Yasuda, K.; Kimijima, K.-i.; Iwashita, N. Scientific Aspects of Polymer Electrolyte Fuel Cell Durability and Degradation. Chem. Rev. 2007, 107 (10), 3904−3951. (8) Tian, Z. Q.; Lim, S. H.; Poh, C. K.; Tang, Z.; Xia, Z.; Luo, Z.; Shen, P. K.; Chua, D.; Feng, Y. P.; Shen, Z.; Lin, J. A Highly Order-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08177. TGA and XRD data, SEM results for PtHS with smaller Pt particles, CV and I−V data in a single cell (Figures S1−S4) (PDF) I

DOI: 10.1021/acsami.6b08177 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Structured Membrane Electrode Assembly with Vertically Aligned Carbon Nanotubes for Ultra-Low Pt Loading PEM Fuel Cells. Adv. Energy Mater. 2011, 1 (6), 1205−1214. (9) Murata, S.; Imanishi, M.; Hasegawa, S.; Namba, R. Vertically Aligned Carbon Nanotube Electrodes for High Current Density Operating Proton Exchange Membrane Fuel Cells. J. Power Sources 2014, 253, 104−113. (10) Hwang, S. M.; Park, J. H.; Lim, S.; Jung, D. H.; Guim, H.; Yoon, Y. G.; Yim, S. D.; Kim, T. Y. Designing an Ultrathin Silica Layer for Highly Durable Carbon Nanofibers as the Carbon Support in Polymer Electrolyte Fuel Cells. Nanoscale 2014, 6 (20), 12111−12119. (11) Ghosh, A.; Basu, S.; Verma, A. Graphene and Functionalized Graphene Supported Platinum Catalyst for PEMFC. Fuel Cells 2013, 13 (3), 355−363. (12) Antolini, E. Graphene as a New Carbon Support for LowTemperature Fuel Cell Catalysts. Appl. Catal., B 2012, 123−124, 52− 68. (13) Krishnan, P.; Advani, S. G.; Prasad, A. K. Magneli Phase TinO2n−1 as Corrosion-Resistant PEM Fuel Cell Catalyst Support. J. Solid State Electrochem. 2012, 16 (7), 2515−2521. (14) Yao, C.; Li, F.; Li, X.; Xia, D. Fiber-Like Nanostructured Ti4O7 used as Durable Fuel Cell Catalyst Support in Oxygen Reduction Catalysis. J. Mater. Chem. 2012, 22 (32), 16560−16565. (15) Kakinuma, K.; Chino, Y.; Senoo, Y.; Uchida, M.; Kamino, T.; Uchida, H.; Deki, S.; Watanabe, M. Characterization of Pt Catalysts on Nb-Doped and Sb-Doped SnO2−δ Support Materials with Aggregated Structure by Rotating Disk Electrode and Fuel Cell Measurements. Electrochim. Acta 2013, 110, 316−324. (16) Maiyalagan, T.; Viswanathan, B. Catalytic Activity of Platinum/ Tungsten Oxide Nanorod Electrodes towards Electro-Oxidation of Methanol. J. Power Sources 2008, 175 (2), 789−793. (17) Shao, Y.; Liu, J.; Wang, Y.; Lin, Y. Novel Catalyst Support Materials for PEM Fuel Cells: Current Status and Future Prospects. J. Mater. Chem. 2009, 19 (1), 46−59. (18) Kloke, A.; von Stetten, F.; Zengerle, R.; Kerzenmacher, S. Strategies for the Fabrication of Porous Platinum Electrodes. Adv. Mater. 2011, 23 (43), 4976−5008. (19) Perry, M. L.; Fuller, T. F. A Historical Perspective of Fuel Cell Technology in the 20th Century. J. Electrochem. Soc. 2002, 149 (7), S59−S67. (20) Chen, Z.; Waje, M.; Li, W.; Yan, Y. Supportless Pt and PtPd Nanotubes as Electrocatalysts for Oxygen-Reduction Reactions. Angew. Chem., Int. Ed. 2007, 46 (22), 4060−4063. (21) Lim, B.; Jiang, M.; Camargo, P. H.; Cho, E. C.; Tao, J.; Lu, X.; Zhu, Y.; Xia, Y. Pd-Pt Bimetallic Nanodendrites with High Activity for Oxygen Reduction. Science 2009, 324 (5932), 1302−1305. (22) Larsen, B. A.; Neyerlin, K. C.; Bult, J. B.; Bochert, C.; Blackburn, J. L.; Kocha, S. S.; Pivovar, B. S. Platinum Nanoplates as Fuel Cell Electrocatalysts. J. Electrochem. Soc. 2012, 159 (10), F622−F627. (23) Su, L.; Shrestha, S.; Zhang, Z.; Mustain, W.; Lei, Y. PlatinumCopper Nanotube Electrocatalyst with Enhanced Activity and Durability for Oxygen Reduction Reactions. J. Mater. Chem. A 2013, 1 (39), 12293−12301. (24) Zhang, G.; Sun, S.; Cai, M.; Zhang, Y.; Li, R.; Sun, X. Porous Dendritic Platinum Nanotubes with Extremely High Activity and Stability for Oxygen Reduction Reaction. Sci. Rep. 2013, 3, 1526. (25) Khudhayer, W. J.; Kariuki, N. N.; Wang, X.; Myers, D. J.; Shaikh, A. U.; Karabacak, T. Oxygen Reduction Reaction Electrocatalytic Activity of Glancing Angle Deposited Platinum Nanorod Arrays. J. Electrochem. Soc. 2011, 158 (8), B1029−B1041. (26) Kibsgaard, J.; Gorlin, Y.; Chen, Z.; Jaramillo, T. F. MesoStructured Platinum Thin Films: Active and Stable Electrocatalysts for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2012, 134 (18), 7758−7765. (27) Papandrew, A. B.; Atkinson, R. W.; Goenaga, G. A.; Kocha, S. S.; Zack, J. W.; Pivovar, B. S.; Zawodzinski, T. A. Oxygen Reduction Activity of Vapor-Grown Platinum Nanotubes. J. Electrochem. Soc. 2013, 160 (8), F848−F852.

(28) Tamaki, T.; Kuroki, H.; Ogura, S.; Fuchigami, T.; Kitamoto, Y.; Yamaguchi, T. Connected Nanoparticle Catalysts Possessing a Porous, Hollow Capsule Structure as Carbon-Free Electrocatalysts for Oxygen Reduction in Polymer Electrolyte Fuel Cells. Energy Environ. Sci. 2015, 8 (12), 3545−3549. (29) Chen, G.; Xia, D.; Nie, Z.; Wang, Z.; Wang, L.; Zhang, L.; Zhang, L. Facile Synthesis of Co−Pt Hollow Sphere Electrocatalyst. Chem. Mater. 2007, 19 (7), 1840−1844. (30) Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486 (7401), 43−51. (31) Debe, M. K. Nanostructured Thin Film Electrocatalysts for PEM Fuel Cells - A Tutorial on the Fundamental Characteristics and Practical Properties of NSTF Catalysts. ECS Trans. 2012, 45 (2), 47− 68. (32) Lai, X.; Halpert, J. E.; Wang, D. Recent Advances in Micro-/ Nano-Structured Hollow Spheres for Energy Applications: From Simple to Complex Systems. Energy Environ. Sci. 2012, 5 (2), 5604− 5618. (33) Kuroki, H.; Tamaki, T.; Yamaguchi, T. Nanostructural Control and Performance Analysis of Carbon-Free Catalyst Layers Using Nanoparticle-Connected Hollow Capsules for PEFCs. J. Electrochem. Soc. 2016, 163 (8), F927−F932. (34) Sun, X.; Li, Y. Colloidal Carbon Spheres and their Core/Shell Structures with Noble-Metal Nanoparticles. Angew. Chem., Int. Ed. 2004, 43 (5), 597−601. (35) Bing, Y.; Liu, H.; Zhang, L.; Ghosh, D.; Zhang, J. Nanostructured Pt-Alloy Electrocatalysts for PEM Fuel Cell Oxygen Reduction Reaction. Chem. Soc. Rev. 2010, 39 (6), 2184−2202. (36) Welch, C.; Labouriau, A.; Hjelm, R.; Orler, B.; Johnston, C.; Kim, Y. S. Nafion in Dilute Solvent Systems: Dispersion or Solution? ACS Macro Lett. 2012, 1 (12), 1403−1407. (37) Anderson, R.; Zhang, L.; Ding, Y.; Blanco, M.; Bi, X.; Wilkinson, D. P. A Critical Review of Two-Phase Flow in Gas Flow Channels of Proton Exchange Membrane Fuel Cells. J. Power Sources 2010, 195 (15), 4531−4553. (38) Yim, S. D.; Sohn, Y. J.; Park, S. H.; Yoon, Y. G.; Park, G. G.; Yang, T. H.; Kim, C. S. Fabrication of Microstructure Controlled Cathode Catalyst Layers and their Effect on Water Management in Polymer Electrolyte Fuel Cells. Electrochim. Acta 2011, 56 (25), 9064− 9073.

J

DOI: 10.1021/acsami.6b08177 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX