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Nanoscale PAPER Cite this: Nanoscale, 2017, 9, 6910 Received 1st March 2017, Accepted 23rd April 2017 DOI: 10.1039/c7nr01491k rsc.li/nanoscale Highly stable nanostructured membrane electrode assembly based on Pt/Nb 2 O 5 nanobelts with reduced platinum loading for proton exchange membrane fuel cells Yachao Zeng, a,b Xiaoqian Guo, a,b Zhiqiang Wang, a,b Jiangtao Geng, a Hongjie Zhang, a,b Wei Song, a Hongmei Yu, * a Zhigang Shao * a and Baolian Yi a Proton exchange membrane fuel cells are promising candidates for the next-generation power sources; however, poor durability and high cost impede their widespread application. To address this dilemma, a nanostructured membrane electrode assembly (MEA) based on Pt/Nb 2 O 5 nanobelts (NBs) was con- structed through hydrothermal synthesis and the physical vapour deposition method. Pt/Nb 2 O 5 NBs were directly aligned with Naon membrane without ionomer as a binder. The prepared catalyst layer is ultra- thin and has ultralow Pt loading. A single cell performance of 5.80 kW g Pt 1 (cathode) and 12.03 kW g Pt 1 (anode) was achieved by the Pt/Nb 2 O 5 NBs-based MEA (66.0 μg Pt cm 2 ). The accelerated durability test indicates that the Pt/Nb 2 O 5 NBs-based MEA is far more stable than conventional Pt/C-based MEA. Introduction Due to their high energy conversion eciency and low environmental impact, proton exchange membrane fuel cells (PEMFCs) have long been considered as one of the most prom- ising candidates for batteries in portable devices and vehicles. 1,2 However, the high consumption and poor dura- bility of Pt-based electrocatalysts in the MEA have hindered the commercialization of PEMFCs. The most commonly employed electrocatalyst in MEA at present consists of 24 nm Pt nano- particles dispersed on a high surface area carbon support such as XC-72 or BP-2000. 3 Under the harsh working conditions within fuel cells (such as strong acidic environments, oxidizing conditions, durative flow of liquids and gases, and high elec- tric current), Pt nanoparticles tend to experience Ostwald ripening/aggregation and detachment from the support. 4 In a classical MEA preparation procedure, a mixture of electrocata- lyst, ionomer, and solvent is first sonicated to form a homo- geneous catalyst ink and then the catalyst ink is coated onto substrates through decal, 5 blade coating, 6 spraying, 7 or brush- ing 8 processes. The prepared catalyst layer is often a random assembly of an electrocatalyst, ionomer, and pores. This con- figuration is unfavorable for the catalyst utilization and mass transport, as the ionomer and also the tortuous porosity impede the transport of reactants to the electrochemically active sites. 9,10 Another issue associated with the conventional MEAs is the poor durability. Intensive research in the past few decades has revealed that carbon corrosion, which occurs in the presence of Pt catalyst and an elevated electric potential, is one of the main failure modes for the catalyst layer. 11 The cor- rosion of the carbon support will eventually lead to a structural collapse of the catalyst layer, 11 further worsening the mass transport in the catalyst layer. Running parallel to the catalyst and support degradation, ionomer degradation/loss can be another critical factor leading to poor durability during the operation of PEMFCs. In a PEMFC, H 2 O 2 , OH radicals, or other contaminants produced from the fuel cell reactions can damage the recast Nafion ionomer employed in the catalyst layer. 1214 To realize the commercialization of PEMFCs, it is important to develop a novel MEA with ultralow Pt consumption, ultra- high durability, and even more so, an ionomer-free catalyst layer. It has been realized that the ecient utilization of Pt in PEMFCs strongly depends on the catalyst layer structure. 15,16 Middelman et al. 17 proposed a nanostrucuted ordered MEA for PEMFC: the MEA features its ultrathin thickness and vertically aligned porosity, and active sites are located at the boundaries of electronic and ionic conductors. Such a configuration favours the mass transport of reactants, electrons and protons. In the development of nanostructured, ordered MEA, the nanostructured thin film (NSTF) electrode, which has been pursued by Debe, has attracted significant interest. 1820 The NSTF electrode is comprised of organic nanowhisker arrays a Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China. E-mail: [email protected], [email protected] b University of Chinese Academy of Sciences, Beijing 100049, China 6910 | Nanoscale, 2017, 9, 69106919 This journal is © The Royal Society of Chemistry 2017 Published on 26 April 2017. Downloaded by Dalian Institute of Chemical Physics, CAS on 26/05/2017 06:14:27. View Article Online View Journal | View Issue

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Page 1: Highly stable nanostructured membrane electrode assembly ...pemfc.dicp.ac.cn/201712.pdfdirectly aligned with Nafion membrane without ionomer as a binder. The prepared catalyst layer

Nanoscale

PAPER

Cite this: Nanoscale, 2017, 9, 6910

Received 1st March 2017,Accepted 23rd April 2017

DOI: 10.1039/c7nr01491k

rsc.li/nanoscale

Highly stable nanostructured membrane electrodeassembly based on Pt/Nb2O5 nanobelts withreduced platinum loading for proton exchangemembrane fuel cells

Yachao Zeng,a,b Xiaoqian Guo,a,b Zhiqiang Wang,a,b Jiangtao Geng,a

Hongjie Zhang,a,b Wei Song,a Hongmei Yu, *a Zhigang Shao *a and Baolian Yia

Proton exchange membrane fuel cells are promising candidates for the next-generation power sources;

however, poor durability and high cost impede their widespread application. To address this dilemma, a

nanostructured membrane electrode assembly (MEA) based on Pt/Nb2O5 nanobelts (NBs) was con-

structed through hydrothermal synthesis and the physical vapour deposition method. Pt/Nb2O5 NBs were

directly aligned with Nafion membrane without ionomer as a binder. The prepared catalyst layer is ultra-

thin and has ultralow Pt loading. A single cell performance of 5.80 kW gPt−1 (cathode) and 12.03 kW gPt

−1

(anode) was achieved by the Pt/Nb2O5 NBs-based MEA (66.0 μgPt cm−2). The accelerated durability test

indicates that the Pt/Nb2O5 NBs-based MEA is far more stable than conventional Pt/C-based MEA.

Introduction

Due to their high energy conversion efficiency and lowenvironmental impact, proton exchange membrane fuel cells(PEMFCs) have long been considered as one of the most prom-ising candidates for batteries in portable devices andvehicles.1,2 However, the high consumption and poor dura-bility of Pt-based electrocatalysts in the MEA have hindered thecommercialization of PEMFCs. The most commonly employedelectrocatalyst in MEA at present consists of 2–4 nm Pt nano-particles dispersed on a high surface area carbon support suchas XC-72 or BP-2000.3 Under the harsh working conditionswithin fuel cells (such as strong acidic environments, oxidizingconditions, durative flow of liquids and gases, and high elec-tric current), Pt nanoparticles tend to experience Ostwaldripening/aggregation and detachment from the support.4 In aclassical MEA preparation procedure, a mixture of electrocata-lyst, ionomer, and solvent is first sonicated to form a homo-geneous catalyst ink and then the catalyst ink is coated ontosubstrates through decal,5 blade coating,6 spraying,7 or brush-ing8 processes. The prepared catalyst layer is often a randomassembly of an electrocatalyst, ionomer, and pores. This con-figuration is unfavorable for the catalyst utilization and masstransport, as the ionomer and also the tortuous porosity

impede the transport of reactants to the electrochemicallyactive sites.9,10 Another issue associated with the conventionalMEAs is the poor durability. Intensive research in the past fewdecades has revealed that carbon corrosion, which occurs inthe presence of Pt catalyst and an elevated electric potential, isone of the main failure modes for the catalyst layer.11 The cor-rosion of the carbon support will eventually lead to a structuralcollapse of the catalyst layer,11 further worsening the masstransport in the catalyst layer. Running parallel to the catalystand support degradation, ionomer degradation/loss can beanother critical factor leading to poor durability during theoperation of PEMFCs. In a PEMFC, H2O2, OH radicals, orother contaminants produced from the fuel cell reactions candamage the recast Nafion ionomer employed in the catalystlayer.12–14

To realize the commercialization of PEMFCs, it is importantto develop a novel MEA with ultralow Pt consumption, ultra-high durability, and even more so, an ionomer-free catalystlayer. It has been realized that the efficient utilization of Pt inPEMFCs strongly depends on the catalyst layer structure.15,16

Middelman et al.17 proposed a nanostrucuted ordered MEA forPEMFC: the MEA features its ultrathin thickness and verticallyaligned porosity, and active sites are located at the boundariesof electronic and ionic conductors. Such a configurationfavours the mass transport of reactants, electrons and protons.In the development of nanostructured, ordered MEA, thenanostructured thin film (NSTF) electrode, which has beenpursued by Debe, has attracted significant interest.18–20 TheNSTF electrode is comprised of organic nanowhisker arrays

aFuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics,

Chinese Academy of Sciences, Dalian, Liaoning 116023, China.

E-mail: [email protected], [email protected] of Chinese Academy of Sciences, Beijing 100049, China

6910 | Nanoscale, 2017, 9, 6910–6919 This journal is © The Royal Society of Chemistry 2017

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covered with continuous PtM (M = Fe, Ni, Co, Mn, etc.) thinfilm. The continous Pt thin film can effectively promote theMEA durability by reducing the surface-energy, which is thedriving force for the Pt nanoparticles agglomerating into largepartcles. Benefitting from the ultrathin thickness, verticallyaligned porosity and non-addition of Nafion ionomer in thecatalyst layer, NSTF have achieved remarkable single cell per-formance and durability with an ultralow Pt loading. Inaddition to the NSTF electrode, highly ordered one dimen-sional nanostructured materials, including Pt nanowirearrays,21–23 carbon nanotube arrays (CNTs),16,24 TiO2 nanorod/nanotube arrays,25,26 and polypyrrole nanowires (PPys),27 havebeen prepared for PEMFCs.

Among these novel support materials, metal oxides standout due to their high chemical stability, thermal stability, andstrong metal-support interaction (SMSI).28 Methods includingthe sol–gel method,29 glancing angle deposition,30 hydro-thermal synthesis26 and anodic oxidization31 have beenapplied in the synthesis of morphology-controlled metaloxides for PEMFCs; however, the poor electronic conductivityof metal oxides have decreased their application in PEMFCs.Although treatments including hydrogenation,31 carboncoating,26 nitrogenization,32 and ion doping33 have beenemployed to address the issue, improvement has beenlimited.

Herein, a facile method is proposed to prepare a nano-structured MEA based on metal oxide nanobelts. The conceptof nanostructured MEA is realized by a template-assisted depo-sition strategy. Typically, Nb2O5 nanobelts (NBs) are employedas catalyst support, and a continuous Pt thin film is coatedonto one side of the Nb2O5 NBs. Co–OH–CO3 nanowire arrays(NWAs), serving as templates, were firstly prepared by a facilehydrothermal synthesis, then Nb2O5 nanobelts (NBs) were pre-pared by the physical vapour deposition (PVD) method. Toendow the Nb2O5 NBs with electroactivity towards the oxygenreduction reaction (ORR) and also to improve the electronicconductivity, continuous Pt thin film was coated onto Nb2O5

NBs by the PVD method. The prepared Pt/Nb2O5 NBs werethen assembled into a catalyst coated membrane (CCM) elec-trode by a facile decal method. The prepared CCM features (1)an ultrathin catalyst layer, (2) vertically aligned porosity, (3) acontinuous Pt thin film and (4) non-addition of Nafionionomer in the catalyst layer. The absence of carbon supporteliminates the concern of carbon corrosion, meanwhile thelow tortuosity and the removal of ionomer in the catalyst layerfacilitate the transport of fuels. To confirm the application ofthe proposed approach, the nanostructured Pt/Nb2O5 NBs-based CCM was evaluated as anode and cathode under a fullsingle cell test. Furthermore, the accelerated durability test(ADT) was performed to investigate the durability of the pre-pared electrode, the result indicated that the Pt/Nb2O5 NBs-based CCM was far more stable than the conventional Pt/C-based CCM.

More importantly, the preparation strategy can be possiblyextended to metal oxides like TiO2, Ta2O5, Cr2O3, SnO2, ITO,etc., and the electrocatalysts for ORR, methanol oxidation,

hydrogen and oxygen evolution reactions are also promising inthis method. This work has provided a general strategy for theapplication of nanostructured metal oxides in electrochemicalenergy conversion and storage devices.

ExperimentalChemicals

All chemicals were of analytical grade and were used asreceived without further purification. Co(NO3)2·6H2O, NH4F,CO(NH2)2, H2SO4, H2O2 and 2-propanol were purchased fromKermel® (Tianjin, China). Nafion® 212 membrane andNafion® ionomer solution (5 wt%) were purchased fromDuPont. Pt/C (70 wt%) was purchased from Johnson Matthey.Nb and Pt targets (Φ 100 mm, with purity >99.95%) were pur-chased from ZhongNuo Advanced Material Technology Co. Ltd(Beijing, China).

Synthesis of Co–OH–CO3 NWAs

Co–OH–CO3 NWAs were synthesized on a stainless steel plateby a facile hydrothermal synthesis method.34 The solutionwas prepared by dissolving 1.5 mmol of Co(NO3)2·6H2O,3 mmol of NH4F, and 7.5 mmol CO(NH2)2 in 70 mL of dis-tilled water. Then, the solution was transferred into a Teflon-lined stainless steel autoclave. The stainless steel plate (2 ×6 cm2 in size) was immersed into the reaction solution. Theliner was sealed in a stainless steel autoclave and maintainedat 120 °C for 5 h and cooled down to room temperature. TheCo–OH–CO3 NWAs supported on stainless steel plate wasthen collected and washed in de-ionized water to remove thecontaminants.

Preparation of Pt/Nb2O5 NBs-based CCM

Nb2O5 film was firstly deposited onto Co–OH–CO3 NWAs byradio frequency magnetron sputtering. After the magnetronsputtering, the Nb target was replaced by Pt target. Pt film wasthen coated onto the prepared nanowire arrays. During themagnetron sputtering, the cathodic power was fixed at 100 W,the argon gas pressure was 0.8 Pa. As the magnetron sputter-ing was terminated, the prepared nanowire arrays were decaledonto Nafion® 212 membrane under the pressure of 3 MPa at140 °C for 3 min, then the stainless steel plate was physicallyremoved. The raw CCM was then purified through a standardpurification process as follows: (1) acid washed in 0.5 M H2SO4

at 80 °C for 1 h; (2) boiled in 3% H2O2 at 80 °C for 1 h; (3)boiled in 0.5 M H2SO4 at 80 °C for 1 h; (4) boiled in distilledwater at 80 °C for 1 h.35

Preparation of conventional CCM

For comparison, conventional Pt/C-based CCM was preparedby air brushing the catalyst ink onto Nafion® 212 membraneon the hot stage at 60 °C. The homogeneous catalyst ink wasprepared by sonicating a mixture of Pt/C, Nafion® ionomer,and 2-propanol with a mass ratio of 5 : 1 : 200. The Pt loadingfor the cathode was set to 0.1 mg cm−2.

Nanoscale Paper

This journal is © The Royal Society of Chemistry 2017 Nanoscale, 2017, 9, 6910–6919 | 6911

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MEA fabrication and single cell tests

MEAs with an active area of 2.56 cm2 were prepared by sand-wiching CCMs between one piece of gas diffusion layer (GDL)and one piece of gas diffusion electrode (GDE) under thepressure of 0.25 MPa at 140 °C for 3 min. The GDL was home-made from carbon paper (Toray®, TGP-H-60) with a microporous layer, the GDE was home-made by brushing Pt/C(70 wt%, Johnson Matthey) onto the GDL, the Pt loading wasset to 0.25 mg cm−2. The prepared MEA was then sandwichedbetween two silver-coated end plates with parallel flow fields,and silicon gaskets were positioned between the MEA andeach of the silver-coated end plates.

The single cell performance and durability was evaluatedon a home-made fuel cell test stand at a cell temperature of80 °C. The anode and cathode were fed with humidified H2

and O2 with a flow rate of 50 mL min−1 and 100 mL min−1.The humidity was 80%. The backpressure for both anode andcathode was 200 kPagauge. A KFM 2030 impedance meter(Kikusui®, Japan) was used to record the I–V curves.Electrochemical impedance spectroscopy (EIS) was performedto investigate the electrode kinetics of the prepared MEAs. TheEIS was carried out on a Solartron 1287 electrochemical inter-face in conjunction with a Solartron 1260 frequency responseanalyser. The measurements were made at a current density of100 mA cm−2 by applying an ac amplitude of 10 mV over theac frequency range from 1 Hz to 10 kHz.

Durability test

Accelerating durability testing (ADT) was performed on a CHI600C by sweeping the potential between 0.6 and 1.0 V vs.RHE at a scan rate of 50 mV s−1 to investigate the durabilityof the prepared MEAs. During the ADT test, the anode sidewas purged with H2, while the cathode side was purged withN2. H2/N2 gases were externally humidified at the dew pointtemperature of 80 °C. The gas flow rate of H2/N2 was50/100 mL min−1. Cyclic voltammetry (CV) was carried out toinvestigate the electrochemical surface areas (ECSAs). CVswere performed between 0.05 V and 1.2 V at a scan rate of500 mV s−1.

Physical characterizations

The morphologies of the prepared samples were investigatedby field emission scanning electron microscopy (FESEM,JSM-7800F, JEOL). The structure and composition of the speci-mens were investigated by a high resolution transmission elec-tron microscope (HRTEM, JEM-2100F, JEOL) equipped with anOxford INCA X-sight energy dispersive X-ray spectrometer.

The crystal structures of Pt/Nb2O5 NBs and Nb2O5 NBs werecharacterized by X-ray diffraction using an X-ray diffractometer(PANalytical EMPYREAN) operating at 40 kV and 200 mA withCu Kα (λ = 1.5405 Å) as a radiation source.

X-ray photoelectron spectroscopy (XPS) analyses were per-formed to reveal the surface properties of the prepared CCMs.The XPS data were obtained with an ESCALab250 Xi electronspectrometer using 300 W Al Kα radiation. The base pressure

was about 1 × 10−7 Pa. The binding energies were referencedto the C 1s line at 284.6 eV from adventitious carbon.

The Pt and Nb loadings of Pt/Nb2O5 NBs-based CCM weredetermined by inductively coupled plasma optical emissionspectroscopy (ICP-OES 7300DV, PerkinElmer).

Results and discussionSynthesis and characterization

The preparation process of the MEA based on Pt/Nb2O5 NBs isillustrated in Scheme 1. Co–OH–CO3 NWAs, serving as tem-plates, were firstly prepared by hydrothermal synthesis. TheNb thin film was then coated onto the Co–OH–CO3 NWAsthrough the PVD method. The Nb thin film was naturally oxi-dized into Nb2O5 when the Nb-coated Co–OH–CO3 NWAs wereexposed to the air, then Pt thin film was coated onto theNb2O5-coated Co–OH–CO3 NWAs. A facile decal method wasadopted to assemble the Pt/Nb2O5-coated Co–OH–CO3 NWAsinto CCM. To remove the templates and possible contami-nants induced during the fabrication process, a standard puri-fication of CCM was performed.35 The structural evolution ofPt/Nb2O5-coated Co–OH–CO3 NWAs into Pt/Nb2O5 NBs will beelucidated later.

Being a powerful technique in the investigation of surfacecomposition and chemical valence, XPS was performed toreveal the surface properties of Pt/Nb2O5 NBs. The XPS spectraare presented in Fig. 1. From Fig. 1a, the binding energy of Nb3d is located at 207.58 eV and 210.35 eV, which is in accord-ance with the oxidation state of NbV in Nb2O5.

36,37 XPS con-firmed the full oxidization of Nb thin film into Nb2O5 after thePt/Nb2O5 NBs-based CCM experienced a series of treatments.For Pt 4f, the most intense peak is assigned to Pt0, the secondmost intense peak is assigned to PtII, which is probably in theform of PtO and Pt(OH)2, and the third peak is assigned toPtIV, which is in the form of PtO2. The Pt0 4f binding energiesfor Pt/Nb2O5 NBs (4f7/2 = 71.47 eV, 4f5/2 = 74.89 eV) were posi-tively shifted in comparison to those of the reference Pt/C(4f7/2 = 71.33 eV, 4f5/2 = 74.70 eV). The positive shift of Pt 4fbinding energies in Pt/Nb2O5 NBs may be ascribed to the SMSIbetween Pt thin film and Nb2O5 NBs.

28,37 The positive shift ofthe Pt 4f energy indicates a down-shift of the d-band centrerelative to the Fermi level.38 This down shift leads to areduction in oxygenated adsorbates (i.e., O and OH) on the Pt

Scheme 1 Schematic of the fabrication process of the nanostructuredMEA based on Pt/Nb2O5 NBs.

Paper Nanoscale

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surface because the antibonding orbitals of the Pt–O bond aremore populated.38

The crystallinity of Pt/Nb2O5 NBs and Nb2O5 NBs was inves-tigated by XRD and selected area electron diffraction (SAED).As shown in Fig. 2(a), for Pt/Nb2O5 NBs, distinct peaks areassigned to Pt(111), (200), (220) and (311) faces; no distinctpeaks are observed for Nb2O5 NBs, indicating that the pre-

pared Nb2O5 NBs are amorphous and no metallic constituentsare present in the metal oxide. To further confirm the consti-tution of Pt/Nb2O5 NBs, HR-TEM and SAED were performed.From Fig. 2(c), the SAED pattern corresponds well to the poly-crystalline Pt, but no SAED pattern was related to niobiumoxide. The XRD patterns are in accordance with the SAEDpattern. From the analysis results of XPS, XRD and SAED,Pt/Nb2O5 NBs was prepared by a facile preparation strategy.

Fig. 3 presents the morphologies of the samples involved inthe magnetron sputtering process.

Fig. 3(a)–(c) are the FESEM images of Co–OH–CO3 NWAs.The formation process of Co–OH–CO3 NWAs on stainless steelplate has been elucidated by Zhang et al.34,39 The area numberdensity of Co–OH–CO3 NWAs is estimated to be 3 to 4 billionper cm2 from Fig. 3(a). Fig. 3(b) is the high resolution SEMimage of Co–OH–CO3 nanowires: ravines can be clearly seenbetween smaller nanowires, indicating that a single Co–OH–

CO3 nanowire is a bundle of smaller Co–OH–CO3 nanowires.Fig. 3(c) is the cross-sectional SEM image of Co–OH–CO3

NWAs, which shows that Co–OH–CO3 NWAs grew relatively ver-tically on the stainless steel plate, the average diameter ofCo–OH–CO3 NWAs is ca. 100 nm, while the length is ca. 3.0 μm.Fig. 3(d)–(f ) are the morphologies of Nb2O5-coated Co–OH–

CO3 NWAs. As the sputtering terminated, Nb was naturally oxi-dized into Nb2O5 when Nb-coated Co–OH–CO3 NWAs wereexposed to air. From Fig. 3(d) and (e), the Nb2O5-coated Co–OH–CO3 NWAs inherited the structural merits from Co–OH–

CO3 NWAs, and ravines can be identified along the nanowires.Fig. 3(f ) is the cross sectional image of Nb2O5-coated Co–OH–

CO3 NWAs, the average length is ca. 3 μm. Bonakdarpouret al.30 prepared nanopillar niobium oxides as support struc-tures for oxygen reduction electrocatalysts by glancing angledeposition; a mimic of 3M’s NSTF offers the potential forsimilar surface area enhancement and mass activates. Herein,nanostructured Nb2O5 was employed as the catalyst support inthe pursuit of constructing a catalyst layer with reduced masstransport resistance and improved durability. To endow Nb2O5

with ORR activity and also electronic conductivity, continuousPt thin film was deposited onto Nb2O5-coated Co–OH–CO3

NWAs. The morphologies of Pt/Nb2O5-coated Co–OH–CO3

nanowire are presented in Fig. 3(g)–(i). As shown in Fig. 3(i),the thickness of the Pt thin film ranges from 17.8 nm to28.7 nm, while the thickness of Nb2O5 film ranges from16.5 nm to 38.8 nm. Although the Co–OH–CO3 NWAs havebeen coated with Nb2O5 and Pt in a sequence, the ravines canstill be vividly seen. Fig. 3(i) shows the cross sectional image ofPt/Nb2O5-coated Co–OH–CO3 NWAs; due to the PVD of Pt andNb, the morphology of Pt/Nb2O5-coated Co–OH–CO3 nano-wires evolved from needle-like to rod-like.

Fig. 4 shows the morphology and elemental distributionsof Pt/Nb2O5-coated Co–OH–CO3 nanowire investigated byHAADF-STEM and EDS mappings. From Fig. 4(d), Nb2O5

formed a compact shell, wrapping the Co–OH–CO3 nanowire.From Fig. 4(e), Pt formed a continuous thin film on Nb2O5 andthe continuous Pt thin layer serves as electrocatalyst and elec-tronic conductor.

Fig. 1 XPS spectra for (a) Nb 3d and (b) Pt 4f of the Pt/Nb2O5 NBs-based CCM.

Fig. 2 (a) XRD patterns of Nb2O5 NBs and Pt/Nb2O5 NBs. (b) Highresolution TEM image of Pt/Nb2O5 nanobelt. (c) Corresponding experi-mental (left) and simulated (right) SAED patterns.

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Fig. 5(a)–(c) are the in panel images of the Pt/Nb2O5 NBs-based CCM at different magnifications. From Fig. 5(a),Pt/Nb2O5 NBs formed a relatively ordered catalyst layer on Nafionmembrane on a large scale. More detailed catalyst layer struc-ture can be seen in Fig. 5(c), where the Pt/Nb2O5-coated Co–OH–CO3 NWAs were cracked during the purification process ofCCM, leaving Pt/Nb2O5 NBs on the membrane. The width ofPt/Nb2O5 NBs is ca. 20 nm, the thickness is ca. 60 nm, and the

length is ca. 2 μm. The elucidation of the formation of thePt/Nb2O5 NBs is as follows: during the decal, the tips of the as-received Pt/Nb2O5-coated Co–OH–CO3 NWAs punctured theNafion membrane. Fig. 3(b) shows that a single Co–OH–CO3

nanowire is a bundle of smaller nanowires; although a multi-layer of Pt and Nb was deposited onto the Co–OH–CO3 NWAs,the unique structure was retained as indicated by the highresolution SEM images (Fig. 3(i)). In the case of ionic contami-nation induced by the dissolution of Co–OH–CO3, the Co–OH–

CO3 NWAs template was removed by sulphuric acid. Duringthe acid wash, Pt/Nb2O5-coated Co–OH–CO3 NWAs crackedalong the ravines. The thickness of the catalyst layer was ca.500 nm, presented in Fig. 5(d). The Pt and Nb loading was66.0 and 228.3 μg cm−2, respectively, determined by ICP-OES.

Single cell performance

Fig. 6(a)–(d) are the I–V curves of Pt/Nb2O5 NBs-based and Pt/C-based CCMs employed as anode and cathode. A maximumpower density of 794.0 mW cm−2 (anode) and 383.0 mW cm−2

(cathode) was achieved by Pt/Nb2O5 NBs-based CCM with a Ptloading of 66.0 μg cm−2. Compared to the Pt/C-based CCM,the performance of Pt/Nb2O5 NBs-based CCM seems to be low.For a better comparison, the I–V curves were normlized to thePt loadings in the prepared CCMs. A maximum mass specificpower density of 5.80 kW gPt

−1 (cathode) and 12.03 kW gPt−1

Fig. 3 (a) The in panel image of Co–OH–CO3 NWAs grown on the stainless steel plate. (b) High resolution SEM image of Co–OH–CO3 nanowires.(c) The cross-sectional image of Co–OH–CO3 NWAs. (d, e) The in panel images of Nb2O5-coated Co–OH–CO3 NWAs arrays at differentmagnifications. (f ) The cross-sectional image of Nb2O5-coated Co–OH–CO3 NWAs. (g) The in panel images of Pt/Nb2O5-coated Co–OH–CO3

NWAs. (h) The cross-sectional image of a single Pt/Nb2O5-coated Co–OH–CO3 nanowire. (i) The cross-sectional image of Pt/Nb2O5-coatedCo–OH–CO3 NWAs.

Fig. 4 (a) HAADF-STEM image of a single Pt/Nb2O5-coated Co–OH–CO3 nanowire. (b) Overlap of elemental mappings and STEM image. EDSmappings of the marked area for (c) Co Kα1, (d) Nb Kα1, and (e) Pt Lα1.

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(anode) was achieved by the Pt/Nb2O5 NBs-based CCM. FromFig. 6(b) and (d), the Pt/Nb2O5 NBs-based CCM seems to bemore applealing in the application as anode in the PEMFCs,since it has a mass specific performance that is 1.3 fold higherthan the conventional CCM. However, the cathode perform-ance of Pt/Nb2O5 NBs-based CCM is not satisfactory.

To gain insight into the single cell performance, EIS andCV were performed during the course of the performanceevaluation. The key parameters are summarized in Table 1.Herein, R0, which is the high frequency resistance, corres-ponds to the ohmic resistance. Rct corresponds to the reactionresistance. For Pt/C-based CCM, R0 is 0.12 Ω cm2 (cathode)and 0.088 Ω cm2 (anode). For Pt/Nb2O5 NBs-based CCM,the value is 0.17 Ω cm2 (cathode) and 0.13 Ω cm2 (anode).By coating a continuous Pt thin film over Nb2O5 NBs, theelectronic conductivity of the Pt/Nb2O5 NBs-based CCMhas been greatly promoted. However, the ohmic resistancesof Pt/Nb2O5 NBs-based CCM are a bit higher than that ofPt/C-based CCM.

It is noteworthy that there is no proton conducting ionomerintroduced into the nanostructured catalyst layer; water thinfilm may be adopted as secondary proton ion-conducting path-ways in the architecture of Pt/Nb2O5 NBs-based CCM.32,40,41

Litster et al. concluded that a similar proton conduction mech-anism may be adopted in liquid saturated Nafion® and bulkwater.42 The proton conductivity on the liquid saturated Ptsurface was estimated to be ∼1 S m−1, which is 1–2 orders ofmagnitude lower than that of the Nafion®.43 The increasedohmic resistances of Pt/Nb2O5 NBs-based CCM may be

ascribed to reduced proton conductivity in water thin film. Tofurther understand the electrode kinetics of Pt/Nb2O5 NBs-based and Pt/C-based CCM, EIS were conducted at two currentdensities.23,44 At a low current density of 100 mA cm−2, the dia-meter of the semi-circle is mainly attributed to the chargetransfer resistance. Serving as cathode, sole semi-circles wereobserved for CCMs based on Pt/Nb2O5 NBs and Pt/C at acurrent density of 100 mA cm−2, indicating that the ORRtaking place on the cathode side is the rate determining step.The diameter of the semi-circle corresponds to Rct, which is abenchmark of electrochemical activity. The Rct of Pt/Nb2O5

NBs-based CCM employed as cathode is almost 2 fold higherthan that of Pt/C-based CCM. It has been suggested that theelectrochemical activity is a product of intrinsic activity andavailable active sites.45 As shown in Fig. 6(f ), the electro-chemical active surface area (ECSA) of Pt/C-based CCM is54.5 m2 gPt

−1, while the ECSA of Pt/Nb2O5 NBs-based CCM is23.9 m2 gPt

−1. The much reduced ECSA of Pt/Nb2O5 NBs-basedCCM resulted in a reduced ORR activity. For a better appli-cation of Pt/Nb2O5 NBs-based CCM in PEMFCs, the electroac-tivity towards ORR should be further improved, which is a taskfor the future. When serving as the anode, dual semi-circleswere observed for CCMs based on Pt/Nb2O5 NBs and Pt/C at acurrent density of 100 mA cm−2, indicating that the HOR andORR simultaneously affected the electrode kinetics. The Rct forHOR of Pt/Nb2O5 NBs-based CCM (0.083 Ω cm2) is very closeto that of Pt/C-based CCM (0.095 Ω cm2). EIS at a high currentdensity of 1000 mA cm−2 was also carried out to gain insightinto the mass transport resistance on single cell performance.

Fig. 5 (a–c) The in panel images of the Pt/Nb2O5 NBs-based CCM at different magnifications. (d) The cross sectional image of the Pt/Nb2O5 NBs-based CCM.

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Unlike the situation in H2-Air PEMFCs, a dual semi-circlerelated to the mass tranport was not detected in the presentwork.22 This is because O2 supply is sufficient in the H2–O2

PEMFCs. From the 7th column of Table 1, the Rct(@1000mA cm−2) of Pt/Nb2O5 NBs-based CCM is higher than that ofPt/C-based CCM. It is supposed that the Rct(@1000 mA cm−2)

of the Pt/Nb2O5 NBs-based CCM should be lower for the elec-trode, which is of ultrathin thickness and reduced tortuosity.The deviation may be ascribed to the fact that proton transportis also vital to ORR processes.40,46 As mentioned above, theproton conductivity of water thin film is in 1–2 orders of mag-nitude lower than that of the Nafion®.43 The much reduced

Fig. 6 (a) I–V curves of CCM based on Pt/Nb2O5 NBs and Pt/C (JM) as anode. (b) I–V curves with current density normalized to the Pt loadings forCCM based on Pt/Nb2O5 NBs and Pt/C (JM) as anode. (c) I–V curves of CCM based on Pt/Nb2O5 NBs and Pt/C (JM) as cathode. (d) I–V curves withcurrent density normalized to the cathodic Pt loadings for CCM based on Pt/Nb2O5 NBs and Pt/C (JM) as cathode. (e) Electrochemical impedancespectroscopy of CCM based on Pt/Nb2O5 NBs and Pt/C (JM) recorded at current density of 100 and 1000 mA cm−2. (f ) Representative CVs ofPt/Nb2O5 NBs and Pt/C (JM, in which the ECSAs have been inserted).

Table 1 A summary of the key parameters derived from the single cell evaluation

CCMsPt loading/μg cm−2

Power density

R0/Ω cm2Rct/Ω cm2

(@100 mA cm−2)Rct/Ω cm2

(@1000 mA cm−2)mW cm−2 kW gPt−1

Pt/C (JM) as cathode 100.0 841.0 8.41 0.12 0.49a 0.13Pt/C (JM) as anode 923.8 9.24 0.088 0.095b N/Ac

Pt/Nb2O5 NBs as cathode 66.0 383.0 5.80 0.17 1.06a 0.38Pt/Nb2O5 NBs as anode 794.0 12.03 0.13 0.083b N/Ac

a The value is Rct towards the ORR process. b The value is Rct towards the HOR process. c The value is negligible.

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proton conductivity in the Pt/Nb2O5 NBs-based CCM limitedits cathode performance.

Though the ECSA of Pt/Nb2O5 NBs-based CCM is muchlower than that of Pt/C-based CCM, a very close electroactivityto that towards HOR was obtained. This is because the electro-activity of Pt towards HOR is six orders of magnitude higherthan that towards ORR. The much faster kinetics of Pt towardsHOR lowered the demand in ECSA. The much improved per-formance of Pt/Nb2O5 NBs-based CCM serving as anode maybe due to the structural advantages. Resulting from the ultra-thin thickness, straight porosity and continuous Pt thin film,H2 molecules can easily reach the active sites, and protons canbe directly transported from the active sites into the bulkmembrane. The introduction of Nb2O5 in the anode may alsobe beneficial to the cell performance, as a trace of CO in H2

flow will be lethal to the Pt-based electrocatalysts. Rochaet al.47 performed a systematic investigation of Nb doping onthe CO tolerance of PEMFC catalysts, and found that a minuteaddition of Nb, most likely in the form of Nb2O5, in Pt basedelectrocatalyst will exert a positive effect on the CO tolerance.

The Nb promotional effect comes from very strong metalsupport interactions between Pt and Nb2O5 that weaken thePt–CO adsorption strength, reducing the CO coverage andfacilitating its oxidation at lower potentials.

Durability test

Fig. 7 shows the durability test results of the prepared CCMs.I–V curves and CVs of the CCMs were recorded every 1000cycles. ECSAs were quantified according to the hydrogenadsorption charges by using the conversion factor of 210mC cm−2

Pt.48 After 5000 CV cycles, the power density retention

of Pt/Nb2O5 NBs-based CCM was 45.8%, which is nearly 2 foldhigher than the conventional Pt/C-based CCM (23.2%).Besides, the ECSA of Pt/C-based CCM dropped 32%, to itsinitial value, while the ECSA of Pt/Nb2O5 NBs-based CCMincreased 160%, compared to its initial value. Similar enhanc-ment in the stability of metal oxide supported Pt electrocata-lysts have been previously reported in the literature.31,49 Thedecrease in ECSA of Pt/C-based CCM may be ascribed to thecarbon support corrosion, Pt nanoparticle dissolution/aggrega-

Fig. 7 (a, b) I–V curves of Pt/Nb2O5 NBs-based and Pt/C-based CCMs recorded during the accelerated degradation test. (c, d) CVs of Pt/Nb2O5

NBs-based and Pt/C-based CCMs every 1000 CV cycles during the accelerated degradation test. (e) ECSA retention for Pt/Nb2O5 NBs-based andPt/C-based CCM during the accelerated degradation test. (f ) EIS of Pt/Nb2O5 NBs-based and Pt/C-based CCM recorded at a current density of100 mA cm−2 before and after the accelerated degradation test.

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tion/Ostwald rippening.4 By employing Nb2O5 NBs as support,there is no concern about carbon corrosion. Compared to Ptnanoparticles, the surface-energy of the continuous Pt thinfilm is much reduced, which alleviates the dissolution of Pt; itwas believed that the SMSI could greatly enhance the dura-bility of Pt-based electrocatalysts.50–53 The SMSI between Ptand supports like Nb2O5, Ti3AlC2,

53 and reduced polyoxometa-lates (POMs),52 contribute to the durability of the Pt-basedelectrocatalysts.

EIS was performed to gain insight into the performancedegradation. From Fig. 7(f ) and Table 2, the R0 of Pt/C-basedCCM increased by 75%. The much increased ohmic resistanceof Pt/C-based CCM may be attributed to the degradation of theionic pathway in the catalyst layer.54 In addition to H2O2 andOH radicals, intensive potential sweeping can also damage theionic pathways, leading to a reduced proton conductivity inthe cathode. In the nanostructured Pt/Nb2O5 NBs-based CCM,water thin film is supposed to be employed as a secondaryproton conductor, thus constant proton conductivity is guaran-teed. After 5000 CV cycles, the R0 of Pt/Nb2O5 NBs-based CCMdecreased by 23.5%. It is possible that the express highway forelectron conductivity has been accomplished after the inten-sive ADT test. The development of ECSA together with reducedohimic resistance needs further research. For Rct at a currentdensity of 100 mA cm−2, the value of Pt/Nb2O5 NBs-basedCCM increased by 34.9%, while the value of the Pt/C increasedby 97.9%. Though an increase in ECSA was realized after ADTfor Pt/Nb2O5 NBs-based CCM, the much reduced intrinsic elec-troactivity supressed the Rct. Hence, Pt alloy based electrocata-lysts are highly recommended to promote the intrinsic electro-activity of the nanostructured electrocatalysts.

The high stability of the nanostructured MEA based onPt/Nb2O5 NBs may have benefitted from (1) the elimination ofcarbon support, (2) the continous Pt thin film, and (3) theSMSI between Nb2O5 and Pt. During the operation of fuel cellsystems, MEA underwent oxidizing conditions, durative flow offluids, large potential gradients, and so on. The harsh operat-ing conditions require the electrode materials to have ultra-high thermal, mechanical and electrochemical durability. Theremarkable durability of Pt/Nb2O5 NBs-based CCM laid thefoundation for the application in PEMFCs.

Conclusion

In this work, a nanostructured MEA based on Pt/Nb2O5 NBs hasbeen constructed for PEMFCs. The MEA was prepared by hydro-

thermal synthesis and the physical vapour deposition method.The prepared nanostructured catalyst layer is an assembly ofPt/Nb2O5 NBs directly aligned with Nafion membrane. Typically,the Nb2O5 NBs serve as catalyst support, while the continuousPt thin film serves as the ORR active site and electron conduct-ing pathway. The Pt/Nb2O5 NBs are directly aligned with theNafion membrane. In the current architecture, the continuousPt thin film serves as electron conducting highway, and thewater thin film is believed to serve as the proton conductingpathway, instead of the Nafion ionomer. The ultrathin thicknessand straight porosity are supposed to be beneficial to the masstransport. The prepared nanostructured MEA was evaluated asanode and cathode, and a maximum power density of 5.80kW gPt

−1 (cathode) and 12.03 kW gPt−1 (anode) were achieved with

a Pt loading of 66.0 μg cm−2. During the accelerated durabilitytest, the Pt/Nb2O5NBs-based MEA presented remarkably higherdurability than the Pt/C-based MEA. By elimination of thecarbon support, there is no concern for support corrosion. Thecontinuous Pt thin film alleviates the Pt dissolution. Meanwhilea SMSI between Pt and Nb2O5 NBs is supposed to have a posi-tive effect on the MEA durability. This work provides a facilemethod for the preparation of electrocatalysts supported onmetal oxides. By virtue of the structural advantages and SMSI, ananostructured catalyst layer with ultrahigh durability andenhanced catalyst utilization could be constructed for electro-chemical energy conversion and storage systems like PEMFCs,direct methanol fuel cells, super capacitors, and lithium-airbatteries and so on.

Acknowledgements

We gratefully acknowledge the financial support from NationalKey Research and Development Program of China (No.2016YFB0101208) and the National Natural ScienceFoundation of China (No. U1508202, No. 61433013, No.21306190 and No. 21473197).

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