Nanostructured Electrocatalysts for PEM Fuel Cells and Redox Flow

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Nanostructured Electrocatalysts for PEM Fuel Cells and Redox Flow Batteries: a Selected Review Yuyan Shao, Yingwen Cheng, Wentao Duan, Wei Wang, Yuehe Lin, Yong Wang, and Jun Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01737 • Publication Date (Web): 03 Nov 2015 Downloaded from http://pubs.acs.org on November 9, 2015

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Nanostructured Electrocatalysts for PEM Fuel Cells and Redox Flow Batteries: a Selected Review. Yuyan Shao, ǂ, * Yingwen Cheng,ǂ Wentao Duan,ǂ Wei Wang,ǂ Yuehe Lin,¶ Yong Wang, ǂ, ‡,* Jun Liuǂ,* ǂ

Pacific Northwest National Laboratory, Richland WA 99352

‡

Voiland School of Chemical Engineering and Bioengineering, Washington State University,

Pullman, WA 99163, USA ¶

School of Mechanical and Materials Engineering, Washington State University Pullman, WA

99164-2920 Email: [email protected]; [email protected]; [email protected]

Abstract: PEM fuel cells and redox flow batteries are two very similar technologies which share common component materials and device design. Electrocatalysts are the key component in these two devices. In this article, we review recent progress of electrocatalytic materials for these two technologies with a focus on our research activities at Pacific Northwest National Laboratory (PNNL) in the past years. This includes 1) nondestructive functionalization of graphitic carbon as Pt support to improve its electrocatalytic performance, 2) triple-junction of metal-carbon-metal oxides to promote Pt performance, 3) nitrogen-doped carbon and metal (metal oxides) doped carbon to improve redox reactions in flow batteries. A perspective on future research and the synergy between the two technologies are also discussed. Key words: fuel cells, flow battery, electrocatalyst, graphene, defects, energy storage.

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1. Introduction Electrocatalysis plays a key role for electric energy conversion and storage in a sustainable low-carbon society. For example, electrocatalysts are one key component in fuel cells,1,2 electrolyzers,3-5 redox flow batteries6-9 and rechargeable metal air batteries.10-16 For the past decade, significant progress has been made in this field. Fuel cell electric vehicles (FCEV) start to commercialize, and large scale redox flow batteries have been deployed for renewable energy storage. However, there are still significant challenges for these technologies. For proton exchange membrane (PEM) fuel cells, high cost and limited durability are the two main limiting factors for its wide adoption.17 These issues are closely associated with Pt-based electrocatalysts which, according to DOE recent analysis,18 accounts for nearly half of PEM fuel cell cost for large scale production. Redox flow batteries, essentially an identical technology to regenerative fuel cells,1921

are also limited by cost and reliability. These two technologies share many common features

including component materials such as membrane, electrode, flow field and the similar device design using bipolar plates. In this article, we review recent progress on electrocatalysts design and development for PEM fuel cells and redox flow batteries. We do not intend to provide a comprehensive review in this broad and intensively investigated field since recently there have been many excellent review articles on both fuel cell electrocatalysts22-28 and redox flow batteries.19,20 Instead, we will provide a concise overview with a focus on the electrocatalysts effort at PNNL that aims to improve the durability and power capability of these two technologies. We developed a multidisciplinary approach from materials synthesis, high resolution electron microscopy, computer modeling and simulation to electrochemical characterization to address the key interfacial challenges in catalyst


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activity and durability. Leveraging expertise and research advancement in these two technologies is beneficial for their development. 2. PEM fuel cell electrocatalysts For PEM fuel cells, the cost and durability are still the two biggest issues. The cost of most PEM fuel cell components could be significantly reduced if the production scales up. At the same time, large scale application of fuel cells will increase the demand of precious metals used in the catalysts. Therefore, there is a great need to decrease the usage of Pt in PEM fuel cells for FCEVs (the goal is to push Pt usage down to 8-10 grams per FCEV,23 similar to the amount in an internal combustion engine for emission control). There have been intensive fundamental studies and engineering development in order to achieve this goal.1,23,29-34 Some recent articles provide excellent review of this.23,24,26,35 Figure 11,29 summarizes the dramatic improvement on electrocatalyst performance that has been achieved in different research groups; as can be seen, great progress has been made in the past few years. With these significantly improved electrocatalyst activities, one would expect significantly decrease in the usage of precious metals if the performance (particularly durability) can be realized in real fuel cell devices.


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Figure 1. Left) Turnover frequencies for different oxygen reduction catalysts;1 Right) Rising performance/cost ratios for Pt nanoparticles with octahedral shapes.29 However, it has been shown that PEM fuel cells suffer faster performance degradation under ultra-low Pt usage.36 The degradation comes from many factors.37 Electrocatalyst degradation is one of the key contributors (if not the biggest contributor) to performance loss. The degradation model of electrocatalysts can be summarized as:38-43 1) Pt sintering through dissolution/redeposition process, 2) Pt agglomeration through nanoparticle diffusion and coalescence, 3) Pt detachment (from support) and dissolution into electrolyte, 4) support corrosion. Intensive effort has been devoted to develop durable electrocatalysts, such as using Pt alloys30,32,34,44,45 and durable support.46-50 Catalyst support not only plays a key role in stabilizing Pt nanoparticles, but is also very critical for mass transfer management in fuel cells.51 The corrosion of carbon has been linked with Pt nanoparticle agglomeration and detachment from support which decrease catalytic performance;38,39 it has also been related to mass transfer loss and the increase in both electric resistance and ionic resistance in catalyst layers which particularly affect the high power performance of fuel cells.52-55 Stable ceramic support materials are one option to avoid the corrosion issue,46-48,56,57 but they usually suffer inconsistence between half-cell test results (based


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on rotating disk electrode (RDE) test) and real fuel cell device performance, probably due to the conductivity issue and interface incompatibility issue between ceramic materials and ionomer. Graphitized carbon has been shown to be more resistant to corrosion thus improving the durability of Pt catalysts.58-60 Graphitic carbon supported electrocatalysts are also easier to be deployed in real fuel cell devices since most of existing knowledge of PEM fuel cells is based on carbon-type catalysts. However, the problem with graphitic carbon is metal nanoparticle coating, i.e., it is very difficult to uniformly coat metal nanoparticles on its surface because of the hydrophobicity and very limited nucleation sites. One approach is to create defects/functional groups on graphitic carbon through oxidation process.61 However, carbon corrosion starts from defect sites such as vacancies, edges, oxygen functional group;58,60,62 the approach of creating defects/functional groups on graphitic carbon undoubtedly compromises the durability. 2.1. Functionalizing graphitic carbon for highly dispersed Pt catalysts We developed a nondestructive functionalization method using polyelectrolytes to facilitate Pt coating based on previously reported methods.63 It was late confirmed that the polyelectrolytes (e.g., poly(diallyldimethylammonium chloride): PDDA; poly(allylamine hydrochloride): PAH) used for the functionalization of graphitic carbon not only disperses Pt nanoparticle very well,64,65 stabilizes Pt nanoparticles,66 but also improves the catalytic performance.67 The electric conductivity of these polyelectrolytes should not be an issue since they form a very thin and porous film on carbon substrates which does not block the electric contact between metal catalysts and carbon support. We choose various graphitic carbons as catalyst supports, ranging from 1D (carbon nanotubes),65,68 2D (graphene)69 to 3D (mesoporous carbon) materials.64 This polyelectrolyte


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functionalization approach works very well for all these materials. Figure 2 shows TEM images of Pt nanoparticle on polyelectrolyte functionalized carbon nanotube (Figure 2b), graphene nanoplatelets (Figure 2d) and graphitic mesoporous carbon (Figure 2f). It is very clear that small Pt nanoparticle of 2-3 nanometers are uniformly dispersed on these graphitic carbon materials. Figure 2a shows the scheme of polyelectrolyte (PAH as an example) functionalization of carbon nanotube and Pt nanoparticle loading process. In brief, the positively charged polyelectrolyte molecules wrap around hydrophobic graphitic carbon due to the dipolar property of these molecules. This not only helps disperse graphitic carbon in the solvents, but also provides nucleation sites for Pt nanoparticles due to the electro-static interaction between positively charged polyelectrolyte and negatively charged Pt precursor which is then reduced by ethylene glycol at elevated temperatures. All these electrocatalysts demonstrated enhanced electrochemical performance (activity and durability).64,65,68,69 This is expected due to the high dispersion of Pt, the graphitic carbon supports which are more resistant to corrosion than regular carbon black, and the likely interaction between Pt and polyelectrolyte (which will be discussed later). We also found that this approach not only works well for Pt nanoparticles, but also for alloy nanoparticles.70


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a)

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Figure 2. a) Scheme of polyelectrolyte (PAH as an example) functionalization of carbon (CNT as an example) and subsequent Pt nanoparticle loading,68 b) TEM image of Pt/PAH-CNT,68 c, d) TEM image of graphene nanoplatelets and Pt/PDDA-graphene nanoplatelets,69 e, f) TEM image of graphitic mesoporous carbon and Pt/PDDA-graphitic mesoporous carbon.64


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40

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Figure 3. a) TEM image of PDDA-Pt/C, b) performance degradation (%) for Pt/C with/without PDDA (ECSA=electrochemical surface area, ORR=oxygen reduction reaction), c) Pt4f XPS in Pt/C and PDDA-Pt/C.66 We also found that there is a specific interaction between Pt and polyelectrolyte PDDA, i.e., between Pt and positively charged nitrogen atom in PDDA.66 This interaction results from electron transfer from Pt to N+ in PDDA molecules as indicated from the slight shift of the binding energy of Pt (Figure 3), similar to the so-called strong metal-support interaction in other supported catalyst systems such as Pt-TiO2 which was discovered decades ago.71,72 This kind of interaction was also found in pyrrolidone rings of PVP and Au cluster.73 This interaction (electron transfer between Pt and N+) may facilitate the electron transfer in ORR process,74 decrease the oxidation degree of Pt,75,76 and increase the durability of Pt nanoparticles (as in the case of Pt alloys77) by reducing Pt dissolution (Figure 3). We notice that PDDA itself presents ORR catalytic activity when functioning together with carbon nanotubes67 or graphene.78 This presents a promising approach to stabilize small Pt nanoparticles (or more generally metal (alloy) nanoparticles) on hydrophobic substrates which may have important applications beyond electrocatalysts. 2.2. Triple-junction stabilizing Pt We developed a new approach to highly disperse Pt nanoparticles on hydrophobic graphitic carbon by creating nucleation sites with metal oxide coating.79 As the first step, we chose indium 8 ACS Paragon Plus Environment

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tin oxide (ITO) and graphene because this is a good model system to study the structure-property relationship due to the well-defined 2D structure of graphene. ITO was chosen for a model metal oxide coating because ITO is a chemically stable and electrically conductive metal oxide. ITO nanoparticles of 6-8 nm were grown on the surface of graphene (Figure 4a-3b). More interestingly, when Pt nanoparticles were loaded onto ITO-graphene nanocomposite, uniformly dispersed Pt nanoparticles had the preference to deposit at the corner of ITO-graphene junction, thus forms a so-called triple-junction structure as we can see from TEM characterization (Figure 4c-3f). However, if no pre-coated ITO nanoparticle, severe agglomeration of Pt particles is observed on graphene. It is not clear yet as for why ITO nanoparticles can be uniformly coated on graphene, but Pt nanoparticles cannot be uniformly dispersed on pure graphene. This difference could be attributed to heterogeneous nucleation phenomena on graphene with different surface chemistry.80 We have a good understanding on why Pt, ITO, graphene form triple junction structures and why ITO nanoparticle coating helps to uniformly disperse Pt nanoparticles. From thermodynamic point of view, our DFT calculation suggests that the system energy of Pt-ITO-graphene is the lowest one when the triple junction is formed. Basically, the ITO-graphene junction can be viewed as artificial structural defects which provide nucleation sites for Pt nanoparticle growth from solution. Since ITO nanoparticles are uniformly dispersed on graphene, these artificial structural defects are also well dispersed which ensures the high dispersion of Pt nanoparticles. The effects of this structure on electrochemical performance, i.e., the structure-property relationship, were systematically studied. Figure 5 shows the comparison of the activity and durability of Pt-ITOgraphene and Pt-graphene toward oxygen reduction. It is very clear that Pt-ITO-graphene performs much better than baseline Pt-graphene; the durability is more than doubled in comparison with baseline in our test ― how Pt-ITO-graphene performs in real fuel cell devices


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under protocols such as those the US DOE suggests17 remain to be tested. The reasons for the enhanced durability of triple-junction structured Pt-ITO-graphene lie in three aspects: 1) ITO nanoparticles protect underlying graphene from corrosion―the specific corrosion current (A/g carbon) is much lower for ITO-graphene; 2) DFT calculation suggests that, under oxidizing environment, the interaction energy at Pt-ITO/graphene is much higher; 3) XPS characterization indicates a strong metal-support interaction between Pt and SnO2 (in ITO). Figure 6 shows the XPS spectra of Pt4f and Sn3d5/2. The Pt binding energy shift of -0.72eV and Sn binding energy shift of -0.68eV in Pt-ITO-graphene indicate electron transfer from partially reduced Sn4+ (in SnO2) to Pt0. This leads to the so-called strong metal-support interaction.71,72 The partial reduction of Sn4+ (in SnO2) probably takes place during Pt loading process (a reduction process).


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Figure 6. High-resolution XPS of Pt4f and Sn3d5/2 in Pt/graphene and Pt/ITO-graphene. 12 ACS Paragon Plus Environment

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There has been much effort on alternative durable support materials for PEM fuel cell cathode. Many are focused on noncarbon support such as stable ceramic materials.46 Good progress has been made. For example, TiO248,49 and 3M nanostructured thin film (NSTF)81 electrocatalysts have shown both good activity and durability. We mentioned that sometimes it is difficult to transfer excellent performance measured in half cell (RDE test) into fuel cell devices (MEA test). Our concept of using triple junction to disperse and stabilize Pt (or alloy) nanoparticles while protecting carbon support from corrosion provides an alternative way to address catalyst activity and durability issues associated with PEM fuel cell cathodes. From the structure of Pt-ITOgraphene nanocomposite, we can see that Pt particles directly contact conductive carbon substrate, so metal oxides used here do not have to be electrically conductive; this definitely broadens the choices of materials and provides more room to tune the properties of electrocatalysts. Since it is still carbon-based electrocatalysts, the advantages of carbon substrate and the abundant knowledge of carbon-based electrocatalysts obtained in the past decades can be leveraged. 3. Redox flow battery electrocatalysts Redox flow batteries (RFB) share similar designs and component materials with PEM fuel cells.19-21 It has recently rejuvenated researchers’ interests and revealed themselves as promising candidates for large-scale energy storage systems.82-90 As illustrated in the schematic Fig. 6 (a), RFBs have two reservoirs for storage of energy-bearing electrolytes, and the electrolytes are pumped through the power-generation cell/stack where redox reactions take place on the surface of electrodes to achieve reversible conversion between electrical and chemical energies. As such, the energy output of a RFB is determined by the volume and concentration of its redox active electrolyte, while the power capability is dictated by the surface area and current density of the cell/stack. The unique battery architecture and working mechanism render the RFBs at least two 13 ACS Paragon Plus Environment

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distinctive characteristics that differentiate them from conventional energy storage devices such as Li-ion battery or supercapacitors, decoupling of power and energy output and the separation of the storage and reaction locality, even though its energy density is lower than Li-ion batteries and power is lower than supercapacitors. The former provide great latitude to design a RFB to meet various applications with wide-ranging energy/power output requirements, while the latter give the RFBs high safety, easy thermal management, as well as the possibility to adopt bi-polar plate design. Other advantages of RFBs include short response times (ms) and long service life, etc. By now, the most promising and extensively-studied RFBs system is the all-vanadium redox flow battery (VRB) system that employs different oxidation states of the same element (vanadium), and hence alleviate the cross-contamination problem.91 VRBs are also known for excellent electrochemical reversibility and high efficiencies in charge and energy utilizations, and a recently invented sulfuric-hydrochloric mixed-acid VRB system developed by PNNL has significantly improved energy densities and operational temperature windows by enhancing V ions solubility and stability in electrolytes.92 The electrode plays a critical role in providing electrochemically active surface for the redox reactions to take place. Electrodes with high surface area, high porosity, high electrochemical activity and stability are desired for enhancing performances of RFBs. The electrode surface chemistry and composition have dramatic influence on its electrochemical activity towards the particular redox reaction. This significantly influences the operating current density of a RFB system thus the size of cell/stack for a rated power output which determines the capital cost of the system. A high performance, high efficiency electrode with minimal overpotential is therefore desired. The electrode materials of VRBs are typically graphite felts (GFs) due to their low cost, high electric conductivity and high stability in the concentrated acidic electrolytes. However, low 14 ACS Paragon Plus Environment

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electrochemical activity and kinetic reversibility of pristine GFs limit VRBs operations to low current density (~50 mA/cm2), which in turn hinders capital cost reduction of the system.93 Besides introduction of functional groups like carboxylic, hydroxyl, oxygen and pyridinic nitrogen to GF surface through different treatment methods,94,95 surface modifications with nano-structured electrocatalysts like metal96 or metal oxide97 nanoparticles, synergy of different nanostructures90,98 have also been proven as effect routes to improve electrochemical activities of the electrodes and lower the electron transfer overpotential. For cost and stability purposes, it would be desirable for the electrocatalysts to meet the following requirements: i) low cost, ii) stable in the concentrated acidic electrolytes, and iii) no/low catalytic activities towards hydrogen or oxygen evolution. Two approaches have been studied to achieve these goals: 1) nitrogen-doped carbon6,9 and 2) metal (oxide)/carbon nanocomposites.96,97


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b)

c)

a) Figure 7. Redox flow batteries and their electrocatalysts. a) A schematic illustration of the structure of a redox flow battery, adopted with permission from Ref ..99 b) STEM image of Bi nanoparticles in the anolytes resulting after cycling with electrolytes containing 0.01 M Bi3+ with insert showing HR-TEM image of the Bi nanoparticle lattice structure.7 c) FE-SEM images of Wdoped Nb2O5-modified graphite felts with 0.05 M Nb in the precursor solutions, with the inset showing higher magnification FE-SEM (upper right) and STEM (lower right) images of the Wdoped Nb2O5 nanoparticles.8 3.1 Nitrogen-doped carbon Nitrogen-doped carbon materials have been known to be electrocatalytically active for many reactions for a long time.100-103 Many papers104-107 including some excellent review articles108-112 have been published in past few years. We have investigated nitrogen-doped carbon and its electrocatalytic role for biosensing,101,102 energy conversion101 and energy storage.6 A particular interest is in the application in redox flow batteries.6,98,113 Figure 8 shows the electrochemical performance of nitrogen doped mesoporous carbon toward the redox reaction in vanadium flow 16 ACS Paragon Plus Environment

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battery.6 Nitrogen doped mesoporous carbon catalyzes vanadium redox reaction ([VO]2+ ↔ [VO2]+), as can be seen from the enhanced reaction kinetics (Figure 8a) and lowered charge transfer resistance (Figure 8b) ― the mass transfer behavior changes too; this is probably due to the different surface area (and pore sizes): 1100 m2/g for nitrogen doped mesoporous carbon and 500 m2/g for mesoporous carbon. The N1s XPS indicates abundant nitrogen functional groups: pyridinic-N (N1, 398.7±0.2 eV), pyrrolic-N (N2, 400.3±0.2 eV), quaternary nitrogen (N3, 401.2±0.2 eV), N-oxides of pyridinic-N (N4, 402.8±0.4 eV). Previous studies already provided good insight on how nitrogen doping enhances electrocatalytic activity of carbon from previous investigations,100,114 particularly its role on oxygen reduction reaction.115-118 DFT calculations suggest that carbon atoms adjacent to nitrogen dopants possess a substantially high positive charge density to counterbalance the strong electronic affinity of the nitrogen atom.104 The heteroatom nitrogen induced “positively-charged” carbon atoms function as the active sites for the oxidation reaction (i.e., electrons are transferred from the reactants to the electrode). The five valence electrons of nitrogen atoms contribute the extra charge to the π bond in graphene layers (e.g., pyridinic-N possesses one lone pair of electrons in addition to the one electron donated to the conjugated π bond system). The nitrogen doping could enhance the basicity of carbon119,120 and the electrical conductivity of nitrogen-doped carbon.121 This is beneficial for the reduction process (i.e., electrons are transferred from the electrode to the reactant). For the specific redox chemistry in vanadium flow batteries, i.e. [VO]2+↔[VO2]+, the reaction involves the breaking/formation of V-O bonds.122,123 The nitrogen-induced charge delocalization might also change the chemisorptions behavior of V-O atoms on carbon, like in the case of oxygen reduction104 and decrease the activation energy for V-O bond formation/breaking, which facilitates the redox process. Recently, investigation on [VO]2+↔[VO2]+ reaction on nitrogen-doped graphene


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indicates that nitrogen doping facilitates the adsorption and coordination of the reactant ions through possible formation of a N-V transitional bonding state, which results in a high catalytic activity.124 This work opens up a new application for nitrogen doped carbon. It also indicates some general connection between electrocatalysis of difference materials. 8 N1

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Figure 8. a) Cyclic voltammograms (CV) on different electrodes in 3.0M H2SO4 + 1.0M VOSO4 (50 mV/s), b) Electrochemical impedance spectroscopy (EIS) of different electrodes in 3.0M H2SO4 + 1.0M VOSO4 solution, c) N1s XPS spectra of nitrogen-doped mesoporous carbon. (NDMC=Nitrogen-doped mesoporous carbon, MC=mesoporous carbon).6 3.2 Metal (oxide)/carbon nanocomposites Another approach is to use metal, metal oxides to improve the performance of GF electrode. In one system, Bi nanoparticles were deposited onto the GF electrode surfaces through synchronous electrodeposition during cell running.96 A small amount (0.05 M – 0.2 M) of BiCl3 additives was added to the electrolytes that were composed of 2 M VOSO4 and 5 M HCl. And subsequent reduction of Bi3+ leads to uniform deposition of Bi nanoparticles on GF surfaces, as shown in Figure 7b. These Bi nanoparticles have demonstrated improved catalytic activities towards the V(II)/V(III) redox couple. As shown from the cyclic voltammograms (CV) results in Figure 9a, potential separation between oxidation and reduction peaks that correspond to V(II)/V(III) redox couple was narrowed from 0.31 V to 0.22 V with addition of 0.1 M BiCl3. The additives, however, 18 ACS Paragon Plus Environment

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hardly exhibit any effect on the V(IV)/V(V) redox couple. Such result also suggests that Bi metals, instead of Bi ions, play a main role in providing the catalytic effect. As shown in Figure 9b-9c cell performances have also been enhanced by incorporating BiCl3 additives in the electrolytes, and higher energy efficiencies (EE) and capacities were achieved, especially at higher current densities. It should be noted that an optimal concentration of BiCl3 of 0.1 M has been identified, as higher concentrations like 0.2 M result in larger particle sizes, therefore higher tendency of Bi particles to be displaced into the electrolyte solutions, and hence lower particle coverage on the electrode surfaces. Metal oxides have also been developed as electrocatalysts for VRB.97 The Nb2O5 nanorods were synthesized through hydrothermal methods, and deposited onto GF surfaces by submerging the felts in the autoclave during preparation. Addition of W precursors facilitates precipitation of nanorods in the form of W-doped Nb2O5, limits their agglomeration, and hence enables high loading ratio of the electrocatalysts. The as-synthesized nanorods also exhibit excellent stabilities in the mixed-acid VRB electrolytes. Unlike Bi catalysts, Nb2O5 nanorods are able to promote reaction kinetics of both redox couples. As demonstrated in the CV results in Figure 9d, addition of Nb2O5 nanorods lowers peak potential separations corresponding to V(II)/V(III) and V(IV)/V(V) redox couples by 61 mV and 100 mV, respectively. And once employed in VRB, W-doped Nb2O5 nanorods promoted cell performances, with higher EE and capacities achieved (Figure 9e-9f).


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Figure 9. Catalytic performances of Bi and W-doped Nb2O5 electrocatalysts. a) CV with glassy carbon as working electrode in solutions of 2 M VOSO4 + 5 M HCl with or without 0.01 M BiCl3 at a scan rate of 50 mV·s–1. b) EE of VRFBs employing electrolytes containing different Bi3+ concentrations as a function of charge/discharge current density. c) Specific discharge capacities of VRFBs as a function of cycle number at different charge/discharge current densities employing the electrolytes containing different Bi3+ concentrations. d) CV with or without Nb2O5 nanorods onto glassy carbon as working electrodes in solutions of 2 M VOSO4 + 5 M HCl, at a scan rate of 50 mV·s–1. e) EE of VRFBs with W-doped Nb2O5 nanorods-decorating GFs as both electrodes as a function of charge/discharge current density. f) Specific discharge capacities of VRFBs as a function of cycle number at different charge/discharge current densities employing different surface-modified electrodes. Panels (a)-(c) were adopted with permission from Ref.,7 and Panels (d)-(f) from Ref..8 20 ACS Paragon Plus Environment

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For future work on electrocatalysts in VRB systems, besides extensive exploration of new materials that meet the requirements mentioned above, efforts should also be directed towards elucidating the catalytic mechanisms as well as structure-performance relationships. Such work, in combination with theoretical and modeling efforts, would definitely help future design and optimization of VRB systems, and in principle other RFBs as well. 4. Perspective Electrocatalysts are the key component for both fuel cells and redox flow batteries, and their activity, selectivity and durability critically determine the overall system cost, reliability and therefore practical applications. Developing low cost, durable and active electrocatalysts are required for their large-scale commercialization. For PEM fuel cells, even though Pt is an expensive material, the performance analysis suggests that Pt and Pt-based alloy may still dominate for PEM fuel cells at least in the short- and mid-term. The intensive research efforts during the past years for fuel cells have resulted in many encouraging demonstrations of electrocatalyst systems.32,34,44,75,125,126 The integration of these electrocatalysts with durable supports such as graphitic carbon and triple junction structured nanocomposite that we have shown above will further improve both the activity and durability. However, there are still some scientific and technology gaps. First, good performance of most advanced electrocatalysts has mostly been demonstrated in half-cell RDE test; however, such results may be difficult to translate into device level performances due to different working environment under practical fuel cell conditions (temperature, interface, etc.) and the problem of transport in the device architecture. Therefore, it is necessary to develop testing protocols to simulate the actual working environment under real fuel cell operations. Second, it is still unclear about the function and degradation mechanisms, and the fundamental understandings about the


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dominant factors that control catalyst activity and durability are still limited. Advanced characterization tools, such as in situ electron microscopy, in situ X-ray microscopy in combination with computer simulation and modeling, may be helpful for tracking the structure and morphology evolution of catalysts and supports under operation conditions, and may guide rational design of durable catalyst systems. Third, there are very limited fundamental understandings of the nucleation and growth pathways of Pt (alloy) on support. Also, even though there are many demonstrations in the literature with structure-properties relationships of Pt and Ptalloy nanoparticles, Pt nanowires, branched Pt, the synthesis of such structures is still highly empirical and the developed protocols are difficult to translate into systems that are more relevant to PEM fuel cells. Fundamental understanding of nucleation and growth pathways of Pt-based nanocrystals on substrates, such as interaction of Pt precursors with carbon support, would be beneficial to the improvement of catalyst activity and durability. Fourth, understanding and manipulating the interface between polymer electrolyte ionomer and electrocatalysts will provide the foundation for real-world deployment for the advanced electrocatalysts. Last but not least, cost-effective scalable synthesis of high performance electrocatalysts is very critical for large scale deployment of fuel cells. From long term perspective, Pt may need to be replaced with precious metal-free electrocatalysts. This should be a strategic research direction for PEM fuel cells for which great progress has been made.22,27,28,110,127-130 Even though it is still a long way for precious metal-free catalysts to find applications in today’s real PEM fuel cell products, these catalysts might find applications in today’s redox flow batteries which presents short-term application values of these catalysts.


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For redox flow batteries, graphite fibers (graphite felt) will continue to be the base electrode materials. The advantages of this kind of materials are low cost, high stability; however, low surface area is a challenge particularly for high power application for which electrocatalysts will become even more crucial. Some of the above mentioned precious metal-free electrocatalysts have shown promise in enhancing the kinetics for redox flow battery electrode reactions.6,9,113,124,131 There is still a gap on how to apply these electrocatalysts onto graphite felt electrode. Future direction should be focused cost-effective scalable in-situ synthesis and processing methods of electrocatalysts onto graphite felt electrode. In summary, PEM fuel cells and redox flow batteries share many common components and design. For the past 15 years, good progress has been made on PEM fuel cells thanks to the extensive investigation and investment around the world. Recently, some of the capabilities developed for PEM fuel cells have been leveraged for redox flow battery development for large scale renewable energy storage. For example, the electrocatalytic materials such as functionalized carbon and nanostructured carbon which have been developed for oxygen electrochemistry (for fuel cell applications) have been adopted in redox flow batteries;6,9,117,124,125 while the flow battery electrocatalysts such as W-Nb2O5 coated carbon fiber can also find applications in fuel cells as demonstrated here showing that metal oxide and metal oxide coated carbon are good catalyst supports for fuel cells.46-49,56 The synergy between PEM fuel cells and redox flow batteries research will promote the progress in both fields and may produce more encouraging outcomes beyond electrocatalysts.


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Acknowledgement This PEM fuel cell research is supported by the U.S. Department of Energy's (DOE's) Office of Energy Efficiency and Renewable Energy Fuel Cell Technologies Office and the redox flow battery research is supported by DOE Office of Electricity Energy Storage Program. PNNL is operated by Battelle for DOE under Contract DE-AC05-76L01830. References: (1) Gasteiger, H. A.; Markovic, N. M. Science 2009, 324, 48. (2) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal. B-Environ. 2005, 56, 9. (3) Park, S.; Shao, Y. Y.; Liu, J.; Wang, Y. Energy Environ. Sci. 2012, 5, 9331. (4) Carmo, M.; Fritz, D. L.; Merge, J.; Stolten, D. Int. J. Hydrog. Energy 2013, 38, 4901. (5) McCrory, C. C. L.; Jung, S. H.; Peters, J. C.; Jaramillo, T. F. J. Am. Chem. Soc. 2013, 135, 16977. (6) Shao, Y. Y.; Wang, X. Q.; Engelhard, M.; Wang, C. M.; Dai, S.; Liu, J.; Yang, Z. G.; Lin, Y. H. J. Power Sources 2010, 195, 4375. (7) Li, B.; Gu, M.; Nie, Z. M.; Shao, Y. Y.; Luo, Q. T.; Wei, X. L.; Li, X. L.; Xiao, J.; Wang, C. M.; Sprenlde, V.; Wang, W. Nano Lett. 2013, 13, 1330. (8) Li, B.; Gu, M.; Nie, Z. M.; Wei, X. L.; Wang, C. M.; Sprenkle, V.; Wang, W. Nano Lett. 2014, 14, 158. (9) Shao, Y. Y.; Engelhard, M.; Lin, Y. H. Electrochem. Commun. 2009, 11, 2064. (10) Lee, J. S.; Kim, S. T.; Cao, R.; Choi, N. S.; Liu, M.; Lee, K. T.; Cho, J. Adv. Energy Mater. 2011, 1, 34. (11) Cheng, F. Y.; Chen, J. Chem. Soc. Rev. 2012, 41, 2172. (12) Lu, Y. C.; Gallant, B. M.; Kwabi, D. G.; Harding, J. R.; Mitchell, R. R.; Whittingham, M. S.; Shao-Horn, Y. Energy Environ. Sci. 2013, 6, 750. (13) Shao, Y. Y.; Ding, F.; Xiao, J.; Zhang, J.; Xu, W.; Park, S.; Zhang, J. G.; Wang, Y.; Liu, J. Adv. Funct. Mater. 2013, 23, 987. (14) Shao, Y. Y.; Park, S.; Xiao, J.; Zhang, J. G.; Wang, Y.; Liu, J. ACS Catal. 2012, 2, 844. (15) Lu, J.; Li, L.; Park, J. B.; Sun, Y. K.; Wu, F.; Amine, K. Chem. Rev. 2014, 114, 5611. (16) Luntz, A. C.; McCloskey, B. D. Chem. Rev. 2014, 114, 11721. (17) U.S. DRIVE Partnership Fuel Cell Technical Team Roadmap, 2013. (18) Spendelow, J.; Marcinkoski, J.; Satyapal, S. DOE Hydrogen and Fuel Cells Program Record-Fuel Cell System Cost - 2014, 2014. (19) Weber, A. Z.; Mench, M. M.; Meyers, J. P.; Ross, P. N.; Gostick, J. T.; Liu, Q. H. J. Appl. Electrochem. 2011, 41, 1137. (20) Wang, W.; Luo, Q. T.; Li, B.; Wei, X. L.; Li, L. Y.; Yang, Z. G. Adv. Funct. Mater. 2013, 23, 970. (21) Yang, Z. G.; Zhang, J. L.; Kintner-Meyer, M. C. W.; Lu, X. C.; Choi, D. W.; Lemmon, J. P.; Liu, J. Chem. Rev. 2011, 111, 3577. (22) Chen, Z. W.; Higgins, D.; Yu, A. P.; Zhang, L.; Zhang, J. J. Energy Environ. Sci. 2011, 4, 3167. (23) Debe, M. K. Nature 2012, 486, 43. (24) Katsounaros, I.; Cherevko, S.; Zeradjanin, A. R.; Mayrhofer, K. J. J. Angew. Chem.-Int. Edit. 2014, 53, 102. (25) Wang, C.; Markovic, N. M.; Stamenkovic, V. R. ACS Catal. 2012, 2, 891. 24 ACS Paragon Plus Environment

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TOC graphic:


Nanostructured Electrocatalysts for PEM Fuel Cells and Redox Flow

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