lifetime of fuel cell
catalysts
lifetime of fuel cell catalysts
Proefschrift
ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus Prof. ir. K. C. A. M. Luyben,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen op
vrijdag 6 Maart 2015 om 10:00 uur
door
Master of Science in Chemical Engineering
at the University of Manchester
geboren te Saratov, Rusland
Roman Latsuzbaia
Dit proefschrift is goedgekeurd door de promotoren: Prof. dr. J. H. van Esch
Prof. dr. S. J. Picken
Copromotor: Dr. ing. G. J. M. Koper Samenstelling promotiecommissie: Rector Magnificus
Prof. dr. J. H. van Esch Prof. dr. S. J. Picken Dr. ing. G. J. M. Koper Prof. dr. J.J.C. Geerlings Prof. dr. P.E. de Jongh Prof. dr. R. van de Krol Dr. W. Briscoe
The work presented in this thesis was carried out in the group of Advanced Soft Matter, department of Chemical Engineering of the Delft University of Technology and was financially supported by the Innovation Oriented Programme (IOP) on Self-Healing Materials (funded by the Dutch Ministry of Economic Affairs). Project: SHM1050, Self-Healing and -Assembling Regenerative Electrode for Proton-Exchange Mem-brane Fuel Cells.
ISBN: 978-94-6186-430-7
Copyright © 2015 ROMAN LATSUZBAIA Cover design by Lisa Schmidt
Printed by Ipskamp Drukkers, Enschede
All right reserved. The author encourages the communication of scientific contents and explicitly allows reproduction for scientific purposes, provided proper citation of the source. Parts of the thesis have been published in scientific journals and copyright is subject to different terms and conditions.
An electronic version of this thesis is freely available at http://repository.tudelft.nl Technische Universiteit Delft, voorzitter
Technische Universiteit Delft, promotor Technische Universiteit Delft, promotor Technische Universiteit Delft, copromotor
Technische Universiteit Delft / Shell Technology Center Amsterdam
Utrecht Universiteit
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH
“I am still learning.”
Michelangelo
“Yes, my friends, I believe that water will one day be employed as fuel, that hy-drogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light, of an intensity of which coal is not capable. Water will be the coal of the future.”
1 Introduction ... 1
1.1 General introduction ... 2
1.2 History of fuel cells ... 3
1.3 Basics of Proton Exchange Membrane Fuel Cells ... 5
1.4 Main challenges of PEMFCs ... 8
1.5 Improving performance of PEMFCs ... 9
1.6 Project idea ... 10
1.7 References ... 11
2 Regeneration possibilities of PEMFC catalysts ... 15
2.1 Durability issues of PEMFC catalysts ... 16
2.2 Recovery of the catalyst performance ... 18
2.2.1 Catalyst recovery after poisoning ... 18
2.2.2 Catalyst recovery after sintering ... 20
2.2.3 Recovery of catalyst utilization ... 23
2.3 Discussion ... 24
2.3.1 Preventive measures ... 24
2.3.2 Regeneration possibilities ... 25
2.4 Conclusions ... 28
2.5 References ... 29
3 PEMFC catalyst dissolution and potential for the regeneration . 33 3.1 Introduction ... 34
3.2 Results and discussion ... 36
3.2.1 Dissolution conditions ... 36
3.2.2 Dissolution rate ... 37
3.2.3 Effect of Oxygen ... 38
3.2.4 Effect of Chloride ... 39
3.2.5 Carbon support stability during the dissolution procedure ... 41
3.2.7 Potential for an in situ Pt fuel cell catalyst regeneration ... 47
3.3 Conclusions ... 48
3.4 Experimental ... 49
3.4.1 Materials ... 49
3.4.2 Platinum dissolution procedure ... 49
3.4.3 Electrochemical characterization ... 50
3.4.4 Instrumentation ... 50
3.5 References ... 52
Appendix A ... 57
4 A novel approach for a PEMFC electrode fabrication. Electrodepo-sition as a redepoElectrodepo-sition step of a catalyst regeneration. ... 77
4.1 Introduction ... 78
4.2 Results and Discussion ... 81
4.2.1 CNNs synthesis: impact on the ESA and corrosion resistance ... 81
4.2.2 Electrochemical functionalization of CNNs... 86
4.2.3 Electrodeposition of Pt NPs on functionalized CNNs-CP ... 90
4.2.4 Accelerated Degradation Tests (ADT) of Pt-CNNs-CP ... 93
4.2.5 Electrodeposition as a technique for the redeposition of Pt NPs ... 95
4.3 Conclusions ... 97
4.4 Materials and methods... 98
4.4.1 Materials ... 98
4.4.2 Growth of CNNs over carbon paper... 98
4.4.3 Carbon support functionalization and electrodeposition of Pt NPs ... 99
4.4.4 Electrochemical characterization ... 100
4.4.5 Instrumentation ... 101
4.5 References ... 102
5 Bicontinuous microemulsions as templates for the robust synthesis method of the monodisperse metal NPs: a general approach. ... 107
5.1 Introduction ... 108
5.2 Results and discussion ... 109
5.3 Conclusions ... 117
5.4 Experimental ... 117
Appendix B ... 123
6 Synthesis, stabilization and activation of Pt nanoparticles for PEM-FC applications ... 141
6.1 Introduction ... 142
6.2 Results ... 145
6.2.1 Catalyst synthesis and extraction ... 145
6.2.2 Catalyst heat treatment ... 146
6.2.3 Catalyst composition and surface characterization ... 148
6.2.4 Catalyst activation and electrochemical performance ... 150
6.2.5 Accelerated Durability Test ... 153
6.3. Discussion ... 156 6.4 Conclusions ... 158 6.5 Experimental ... 159 6.5.1 Materials ... 159 6.5.2 Pt catalyst preparation ... 159 6.5.3 Catalyst characterisation ... 160 6.5 References ... 163 Appendix C ... 167 Summary ... 175 Samenvatting ... 181 Curriculum Vitae ... 187 List of publications ... 189 Acknowledgements ... 195
Abstract
Most of the vehicles are utilizing petroleum-derived fuels, and longing for these fuels has led to political unrests, wars, poverty and environmental dis-asters. Especially, amid growing environmental concerns sustainable energy sources and converters, such as fuel cell technology, batteries, etc., are promis-ing sustainable alternative to the conventional fossil fuel based energy genera-tion. Particularly, Proton Exchange Membrane Fuel Cells (PEMFCs) can offer emission-free power generation for transportation and stationary applications. However, price and durability of the PEMFC components, such as catalysts, and membranes, hampers the full commercialization.
The goal of the research described in the thesis is to investigate possibilities to increase the lifetime of fuel cell catalysts. Two approaches were employed: (i) conventional – by prevention of the degradation processes and (ii) uncon-ventional – regeneration of the degraded catalyst. The second unprecedented approach offers a possibility to significantly prolong the lifetime of the catalyst and save the precious metal consumption.
Introduction
1.1 General introduction
Fuel cells present themselves as a promising sustainable energy converters (chemical to electric) for stationary and transport applications, or portable de-vices, due to their main advantage of sustainable energy generation with high efficiency of energy conversion.1, 2 Additionally, advantages over other compet-ing technologies include possibility of use of renewable fuels, silent and vibra-tion free operavibra-tion at ambient condivibra-tions, delivery of higher power densities, and modularity, which means that similar efficiencies can be obtained from large and small units.3 Fuel cells employ fuels of high energy density, therefore, this allows significant reduction of the system weight, compared, for instance, to heavy batteries.4 Furthermore, even though energy generated by internal combustion engine (ICE) is cheaper, fuel cell technology is still competitive with ICEs due to no pollutant emissions during the operation.1
According to a definition of Connihan a fuel cell ”is an electrical cell, which
unlike of storage cells can be continuously fed by fuel so that the electrical power output is sustained indefinitely”.5 Therefore, the fuel cells can pro-duce electricity continuously as long as fuel is supplied. Various types of the fuel cells have been developed and employed in numerous applications. They mainly differ by the electrolyte used, the fuel consumed (Hydrogen, Metha-nol, Formic acid) and the operation conditions.2 Low temperature fuel cells, such as Proton Exchange Membrane Fuel Cells has attracted much attention due to high potential to deliver a ‘clean’ energy with high efficiency, especially, for portable and transport applications.1 Currently, the fuel cell technology is closer to wider commercialization than ever before, e.g. the PEMFCs in auto-motive applications. Large autoauto-motive manufacturers are active in the last sev-eral years:6, 7 Toyota is planning to launch their hybrid ‘Mirai’ Fuel Cell Sedan in 2015, Honda and its FCX Fuel Cell powered vehicle to be launched in 2016, Audi’s A7 h-tron hybrid, Volkswagen’s hydrogen-powered Golf Estate and Pas-sat showcased on motor shows this year.6, 8 Nevertheless, improvements on various aspects of a PEMFC operation are required in order to make the tech-nology cheaper.
Sophisticated catalyst materials (such as platinum or its alloys) are required in order to catalyze the electrochemical reactions in PEMFCs and achieve high INTRODUCTION
efficiencies.9, 10 That is where the problems with PEMFCs and other types of the fuel cells begin. First of all, platinum as a catalyst material is expensive, hence in order to achieve wide commercialization of the fuel cells platinum loading in the electrodes should be reduced. A requirement is set by US Department of Energy (DOE), based on the scarcity of platinum, to reduce the catalyst load-ing lower than 0.20 mg cm-2 of an electrode area by 2015, and at the same time keep high performance of PEMFCs. The fuel cell operational lifetime of 5000 hours for automotive applications is another requirement.11, 12 Problem of degradation of the catalyst material and other fuel cell components is an important issue hampering the commercialization of PEMFCs. 13, 14
This PhD thesis is mainly focused on the problem of degradation of plati-num electrocatalyst, and two different approaches to prolong the lifetime of the fuel cell catalyst were investigated. First approach is focused on produc-tion of durable catalyst material and novel fabricaproduc-tion methods of the durable electrodes. In the second and unconventional approach it is proposed to in situ regenerate a PEMFC electrode. A method for the catalyst regeneration upon its performance decay is developed. Self-healing approach can be introduced for this process.
1.2 History of fuel cells
Fuel cells have been on the spotlight already for the last few decades as prom-ising and environmentally friendly energy conversion devices. As early as in 1838 Christian Schönbein discovered that, when connected by electrodes oxy-gen, hydrogen and chlorine could react to generate electricity, and he named this effect as a “polarization effect”.15 In the same period of 1930s, a British chemist, trained lawyer and judge, Sir William Robert Grove investigated wa-ter electrolysis process to produce hydrogen and oxygen, when he discovered that the reaction could go both directions. His invention a “gas voltaic battery” is regarded as a first prototype of a fuel cell.16
Fifty years later in 1889, Carl Langer and Ludwig Mond performed series of studies and attempted to construct first practical fuel cell device to operate on coal gas as a fuel and oxygen as an oxidant. Their device with the electrode of platinized Pt with a surface area of 700 m2 delivered current of 2 - 3 A at
volt-age of 0.73 V,16 however performance was not stable and reproducible. At that time, this invention found to be useless, as it did not provide a stable performance and the emergence of the internal combustion engine in 19th cen-tury offered economically viable solution to the energy demand. No practical or commercial application of fuel cells was established for more than a centu-ry.16 In 1960s, Francis Thomas Bacon, an engineering professor at Cambridge University in the UK, had a public demonstration of a fuel cell battery with a power output of 5 - 6 kW, which again attracted a commercial and scientific attention to the technology.16 First PEMFC was produced by General Electric, the technology was used by NASA in their Gemini and Apollo space projects.16
In the past three decades interest in fuel cells and other sustainable tech-nologies grew rapidly due to environmental concerns. The energy consump-tion and use of the fossil fuels increased dramatically. This led to the environ-mental issues, such as increased release of greenhouse gases. From 2000 to 2007 CO2 emissions increased by 22 % - 8.2 billion tons.17 Many world nations are spending a lot of efforts to maintain sustainable development and imple-ment alternative sustainable energy sources or converters, such as solar, wind, fuel cells, hydropower, biofuels, etc. Among different types of the sustainable power generators fuel cells stand out due to high efficiencies and emission free operation.2 Different types of the fuel cells have been developed, out of which five main types can be categorized based on the electrolyte and charge carriers they utilize: proton exchange membrane fuel cells(PEMFCs), solid oxide fuel cells (SOFC), phosphoric acid fuel cells (PAFCs), alkaline fuel cells (AFCs) and molten carbonate fuel cells (MCFCs). PEMFCs are represented with two dif-ferent types based on the fuel consumed: H2-O2 PEMFCs and direct methanol fuel cells (DMFCs). Various types of the fuel cells can deliver wide range of the power output for different applications: starting from portable devices up to power plants, see Figure 1.1.
The work presented in this thesis is focused on H2-O2/Air based PEMFCs, which are highly suited for an automotive application, due to advantages such as high efficiencies, fast start up, compactness and simplicity of the system.1, 18 INTRODUCTION
1.3 Basics of Proton Exchange Membrane Fuel
Cells
Structure of a typical PEMFC is presented on the Figure 1.2. The main com-ponent of this type of fuel cells is a Membrane Electrode Assembly (MEA), which consists of proton exchange polyelectrolyte membrane sandwiched be-tween two electrodes, anode and cathode (Figure 1.2). These electrodes consist of several components: carbon paper or gas diffusion layer (GDL), as a me-chanical and electronic support, a microporous layer (MPL) for the reactant/ product transport and the most important, a catalyst layer (CL) located on the interface with the membrane.3 The CL consists of catalyst nanoparticles sup-ported on conductive carbon supports, such as carbon blacks, carbon nano-tubes, etc.19 Pt nanoparticles are mainly utilized as a state-of-the-art catalyst for oxidation/reduction reactions to take place at temperatures of 50 - 150 0C at the electrodes.
During the operation of a PEMFC hydrogen is supplied to the anode side, where it is catalytically split to protons generating pair of electrons (Eq. 1.1). The protons then diffuse via water channels across the membrane, while the electrons are transferred through an external circuit to the cathode, driven by the potential difference of the electrodes. At the cathode side the electrons re-combine with the protons and reduce oxygen to water (Eq. 1.2).20 The mem-brane separates the electrodes to prevent the mixing of the gases and more
importantly to prevent the short-circuit recombination of the electrons.2 Following electrochemical reactions occur during the operation of a PEM:19 Anode (oxidation): H2 → 2H+ + 2e- E
a0 = 0 V vs RHE (Eq. 1.1)
Cathode (reduction): 1/2O2 + 2H+ + 2e-→ H
2O Ea0 = 0 V vs RHE (Eq. 1.2)
Overall reaction: H2 + 1/2O2 → H2O (Eq. 1.3) The reversible cell voltage for a fuel cell can be calculated from Eq. 1.4:2
where a expresses activities of the products and reactants, E0 is the voltage
under standard conditions, F is a Faraday constant.
Assuming that all of the Gibbs free energy of the overall reaction (Eq. 1.2),
Δĝ, can be converted into electrical energy and considering the enthalpy of the
hydrogen fuel, Δĥ, or its higher heating value (HHV), the maximum theoretical or reversible energy efficiency of a fuel cell can be calculated with the following
Figure 1.2 Scheme of a PEMFC on the left, and scheme of a typical PEMFC electrode, which
consists of 3 layers: GDL, MPL and CL on the top.
INTRODUCTION
equation:2
εthermo = Δĝ / Δĥ (Eq. 1.5) Gibbs free energy itself is related to the reversible cell voltage through Eq. 1.6:
Δĝ = - n F E (Eq. 1.6)
Then for the reaction at room temperature and pressure and H2-O2 fuel, Δĝ0
= -237.17 kJ/mol and Δĥ0
HHV = -286 kJ/mol. This results in 83 % maximum
theoretical energy efficiency of a fuel cell at STP.2 In contrast to a fuel cell, for a heat engine the theoretical efficiency is described by the Carnot cycle. For an engine that operates at 400 0C and rejects heat at 50 0C, the reversible ef-ficiency is only 52 %.2
However, the real fuel cell efficiency is only 50 - 60 %18 due to a number of irreversible voltage losses:2 (i) Activation losses, due sluggish kinetics of ORR reaction, (ii) ohmic losses, due to ionic or electrical resistance, (iii) concen-tration or mass transfer losses, (iv) internal (stray) currents and (v) reactants crossover. Considering these loses a characteristic PEMFC polarization is giv-en on Figure 1.3:
Figure 1.3 Voltage losses in a PEMFC and corresponding polarization curve. Illustration
1.4 Main challenges of PEMFCs
Although considerable effort has been devoted to the PEMFC technology and it is a promising alternative to the conventional energy conversion devices, still few challenges hamper the full-scale commercialization. The two major chal-lenges according to the US Department of Energy, which produces annual re-ports on the status of the FC technology, are cost and durability.21 However, additional issues, such as catalyst activity, onboard hydrogen storage, safety and other issues need to be addressed as well.
Cost of the energy production with PEMFCs for automotive applications needs to be reduced from the current status of $55/kW to the ultimate target of $30/kW in order to compete with the conventional technologies, such as ICEs. The target price is a projection with a condition of a high-volume manu-facturing (500’000 unit/year).21 Importantly, 46 % of the current fuel cell costs are due to the electrode only, because of the expensive Platinum Group Metal (PGM) catalysts. The rest are the costs of membranes, bipolar plates and oth-er.21 Therefore, improvements in the cathode kinetics and reductions of PGM catalyst loading are crucial.
Durability is another important issue, as stable performance of the FC sys-tems has not been established yet. When considering FC technology for trans-portation applications, the FC systems are required to achieve operational life-times comparable to ICE, i.e. 5000 hours/150000 miles.22 For the stationary applications requirement is 40’000 hours of reliable operation time at tem-peratures from -35 to +40 0C.22 During the operation under constant load fuel cells are relatively stable, with a low degradation rate of 1 - 2 μV h.13 However, in reality, if operated for the automotive applications, where the operation conditions are not stable (frequent start up/shut down), the degradation rates increase orders of magnitude. The main operation conditions, which cause the increase of the degradation rate, are: start-stop and load cycles, low or varying humidification, fuel starvation and temperatures higher than 90 0C.13
The components undergoing the degradation processes the most in PEMFCs are the electrodes and the membrane.22 In case of the electrodes, application of highly dispersed and nanostructured catalysts with high surface-to-volume ratio, i.e. increasing the surface area of the electrocatalyst, is crucial. This en-sures lower catalyst loading and higher current output of a fuel cell. Howev-INTRODUCTION
er, small nanoparticles supported on carbon supports, used in fuel cells, are thermodynamically unstable, and are undergoing degradation through several parallel mechanisms. These will be discussed in detail in the next Chapter 2.
Activity enhancement is required mainly for the cathode catalysts of PEM-FCs due to sluggish Oxygen Reduction Reaction (ORR). Improving the activity is the way to increase the efficiency and reduce Pt catalyst loading, therefore, cost of the energy generation by a PEMFC. Hydrogen oxidation reaction is fac-ile on Pt catalysts, whereas ORR is typically kinetically slow due to exception-ally strong O=O bond (498 kJ/mol).23 Of all pure metals Pt is the most active for the ORR reaction,9 on which the oxygen reduction to water occurs through a 4-electron process with a significant overpotential, η ≈ 0.3 V,24 which cor-responds to an open circuit voltage of ≈ 0.9 V. In order to keep the efficiency of a PEMFC as high as possible, it is necessary to keep the potential at which the ORR occurs as close as possible to its thermodynamic electrode value (1.23 V) and satisfactory reaction rates.25 Therefore, ORR catalysts with higher activity need to be developed, that will have direct implications on thermodynamic ef-ficiency, fuel cell operation stability, catalyst loading and, obviously, the price.
1.5 Improving performance of PEMFCs
In order to deal with above-mentioned problems novel durable and active catalyst materials are continuously proposed and assessed for the application in PEMFCs.
In order to improve durability and increase lifetime of the fuel cell catalysts, a strategy of the prevention of degradation processes is usually applied: ma-terials resistant to the degradation processes are developed. Nanostructured Pt catalysts of various shapes, such as nanowires,26 porous dendritic27 or star-like28 exhibited superior stability compared to the state of the art Pt/C cata-lysts. On the other hand, these materials generally have smaller active surface area per gram of the catalyst. Another approach of alloying of Pt with other metals, such as Au,29 Ce30 and La,30 also results in increased durability and in some cases – enhanced activity.30
Catalyst supports resistant to oxidation, such as carbon nanotubes (CNTs),31 carbon nano-networks (CNNs),32 hollow graphitic spheres (HGS),33 signifi-cantly improve durability of a fuel cell electrode. Additionally,
functionaliza-tion of these supports allows attachment of the catalyst nanoparticles to the support surface, therefore, limits their mobility and minimizes the agglomera-tion process, which leads to improved durability of the catalyst layer.
There are several strategies to increase ORR activity of the PEMFC catalyst. In the case of pure Pt, shape modifications influence not only durability, but also activity. Polycrystalline bulk Pt shows greater activity compared to na-noparticles of pure Pt.9 Similarly, Pt nanowires or Pt nanotubes in addition to greater stability, have significantly enhanced ORR kinetics.27 A success-ful strategy for the designing active ORR catalyst on the fundamental basis proposed by Norskov and co-workers9 involves coupling of the intermediate adsorbate states with metal d states. Then, so called “Volcano plots”9 are de-veloped based on the assessment of coupling strength between O, O2, or OH ORR intermediates on various metal surfaces. Successful demonstration of this theory was reported by Stamenkovic and co-workers,34 who achieved un-precedented high activities for Pt3M binary alloys (M = Ti, V, Fe, Co, Ni) with an outer layer of Pt. Alloying Pt with early transition metals, Sc or Y, enhanced the activity for ORR 2-folds or 6-10-folds respectively, while the stability in-creased as well.35 Exceptional activities are achieved for alloys of Pt with Ni36, 37 and other transition metals (Co, Cr, Cu, Mn, etc.). Enhanced activity of Pt was achieved with a hollow Pt nanoparticles with traces of Ni and Co.38 The main mechanisms of ORR activity improvement is due to modified electronic struc-ture Pt by the alloying metals, which results in slightly weaker binding of ORR intermediates compared to pure Pt.34
1.6 Project idea
In this thesis, I discuss a novel idea of FC catalyst regeneration to increase lifetime of the PEM fuel cell electrode/catalyst operation and, therefore, re-duce the catalyst costs. As many of the catalyst degradation mechanisms are difficult to avoid, the regeneration is alternative option to prolong catalyst life-time. In this thesis, we investigate fundamental aspects of Pt catalyst regenera-tion ex situ, which can be potentially applied in situ later on. The regeneraregenera-tion strategy consists of two steps: (i) Full or partial dissolution of coarsened Pt catalyst nanoparticles and (ii) Chemical or electrochemical redeposition of the dissolved Pt, see Figure 1.4. In the following thesis chapters I discuss disso-INTRODUCTION
lution aspects of Pt nanoparticles in mild and carbon-preserving conditions, which are essential for the subsequent redeposition of the nanoparticles on the same support and potential redeposition strategies. For the redeposition of Pt nanoparticles, we investigate electrodeposition technique and chemical reduc-tion in a templated medium, such as microemulsions.
1.7 References
1. Barbir, F., Chapter 1 - 1. Introduction. In PEM Fuel Cells, Barbir, F., Ed. Academic Press: Burlington, 2005; pp 1-16.
2. O’Hayre, R.; Cha, S. W.; Colella, W.; Prinz, F. B., Fuel Cell Fundamentals. Wiley:
2009.
3. Barbir, F., Chapter 4 - 4. Main Cell Components, Materials Properties and Pro-cesses. In PEM Fuel Cells, Barbir, F., Ed. Academic Press: Burlington, 2005; pp 73-113.
4. Sørensen, B., Chapter 3 - Fuel cells. In Hydrogen and Fuel Cells, Sørensen, B., Ed. Academic Press: Burlington, 2005; pp 113-207.
5. Cook, B. Engineering Science and Education Journal 2002, 11, (6), 205-216. 6. Hyundai FCEV out next spring, concept cars shown by Toyota, Daihatsu and
Hon-da. Fuel Cell Bulletin 2013.
7. Pollet, B. G.; Staffell, I.; Shang, J. L. Electrochimica Acta 2012, 84, (0), 235-249. 8. Verhelst, S. Proceedings of the IEEE 2014, 102, (10), 1399-1403.
Figure 1.4 A scheme of proposed in situ fuel cell regeneration strategy: coarsened/sintered
Pt catalyst on a carbon black support is partially/fully dissolved in a fuel cell and then rede-posited back on the same electrode via chemical/electrochemical deposition.
9. Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. The Journal of Physical Chemistry B 2004, 108, (46), 17886-17892. 10. Xu, Y.; Shao, M.; Mavrikakis, M.; Adzic, R. R., Recent Developments in the Electro-catalysis of the O2 Reduction Reaction. In Fuel Cell Catalysis, John Wiley & Sons, Inc.: 2008; pp 271-315.
11. Garland, N. L.; Benjamin, T. G.; Kopasz, J. P. In DOE fuel cell program: Durability technical targets and testing protocols, ECS Transactions, 2007; pp 923-931. 12. Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Applied Catalysis B:
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16. Bagotsky, V. S., The Long History of Fuel Cells. In Fuel Cells, John Wiley & Sons, Inc.: 2012; pp 25-40.
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20. Barbir, F., Chapter 3 - 3. Fuel Cell Electrochemistry. In PEM Fuel Cells, Barbir, F., Ed. Academic Press: Burlington, 2005; pp 33-72.
21. Spendelow, J. S.; Papageorgopoulos, D. C. Fuel Cells 2011, 11, (6), 775-786. 22. Borup, R.; Meyers, J.; Pivovar, B.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers,
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4, (6), 1685-1698.
25. Song, C.; Zhang, J., Electrocatalytic Oxygen Reduction Reaction. In PEM Fuel Cell Electrocatalysts and Catalyst Layers, Zhang, J., Ed. Springer London: 2008; pp 89-134.
26. Ruan, L.; Zhu, E.; Chen, Y.; Lin, Z.; Huang, X.; Duan, X.; Huang, Y. Angewandte Chemie International Edition 2013, 52, (48), 12577-12581.
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L.; Chorkendorff, I. Journal of Materials Chemistry A 2014, 2, (12), 4234-4243. 31. Hafez, I. H.; Berber, M. R.; Fujigaya, T.; Nakashima, N. Sci. Rep. 2014, 4.
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33. 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. Beilstein Journal of Nanotechnology 2014, 5, 44-67.
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38. Cantane, D. A.; Oliveira, F. E. R.; Santos, S. F.; Lima, F. H. B. Applied Catalysis B: Environmental 2013, 136-137, 351-360.
Abstract
PEMFC catalysts tend to degrade during the operation. This is one of the ma-jor issues to be resolved before the full commercialization. Prevention of the degradation processes is the main strategy proposed in the literature to solve the poor durability of the catalysts. Post-degradation treatment of PEMFC catalysts or regeneration, in order to increase the lifetime, has not been in-vestigated adequately. However, the regeneration approach can offer not only the lifetime increase, but also save the precious platinum catalyst, significant amounts of which are lost during the recovery and deposition processes. In this chapter we review the processes described in the literature to regener-ate fuel cell and other transition metal catalysts in order to compare to our ap-proach of regeneration through dissolution-redeposition process and to iden-tify future opportunities for the improvement. More importantly, examples of several regeneration approaches of Pt catalysts in other catalytic applications, which could be potentially implemented in fuel cells, are reviewed.
Regeneration
possibilities of
PEMFC catalysts
2.1 Durability issues of PEMFC catalysts
Pt and Pt based metal alloys have been typically proposed as electrocatalysts for the oxygen reduction reaction in PEMFCs.1 Usually, these catalysts are sup-ported on high surface area conductive carbon materials, such as Vulcan XC-72 carbon black, Ketjen black, carbon nanotubes, carbon nanofibers, etc.2 The catalyst layer of a typical PEMFC electrode consists of catalyst nanoparticles, carbon support and ionomer to ensure the proton conductivity.3 Under harsh PEMFC operation conditions, especially at the cathode, the catalyst layer of an electrode undergoes degradation processes, which can be categorized in fol-lowing types:4 (i) nanoparticle coalescence, (ii) Pt dissolution, (iii) Ostwald Ripening (iv) nanoparticle detachment, (v) carbon support corrosion, and in presence of impurities (vi) catalyst poisoning, see Figure 2.1.
Nanoparticle coalescence and agglomeration take place during operation of a PEMFC. Typically Pt nanoparticles of a small size (<10 nm) are used in PEM-FCs. The nanoparticles of such small size have increased curvature and there-fore, high surface energy (Gibbs-Thompson effect).5 That is why nanoparticles of few nanometers in size are not stable and tend to agglomerate and grow in size. Typically the catalyst nanoparticles are not attached to the carbon sup-port. Then the coalescence can take place, when nanoparticles migrate along the surface of a carbon support and get in contact.4
Pt dissolution can occur during the operation of a PEMFC on the cathode side due to an acidic and corrosive environment. Numerous studies were con-ducted on Pt dissolution in PEMFCs to confirm significant rates of chemical/ electrochemical Pt dissolution in acidic environment and transient potential conditions, such as load change or during the start-up and shutdown of a fuel cell.6, 7 The dissolution is especially severe for small Pt catalyst nanoparticles, which dissolve at potentials lower than bulk platinum, due to the above men-tioned Gibbs-Thompson effect.6
Ostwald Ripening processes are enhanced when polydisperse catalyst nano-particles are used in PEMFCs. During this process smaller nanonano-particles are dissolved and re-deposited on the larger ones. As a result nanoparticles grow in size, and similarly to the above mentioned mechanisms, the process takes place due to the driving force to reduce the surface energy of the nanoparticles.5
Detachment of nanoparticles mainly takes place due to weak interactions of REGENERATION POSSIBILITIES OF PEMFC CATALYSTS
Pt nanoparticles with the support. It is believed that carbon corrosion may lead to further weakening of those interactions.4, 8
Carbon support corrosion, thermodynamically, should take place at relative-ly low potentials E0
CO2/C = 0.207 V vs RHE.9 However, according to Kinoshita et al the surface groups passivate the surface of carbon for the further reaction,10 thus in practice the oxidation to CO2 takes place at potentials above 0.8 V and becomes severe at potentials above 1.2 V.9 The effects are: loss of structural integrity and reduced porosity, triggering the above mentioned degradation mechanisms. The corrosion also leads to increased hydrophilicity of the elec-trode and flooding effects, which on the other hand result in mass transport limitations for the reactant and products.4
Figure 2.1 Mechanisms of degradation of Pt catalyst nanoparticles in a PEMFC.
Catalyst poisoning is the fastest occurring degradation mechanism, which can significantly deteriorate catalyst activity during the operation of a PEMFC in presence of impurities, especially for the anode catalyst, due to impurities present in the hydrogen fuel.11 Hydrogen fuel produced from reformed hydro-carbons is known to contain impurities such as CO, H2S, SO2, etc.11 Addition-ally, fuel cell electrodes can be contaminated from the airborne impurities such as SO2, H2S, NO2 and COS.12, 13 Additionally, chlorinated greasing and cleaning agents, such as tetrachloroethylene (TTCE), may be introduced into a fuel cell during fuelling or vehicle maintenance.14 Furthermore, metal contamination of a PEMFC catalyst can take place when other components of a fuel cell start to corrode. If exposed to the above-mentioned contaminants, both catalysts, the cathode and the anode, would experience reduced activity.11, 13
2.2 Recovery of the catalyst performance
2.2.1 Catalyst recovery after poisoning
Various recovery strategies after catalyst poisoning are proposed depending on the type of poisoning. The regeneration of a fuel cell catalyst after poisoning can be classified in two main strategies: (i) chemical/electrochemical removal of the contaminants and (ii) physical desorption of the contaminants. Nor-mally both processes occur simultaneously, see Table 2.1.
When Pt catalyst is exposed to a fuel contaminated with SO2 during the ORR, it reacts with both O2 and SO2 simultaneously, the latter one forms chem-isorbed SOx species on the Pt surface.12, 16 The exact coordination of the SO
x on Pt surface is still unclear,12 however, it can completely block active sites of the catalyst leading to a fast performance decay.13
One of the strategies reported for the removal of sulfur contaminants is a po-tential cycling at room or elevated temperatures (<100 0C).12, 16-18 For instance, Gould et al reported a two step cleaning procedure. During the first step the feed, H2 and O2, is normally switched to H2-N2 couple, which leads to a drop of the cell voltage to 0.05 V vs RHE.12 This results in a reduction of the adsorbed sulfur species on the cathodic side to S0. In the next step, potential cycling REGENERATION POSSIBILITIES OF PEMFC CATALYSTS
procedure is applied, and the voltage sweep above 0.9 V vs RHE leads to an oxidation of S0 to form water soluble and easily removable sulfate:16
Pt-S0 + 4H
2O → SO42- + 8H+ + 6e- + Pt (Eq. 2.1) Another approach of the recovery from sulfur contamination (H2S, SO2) in-volves potential cycling in oxidative medium, in presence of oxygen, at various temperatures.18 Almost full recovery is achieved at potential cycling between 0.2 and 1.0 V at 70 0C in O
2-saturated 0.1 M H2SO4 solution. However, at lower temperatures (25 0C), similar recovery degree could be only achieved with higher upper potential limit, by potential cycling between 0.4 and 1.7 V. The authors suggested that faster recovery of the poisoned catalyst at higher temperatures occurs due to the parallel process of desorption of weakly bound sulfur species and their replacement with O2.18 Pt alloyed with transition met-als, such as Co and Ni, leads to a greater extent of poisoning, that is greater sulfur coverage on the catalyst surface. Interestingly, presence of those tran-sition metals also accelerates the performance recovery during the cleaning procedure. Pt3Co/C fully recovers the activity twice faster then Pt/C catalyst.16 Similarly, when FC catalysts are poisoned by CO, the electrochemical poten-tial cycling procedure allows full recovery from the poisoning.19, 20 Researchers in several studies managed to steadily operate a PEMFC with a hydrogen feed, contaminated with CO, by application of short potential pulses (tens of mil-liseconds) to strip off CO at room temperature and atmospheric pressure.19, 20 The transient on-line recovery of the catalyst poisoned with CO occurs due to fast electrochemical oxidation of CO during anodic polarization.19 It was also shown that it is enough to switch to a pure hydrogen feed for only 20 minutes during a FC operation to achieve almost full recovery from the poisoning.19 Likewise, when poisoned by NO2 exposure to a neat air/O2 during a FC opera-tion assists in a full recovery of activity in 24 hours.13
Striking, 98 % degradation of a PEMFC current density in 3 hours takes place when catalyst is exposed only to 30 ppm of TTCE.14 However, almost complete recovery of the electrochemical surface area (ECSA) is achieved in 1.5 hours by exposure of the catalyst (cathode or anode) to H/Ar (1/1) mixture at 60 0C and
2 atm.14 Important to note that, after prolonged exposure to TTCE it was im-possible to achieve complete recovery even after longer cleaning treatments.21
2.2.2 Catalyst recovery after sintering
Much more challenging is the regeneration of sintered PEMFC catalyst na-noparticles. The summary of recovery possibilities after sintering of Pt and few other metal catalyst nanoparticles is given in Table 2.2. One of the strategies of the ECSA recovery could be a roughening of the Pt catalyst surface, which would increase the catalyst surface area. This approach was actively studied in the 1980s. Kinoshita and co-workers22 showed that potential cycling between 0.05 and 1.4 V vs RHE, at a sweep rate of 45 mV s-1, in 1 M H
2SO4 and room temperature, led to a roughening of Pt foil, and therefore, increase of the ECSA. However, this strategy did not work on Pt nanoparticles supported on carbon (Pt/C) and resulted in the ECSA decay. Untereker et al23 achieved roughening of smooth Pt electrodes using Rotating Ring Disc Electrode (RRDE) technique via simultaneous dissolution/re-precipitation of Pt at very high scan rates (90 V s-1, 25 Hz, 0 – 1.8 V vs RHE). The authors showed that low rotation speed led to a rougher surface since higher concentrations of dissolved Pt were available in the vicinity of Pt electrode surface for re-precipitation. An increase of the ECSA by ≈ 140 cm2 mg-1 of Pt was achieved. Therefore, inability to increase the ECSA of Pt/C catalyst by Kinoshita et al22 is due to the slow potential cycling procedure, so diffusion of the dissolved Pt away from the catalyst surface pre-vented re-precipitation.
One of the few investigations, where above mentioned approach was applied for recovery of the ECSA of a platinum catalyst, was reported by Olender and co-workers.24 Following modified conditions were used: triangular wave po-tential cycling between 0 and 1.3 V, sweep rate from 1 V s-1 up to 100 V s-1 for 10 – 60 minutes. The authors could not achieve an increase of the ECSA of a fresh catalyst, however, they could recover 70 - 97 % of the ECSA for the aged catalyst with a periodic fast potential cycling treatment. The authors suggested that recovery of the ECSA was achieved because of the breakage of Pt agglom-erates and resulting re-dispersion of the nanoparticles. The breakage of the nanoparticles occurs due to a high charge injection during the fast potential cycling up to high voltages before the catalyst nanoparticles are fully sintered.24
In conventional heterogeneous catalysis based on metal nanoparticles sup-REGENERATION POSSIBILITIES OF PEMFC CATALYSTS
ported on solid supports the regeneration by redispersion of the sintered na-noparticles have been already reported for metals such as Pt,25-27 Au,28-30 Pd,31, 32 Rh,33, 34 Ni,30 etc., summarized in the Table 2.2.
An elegant approach of breaking nanoparticle agglomerates without etch-ing and successive reduction was proposed by Kartusch et al.28 Authors re-ported breakage of Au NP agglomerates (supre-ported on TiO2) of 8 - 10 nm into smaller nanoparticles of 2 - 3 nm in a liquid-phase during hydrogenation of 4-nitrobenzaldehyde, nitrobenzene and nitrosobenzene at 100 0C and 10 bar H2. Similar effects were also achieved in pure solvents, such as Toluene or THF in presence of H2 at room temperature or at 100 0C in presence of an inert gas. According to the authors, under reductive treatment the nanoparticle agglom-erates were unstable and broke into smaller, 2 nm size nanoparticles, because of the stronger interaction of Au with the support, across the oxygen vacancies of TiO2, than Au-Au interparticle interactions.28
Once nanoparticles are agglomerated and then fused, the breaking of the agglomerates is no longer possible and etching techniques are applied. For instance, Banerjee et al30 demonstrated a general approach for redispersion of transition metal nanoparticles (Au, Pd, Pt, Ru, Rh, Ni, Co, Fe, Ag, Cu) in a liquid-phase of halide tetraalkylphosphonium ionic liquids. The method in-volved oxidation of the nanoparticles and successive reduction in ionic liquids. The oxidation takes place when the nanoparticles are heated to 60 0C in the ionic liquid under O2 atmosphere roughly from 2 to 12 hours, depending on the metal, slowest for Pt and Pd.30 Afterwards, the reduction step involved a treatment of the formed solution of metal salts in the ionic liquid with excess of a reducing agent, LiBH4. According to the authors, presence of the halide ions in combination with the oxidative O2 environment at elevated tempera-tures (60 0C) allows facile etching of the large nanoparticles. Similarly, Sa et al29 instead of an application of the halide containing ionic liquids, used alkyl
halides, CH3J, for the oxidative redispersion of Au NPs performed at 240 0C. Thermal treatment of metal nanoparticles in various gas atmospheres was also applied in several studies for the regeneration of sintered catalysts.25, 27, 35-37 Nagai et al35 proposed a rapid oxidative re-dispersion of Pt exhaust catalyst (supported on CeZrY oxides) as an on-line approach to increase the catalyst lifetime. The procedure involved cycling of oxidative/reducing conditions at 400 - 800 0C. However, instead of the voltage cycling, as in one of the
above-mentioned techniques, the authors cycled gas mixtures: oxidative, 4 or 20 % O2 in He, and reducing, 3 % H2 in He. The alternation time was 60 s, and as a result the average size of Pt nanoparticles reduced from 4.4 to 2.7 nm. Ac-cording to the authors35 during the oxidation cycle PtO
x are formed, which have relatively strong interaction with the support (Pt-O-Ce bond), as a result PtOx are re-dispersed on the support. Afterwards, during the reduction cycle re-dispersed Pt oxides are reduced to yield smaller Pt nanoparticles, see Fig-ure 2.2. Galisteo et al25 reported a similar approach, using high temperature oxidation-redeposition of Pt NPs in the air stream at 550 0C in presence of 1,2-dichloropropane (DCP). Subsequently, the Pt oxides are reduced at 400 0C in presence of H2.
Recently, a phenomenon of catalyst disintegration into adatom complexes in the presence of oxidative gases at low temperatures and low pressures was reported in several studies.38 Reactant induced disintegration of catalyst nanoparticles takes place when metal-reactant bond strength is greater than metal-support or metal-metal interactions,38 see Figure 2.3. It was reported that CO disintegrates or breaks up Pt catalyst nanoparticles supported on vari-ous supports (KL zeolite,39, 40 TiO
2,41, 42 ZrO243) into nano-sized clusters, which return to their original morphology after removal of CO. Formed metal adatom complexes are not stable. Berko et al41 reported disintegration of Pt catalyst supported on TiO2 at low temperature of 300 K and 0.1 mbar pressure which leads to Ostwald ripening and growth instead of reduction of the size. Gold-smith and co-workers38 studied this phenomenon and presented in a recent report an ab initio thermodynamics study of metal nanoparticle (Pt, Pd, Rh) disintegration induced by CO and NOx at different temperature, pressure, and various NP size conditions. The motivation of the investigation was to under-stand how to prevent reactant-induced disintegration of catalyst nanoparticles or, more importantly in the context of this review, how to induce it for the re-dispersion of sintered catalyst nanoparticles, which can be done at relatively low temperatures.
In order to break transition metal agglomerates (Pt, Ag, Ni) into a smaller nanoparticles defragmentation or high velocity impaction was also report-ed.44-46 However, the technique requires complex devices and relatively harsh conditions, which would be problematic to apply for an in situ regeneration purposes.
2.2.3 Recovery of catalyst utilization
The state of the art catalyst materials also suffer from low catalyst utilization, which means lower electrochemically active surface area (ECSA) and, there-fore lower efficiencies.47 Additionally, the ECSA decays during the operation of a fuel cell, however, if the utilization can be increased after the decay, it can be used for the performance recovery. He et al reported a method in order to
Figure 2.2 Proposed mechanisms of Pt catalyst regeneration by an in situ re-dispersion: (i)
Formation of Pt oxides on the NP surface, (ii) Migration of PtOx on the support and attach-ment to it via Pt-O-Ce bond of the CeZrY support, (iii) breakage of the Pt-O-Ce bond, reduc-tion of the oxides and formareduc-tion of smaller Pt NPs. Illustrareduc-tion from Ref 38.
Figure 2.3 Disintegration of metal nanoparticles into adatom complexes induced by
re-actants when the metal-reactant bonding is stronger then metal-metal and metal-support interactions. The disintegration takes place when the Gibbs free energy of disintegration is negative. Illustration from Ref 41.
recover the fuel cell performance by increasing the utilization of the catalysts on both sides of a PEMFC, anode and cathode. The procedure for the catalyst performance increase involved: (i) filling anode compartment with H2, cathode with N2 and application of a 200 mA cm-2 current for hydrogen evolution for 30 minutes; same procedure is repeated for the other side. The authors explain an increase in the catalyst utilization, by an increase of the porosity and tortu-osity, and related improvement of a three-phase contact (electrolyte, catalyst, reactants) of the electrode after the recovery procedure.48
2.3 Discussion
2.3.1 Preventive measures
Most of the PEMFC catalyst degradation mechanisms can be prevented, and currently this is the main strategy to deal with them.4 Often proposed preven-tion approaches lead to more complex electrode designs, complex catalyst formulations, additives and extra costs. For instance, catalyst poisoning can be prevented by introducing additional reformer of the fuel before the feed enters a fuel cell20 or by utilization of Pt-based alloy catalysts, which are resistant to poisoning, e.g. Pt-Ru against CO poisoning.20 In order to prevent coalescence, several preventive measures can be employed: (i) homogenous dispersion and relatively large interparticle distance on a support, so the NPs are not in con-tact and do not coalesce,4 (ii) restricting nanoparticle mobility on the support by improving support-catalyst interaction.4, 8 Latter one also prevents nano-particle detachment. Ostwald ripening can be prevented by employing highly monodisperse catalyst nanoparticles with good dispersion on a support and large inter-particle distances.4 Much more problematic is prevention of the fuel cell catalyst dissolution, which is caused by a harsh and corrosive envi-ronment and transient voltage fluctuations during the operation, start-up and shutdown of a fuel cell.4 Adzic and co-workers49 offered an original solution to this problem, which involved introduction of a sacrificial metal, such as Pd, alloyed with Pt, then Pd is dissolved during the operation of a fuel cell, whereas Pt is undamaged. Nevertheless, when using sacrificial metal to prevent Pt dis-solution, one should take into account the fact, that dissolved sacrificial metal can precipitate in the membrane or on the anode side of the fuel cell, where H2 REGENERATION POSSIBILITIES OF PEMFC CATALYSTS
is supplied, causing undesired effects.
The degradation of a fuel cell electrode via carbon corrosion mecha-nism is often prevented when using novel oxidation resistant carbon supports (carbon nanotubes, carbon nano networks, etc.)2 and non-carbon support ma-terials (carbides,50 oxides51).
2.3.2 Regeneration possibilities
The degradation mechanisms mentioned above can be prevented. However, so far, to our best knowledge, the PEMFC catalyst regeneration or recovery approaches mainly deal with the poisoning issue. The problem of poisoning of the catalysts can be resolved with the strategies described above, however the problem of coarsening/sintering and dissolution is mainly resolved through mitigation strategies.
The contamination problem is a short-term problem, which occurs relatively fast, and therefore it is essential to tackle it in an easy manner. When compar-ing, the recovery efficiencies and complexity of the procedures, one can notice that CO, NO2 removal can be easily performed even during the operation of a PEMFC, just by switching to a neat fuel, however, additional potential cycling steps are required for the recovery from sulfur poisoning (Table 2.1). Neverthe-less, prolonged exposure to CO, for instance, can cause more severe and ir-reversible damage, because presence of CO is found to promote Pt dissolution processes.52 Therefore, it is of a great importance to prevent CO poisoning by removing the CO from the fuel. Alternatively, continuous recovery treatment can be performed, as in the procedure of Adams et al20 and Lu et al,19 (Table 2.1).
The problem of sintering is a long-term problem. Little work has been done on recovery of the coarsened PEMFC catalyst. However as mentioned above various approaches have been applied for the transition metal catalyst redis-persion in other applications, see Table 2.1. Low temperature approaches men-tioned above can be applied in fuel cells as well. Many of those regeneration processes are based on specific support-NP interaction. Therefore, application of non-carbon supports in PEMFCs, e.g. TiO2,51 can introduce strong support - NP interactions, and allow in situ redispersion of Pt with above mentioned techniques, for instance as the procedure of redispersion of Au NPs on TiO2.28 On the other hand, oxide supports would also cause the fixation Pt NPs due
to stronger interactions,51 decrease nanoparticle mobility and, therefore, sta-bilize them against degradation. Additionally, application of the metal oxides as catalyst supports for PEMFCs could allow regeneration treatments at much higher temperatures (Table 2.1), compared to the carbon supports, which are easier to oxidize. However, an in situ treatment at high temperatures (>100 0C) would be still challenging, as it would damage the proton exchange membrane (PEM). Application of halide containing ionic liquids proposed by Banerjee et
al,30 could be an option for an in situ recovery of the ECSA of Pt NPs, as the conditions applied are relatively mild. However, one should definitely consider two factors: (i) complete removal of the halides after the regeneration, as they promote dissolution of Pt6 and (ii) the effects of halides on the gas diffusion layer (GDL) and PEM of a fuel cell.
Additionally, novel regenerative electrode designs could be a solution for the in situ regeneration of the PEMFC catalysts. For instance, reversible Pt cata-lyst assembly could be potentially used for the regeneration purposes. Chen et
al53 reported reversible assembly of Pt nanoparticles into nanochains (PtNCs). Another option could be an implementation of a catalyst slurry, which could be circulated in a fuel cell and an additional regenerative chamber, where high temperature or a etching-reduction treatment approaches mentioned above could be applied.
Few of the degradation mechanisms of NPs, such as carbon corrosion, nano-particle detachment or dissolution and redeposition in the fuel cell membrane cause irreversible damage. For instance, once parts of the carbon support are corroded to CO2 causing Pt detachment, problems with the electron transfer and increased hydrophilicity of the support, none of the above mentioned re-generative techniques or any other approach would allow recovery.
2.4 Conclusions
Numerous studies on the fuel cell catalyst degradation mechanisms and the ways to prevent them are reported in the literature. Typically, these ap-proaches involve introduction of additional components in the fuel cell system, application of complex catalyst formulations, such as various metal alloys, functionalized supports, etc. Often not all the degradation mechanisms can be addressed at the same time, and degradation still takes place. Therefore, it is REGENERATION POSSIBILITIES OF PEMFC CATALYSTS
important to introduce regenerative approaches for PEMFC catalysts in order to further prolong catalyst lifetime. The topic was not investigated adequately for the fuel cell catalysts and only few regeneration methods were reported in the literature. These methods mainly focus on solving the problem of cata-lyst poisoning, by application of cleaning procedures, such as oxidation of the impurities via potential cycling. Only few of the methods can deal with the degradation issues such as nanoparticle agglomeration, one of the major fuel cell degradation mechanism. Successful regeneration approaches of catalyst nanoparticles in other heterogeneous catalysis applications, e.g. automotive exhaust catalyst nanoparticles (Pt, Rh, Pd), can be potentially applied in fuel cells. The regeneration strategies presented in this chapter may demand modi-fications of the current PEMFC setup before implementation. Once a proper in situ regeneration process is proposed not only the lifetime of a fuel cell will be prolonged resulting in reduced costs, but also fuel cell catalyst handling can be optimized, as currently, significant amounts of nanoparticles are lost and released into the atmosphere during catalyst handling, operation and recovery processes.
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Part of the Chapter 3 is submitted for publication in ChemSusChem. Environmentally friendly carbon-preserving recovery of noble metal from supported catalysts.
Abstract
The electrochemical dissolution of platinum can be used as an environmen-tally friendly way to recover or regenerate platinum fuel cell catalyst. We man-aged fully dissolve platinum nanoparticles from a fuel cell electrode in very mild conditions without damaging other parts of the electrode, such as the car-bon support. Following conditions were employed: electrochemical potential cycling between 0.5 - 1.1 V and at a 50 mV s-1 scan rate, at room temperature in 0.1 M HClO4 and 0.1 M HCl. Dissolution rates as high as 22.5 μg cm-2 cycle-1 were achieved, which ensured a relatively short dissolution timescale of 3 - 5 hours for a platinum loading of 0.35 mg cm-2 on carbon.
This method has two significant implications:
i. In situ regenerate fuel cell catalyst, this could potentially double the life-time and avoid platinum losses during its handling in the nanoparticle deposition and recovery processes.
ii. Recovery of the degraded platinum nanoparticles from the fuel cell in a simple and environmentally friendly way.
