Proton exchange membrane fuel cells (PEMFCs), in their low temperature (LT, 60-80 °C) and high temperature (HT, 120-200 °C) variants, can be used for broad applications in the automotive and stationary sector. However, the most commonly used PEMFC catalyst, consisting of platinum nanoparticles supported on a carbon black, suffers from degradation under the relevant working conditions e.g. low pH environment and potentials in the range of 0.6-1.5 V vs. RHE. These processes include carbon corrosion, Pt dissolution and detachment as well as agglomeration.[1-3] Especially, carbon corrosion leads to an increase of the support hydrophilicity, decreased conductivity and loss of Pt particles resulting in an overall performance loss of PEMFCs.[1] Different studies already reported a positive effect of metal oxide-carbon composite supports using TiO2-Vulcan®XC-72[1,2], SnO2-Vulcan®XC-72[2,3], or fluorine-doped SnO2 on reduced graphene oxide[4] on the carbon and Pt stability. Moreover, Ruiz Camacho et al. showed higher ORR activity for Pt/TiO2-Vulcan and Pt/SnO2-Vulcan compared to Pt/C and a lower potential for CO oxidation during stripping experiments.[2] This can be beneficial for PEMFC operation with reformate. To further push the activity and stability of Pt/metal oxide-carbon catalysts homogeneous distribution of metal oxide and Pt nanoparticles is necessary. In contrast to previous studies, that uses low surface area Vulcan®XC-72[1-3] the implementation of high surface area Black Pearls (BPs) can enable more homogenous distribution of metal oxides and Pt nanoparticles which can positively impact the activity. In this comparative study, Pt/metal oxide-carbon catalysts using SnO2 and TiO2 nanoparticles on BPs are analyzed towards their physical properties and electrochemical ORR activity and stability. Metal oxide/carbon composites were fabricated by deposition of 50 wt.% commercial SnO2 or TiO2 nanoparticles on Black Pearls® 2000. Thermogravimetric analysis (TGA) reveals the successful deposition of metal oxides TiO2 (41 wt.%) and SnO2 (47 wt.%) on BP. Next, deposition of 40 wt.% Pt nanoparticles with diameters between 1-2 nm on the metal oxide-BP composites is done. Transmission electron microscope (TEM) images display successful deposition of Pt with uniform distribution of Pt for both composite catalysts in Figure 1 a) and c). The elemental mapping of Pt and Sn or Ti, using scanning TEM with energy dispersive spectroscopy (EDS) for analysis of the interaction between metal oxide displays homogenous distribution of Pt over the metal oxide-BP supports (Fig 1, b), d)). In the case of Pt/SnO2-BP also uniform Sn distribution is observed whereas for Pt/TiO2-BP partial agglomeration of TiO2 is found. Further analysis of Pt and metal oxides will be given using high resolution-TEM for analysis of lattice distance and ICP-MS for determination of Pt content. Moreover, electrochemical characterization using rotating ring disc electrode will be carried out for comparison of ORR activity and selectivity. Furthermore, an accelerated stress test including 5000 cycles in the range of 0.6-1.5 V vs. RHE in N2-saturated 0.1 mol L-1 HClO4 is applied to analyze the overall catalyst stability. The results will reveal the most promising candidate in terms of activity and stability for future application in HT-PEM half- and single-cell setups. References: [1] S. von Kraemer, K. Wikander, G. Lindbergh, A. Lundblad, A. E. C. Palmqvist, J. Power Sources, 180, 185 (2008). [2] B. Ruiz Camache, C. Morais, M.A. Valenzuela, N. Alonso-Vante, Catal. Today, 202, 36 (2013). [3] J. Parrondo, F. Mijangos, B. Rambabu, J. Power Sources, 195, 3977 (2010). [4] D. Schonvogel, J. Hülstede, P. Wagner, A. Dyck, C. Agert, M, Wark, J. Electrochem. Soc., 165 (6) 3373 (2018). Figure 1
Polymer electrolyte membrane fuel cells (PEMFCs) are recognized as one of the renewable energy sources in portable, automobile, and stationary applications. Currently, both low temperature (LT) and high temperature (HT) PEMFCs use platinum group metal (PGM) based catalysts for oxygen reduction reaction (ORR) containing usually Pt nanoparticles on carbon black. To reduce the total cost of PEMFC stack, worldwide researchers have considerable attention to find an inexpensive alternative catalyst. Recently, metal-nitrogen-carbon (M-N-C) compounds such as Fe-N-Cs are the most promising PGM-free catalysts for ORR. However, they suffer from insufficient volumetric activity and electrochemical stability in PEMFCs. [1,2] Xiao et al. have proven an improved electrochemical stability of Pt-Fe-N-C electrocatalysts consisting of atomically dispersed Pt and Fe atoms or Pt-Fe alloy nanoparticles in comparison with Fe-N-C only. [3] Mechler et al. have reported that the addition of 1-2 wt.% Pt in hybrid Pt/Fe-N-C catalyst performs well with an increased stability in LT-PEMFCs. [4] Liao et al. have recently shown better ORR activity of Pt/Fe-N-C than Pt/C. [5] With the overriding goal of reducing LT- and HT-PEMFC production costs, Pt/Fe-N-C activity, selectivity and stability with systematically reducing the Pt-content has not been investigated yet. In this study, Pt/Fe-N-C hybrids are synthesized using PMF-011904 from Pajarito Powder (USA) as catalyst support and wet-chemically precipitated Pt nanoparticles with targeted Pt-contents of 40, 5, 1 and 0 wt.%. First, ICP mass spectrometry is used for Pt quantification along all catalysts, and transmission electron microscopy is carried out to investigate morphology and Pt nanoparticle diameter and distribution on the Fe-N-C catalyst. Second, ORR activity and selectivity is investigated by rotating ring disc electrode (RRDE) experiments in 0.1 mol L-1 HClO4, and electrochemical surface area of Pt is calculated by hydrogen underpotential deposition and CO stripping voltammetry. Last, accelerated stress testing (AST) is performed with 5,000 triangle potential cycles between 0.6–1.5 VRHE to evaluate the catalyst stability. Figure 1 shows the initial ORR curves during the RRDE experiments. The mass activities of 40, 5 and 1 wt.% Pt/Fe-N-C determined at 0.9 VRHE are 222.5 ± 41.2 A gPt -1, 170.8 ± 80.4 A gPt -1 and 49.0 ± 8.6 A gPt -1, respectively. Thus, the mass activity depends on Pt-content in the hybrid catalyst strongly. In addition, the other electrochemical characterization results are also going to be discussed in terms of catalyst selectivity, stability and the correlation with morphological aspects during the presentation. Figure 1 ORR curves in O2-saturated 0.1 mol L-1 HClO4 electrolyte with rotation speed of 1,600 rpm and scan rate of 5 mV s-1 (averaged results of three separate electrodes per each catalyst). [1] Y. He, S. Liu, C. Priest, Q.Shi , G. Wu, Chem. Soc. Rev. 2020, 11, 3484–3524. [2] T. Reshetenko, A. Serov, M. Odgaard, G. Randolf, L. Osmieri, A. Kulikovsky, Electrochem. Commun. 2020, 118, 106795. [3] F. Xiao, G.-L. Xu, C.-J. Sun, I. Hwang, M. Xu, H.-W. Wu, Z. Wei, X. Pan, K. Amine, M. Shao, Nano Energy 2020, 77, 10592. [4] A. K. Mechler, N. R. Sahraie, V. Armel, A. Zitolo, M. T. Sougrati, J. N. Schwämmlein, D. J. Jones, F. Jaouen, J. Electrochem. Soc. 2018, 165, F1084. [5] W. Liao, S. Zhou, Z. Wang, F. Liu, H. Pan, T. Xie, Q. Wang, ChemCatChem 2021, 13, 23, 4925-4930. Figure 1
For further commercialization of the proton exchange membrane fuel cell technology an improvement of the stability of commonly used Pt-based oxygen reduction reaction (ORR) catalysts is necessary. In this study SnO2- and TiO2-carbon nanocomposites using high surface area Black Pearls (BP) and commercial metal oxide nanoparticles were investigated as support materials for Pt. Homogenously distributed 40 wt.% Pt catalysts were obtained. When using TiO2-BP support an outstanding intrinsic activity but no stability improvement was observed, whereas the use of SnO2-BP as Pt support lead to a slightly lower ORR activity but an increased catalyst stability.
Two types of graphite composite based materials, BPP-01 with higher amount of polymer binder in comparison to BPP-02, are investigated and reported for their suitability as bipolar plates (BPPs) in polymer electrolyte membrane fuel cells (PEMFC). Through-plane electrical conductivity of pristine BPP-01 and BPP-02 are 10.31 and 11.38 S cm-1, respectively. The electrochemical behaviour of aged BPPs is examined by cyclic voltammetry (CV) in sulfuric acid. Chemically aged BPP-01 exhibits lower double layer capacitance than BPP-02. In order to observe cathodic and anodic corrosion behaviour of BPPs, potentiostatic polarisation experiments are also performed. BPP-02 undergoes higher corrosion rate than BPP-01 during cathodic polarisation. BPP-02 surface change can be observed even with bare eyes. Conversely, physical changes do not occur after anodic polarisation for both BPPs which were thoroughly characterised by profilometer, optical microscope and micro-computed tomography (μ-CT). The higher polymer content based graphite composite bipolar plate (BPP-01) endures the potential applied in PEMFC applications.
Nowadays polymer electrolyte membrane fuel cells (PEMFC) become commercially established systems used in automobile, stationary and portable power generation industry which provide an improved kinetic, high efficiency, zero emission and high tolerance to the impurities. A standard catalyst used in PEMFC is based on Pt nanoparticles on carbon support materials. However, high cost of this critical raw material stimulates interest in the development of platinum group metals-free electrocatalysts (1). Fe-N-C materials are one of the promising non-precious metal electrocatalysts for these fuel cells which have recently reached outstanding performance in terms of oxygen reduction reaction (ORR) activity (2). However, the volumetric activity and durability of Fe-N-C catalyst is still significantly lower compared to Pt/C in PEMFC mainly due to their lower turnover frequency and active sites density (3).Therefore, in this work, a Black Pearl (BP) based Fe-N-C catalyst is used as ORR active support material for Pt-nanoparticles to investigate synergetic effects of active Pt and Fe-N sites towards stability and volumetric activity. We present a comparative electrochemical characterization of Pt/Fe-N-C and Pt/BP electrocatalysts with loadings of 0.01, 0.1 and 1 wt.% Pt. The measurements were done in terms of rotating ring-disk electrode experiments in 0.1 M HClO4 electrolyte under accelerated stress test (AST) during 5000 cycles in potential range 0.6 – 1.5 V vs RHE in N2-saturated electrolyte, in order to provoke Pt as well as carbon support degradation. The electrochemical surface area (ECSA) of the Pt was determined by hydrogen underpotential deposition (HUPD) and CO stripping methods before and after AST for Pt/BP catalysts. However, for Pt/Fe-N-C the ECSA determination was impossible because for metal-oxide supported Pt catalysts, a straightforward analysis of HUPD and CO methods fails to give meaningful values. The RRDE experiments revealed that ORR mass and specific activities were significantly decreased after AST especially for Pt/BP (Fig. 1) which was attributed to the platinum dissolution as well as carbon support degradation during cycling.1. E. Eren, N. Özkan, Y. Devrim, Int J Hydrogen 2020, 45 (58), 33957-33967.2. Y. He, S. Liu, C. Priest, Q.Shi , G. Wu, Chem. Soc. Rev., 2020, 11, 3484–3524.3. T. Reshetenko, A. Serov, M. Odgaard, G. Randolf, L. Osmieri, A. Kulikovsky, Electrochem Commun 2020, 118, 106795.Figure 1. Cathodic scan of ORR curves of 0.1 wt.% Pt/BP catalyst with 40 µg cm-2 Pt loading in O2-saturated 0.1 M HClO4 at scan rate 5 mV s-1 by 1600 rpm before and after AST. Figure 1
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