A Pathway towards Pt-free Cathodes in High-Temperature Proton Exchange Membrane Fuel Cells

The high temperature proton exchange membrane fuel cell (HT-PEMFC) has several advantages compared to its low temperature (LT) counterpart. The typical operation temperature of 160 °C enables an easier heat management, and the omission of humidification. However, due to the partial blocking of the cathodic and anodic Pt catalyst by phosphates from the phosphoric acid-doped membrane, higher catalyst loadings compared to LT-PEMFC of 0.86 mgPt cm-2 per electrode are commonly employed.[1] To increase the competitiveness of HT-PEMFCs implementation of Fe-N-Cs is a promising option for reduction of catalyst costs. In this study, we give an overview about the application of different Fe-N-C catalysts in Pt free HT-PEMFC cathodes.[2] Moreover, we show their application in hybrid PtNi/C+Fe N C cathodes.[1] The complete replacement of Pt catalyst by Fe N C in the cathode results in low performance[2] and a strong voltage decay within the first 60 hours of HT-PEMFC operation. In contrast, a hybrid MEA with reduced Pt-loading displayed a more comparable performance to commercial MEA (Celtec®-P1200) and constant voltage over 60 h. Furthermore, it was found that the typical activation procedure of HT-PEMFC MEAs (around 60 h constant load) is not sufficient for hybrid MEAs. There, a voltage increase over the first 240 h of operation was observed.[1] These results give the basis for further optimization of Pt-free Fe N C electrodes. Furthermore, the potential of hybrid MEAs for Pt-loading reduction in HT-PEMFC is pointed out

Investigation of Pt/Fe-N-C Hybrids Towards ORR in Acidic Environment

Metal-nitrogen-carbon (M-N-C) compounds such as Fe-N-Cs are currently the most promising platinum group metal free catalysts for oxygen reduction. Regarding the overriding goal of reducing PEM fuel cell production costs by reducing the amount of platinum, the use of Fe-N-Cs as catalytic active support is investigated in this study. Activity and stability of Pt in different contents on a commercial Fe-N-C is compared to Pt on a typical carbon black. Pt nanoparticles are well-distributed on both support classes. However, electrochemical surface and mass activity of Pt is lower on Fe-N-C compared to carbon black. Although Pt does not profit in any catalytic matter from interaction with Fe-N-C, the Pt/Fe-N-C in total has a boosting effect on ORR activity being important for future strategies to lower the Pt content in PEM fuel cells

Effect of Fe-N-Cs as Catalytic Active Support for Platinum towards ORR in Acidic Environment

Metal-nitrogen-carbon (M-N-C) compounds such as Fe-N-Cs are currently the most promising platinum group metal free catalysts for oxygen reduction in acidic environment. Regarding the overriding goal of reducing PEMFC production costs by reducing the platinum content, the use of Fe-N-Cs as catalytic active support for low Pt amounts is investigated in this study. Activity and stability of Pt in different contents on a commercial Fe-N-C is compared to Pt on a typical carbon black. Pt nanoparticles are well-distributed on both support substrate classes. Although the electrochemical surface and mass activity of Pt is lower on Fe-N-C compared to carbon black, the Fe-N-C has a contribution to total ORR activity depending on the Pt/Fe-N-C ratio, which is quantified. In the low Pt content case of 1 wt, the ORR activity is increased by factor of two in presence of Fe-N-C. This boosting effect on ORR activity is important for future strategies to lower the Pt content in PEMFCs

Cross-linked Polybenzimidazole/sulfonated copper (II) phthalocyanine tetrasulfonic acid based composite membranes for HT-PEMFCs

Till now, phosphoric acid-doped polybenzimidazole (PBI) is the most promising high temperature proton exchange membrane (HT-PEM) for HT-PEMFC applications. However, PBI membranes displayed a tradeoff relationship between phosphoric acid (PA) uptake and mechanical stability. Mainly, membrane creep under compressive loads in the fuel cell has been considered for fuel cell performance failure [1]. In order to overcome these issues, significant research focused on the development of crosslinked (i.e., ionic, covalent, or both ionic and covalent bonds) PA doped PBI membranes. In our earlier study, ionically crosslinked HT-PEM modified into covalently crosslinked one via thermal curing technique [2]. In this workshop, the authors present the successful incorporation of water-soluble copper (II) phthalocyanine tetrasulfonic acid tetrasodium salt (IEC; 4.46 meq/g) as a crosslinking inorganic filler in the synthesized PBI polymer matrix. DLR membrane based MEA’s H2/air fuel cell performance is compared with commercial Celtec® MEA at 160 ºC (Fig.1). In addition, the newly developed composite membranes properties are going to be discussed during the presentation

Operando X-ray absorption spectroscopy of Fe–N–C catalysts based on carbon black and biomass-derived support materials for the ORR

Iron nitrogen carbon (Fe–N–C) catalysts are among the most promising non-platinum group metal catalysts for the oxygen reduction reaction (ORR). Their activity and stability are considerably influenced by the structure of the C-support. New biochar materials offer native heteroatom doping, making them a promising precursor for Fe–N–C catalysts. In this study, we apply operando X-ray absorption spectroscopy at the Fe K-edge to characterize the atomic Fe-based active sites of a commercial Fe–N–C catalyst, a carbon black-based catalyst as well as a novel biomass-based Fe–N–C catalyst. We compare the density and the potential-dependent nature of the FeNx-type active sites during operation. Our results demonstrate that the novel biomass-based catalyst exhibits a higher active-site density compared to commercial and carbon black-based Fe–N–C catalysts. Moreover, dynamic detection of the Fe K-edge intensity during potential cycling reveals that their reversible iron redox potential is lower compared to that of conventional catalysts. Evaluation of the Fe K-edge shift as well as of the extended X-ray absorption fine structure (EXAFS) suggests hetero-atom doping and iron under-coordination as potential causes for the observed differences. These insights open the pathway to develop new optimization strategies for Fe–N–C catalysts based on biomass support materials

Non-Precious Metal Catalysts Based on Activated Biochar for the Oxygen Reduction Reaction in High Temperature Proton Exchange Membrane Fuel Cell

The high-temperature proton exchange membrane fuel cell (HT-PEMFC) plays an essential role for energy conversion of green hydrogen to electricity contributing to the target of CO2 neutral energy supply. However, the high platinum catalyst loading which is necessary in HT-PEMFC due to partial poisoning of the catalysts by phosphates originating from the phosphoric acid-doped membrane, is one major contribution to the material costs. Therefore, the reduction but also replacement of Pt catalysts in HT-PEMFCs is in focus of research. In case of the oxygen reduction reaction Fe-N-C catalysts are promising candidates to replace Pt-based catalysts, since they show no poisoning by phosphates and activities close to Pt/C. However, this catalyst class suffers from low volumetric activity and stability mainly due to carbon corrosion. In this work, activated lignocellulosic biomasses are investigated as novel sustainable carbon supports with native heteroatom doping in Fe-N-Cs for application in HT-PEMFC gas diffusion electrodes (GDE). Two chemical activation procedures are used for generation of activated biomasses with different porosities. Pyrolysis and KOH activation for rye straw is done for generation of a microporous high surface area carbon. H3PO4 activation of rye straw and coconut shells is carried out to yield a mesoporous carbon. Implementation of the three biomass-based supports and two common oxidised carbon blacks Vulcan® and Black Pearls® in Fe-N-C synthesis, reveals a homogeneous incorporation of Fe and N in the H3PO4 activated biomasses and oxidised Black Pearls. In the case of oxidised Vulcan and the KOH activated biochar iron containing particles like iron carbide are found. Comparison of the physical parameters of the supports and Fe-N-Cs reveals that a surface area higher than 800 m² g-1, presence of mesopores and low amounts of amorphous carbon are beneficial for Fe-Nx site incorporation. The atomically dispersed Fe-N-Cs reveal high mass activity whereas low mass activities for catalysts containing iron particles are shown. A 50 % higher stability in terms of mass activity loss for the H3PO4 activated biomass-based Fe-N-Cs against carbon corrosion is observed. This is attributed to the presence of P species which inhibit electron withdrawal from the carbon network. GDEs are fabricated using ultrasonic spray coating and doctor blade coating. The GDEs fabricated by ultrasonic spray coating display thinner catalyst layers that lead to low performances in GDE half-cell measurements in conc. H3PO4 at 140 °C due to suspected H3PO4 flooding of the GDE which was not the case in HT-PEMFC for the Black Pearl-basedand commercial Fe-N-C. Biomass-based GDEs display poor performance in GDE half-cell measurement and HT-PEMFC independent of fabrication method. This is attributed to a rough catalyst layer surface due to larger catalyst agglomerate sizes and a higher hydrophilic character which also can induce phosphoric acid flooding. A 90 h constant load HT-PEMFC test shows fast activity decay for all tested catalysts within the first 24 h of operation which is attributed to carbon corrosion and deactivation of active sites according to electrochemical characterisation and post-mortem analysis. In conclusion, the suitability of H3PO4 activated lignocellulosic biomasses as sustainable support for Fe-Nx sites is shown, whereas KOH activated biochars are not suitable. A higher stability against carbon corrosion due to the presence of phosphor species and higher amounts of nitrogen species in the biomass-based Fe-N-Cs compared to Black Pearl-based Fe-N-C is revealed. However, the agglomerate size of the biomass-based Fe-N-Cs and hydrophilic character has to be adjusted to overcome challenges like sedimentation of catalyst ink or distribution of hydrophobic binder during GDE fabrication to increase the performances in HT-PEMFC

PAFC, HT-PEMFC cathode

HT-PEMFC and PAFC cathodes are constituted similarly in terms of gas diffusion layers and applied catalysts. In general, Pt-alloy catalysts with loadings up to 1 mgPt cm−2 are commonly employed due to partial poisoning of the Pt catalyst through the adsorption of phosphates present from the phosphoric acid electrolyte in both FC types. This chapter gives an overview of the theoretical background of oxygen reduction reaction and discusses the state-of-the art cathodes with catalysts and supports. Relevant catalyst degradation pathways present in HT-PEMFC as well as PAFC are shown. The specification of gas diffusion electrodes as well as their fabrication and characterization are described followed by an overall trend of current research topics in HT-PEMFC toward reduction of Pt loading

Towards the Performance and Stability of Different Fe-N-C Catalysts in the High Temperature Proton Exchange Membrane Fuel Cell

The operation temperature of around 160 °C of the high temperature proton exchange membrane fuel cell (HT-PEMFC) enables a higher tolerance towards contaminants like CO. This makes the operation with reformate from green methanol possible, which is beneficial for volume critical application. However, due to the presence of phosphates from the acid-doped polymer membrane the commonly used expensive Pt catalyst is partly poisoned making high Pt loadings up to 1 mgPt cm-2 per electrode inevitable [1]. At this point, less expensive Fe-N-Cs provide a promising alternative to Pt based catalysts with a better tolerance towards phosphates. In this study, we present a comparison of four Fe-N-Cs towards their application in HT-PEMFC. This includes a commercial PMF catalyst (Pajarito Powder, 011904), a Black-Pearl (BP)- and two activated biomass-based Fe-N-Cs. Physical characterization reveals homogenous Fe and N distribution, higher contents of oxygen-containing functional groups for biomass-based Fe-N-Cs and differences in porosity [1,2]. Mass activities of the Fe N Cs determined by thin-film analysis in 0.5 M H3PO4 display values in range of 1.2-2.5 A g-1 (at 0.8 VRHE). The implementation of the Fe N Cs in gas diffusion electrodes (GDE) results in more inhomogeneous GDE structures for the biomass-based Fe N Cs [3]. Moreover, the application of these GDEs as cathodes in HT-PEM single-cells show the lowest performances within this series. A constant load HT-PEMFC test over 90 h shows strong performance losses of 15 26 % within the first 24 h for all Fe-N-Cs. Cyclic voltammetry after 50 h, end of test characterization as well as post-mortem analysis of membrane electrode assembly (MEA) e.g. using µ computed tomography (µ-CT) are used to identify degradation processes like carbon corrosion. This study helps to understand the impact of catalyst and GDE structure on HT-PEMFC performance and stability which is necessary for further catalyst and GDE developments. [1] R. Zeis, Beilstein J. Nanotechnol. 2015, 6, 68. [2] J. Müller-Hülstede, D. Schonvogel, H. Schmies, P. Wagner, A. Dyck, M. Wark, ACS Appl. Energy Mater. 2021, 4, 7, 6912. [3] J. Müller-Hülstede, T. Zierdt, H. Schmies, D. Schonvogel, Q. Meyer, C. Zhao, P. Wagner, M. Wark, J. Power Sources 2022, 537, 231529

Effect of Different Fe-N-C Catalysts on the Performance and Stability of HT-PEMFCs

High electrode loadings up to 1 mgPt cm-2 are commonly used in high temperature proton exchange membrane fuel cells (HT-PEMFC) due to partial deactivation of the Pt catalyst by phosphates [1]. At this point, the implementation of less expensive Fe-N-C catalysts for the oxygen reduction reaction in HT-PEMFCs can significantly reduce the material costs. However, a decrease in performance of Fe-N-C-based HT-PEMFCs within the first hours of operation was reported for Black Pearls (BP)-based Fe-N-Cs [2]. In this study, the effect of different Fe N-C types on their stability in HT-PEMFCs is evaluated and degradation processes are identified. A commercial catalyst (PMF 011904, Pajarito Powder), a BP- and two activated biomass-based Fe-N-Cs are implemented as cathode catalysts [3]. Single-cell tests with begin of test characterization followed by 90 h operation at a constant load of 0.1 A cm-2, cyclic voltammetry (CV) and end of test characterization are performed. While initial cell performances display significant differences, the performance decay after 24 h of operation is comparable for all cells. There, an overall performance loss of up to 26 % is observed. Electrochemical and physical post mortem analysis of the cathodes using CV, transmission electron microscopy and mass spectroscopy with inductively coupled plasma reveal, that deactivation of Fe-Nx sites is the main degradation process and corrosion of carbon support is negligible. This study helps to understand degradation phenomena of Fe-N-Cs in HT PEMFC. Moreover, approaches for the optimization of this catalyst class to increase the stability and to the overall Pt loading in HT-PEMFCs is given. [1] R. Zeis, Beilstein J. Nanotechnol. 2015, 6, 68. [2] Y. Hu et al., Appl. Catal. B 2018, 234, 357. [3] J. Müller-Hülstede et al., J. Power Sources 2022, 537, 231529