K. Andreas Friedrich

Improving the environmental impact of civil aircraft by fuel cell technology: concepts and technological progress

Nowadays, new technologies and breakthroughs in the fields of energy efficiency, alternative fuels and added-value electronics are leading to improved, more environmentally sustainable and green thinking applications. Due to the forecasted rapid increase of volume of air traffic, future aircraft generations have to face enhanced requirements concerning productivity, environmental compatibility and higher operational availability, thus effecting technical, operational and economical aspects of in-flight and on-ground power generation systems, even if air transport is responsible for only about 2% of all anthropogenic CO2 emissions. The trend in new aircraft development is toward “more electric” architectures which is characterized by a higher proportion of electrical systems substituting hydraulically or pneumatically driven components, and, as a result, increasing the amount of electrical power. Fuel cell systems in this context represent a promising solution regarding the enhancement of the energy efficiency for both cruise and ground operations. For several years the Institute of Technical Thermodynamics of the German Aerospace Center (Deutsches Zentrum fur Luft- und Raumfahrt, DLR) in Stuttgart and Hamburg has developed fuel cell systems for aircraft applications. The activities of DLR focus on: identification of fuel cell applications in aircraft in which the properties of fuel cell systems, namely high electric efficiency, low emissions and silent operation, are capitalized for the aircraft application; design and modeling of possible and advantageous system designs; theoretical and experimental investigations regarding specific aircraft relevant operating conditions; qualification of airworthy fuel cell systems; set up and full scale testing of fuel cell systems for application in research aircraft. In cooperation with Airbus, several fuel cell applications within the aircraft for both ground and cruise operation have been identified. As a consequence, fuel cell systems capable of supporting or even replacing existing systems have been derived. In this context, the provision of inert gas for the jet fuel (kerosene) tank and electrical cabin power supply, including water regeneration, represent the most promising application fields. This paper will present the state of development and the evolution discussing the following points: modeling of different system architectures and evaluation of promising fuel cell systems; experimental evaluation of fuel cell systems under relevant conditions (low pressure, vibrations, reformate operation, etc.); fuel cell test in DLRs research aircraft ATRA (A320) including the test of an emergency system based on hydrogen and oxygen with 20 kW of electrical power. The fuel cell system was integrated into an A320 aircraft and tested up to a flight altitude of 25 000 feet under several acceleration and inclination conditions; fuel cell tests in Antares-H2—DLRs new flying test bed.

Uncovering the Stabilization Mechanism in Bimetallic Ruthenium-Iridium Anodes for Proton Exchange Membrane Electrolyzers.

Proton exchange membrane (PEM) electrolyzers are attracting an increasing attention as a promising technology for the renewable electricity storage. In this work, near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) is applied for in situ monitoring of the surface state of membrane electrode assemblies with RuO2 and bimetallic Ir0.7Ru0.3O2 anodes during water splitting. We demonstrate that Ir protects Ru from the formation of an unstable hydrous Ru(IV) oxide thereby rendering bimetallic Ru-Ir oxide electrodes with higher corrosion resistance. We further show that the water splitting occurs through a surface Ru(VIII) intermediate, and, contrary to common opinion, the presence of Ir does not hinder its formation.

AFM as an analysis tool for high-capacity sulfur cathodes for Li-S batteries.

Summary In this work, material-sensitive atomic force microscopy (AFM) techniques were used to analyse the cathodes of lithium–sulfur batteries. A comparison of their nanoscale electrical, electrochemical, and morphological properties was performed with samples prepared by either suspension-spraying or doctor-blade coating with different binders. Morphological studies of the cathodes before and after the electrochemical tests were performed by using AFM and scanning electron microscopy (SEM). The cathodes that contained polyvinylidene fluoride (PVDF) and were prepared by spray-coating exhibited a superior stability of the morphology and the electric network associated with the capacity and cycling stability of these batteries. A reduction of the conductive area determined by conductive AFM was found to correlate to the battery capacity loss for all cathodes. X-ray diffraction (XRD) measurements of Li2S exposed to ambient air showed that insulating Li2S hydrolyses to insulating LiOH. This validates the significance of electrical ex-situ AFM analysis after cycling. Conductive tapping mode AFM indicated the existence of large carbon-coated sulfur particles. Based on the analytical findings, the first results of an optimized cathode showed a much improved discharge capacity of 800 mA·g(sulfur)−1 after 43 cycles.

Investigation of Locally Resolved Current Density Distribution of Segmented PEM Fuel Cells to Detect Malfunctions

With the aid of PCB technology, current density distribution could be achieved by measuring the local current density inside the fuel cell, which would be more precisely reflecting the local electrochemical reaction. An important application of segmented cell technology was used for in-situ error detection of the evolution of membrane leakage. From the evolution of current density distributions, starting locations of membrane leakages and their spreading could be detected. The effect of operation conditions could be found: the steeply decreased pressure and the fast cooling of the local membrane was the main reason for the degradation of the membrane. It caused large temperature gradient in different local membranes, which resulted in inhomogeneous current density distribution and mechanical strain and stress. With the aid of segmented cell, malfunction could be detected at an early stage and thereby catastrophic failure of the whole stack may be avoided.

Improving the activity and stability of Ir catalysts for PEM electrolyzer anodes by SnO2:Sb aerogel supports: does V addition play an active role in electrocatalysis?

Low Ir loading oxygen evolution reaction (OER) catalysts with superior activity and durability for proton exchange membrane (PEM) electrolyzers are an important topic in industry and academia. One possible strategy for addressing this challenge is the use of support materials that are stable under highly corrosive acidic environments at a high working potential (>1.4 V). Moreover, highly porous structure is another key criteria for OER catalyst support to achieve a high electrochemical surface area. Here, we report a novel Ir supported on a SnO2:Sb aerogel OER catalyst (Ir/SnO2:Sb-mod-V), which was prepared under ambient pressure by using vanadium additives. It shows an unrivaled activity and enhanced stability, on which vanadium does not play any active role but demonstrates the influences that changes the porosity of the aerogel support and affects the impurity content of the chlorine. By taking advantage of the high porosity of the aerogel substrate, Ir/SnO2:Sb-mod-V allows a decrease of more than 70 wt% for precious metal usage in the catalyst layer while keeping a similar OER activity compared to its unsupported counterpart.

Evaluation of the Effect of Sulfur on the Performance of Nickel/Gadolinium-Doped Ceria Based Solid Oxide Fuel Cell Anodes.

Abstract The focus of this study is the measurement and understanding of the sulfur poisoning phenomena of Ni/gadolinium‐doped ceria (CGO) based solid oxide fuel cells (SOFC). Cells with Ni/CGO10 and NiCu5/CGO40 anodes were characterized by using impedance spectroscopy at different temperatures and H2/H2O fuel ratios. The short‐term sulfur poisoning behavior was investigated systematically at temperatures of 800–950 °C, current densities of 0–0.75 A cm−2, and H2S concentrations of 1–20 ppm. A sulfur poisoning mitigation effect was observed at high current loads and temperatures. The poisoning behavior was reversible for short exposure times. It was observed that the sulfur‐affected processes exhibited significantly different relaxation times that depend on the Gd content in the CGO phase. Moreover, it was demonstrated that the capacitance of Ni/CGO10 anodes is strongly dependent on the temperature and gas‐phase composition, which reflects a changing Ce3+/Ce4+ ratio.

Insights into solid electrolyte interphase formation on alternative anode materials in lithium-ion batteries

To gain new insights into the formation of the solid electrolyte interphase (SEI) as a basis for the safe and efficient use of new anode materials, SEI formation on silicon and lithium titanate (LTO) anodes was studied by electrochemical impedance spectroscopy (EIS) and ex situ X-ray photoelectron spectroscopy (XPS) measurements. EIS measurements performed at equidistant voltage intervals provided insights into SEI formation; ex situ XPS measurements supplied data on the chemical composition of the SEI layer. On the silicon anodes, a reduction of the resistance in the second cycle was observed, which suggests the formation of a stable SEI with SiO2, Li4SiO4, LiF, and different carbonates as its main components. On the LTO anodes, however, the resistance increased by a factor of two, indicating incomplete SEI formation. Here, LiF and different carbonates were identified as the SEI’s main components.Graphical Abstract

Cathode Materials for Polymer Electrolyte Fuel Cells Based on Vertically Aligned Carbon Filaments

Polymer electrolyte fuel cells (PEMFC) suffer dramatic loss of an operating voltage due to a significant cathode overpotential calling for economically unviable noble metal catalyst loadings in the cathode active layers. Therefore significant effort is put into the investigation of the cathode oxygen reduction reaction (ORR) and the development of novel electrocatalytic materials. In this presentation we discuss catalytic materials based on platinum nanoparticles supported on vertically aligned carbon nanofilaments. They are intended to improve mass transport, Pt utilization and corrosion stability of the cathode. They are also of interest as a model system for shedding light on the kinetics and mechanisms of fuel cell reactions.

Relations of Water Management and Degradation Processes in PEFC

The life-time of fuel cells is one of the main barrier for using fuel cells as electric power generators or sources in mass market products. Therefore, in the last years the research activi-ty to investigate the degradation processes and mechanisms of fuel cells – especially of polymer electrolyte membrane fuel cells (PEFC) has intensified significantly. Consequently, many degradation processes in PEFC have been identified and some of the degradation mechanisms are understood qualitatively. However, not all significant degradation mecha-nisms are understood sufficiently and the relation of the degradation rates and the operating conditions are unclear for most degradation processes. For many degradation processes the water management especially a liquid water phase inside the cell is a crucial factor for the degradation processes. Although for some of the degradation mechanisms the influence of water is well-understood most degradation processes still need clarifying in this respect. In this contribution the known degradation processes are summarized and evaluated with respect to the influence of liquid water. Some processes, in particular the agglomeration of the catalyst particles and the decomposition of PTFE are discussed in detail. In addition to the direct influence of the water on the individual degradation, the water man-agement will also changed by the degradation processes, e.g. by the decrease of the hydropho-bicity. Consequently, the water induces complex interactions involving various degradation processes.

Operando Evidence for a Universal Oxygen Evolution Mechanism on Thermal and Electrochemical Iridium Oxides

Progress in the development of proton exchange membrane (PEM) water electrolysis technology requires decreasing the anode overpotential, where the sluggish multistep oxygen evolution reaction (OER) occurs. This calls for an understanding of the nature of the active OER sites and reaction intermediates, which are still being debated. In this work, we apply synchrotron radiation-based near-ambient pressure X-ray photoelectron and absorption spectroscopies under operando conditions in order to unveil the nature of the reaction intermediates and shed light on the OER mechanism on electrocatalysts most widely used in PEM electrolyzers-electrochemical and thermal iridium oxides. Analysis of the O K-edge and Ir 4f spectra backed by density functional calculations reveals a universal oxygen anion red-ox mechanism regardless of the nature (electrochemical or thermal) of the iridium oxide. The formation of molecular oxygen is considered to occur through a chemical step from the electrophilic OI- species, which itself is formed in an electrochemical step.