Ragone plots revisited: A review of methodology and application across energy storage technologies

The term “Ragone plot” refers to a popular and helpful comparison framework that quantifies the energy–power relationship of an energy storage material, device, or system. While there is consensus on the general Ragone plot concept, many implementations are found in the literature. This article provides a systematic and comprehensive review of the Ragone plot methodology in the field of electric energy storage. A faceted taxonomy is developed, enabling existing and future Ragone plots to be unambiguously classified and contextualized. This review focuses on disseminating the methodology, discussing technology-specific aspects, and giving an overview of the further sizing and design methods developed based on Ragone plots. Additionally, this article identifies best practices for obtaining and presenting Ragone plots. This review is not limited to electrochemical energy storage, where the framework is traditionally applied, but also encompasses all other electric energy storage. Here, the Ragone plot can compactly quantify off-design performance and operational flexibility, independent of technology-specific performance indicators. This review is the first of its kind and can, therefore, guide future application of the Ragone plot framework in a consistent manner

Degradation of proton exchange membrane (PEM) water electrolysis cells: Looking beyond the cell voltage increase

The degradation of proton exchange membrane water electrolysis cells is usually measured in a temporal increase of the cell voltage. Although this is sufficient to evaluate the stability of a system, it is less suitable for targeted material development. Thus, an overpotential-specific and temporally resolved electrochemical characterization protocol is proposed. In this the ohmic overpotential is determined with high frequency resistance measurements. These are also used in combination with polarization curves to distinguish between the kinetic and mass transport overpotentials and to determine kinetic key parameters, according to the Butler-Volmer and transition state theory. Complementary electrochemical impedance spectroscopy measurements further unravel the individual resistances. On this basis, the following statements can already be issued. The major share of the measured cell voltage increase, i.e. degradation, is of apparent nature as it is recovered once lower potentials are applied. It is suggested that this is due to changes in the oxidation states of the iridium-based catalyst. Real degradation occurs in the ohmic and mass transport overpotential mainly at higher current densities and longer operating times. The increasing kinetic overpotential with increasing operating time is primarily potential-driven. Interestingly, both the Tafel slope and the apparent exchange current density slightly increase over time. © 2019 The Author(s). Published by ECS

Theoretical dimensioning and sizing limits of hybrid energy storage systems

Aim of a storage hybridisation is a beneficial usage or combination of different storage technologies with various characteristics to downsize the overall system, decrease the costs or to increase the lifetime, system efficiency or performance. In this paper, the point of interest is a different ratio of power to energy (specific power) of two storages to create a hybrid energy storage system (HESS) with a resulting specific power that better matches the requirements of the application. The approach enables a downsizing of the overall system compared to a single storage system and consequently decreases costs. The paper presents a theoretical and analytical benchmark calculation that determines the maximum achievable hybridisation, i.e. possible spread in specific power, while retaining the original total energy and power capacities of an equivalent single storage system. The theory is independent from technology, topology, control strategy, and application and provides a unified view on hybrid energy storage systems. It serves as a pre-dimensioning tool and first step within a larger design process. Furthermore, it presents a general approach to choose storage combinations and to characterize the potential of an application for hybridisation. In this context, a Hybridisation Diagram is proposed and integral Hybridisation Parameters are introduced

Model-Based Analysis of Low Stoichiometry Operation in Proton Exchange Membrane Water Electrolysis

Proton exchange membrane water electrolysis cells are typically operated with high water flow rates in order to guarantee the feed supply for the reaction, the hydration of the ionomer phase and to homogenize the temperature distribution. However, the influence of low flow rates on the cell behavior and the cell performance cannot be fully explained. In this work, we developed a simple 1+1-dimensional mathematical model to analyze the cell polarization, current density distribution and the water flow paths inside a cell under low stoichiometry condition. The model analysis is in strong context to previous experimental findings on low water stoichiometry operations. The presented analysis shows that the low water stoichiometry can lead to dry-out at the outlet region of the anode channel, while a water splitting reaction is also present there. The simulation results show that the supply with water in this region is achieved by a net water transport from the cathode to the anode catalyst layer resulting in higher local proton resistances in the membrane and the anode catalyst laye

Hydrogen-powered aviation in Germany: A macroeconomic perspective and methodological approach of fuel supply chain integration into an economy-wide dataset

The hydrogen (H2) momentum affects the aviation sector. However, a macroeconomic consideration is currently missing. To address this research gap, the paper derives a methodology for evaluating macroeconomic effects of H2 in aviation and applies this approach to Germany. Three goals are addressed: (1) Construction of a German macroeconomic database. (2) Translation of H2 supply chains to the system of national accounts. (3) Implementation of H2-powered aviation into the macroeconomic data framework. The article presents an economy-wide database for analyzing H2-powered aviation. Subsequently, the paper highlights three H2 supply pathways, provides an exemplary techno-economic cost break-down for ten H2 components and translates them into the data framework. Eight relevant macroeconomic sectors for H2-powered aviation are identified and quantified. Overall, the paper contributes on a suitable foundation to apply the macroeconomic dataset and to conduct macroeconomic analyses on H2-powered aviation. Finally, the article highlights further research potential on job effects related to future H2 demand

Hydrogen Crossover in PEM Water Electrolysis at Current Densities up to 10 A cm−2

Hydrogen crossover poses a critical issue in terms of the safe and efficient operation in polymer electrolyte membrane water electrolysis (PEMWE). The impact of key operating parameters such as temperature and pressure on crossover was investigated in the past. However, many recent studies suggest that the relation between the hydrogen crossover flux and the current density is not fully resolved. This study investigates the hydrogen crossover of PEMWE cells using a thin Nafion 212 membrane at current densities up to 10 A cm−2 and cathode pressures up to 10 bar, by analysing the anode product gas with gas chromatography. The results show that the hydrogen crossover flux generally increases over the entire current density range. However, the fluxes pass through regions with varying slopes and flatten in the high current regime. Only considering hydrogen diffusion as the single transport mechanism is insufficient to explain these data. Under the prevailing conditions, it is concluded that the electro-osmotic drag of water containing dissolved hydrogen should be considered additionally as a hydrogen transport mechanism. The drag of water acts opposite to hydrogen diffusion and has an attenuating effect on the hydrogen crossover in PEMWE cells with increasing current densities

Evaluating the influence of requirements in fuel cell system design using Design Requirement Maps

Finding a combination of design variables for an optimized design target is the main aspect in fuel cell system design. Beside that, it has to be ensured that all requirements, on component and vehicle level, are met. Using a visualization approach, called Design Requirement Map, as a graphical presentation of the design target and the requirements of two degrees of freedom, helps to answer certain design questions and enable an estimation of the influence of requirements and operating points on the optimal system design. In this paper, first, the general fuel cell system design problem is formulated and, second, the Design Requirement Map is used to study the influence of requirements on the optimal combination of humidifier scale and air compression ratio. Designs with too small or too large humidifiers reveal as designs, which are constrained by at least one of the considered requirements. In addition, the influence for a multi-objective design target and different ambient temperatures and pressures are addressed. For certain design questions using Design Requirement Maps can be very helpful to evaluate the impact of requirements on the system design especially when considering different operating points. © 2021 The Authors. Fuel Cells published by Wiley-VCH Gmb

Is iridium demand a potential bottleneck in the realization of large-scale PEM water electrolysis?

Proton exchange membrane water electrolysis (PEMWE) is a key technology for future sustainable energy systems. Proton exchange membrane (PEM) electrolysis cells use iridium, one of the scarcest elements on earth, as catalyst for the oxygen evolution reaction. In the present study, the expected iridium demand and potential bottlenecks in the realization of PEMWE for hydrogen production in the targeted GW a−1 scale are assessed in a model built on three pillars: (i) an in-depth analysis of iridium reserves and mine production, (ii) technical prospects for the optimization of PEM water electrolyzers, and (iii) PEMWE installation rates for a market ramp-up and maturation model covering 50 years. As a main result, two necessary preconditions have been identified to meet the immense future iridium demand: first, the dramatic reduction of iridium catalyst loading in PEM electrolysis cells and second, the development of a recycling infrastructure for iridium catalysts with technical end-of-life recycling rates of at least 90%. © 2021 The Author(s

Transformation der Energiesysteme : Ein kompakter Überblick zur Energieforschung an der Leibniz Universität Hannover

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