Developments in Dilatometry for Characterisation of Electrochemical Devices

Since the 1970s, electrochemical dilatometry (ECD) has been used to investigate the dilation of layered host materials due to the intercalation of guest ions, atoms or molecules, and has recently gained traction in application to various electrochemical devices, such as lithium-ion batteries (LiBs), which have electrodes that undergo volume changes during cycling, resulting in particle cracking and electrode degradation. With resolution capabilities spanning tens of microns down to a few nanometres, dilatometry is a valuable tool in understanding how commonly used electrodes dilate and degrade and can therefore be of critical value in improving their performance. In recent years, there has been a plethora of studies using dilatometry as a monitoring tool for understanding operating performance in various electrochemical devices; however, to our knowledge, there has been no in-depth review of this body of research to date. This paper seeks to address this by reviewing how dilatometry works and how it has been used for the characterisation of electrochemical energy storage devices

Efficient electrocatalytic oxygen reduction reaction of thermally optimized carbon black supported zeolitic imidazolate framework nanocrystals under low-temperature

Turning commercially available low-cost conducting carbon black materials into functional electrocatalytic electrode media using simple surface chemical modification is a highly attractive approach. This study reports on remarkably enhanced oxygen electrocatalytic activity of commercially available Ketjenblack (KB) by growing a non-precious cobalt metal-based zeolitic-imidazolate framework (ZIF-67) at room temperature in methanol solution followed by a mild thermolysis. The resulting Co@CoOx nanoparticle decorated nitrogen-doped KB derived from the optimized ZIF-67 : KB weight ratio of hybrid samples at 500-600 °C shows high performance for the oxygen reduction reaction (ORR) with impressive Eonset and E1/2 values of ∼0.90 and ∼0.83 V (vs. RHE), respectively in 0.1 M KOH electrolyte. Such ORR activity is comparable to, or better than many metal@metal-oxide-carbon based electrocatalysts synthesized under elevated carbothermal temperatures and using multicomponent/multistep chemical modification conditions. Therefore, a simple electrocatalyst design reported in this work is an efficient synthesis route that not only utilises earth-abundant carbon black but also comprises scalable room temperature synthesized ZIF-67 following mild thermolysis conditions under 600 °C

Comparative study of energy management systems for a hybrid fuel cell electric vehicle - A novel mutative fuzzy logic controller to prolong fuel cell lifetime

Hybrid fuel cell battery electric vehicles require complex energy management systems (EMS) in order to operate effectively. Poor EMS can result in a hybrid system that has low efficiency and a high rate of degradation of the fuel cell and battery pack. Many different types of EMS have been reported in the literature, such as equivalent consumption minimisation strategy and fuzzy logic controllers, which typically focus on a single objective optimisations, such as minimisation of H2 usage. Different vehicle and system specifications make the comparison of EMSs difficult and can often lead to misleading claims about system performance. This paper aims to compare different EMSs, against a range of performance metrics such as charge sustaining ability and fuel cell degradation, using a common modelling framework developed in MATLAB/Simulink - the Electric Vehicle Simulation tool-Kit (EV-SimKit). A novel fuzzy logic controller is also presented which mutates the output membership function depending on fuel cell degradation to prolong fuel cell lifetime – the Mutative Fuzzy Logic Controller (MFLC). It was found that while certain EMSs may perform well at reducing H2 consumption, this may have a significant impact on fuel cell degradation, dramatically reducing the fuel cell lifetime. How the behaviour of common EMS results in fuel cell degradation is also explored. Finally, by mutating the fuzzy logic membership functions, the MFLC was predicted to extend fuel cell lifetime by up to 32.8%

X-ray Computed Tomography for Failure Mechanism Characterisation within Layered Pouch Cells

Lithium-ion battery (LIB) safety is a multi-scale problem: from the whole-cell architecture to its composite internal 3D microstructures. Substantial research is required to standardise failure assessments and optimise cell designs to reduce the risks of LIB failure. In this work, the failure response of a 1 Ah layered pouch cell with a commercially available NMC cathode and graphite anode at 100 % SOC (4.2 V) is investigated. The mechanisms of two abuse methods; mechanical (by nail penetration) and thermal (by accelerating rate calorimetry) are compared by using a suite of post-mortem analysis methods. Post-mortem whole-cell architectural changes and electrode layer deformations were analysed for both mechanisms using non-invasive X-ray computed tomography. Furthermore, changes to electrode surfaces, bulk microstructures and particle morphologies are compared by following a proposed cell disassembly and post-mortem sample preparation methodology. Building on the insights into critical architectural weak points, electrode behaviours and particle cracks, the reliability of X-ray computed tomography as a guide for LIB failure assessment is demonstrated

Fault diagnosis of PEMFC based on the AC voltage response and 1D convolutional neural network

Real-time diagnosis is required to ensure the safety, reliability, and durability of the polymer electrolyte membrane fuel cell (PEMFC) system. Two categories of methods are (1) intrusive, time consuming, or require alterations to the cell architecture but provide detailed information about the system or (2) rapid and benign but low-information-yielding. A strategy based on alternating current (AC) voltage response and one-dimensional (1D) convolutional neural network (CNN) is proposed as a methodology for detailed and rapid fuel cell diagnosis. AC voltage response signals contain within them the convoluted information that is also available via electrochemical impedance spectroscopy (EIS), such as capacitive, inductive, and diffusion processes, and direct use of time-domain signals can avoid time-frequency conversion. It also overcomes the disadvantage that EIS can only be measured under steady-state conditions. The utilization of multi-frequency excitation can make the proposed approach an ideal real-time diagnostic/characterization tool for fuel cells and other electrochemical power systems

A Review of Drive Cycles for Electrochemical Propulsion

Automotive drive cycles have existed since the 1960s. They started as requirements as being solely used for emissions testing. During the past decade, they became popular with scientists and researchers in the testing of electrochemical vehicles and power devices. They help simulate realistic driving scenarios anywhere from system to component-level design. This paper aims to discuss the complete history of these drive cycles and their validity when used in an electrochemical propulsion scenario, namely with the use of proton exchange membrane fuel cells (PEMFC) and lithium-ion batteries. The differences between two categories of drive cycles, modal and transient, were compared; and further discussion was provided on why electrochemical vehicles need to be designed and engineered with transient drive cycles instead of modal. Road-going passenger vehicles are the main focus of this piece. Similarities and differences between aviation and marine drive cycles are briefly mentioned and compared and contrasted with road cycles. The construction of drive cycles and how they can be transformed into a ‘power cycle’ for electrochemical device sizing purposes for electrochemical vehicles are outlined; in addition, how one can use power cycles to size electrochemical vehicles of various vehicle architectures are suggested, with detailed explanations and comparisons of these architectures. A concern with using conventional drive cycles for electrochemical vehicles is that these types of vehicles behave differently compared to combustion-powered vehicles, due to the use of electrical motors rather than internal combustion engines, causing different vehicle behaviours and dynamics. The challenges, concerns, and validity of utilising ‘general use’ drive cycles for electrochemical purposes are discussed and critiqued

Structure‐guided Capacitance Relationships in Oxidized Graphene Porous Materials Based Supercapacitors

Supercapacitors formed from porous carbon and graphene-oxide (GO) materials are usually dominated by either electric double-layer capacitance, pseudo-capacitance, or both. Due to these combined features, reduced GO materials have been shown to offer superior capacitance over typical nanoporous carbon materials; however, there is a significant variation in reported values, ranging between 25 and 350 F g−1. This undermines the structure (e.g., oxygen functionality and/or surface area)-performance relationships for optimization of cost and scalable factors. This work demonstrates important structure-controlled charge storage relationships. For this, a series of exfoliated graphene (EG) derivatives are produced via thermal-shock exfoliation of GO precursors and following controlled graphitization of EG (GEG) generates materials with varied amounts of porosity, redox-active oxygen groups and graphitic components. Experimental results show significantly varied capacitance values between 30 and 250 F g−1 at 1.0 A g−1 in GEG structures; this suggests that for a given specific surface area the redox-active and hydrophilic oxygen content can boost the capacitance to 250–300% higher compared to typical mesoporous carbon materials. GEGs with identical oxygen functionality show a surface area governed capacitance. This allows to establish direct structure-performance relationships between 1) redox-active oxygen functional concentration and capacitance and 2) surface area and capacitance

Lithium-sulfur battery diagnostics through distribution of relaxation times analysis

Electrochemical impedance spectroscopy (EIS) is widely used in battery analysis as it is simple to implement and non-destructive. However, the data provided is a global representation of all electrochemical processes within the cell and much useful information is ambiguous or inaccessible when using traditional analysis techniques. This is a major challenge when EIS is used to analyse systems with complex cell chemistries, like lithium-sulfur (Li-S), one of the strongest candidates to supersede conventional Li-ion batteries. Here we demonstrate the application of distribution of relaxation times (DRT) analysis for quantitative deconvolution of EIS spectra from Li-S batteries, revealing the contributions of (eight) distinct electrode processes to the total cell polarisation. The DRT profile is shown to be strongly dependent on cell state-of-charge, offering a route to automated and on-board analysis of Li-S cells

Self-assembled carbon nanoribbons with the heteroatom doping used as ultrafast charging cathodes in zinc-ion hybrid supercapacitors

Zinc-ion hybrid supercapacitors (ZHSs) are highly desirable for large-scale energy storage applications owing to the merits of high safety, low cost and ultra-long cycle life. The poor rate performance of cathodes, however, severely hinders their application. Herein, aqueous ZHSs with superior performance were fabricated by employing a series of ultrathin carbon nanobelts modified with B, N, O (CPTHB-Bx). The heteroatom doping can significantly modify the chemical behaviors of carbon frameworks, which could generate numerous active sites and accelerate the charge transport. The systematic investigation reveals that the B–N groups are active species for fast Zn-ion adsorption and desorption. As a result, the best-performed CPTHB-B2 exhibits an excellent electrochemical performance as cathodes in ZHSs, delivering a high specific capacitance of 415.3 F g−1 at 0.5 A g−1, a record high capacitance retention of 81% when increasing the current densities from 0.5 to 100 A g−1, an outstanding energy density of 131.9 W h kg−1 and an exceptionally high power density of 42.1 kW kg−1. Our work provides a new cathode design for ultrafast charging Zn-ion storage devices

Electrochemical recovery of lithium-ion battery materials from molten salts by microstructural characterization using X-ray imaging

Recycling spent lithium-ion batteries (LiBs) guarantees the conservation of important metal resources by reducing demands on raw supply and offsetting the energy and environmental costs associated with its manufacture. Employing a molten salt as a solvent for extraction affords a much greener and simpler route to metal recovery by electrochemical means. The current mechanistic understanding of the electrochemical recovery of metals in molten salts needs to be improved for the process to be optimized. X-ray computed tomography offers a non-destructive approach for 3D microstructure visualization and subsequent quantification. Here, we study the electrochemical deposition of recovered cobalt metal from lithium cobalt oxide, LiCoO2 in LiCl-KCl eutectic (LKE). This diagnostic approach has been applied to LiCoO2-LKE samples before and after electrolysis at 450°C, yielding key insights into the morphological evolution of product formation