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Ionic Liquid-based Hybrid Power Supercapacitors

Final Report Summary - ILHYPOS (Ionic Liquid-based Hybrid Power Supercapacitors)

The ILHYPOS project aimed at developing green, safe, and high specific energy and power hybrid Super-capacitors (SC) for application as peak power smoothing device in fuel cell Powered electric vehicles (PEM) and, as an additional second option, in delocalised small energy production plants based on PEM fuel cells.

The hybrid SCs to be developed are based on the use of ionic liquids as electrolytes. Ionic liquids are excellent ionic conductors, virtually non volatile and thermally stable up to 300 degrees Celsius.

The scientific objectives were:
(1) synthesis and characterisation of an ionic liquid (or a mixture of ionic liquids) having improved properties (overall ionic conductivity, electrochemical, chemical and thermal stabilities) at low temperatures (down to - 20 degrees Celsius), while maintaining its superior performance at 60 degrees Celsius and above with respect to present ionic liquids;
(2) synthesis of Electronically conducting polymers (ECPs) optimised for the use as positive electrode in ionic liquid-based SCs by electrochemical techniques;
(3) identification of high surface area carbons (e.g. activated and aerogel carbons) optimised for the use as negative electrode in ionic liquid-based SCs;
(4) investigations of the electrochemical performance of current collectors in ionic liquids based SCs.
Surface treatments would be developed onto the Al current collectors used in these hybrid SCs to decrease the series resistance of the cells.

Commercially available SCs based on organic electrolytes suffer of limitations associated with the operating temperature. Temperatures above 40 degrees Celsius, frequently encountered within fuel cell powered vehicles and stationary (Combined heat and power (CHP) or other possible usage, such as Uninterruptible power systems (UPS)) systems, may cause the degradation of the commercial SCs in terms of performance and safety. The volatility of organic solvents such as acetonitrile increases sharply with temperature making the devices containing them unsafe at 50-60 degrees Celsius. Moreover, ILHYPOS SCs relieved from more polluting chemicals largely used in present SC (organic electrolytes substituted by 'green' ionic liquids).

The project was organised in four phases. During phase 1 (Electrode and electrolyte materials R&D), academic and basic research organisations work was concentrated on the optimisation of the electrode and electrolyte materials in order to significantly improve the overall technical performances of each single component with the respect to present state of the art. With phase 2 (Development and production (D&P) of SC materials), the focus was on the scale up processes for optimising the materials production. In phase 3 (Application requirements and full-scale prototype production), an application specific study was performed by two end users in collaboration with a research organization as hybrid vehicle configuration investigator, and, based on these studies, hybrid SC components were designed and assembled in the final prototypes. In phase 4 (Application testing), testing procedures were developed and used to experimentally verify the performance of the prototype with the respect to the project targets.

In 3,5 years of the project, the activities were, in-line with the planning, mainly devoted to the research and development of key materials for the preparation of SCs, together with the scale-up process analysis for the realisation of SC components and the study of the application technical requirements. Finally, cell prototypes were produced and tested according to three different designs confirming most of the project targets, with significant new inputs from novel asymmetric configuration and wound stack technology for cell assembly.

Investigation and development of electrode materials and their compatibility with ionic liquid electrolyte was carried out. Furthermore, experimental activities were carried out for scaling up ECP production, as well as to select separators and to optimise current collectors.

In parallel to optimised electrode materials investigation, new ionic liquids (and mixtures) were synthesised using simple processes (one is an original aqueous route), fully characterised and then prepared in suitable quantities at laboratory scale (many batches up to 30 g each) for verifying the compatibility (electrochemical stability up to 5 V) and performance characteristics of electrodes materials with these new compounds. After selection and characterisations at ENEA, scale up processes were studied and optimised at Evonik up to the production of ionic liquid for sample productions.

The ILHYPOS project addressed key applications for SCs, both in conjunction with fuel cells (FCs):
- the use in hybrid electric vehicles (HEVs) with or without batteries; and
- the stationary application in UPS, which was selected during the project, as more commercially interesting with respect to the initial CHP productions. For both applications, a preliminary analysis, supported by experimental data and simulations, was carried out to identify the technical requirements and the potential advantages achievable with the introduction of SCs. In order to establish the guidelines for the ILHYPOS SCs, the research team investigated:
- SCs sizing both for HEV and FC-UPS;
- the application of SC for HEV - and FC-UPS;
- the modelling and simulation of energy storage systems for HEV and FC-UPS.

Three generations of prototype cells were finally assembled and tested according to a defined test procedure:
1. Symmetric carbon / carbon cells with ionic liquid-based electrolyte, mainly aimed to analyse the functionality and repeatability of the assembly process. Non-optimised carbon electrodes were used. A set of six cells were initially assembled for such purpose.
2. Hybrid carbon / ECP cells with ionic liquid-based electrolyte. This was the cell design planned at the beginning of the project. Some prototype cells were finally assembled to verify basic performances and the effect of assembly process on the stability of ECP.
3. Asymmetric carbon / carbon cells with ionic liquid-based electrolyte, a novel concept, aimed at meeting project targets by optimising performances of carbon electrodes and the peculiar features of ionic liquid-based electrolyte.

The ILHYPOS SC test procedure was finalised to the following objectives:
1. evaluation of performance characteristics of SC;
2. comparison of SC in a consistent and reliable way;
3. identification of weaknesses of technology for their development.

The testing work was at the end used, in combination with more electrochemical testing, to characterise prototype cells and to verify their capability to meet project technical targets, mainly in terms of specific power, energy and internal resistance (Equivalent series resistance (ESR)).

The dissemination activities followed three main channels, mostly related to the characteristics and the mission of the participating organisations in the ILHYPOS project:
1. Scientific papers for international refereed journals for the scientific community were largely prepared, together with various presentations to national and international conferences. The scope was to disseminate the results and have possible feedback from the scientific and industrial target groups to improve and overcome project results, maintaining the activities really challenging at the research and development frontier.
2. The preparation of materials and support to the European Commission to use more official channel, already available, to reach the large audience attending and using the European Union facilities and events, such as: interviews, brochures, posters and other institutional events (review days, TRA conferences) as well as support to the preparation of periodic European Commission technical reports.
3. The general public was mostly addressed in general purpose events, with contribution to questionnaires and interviews and with preparation of videos containing information about the project or the technologies studied in the project.