ILHYPOS - Ionic Liquid-based Hybrid Power Supercapacitors
Overview
Background & policy context:
The demand for clean energy is rapidly expanding worldwide and one of the most promising solutions is non-polluting energy production by fuel cells. Supercapacitors (SCs), due to their capability to deliver high specific power in a few seconds, are considered as electrical energy storage devices for smoothing the short-time power burst required in transport and stationary applications of fuel cells.
Commercial SCs are double-layer carbon SCs (DLCS) which make use of electrolyte solutions consisting of a salt dissolved in an organic solvent, which permits relatively high operating voltages (around 2.5 V). The main drawback with these SCs is that the organic solvents do not often fulfil the requirements of environmental compatibility and safety for vapour generation, flammability and explosions. This is the case for DLCSs with acetonitrile-based electrolytes, which are the most common high-voltage DLCSs on the market.
The high vapour pressure of these electrolytes requires a careful and expensive thermal control. Temperatures above 40° C, normal in fuel cell vehicles and CHP (combined heat and power) systems, may cause the degradation of present SCs in terms of performance and safety. The volatility of organic solvents increases sharply with temperature, making SCs potentially unsafe beyond 50-60° C and, generally, non-environmentally friendly with the presence of polluting chemicals.
The hybrid SCs to be developed in the project ILHYPOS are based on the use of ionic liquids as electrolytes and on a novel hybrid configuration using electronic conducting polymers (ECPs) as positive electrodes. Ionic liquids are excellent ionic conductors, virtually non-volatile and thermally stable up to 300° C, with a high working voltage (in excess of 5 V). These properties make ionic liquids excellent candidates as electrolytes in SCs with improved performances: specific energy and power of about 15 Wh/kg and 7 kW/kg can be reached.
Objectives:
The ILHYPOS project aimed at developing green, safe, and high specific energy and power hybrid SuperCapacitors (SC) for application as peak power smoothing device in Polymer Electrolyte Membrane (PEM) fuel cell-powered electric vehicles 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° C.
The scientific objectives were:
- 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° C), while maintaining its superior performance at 60° C and above with respect to present ionic liquids;
- Synthesis of Electronically Conducting Polymers (ECPs) optimised for the use as positive electrode in ionic liquid-based SCs by electrochemical techniques;
- Identification of high surface area carbons (e.g. activated and aerogel carbons) optimised for the use as negative electrode in ionic liquid-based SCs;
- 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.
Methodology:
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 organisation 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.
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