Periodic Reporting for period 4 - IMMOCAP ('If immortality unveil…'– development of the novel types of energy storage systems with excellent long-term performance)
Période du rapport: 2022-04-01 au 2023-09-30
Résumé du contexte et des objectifs généraux du projet
The IMMOCAP project aimed to find the reasons of performance fade of electrochemical capacitors during the long-term operation and to propose the solutions for their further development. Primarily, the project was focused on aqueous-based systems. Generally, the work was done in parallel on both - capacitive and redox-based systems, in order to find similarities and differences among their ageing mechanisms. It has to be noted that aqueous electrolytic solutions appear to be eco-friendly, green electrolytes; in combination with non-toxic, cheap, sustainable carbon materials, they seem to emerge on the energy storage market as the systems very competitive to the ones that are already proposed and present on the market. Nevertheless, the energy density provided by water-based capacitors is still remarkably lower than for systems operating in aprotic media, thus, both need to be considered to provide a competitive solution.
An overall objective was to find and describe the ageing process of electrochemical capacitors. It was assumed that knowing the reason of performance fade, we should be able to propose reasonable improvement and obtain the system with energy density up to 20 Wh/kg (through the involvement of redox active counterparts), satisfactory long-term performance (up to 50 000 cycles) being not at the expense of its power, i.e. up to 5 kW/kg (at peak demand). These parameters have been reached for capacitors exploiting redox-active additives.
In our work, we considered the electrode materials (mostly carbon-based materials of various origin), electrolytes (protic and aprotic, including ionic liquids), separators (polymer membranes) and the current collectors. A valuable aspect of this research is sought in the experiments realized in so-called operando mode. With this approach, we were able to determine the changes in the system directly under electrochemical polarization. These experiments allowed us to identify several, peculiar phenomena, described in more than 30 papers published as a project outcome.
We have reported that the ionic fluxes, both in the volumetric and gravimetric aspect, might not be symmetric and are governed by the textural parameters of the electrodes together with the ion dimension. Briefly, it has been found that on the negative electrode, the cations (either partially or totally hydrated) are most likely adsorbed at the electrode/electrolyte interface and their adsorption is a multistep process. On the positive electrode, in the majority of cases, the presence of cations and anions has been found. Furthermore, local pH changes indicated the ambiguous role of water (solvent). Such results have been obtained with dilatometric, electrogravimetric and microscopic (electrochemical scanning microscope) experiments. In this context, we developed the tool for operando monitoring of the pH during capacitor operation and we have described the changes induced at the interface at various pH values.
Generally, we have found that the aqueous electrolytes might trigger different ageing mechanisms and require special attention during system design.
As already mentioned, several works performed during project realization were addressed towards the improvement of the environmentally friendly electrochemical capacitors, and in the majority of cases, the research concerns the systems with water-based electrolytes. The main problem of such constructions is the limited performance during continuous charge/discharge loops, due to the specific interactions between electrolyte and electrode surface. This is mainly observed for the systems that exploit the redox activity of the ions from electrolyte, such as iodides, bromides or thio- and selenocyanates. However, it has also been found that the electrolytes that primarily do not demonstrate the redox activity (such as solutions of sulfates and nitrates), contribute to the charge storage with the processes demonstrating clear redox signature. Improvement of their performance in terms of cycling stability is one of the issues considered crucial for their implementation to the market. We have described several interactions between electrolytes of different nature (protic and aprotic) with carbon electrode. We have also indicated that mixture of ions (especially cations) might have a positive impact on the capacitor performance.
To improve the parameters of the electrochemical capacitors, we have implemented several modifications to the electrolytes that also include incorporation of the nanoparticles that might play a role of electrocatalyst and improve the efficiency of the redox process. For instance, thiocyanate-based solutions have been modified by gold nanoparticles addition at different concentrations and particle sizes. It has to be said that the concentration of the nanoparticles has been kept at an extremely low level in order to ensure a reasonable/affordable price. In this way, we remarkably improved the cyclability but also the power of the device that allowed us to reach (at certain conditions) 5 kW/kg of power density.
In the field of the separators and membranes, we have also proposed the novel types of gel-like electrolytes that can play a bifunctional role – of the separator and the electrolyte. It is worth to say that the proposed solutions are environmentally friendly as they exploit the naturally abundant biopolymers.
For the current collectors' issue, we have proposed the protective layer that remarkably prolonged the cycle-life of the device and protected the carbon material against physical deterioration. This study has been also extended to the corrosion investigation under various conditions and we demonstrated that the type of polarisation induces different corrosion mechanisms. We have also indicated that the corrosion of the electrode might precede the corrosion of other cell elements and could also be a trigger of other failure mechanisms.
For the aprotic systems, we have also demonstrated the first hybrid, metal-ion capacitor that can be pre-metalated without using external source of Li (Na or K too), i.e. directly from electrolytic solution, without loss of electrolyte conductivity. We indicated the role of right charge balance and the mechanisms that are responsible for their insertion.
Besides typically “applied” aspects that concern the parameters of the electrochemical capacitors, the work in the project also involves in-depth fundamental studies that allow us to understand better the phenomena that are on-going at the electrode/electrolyte interface. In these contexts, we develop in-situ and operando techniques (for instance, to see how the wettability of carbon electrode changes with potential), that brings essential knowledge about the mysterious electrode/electrolyte interface. We have also demonstrated that the population of ions in water-based electrolytes strongly depends on pH at the interface, that is usually much different than in electrolyte bulk.
We also worked on mathematical models that describe the viscoelastic properties of the electrolytes, electrode slurries and also the membranes. In this aspect, we found the optimal conditions for electrode and electrolyte formulation at a larger scale.
In the context of societal impact, the main issue is solving the problem of efficient energy storage. With our efforts, we propose a device and its components (e.g. electrolyte, electrode material) that will work better than currently used batteries and capacitors.
For society, this might be a huge advantage because effective energy storage is largely limiting the development of humanity in almost every field. We could reduce the number of short-acting batteries and accumulators with the devices of remarkably better long-time performance. This also means that the number of wastes could be reduced. Having an efficient energy storage system, we would limit the number of charging events for our cell phones, laptops, remote controllers, drones or other electric vehicles and devices.
An overall objective was to find and describe the ageing process of electrochemical capacitors. It was assumed that knowing the reason of performance fade, we should be able to propose reasonable improvement and obtain the system with energy density up to 20 Wh/kg (through the involvement of redox active counterparts), satisfactory long-term performance (up to 50 000 cycles) being not at the expense of its power, i.e. up to 5 kW/kg (at peak demand). These parameters have been reached for capacitors exploiting redox-active additives.
In our work, we considered the electrode materials (mostly carbon-based materials of various origin), electrolytes (protic and aprotic, including ionic liquids), separators (polymer membranes) and the current collectors. A valuable aspect of this research is sought in the experiments realized in so-called operando mode. With this approach, we were able to determine the changes in the system directly under electrochemical polarization. These experiments allowed us to identify several, peculiar phenomena, described in more than 30 papers published as a project outcome.
We have reported that the ionic fluxes, both in the volumetric and gravimetric aspect, might not be symmetric and are governed by the textural parameters of the electrodes together with the ion dimension. Briefly, it has been found that on the negative electrode, the cations (either partially or totally hydrated) are most likely adsorbed at the electrode/electrolyte interface and their adsorption is a multistep process. On the positive electrode, in the majority of cases, the presence of cations and anions has been found. Furthermore, local pH changes indicated the ambiguous role of water (solvent). Such results have been obtained with dilatometric, electrogravimetric and microscopic (electrochemical scanning microscope) experiments. In this context, we developed the tool for operando monitoring of the pH during capacitor operation and we have described the changes induced at the interface at various pH values.
Generally, we have found that the aqueous electrolytes might trigger different ageing mechanisms and require special attention during system design.
As already mentioned, several works performed during project realization were addressed towards the improvement of the environmentally friendly electrochemical capacitors, and in the majority of cases, the research concerns the systems with water-based electrolytes. The main problem of such constructions is the limited performance during continuous charge/discharge loops, due to the specific interactions between electrolyte and electrode surface. This is mainly observed for the systems that exploit the redox activity of the ions from electrolyte, such as iodides, bromides or thio- and selenocyanates. However, it has also been found that the electrolytes that primarily do not demonstrate the redox activity (such as solutions of sulfates and nitrates), contribute to the charge storage with the processes demonstrating clear redox signature. Improvement of their performance in terms of cycling stability is one of the issues considered crucial for their implementation to the market. We have described several interactions between electrolytes of different nature (protic and aprotic) with carbon electrode. We have also indicated that mixture of ions (especially cations) might have a positive impact on the capacitor performance.
To improve the parameters of the electrochemical capacitors, we have implemented several modifications to the electrolytes that also include incorporation of the nanoparticles that might play a role of electrocatalyst and improve the efficiency of the redox process. For instance, thiocyanate-based solutions have been modified by gold nanoparticles addition at different concentrations and particle sizes. It has to be said that the concentration of the nanoparticles has been kept at an extremely low level in order to ensure a reasonable/affordable price. In this way, we remarkably improved the cyclability but also the power of the device that allowed us to reach (at certain conditions) 5 kW/kg of power density.
In the field of the separators and membranes, we have also proposed the novel types of gel-like electrolytes that can play a bifunctional role – of the separator and the electrolyte. It is worth to say that the proposed solutions are environmentally friendly as they exploit the naturally abundant biopolymers.
For the current collectors' issue, we have proposed the protective layer that remarkably prolonged the cycle-life of the device and protected the carbon material against physical deterioration. This study has been also extended to the corrosion investigation under various conditions and we demonstrated that the type of polarisation induces different corrosion mechanisms. We have also indicated that the corrosion of the electrode might precede the corrosion of other cell elements and could also be a trigger of other failure mechanisms.
For the aprotic systems, we have also demonstrated the first hybrid, metal-ion capacitor that can be pre-metalated without using external source of Li (Na or K too), i.e. directly from electrolytic solution, without loss of electrolyte conductivity. We indicated the role of right charge balance and the mechanisms that are responsible for their insertion.
Besides typically “applied” aspects that concern the parameters of the electrochemical capacitors, the work in the project also involves in-depth fundamental studies that allow us to understand better the phenomena that are on-going at the electrode/electrolyte interface. In these contexts, we develop in-situ and operando techniques (for instance, to see how the wettability of carbon electrode changes with potential), that brings essential knowledge about the mysterious electrode/electrolyte interface. We have also demonstrated that the population of ions in water-based electrolytes strongly depends on pH at the interface, that is usually much different than in electrolyte bulk.
We also worked on mathematical models that describe the viscoelastic properties of the electrolytes, electrode slurries and also the membranes. In this aspect, we found the optimal conditions for electrode and electrolyte formulation at a larger scale.
In the context of societal impact, the main issue is solving the problem of efficient energy storage. With our efforts, we propose a device and its components (e.g. electrolyte, electrode material) that will work better than currently used batteries and capacitors.
For society, this might be a huge advantage because effective energy storage is largely limiting the development of humanity in almost every field. We could reduce the number of short-acting batteries and accumulators with the devices of remarkably better long-time performance. This also means that the number of wastes could be reduced. Having an efficient energy storage system, we would limit the number of charging events for our cell phones, laptops, remote controllers, drones or other electric vehicles and devices.
Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent
As already mentioned, the research in the project concerned the development of all electrochemical capacitor components.
Firstly, the viscoelastic properties of the electrolytes and the electrode’ slurries have been determined. As a result, the relation between viscosity and the shear rate used during electrode formulation was described. In this research, different concentrations/compositions of individual material components were taken into account. It should be pointed out that we have included several materials (such as activated carbons, carbon nanotubes, activated carbon blacks) in the study. We found the correlation between the solvent volume, carbon material porosity and the final electrode performance.
This allowed us to propose the equation which enables the optimal selection of the proportions of individual components with respect to the planned geometric dimensions of the electrodes. Furthermore, the optimal viscosity window for the material was determined. This knowledge allowed for making smooth and evenly covered electrode films on the current collectors and remarkably improved the reproducibility of the results.
The electrochemical and viscoelastic characteristics of capacitors operating with highly viscous electrolytes (such as “water-in-salt” formulations) have allowed the advantages associated with their high viscosity to be verified and pointed out. Usually, high viscosity has been considered as rather negative property in terms of the operation of such systems, mainly aggravating their power. This research allowed to conclude that the conditioning step before the regular operation is required if adequate power is requested. Furthermore, the electrochemical characteristics of devices with “water-in-salt” electrolytes allowed for a detailed description of the electrode/electrolyte interactions in highly concentrated electrolytes.
In the context of ageing mechanisms investigation, various capacitor systems were tested until 20% of initial capacitance loss; at such conditions, regarding international standards, the device is considered as damaged. Afterwards, comprehensive post-mortem physico-chemical analyses (N2 sorption at 77 K, SEM, XPS) were performed for different electrolytes. The studies evidenced that the main problem with the “immortality” of the electrochemical capacitor is the oxidation of the positive electrode (e.g. for the cells with 1 mol/L LiNO3 electrolyte). However, it seems that rather the solid-state deposit present on both electrodes is one of the failure reasons. Notably, the mechanism of the performance fading strongly depends on the voltage applied and remarkably changes after U>1.5 V. With sophisticated techniques developed in the project (like operando monitoring of pH), we discovered the reasons of performance fade and we were able to propose preventive measures to prolong the capacitor lifetime.
Additionally, it has been noticed that when redox-active electrolyte (1 mol/L KI) is used for capacitor construction, iodide/polyiodides anions hinder the access to the porosity on both, positive and negative, electrodes. Therefore, the cation exchange membrane was used to inhibit the iodides movement towards the negative electrode. Also, it has been evaluated that ageing protocols impact the lifespan of ECs differently.
As already mentioned, the electrolytes selected for research included the capacitive and redox-active ones. Thus, we covered ageing mechanisms in iodide and bromide-based electrolytes (MeI, where Me= Na+, K+, Rb+) and sulfate or nitrate-based ones (Me2SO4 where Me=Li+, Na+, K+, Rb+, Cs+ and LiNO3).
The electrolytic solutions containing redox active species (KSeCN; also mixed with neutral sulphate salts) have been proposed as electrolytes for ECs giving maximum operating voltage 1.4 V, high capacitance and relatively long cycle life. Moreover, KSCN-based electrolytes modified by gold nanoparticles (AuNPs) were tested. Various conditions were taken into account – different gold nanoparticles size (10, 50 and 100 nm) and their concentration in solutions (in the range from 0.1 – 4 nmol/L). 10 nm nanoparticles in 2 nmol/L concentration were chosen as optimal ones. Thanks to nanoparticles addition to redox active electrolyte power of the device was enhanced and cyclability was prolonged. Other electrolytes (Li2SO4 and KI solutions) were also modified by AuNPs. However, this additive has a meaningful effect only on KSCN-based electrolytes in ECs.
It should be said that IMMOCAP project interest was not only focused on the full cell performance, but also on gaining fundamental knowledge (in three-electrode or in-situ cells) to determine interactions at the electrode/electrolyte interface. Hence, electrochemical quartz crystal microbalance (EQCM) has been implemented. It has been found that aqueous-based electrochemical capacitors reveal the different ageing mechanism and failure reason among various electrolytes. Furthermore, the ageing protocol used during studies impacts the cell’s performance fade. So far, various voltages, current densities and ‘ageing protocols’ have been implemented, showing their influence on physicochemical changes of components and, as a consequence, system fade.
Electrochemical dilatometry experiments were also performed. Microporous activated carbon electrodes were polarised positively and negatively in Li2SO4 aqueous solution. Simultaneously, volumetric changes of the electrode were recorded. Definitely, higher electrode expansion was observed upon a negative polarisation than in the case of a positive one. The results are comparable regardless of the technique used. Interestingly, we found also, that the dilatometric changes depend on the pH value that is significantly changing during capacitor operation and governs the failure mechanisms the cell is subjected to.
In-situ and operando techniques supported the research on the influence of electrolyte viscosity on the electrochemical capacitor. It was performed by the controlled addition of viscosity agent to aqueous Li2SO4 solution. For this task, the electrowetting investigation procedure was designed and realised – it included the on-line investigation of the contact angle change upon polarisation of the electrodes.
Interesting results were obtained with X-Ray Tomography, performed at Kansai University (Japan). It has been proved that the solid-state deposit is being formulated during long-term operation. The mechanism proposed postulates the transfer of OH- through the separator and precipitation of lithium carbonate (as CO2 might evolve on the positive side).
We have also performed on-line observation of microscopic changes of carbon structure with Scanning Electrochemical Microscopy (SECM), operation methods development. Additionally, for further examination, new customised cell was designed and manufactured.
The analysis of carbon/electrolyte interaction by deconvolution of the current components in the electrochemical capacitor by the implementation of novel Step Potential Electrochemical Spectroscopy (SPECS) technique was done, to identify the mechanisms of the charge storage. It has been realised in cooperation with our partners from Australia (Prof. Scott Donne, University of Newcastle).
The electrochemical capacitor behaviour under polarisation was also studied with simultaneous internal pressure measurement. This provided important information on the volumetric changes within the capacitor device operating with an aqueous medium and gave the supporting data for the analysis of data originating from electrochemical measurements. The extension of the voltage window was proved when the electrodes were oxidised and ammonia-treated. High cell voltage brought about the demand for experiments on pH fluctuations inside the cell, and the preliminary results on this topic were obtained.
Determination of capacitor power properties under various testing conditions was also realised for highly viscous electrolytes. It appeared that higher viscosity does not necessarily mean lower conductivity. For this research, we designed and produced various devices for investigation.
Finally, we developed hybrid electrochemical capacitor operating in aprotic media that could be assembled in a very attractive way, without any external pre-metalation step. This device demonstrated very attractive performance, with energy density reaching 100 Wh/kg and cyclability thousands of cycles. We investigated and described the optimal assembly conditions and major failure mechanisms.
Our results have been presented in more than 30 papers published to date, 10 patent applications and 40 conference contributions (invited or keynote lectures, regular orals and posters) worldwide. We were also active in sharing our experience during seminars and workshops organized during international conferences, transferring knowledge mostly to less experienced researchers. Finally, the concept of hybrid electrochemical capacitor is being further developed in Proof of Concept project, as its performance is commercially attractive but requires verification in industrial environment.
Firstly, the viscoelastic properties of the electrolytes and the electrode’ slurries have been determined. As a result, the relation between viscosity and the shear rate used during electrode formulation was described. In this research, different concentrations/compositions of individual material components were taken into account. It should be pointed out that we have included several materials (such as activated carbons, carbon nanotubes, activated carbon blacks) in the study. We found the correlation between the solvent volume, carbon material porosity and the final electrode performance.
This allowed us to propose the equation which enables the optimal selection of the proportions of individual components with respect to the planned geometric dimensions of the electrodes. Furthermore, the optimal viscosity window for the material was determined. This knowledge allowed for making smooth and evenly covered electrode films on the current collectors and remarkably improved the reproducibility of the results.
The electrochemical and viscoelastic characteristics of capacitors operating with highly viscous electrolytes (such as “water-in-salt” formulations) have allowed the advantages associated with their high viscosity to be verified and pointed out. Usually, high viscosity has been considered as rather negative property in terms of the operation of such systems, mainly aggravating their power. This research allowed to conclude that the conditioning step before the regular operation is required if adequate power is requested. Furthermore, the electrochemical characteristics of devices with “water-in-salt” electrolytes allowed for a detailed description of the electrode/electrolyte interactions in highly concentrated electrolytes.
In the context of ageing mechanisms investigation, various capacitor systems were tested until 20% of initial capacitance loss; at such conditions, regarding international standards, the device is considered as damaged. Afterwards, comprehensive post-mortem physico-chemical analyses (N2 sorption at 77 K, SEM, XPS) were performed for different electrolytes. The studies evidenced that the main problem with the “immortality” of the electrochemical capacitor is the oxidation of the positive electrode (e.g. for the cells with 1 mol/L LiNO3 electrolyte). However, it seems that rather the solid-state deposit present on both electrodes is one of the failure reasons. Notably, the mechanism of the performance fading strongly depends on the voltage applied and remarkably changes after U>1.5 V. With sophisticated techniques developed in the project (like operando monitoring of pH), we discovered the reasons of performance fade and we were able to propose preventive measures to prolong the capacitor lifetime.
Additionally, it has been noticed that when redox-active electrolyte (1 mol/L KI) is used for capacitor construction, iodide/polyiodides anions hinder the access to the porosity on both, positive and negative, electrodes. Therefore, the cation exchange membrane was used to inhibit the iodides movement towards the negative electrode. Also, it has been evaluated that ageing protocols impact the lifespan of ECs differently.
As already mentioned, the electrolytes selected for research included the capacitive and redox-active ones. Thus, we covered ageing mechanisms in iodide and bromide-based electrolytes (MeI, where Me= Na+, K+, Rb+) and sulfate or nitrate-based ones (Me2SO4 where Me=Li+, Na+, K+, Rb+, Cs+ and LiNO3).
The electrolytic solutions containing redox active species (KSeCN; also mixed with neutral sulphate salts) have been proposed as electrolytes for ECs giving maximum operating voltage 1.4 V, high capacitance and relatively long cycle life. Moreover, KSCN-based electrolytes modified by gold nanoparticles (AuNPs) were tested. Various conditions were taken into account – different gold nanoparticles size (10, 50 and 100 nm) and their concentration in solutions (in the range from 0.1 – 4 nmol/L). 10 nm nanoparticles in 2 nmol/L concentration were chosen as optimal ones. Thanks to nanoparticles addition to redox active electrolyte power of the device was enhanced and cyclability was prolonged. Other electrolytes (Li2SO4 and KI solutions) were also modified by AuNPs. However, this additive has a meaningful effect only on KSCN-based electrolytes in ECs.
It should be said that IMMOCAP project interest was not only focused on the full cell performance, but also on gaining fundamental knowledge (in three-electrode or in-situ cells) to determine interactions at the electrode/electrolyte interface. Hence, electrochemical quartz crystal microbalance (EQCM) has been implemented. It has been found that aqueous-based electrochemical capacitors reveal the different ageing mechanism and failure reason among various electrolytes. Furthermore, the ageing protocol used during studies impacts the cell’s performance fade. So far, various voltages, current densities and ‘ageing protocols’ have been implemented, showing their influence on physicochemical changes of components and, as a consequence, system fade.
Electrochemical dilatometry experiments were also performed. Microporous activated carbon electrodes were polarised positively and negatively in Li2SO4 aqueous solution. Simultaneously, volumetric changes of the electrode were recorded. Definitely, higher electrode expansion was observed upon a negative polarisation than in the case of a positive one. The results are comparable regardless of the technique used. Interestingly, we found also, that the dilatometric changes depend on the pH value that is significantly changing during capacitor operation and governs the failure mechanisms the cell is subjected to.
In-situ and operando techniques supported the research on the influence of electrolyte viscosity on the electrochemical capacitor. It was performed by the controlled addition of viscosity agent to aqueous Li2SO4 solution. For this task, the electrowetting investigation procedure was designed and realised – it included the on-line investigation of the contact angle change upon polarisation of the electrodes.
Interesting results were obtained with X-Ray Tomography, performed at Kansai University (Japan). It has been proved that the solid-state deposit is being formulated during long-term operation. The mechanism proposed postulates the transfer of OH- through the separator and precipitation of lithium carbonate (as CO2 might evolve on the positive side).
We have also performed on-line observation of microscopic changes of carbon structure with Scanning Electrochemical Microscopy (SECM), operation methods development. Additionally, for further examination, new customised cell was designed and manufactured.
The analysis of carbon/electrolyte interaction by deconvolution of the current components in the electrochemical capacitor by the implementation of novel Step Potential Electrochemical Spectroscopy (SPECS) technique was done, to identify the mechanisms of the charge storage. It has been realised in cooperation with our partners from Australia (Prof. Scott Donne, University of Newcastle).
The electrochemical capacitor behaviour under polarisation was also studied with simultaneous internal pressure measurement. This provided important information on the volumetric changes within the capacitor device operating with an aqueous medium and gave the supporting data for the analysis of data originating from electrochemical measurements. The extension of the voltage window was proved when the electrodes were oxidised and ammonia-treated. High cell voltage brought about the demand for experiments on pH fluctuations inside the cell, and the preliminary results on this topic were obtained.
Determination of capacitor power properties under various testing conditions was also realised for highly viscous electrolytes. It appeared that higher viscosity does not necessarily mean lower conductivity. For this research, we designed and produced various devices for investigation.
Finally, we developed hybrid electrochemical capacitor operating in aprotic media that could be assembled in a very attractive way, without any external pre-metalation step. This device demonstrated very attractive performance, with energy density reaching 100 Wh/kg and cyclability thousands of cycles. We investigated and described the optimal assembly conditions and major failure mechanisms.
Our results have been presented in more than 30 papers published to date, 10 patent applications and 40 conference contributions (invited or keynote lectures, regular orals and posters) worldwide. We were also active in sharing our experience during seminars and workshops organized during international conferences, transferring knowledge mostly to less experienced researchers. Finally, the concept of hybrid electrochemical capacitor is being further developed in Proof of Concept project, as its performance is commercially attractive but requires verification in industrial environment.
Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)
Understanding the mutual correlations between the components of the electrode materials allows the electrode production conditions to be specified and further optimised. The high degree of controllability of their production is now considered to be reached. Furthermore, the results obtained from the study on EDLCs operating with highly viscous electrolytes in some way renew the information that might be found in the literature. It has been claimed that the use of such electrolytes allows the self-discharge to be reduced or the gas generated in the system to be diminished. Besides, essential knowledge about highly concentrated and viscous electrolytes helps to understand the phenomena associated with the formation of an electric double-layer at the electrode/electrolyte interface. Deep understanding of the phenomenon of charge accumulation in highly viscous media is found, including quantitative distribution between the electric-double layer storage and charge-transfer processes in total EDLC capacitance.
Besides deep research on highly concentrated electrolytes, we managed to choose the right compounds and select their molar ratio to obtain a deep eutectic mixture of acetamide and lithium nitrate and use it as an electrolyte in electrochemical capacitors, which no one has ever done before. At 100°C, the resulting device works better than commonly used capacitors based on aqueous solutions. We deeply believe that the knowledge acquired during the implementation of this topic will contribute to finding more formulations that will turn out as important for further development of safe and sustainable electrolytes. The recent literature review indicates that the subject is very attractive and deeply researcher further, thus our findings are timely and pertinent. To conclude this aspect, we were looking for cheap and effective solutions that will economically be able to replace currently used, expensive organic electrolytes. Moreover, we found an eco-friendly solution. We were also considering the use of immiscible solvents that will separately affect the positive and negative electrode of sustainable electrochemical capacitors, however, this study is still ongoing, as there are several aspects to be finally clarified. It is however clear that immiscible electrolytes require special research attention to become attractive in electrochemical energy conversion and storage systems.
Besides the fundamental research on specific electrolytes, a broad review concerning sustainable electrode materials has been done (and published) for a deep understanding of the electrode material side. Furthermore, the synthesis of sustainable carbon-based material, that plays a role of electrode’s active component has been conducted. For this purpose, the combination of salt- and soft-template methodologies have been coupled. As a result, materials of characteristics very competitive to the commercial activated carbons have been proposed. This study has been done in cooperation with partners from IS2M in Mulhouse, France. This scientific direction is planned to be further explored within the project to tune the material properties with respect to the utilised electrolyte. Moreover, as operating conditions have been proven to impact the ECs ageing (voltage, current, various protocols), a detailed study on the ‘ideal’ ECs application is ongoing. For each electrolyte studied, all discoveries come along with an improvement recipe, either from the side of cell construction, electrolyte composition or cell reuse.
In this context, the research performed allowed the aqueous medium capacitor failure reasons to be identified. Comparison of different capacitor ageing processes has been carried out. It was found out the ageing mechanisms strongly depend on the electrolyte, electrode composition and electrochemical protocol applied (a type of life-time test, voltage and current density). The conclusion drawn from the studies show that each system must be optimised independently, and to inhibit capacitor failure, it is necessary to counteract the causes. Here, we also proposed numerous carbon-based materials of tailored properties that can sustain high current loads.
The research was focused on such a target. Firstly, an electrochemical protocol which successfully extends the ECs operation time has been developed. Moreover, according to the “zero waste” principle, the trials with the chemical recovery of the positive electrode or reuse of negative one resulted in novel protocol for electrode re-use.
A part of the conducted research aims at the improvement of the fundamental knowledge about the interaction of electrolyte/electrode contact behaviour. Especially regarding the ageing of carbons material in conditions of constant load. Application of the SPECS technique, together with in-situ dilatometry, SECM, and electrowetting investigations, will allow the specific interactions of electrolyte with the polarised electrode to be identified. The electrolyte behaviour depends on the current/polarisation direction, which leads to differences in the carbon structure and the ageing process. The anticipated results will bring closer the possibility of developing the electrochemical capacitors with improved life-span.
Based on electrochemical dilatometry studies, coupled with scanning electrochemical microscopy (SECM) and electrochemical quartz crystal microbalance (EQCM), a detailed description of ionic fluxes and EDL formation at carbon electrode/electrolyte interface in aqueous electrolytes was presented. It is demonstrated that not only anions and cations take part in EDL formation but also ions from water dissociation. This phenomenon is more than complex in aqueous solutions because of the presence of the hydration shell, which can be partially or totally removed when ions are entering into the pores of the electrode. These results bring valuable insight into the ageing of the carbon electrodes, as the origin of structural changes was clearly identified.
The progress beyond state of the art includes the pH measurements under capacitor polarisation. This phenomenon is known but has not been measured and presented so far. The only knowledge to date concerns the post-mortem analysis. Our approach provided an in-depth understanding of internal ion flow and helps in the determination of the mechanisms which govern the cell behaviour at increased voltage. It has been proved that the proper modification of electrodes increases the overvoltage for aqueous electrolyte decomposition. The unique strategy for the extension of cycle life includes the pressurisation of the cell, first using carbon dioxide (as it is the main product of carbon oxidation at positive electrode) and then other gases, to take control over the equilibria, not only at the electrode/electrolyte interface but also in the electrolyte. Another operando measurement is gas chromatography coupled with mass spectrometry which provides important information on the evolved gaseous species during capacitor ageing.
Finally, we proposed novel approach to pre-metalation of electrochemical capacitors exploiting insertion processes (in aprotic media) that makes them commercially attractive. We proposed also optimal charging and operating conditions that ensure extraordinary cycle life.
Besides deep research on highly concentrated electrolytes, we managed to choose the right compounds and select their molar ratio to obtain a deep eutectic mixture of acetamide and lithium nitrate and use it as an electrolyte in electrochemical capacitors, which no one has ever done before. At 100°C, the resulting device works better than commonly used capacitors based on aqueous solutions. We deeply believe that the knowledge acquired during the implementation of this topic will contribute to finding more formulations that will turn out as important for further development of safe and sustainable electrolytes. The recent literature review indicates that the subject is very attractive and deeply researcher further, thus our findings are timely and pertinent. To conclude this aspect, we were looking for cheap and effective solutions that will economically be able to replace currently used, expensive organic electrolytes. Moreover, we found an eco-friendly solution. We were also considering the use of immiscible solvents that will separately affect the positive and negative electrode of sustainable electrochemical capacitors, however, this study is still ongoing, as there are several aspects to be finally clarified. It is however clear that immiscible electrolytes require special research attention to become attractive in electrochemical energy conversion and storage systems.
Besides the fundamental research on specific electrolytes, a broad review concerning sustainable electrode materials has been done (and published) for a deep understanding of the electrode material side. Furthermore, the synthesis of sustainable carbon-based material, that plays a role of electrode’s active component has been conducted. For this purpose, the combination of salt- and soft-template methodologies have been coupled. As a result, materials of characteristics very competitive to the commercial activated carbons have been proposed. This study has been done in cooperation with partners from IS2M in Mulhouse, France. This scientific direction is planned to be further explored within the project to tune the material properties with respect to the utilised electrolyte. Moreover, as operating conditions have been proven to impact the ECs ageing (voltage, current, various protocols), a detailed study on the ‘ideal’ ECs application is ongoing. For each electrolyte studied, all discoveries come along with an improvement recipe, either from the side of cell construction, electrolyte composition or cell reuse.
In this context, the research performed allowed the aqueous medium capacitor failure reasons to be identified. Comparison of different capacitor ageing processes has been carried out. It was found out the ageing mechanisms strongly depend on the electrolyte, electrode composition and electrochemical protocol applied (a type of life-time test, voltage and current density). The conclusion drawn from the studies show that each system must be optimised independently, and to inhibit capacitor failure, it is necessary to counteract the causes. Here, we also proposed numerous carbon-based materials of tailored properties that can sustain high current loads.
The research was focused on such a target. Firstly, an electrochemical protocol which successfully extends the ECs operation time has been developed. Moreover, according to the “zero waste” principle, the trials with the chemical recovery of the positive electrode or reuse of negative one resulted in novel protocol for electrode re-use.
A part of the conducted research aims at the improvement of the fundamental knowledge about the interaction of electrolyte/electrode contact behaviour. Especially regarding the ageing of carbons material in conditions of constant load. Application of the SPECS technique, together with in-situ dilatometry, SECM, and electrowetting investigations, will allow the specific interactions of electrolyte with the polarised electrode to be identified. The electrolyte behaviour depends on the current/polarisation direction, which leads to differences in the carbon structure and the ageing process. The anticipated results will bring closer the possibility of developing the electrochemical capacitors with improved life-span.
Based on electrochemical dilatometry studies, coupled with scanning electrochemical microscopy (SECM) and electrochemical quartz crystal microbalance (EQCM), a detailed description of ionic fluxes and EDL formation at carbon electrode/electrolyte interface in aqueous electrolytes was presented. It is demonstrated that not only anions and cations take part in EDL formation but also ions from water dissociation. This phenomenon is more than complex in aqueous solutions because of the presence of the hydration shell, which can be partially or totally removed when ions are entering into the pores of the electrode. These results bring valuable insight into the ageing of the carbon electrodes, as the origin of structural changes was clearly identified.
The progress beyond state of the art includes the pH measurements under capacitor polarisation. This phenomenon is known but has not been measured and presented so far. The only knowledge to date concerns the post-mortem analysis. Our approach provided an in-depth understanding of internal ion flow and helps in the determination of the mechanisms which govern the cell behaviour at increased voltage. It has been proved that the proper modification of electrodes increases the overvoltage for aqueous electrolyte decomposition. The unique strategy for the extension of cycle life includes the pressurisation of the cell, first using carbon dioxide (as it is the main product of carbon oxidation at positive electrode) and then other gases, to take control over the equilibria, not only at the electrode/electrolyte interface but also in the electrolyte. Another operando measurement is gas chromatography coupled with mass spectrometry which provides important information on the evolved gaseous species during capacitor ageing.
Finally, we proposed novel approach to pre-metalation of electrochemical capacitors exploiting insertion processes (in aprotic media) that makes them commercially attractive. We proposed also optimal charging and operating conditions that ensure extraordinary cycle life.