Supervisors: Prof. Maria de Fátima Grilo da Costa Montemor

The main objective of the thesis was to design and test a new energy storage device, a hybrid between a battery and supercapacitor, to evaluate its performance and to compare it with currently available electrochemical devices, namely lead-acid batteries and electrochemical double-layer supercapacitors . Energy storage is one of the key solutions that will lead to the implementation of sustainable energy systems. In this thesis, new hybrid electrochemical energy storage device, between a battery and supercapacitor, was tested to check its performance and to compare it with currently available electrochemical devices, especially with lead-acid batteries and electrochemical double-layer supercapacitors.

Energy storage

Mechanical energy storage

A pumped hydro storage system is today the most efficient and mature way of large-scale energy storage. Flywheels are among the most widely used kinetic energy storage technologies and have attracted much attention from researchers. Compressed air energy storage is a well-known technology that uses internal energy storage and can be divided into three main parts: diabatic, adiabatic (isentropic) and isothermal.

Chemical energy storage

The operating principle of the solar power plant, schematically shown in Figure 7, is similar to that of a conventional power plant, with the difference that no fuel is burned. Secondary batteries, due to the properties of the electrode material, can be charged and, depending on the type of rechargeable battery, used several times from tens to thousands of times. Lithium ions are stored only in the layered structure of the host material, without any chemical reaction.

Since the thickness of the electrochemical double layer is very small (only a few nanometers) compared to electrostatic and electrolytic capacitors, the capacitance (which is inversely proportional to the thickness) is very high. The nature of the electrolyte depends mainly on the final use of the specific supercapacitor because it determines its voltage. The charge/discharge processes may be longer due to the kinetics of the reactions taking place.

Simultaneously with the Faradaic processes, there is formation of the electrochemical double layer that occurs on the surface of active material exposed to the electrolyte. Typically, about 2/3 of the stored charge comes from the redox reactions, while up to 1/3 can be contributed by EDL. In this figure, at the extremes of the development path, it is possible to find Thermochemical Energy Storage (left) and Pumped Storage Hydropower (right).

Figure 15 shows that when there is a need for fast system response of the specific technology, the higher the capital cost of the technology is due to the high reliability need of the system.

Batteries and supercapacitors

From the analysis of tab.3, it can be concluded that batteries have the worst performance in terms of power density, lifetime and efficiency, but higher energy density compared to supercapacitors. It can be seen from Figure 18 that battery technologies are suitable for almost all mentioned applications, while supercapacitors are only used in power system start-up, voltage support and power quality. One of the electrodes used to assemble the hybrid device is based on Ni-Co transition metal oxide/hydroxide coatings with a 3D dendritic structure and was fabricated directly on thin stainless steel current collectors in a single and rapid step with no need to introduce binders and foreign additives.

Ni-Co electrodes were produced by a very simple, inexpensive and environmentally friendly process called electrodeposition. Figure 19 shows a cross-section of the Ni-Co structures acting as the active material formed directly on the stainless steel collector. It can be seen that the formation of dendritic structures significantly increases the specific surface area of ​​the active material.

Fig.20 shows Ragone plot in which the prototype developed by C2C at laboratory scale (with 1 cm2 electrodes) is compared with different technologies. The next step of the C2C technology is scaling up the electrode area for 50 times. C2C aims to test two different types of devices: a Faradaic supercapacitor using two identical electrodes made of the Ni-Co high-area dendritic coatings and a hybrid (or asymmetric) device using one carbon electrode (the anode) and the same Ni- Co uses dendritic coating as cathode.

Fig.21 shows the schematic view of the NiCo-NiCo and NiCo-C electrodes and also Fig.22 shows the schematic view of the NiCo-C prototype.

Aim of the thesis

Description of the experiment

Electrode quality test

Electrode quality testing was performed using a VoltaLab PGZ 100 potentiostat with integrated VoltaLab software, which is shown in Fig.24. Tests were performed by keeping the electrodes inside a container specially prepared for the test of cells made from these electrodes (Fig. 24 and Fig. 25). As received (new) Nickel Cobalt (NiCo) electrodes were placed in the container, wired and connected to VoltaLab PGZ 100 (Fig. 25), .. b) test parameters were set in Voltalab software (Fig. 26), .. applied charge- /discharge current: 40mA, .. c) electrodes were accepted as good if the previously set limits were achieved within a time frame close to 5 min; if the cycle took more than this time or less than 4 minutes the electrodes were rejected;.

NiCo-NiCo and NiCo-C electrode test

In the first task, a high voltage drop was noted for NiCo-C electrodes, as well as a significant loss of active material from the NiCo electrode. The cell was tested using the same protocol as for the previous NiCo-NiCo, NiCo-C and NiCo-C electrodes, in a higher potential window. Fig.28 shows the assembly of the electrodes and separator and Fig.29 and Fig.30 show the setup for Test 2.

It is worth mentioning that to avoid excessive destruction of active material electrodes, the cellulose separator and electrodes were wetted with KOH solution before mounting.

Capacitance fade

Calculations

32 Illustrative charge/discharge curve illustrating the ohmic drop and the potential window for extracting the capacitance. In addition, the ΔU value compared to the discharge curve (Fig. 33) was obtained in one of the tests, and it appears that the method used is sufficiently accurate. ΔU is the change in potential, V, calculated as before, R is the resistance, Ω, and was calculated as follows.

Vdrop is the voltage drop, V, value is taken from obtained results as the difference in maximum potential value and value of first discharge step,. Maximum Specific Energy and Power were calculated in the same way as Average Specific Energy and Power with one difference: ΔV was changed to Vmax which for NiCo-NiCo and NiCo-C was 1.45 V and for NiCo-C in higher potential window was equal to 1.60 V 3) Efficiency.

Electrode Test1 and Test2

It can be observed that the NiCo-NiCo cell shows a longer charge/discharge time for the lowest applied current compared to NiCo-C, but when the applied current was higher, the charge/discharge time for NiCo-C was longer compared to NiCo-NiCo . The highest values ​​of charge/discharge times were obtained for NiCo-C in the higher potential window. The lowest resistance was obtained for the NiCo-NiCo cell and decreased slightly with increasing applied current.

Moreover, it can be seen that the charge/discharge curve for NiCo-NiCo is more of the battery type (a curve) and for NiCo-C more of the supercapacitor type (a sloping line). The results show that the average specific power obtained for NiCo-NiCo plates in both tests is comparable due to the similar voltage drop values ​​obtained. Comparing the results of Test1 and Test2 for NiCo-C electrodes, it can be seen that the specific power values ​​obtained in the second test are much higher compared to the first test.

A comparison of the results from Test 1 and Test 2 for NiCo-C electrodes in a larger potential window shows that the specific power values ​​obtained in the second test are much higher than in the first test. The highest value of specific power (57.77 W/kg) and energy (11.4 Wh/kg) was achieved for NiCo-C electrodes in the highest potential window in Test 2. For NiCo-C electrodes, higher values ​​were achieved in Test 2 efficiencies, an effect that is also related to the voltage drop.

13 Charge/discharge times and efficiency for NiCo-C in a higher potential window for Test1 and Test2.

Capacitance fade experiment

It is worth emphasizing that while testing the active material, electrochemical activation was not taken into account and many electrodes were rejected because the results were not fully reproducible. In that case, consideration should be given to checking each electrode individually or alternatively disconnecting from the produced batch due to the fragility of the electrodes. This will not affect the performance of further used electrodes, because electrodes used in quality tests should not be used to assemble the hybrid cell.

In addition, the applied current for quality testing can be higher to reduce the time required for the test. The best results achieved for NiCo-NiCo (40 W/kg and 5 Wh/kg) NiCo-C (approx. 45 W/kg and 8 Wh/kg) NiCo-C in higher potential window (approx. 57 W/kg 11 Wh/ kg) was quite satisfactory compared to other available supercapacitor technologies. To avoid losing the excess material in the future, it is worth considering testing electrodes horizontally rather than vertically, as was done in all experiments.

Nevertheless, the decrease in capacitance was not significant for NiCo-C electrodes, but the most interesting was the result for NiCo-NiCo electrodes. Values ​​achieved for power and energy density were 352 W/kg and 32 Wh/kg respectively and were relatively high when placed on Ragone grounds and compared to others. This result places this device between the range of lead-acid batteries and close to lithium-ion batteries, with the advantage that the device can be charged and discharged at least up to 1000 cycles with very little capacitance decay and charged with very high efficiency (about 90% ).

These results are very promising for the future development of C2C prototypes. Figure 46 shows the maximum values ​​obtained in Test 2 and an additional calculated value for the maximum capacitance compared to other technologies.