The SunStorage project was a great collaborative adventure, but our journey has come to an end. It was a hard and perilous road, but we can say proudly that WE MADE IT!
Our SunStorage was a very ambitious and multidisciplinary project focused on the development of emergent technologies for sustainable, efficient and cost-effective energy storage. The research team involved top research laboratories and scientists in Portugal: LEPABE (UPORTO), CQC (UCoimbra) and NOVA.ID. FCT (LAQV-REQUIMTE, FCT-NOVA). The project was divided into 7 tasks, had 29 milestones and a total duration of 51 months. At the end, all indicators were surpassed and the achieved results allowed to place Portugal as a leading country in the developed technologies: Redox Flow Batteries (RFB), Solar Redox Flow cells (SRFC) and electro photoelectrochemical (PEC) CO2 reduction.
Activity 1 concerned the project management, financial and technical reporting and the communication among the consortium, FCT and PT2020. During the course of the SunStorage project, all partners communicated and exchanged information also without any concerns. The results of the project were highly disseminated throughout mainly peer-reviewed scientific publications, patents, workshops, conferences, and many other forms of dissemination. NOVA.ID.FCT promotor organized a National Workshop on “Carbon Capture and Conversion” at the Chemistry Department of FCT-NOVA and UPORTO promotor also organized a National Workshop on “Energy Storage Solutions”. The Workshops aimed to be used as a valuable communication tool for the general public, national and international scientific communities. Moreover, the team developed a dissemination video to increase the visibility of the project.
Activity 2 aimed at the development of photoelectrodes for PEC cells for solar charging RFB and for CO2 electroreduction, namely hematite, tantalum nitride and tungsten trioxide, combined in a tandem arrangement with emergent PV solar cells, such as DSSC and PSC. For their optimization, electrochemical impedance and transient absorption spectroscopy analyses proved to be powerful techniques. For the SRFC, a hematite photoelectrode was considered mainly due to the reported unprecedented photovoltage of 1.2 V and the longest stability (1000 h) with ferrocyanide/AQDS redox pairs – Figure 1a. Moreover, the control of hematite morphology by nanostructuring allowed to obtain a high photocurrent of ca. 4.5 mA/cm2 (Figure 1b); the previously reported hematite-based SRFC showed only 0.3 mA/cm2. This photoelectrode proved to be stable for > 1500 h under continuous illumination; at the time of the report submission the stability test was still running. A new SRFC based on iodide/AQDS redox pairs was also studied with success. The hematite yielded a stable performance in contact with iodide/iodine solution with pH 5.5 and by coupling in series a DSSC (tandem arrangement) a photovoltage of 1.6 V was reached that was sufficient to fully charge the SRFC. A new family of low-cost and high-performing photoelectrodes was also prepared – Ta3N5. Two types of photoelectrodes were synthetized: opaque and semi-transparent. For the opaque Ta3N5, a systematic study was performed that allowed to reach photocurrents of > 7.5 mA/cm2 and > 1 V photovoltage (Figure 1c). The thin and semi-transparent photoelectrodes were prepared by electrophoretic deposition, showing already > 4.5 mA/cm2 with reasonable stability and a photovoltage of > 1.5 V using a tandem configuration with a PSC. Both hematite and Ta3N5 photoelectrodes displayed world performance records on this field. Multi-layered WO3 photoelectrodes were also prepared for PEC CO2 reduction, displaying photocurrents of ca. 1.9 mA/cm2.
Figure 1. a) Hematite thin film operating stable over 1000 h in a ferrocyanide/AQDS. b) Best performing hematite photoelectrode with and without the Co-Pi co-catalyst displaying a photocurrent of ca. 4.5 mA/cm2 using 0.1 M K4[Fe(CN)6]·3H2O on 1 M NaOH (pH 14). c) Opaque Ta3N5 photoelectrode in ferrocyanide showing a record-breaking photocurrent of 7.5 mA·cm-2and photovoltage > 1 V.
Activity 3 addressed the optimization of SRFCs, aiming at studying new and promising redox pairs and electrolytes suitable for the prepared photoelectrodes, such as anthraquinones, acridones, ortho-diones, tryptanthrines and isatin derivatives as negolytes and ferrocyanide as posolyte; their electrochemical and photophysical properties were investigated. Two patents were filled, one regarding the concept of a SRFC and the other aiming at water-soluble trytanthrin sulfonic derivatives for RFB. Since the reactor design greatly influences their overall performance, several cell arrangements were also studied for the design and construction of innovative lab scale SRFC devices (from < 1 up to 25 cm2), as shown in Figure 2. The first ever dynamic multi-dimensional model for a vanadium SRFC was developed, incorporating charge conservation equations, species and charge transport equations and photoelectrochemical and electrochemical reaction equations. The simulation and experimental matched closely.
Figure 2. SRFC devices developed at LEPABE – UPORTO during SunStorage, presenting different active areas (4 cm2; 0.28 cm2 and 25 cm2).
Activity 4 aimed at the development of PEC reduction of CO2. Several tasks were performed, namely the spectroscopic and photophysical investigation of hybrid membranes of sulfonated poly(ether ether ketone), SPEEK, and porphyrins and synthesis of ionic liquid electrolytes. A disruptive electrochemical PEM reactor was developed for gas-phase CO2 electroreduction using modified Nafion membranes demonstrating high efficiencies and current densities – patent filled. Moreover, the liquid-phase CO2 reduction at high pressure was reported for the first time using an innovative design (Figure 3), which is advantageous for gases transportation and storage. Further work was focused in replacing the commercial Zn foil by a more active catalytic cathode based on 3D monolithic porous aerogels. This work culminated in filling a patent. Zn-Cu catalysts with proved high activity for syngas production were prepared in the form of bimetallic Zn-Cu aerogels of high surface area aiming at enhancing current density of CO2 electroreduction.
Figure 3. Photoelectrochemical cell for solar-driven electroreduction of CO2, allowing the illumination of the photoanode by a Xenon lamp through an optical fibber and the projected light on a paper sheet.
Within activity 5, a systematic assessment of operating conditions (charge/discharge current density and electrolyte flow rate) and battery components (electrode compression and membrane) influencing the performance of a lab-scale RFB was conducted, envisaging the upscale of the RFBs. A new design, named Oriented Distribution Path was developed; charge/discharge cycling, polarisation curves and EIS analysis, as well as CFD simulations were performed for its optimization. As the flow rate increases from 10 to 300 cm3/min, the limiting current density increased from 329.2 mA/cm2 up to 1178.0 mA/cm2 and more recently up to 1.8 A/cm2, with >150 mA/cm2 @ 80 % of round-trip-efficiency (> 400 mA/cm2 @ 1 V) – the target value was 100 mA/cm2 @ 1 V. Moreover, very selective membranes of SPEEK were developed that showed lower proton conductivity and cost when compared with commercial Nafion115 at basic pH (publication under submission).
Activity 6 aimed at the development of a RFB prototype (1 kW-class stack) with current density of 100 mA/cm2 at 1 V. Different prototypes were constructed: i) a 300 cm2 VRFB (37-cell stack) able to operate at 80 mA/cm2 with 80 % energy efficiency and with ca. 200 mA/cm2 @ 1 V/cell – Figure 4; ii) a 1250 cm2 stack displaying similar performances to the ones available in the market, i.e. round-trip-efficiencies ranging from 81 % at 60 mA/cm2 to 75 % at 96 mA/cm2; and iii) two additional 10 kW stacks with 55 cells and one with 37 cells creating a 30 kW system – Figure 5. The latter generation based on a stack of 5 cells yielded ca. 250 mA/cm2 @ 1 V/cell, more energy efficient than the previous generations. This prototype is able to operate at 10 kW with 66 % of round-trip-efficiency and at the maximum current of 110 A per stack – Figure 6. A patent disclosing the design of this stack was filed and licensed to VisBlue. For the SRFC technology, the constructed PEC cell was named SolarFlow25 cell and its photocharging performance was demonstrated for the first time using a 25 cm2 photoelectrode (hematite photoelectrode paired with a DSSC) and a ferrocyanide-anthraquinone chemistry (with a pH gradient of 10 vs 14). An average unbiased photocurrent of ca. 11 mA was recorded during photocharge, followed by discharge at -15 mA; coulombic efficiencies > 80 % were obtained – Figure 7. For PEC CO2 reduction, an innovative PEC cell with ca. 36 cm2 was demonstrated operating at high pressures and with a current range of 1.7–1.9 mA/cm2. A scaled up 50 cm2 cell, named CoolPEC cell, was also designed and tested over 1008 h at 45 °C. Very recently, a new 200 cm2 PEC module, comprising four 50 cm2 PEC cells, was optimized and tested under outdoor conditions. The performance values obtained for the three technologies are very competitive given the large-area adopted and represent outstanding landmarks in the field.
Figure 4. a) 37-cell stack (Version 5); b) Polarization curve for charge and discharge phases for the 37 cell stack.
Figure 6. Three 10 kW batteries installed and ready to be operated.
Figure 7. SolarFlow25 cell: a) Representative device cycling behavior; b) Efficiency-related metrics recorded for 10 cycles.
Activity 7 concerned the life-cycle assessment and sustainability evaluation of the developed technologies. It was found that the PEC devices for charging electrochemical fuels and producing solar fuels should improve the current density before reaching commercial competitiveness. The RFC are already commercial, but it was recommendable to reduce the content in copper, for the end planes, as it has a high environmental impact.
We bid you all farewell and best of luck on all future endeavors,