The application of multijunction solar cells in photoelectrochemical (PV-EC) devices is addressed. Integrated PV-EC devices can be composed of a ‘traditional’ photovoltaics (PV) cell combined with electrochemical (EC) cell, presenting a promising approach to produce fuels, for example hydrogen. The requirements for PV-EC devices and strategies for the solar cell development will be addressed. The results on the photovoltaic development of multijunction silicon based cells will be presented, focusing on a wide range of photovoltages and photocurrents in various systems, including the adoption of the cells to function on either cathode or anode sides of the system. A prototype integrated PV-EC system based on silicon multijunction solar cells can yield solar to hydrogen efficiencies (STH) of 9.5%.

The strategies of device upscaling beyond laboratory size and the influence of varied illumination conditions close to obtained outdoor will also be discussed. This includes the effects of spectral quality, intensity and incident angle on the performance of both photovoltaic part and PV-EC devices.

            Imagine an energy vector with a round-trip-efficiency > 80 % that is easily and efficiently chargeable using either sunlight or biomass. Imagine that this energy vector can produce dispatchable electricity and chemical fuels. When “solar rechargeable battery” concept was first proposed at the beginning of the 80s [1], it aimed at to play an important role on harvesting and storing sunlight energy[2].[1] However, there was no more research going on in this topic until 2013 when Yang et al. [3] published an article integrating a dye sensitized solar cell (DSSC) with a redox flow battery (RFB) for converting sunlight into storable chemical energy, displaying a solar-to-chemical energy conversion efficiency of ca. 0.09 %. In 2016, Wedge et al. [4] reported the first solar aqueous alkaline RFB using low cost, environmentally safe and stable materials: hematite photoelectrode and ferrocyanide/AQDS redox pairs. This revamped the idea of using photoelectrochemical (PEC) cells for charging two electrochemical fuels in an integrated technology, which was named solar redox flow cell (SRFC). In 2020, the world record solar-to-electrochemical energy conversion efficiency reached 20 % [5], demonstrating the potential of SRFCs over more conventional PEC technologies and over conventional PV-redox flow battery approach – Figure 1.

            This talk will present the latest developments in the field, addressing critical sustainability pathways: i) stable and efficient earth-abundant photo-absorber materials; ii) stable and high energy density redox pairs, made of earth abundant elements, matching with the energy levels of the semiconductors; and iii) optimized SRFC device architectures suitable for large-scale solar fuels production. Mimicking the nature, a SRFC device can be combined with a very recently disclosed Microbial Redox Flow Cell (MRFC) [6] to produce continuously – day and night, summer and winter – the electrochemical fuels. Finally, charged redox pairs can be used to produce dispatchable electricity but also to produce chemicals either through direct redox reactions or following an electrochemical approach.

[1] In 1991, Kumar et al. [2] asked “The entire discussion is geared towards answering a relevant question: what has gone wrong to result in the stagnation and failure in commercialization of a PEC based solar cell?”

References

1.         Licht, S., et al., A light-variation insensitive high efficiency solar cell. Nature, 1987. 326(6116): p. 863-864.

2.         Sharon, M., et al., Solar rechargeable battery—principle and materials. Electrochimica Acta, 1991. 36(7): p. 1107-1126.

3.         Liu, P., et al., A Solar Rechargeable Flow Battery Based on Photoregeneration of Two Soluble Redox Couples. ChemSusChem, 2013. 6(5): p. 802-806.

4.         Wedege, K., et al., Direct Solar Charging of an Organic–Inorganic, Stable, and Aqueous Alkaline Redox Flow Battery with a Hematite Photoanode. Angewandte Chemie International Edition, 2016. 55(25): p. 7142-7147.

5.         Li, W., et al., High-performance solar flow battery powered by a perovskite/silicon tandem solar cell. Nature Materials, 2020.

6.         Santos, M.S.S., et al., Microbially-charged electrochemical fuel for energy storage in a redox flow cell. Journal of Power Sources, 2020. 445: p. 227307.

In terms of technological readiness, the most feasible approach for solar energy utilization would be a photovoltaic (PV) panel. However, the PV is faced with challenges concerning the security of supply because of the intermittent nature of the sun. In this context, solar rechargeable redox flow battery (SRFB) technology is being in the spotlight as a mean of simultaneously storing the solar energy into chemicals, which can be readily utilized to generate electricity via reversible reactions [1,2].

However, the plain fact is that there is that most studies overlook practical challenges arising from the inherent instability and degradation of the system under the light and heat. According to our recent theoretical modeling, silicon-based solar PV system shows a severe power-loss exceeding 20% compared to the ideal case [3]. In this work, above described thermal degradation is quantified by introducing a thermo-electrochemical model, which covers both heat-transfer and electrochemical studies to minimize the gap in performances between the laboratory and practical working environments with drastic temperature change and thermal shocks.

The main focus of the study is on avoiding the thermal efficiency loss at a high temperature, which has been a critical technical barrier for the practical application. In our model system, the electrolyte flow acts as a coolant-storage multi-functional medium, and it stabilizes the operating temperature of the photo-charging system via a heat-transfer regime at solid/liquid interface between the solar-driven device and electrolyte.

Energy generation devices have been grown tremendously from fast few decades, but the system is still limited with energy storage devices. Storing energy in the batteries is not the permanent solution, nevertheless energy can’t be store longer period. Only way to store energy for longer period is storing energy in chemical bonds, such as fuels, gas or chemicals. Therefore, concept of electrochemical energy storage device has been rising recently, especially electrochemical organic synthesis, has made a footprint for the green synthesis of value-added chemicals. Impotently these electrochemical processes can be renewable.  Considering, carbon neutral industry promise – biomass has great potential for providing many flatform chemicals. The one of the elements of biomass, 5-hydroxymethylfurfural (HMF) and its oxidized form 2,5-furandicarboxylic acid (FDCA) is a monomer of biobased polymer and its properties are superior than the PET. We have investigated efficient electrochemical conversion of HMF to FDCA using abundant nickel as a catalyst. We investigated the key factors for tuning the chemical selectivity for HMF oxidation over the competing oxygen evolution reaction (OER) at the catalyst surface. We show that the selectivity for HMF oxidation is enhanced by removing trace impurities of iron species as well as adjusting the composition of the alkali hydroxide electrolyte solution. LiOH electrolyte without iron impurities is more favorable for HMF oxidation and whereas CsOH with iron species present is more active for OER and unfavorable for HMF oxidation. Under optimized condition we have achieved 98% faradaic efficiency for the production of FDCA from HMF, with iron free 1M LiOH electrolyte (pH 14). This simple approach can be used as model system for other electrochemical organic synthesis, where OER is competing process.

Redox flow batteries offer a reliable solution for future grid-scale energy storage. In comparison with mostly investigated and commercialized vanadium flow batteries, aqueous zinc-iodide flow battery is a highly promising contender due to its cost-effectiveness, environment friendliness, safety, and high energy density. However, there are still obstacles to overcome before utilizing its full potential. Therefore, various research is under investigation to improve the battery design by optimizing its components towards the development of high-performance batteries. Among those cell components, electrolyte has a major contribution in scaling up cell performance. Due to an imbalance of concentration of solutes between the anode and cathode compartments during electrochemical cycling, the differential osmotic pressure results in water migration through the ion-permeable membrane from the compartment with lower ionic strength to the compartment with higher ionic strength, which is usually overlooked by using a static positive compartment, limiting its capacity. The addition of extra solute to the electrolyte solution is a way out to resolve this issue and reach an ionically balanced situation between two compartments. In this work, we have carried out the experimental analysis of a lab-scale Zn-I flow battery as per our theoretical ionic strength calculations to reach an overall balanced system. From the theoretical calculation, it has been proven that by adding extra 2 M potassium iodide (KI) to an electrolyte of 1.5 M ZnI₂: KI, the ionic balance between the compartments could be achieved. We have performed electrochemical impedance spectroscopic (EIS) technique as a tool to study the solution resistance and ionic conductivity of the half-cell compartments. Further full cell cycling following post-mortem analysis of the half-cell electrolytes ensured no water migration between compartments during cycling, also excellent cycling efficiencies have been achieved. Overall, this mechanism shows an efficient path for the improvement in the electrolyte design in the development of high-performance Zn-I flow batteries, which enlighten a step forward to the research in next-generation aqueous flow batteries.

Keywords: Aqueous Zn-iodide flow batteries, water migration, osmotic pressure, electrochemical impedance spectroscopy

Organic-inorganic and all-inorganic lead halide perovskites (APbX₃) have made a huge step towards highly efficient emergent photovoltaic (PV) technology with already 25.2% power conversion efficiency. The high defect tolerance as well as their suitable and tunable optoelectronic properties made them an attractive alternative to the silicon and thin film PV technologies. Apart from that, perovskites show versatility in their applications fields e.g. in LEDs or photoconductors but also in energy storage such as perovskite solar cell coupled self-rechargeable batteries. However, one of the major problems encountered with Pb halide perovskites, apart from structural and chemical stability, is the toxicity associated with heavy metal lead. Potentially less toxic bismuth (Bi) halide perovskites were reported to possess promising optoelectronic properties including a high absorption coefficient and can be processed from solution using a variety of wet chemical deposition techniques and additives. In this work, different Bismuth based perovskite-like materials were screened by varying the A-site of the A₃Bi₂I₉ with organic monovalent cations like CH₃NH₃⁺ (methylammonium, MA), CH(NH₂)₂⁺ (formamidinium, FA), (CH₃)₂NH₂⁺ (dimethylammonium, DMA), (guanidinium, NH₂)₃⁺ (GA), C₅H₆N (pyridinium, Pyr), new C₄H₅N₂ (pyridazinium, Pyz) and inorganic alkali metal cations for photovoltaic application. Furthermore, Silver bismuth iodides have gained recent attention as a stable alternative to lead perovskite photovoltaics. Ag₃BiI₆ represents a rudorffite structure and can be used in the same solar cell architecture. Herein, Ag₃BiI₆ thin films were investigated experimentally and theoretically as solar cell absorbers and their degradation was attributed to ion diffusion of highly mobile AgI.

Tactile or electronic skin is needed to provide critical haptic perception to robots and amputees, as well as in wearable electronic systems that are used for health monitoring and wellness applications. Energy autonomy of skin is a critical factor in these application to enable portability and longer operation times. This lecture will present an energy autonomous electronic skin based on a novel structure, consisting of graphene based transparent tactile sensitive layer integrated on photovoltaic cells. Transparency of the touch sensitive layer allows the photovoltaic cell to effectively harvest light. The touch sensitive layer requires ultralow power (20 nW/cm²) for its operation and this leads to surplus energy generation by the photovoltaic cells underneath. The lecture will also present our advanced version of electronic skin, where no touch sensor is used and yet the innovative arrangement of solar cells allows touch sensing from large areas. Given that there is not separate touch sensor, this advanced version of electronics skin does not consume any energy for sensing operation and instead only produces energy. If this skin is present over large areas, as human skin is present over whole body, then it could generate sufficient energy to power devices such as actuators used in robotics and prosthetics. Such scenarios enabled by the energy autonomous electronics skin integrated on robotic hand will be presented in the lecture along with the tasks such as grabbing of soft objects.

The photovoltaic (PV) module directly integrated of solar conversion and electrochemical energy storage provides a new and promising insight towards solar energy utilization. Particularly, solar rechargeable redox flow batteries (SRFB) offer a cost-effective and compact solution to solve the photovoltaic intermittency issues. Despite of the large state of the art of SRFB, the actual technology is hampered by low efficiencies and “extra” potential is required for charging the battery. Among, all RFB, all vanadium is the chemistry most development and most mature, being worldwide commercialized. The vanadium redox flow battery (VRFB) presents a standard cell potential is 1.26 V, which in practise could arise up to 1.7V. This operational potential is extremely high to pair with PV commercial modules, supposing a great challenge for the solar recharging process. Herein, the design of several configurations using different materials will be presented, obtaining results quite encouraging towards the realization of the Solar-charged -VRFB (SVRFB). Two approaches have been followed: (1) inexpensive Cu(In,Ga)Se₂ modules disposed in 3 and 4 series-connected cells, allowing the full unbiased photocharge under 1 Sun illumination. The resulted SVRFB device can deliver excellent energy efficiency (77%) and solar-to-charge efficiency (7.5%) [1]; (2) Triple junction TF silicon solar cell under illumination (300 mW cm^-2), operating at 25 mA cm^-2 as bias free-photocurrent [2]. In that case, the energy density achieved values up to 54 Wh L^-1, while the solar-to-output electricity efficiency was roughly 10%. Finally, several strategies to increase the photocharge of the negative half-cell reaction in Vanadium redox flow batteries is discussed [3].