Energy storage plays, undoubtedly, a fundamental role in the process of total decarbonization of the global economy that is expected to take place in the coming decades. The energy transition to a renewable and sustainable generation is the solution to reduce greenhouse gas emissions and thus achieve the European Commission´s goal of becoming the world´s first decarbonized economy. In this transition, beyond li-ion technologies will play a key role to meet the increasing energy demand that cannot be covered by Li-ion batteries solely. In this scenario, sodium-oxygen and zinc air batteries have become an attractive alternative due to their high gravimetric energy densities (1605 or 1108 Wh/kg based on Na₂O₂ or NaO₂, respectively; and 1086 Wh/kg based on ZnO discharge products) resulting from the use of an oxygen-based phase-change reaction (potentially reducing the weight and freeing up space for other components).

In this talk, a general overview of these systems will be given along with the materials requirement and system performance. These batteries have been considered as the holy grail of battery research due to their high theoretical energy densities; however, several challenges remain to be solved before commercialization.

Renewables are essential for the energy transition to a sustainable and eco-friendly energy system that avoids the use of fossil fuels. One of the major limitations of renewables is their intermittent character which depends on climate conditions. This climate dependence leads to a complete disconnection between energy production and demand. Electrochemistry, fed by renewables, has emerged as a smart procedure that can pave the way for the energy transition. The main chemical procedures studied in electrochemistry are water splitting and CO₂ reduction. In both cases, the oxygen evolution reaction (OER) takes place at the anode, providing the electrons and protons needed in the cathode for the generation of H₂ in water splitting or the conversion of CO₂ into valuable species, such as CH₄ or CH₃OH. The OER produces O₂ that despite being a very important compound, its price in the market is very low, which limits the economic viability of the process and ultimately reduces the interest in this technology.¹ Furthermore, this reaction exhibits large overpotentials when using catalysts based on earth abundant materials, limiting energy conversion efficiency.² For these two reasons, there is a growing interest to find alternative reactions to OER at the anode that, by one side, reduce the overpotentials needed and by the other, produce compounds with higher added-value and interest for the chemical industry.^1,3 In this framework biomass valorization, has emerged as an attractive substitute to the oxidation of H₂O.⁴ Herein we present our more recent results in the electrochemical transformation of biomass, as well as the use of electrochemistry for energy storage.

Solid oxide fuel cells (SOFCs) are capable of providing high energy efficiency and fuel flexibility leading to use of hydrogen and hydrocarbons due to high operation temperature (600–800°C). Recently, ammonia (NH₃) is considered as one of the most promising hydrogen carriers because of great advantages of zero-carbon emission, a high hydrogen content and worldwide infrastructures established for production, transport and storage. Thus, much effort has been given to directly utilize ammonia as a fuel for SOFCs, so called direct ammonia (DA)-SOFCs in which the fuel is decomposed hydrogen and nitrogen, and the released hydrogen is electrochemically oxidized in the anode consecutively. Thus, DA-SOFCs are believed to accomplish a CO₂-free power generation with high efficiency. In this study, the current status on the development of SOFCs fed with ammonia, and strategical approaches to overcome technical issues limiting the development of DA-SOFCs will be introduced. In addition, results on the application of an exsolution catalyst to decompose the fuel in the anode of DA-SOFCs are present, which has proven the feasibility of ammonia fuel for high performance and reliable SOFCs.

Abstract

One of the challenges faced by photoelectrochemical (PEC) water splitting technology is the relatively high levelized cost of hydrogen (LCOH) compared with other hydrogen generation methods. A potential strategy to increase its competitiveness is to couple PEC water splitting with hydrogenation reactions that produce valorized chemicals[1][2]. In this study, we evaluate the economic potential of co-producing hydrogen and methylsuccinic acid (MSA) by coupling the hydrogenation of itaconic acid (IA) inside a PEC water splitting device based on BiVO₄ and silicon heterojunction (SHJ) absorbers[3][4].

This study examines the economics of a 1000 kg H₂/day rated capacity PEC plant with the coupled co-production feature by conducting a techno-economic assessment (TEA) under a base case scenario where PEC devices with solar-to-hydrogen (STH) efficiency of 10% and longevity of 20 years are considered. When H₂ is the only product, the obtained cradle-to-gate LCOH is 22.3 €/kg. However, with the coupled hydrogenation reaction, the LCOH can be reduced to 2.8 €/kg when only 3.9% of the generated H₂ molecules are converted to MSA. This LCOH value is already competitive with the benchmark H₂ derived from steam methane reforming (SMR)[5]. This means that the technical advancements based on currently demonstrated coupled catalyst PEC technology can provide sufficient cost reductions to allow solar hydrogen to directly compete on a levelized cost basis with hydrogen produced from fossil energy.

Our sensitivity analysis further indicates that the H₂-to-MSA conversion efficiency and MSA sales price have the greatest impact on the reduction of LCOH. Therefore, the system can take advantage of its flexibility to maximize benefits by adjusting the conversion rates accordingly to market conditions. Further optimization is suggested regarding the substitution of expensive system components, i.e., PV absorber and membrane, with envisioned cheaper alternatives, which might substantially lower the overall system cost.

Key words:  water splitting, (photo)electrochemistry, technoeconomic analysis, coupled catalysis, hydrogenation

Oxygen reduction reaction activity is governed by the oxygen adsorption/dissociation, proton conduction, and electron transfer kinetics in protonic ceramic fuel cells (PCFCs). Various strategies have been explored to enhance the proton and electron conductivity via tuning oxygen vacancy concentration in the electrode materials and introducing electronic conducting agents. However, there are few studies on improving surface exchange reaction (oxygen adsorption/dissociation) kinetics in protonic ceramic fuel cells. In this study, we report uniformly distributed nickel oxide (NiO) nanoparticles as a catalyst to enhance the electrochemical performance of BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ-BaZr_0.1Ce_0.7Y_0.1Yb_0.1O₃ (BCFZY-BZCYYb) composite cathode by improving surface exchange reaction kinetics. The 0D NiO nanoparticles with high adsorption and fast dissociation ability of oxygen could enlarge the active sites for surface exchange reaction without fading the BCFZY surface and triple-phase boundaries where H₂O formation reaction occurs. The cathode employing NiO nanoparticles exhibits largely reduced polarization resistance and superior power density of 780 mW/cm² at 600 °C.  In addition, the water-stable characteristic of NiO and reduced polarization resistance of composite cathode drastically improved the cell stability.

Much research has shown that crystal structure and dimensionality in materials play an important role in determining their functional properties besides material composition itself. From this aspect, attention to two-dimensional (2D) materials with different three-dimensional (3D) crystallography has been given. The initial boost in research of 2D materials is largely attributed to graphene, which shows interesting phenomena such as super-electrical conductivity and high flexibility with good strength that are absent in bulk graphite. After the discovery of graphene, relevant research on single layers of transition metal dichalcogenides (TMDCs) with lamellar structures similar to that of graphite generated a second wave of interest. Accordingly, many unusual physical, chemical, or electronic properties compared to their bulk counter-parts have been reported. On the other hand, whereas much research effort has been devoted to graphene and TMDC, to date, relatively little research has been conducted in the area of 2D oxide nanosheets because most oxides have 3D crystal structures (it is noted that graphene and TMDC are inherently 2D structures, which are strongly bonded in plane but weakly bonded out of plane with van der Waals force). However, oxides have advantages such as chemical stability and low cost compared to the above materials, and thus, if synthesized to 2D structures, they can be another potential group with valuable functionality. In this presentation, the synthesis of 2D materials based on oxides and, further, their usage for various applications such as energy, sensor, and display will be dealt with.  

Characterization of multi-component slurry such as cathode and anode slurries of Li-ion battery is important, however, it is very difficult task. Thus, we investigated both the flow ability and the packing (settling) ability to discuss the particles dispersion and aggregation state for the cathode slurry of Li-ion battery. The slurries were prepared by changing the mixing condition of acetylene black particles and then the prepared slurry was casted and dried to fabricate the cathode layer. The flow curve of the slurry was measured by a rheometer and the time change of settling hydrostatic pressure was measured by a hydrostatic pressure analyzer. The density and electric resistance of the fabricated cathode layer were measured as well. It was found that a slurry in which acetylene black particles forms a network structure, with sufficient strength and the ability to rapidly recover after breaking by an external force, yields a cathode with comparatively high density and comparatively low volume resistivity [1]. It was also found that the normalized settling time of a cathode slurry determined from its change in hydrostatic pressure over time correlates well with both the density and volume resistivity of a resulting as-cast cathode [1].

The demand for rechargeable Li batteries having more safety and higher energy density has been increased to meet the strong demand of novel applications such as electric vehicles and energy storage system. In this aspect, Li ion batteries containing typical liquid electrolytes have fundamental limitations because liquid electrolytes can act as fuels in thermal runaway behavior leading to a fire or an explosion of battery and can be decomposed at high potential (> 4.5V) leading to the restricted use of high potential cathodes. To address these problems, there are several approaches. One of promising approaches is to apply proper oxide-based solid electrolytes (SEs) instead of liquid electrolytes because they can enable to deliver superior safety with Li metal and to achieve high energy density simultaneously.

In this talk, I will discuss about the progress of the development of the oxide-based SEs, especially, focusing on the newly developed garnet-type SE that has superior electrochemical/chemical properties with respect to the wettability with Li metal, ionic conductivity, and chemical stability with air, and the revisited Lisicon-type SE that has superior compatibility of most of active materials. Also, I will talk about the efforts in our group to build up all solid-state battery by using the developed oxide-based SEs.

Solid oxide regenerative fuel cells (SORFCs), which perform the dual functions of power generation and energy storage at high temperatures, offer one of the most efficient and environmentally friendly options for future energy management systems. Although the functionality of SORFC electrodes could be significantly improved by reducing the feature size of electrode to nanoscale, the practical use of nanomaterials has been limited due to the lack of stability and controllability at high temperatures. Herein, we demonstrate an advanced infiltration technique that allows nanoscale control of highly active and stable catalysts at elevated temperatures. Homogeneous precipitation in chemical solution, which is induced by urea decomposition, allows the precise tailoring of the phase purity and geometric properties. Particularly, effective complexing followed by instantaneous precipitation enables the atomic-scale dispersion of active catalysts on support. Controlling the key characteristics of nanocatalysts yields an electrode that is very close to the ideal electrode structure. Consequently, outstanding performance and durability are demonstrated in both fuel cell and electrolysis modes [1, 2].

References

[1] K.J. Yoon, M. Biswas, H.-J. Kim, M. Park, J. Hong, H. Kim, J.-W. Son, J.-H. Lee, B.-K. Kim, H.-W. Lee, Nano Energy 2017, 9.

[2] J. Shin, Y.J. Lee, A. Jan, S.M. Choi, M.Y. Park, S. Choi, J.Y. Hwang, S. Hong, S.G. Park, H.J. Chang, M.K. Cho, J.P. Singh, K.H. Chae, S. Yang, H.-I. Ji, H. Kim, J.-W. Son, J.-H. Lee, B.-K. Kim, H.-W. Lee, J. Hong, Y.J. Lee, K.J. Yoon, Energy & Environmental Science 2020 13 4903

Irreversible phase transformations of layered oxide cathodes during charging have been detrimental for most of them. Even if a lot of efforts have been made to relieve this highly irreversible phase transformation, there have been just a few successful results, which definitely limit the amount of extracted alkali ions and therey the available capacitie of the layered oxides. So, this presentation will suggest two strategies to get over the limitation of previous researches.  

As an inverse conceptual strategy, we first observed the possibility to make this irreversible phase transformation extremely reversible by utilizing crystal water as a pillar. Although we found a few cyrstal structures working with this reversible phase transition, the works using Na-birnessite (Na_xMnO₂•yH₂O; Na-bir) or Li-birnessite (Li-bir) as basic structural units will be highlighted here. The crystal water in the structure contributes to generating metastable spinel-like phase, which is the key factor for making this unusual reversibility happen. The reversible structural rearrangement between layered and spinel-like phases during electrochemical reaction could activate new cation sites and enhance ion diffusion with higher structural stability. This unprecedented reversible phase transformation between spinel and layered structure was maximized through modulating the steric coordination or amount of cyrstal water in the lattice resultantly optimizing the electrochemical performances of the birnessite layered oxides.

Pseudo Jahn-Teller effect (Pseudo JTE) will be also stressed out as a fundamental reason behind lattice distortion of layered oxide cathodes. Even though Jahn-Teller effect (JTE) has been regarded as one of the most important determinators of how much stress layered cathode materials undergo during charge and discharge, there have been many reports that traces of superstructure exist in pristine layered materials and irreversible phase transitions occur even after eliminating the JTE. A careful consideration of the energy of cationic distortion using a Taylor expansion indicated that second-order JTE (Pseudo JTE) is more widespread than the aforementioned JTE because of the various bonding states that occur between bonding and anti-bonding molecular orbitals in transition metal octahedra. As a model case, some of layered oxide cathodes including P2-type Mn-rich cathode (Na_3/4MnO₂) will be dealt with in this presentation

The two new insights provide deep insight into novel class of intercalating materials which can deal with highly reversible framework changes, and thus it can break up some typical prejudices which we have about the layered cathodes for Li or Na secondary batteries.

The intermittent nature of renewable energies inevitably leads to energy surpluses to maintain energy demand at peak times. Consequently, the storage of energy surpluses from renewables becomes a necessity. To tackle this problem, in this work we propose an efficient way of energy storage based on using green hydrogen as an energy vector. Green hydrogen can be store using different technologies but among them, the storage forming chemical bonds represents an unexplored technology that will have an important impact in handling, management and transporting hydrogen. Our aim is to develop efficient systems for hydrogen storage in the form of chemical bonds based on Liquid Organic Hydrogen Carriers (LOHCs) [1].

The key point of efficient LOHCs lies in the design of suitable catalysts and processes for the conversion and reconversion of hydrogen into chemical bonds. In this work, we describe the state of the art in the field [2] [3] and our last results in for hydrogen storage in the form LOHCs.

All-Inorganic Halide Perovskite Nanocrystals (QDs) have emerged as a new class of fascinating nanomaterials with outstanding optoelectronic properties, with promise to revolutionize different fields like photovoltaics, lasing and emission. In the present talk, we will describe our efforts towards the application of these materials for solar-driven hydrogen production coupled to other processes like organic transformations and waste valorization.[1-2] We will discuss on the rational design of these fascinating materials towards photoelectrochemical processes, and the importance of extracting basic electronic and optical information to understand the carrier dynamics,[3] the influence of trap states and to define adequate defect passivation strategies to maximize the performance and stability of these materials.[4] Moreover, proper interrogation tools are needed to validate their photoelectrocatalytic activity and selectivity. The need for integration of the developed materials into tailored photoelectrochemical devices highlight the urgent need for stabilization strategies to move beyond the proof-of-concept stage to relevant technological developments.

All-solid-state batteries (ASSBs) with sulfide-based solid electrolytes (SEs) have been considered the most promising next-generation energy storage system due to their high energy density. Silicon (Si) has been intensively studied as an anode material due to the high theoretical capacity (3572 mAh g^-1, Li₁₅Si₄) and a relatively low working voltage (~0.5 V vs Li/Li⁺) in ASSBs. However, poor electrochemical properties caused by the large volume change (~ 300%) and low electronic conductivity of Si have limited its practical use. Here, we report Si nanoparticles embedded in carbon nanofiber (CNF) coated with solid electrolyte (LPSCl) (Si/CNF@LPSCl) as anode material to achieve high energy density and cycle stability for ASSBs. The CNF is a desirable host matrix for the Si nanoparticles due to its good mechanical property and high electronic conductivity. The conformal coating of SE on the surface of Si/CNF composite enhances the interfacial stability between the active material and the SE, which leads to the improvement in electrochemical properties by suppressing the contact loss. The Si/CNF@LPSCl composite electrode exhibits a reversible capacity of ~ 1172 mAh g^-1 at 0.1C and stable cyclability of ~ 84.3% at 0.5C after 50 cycles.