Energy Conversion Devices Research Articles

(Invited) Manipulation of Locally-Excited, Charge-Transfer, and Charge-Separated States at Organic-Inorganic Heterointerfaces

Precise engineering of excited-state interactions between an organic conjugated molecule and a two-dimensional semiconducting inorganic nanosheet, specifically the manipulation of locally-excited (LE), charge-transfer (CT), and charge-separated (CS) states, still remains a challenge for state-of-the-art photochemistry. In this study we successfully observed a long-lived, highly emissive CT state at structurally well-defined hetero-nanostructure interfaces of photoactive pyrene and two-dimensional (2D) MoS2 nanosheet via N-benzyl succinimide bridge (Py-Bn-MoS2). Spectroscopic measurements revealed that the LE state of pyrene in Py-Bn-MoS2 efficiently generates an unusual emissive CT state without further yielding the CS state. Theoretical studies supported the interaction of MoS2 vacant orbitals with the pyrene excited state to form a CT state. This is the first example of realizing a mixing luminescence property of organic–inorganic 2D hetero-nanostructures by an accurate design of bridge structure, and therefore represents an important step in their applications for energy conversion and optoelectronic devices and sensors.[1] J. Baek, T. Umeyama, W. Choi, Y. Tsutsui, H. Yamada, S. Seki, H. Imahori, Chem. Eur. J., 24, 1561 (2018).[2] T. Umeyama, T. Ohara, Y. Tsutsui, S. Nakano, S. Seki, H. Imahori, Chem. Eur. J., 26, 6726 (2020).[3] T. Umeyama, H. Xu, T. Ohara, Y. Tsutsui, S. Seki, and H. Imahori, J. Phys. Chem. C, 125, 13954 (2021).[4] T. Umeyama, D. Mizutani, Y. Ikeda, W. R. Osterloh, F. Yamamoto, K. Kato, A. Yamakata, M. Higashi, T. Urakami, H. Sato, H. Imahori, Chem. Sci., 14, 11914 (2023).

(Invited) Zwitter Ion-Promoted Hybrid Sepiolite Membranes for Energy Applications

Functionalized natural clays exhibit significant potential as electrode materials, solid-state electrolytes and separators in energy storage and conversion devices. Features that evoke interest in clay minerals are their (1) porous structures and adsorption properties, (2) tunable surface areas, (3) remarkable thermal and mechanical stabilities, (4) abundant natural reserves, and (5) cost. By tweaking such features along with functionalization with desired molecules, they can be used as an ideal candidate for solid-state electrolyte and battery separators. Naturally occurring clays possess layered silicate structures with interchangeable, intercalated ions and tunable chemical properties. Among them, sepiolite, which is a hydrated magnesium silicate, is one of the most appealing clays having a high-aspect ratio. The blocks and channels in sepiolite with discontinuity in the external silica sheet expose a significant number of surface-silanol groups directly accessible to functionalize with organic and inorganic species. Moreover, its exceptionally high surface area and high cation exchange capacity (CEC) compared to other clays enables easy functionalization, thereby imparting exceptional mechanical durability and material properties that can be used for myriads of applications.In this work a sustainable, cost-effective, environmentally benign, carbon-neutral, and bio-friendly sepiolite clay composites have been developed using betaine and ionic liquid without the use of polymers, metals, or carbon-based precursors.Betaine-modified sepiolite clay composites have been synthesized to fabricate flexible and mechanically robust membranes. These membranes are characterized using powder X-ray diffraction, ATR-FTIR, XPS, surface area analyses, rheology, and imaging techniques (e.g., SEM). Morphological analyses reveal sepiolite possesses needle-shaped morphology in a highly aggregated state. The addition of betaine disaggregates such bundles, forming a 3D network with unique “clay anemone” structures. The XRD patterns are consistent with the literature reflecting no change in the d-spacing and corresponding 2θ value (d100 2θ = 7.4 ͦ) before and after betaine treatment. This provides strong evidence of modification limited to the external surface. ATR-FTIR spectra of the sepiolite-betaine composite membranes show the presence of 935, 1330,1391, and 1616 cm-1 frequencies that correspond to C-N-C, N-C-H, C-N and C=Ostretching respectively. This suggests the successful functionalization of sepiolite clay by betaine through an intercalation mechanism. Further, the electrochemical properties of betaine-modified sepiolite membranes are investigated by electrochemical impedance spectroscopy (EIS) measurements to study the ion dynamics of the hybrid membranes. The room temperature ionic conductivities of these membranes are measured using lithium perchlorate (LiClO4) in ethylene carbonate (EC) and dimethyl carbonate (DMC) mixed solvent (1:1). The concentration of LiClO4 is varied to monitor the change in ionic conductivity and ease of ion transfer through the membranes. The highest ionic conductivity is observed in 0.1M LiClO4 concentration which is superior to the ionic conductivity of sepiolite or hybrid materials with carbon-based precursors, metal ions, quantum dots, or polymers. Viscoelastic properties of the sepiolite slurries with varying concentrations of betaine are also studied to investigate the interaction mechanism between the sepiolite-betaine system and correlate the impact of these interactions on the structure and ionic conductivity of the hybrid sepiolite membranes. Contrary, the IL-sepiolite membranes exhibit superior ionic conductivity compared to betaine-sepiolite membranes, which gives a distinct advantage in the physicochemical and electrochemical properties. The composites are synthesized with varying concentrations of ionic liquid-to-clay ratios and pressed into pellets for conductivity measurements. The highest ionic conductivity of 1 x 10-2 S/cm is achieved by modifying the clay with 10 wt. % IL specimens. This helps to improve the ionic conductivity of the IL-clays. The XRD spectra of sepiolite-IL composites show almost no change in the 2 theta values and peak intensities compared to the pristine sepiolite, confirming the crystalline structure of the sepiolite clay is intact after functionalization with IL. The ATR-FTIR confirms the IL-sepiolite modification with the appearance of a new band at 1345 cm-1 that is attributed to -CH stretching frequencyfrom the alkyl chains of IL present in the clay structures.We have developed a new approach using earth-abundant silicate-based materials to synthesize hybrid zwitterion-modified sepiolite clay membranes. Such materials are not only flexible and mechanically durable, but they also exhibit enhanced ionic conductivity and show exceptional thermal and chemical durability at temperatures above 300 °C and with most aggressive organic solvents, making them a superior candidate for an ergonomic and eco-friendly membrane-based separator for energy devices and gas absorption. Current work is in progress to understand the fundamentals underlying the unique structure-property relationship of hybrid sepiolite, zwitterion-silicate interactions, and conductivity variance with sepiolite architectures, thereby exploiting newer and better properties for application in catalysis, flexible electronics, and biomedical fields. Figure 1

Assessing Nonlinear Polarization in Electrochemical Cells Using AC Impedance Spectroscopy

Electrochemical energy storage and conversion devices are used to power a range of electrical loads including portable electronics, electric vehicles, and utility grids. The energy efficiency of these systems decreases with increasing current load due to energy losses (in the form of heat) associated with different processes. Electrochemical impedance spectroscopy (EIS) is a nondestructive technique commonly used to assess rate-limiting steps in electrochemical devices. Most EIS measurements for energy storage devices are only performed around open-circuit and do not probe system behavior under non-zero biases where these devices operate. Despite its widespread use, this approach does not provide information about the relative overpotentials associated with coupled nonlinear processes (e.g., diffusion limited redox reactions) commonly encountered in these systems. Furthermore, ambiguous terms such as equivalent series resistance and interfacial resistance can lead to confusion and data misinterpretation. For example, Ohm’s law cannot be applied to calculate nonlinear overpotentials using a resistance that was measured from a locally linear response.To determine overpotentials in nonlinear systems using EIS, one must integrate the individual resistive elements of the equivalent circuit as a function of steady state current. Recent experimental work has used this method to decouple ohmic, kinetic, and transport overpotentials in redox flow batteries. In these prior studies, impedance elements were derived, but analytical expressions for the corresponding overpotentials were not presented.While integrated resistance measurements are a powerful experimental tool, a more rigorous mathematical framework describing the approach is needed. The present work bridges this gap by: (i) deriving a general impedance expression for coupled charge transfer and diffusion processes and (ii) integrating the individual resistive elements as a function of steady-state current to obtain global polarization relationships. Analytical expressions for diffusion overpotentials are obtained for special cases, and numerical methods are applied when closed-form expressions do not exist. The work considers three model electrode geometries (planar, cylindrical, and spherical) and two diffusion boundary types (reflective and transmission). Finally, this treatment is extended to macroscopically homogenous porous electrodes which are relevant for describing practical devices.This presentation will highlight how measuring impedance at different steady-state biases provides critical insights on the rate-limiting processes in energy storage devices. While this work focuses on a simple Faradaic charge transfer process, the method can be used to evaluate other nonlinear phenomena in electrochemical power supplies. For example, the resistance of passive films (e.g., solid electrolyte interphase layer) in Li-ion batteries is typically only probed near open-circuit. Similarly, literature on solid-state batteries commonly uses ambiguous terms such as interfacial resistance to describe rate-limiting processes. For these more complex systems, integrating the resistive elements can provide insights on how these phenomena influence device efficiency. Overall, combining these experimental observations with physical models derived using the approach illustrated herein represents a powerful tool to guide component/system design and improve device performance. Acknowledgements This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE). This work is sponsored by the U.S. Department of Energy in the Office of Electricity through the Energy Storage Research Program and the Office of Energy Efficiency and Renewable Energy (EERE) in the Vehicle Technologies Office (VTO) through the Advanced Battery Materials Research (BMR) Program.

(Invited) 3D Printing of 2D Materials for Optimized Electrochemical Performance

Electrochemical energy storage (EES) and conversion devices (e.g. batteries, supercapacitors, and reactors) are emerging as primary methods for global efforts to shift energy dependence from limited fossil fuels towards sustainable and renewable resources. These devices, while showing great potential for meeting some key metrics set by conventional technologies, still face significant limitations. For example, an EES device tends to exhibit large energy density (e.g. lithium-ion battery) or power density (e.g. supercapacitor), but not both. This inability of a single device to simultaneously achieve both metrics represents a major obstacle to widespread adoption of EES devices. Similarly, many catalytic processes depend on expensive platinum group metals (PGM) because non-PGM materials have poor performance or durability. Though the integration of 2D materials (e.g. graphene, dichalcogenides, MXene, etc.) into electrochemical devices has yielded some exciting results towards tackling these issues, significant improvements are still needed. One approach to optimizing the performance of these devices is to focus on one of the fundamental processes that occur in these systems: mass (or charge) transport. The efficient transport of ions within EES and conversion devices is critical to realizing better performance and durability. The pore structure of the electrode is a key factor in determining this transport phenomena, but in many cases, engineering the pore structure in a highly deterministic fashion can be challenging. In this work, we explore a number of additive manufacturing methods (e.g. direct ink write, projection microstereolithography, etc.) to engineer the pore structure of device electrodes. We also determine effective electrode geometries using both simple theory and topology optimization techniques. The topology optimization couples the solution of the forward electrochemical problem over the full electrode domain with gradient-based optimization. The output of our code is a three-dimensional CAD representation which optimizes over specific performance metrics and which can be used to print functional electrodes. This work provides a systematic path toward automated design and fabrication of engineered electrodes with precise control over the fluid and species distribution.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Changing the Linking Functionality in a Triblock Co-Polymer for Electrolyzer or Fuel Cell Membrane or Ionomer for Chemical Stability Improvements and Potentially Other Consequences.

We have developed a versatile scalable triblock polymer system for anion exchange membrane (AEM) based electrochemical energy conversion devices. This system is uniquely suited to solving both the membrane and ionomer challenges for the commercialization of AEM water electrolysis or fuel cells. Using our advanced fundamental understanding of AEMs based on our work which began in 2010, we can design systems from first principles to match the needs of the device. A triblock ABA polymer is readily fabricated that has both chemical and mechanical stability. The synthesis of these systems is robust, high yield and amenable to high volume production. It utilizes a novel dual chain transfer agent to mediate the chain-transfer ring-opening metathesis polymerization (ROMP) of cyclooctene (COE) or the polymerization of isoprene (IP) to give the hydrophobic B block of the polymer. This is followed by the reversible addition-fragmentation chain transfer radial polymerization of chloromethylstyrene (CMS). The polyCMS (pCMS) blocks are then quaternized with an amine to give the cationic hydrophilic A block. Before quaternization the B block is hydrogenated to give either semi-crystalline polyethylene (pE) or amorphous polymethylbutylene (pMB) from pCOE and pIP respectively. The obvious achilles heal in this system from a chemical durability standpoint is the linking ether functionality between the hydrophobic and hydrophilic blocks. The material we have synthesized to date have exceptional chemical and mechanical stability. It performs very well in ex-situ testing with KOH solutions and in device testing we have demonstrated >500 h at 50°C in water electrolysis in 5 cm2 cells in 1M carbonate electrolyte at 0.5 A cm-2. We attribute the chemical stability to the fact that we post quaternize the membrane to give methylpipiridinium cations and we assume that this reaction does not fully penetrate down the hydrophilic chain. This leaves a hydrophobic un-quaternized region close to the hydrophobic block that does not allow access to the ether linkage by the hydrated hydroxide anion. Recently a new approach to the telehelic mid block pCOE has been reported that has no ether linkages and allows the hydrophobic outer blocks to be linked by a methylene linker. This gives use an opportunity to directly contrast the chemical stability of two identical triblock co-polymers one with an ether linkage and one with a methylene linkage between blocks. In this presentation we will show the results from ex-situ chemical stability test one at low temperatures and higher hydroxide concentration and one at higher temperature and lower hydroxide concentration. There are many examples in polymer electrolyte chemistry where a simple change leads to unintended consequences. The geometry of the ether linkage versus the methylene linkage will change the way that the polymer chains will organize and entangle in ways that are not currently predictable. To probe the mechanical, chemical and morphological difference between these two polymers we will fully characterize and contrast both polymers physical chemical properties including water uptake, ionic conductivity, DSC, TGA, tensile strength and morphology through SAXS, WAXS and HRTEM. Data for the two polymer membranes will also be contrasted and compared in small laboratory 5 cm2 single cells for both electrolysis in dilute carbonate and H2/O2 fuel cells using conventional catalysts.

Operando X-Ray Scattering for Electrocatalysis and Beyond

The energy transition stands as one of the greatest challenges of today’s society. In this context, electrocatalyst materials at the heart of electrochemical energy conversion devices such as fuel cells and water electrolyzers are expected to play an increasing crucial role in the near future. The urgent bottleneck to be overcome in electrocatalyst materials development to allow the widespread deployment of electrochemical systems is thus reaching combined high activity and long-term stability at low cost. Despite the diversity in electrocatalyst materials, the latter being largely imposed by the various types of electrochemical systems (noble vs. non-noble metals in acidic vs. alkaline media for example) and the prerequisites of the different electrochemical processes (oxidation or reduction reactions of various species at different electrode potential ranges), most activity and stability properties of electrocatalysts directly derive from their (surface) chemistry and structure [1]. Such properties (and their temporal evolution) can thus be directly investigated by means of in situ or operando high energy X-ray scattering (XRD) technique [2].In this presentation, the versatility of in situ and operando XRD technique in addressing key bottlenecks in electrocatalyst materials development, notably by probing adsorption and oxidation trends [3], will be showcased. After an overview of previous contributions from the literature, the talk will focus on our ongoing works which encompass the study of Pt-based nanocatalysts for the oxygen reduction reaction (ORR) at proton-exchange membrane fuel cells (PEMFCs) anode, carbon-capped Ni@C catalyst for the hydrogen oxidation reaction (HOR) at anion-exchange membrane fuel cells (AEMFCs) anode and IrCu unsupported aerogel catalyst for the oxygen evolution reaction (OER) at proton-exchange membrane water electrolyzers (PEMWEs) anode.Finally, the ability of operando XRD to provide device-relevant insights at the macroscale (such as ionomer hydration in PEMFCs or water distribution in AEMFCs) beyond electrocatalysts microstructural properties will be presented.

Confocal Raman Microscopy – Shedding Light on the Ionomer Properties of PFSA Membranes

Perfluorinated sulfonic acid (PFSA) polymers are the state-of-the-art material for the proton exchange membrane (PEM) in various electrochemical energy conversion devices, e.g., PEM fuel cells, PEM water electrolyzers, or redox flow batteries, combining chemical and mechanical durability with high proton conductivity.The most prominent example of a PFSA material is NafionTM, which is a so-called long side chain (LSC) ionomer due to its comparably long side chain between the polymer backbone and the charged sulfonyl end group. On the other hand, short side chain (SSC) ionomers like Aquivion® and 3M Ionomer or composite membranes are promising approaches to improve performance in harsh conditions, such as low humidity or high operating temperatures.1 SSC PEMs feature increased water uptake, proton conductivity,2 and crystallinity3 compared to LSC PEMs. Composite PEMs can be specifically tailored to the required operating conditions by carefully designing the interlayers or additives. As the performance and longevity of electrochemical cells are a delicate convolution of various (composite) membrane properties and parameters, a method for evaluating ionomer membrane properties is crucial for rationally engineering optimized PEMs.Here, we use confocal Raman microscopy (CRM) to investigate application-relevant properties of PFSA PEMs and for high-resolution imaging of composite membranes.4 Thus, the high spatial resolution of confocal optical microscopy and the chemical sensitivity of Raman scattering are combined in a single measurement approach. NafionTM, 3M Ionomer, and Aquivion® at varying equivalent weights (EW) feature distinctive Raman spectra (Fig. 1) that show characteristic spectral changes depending on the ionomer’s side chain structure and EW. We show that the EW of these PFSA types can be reliably measured using Raman spectroscopy. Further, parameters such as swelling, ionizable group content, and water uptake can be quantified by CRM non-destructively and contact-free. The diffraction-limited resolution of CRM of less than 2 µm enables a view of the local distribution of these properties and, therefore, allows to image and analyze composite membranes. In summary, we comprehensively study ionomer properties of (composite) membranes and establish CRM as a powerful platform for characterizing advanced PEMs for electrochemical energy deviceReferences A. S. Aricò, A. Di Blasi, G. Brunaccini, F. Sergi, G. Dispenza, L. Andaloro, M. Ferraro, V. Antonucci, P. Asher, S. Buche, D. Fongalland, G. A. Hards, J. D. B. Sharman, A. Bayer, G. Heinz, N. Zandonà, R. Zuber, M. Gebert, M. Corasaniti, A. Ghielmi and D. J. Jones, Fuel Cells, 10(6), 1013–1023 (2010).Y.-C. Park, K. Kakinuma, H. Uchida, M. Watanabe and M. Uchida, Journal of Power Sources, 275, 384–391 (2015).K. D. Kreuer, M. Schuster, B. Obliers, O. Diat, U. Traub, A. Fuchs, U. Klock, S. J. Paddison and J. Maier, Journal of Power Sources, 178(2), 499–509 (2008).M. Maier, D. Abbas, M. Komma, M. S. Mu'min, S. Thiele and T. Böhm, Journal of Membrane Science, 669, 121244 (2023). Figure 1

Deep Eutectic Solvent Supported Polymer-Based High Performance Anion Exchange Membrane for Alkaline Fuel Cells

Alkaline anion exchange membrane fuel cells (AEMFC) have gained interest due to their potential as electrochemically energy conversion devices and a competitive alternative to the more extensively studied commercialized proton exchange membrane fuel cells (PEMFCs). Nevertheless, their development impedes the limited ion conductivity, chemical and mechanical stability in an alkaline medium of anion exchange membranes (AEMs). The ionic conductivity of AEM is lower than that of a proton exchange membrane (PEM) because the mobility of hydroxide ions is inherently lower than that of protons [1]. Deep eutectic solvents (DESs) as green solvents can overcome this issue.The main idea of this research is to fabricate a deep eutectic solvent-supported polymer-based AEM for alkaline fuel cell application by electrospinning technique. Poly(vinyl alcohol) (PVA) is one of the potential polymers for the development of AEM due to its water solubility, biodegradability, high fiber forming ability, non-toxicity, and high chemical and thermal stability [2]. This work aims to achieve high ionic conductivity, chemical and mechanical stability by two methods: swelling polyvinyl alcohol (PVA)-based nanofibers into DES and encapsulating DES inside the PVA-nanofibers together with optimizing fabrication methods.In this study, polymer electrolytes as potential AEMs were first prepared using the swelling method, where electrospun PVA nanofibers were fabricated via the electrospinning technique, followed by swelling in a series of DESs. To prepare the DES solutions, choline chloride (ChCl) and ethylene glycol (EG) were mixed at different molar ratios. The cross-linking of pure PVA fibers with glutaraldehyde in ethanol (see Figure 1B) and DES uptake and the presence of voids as channels for ion conduction (see Figure 1C) were successfully confirmed with a scanning electron microscopy (SEM, Crossbeam500, Zeiss). The ionic conductivity of swelled in DES PVA-based membranes were evaluated by electrochemical impedance spectroscopy (Metrohm Autolab) at room temperature and was equal to 0.336 mS/cm. The presence of DES in the membrane was confirmed with FTIR spectroscopy (FT-IR spectrometer, Nicolet iS10): a slight C-N+ peak at 953 cm-1 from ChCl has proven interaction between PVA and DES with FTIR spectroscopy, meaning that DES is present in the fiber. In addition, the nitrogen content in the obtained membrane was determined by CHNS elemental analysis (Elemental Vario micro cube) as 3.32 wt.%.In order to overcome high weight loss in the swelling method, the encapsulation method was suggested as an alternative option, which is based on the encapsulation of DES into the PVA solution by mixing them, varying the amount of DES added, and then preparing fibers using electrospinning. Observing the morphology of DES-encapsulated PVA nanofibers with a scanning electron microscope (SEM, Crossbeam500, Zeiss), as the content of DES in the solution increased, there was an observed rise in the average diameter of the nanofibers (see Figure 1D). Encapsulation of DES in PVA nanofibers was primarily confirmed by transmission electron microscope (TEM, JEM -1400 Plus, JEOL): PVA appears lighter due to its decreased density, proving that DES is present inside the fibers. Also, the same tendency was observed with the results of FTIR spectroscopy (FT-IR spectrometer, Nicolet iS10) as for the swelling method, meaning that DES is present in the fiber. According to CHNS elemental analysis (Elemental Vario micro cube), nitrogen content was lower than for the swelling method - 0.70 wt.%. The ionic conductivity was evaluated as 0.364 mS/cm by electrochemical impedance spectroscopy (Metrohm Autolab) at room temperature, which was almost the same as for the swelling method.As a result, very flexible and visually transparent electrospun DES-supported PVA-based AEMs were obtained via the swelling and encapsulation method.

Dissolution Stability of Graphene-Modified Pt(111)

Pt plays a leading role in electrocatalysis, being the best pure metal catalyst for the oxygen reduction reaction (ORR) taking place at the cathode of the commercial polymer electrolyte membrane fuel cells (PEMFCs). In addition, it is usually used in the cathode to catalyze the hydrogen evolution reaction (HER) in water electrolyzers for hydrogen production. Apart from activity, stability is another critical parameter to consider when evaluating the performance of electrochemical energy conversion devices. Even pure Pt catalysts degrade under real-life conditions due to electro-oxidation and dissolution processes. Fundamental studies with well-defined Pt surfaces are mandatory for understanding these phenomena and developing new electrocatalysts with improved activity and stability.1 Different types of protection on the surface of electrocatalysts, such as carbon layers or ceria surrounding metal nanoparticles,2-4 have been investigated to enhance their stability while maintaining or improving their activity towards a reaction of interest. Here, we propose the modification of Pt(111) single crystal electrode with an adlayer of graphene as a model system to evaluate the protection mechanism of carbon-capped catalysts.In this work, the stability of graphene-modified Pt(111) in 0.1 M HClO4 is studied by on-line inductively coupled plasma mass spectrometry (ICP-MS) measurements using an electrochemical scanning flow cell (SFC). The Pt(111) surface was modified by a fast and easy transfer method of a commercially available chemical vapor deposited (CVD) graphene single-layer. The cyclic voltammetry results are comparable to those previously reported for surfaces prepared by direct CVD on the Pt(111) electrode.5 To test the quality of the graphene adlayer we checked the electrocatalytic activities towards different reactions of interest, such as ORR, HER and hydrogen oxidation reaction (HOR), and compared the results with the previous works.6 The graphene-modified Pt(111) surface presents enhanced stability since no dissolution is detected for a cyclic voltammetry experiment at 10 mV s-1 with an upper potential limit (UPL) of 1.3 V vs. RHE. In contrast, evident Pt dissolution is observed for the unmodified surface. Different accelerated stress test (AST) protocols consisting in groups of 50 cyclic voltammetries at 200 mV s-1 were performed up to three different UPL, namely 1.3 V, 1.5 V and 1.7 V vs. RHE, showing almost no dissolution over the protocol when UPL = 1.3 V, while the integrity of the graphene adlayer was quickly compromised for UPL = 1.7 V as pointed out by the quick increase in Pt dissolution. The degradation of the graphene layer in the different conditions was additionally characterized by Raman spectroscopy and atomic force microscopy (AFM). The results from this work provide useful information about the behavior of carbon capping agents on Pt surfaces, which can help to design new electrocatalytic materials with improved stability. References (1) Fuchs, T.; Briega-Martos, V.; Drnec, J.; Stubb, N.; Martens, I.; Calle-Vallejo, F.; Harrington, D. A.; Cherevko, S.; Magnussen, O. M. Anodic and Cathodic Platinum Dissolution Processes Involve Different Oxide Species. Angew. Chem. Int. Edit. 2023, 62, e202304293.(2) Sgarbi, R.; Doan, H.; Martin, V.; Chatenet, M. Tailoring the Durability of Carbon-Coated Pd Catalysts Towards Hydrogen Oxidation Reaction (HOR) in Alkaline Media. Electrocatalysis 2023, 14 (2), 267-278.(3) Kim, H.; Kwon, G. H.; Han, S. O.; Robertson, A. Platinum Encapsulated within a Bacterial Nanocellulosic-Graphene Nanosandwich as a Durable Thin-Film Fuel Cell Catalyst. ACS Appl. Energy Mater. 2021, 4 (2), 1286-1293.(4) Miller, H. A.; Vizza, F.; Marelli, M.; Zadick, A.; Dubau, L.; Chatenet, M.; Geiger, S.; Cherevko, S.; Doan, H.; Pavlicek, R. K.; et al. Highly active nanostructured palladium-ceria electrocatalysts for the hydrogen oxidation reaction in alkaline medium. Nano Energy 2017, 33, 293-305.(5) Fu, Y. C.; Rudnev, A. V.; Wiberg, G. K. H.; Arenz, M. Single Graphene Layer on Pt(111) Creates Confined Electrochemical Environment via Selective Ion Transport. Angew. Chem. Int. Edit. 2017, 56 (42), 12883-12887.(6) Shih, A. J.; Arulmozhi, N.; Koper, M. T. M. Electrocatalysis under Cover: Enhanced Hydrogen Evolution via Defective Graphene-Covered Pt(111). ACS Catal. 2021, 11 (17), 10892-10901.

(Invited) Semiconductor Nanostructures for Light Energy Conversion

This talk will review some of the key developments of semiconductor quantum dots based light energy conversion. Semiconductor nanostructures are finding new ways to design light energy conversion devices (e.g., thin film solar cells and light emitting devices). The decreased consumption of energy during the manufacture and the lessened use of semiconductor materials lowers the overall carbon footprint with energy payback time less than a year for such devices. The early studies which focused on the synthesis of various semiconductor nanostructures and exploration of their size dependent optical and electronic properties have led to their their integration in high efficiency thin film solar cells.. Unlike solar cell devices, photocatalytic processes are yet to deliver conversion efficiencies and stability that can make them compatible for practical applications. Understanding the limitations imposed by interfacial charge transfer processes remains a key to further advance photocatalytic systems for chemical energy storage. Recent developments in utilizing semiconductor nanostructures for light energy conversion and storage will be discussed.

Surface Reconstruction of La0.5Sr0.5CoO3-δ Ceramic toward High Solar Selectivity for High-Temperature Air-Stable Solar-Thermal Conversion.

As a key device for solar energy conversion, solar absorbers play a critical role in improving the operating temperature of concentrated solar power (CSP) systems. However, solar absorbers with high spectral selectivity and good thermal stability at high temperatures in air are still scarce. This study presents a novel surface reconstruction strategy to improve the spectral selectivity of La0.5Sr0.5CoO3-δ (LSC5) for enhanced CSP application. The strategy could efficiently enhance the solar absorptance due to the existence of a high-absorption thin layer composed of nanoparticles on the LSC5 surface. Meanwhile, the crystal facet with low emittance on the LSC5 surface was exposed. Thus, the LSC5 that underwent surface reconstruction achieved a higher solar absorptance (∼0.75) and lower infrared emittance (∼0.19) compared to the original LSC5 (0.63/0.21), representing an improvement of nearly 32%. Additionally, the surface reconstructed LSC5 demonstrated a lower infrared thermographic temperature and a higher solar-thermal conversion equilibrium temperature compared to those of LSC5 and SiC. Moreover, the reconstructed LSC5 could maintain stable performance up to 800 °C in air, which might simplify the complexity of the CSP systems. The surface reconstruction strategy provided a new method to optimize the spectral selectivity of high-temperature stable ceramics, contributing to advancements in solar energy conversion technologies.

A high-performance watermelon skin ion-solvating membrane for electrochemical CO2 reduction

Ion-solvating membranes have been gaining increasing attention as core components of electrochemical energy conversion and storage devices. However, the development of ion-solvating membranes with low ion resistance and high ion selectivity still poses challenges. In order to propose an effective strategy for high-performance ion-solvating membranes, this study conducted a comprehensive investigation on watermelon skin membranes through a combination of experimental research and molecular dynamics simulation. The micropores and continuous hydrogen-bonding networks constructed by the synergistic effect of cellulose fiber and pectin enable the hypodermis of watermelon skin membranes to have a high ion conductivity of 282.3 mS cm−1 (room temperature, saturated with 1 M KOH). The negatively charged groups and hydroxyl groups on the microporous channels increase the formate penetration resistance of watermelon skin membranes in contrast to commercially available membranes, and this is crucial for CO2 electroreduction. Therefore, the confinement of proton donors and negatively charged groups within three-dimensional microporous polymers gives inspiration for the design of high-performance ion-solvating membranes.

Shortcut Approximation to the Coupled Equations of Heat and Electric Currents

A self-consistent approximation to the coupled equations of heat and electric current densities is developed to facilitate the integration of the relaxation time approximation of the Boltzmann transport equation in a realistic design, analysis, and optimization of energy conversion devices. By introducing two auxiliary energies in the empirical Boltzmann transport equations, the coupled equations can be simply expressed in these auxiliary energies. These auxiliary energies are then determined by optimizing both the heat current through charge neutrality equation without the presence of electric current and the carrier concentrations with the presence of electric current. The heat and the electric current densities as well as the transport properties can then be calculated. The calculated transport properties agreed reasonably well with existing experimental and computational data.

Surface structure of MOVPE-prepared As-modified Si(100) substrates

In the pursuit of high-efficiency tandem devices for solar energy conversion based on III-V-semiconductors, low-defect III-V nucleation on Si(100) substrates is essential. Here, hydrogen and arsenic are key ingredients in all growth processes with respect to industrially scalable metalorganic vapor phase epitaxy. Our study provides insight into Si(100) surface preparation for the initial stage of III-V nucleation. The samples investigated, prepared on substrates with different offcut angles, show single domain surfaces consisting of rows of preferentially buckled dimers. Low energy electron diffraction and reflection anisotropy spectroscopy confirm well-defined (1 × 2)/(2 × 1) majority domains. Fourier-transform infrared spectroscopy revealed hydrogen bonding to the surface dimers, while no impurities were found by XPS. Density functional theory calculations support the experimental results and reveal a novel surface motif of H-passivated Si-As mixed dimers.

DFT study of optoelectronic and thermoelectric properties of pure and doped double perovskite Cs2SnI6 for solar cell applications

The global need of energy is raising day by day. To meet this demand double perovskites (DPs) can play a vital role for energy conversion devices. In this letter, we formulated and examined lead-free defect perovskites using a combination of tellurium and tin, denoted as Cs2Sn1-xTexI6 (0, 0.25), utilizing density functional theory which include structural, optical and thermal behavior. Both the optimized volume and lattice parameter exhibit a linear increase with the Te content in Cs2Sn1-xTexI6 (CsSnTeI). The negative value of formation energy for both pure and doped materials along with their Poisson and Pugh ratio confirm the stability of these materials. The band structure analysis reveals its semiconducting behavior, with the band gap expanding proportionally to the increased Te concentration with band gap value of pure (1.20 eV) and doped 91.43 eV) double perovskites. Optical characteristics, including the dielectric function and absorption coefficient, were also analyzed. To expose their potential use of Cs2Sn1-xTexI6 (0, 0.25) in thermoelectric devices, temperature dependent thermoelectric features are investigated reveal that suitability of these materials for energy conversion purposes.

Cation/anion-doping induced electronic structure modulation of CoMoO4 for enhanced hydrogen evolution

The design and fabrication of high-performance, inexpensive and durable electrocatalyst toward hydrogen evolution reaction (HER) is supremely significant for alleviating energy crisis and environmental concerns, but still remaining challenging. Herein, we develop an experimental work based on etching and reduction strategy to reveal the remarkable effect of cation/anion co-doping in CoMoO4 on its intrinsic HER activity. The CoMoO4 with Fe and B incorporation (Fe/B-CoMoO4) exhibits a current density of 10 mA cm−2 with strikingly low potential of 38 mV coupling with Tafel slope of 51 mV dec-1, and manifesting a robust durability for 100 h with no attenuation, which is comparable to the state-of-the-art commercial Pt/C catalyst. The collective experimental and theoretical findings concomitantly illustrate that the enhanced performances are due to the strong synergistic effect resulting from the co-doping of Fe and B, which plays a pivotal role in finely tuning the electronic structure of CoMoO4, further optimizing the adsorption free energy of H intermediates and shifting the center of the d-band of Fe/B-CoMoO4 away from the Fermi level. This fantastic work highlights the critical role of foreign element incorporating for optimizing electronic structure of transition metal oxides toward HER, and offers valuable guiding principles for rational design of more efficient energy conversion devices.

Influence of Rare-Earth Doping Content and Type on Phase Transformation and Transport Properties in Highly Doped CeO2.

Rare-earth doped CeO2 materials find extensive application in high-temperature energy conversion devices such as solid oxide fuel cells and electrolyzers. However, understanding the complex relationship between structural and electrical properties, particularly concerning rare-earth ionic size and content, remains a subject of ongoing debate, with conflicting published results. In this study, we have conducted comprehensive long-range and local order structural characterization of Ce1-xLnxO2-x/2 samples (x ≤ 0.6; Ln = La, Nd, Sm, Gd, and Yb) using X-ray and neutron powder diffraction, Raman spectroscopy, and electron diffraction. The increase in the rare-earth dopant content leads to a progressive phase transformation from a disordered fluorite structure to a C-type ordered superstructure, accompanied by reduced ionic conductivity. Samples with low dopant content (x = 0.2) exhibit higher ionic conductivity in Gd3+ and Sm3+ series due to lower lattice cell distortion. Conversely, highly doped samples (x = 0.6) exhibit superior conductivity for larger rare-earth dopant cations. Thermogravimetric analysis confirms increased water uptake and proton conductivity with increasing dopant concentration, while the electronic conductivity remains relatively unaffected, resulting in reduced ionic transport numbers. These findings offer insights into the relationship between transport properties and defect-induced local distortions in rare-earth doped CeO2, suggesting the potential for developing new functional materials with mixed ionic oxide, proton, and electronic conductivity for high-temperature energy systems.

Nb-MXene as promising material for electrocatalysis in energy conversion (OER/ORR) and storage

Low-cost non-noble metal electrocatalysts are currently the focus of research and development for energy conversion and storage devices. MXenes, the newest class of two-dimensional materials, have high surface area, nanometer layer thickness, hydrophilicity and high electrical conductivity that favor their performance for electrocatalytic reactions. Herein, niobium carbide MXene (Nb-MXene) was obtained from its MAX phase by chemical synthesis and characterized regarding its morphology, structure and electrochemical activity for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) as well as for charge storage in different electrolytic media. The results demonstrate the good performance of Nb-MXene for storing charge (104 F g−1 at 5 mV s−1) that increased remarkably with continuous cycling in an acidic medium. Current density for oxygen evolution of ∼32.5 mA cm−2 was reached in alkaline medium. The oxygen reduction reaction in the same media was observed to proceed via 2e− mechanism with a Tafel slope of 114 mV dec−1. Therefore, Nb-MXene presents characteristics and performance of a promising material for electrocatalytic reactions.

Optimal scheduling of virtual power plants considering integrated demand response

Abstract In contemporary power system management, leveraging multilevel demand response strategies plays a pivotal role in balancing energy supply with demand, thereby enhancing system efficiency and flexibility. This paper introduces an innovative optimization model for virtual power plant operations that anticipates the scheduling needs several days in advance. It incorporates a comprehensive approach to demand response on the consumer side, highlighting two main types: price-based and substitution-based responses. Furthermore, the model integrates the functionalities of energy storage and conversion devices. This integration aims to facilitate effective load management, ensuring an optimized load distribution while minimizing the overall system costs.

Colloidal hot injection synthesis of Cu2FeSnS4 nanoparticles towards an enhanced capacity anode material for Li-ion batteries

The increasing demand for portable energy storage devices has prompted a considerable exploration into advanced anode material with higher Li-ion battery capacities. Quaternary chalcopyrite semiconductors have generated significant attention as promising materials in applications such as optoelectronic devices, energy conversion, and energy storage devices. In this paper, Cu2FeSnS4 (CFTS) nanoparticles were prepared through a colloidal hot injection method (HIM). The structural analysis indicates that the CFTS nanoparticles exhibit a pure tetragonal structure characterized by high crystallinity. The morphological and elemental analysis shows the stoichiometric synthesis of CFTS nanoparticles with significant agglomeration of crystallites, accompanied by variation in size. The CFTS nanoparticles, employed as an anode material for Li-ion batteries, demonstrated significant performance characterized by high reversible capacity and stability at ambient temperature. The initial discharge capacity is achieved at 1181 mAhg−1 and retains 528 mAhg−1 during charge-discharge cycles within the voltage range of 0.005 to 3.0 V (versus Li/Li+) after 5 cycles. The CFTS nanoparticle performance findings indicate its promising implementation as an anode electrode in Li-ion batteries.