Electrochemical Energy Conversion Research Articles

(Invited) Electrochemical Characterization of Catalyst/Ionomer Interfaces

Electrochemical energy conversion devices such as polymer-electrolyte fuel cells, water-splitting or carbon dioxide reduction electrolyzers, utilize ionically-conductive polymers (ionomers) in their electrode structure where an ionomer forms interfaces with the catalysts. The ionomers exist as nanometer-thick film to cover the catalyst particles and act as a binder while enabling the transport of active species necessary for the reactions. While the nature of species and operational environment differs across these devices, their overall efficiency and performance is influenced by the electrode architecture, where the interfaces and interactions between the ionomers and catalysts play an important role. In hydrogen technologies, for example, ionomer thin films bind the catalyst sites and carbon supports to facilitate transport of ionic (e.g. protons), liquid (water) and gaseous (e.g., oxygen) species under a nano-confined environment. Such confinement effects not only change the ionomer structure and properties, usually with a tendency to increase transport resistances, but also amplify the impact of the electrocatalyst-ionomer interactions in the electrodes, thereby reducing the catalyst activity. Thus, understanding the behavior of nano-confined ionomers and their interactions with the catalyst surfaces is key for mitigating the transport resistances in catalyst ionomers and improving the electrode performance and cell efficiency.This talk will provide insights into electrochemical characterization of catalyst ionomers by focusing on varying chemistries of perfluorosulfonated cation-exchange ionomers with additional examples of anion-exchange ionomers for emerging technologies. The hydration and transport properties of proton-exchange catalyst-ionomers will be examined with the effects of chemistry, processing and thickness (degree of confinement), along with their impact on the morphological changes governing the film function. Through a systematic variation of material chemistries and environmental parameters, primary factors impacting the structure-property relationship of catalyst ionomers are described along with a discussion of secondary factors altering the cation-anion interactions. Then, the interplay between the effects of chemistry and ion exchange (capacity) on modifying the transport function and the strength of ionomer-catalyst interactions will be presented. The results will be collated to elucidate the underlying origins of the transport resistances occurring in electrodes, and to develop materials and processing strategies for their mitigation for improved performance and efficiency in electrochemical energy devices.

(Invited) Crosscutting Materials Innovation for Transformational Chemical and Electrochemical Energy Conversion Technologies—Solid Oxide Cells

The Solid Oxide Cell (SOC) primarily operated as Solid Oxide Fuel Cell (SOFC) converts the chemical energy of a fuel such as hydrogen directly into electricity with very high efficiencies. Reverse operation of the exact same SOCs—as Solid Oxide Electrolysis Cells (SOEC) converts electrical energy into chemical energy by using the electricity to split chemical molecules such as steam, carbon dioxide or mixtures of both into hydrogen, carbon monoxide or mixtures of both respectively.The SOC is basically a gas-tight and oxide-ion conducting solid electrolyte, sandwiched between porous oxide ion – and electron-conducting electrode materials—the oxygen electrode and the fuel electrode where the electrochemical conversions occur.Irrespective of operation mode, the choice of SOC materials and processing routes generally aim for maximum performance and lifetime at chosen conversion rate and efficiency at nominal operation. This is as much as the technology can intrinsically offer and by far not enough to guarantee either profitable production of electricity or chemicals hydrogen, carbon monoxide or both. Ultimately the materials and manufacturing costs of the cell, stack, module, and system play a key role in the involved invest. Generally, operation costs such as cost of natural gas or electricity as well as maintenance costs are much more significant in determining the profitability of the technology than the investment costs. These technology-independent factors can in turn orientate the technologist to optimize the cell for either for maximum efficiency or conversion rate capability.In this talk we will discuss specific SOC materials such as doped zirconia, doped ceria, manganates, cobaltites, and titanates. Intrinsic limitations and advantages with respect to performance, durability and costs; how they are withstanding the test of time and the ever decreasing operation temperature from micro/nano structuration, infiltration and exsolution of electrocatalysts through switching of cell supports. We will give a perspective on the dependency of cell material choices based on technology use cases and boundary conditions Figure 1

Battery Modeling: Fusing Equivalent Circuit Models with Data-Driven Surrogate Modeling

Electrochemical impedance spectroscopy (EIS) is widely used to characterize electrochemical energy conversion systems. The traditional analysis with equivalent circuit models (ECM) has recently been augmented by a transform based distribution of relaxation times (DRT) analysis which allows one to reduce the ambiguity in the construction of ECMs and thus overfitting. Experimentally determined ECM parameters vary depending on operating conditions and the lifetime history of battery usage. Here we focus on State of Health (SOH) and State of Charge (SOC) as a basis for operando diagnosis and functionality optimization in the setting of fast-charging. Within pure ECM approaches, aging effects can only be represented to a limited extent, as aging is related to a variety of different factors whose impact on cell impedance are not sufficiently understood, yet. The highly complex interplay of factors motivates the development of data-driven Machine Learning (ML) models as a basis for future battery management systems. We present ML enabled ECMs based on experimental impedance analyses and a data-driven ML approach that computationally samples an abstract target space for classification and recognition of cells at vastly different SOC/SOH combinations.

Real-Time Mass Spectrometry for Simultaneous Monitoring of Electrode Dissolution and Reaction Product Distribution during Organic Electrosynthesis

An organic electrosynthesis is an emerging tool for modern chemical manufacturing fitting the environmental and sustainability agenda. It utilizes polarized electrode surfaces as “traceless” oxidants or reductants, whereas its heterogeneous nature and exceptional tunability enable easy access to radical intermediates and novel mechanistic pathways. In the line of further energy input improvement, there is a growing demand for appropriate catalytic materials devoted to organic molecule transformations, for which the activity and selectivity have so far been most studied. Although the investigation of electrodes’ and electrocatalysts’ stability in the field of electrochemical energy conversion is well-established, there is still a lack of attention to the symmetrical studies for organic electrosynthesis. Nevertheless, electrode material losses shorten the reactor’s lifetime and can cause metal cathodic re-depositions, severely altering reaction product selectivity and final product contaminations. Although some reports on the ex-cell metal dissolution analysis are available, no information about its mechanism can be gained in such a way. To highlight the issue numerically, an analytical method to characterize rates and mechanisms of catalyst dissolution within a real-time response-enabled potential-, temperature-, or time-resolved parametrization of the electrocatalyst is highly demanded.Herein we present a system to study organic electrosynthetic protocols under dynamic operations to quantitatively monitor the electrode dissolution aligned with reaction products during organic molecules transformations. The instrument comprises an electrochemical flow cell (EFC) with exchangeable working electrodes, the outlet of which is interfaced with an online quadrupole mass spectrometer (OQMS) for the analysis of extracted by solid membrane gaseous products and an inductively coupled plasma mass spectrometer (ICP-MS) for the detection of dissolved metals. The system is exemplified by a benchmark Kolbe electrolysis over platinum Pt anode, commonly considered a stable and inert electrode material. Based on the literature, methanol has been used as a solvent of choice, while partially neutralized acetic acids with different bases were studied as objects. The methodology shed some light on understanding the role of platinum oxide formation on the electrode surface and can be used to study time- and potential-resolved catalyst stability and the possibility for dissolved transition metal-catalyzed electroorganic transformations.

(Invited) Importance of Accelerated Stress Tests in Accelerated Discovery of Energy Materials

Accelerated Stress Tests (ASTs) are critical to the rapid discovery and deployment of new stable materials in electrochemical energy conversion technologies. The widespread use of validated ASTs can spur materials development efforts that address the key shortcomings of today’s materials. An excellent example of the use of ASTs has been in polymer electrolyte membrane fuel cells (PEMFCs). This talk will describe the various ASTs that have been utilized to evaluate oxygen reduction reaction (ORR) catalysts, catalyst supports and membranes in fuel cells. As better materials get developed, the ASTs have evolved over time to provide quick feedback on the degradation rates of these materials. Finally, the ASTs can also provide insights into the tradeoffs between cost, performance and durability of these electrochemical systems. Results from the Million Mile Fuel Cell Truck Consortium (M2FCT) will be presented with particular emphasis on heavy duty application of fuel cells. This talk will also address the ASTs that are currently under development for polymer electrolyte membrane water electrolyzers (PEMWEs). The Department of Energy (DOE) has recently set performance and durability targets for PEMWEs. The Hydrogen from Next generation Electrolyzers of Water (H2NEW) consortium is currently engaged in developing ASTs for PEMWEs and these results will be presented and discussed. By providing these two specific examples, the importance of ASTs in materials discovery will be highlighted. Acknowledgement: This work was supported by the Hydrogen and Fuel Cell Technologies Office (HFTO), Office of Energy Efficiency and Renewable Energy, US Department of Energy (DOE). Funding is through the Million Mile Fuel Cell Truck (M2FCT) consortia, technology managers G. Kleen and D. Papageorgopoulos and Hydrogen from Next generation of Electrolyzers of Water (H2NEW) consortia, technology manager Dave Peterson.REFERENCES: https://www.energy.gov/eere/fuelcells/technical-targets-proton-exchange-membrane-electrolysis

A High-Entropy Oxide as High-Activity Electrocatalyst for Water Oxidation

High-entropy materials are an emerging pathway in the development of high-activity (electro)catalysts because of the inherent tunability and coexistence of multiple potential active sites, which may lead to earth-abundant catalyst materials for energy-efficient electrochemical energy storage. In this report, we identify how the multication composition in high-entropy perovskite oxides (HEO) contributes to high catalytic activity for the oxygen evolution reaction (OER), i.e., the key kinetically limiting half-reaction in several electrochemical energy conversion technologies, including green hydrogen generation. We compare the activity of the (001) facet of LaCr0.2Mn0.2Fe0.2Co0.2Ni0.2O3-δ with the parent compounds (single B-site in the ABO3 perovskite). While the single B-site perovskites roughly follow the expected volcano-type activity trends, the HEO clearly outperforms all of its parent compounds with 17 to 680 times higher currents at a fixed overpotential. As all samples were grown as an epitaxial layer, our results indicate an intrinsic composition–function relationship, avoiding the effects of complex geometries or unknown surface composition. In-depth X-ray photoemission studies reveal a synergistic effect of simultaneous oxidation and reduction of different transition metal cations during the adsorption of reaction intermediates. The surprisingly high OER activity demonstrates that HEOs are a highly attractive, earth-abundant material class for high-activity OER electrocatalysts, possibly allowing the activity to be fine-tuned beyond the scaling limits of mono- or bimetallic oxides.

The Effect of Junction Composition of Polarization Behaviour of Bipolar Membranes

Bipolar membranes (BPMs), i.e, laminates cationic and anionic polymer membranes, have a wide range of applications. Starting from an early application in electrodialysis systems, the application scope has more recently broaden to electrochemical energy conversion systems such as water electrolyzer and CO2 reduction reaction, fuel cells and flow batteries.1 In connection to CO2 reduction reaction, the implementation of BPM effectively mitigates CO2 crossover driven by (bi)carbonate migrations, but the rate capability is limited by the large overpotential across current state-of-the-art BPMs.2 The large overpotential primarily originates from the slow water dissociation (WD) kinetics at the interface between the cation exchange layer (CEL) and anion exchange layer (AEL), and largely depends on junction morphology. The detailed structure-properties relationships remain poorly understood, but physical properties of individual polymeric membranes and the composition and morphology of the WD catalyst are important parameters and the optimal combination of materials may be different for different systems. Furthermore, the WD performance of BPM in MEA water electrolyzer not only depends on BPM’s junction contact, but also the interface contact between electrodes and CEL/AEL.In this work, a series of BPM system featuring different compositions of membrane layers and WD catalysts are explored and characterized by polarization behaviour (J-V curve) via H-cell and MEA water electrolyzer. We attempt to link the junction composition created by different BPM systems to its WD performance and provide a systematic choice of CEL/AEL and WD catalyst in the BPM system.(1) Blommaert, M. A.; Aili, D.; Tufa, R. A.; Li, Q.; Smith, W. A.; Vermaas, D. A. Insights and Challenges for Applying Bipolar Membranes in Advanced Electrochemical Energy Systems. ACS Energy Lett. 2021, 2539–2548. https://doi.org/10.1021/acsenergylett.1c00618.(2) Garg, S.; Giron Rodriguez, C. A.; Rufford, T. E.; Varcoe, J. R.; Seger, B. How Membrane Characteristics Influence the Performance of CO 2 and CO Electrolysis. Energy Environ. Sci. 2022. https://doi.org/10.1039/D2EE01818G.

Rationalizing the Effect of the Redox-Inactive Metals on the Activity of Oxides in Oxygen Electrocatalysis

The large-scale implementation of renewable energy conversion and storage technologies is a key challenge to transition to a sustainable society. One of the most critical elements towards this goal is the discovery of active, stable and inexpensive catalysts for electrochemical energy conversion processes, such as the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which are central for the efficiency of water-splitting electrolyzers, fuel cells and metal–air batteries.1 Manganese-based spinels, MxMn3-xO4, and ruthenium pyrochlores, M2Ru2O7 − δ, are known as active catalysts for the ORR and OER, respectively, and were studied here as model systems to rationalize the effect of the incorporation (doping) of redox-inactive elements (M) into the structure on the catalytic performance of complex oxides in oxygen electrocatalysis.We found a direct correlation between the ORR/OER activity of Mn3O4:M spinels (M = Mn, Cu, Zn, Mg, Ca, Sr),2 and Y1.8M0.2Ru2O7 − δ pyrochlores (M = Y, Fe, Co, Ni, Cu),3 the lattice oxygen binding strength/lability and the formation enthalpy of the binary MO x oxides. We found experimentally that the activity of Mn3O4:M and Y1.8M0.2Ru2O7 − δ in, respectively, ORR and OER, varies significantly depending on the nature of M. We observed that metals M with higher enthalpies of formation for the respective oxide weaken the oxygen binding on the surface of the complex oxide that correlates in turn with changes in ORR/OER activity. DFT studies captured changes in the electronic structure upon M substitution and allowed to correlate trends in the electronic band structure and the oxygen vacancies formation energies with the ORR/OER performance of the Mn3O4:M/Y1.8M0.2Ru2O7 − δ catalysts. These activity trends are subsequently discussed in the framework of theoretically established correlations between the electrocatalytic activity and adsorption energies of catalytic intermediates. We demonstrated that the introduction of redox-inactive metal substituents/dopants into the host structure of a metal oxide is a highly versatile approach to tune the binding strength of reaction intermediates (determining the position of the catalyst on the ORR/OER activity volcano), and in turn improve the ORR/OER activity and also selectivity. This approach is a general concept that is transferrable to a broad selection of materials and processes associated with adsorption and redox reactions involving oxygen species.References(1) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Science 2017, 355, eaad4998.(2) Wu, Y.-H.; Mehta, H.; Willinger, E.; Yuwono, J. A.; Kumar, P. V.; Abdala, P. M.; Wach, A.; Kierzkowska, A.; Donat, F.; Kuznetsov, D. A.; Müller, C. R. Angew. Chem. Int. Ed. 2023, 62, e202217186.(3) Kuznetsov, D. A.; Naeem, M. A.; Kumar, P. V.; Abdala, P. M.; Fedorov, A.; Müller, C. R. J. Am. Chem. Soc. 2020, 142, 7883-7888.

(Invited) Ceramic-Based Electrochemical Systems for Sustainable Transition to Zero-Carbon Energy

Realizing a sustainable energy future pose multiple challenges, especially with respect to our heavy dependence on fossil fuels. In that regard, electrochemical energy conversion systems based on ionically and mixed conducting ceramics offer unique opportunities for clean energy production from fossil fuels by abandoning the undesired air-based combustion that emits CO2 into the atmosphere and replacing it with oxygen-based electrochemical conversion processes, which produce capture-ready CO2 instead and avoid emissions. In the context of climate change, this distinction is critically important as processes that release CO2 unabatedly into the atmosphere must be avoided entirely. But that does not necessarily require that we should rush to abandon fossil fuels prematurely before we can replace them at-scale with clean energy alternatives.After framing the global energy landscape and threats to climate change, this presentation will discuss the opportunities and technical challenges of converting fossil fuels in solid oxide-base fuel cells with focus on solid fuels including coal. It will also introduce the new concept of electrochemical gasification to produce ‘blue’ hydrogen from fossil fuels without release of CO2 into the atmosphere. These technologies can help avoid major disruptions to energy supply and offer a smooth transition into a zero-carbon energy economy.

Ni-Fe-S Supported on Mo-Based Mxene (Mo2CTx) As a High-Performance Bifunctional Electrocatalyst for Rechargeable Zinc–Air Batteries

Rechargeable Zn-air batteries are a promising alternative to lithium-ion batteries due to their high theoretical energy density (1350 Wh kg−1), low cost, and environmental benefits. To meet the growing demand on renewable power for electric vehicles and sustainable goals, inexpensive and efficient bifunctional catalysts with both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities have recently garnered a great interest in energy conversion fields. MXene, a rising family of two-dimensional (2D) materials, particularly Mo2CTx, has been identified as a promising electrocatalyst supporter. The tunable surface chemistry, atomic thickness, and high surface area of the MXene architecture reduce the reaction barrier by enhancing the conductivity, exposing active sites and increasing the reaction rate in charge transfer and gas/ion diffusion process. Therefore, with coexistence of MXene platform, favorable reaction conditions can be created to facilitate the catalyst's performance.In this study, a novel bifunctional electrocatalyst of Ni-Fe sulfide uniformly embedded in a Mo-based MXene (Mo2CTx) sheet structure is designed for Zn-air battery applicaiton. Surface characterization shows that 2-D Mo2CTx nanosheets can be successfully fabricated by hydrothermal etching method. The deposition of Ni-Fe sulfide nanocatalysts onto Mo2CTx MXene nanosheets exhibits excellent electrochemical performance, which is attributed to the enhanced intrinsic activity due to rapid electron transfer from MXene, abundant functional groups on the surface to lower the reaction activity energy. In addition, the architecture of MXene support provides a large specific surface area, high porosity, good conductivity and highly exposed active sites. When using MXene supported Ni-Fe-S nanocatalysts as the electrode materials for Zn-air battery application, a high peak power density at 107.24 mW cm−2 with superior cycle stability within 200h is obtained. Moreover, the possible mechanisms on energy storage is also proposed. These results clearly indicates the successfully development of an exciting bifunctional electrocatalyst for electrochemical energy conversion applications.

Modification of Fuel Electrode Microstructure for Protonic Ceramic Cells by the Infiltration of Various Metal Nanoparticles

Proton ceramic cells (PCCs) are promising eco-energy electrochemical energy conversion systems. Proton conducting perovskite oxide electrolytes of PCCs have high ion conductivity at intermediate and low temperatures because of their low activation energy. Most of electrode infiltration studies for fuel electrode supported PCCs were carried out on a thin air electrode layer rather than a thick fuel electrode. In the present study, various metallic nanoparticles (such as Ni, Pd and Pt) were infiltrated in fuel electrode scaffolds to increase the thriple phase boundary (TPB, electrode-electrolyte-gas phase). The fabricated fuel electrode supported PCC consisted of Ni-BaCe0.7Zr0.1Y0.1Yb0.1O3-ẟ cermet fuel electrode, BaCe0.7Zr0.1Y0.1Yb0.1O3-ẟ (BCZYYb) electrolyte, and BCZYYb-La0.6Sr0.4Co0.2Fe0.8O3-ẟ composite air electrode. SEM analysis results revealed that metallic nanoparticles can be infiltrated up to the fuel electrode functional layer and their distribution was uniform through the layer. The PCC with metal nanoparticle infiltrated fuel electrode exhibited higher power density and lower polarization resistance than that with bare fuel electrode. This study shows the electrochemical performance of fuel electrode supported PCCs can be further improved by the facile infiltration method.

Solvent‐Mediated Synthesis of Functional Powder Materials from Deep Eutectic Solvents for Energy Storage and Conversion: A Review

AbstractDeveloping advanced electrochemical energy storage and conversion (ESC) technologies based on renewable clean energy can alleviate severe global environmental pollution and energy crisis. The efficient preparation of functional electrode materials via a simple, green, and safe synthesis process is the key to the commercial feasibility of these ESC systems. Deep eutectic solvents (DESs) with easy‐tunable solvent properties and recyclable features have emerged as novel solvent systems for designing and synthesizing various functional powder materials for ESC devices. In this paper, the application of DESs in the synthesis of energy‐related functional powder materials is systematically reviewed. After briefly introducing the classification and synthesis of DESs, their critical roles in synthesizing powder materials are discussed. Then, the recent advances of DES‐derived powder materials in ESC, including batteries, fuel cells, supercapacitors, and water splitting, are described in detail from the perspective of preparation‐structure‐activity. Finally, some challenges and development directions of the DESs‐mediated synthesis of powder materials with high electrochemical performance for ESC applications are outlined.

Antimony Sulfide-Based Materials for Electrochemical Energy Conversion and Storage: Advances, Challenges, and Prospects

Antimony Sulfide-Based Materials for Electrochemical Energy Conversion and Storage: Advances, Challenges, and Prospects

Oxygen vacancies and tailored redox activity encountered with NiCo2O4 nanostructures for promising applications in supercapacitor and water oxidation

It is well known that transition metal oxides play a significant role in the development of efficient electrochemical energy storage and conversion systems. However, their application is limited by low electrical conductivity, a limited number of electroactive sites, and limited stability. In this study, nickel‑cobalt bimetallic oxide (NiCo2O4) nanostructures were prepared using orange peel extract and its fruit juice during hydrothermal method. The use of green chemical components from orange peel extract and its fruit juice was to demonstrate their ability to manipulate NiCo2O4's structural and functional properties. For this reason, the optimized 1 mL of orange peel extract and its fruit juice was found highly active for improving the electrochemical properties of NiCo2O4 nanostructures. The electrochemical supercapacitor performance of optimized NiCo2O4 nanostructures showed a specific capacitance of 997.50 Fg−1 with a cycling stability of more than thirty thousand cycles, resulting in a capacitance retention rate of almost 100 % at 1.5 Ag−1. In addition, during the oxygen evolution reaction (OER), the overpotential was 120 mV at 20 mA/cm2 and the Tafel slope was 110 mVdec−1, with a prolonged stability exceeding 45 h were observed for optimized NiCo2O4 nanostructures. With an electrode material comprising NiCo2O4 nanostructures and an orange peel extract, outstanding electrochemical performance was obtained. This could be explained by several factors, including high electroactive site exposure, rapid charge transfer kinetics, high hydroxide adsorption, oxygen vacancies on the surface, and abundance of Co(III). The approach described here can be used for the facile synthesis of a wide range of nanostructured functional materials that exhibit impressive electrochemical properties, particularly for energy storage and conversion applications.

Atomically precise Au15Ag23 nanoclusters co-protected by alkynyl and bromine: Structure analysis and electrocatalytic application toward overall water splitting

We report the structure analysis and electrocatalytic application toward overall water splitting of atomically precise Au15Ag23(tBuC ≡ C)18Br6 nanoclusters (Au15Ag23 in short). Au15Ag23 possesses a triple-layered core-shell-shell metal core structure of Au6@Au6Ag23@Au3, with the core centre being a tetragonal bipyramidal Au6 structure unit, and the outer layer of one Ag atom linked with two alkynyl ligands to form a linear structural motif covering the surface of the metal nucleus. Au15Ag23 is a superatom with 14 free electrons and it displays a unique absorption profile. Remarkably, the Au15Ag23 catalyst exhibits excellent electrocatalytic properties in hydrogen evolution reaction (HER), manifested by a low overpotential of 125 mV at 10 mA cm−2 and a negligible current decrease for 16 h in 0.5 M H2SO4, while the Au15Ag23/NiFe-LDH catalyst demonstrates outstanding electrocatalytic performance in oxygen evolution reaction (OER), evidenced by an ultralow overpotential of 250 mV at 10 mA cm−2 and a ∼5% current decrease for 30 h in 1.0 M KOH. Such promoting effect is ascribed by the electron transfer from NiFe-LDH to the Au15Ag23 clusters, which results in generating high valent Fe species as active sites. When using the two catalysts for overall water splitting (OWS), only 1.51 V was required to achieve a current density of 10 mA cm−2, and it performed well in the 50 h's stability test. This study enriches the family member of alkynyl-protected AuAg bimetallic nanoclusters, and further highlights the dual functionalities (catalyzing HER and promoting OER) that metal nanoclusters can make contribution for electrochemical energy conversion and beyond.

Ultrafast Proton Conduction in an Aqueous Electrolyte Confined in Adamantane-like Micropores of a Sulfonated, Aromatic Framework.

Sulfonated, cross-linked porous polymers are promising frameworks for aqueous high-performance electrolyte-host systems for electrochemical energy storage and conversion. The systems offer high proton conductivities, excellent chemical and mechanical stabilities, and straightforward water management. However, little is known about mass transport mechanisms in such nanostructured hosts. We report on the synthesis and postsynthetic sulfonation of an aromatic framework (SPAF-2) with a 3D-interconnected nanoporosity and varying sulfonation degrees. Water adsorption produces the system SPAF-2H20. It features proton exchange capacities up to 6 mequiv g-1 and exceptional proton conductivities of about 1 S cm-1. Two contributions are essential for the highly efficient transport. First, the nanometer-sized pores link the charge transport to the diffusion of adsorbed water molecules, which is almost as fast as bulk water. Second, continuous exchange between interface-bound and mobile species enhances the conductivities at elevated temperatures. SPAF-2H20 showcases how to tailor nanostructured electrolyte-host systems with liquid-like conductivities.

Construction of Core-Shelled Covalent/Metal-Organic Frameworks for Oxygen Evolution Reaction.

Oxygen evolution reaction (OER) is the half-reaction in zinc-air batteries and water splitting. Developing highly efficient catalysts toward OER is a challenge due to the difficulty of removing four electrons from two water molecules. Covalent organic frameworks (COFs) provide the new chance to construct the highly active catalysts for OER, because they have controlled skeletons, porosities, and well-defined catalytic sites. In this work, core-shell hybrids of COF and metal-organic frameworks (MOFs) have first demonstrated to catalyze the OER. The synergetic effects between the COF-shell and MOF-core render the catalyst with higher activity than those from the COF and MOF. And the catalyst achieved an overpotential of 328mV, with a Tafel slope of 43.23mV dec-1 in 1m KOH. The theoretical calculation revealed that the high activity is from the Fe sites in the catalyst, which has suitable binding ability of reactant intermediate (OOH*), and thus contributed high activity. This work gives a new insight to designing COFs in electrochemical energy storage and conversion systems.

Metal-Organic-Framework-Based Nanoarrays for Oxygen Evolution Electrocatalysis.

The development of highly active and stable electrode materials for the oxygen evolution reaction (OER) is essential for the widespread application of electrochemical energy conversion systems. In recent years, various metal-organic frameworks (MOFs) with self-supporting array structures have been extensively studied because of their high porosity, abundant metal sites, and flexible and adjustable structures. This review provides an overview of the recent progress in the design, preparation, and applications of MOF-based nanoarrays for the OER, beginning with the introduction of the architectural advantages of the nanoarrays and the characteristics of MOFs. Subsequently, the design principles of robust and efficient MOF-based nanoarrays as OER electrodes are highlighted. Furthermore, detailed discussions focus on the composition, structure, and performance of pristine MOF nanoarrays (MOFNAs) and MOF-based composite nanoarrays. On the one hand, the effects of the two components of MOFs and several modification methods are discussed in detail for MOFNAs. On the other hand, the review emphasizes the use of MOF-based composite nanoarrays composed of MOFs and other nanomaterials, such as oxides, hydroxides, oxyhydroxides, chalcogenides, MOFs, and metal nanoparticles, to guide the rational design of efficient OER electrodes. Finally, perspectives on current challenges, opportunities, and future directions in this research field are provided.

Advances in boron nitride‐based materials for electrochemical energy storage and conversion

AbstractEnergy storage and conversion (ESC) devices are regarded as predominant technologies to reach zero emission of carbon dioxide, which still face many challenges, such as poor safety, limited cycle life, low efficiency, etc. Hexagonal boron nitride (h‐BN), distinguished by its robust mechanical strength, chemical inertness, exceptional thermal stability, and superior ion conductivity, has appeared to meet some challenges of ESC devices. Typically, h‐BN can act as a perfect modifier to enhance the safety of batteries by improving the mechanical strength and heat dissipation of separators, extend cycle life of Li metal batteries by protecting solid state electrolyte from reducing and increase efficiency of fuel cells by improving the proton conductivity of membranes. Besides, recent progress on doping, surface modification, tailoring quantum dots, heterostructures, and hybridizations with other nanomaterials has made it possible to extensively apply h‐BN to other ESC technologies. This review provides a comprehensive overview of the up‐to‐date synthetic strategies for BN‐based materials and discusses the most recent breakthroughs on their application in electrochemical ESC technologies. Also, the challenges and future development for BN‐based materials in these fields are assessed.

Measuring three-dimensional bubble dynamics for hydrogen production via water electrolysis

The commercialization of mass hydrogen production via water electrolysis is presently limited by low operational current densities. The optimal performance of electrolysis cells is significantly influenced by the substantial formation and residence of bubbles at high current densities. Thus, it is crucial to design electrodes with the ability for rapid bubble discharge to ensure appropriate bubble management. However, the quantitative volumetric measurements required to determine the bubble discharge ability of an electrode are not yet sufficiently accurate. This paper describes a quantitative volumetric method that combines a stereoscopic shadowgraph imaging system with particle tracking velocimetry (PTV) to measure the three-dimensional position, size, and velocity of micrometer-sized bubbles. The proposed method successfully captures hydrogen bubbles larger than 30 μm bubbles in an alkaline water electrolyzer. Considering the different luminance patterns of small bubbles (r ≤ 4 pixels) and large bubbles (r > 4 pixels) in the current imaging system, a bubble-size adaptive detection algorithm is established based on the cascade correlation method to obtain the two-dimensional centroid coordinates and radius of the observed bubbles. The bubble size information is also introduced into a two-view PTV algorithm for retrieving the Lagrangian trajectory of each bubble. Both the bubble detection and PTV algorithms are validated using synthetic datasets. Once the bubble trajectories are resolved successfully, the three-dimensional bubble velocity is obtained, and the actual bubble sizes are further corrected using the depth information. Analysis of the trajectory and velocity components indicates the existence of lateral bubble motion, reflecting interactions among bubbles. The rise velocity is positively correlated with the bubble radius in two regions, respectively, and the deviation from the theoretical value reveals the influence of non-buoyancy factors. The proposed technique provides effective diagnostics of the three-dimensional dynamic characteristics of micrometer-sized bubbles and can be used to evaluate and design bubble management systems for various electrochemical energy conversion devices.