Electrochemical Energy Conversion Systems Research Articles

Advancing Energy Sustainability Through Solar-to-Fuel Technologies: From Materials to Devices and Systems.

To achieve carbon neutrality and sustainable development, innovative solar-to-fuel systems have been designed through the integration of solar energy harvesting and electrochemical devices. Over the last decade, there have been notable advancements in enhancing the efficiency and durability of these solar-to-fuel systems. Despite the advancements, there remains significant potential for further improvements in the performance of systems. Enhancements can be achieved by optimizing electrochemical catalysts, advancing the manufacturing technologies of photovoltaics and electrochemical cells, and refining the overall design of these systems. In the realm of catalyst optimization, the effectiveness of materials can be significantly improved through active site engineering and strategic use of functional groups. Similarly, the performance of electrochemical devices can be enhanced by incorporating specific additives into electrolytes and optimizing gas diffusion electrodes. Improvements in solar harvesting devices are achievable through efficient passivant and self-assembled monolayers, which enhance the overall quality and efficiency of these systems. Additionally, optimizing the energy conversion efficiency involves the strategic use of DC converters, photoelectrodes, and redox media. This review aims to provide a comprehensive overview of the advancements in solar-powered electrochemical energy conversion systems, laying a solid foundation for future research and development in the field of energy sustainability.

Three-dimensional porous structured cobalt- and nitrogen-doped carbon nanotube electrocatalyst derived from cobalt-based zeolitic imidazolate framework nanoleaves for high performance zinc-air battery

The development of efficient and cost-effective electrocatalysts to overcome the intrinsic sluggish kinetics of the oxygen reduction reaction (ORR) in zinc-air batteries is crucial. In this study, we introduce a strategy that integrates a template-assisted synthesis with subsequent thermal treatment to fabricate an active and stable cobalt-based nitrogen-doped carbon electrocatalyst, denoted as Co-N-CNT. The strategy adjusts the disordered architecture of the zeolitic imidazolate framework (ZIF) through the synergistic effect of bimetallic species, restricted the growth of zeolitic imidazolate framework nanoleaves (ZIF-L) using salt templates, and directed the transformation from a two-dimensional blade-like morphology to a three-dimensional multi-tiered composite structure. Notably, the Co-N-CNT-800 sample, synthesized at an optimized pyrolysis temperature of 800 °C, exhibits a half-wave potential of 0.89 V and demonstrates stability with sustained cycling over 21 h, which is comparable to the performance of commercial Pt/C electrocatalysts. Moreover, when employed as the cathode in zinc-air batteries, Co-N-CNT-800 not only surpasses Pt/C in terms of power density but also exhibits long-term charge/discharge stability. This findings offer a viable pathway for the design of active and cost-effective ORR electrocatalysts, holding promise for applications in the electrochemical energy storage and conversion systems.

LDHs and their Derivatives for Electrochemical Energy Storage and Conversion Systems: Design, Insights and Applications

AbstractElectrochemical energy storage and conversion systems (EESCSs), including batteries, supercapacitors, fuel cells, and water electrolysis technologies, enabling the direct conversion between chemical and electrical energies. They are key to the flexible storage and utilization of renewable energy and play an important role in future energy technologies. The physical and chemical properties of electrode materials significantly influence the performance of EESCSs, necessitating the development of materials with both high cost‐effectiveness and superior electrochemical storage and catalytic capabilities. Layered double hydroxides (LDHs), due to their unique properties as high surface area, evenly distributed active sites, and tunable compositions, makes themselves and the derivatives have become focal points in the EESCSs. This review summarizes recent advances of LDH and their derivates for diverse EESCSs applications, aims to elucidate the modification strategies of LDH materials and provide valuable insights for exploring LDH‐derived materials in various electrochemical devices, offering interdisciplinary perspectives for the rational design of LDH‐based materials.

Exploring electron transfer: Bioinspired, biomimetics, and bioelectrochemical systems for sustainable energy and Value-Added compound synthesis

Electron transfer (ET) is a fundamental process that underlies various phenomena in physics, chemistry, and biology. Understanding ET mechanisms is crucial for developing sustainable energy solutions and synthesizing value-added compounds efficiently. In this context, the present review provides the fundamental aspects of ET involving bioinspired, biomimetics, and biological entities and its significance for sustainable energy and green electrosynthesis fields. Among the theoretical and experimental cornerstones, Marcus Theory, electronic conductance, computational modeling, biomolecular thermodynamics, electrochemical and kinetic theories, protein film voltammetry, and the emergence of in situ and operando techniques are explored. Theoretical modeling is vital for understanding and predicting ET processes. Additionally, the significance of experimental techniques for investigating the ET process in biological entities and interfaces is discussed. Protein film voltammetry is a valuable and consolidated technique for studying ET processes at the protein-electrode interface, whereas in situ and operando techniques for interrogating ET processes in real time provide insights into the dynamics and mechanisms of ET. The concept of quantum conductance in biological structures is addressed, evidencing a trend and power of single-entity analysis. Aspects of extracellular and interfacial ET processes are presented and discussed in the electrochemical energy conversion systems. A deep understanding of these processes can improve the design of efficient bioinspired catalysts. Therefore, this multidisciplinary work aims to fill the gaps between different scientific fields related to ET involving bioentities to develop innovative energy and value-added compound synthesis solutions.

Anisotropic polyphenylene-based anion-exchange membranes with flexible side-chains via click reaction for high-performance water electrolysis

Anion-exchange membrane water electrolysis (AEMWE) represents a promising and cost-effective electrochemical energy conversion system for green hydrogen production owing to its utilization of earth-abundant metals for various cell components (catalysts, porous transport layers, bipolar plates) and the zero-gap design. However, the lack of high-performance anion-exchange membranes (AEMs) has hampered the advancement of AEMWE technology. Here, by using colon coupling and click reactions, the ether-free polyphenylene-based AEMs (QPPBTF-TMA-w) with flexible side-chains containing triazole linkers. The QPPBTF-TMA-w membranes stand out for their lower activation energy for hydroxide conduction which is attributed to the well-structured ion-conducting channels via distinct phase separation facilitated by the triazole linker, which optimizes hydrophilicity and ensures efficient ion transport within the membrane. In addition, the QPPBTF-TMA-w membranes exhibit unique anisotropic swelling and ion-conduction behaviors, such as lower in-plane swelling and higher through-plane conductivity, which are advantageous for practical cell applications. Therefore, despite having a modest ion exchange capacity (IEC) value of 2.0, the QPPBTF-TMA-w membrane delivers high-performance in anion-exchange membrane water electrolysis (AEMWE), achieving 6.4 A/cm2 at 2.0 V at 70 °C using a 1.0 M KOH solution.

Selectivity-Enhanced Electroreduction of CO2 to CO at Novel Ru-Linked-GO Nanohybrids: the Role of Nanoarchitecture.

Recently, global-scale efforts have been conducted for the electroreduction of CO2 as a potentially beneficial pathway for the conversion of greenhouse gases to useful chemicals and renewable fuels. This study focuses on the development of selective and sustainable electrocatalysts for the reduction of aqueous CO2 to CO. A RuIIcomplex [Ru(tptz)(ACN)Cl2] (RCMP) (tptz = 2,4,6-tris(2-pyridyl)-1,3,5-triazine, ACN = acetonitrile) was prepared as a molecular electrocatalyst for the CO2 reduction reaction in an aqueous solution. Density functional theory-calculated frontier molecular orbitals suggested that the tptz ligand plays a key role in dictating the electrocatalytic reactions. The RCMP electrocatalyst was grafted onto the graphene oxide (GO) surface both noncovalently (GO/RCMP) and covalently (GO-RCMP). The field emission scanning electron microscopy and elemental distribution analyses revealed the homogeneous distribution of the complex onto the GO sheet. The photoluminescence spectra confirmed accelerated charge-transfer in both nanohybrids. Compared to the bare complex, the GO-RCMP and GO/RCMP nanohybrids showed enhanced electrocatalytic activity, achieving >95% and 90% Faradaic efficiencies for CO production at more positive onset potentials, respectively. The GO-RCMP nanohybrid demonstrated outstanding electrocatalytic activity with a current of ∼84 μA. The study offers a perspective on outer- and inner-sphere electron-transfer mechanisms for electrochemical energy conversion systems.

Chitosan Composite Membrane with Efficient Hydroxide Ion Transport via Nano-Confined Hydrogen Bonding Network for Alkaline Zinc-Based Flow Batteries.

The development of membranes with rapid and selective ionic transport is imperative for diverse electrochemical energy conversion and storage systems, including fuel cells and flow batteries. However, the practical application of membranes is significantly hindered by their limited conductivity and stability under strong alkaline conditions. Herein, a unique composite membrane decorated with functional Cu2+ cross-linked chitosan (Cts-Cu-M) is reported and their high hydroxide ion conductivity and stability in alkaline flow batteries are demonstrated. The underlying hydroxide ions transport of the membrane through Cu2+ coordinated nano-confined channels with abundant hydrogen bonding network via Grotthuss (proton hopping) mechanism is proposed. Consequently, the Cts-Cu-M membrane achieves high hydroxide ion conductivity with an area resistance of 0.17Ωcm2 and enables an alkaline zinc-based flow battery to operate at 320mAcm-2, along with an energy efficiency of ≈80%. Furthermore, the membrane enables the battery for 200cycles of long-cycle stability at a current density of 200mAcm-2. This study offers an in-depth understanding of ion transport for the design and preparation of high-performance membranes for energy storage devices and beyond.

Geometric and electronic engineering of cobalt, manganese oxide sites in porous nitrogen-doped carbon for boosted oxygen reduction activity in microbial fuel cell

The development microbial fuel cell relies heavily on cathodic catalysts to provide excellent activity and fast electron rotor transfer rates. However, the design of such electrocatalysts is extremely challenging. Here, a strategy is proposed to regulate the electrocatalyst comprised of cobalt and manganese oxides embedded the porous nitrogen-doped carbon (Co3O4-MnO2/NC) with dominant crystal surface by geometric and electronic engineering. Experimental results and theoretical calculations show that the coordination of bimetallic oxides with N atoms induces an asymmetric distribution of charge of central metal sites, which promotes the adsorption/desorption of oxygen intermediates. This electrocatalyst exhibits excellent oxygen reduction reaction (ORR) activity, long-term stability and antitoxic tolerance. Importantly, microbial fuel cell equipped with Co3O4-MnO2/NC generates excellent performance in power density, chemical oxygen demand (COD) removal rate and coulomb efficiency. This work demonstrates the importance of geometric and electronic engineering design of bimetallic oxide to expose more dominant crystal faces to improve electrocatalytic activity in electrochemical energy conversion systems.

Recent Novel Fabrication Techniques for Proton-Conducting Solid Oxide Fuel Cells

Research has been conducted on solid oxide fuel cells (SOFCs) for their fuel flexibility, modularity, high efficiency, and power density. However, the high working temperature leads to the deterioration of materials and increased operating costs. Considering the high protonic conductivity and low activation energy, the proton conducting SOFC, i.e., the protonic ceramic fuel cell (PCFC), working at a low temperature, has been wildly investigated. The PCFC is a promising state-of-the-art electrochemical energy conversion system for ecological energy; it is characterized by near zero carbon emissions and high efficiency, and it is environment-friendly. The PCFC can be applied for the direct conversion of various renewable fuels into electricity at intermediate temperatures (400–650 °C). The construction of the PCFC directly affect its properties; therefore, manufacturing technology is the crucial factor that determines the performance. As a thinner electrolyte layer will lead to a lower polarization resistance, a uniformly constructed and crack-free layer which can perfectly bond to electrodes with a large effective area is challenging to achieve. In this work, different fabrication methods are investigated, and their effect on the overall performance of PCFCs is evaluated. This article reviews the recent preparation methods of PCFCs, including common methods, 3D printing methods, and other advanced methods, with summarized respective features, and their testing and characterization results.

Deciphering interfacial charge transfer mechanisms in electrochemical energy systems through impedance spectroscopy

Employing electrochemical impedance spectroscopy as a non-destructive analytical tool for elucidating the interfacial charge transfer in electrochemical energy conversion/storage systems.

Conductive metal–organic frameworks for zinc–air battery application: design principles, recent trends and prospects

Electrochemical energy conversion and storage systems have an indispensable place in the modern world to fulfill the increasing energy demands.

Kinetic Insights into the Mechanism of Oxygen Reduction Reaction on Fe2O3 /C Composites

Electrochemical energy storage and conversion systems are the appealing replacement for nonrenewable resources to provide clean and sustainable energy. Of particular interest are fuel cells and metal-air batteries, both of which rely on the oxygen reduction reaction (ORR) for their operation. Fe-N-C catalyst has been widely used as a prominent catalyst towards ORR, but there is still some ambiguity regarding its active sites. This study describes the role of Fe2O3, an inevitable species formed in Fe-N-C catalyst towards the ORR. The Fe2O3 is carefully synthesised on various carbon supports and characterised using XPS spectra, HRTEM images, and surface area analysis. The particle size of the Fe2O3 nanoparticles synthesised on the pores of the carbon support was determined to be ~10 nm from the HRTEM analysis. The current density of ORR and the number of electrons transferred on Fe2O3/C catalysts is increased compared with bare carbon supports. Based on the kinetic analysis, the ORR on Fe2O3/C follows pseudo-4-electron (or) 2+2-electron ORR, where the first 2-electron ORR to H2O2 and second 2-electron H2O2 reduction reaction (HPRR) to H2O are assigned to the graphitic carbon (carbon defects) and Fe2O3 active sites, respectively. The role of Fe2O3 is further studied using the Theoretical method, which reveals the decreased formation of free energy of O2 adsorption. The Fe2O3 reduces the energy barrier for the reduction of *OOH to OH-. The Fe2O3/C shows higher stability by retaining 95% of the initial activity even after 20,000 cycles. Figure 1

Developing Hybrid Aqueous Li/Na-Ion Capacitors by Electrodes Synthesized by Facile Recycling and Reuse Process: Waste to Wealth

The explosive evolution of industrialization and technologies has grabbed eyeball attention inthe current scenario for improving the living standard of the human beings irrespective of the threat and risk possessed by the environment. So to overcome this catastrophe, there has been a significant focus on progressive development of electrochemical energy storage and conversion systems to resolve the intermittency nature of renewable energy sources. The electrochemical energy storage devices such as batteries and supercapacitors have been relied upon the ability to store the charge in the wide potential region with high Coulombic efficiency and long-term stability. However, transition metal oxides are well known, for their high theoretical specific capacity but suffer from low electronic conduction and capacity fading during cycling. To address these shortfalls metal oxides are usually, embedded within a conductive carbon-based matrix (for instance graphene). The use of graphene would afford easy pathway for electron movement and increases the surface area for higher interaction of active electrode materials with electrolyte. Subsequently, graphene based transition metal oxides composites have attracted active research pursuits in synthesizing the low cost effective electrode materials for energy storage devices. And if graphene is recycled and reused then it’s very much cheap and efficient electrodes for hybrid energy storage devices. In this work, electrode materials are developed by facile and economic approach with an aim to achieve improved electrochemical activity. The conversion of utilized electrode materials (e-waste) to efficient graphene electrodes (wealth) is validated in this research. The challenges in this work are (i) recycling pure samples, (ii) resynthesizing the graphene and tuning its layers, (iii) stability and efficiency of electrode materials, (iv) optimization of electrode materials and (v) practical demonstration of hybrid-ion capacitors. The fascinating part of the research finding is both the electrode materials are derived via. recycling, for which device can be promise to be sustainable and clean energy. The details will be presented in ECS conference. Such scheme could serve as precedent to the development of potential electrode materials for energy storage devices.

Metal(Pt, Pd)-Perovskite Oxide(Ba0.5Sr0.5Co0.8Fe0.2O3-δ) Hybrid Material As a Bifunctional Electrocatalyst for Lithium-Air Battery

The rapid depletion of fossil fuel resources and growing concerns over climate change have made the development of next-generation batteries a top priority. Among these, Li-O2 batteries have attracted significant interest due to their potential for much higher gravimetric energy storage density compared to other chemical batteries. However, efficient and cost-effective catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are critical for the efficient functioning of these batteries.In recent years, complex perovskite oxides, such as Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), have been identified as promising candidates for bifunctional catalysts due to their defective structures and excellent oxygen mobility. However, while BSCF exhibits high OER activity, its ORR performance is poor. To address this issue, researchers have turned to metal-oxide composite catalysts, where nanosized metals are deposited on the surface of perovskite oxides, to increase activity and stability towards ORR due to the synergistic effect between metal and oxide support.In this study, we report the development of bifunctional electrocatalysts with highly electrocatalytic activity for ORR and OER. We used a simple wet-chemical processing technique to deposit metal catalysts with superior ORR activity on perovskite oxide-based catalysts with high OER activity. The direct deposition method is an effective way to create a good distribution of metal catalysts, intimate contact between metal catalysts and BSCF perovskite oxide, which may contribute to the creation of a synergistic effect between them. Our results showed that the metal catalysts deposited on perovskite oxides of Pd@BSCF and Pt@BSCF exhibited better ORR activity than others and higher OER activity than the perovskite oxides alone, resulting in much-enhanced bifunctionality through a synergistic effect between the metal catalysts and perovskite oxides.We further investigated the enhanced bifunctional electrocatalytic activity of the metal catalysts deposited perovskite oxides for Li-O2 batteries. The assembled Li-O2 batteries with Pd@BSCF cathode demonstrated lower discharge/charge overpotential (0.8 V at the cutoff capacity of 500 mAh gcat -1) and improved cycling stability (over 70 cycles), indicating the effectiveness of our approach in enhancing the bifunctional activities of ORR and OER for application in electrochemical energy storage and conversion systems. The encouraging results obtained from this study might have been attributed to two mechanisms, namely, the synergistic effect of electronic transfer and RDS/spillover. Our study suggests that the deposition of metal catalysts on the surface of perovskite oxides (Pd@BSCF and Pt@BSCF) is an effective approach to enhance bifunctional activities of ORR and OER, making it a promising strategy for the development of next-generation batteries. 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.

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.

(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.

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.

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.

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.