Interconnected Copper Cobaltite Nanochains as Efficient Electrocatalysts for Water Oxidation in Alkaline Medium

The influence of nanostructure and electrolyte concentration on the performance of nickel sulfide (Ni3S2) catalyst for electrochemical overall water splitting

Developing non-precious nanostructured electrocatalysts, exhibiting high catalytic activity in combination with excellent stability, has an enormous potential to replace noble-metal-based catalysts for Hydrogen production through electrochemical water splitting. In this study, a facile method is used for the synthesis of three different hierarchical nanostructures of nickel sulfide (Ni3S2) including nanosheets, nanorods, and multiconnected nanorods that are directly grown on 3D nickel foam (NF). These nanostructured electrocatalysts are evaluated for electrochemical water splitting in alkaline media using four different concentrations to understand the effect of nanostructure and ion concentration on the efficiency. Among different combinations of structure and electrolyte concentration, the Ni3S2 in the form of nanosheets exhibited the best electrocatalytic performance for hydrogen evolution reaction (HER) as well as oxygen evolution reaction (OER) in 3.0 M alkaline solution. The hierarchical Ni3S2 nanosheets exhibited a high electrochemically active surface area, which facilitated the charge transport phenomenon along the electrode-electrolyte interface in a higher electrolyte concentration that improved the reaction kinetics so as overall water splitting. The developed Ni3S2 nanosheets required an overpotential of 110 mV (@10 mA cm-2) and 211 mV (@100 mA cm-2) for HER and OER, respectively in 3.0 M electrolyte concentration. This work provides insight into how the materials' nanostructures and electrolyte concentration could be utilized to improve the electrocatalytic performance for an overall water-splitting process, and the concept could be applied for material designing and conditions optimization for other catalytic applications.

Experimental and Theoretical Insights of Anion Regulation in MOF-Derived Ni-Co-Based Nanosheets for Supercapacitors and Anion Exchange Membrane Water Electrolyzers

The anionic components have a significant role in regulating the electrochemical properties of mixed transition-metal (MTM)-based materials. However, the relationship between the anionic components and their inherent electrochemical properties in MTM-based materials is still unclear. Herein, we report the anion-dependent supercapacitive and oxygen evolution reaction (OER) properties of in situ grown binary Ni-Co-selenide (Se)/sulfide (S)/phosphide (P) nanosheet arrays (NAs) over nickel foam starting from MOF-derived Ni-Co layered double hydroxide precursors. Among them, the Ni-Co-Se NAs exhibited the best specific capacity (289.6 mA h g^-1 at 4 mA cm^-2). Furthermore, a hybrid device constructed with Ni-Co-Se NAs delivered an excellent energy density (74 W h kg^-1 at 525 W kg^-1) and an ultra-high power density (10 832 W kg^-1 at 46 W h kg^-1) with outstanding durability (∼94%) for 10 000 cycles. Meanwhile, the Ni-Co-Se NAs showed superior electrocatalytic OER outputs with the lowest overpotential (235 mV at 10 mA cm^-2) and Tafel slope. In addition, Ni-Co-Se NAs outperformed IrO₂ as an anode in an anion exchange membrane water electrolyzer at a high current density (>1.0 A cm^-2) and exhibited a stable performance up to 48 h with a 99% Faraday efficiency. Theoretical analyses validate that the Se promotes OH adsorption and improves the electrochemical activity of the Ni-Co-Se through a strong electronic redistribution/hybridization with an active metal center due to its valence 4p and inner 3d orbital participations. This study will provide in-depth knowledge of bifunctional activities in MTM-based materials with different anionic substitutions.

Structure and oxygen vacancy engineered CuCo-layered double oxide nanotube arrays as advanced bifunctional electrocatalysts for overall water splitting

In recent years, as a green renewable energy production technology, electrochemical water splitting has demonstrated high development potential. Many materials have been reported as successful catalysts in the water-splitting field. However, it is still a huge challenge to produce bifunctional electrocatalysts for the efficient and sustainable generation of hydrogen and oxygen simultaneously. Herein, we successfully developed oxygen vacancies abundant CuCo layered double oxide (O_v-CuCo-LDO) hollow nanotube arrays (HNTAs) loaded on nickel foam as advanced electrocatalysts for total water splitting. When the current density was 10 mA cm^-2, the O_v-CuCo-LDO HNTAs exhibited outstanding onset overpotentials of 53.9 and 72.5 mV for the hydrogen evolution and oxygen evolution reactions (HER and OER) in alkaline medium, respectively, because of the bimetallic synergistic effect between the cobalt and copper and the unique hollow porous structure. In addition, an as-assembled O_v-CuCo-LDO||O_v-CuCo-LDO electrolytic cell showed a small potential of 1.55 V to deliver a current density of 10 mA cm^-2. Moreover, it also showed remarkable durability after long-term overall water splitting for more than 20 h. The research results in this paper are of great interest to practical applications of the water decomposition process, providing clear and in-depth insights into preliminary robust and efficient multifunctional electrocatalysts for overall water splitting.

Anion-Exchange Membrane Water Electrolyzers

This Review provides an overview of the emerging concepts of catalysts, membranes, and membrane electrode assemblies (MEAs) for water electrolyzers with anion-exchange membranes (AEMs), also known as zero-gap alkaline water electrolyzers. Much of the recent progress is due to improvements in materials chemistry, MEA designs, and optimized operation conditions. Research on anion-exchange polymers (AEPs) has focused on the cationic head/backbone/side-chain structures and key properties such as ionic conductivity and alkaline stability. Several approaches, such as cross-linking, microphase, and organic/inorganic composites, have been proposed to improve the anion-exchange performance and the chemical and mechanical stability of AEMs. Numerous AEMs now exceed values of 0.1 S/cm (at 60–80 °C), although the stability specifically at temperatures exceeding 60 °C needs further enhancement. The oxygen evolution reaction (OER) is still a limiting factor. An analysis of thin-layer OER data suggests that NiFe-type catalysts have the highest activity. There is debate on the active-site mechanism of the NiFe catalysts, and their long-term stability needs to be understood. Addition of Co to NiFe increases the conductivity of these catalysts. The same analysis for the hydrogen evolution reaction (HER) shows carbon-supported Pt to be dominating, although PtNi alloys and clusters of Ni(OH)₂ on Pt show competitive activities. Recent advances in forming and embedding well-dispersed Ru nanoparticles on functionalized high-surface-area carbon supports show promising HER activities. However, the stability of these catalysts under actual AEMWE operating conditions needs to be proven. The field is advancing rapidly but could benefit through the adaptation of new in situ techniques, standardized evaluation protocols for AEMWE conditions, and innovative catalyst-structure designs. Nevertheless, single AEM water electrolyzer cells have been operated for several thousand hours at temperatures and current densities as high as 60 °C and 1 A/cm², respectively.

Electro-Reforming Polyethylene Terephthalate Plastic to Co-Produce Valued Chemicals and Green Hydrogen

Upcycling plastic waste pollution for sustainable resources and energy is an ideal solution to plastic waste-related environmental issues. Polyethylene terephthalate (PET), one of the most prominent single-use daily plastics with up to millions of tons produced annually, has recently been explored with respect to chemical recycling to ameliorate its environmental impact. In this work, we report an electrochemical upcycling approach to electrocatalytic oxidation of PET hydrolysate using Cu-based nanowire catalysts. We demonstrate that the electrocatalyst can catalyze the ethylene glycol (EG) molecule derived from PET waste toward formate with high selectivity and exhibit a lower onset potential for EG oxidation than for water oxidation. Experimental and density functional theory calculation results reveal that the oxidation pathway of EG on CuO can selectively break the C-C bond to generate formic acid. This work sheds light on employing earth-abundant metal catalysts to convert PET plastic waste to produce valued chemicals and green hydrogen.

Superior supercapacitor performance of Bi2S3 nanorod/reduced graphene oxide composites

The present work is focused on the synthesis of bismuth sulfide (Bi2S3) nanorod/reduced graphene oxide (RGO) composites via a one-step hydrothermal method using GO and bismuth nitrate in 5 : 1, 3 : 1 and 2 : 1 weight ratios and their characterization. The morphological studies revealed the formation of homogeneously dispersed Bi2S3 nanorods on RGO sheets along with occasional wrapping in the Bi2S3 nanorod/RGO (3 : 1) composite. XRD, FTIR, Raman and XPS studies suggested the incorporation of Bi2S3 in RGO sheets. The galvanostatic charge-discharge measurements showed that the Bi2S3 nanorod/RGO (3 : 1) composite exhibited the highest specific capacitance (1932 F g-1) at 1 A g-1 in the presence of 2 M aqueous KOH in a three-electrode cell. This is ascribed to the enhanced contact area between metal sulfide nanoparticles and RGO, increased conductivity and synergistic effect of Bi2S3 and RGO. The optimized Bi2S3 nanorod/RGO (3 : 1) composite also maintained an excellent cycling stability with ∼100% capacitance retention after 700 cycles. It is noted that the supercapacitor performance of the Bi2S3 nanorod/RGO (3 : 1) composite was better than group V and VI metal chalcogenides and their nanocomposites reported in several previous studies.

N, Ru Codoped Pellet Drum Bundle-Like Sb2S3: An Efficient Hydrogen Evolution Reaction and Hydrogen Oxidation Reaction Electrocatalyst in Alkaline Medium

Though investigations have been made on several metal chalcogenides in hydrogen evolution reactions (HERs) and hydrogen oxidation reactions (HORs), antimony sulfide (Sb₂S₃) has not generated much attention. In this direction, the present work reports on the synthesis of N, Ru codoped pellet drum bundle-like antimony sulfide (Sb₂S₃) via a simple reflux method. Subsequent HER and HOR electrocatalytic investigations in 1 M KOH revealed their suitability as an efficient and inexpensive alternative to platinum, as is evident from the overpotential (72 mV at a current density of 10 mA cm^-2), Tafel slope (193 mV/decade), exchange current density (1.42 mA/cm²), and breakdown potential at ∼0.6 V vs RHE, respectively. Such remarkable HER and HOR performance of N, Ru codoped Sb₂S₃ could be ascribed to the presence of relatively larger active sites compared to Sb₂S₃ and N-doped Sb₂S₃ individually due to synergistic effects arising from N and Ru dopants. Further, N, Ru codoped Sb₂S₃ demonstrated high intrinsic catalytic activity as indicated by its turnover frequency (2.03 s^-1) and current loss, corresponding to 35% after 10 h of continuous amperometric i-t operation. Alternatively, such excellent catalytic performance of N, Ru codoped Sb₂S₃ arises due to geometric lattice defects with surface oxygen vacancy, and the availability of abundant edges and its pellet drum-like morphology also cannot be overruled.

Facile Solvothermal Preparation of Mn2CuO4 Microspheres: Excellent Electrocatalyst for Real-Time Detection of H2O2 Released from Live Cells

Hydrogen peroxide (H₂O₂) is an eminent biomarker in pathogenesis; a selective, highly sensitive real-time detection of H₂O₂ released from live cells has drawn a significant research interest in bioanalytical chemistry. Binary transition-metal oxides (BTMOs) displayed a recognizable benefit in enhancing the sensitivity of H₂O₂ detection; although the reported BTMO-based H₂O₂ sensor's detection limit is still insufficient, it is not appropriate for in situ profiling of trace amounts of cellular H₂O₂. In this paper, we describe an efficient, reliable electrochemical biosensor based on Mn₂CuO₄ (MCO) microspheres to assay cellular H₂O₂. The Mn₂CuO₄ microspheres were prepared through a superficial solvothermal method. It is obvious from impedance studies, introduction of manganese into copper oxide lattice significantly improved the ionic conductivity, which is beneficial for the electrochemical sensing process. Thanks to the distinct microsphere structure and excellent synergy, MCO-modified electrode exhibited excellent nonenzymatic electrochemical behavior toward H₂O₂ sensing. The MCO-modified electrode delivered a broad working range (36 nM to 9.3 mM) and an appreciable detection limit (13 nM), with high selectivity toward H₂O₂. To prove its practicality, the developed sensor was applied in the detection of cellular H₂O₂ released by RAW 264.7 cells in presence of CHAPS. These results label the possible appliance of the sensor in clinical analysis and pathophysiology. Thus, BTMOs are evolving as a promising candidate in designing catalytic matrices for biosensor applications.

Rational Design of Co(II) Dominant and Oxygen Vacancy Defective CuCo2O4@CQDs Hollow Spheres for Enhanced Overall Water Splitting and Supercapacitor Performance

The hierarchical CuCo₂O₄@carbon quantum dots (CQDs) hollow microspheres constructed by 1D porous nanowires have been successfully prepared through a simple CQDs-induced hydrothermal self-assembly technique. XPS analysis shows the CuCo₂O₄@CQDs possesses the Co(II)-rich surface associated with the oxygen vacancies, which can effectively boost the Faradaic reactions and oxygen evolution reaction (OER) activity. For example, the as-synthesized 3D porous CuCo₂O₄@CQDs electrode exhibits high activity toward overall electrochemical water splitting, for example, an overpotential of 290 mV for OER and 331 mV for hydrogen evolution reaction (HER) in alkaline media have been achieved at 10 mA cm^-2, respectively. Furthermore, an asymmetric supercapacitor (ASC) (CuCo₂O₄@CQDs//CNTs) delivers a high energy density of 45.9 Wh kg^-1 at 763.4 W kg^-1, as well as good cycling ability. The synergy of Co(II)-rich surface, oxygen vacancies, and well-defined 3D hollow structures facilitates the subsequent surface electrochemical reactions. This work presents a facile method to fabricate energetic nanocomposites with highly reactive, durable, and universal functionalities.