Transition metal based heterogeneous electrocatalysts for the oxygen evolution reaction at near-neutral pH

Hierarchical CoMn-LDH and Heterostructured Composites for Advanced Supercapacitors and Electrocatalysis Applications

In the present study, self-assembled hierarchical CoMn-LDH, CoMn@CuZnS, and CoMn@CuZnFeS heterostructured composites were synthesized for bifunctional applications. As an electrode for a supercapacitor, CoMn-LDH demonstrated superior areal and specific capacitance of 5.323 F cm-2 (279.49 mAh/g) at 4 mA cm-2, comparable to or even higher than other LDHs. The assembled AC//CoMn-LDH hybrid supercapacitor device further demonstrated better stability with 63% original capacitance over 20,000 cycles. Later, as a catalyst, CoMn-LDH, CoMn@CuZnS, and CoMn@CuZnFeS electrodes revealed better performance, with overpotentials of 340, 350, and 366 and -199, -215, and -222 mV to attain 10 mA cm-2 of current density for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. Moreover, for CoMn-LDH, small Tafel slopes of 102 and 128 mV/dec were noticed for OER and HER with good stability compared to heterostructured electrodes.

Spin Manipulation of Heterogeneous Molecular Electrocatalysts by an Integrated Magnetic Field for Efficient Oxygen Redox Reactions

Understanding the spin-dependent activity of nitrogen-coordinated single metal atom (M-N-C) electrocatalysts for oxygen reduction and evolution reactions (ORR and OER) remains challenging due to the lack of structure-defined catalysts and effective spin manipulation tools. Herein, both challenges using a magnetic field integrated heterogeneous molecular electrocatalyst prepared by anchoring cobalt phthalocyanine (CoPc) deposited carbon black on polymer-protected magnet nanoparticles, are addressed. The built-in magnetic field can shift the Co center from low- to high-spin (HS) state without atomic structure modification, affording one-order higher turnover frequency, a 50% increased H2O2 selectivity for ORR, and a ≈4000% magnetocurrent enhancement for OER. This catalyst can significantly minimize magnet usage, enabling safe and continuous production of a pure H2O2 solution for 100 h from a 100 cm2 electrolyzer. The new strategy demonstrated here also applies to other metal phthalocyanine-based catalysts, offering a universal platform for studying spin-related electrochemical processes.

Enhanced performance of molecular electrocatalysts for CO2 reduction in a flow cell following K+ addition

Electrocatalytic CO2 reduction is a key aspect of artificial photosynthesis systems designed to produce fuels. Although some molecular catalysts have good performance for CO2 reduction, these compounds also suffer from poor durability and energy efficiency. The present work demonstrates the improved CO2 reduction activity exhibited by molecular catalysts in a flow cell. These catalysts were composed of a cobalt-tetrapyridino-porphyrazine complex supported on carbon black together with potassium salt and were both stable and efficient. These systems were found to promote electrocatalytic CO2 reduction with a current density of 100 mA/cm2 and generated CO over at least 1 week with a selectivity of approximately 95%. The optimal catalyst gave a turnover number of 3,800,000 and an energy conversion efficiency of more than 62% even at 200 mA/cm2.

Interface strong-coupled 3D Mo-NiS@Ni-Fe LDH flower-cluster as exceptionally efficient electrocatalyst for water splitting in wide pH range

It is crucial to create a bifunctional catalyst with high efficiency and low cost for electrochemical water splitting under alkaline and neutral pH conditions. This study investigated the in-situ creation of ultrafine Mo-NiS and NiFe LDH nanosheets as an effective and stable electrocatalyst with a three-dimensional (3D) flower-cluster hierarchical structure (Mo-NiS@NiFe LDH). The strong interfacial connection between Mo-NiS and NiFe LDH enhances the formation of metal higher chemical states in the material, optimizes the electronic structure, increases OH^- adsorption capacity improves electron transfer/mass diffusion, and promotes O₂/H₂ gas release. As a result, at 10 mA cm^-2, Mo-NiS@NiFe LDH/NF demonstrates the outstanding bifunctional electrocatalytic activity of just 107 mV (HER, hydrogen evolution reaction) and 184 mV (hydrogen evolution reaction) (OER, oxygen evolution reaction). The catalytic performance is remarkably stable after 72 h of continuous operation in 1 M KOH at high current densities (300 mA cm^-2). More interestingly, in the overall water splitting system, the cell voltages for anode and cathode in both alkaline and neutral electrolytes for Mo-NiS@NiFe LDH/NF are only 1.54 V (alkaline) and 2.06 V (neutral) at 10 mA cm^-2. These results demonstrated that the bifunctional electrocatalyst design concept is a viable solution for water splitting in both alkaline and neutral systems.

Selectively Enhanced Electrocatalytic Oxygen Evolution within Nanoscopic Channels Fitting a Specific Reaction Intermediate for Seawater Splitting

Abundant availability of seawater grants economic and resource-rich benefits to water electrolysis technology requiring high-purity water if undesired reactions such as chlorine evolution reaction (CER) competitive to oxygen evolution reaction (OER) are suppressed. Inspired by a conceptual computational work suggesting that OER is kinetically improved via a double activation within 7 Å-gap nanochannels, RuO₂ catalysts are realized to have nanoscopic channels at 7, 11, and 14 Å gap in average (d_gap ), and preferential activity improvement of OER over CER in seawater by using nanochanneled RuO₂ is demonstrated. When the channels are developed to have 7 Å gap, the OER current is maximized with the overpotential required for triggering OER minimized. The gap value guaranteeing the highest OER activity is identical to the value expected from the computational work. The improved OER activity significantly increases the selectivity of OER over CER in seawater since the double activation by the 7 Å-nanoconfined environments to allow an OER intermediate (*OOH) to be doubly anchored to Ru and O active sites does not work on the CER intermediate (*Cl). Successful operation of direct seawater electrolysis with improved hydrogen production is demonstrated by employing the 7 Å-nanochanneled RuO₂ as the OER electrocatalyst.

Morphology-Dependent Electrocatalytic Behavior of Cobalt Chromite toward the Oxygen Evolution Reaction in Acidic and Alkaline Medium

Exploiting an affordable, durable, and high-performance electrocatalyst for the oxygen evolution reaction (OER) under lower pH condition (acidic) is highly challengeable and much attractive toward the hydrogen-based energy technologies. A spinel CoCr₂O₄ is observed as a potential noble-metal-free candidate for OER in alkaline medium. The presence of Cr further leads to electronic structure modulation of Co₃O₄ and thereby greatly increases the corrosive resistance toward OER in acidic environment. Herein, a typical CoCr₂O₄ with three different morphologies was synthesized for the very first time and employed as an electrocatalyst for OER in alkaline (1 M KOH) and acidic (0.5 M H₂SO₄) medium. Moreover, different morphologies display a different intrinsic exposed active site and thereby display different electrocatalytic activities. Likewise, the CoCr₂O₄ Mic (synthesized by the microwave heating method) displays a higher catalytic activity toward OER and delivers a low overpotential of 293 and 290 mV to attain 10 mA/cm² current density and smaller Tafel slope values of 40 and 151 mV/dec, respectively, in alkaline and acidic environment than the synthesized CoCr₂O₄ Wet (wet-chemically synthesized) and CoCr₂O₄ Hyd (hydrothermally synthesized). Moreover, CoCr₂O₄ Mic exhibits a long-term durability of 24 h (1 M KOH) and 10.5 h (0.5 M H₂SO₄). The optimized Co-O bond energy in OER condition makes the CoCr₂O₄ Mic superior than the CoCr₂O₄ Hyd and CoCr₂O₄ Wet. Moreover, the substitution of Cr induces the electron delocalization around the Co active species and thereby, positive shifting of the redox potential leads to providing an optimal binding energy for OER intermediates. Also, interestingly, this work represents a catalytic activity trend by a simple experimental result without any complex theoretical calculation. The morphology-dependent electrocatalytic activity obtained in this work will provide a new strategy in the field of electrochemical conversion and energy storage application.

Anode Catalysts in CO2 Electrolysis: Challenges and Untapped Opportunities

The field of electrochemical carbon dioxide reduction has developed rapidly during recent years. At the same time, the role of the anodic half-reaction has received considerably less attention. In this Perspective, we scrutinize the reports on the best-performing CO2 electrolyzer cells from the past 5 years, to shed light on the role of the anodic oxygen evolution catalyst. We analyze how different cell architectures provide different local chemical environments at the anode surface, which in turn determines the pool of applicable anode catalysts. We uncover the factors that led to either a strikingly high current density operation or an exceptionally long lifetime. On the basis of our analysis, we provide a set of criteria that have to be fulfilled by an anode catalyst to achieve high performance. Finally, we provide an outlook on using alternative anode reactions (alcohol oxidation is discussed as an example), resulting in high-value products and higher energy efficiency for the overall process.

Revealing the pH-Universal Electrocatalytic Activity of Co-Doped RuO2 toward the Water Oxidation Reaction

Electrocatalytic water splitting has gained vast attention in recent decades for its role in catalyzing hydrogen production effectively as an alternative to fossil fuels. Moreover, the designing of highly efficient oxygen evolution reaction (OER) electrocatalysts across the universal pH conditions was more challengeable as in harsh anodic potentials, it questions the activity and stability of the concerned catalyst. Generally, geometrical engineering and electronic structural modulation of the catalyst can effectively boost the OER activity. Herein, a Co-doped RuO₂ nanorod material is developed and used as an OER electrocatalyst at different pH conditions. Co-RuO₂ exhibits a lower overpotential value of 238 mV in an alkaline environment (1 M KOH) with a Tafel slope value of 48 mV/dec. On the other hand, in acidic, neutral, and near-neutral environments, it required overpotentials of 328, 453, and 470 mV, respectively, to attain a 10 mA/cm² current density. It is observed that doping of Co into the RuO₂ could synergistically increase the active sites with the enhanced electrophilic nature of Ru⁴⁺ to accelerate OER in all of the pH ranges. This study finds the applicability of earth-abundant-based metals like Co to be used in universal pH conditions with a simple doping technique. Further, it assured the stable nature in all pH electrolytes and needs to be further explored with other metals in the future.

Electrocatalytic desalination with CO2 reduction and O2 evolution

Multifunctional electrocatalytic desalination is a promising method to increase the production of additional valuable chemicals during the desalination process. In this work, a multifunctional desalination device was demonstrated to effectively desalinate brackish water (15 000 ppm) to 9 ppm while generating formate from captured CO₂ at the Bi nanoparticle cathode and releasing oxygen at the Ir/C anode. The salt feed channel is sandwiched between two electrode chambers and separated by ion-exchange membranes. The electrocatalytic process accelerates the transportation of sodium ions and chloride ions in the brine to the cathode and anode chamber, respectively. The fastest salt removal rate to date was obtained, reaching up to 228.41 μg cm^-2 min^-1 with a removal efficiency of 99.94%. The influences of applied potential and the concentrations of salt feed and electrolyte were investigated in detail. The current research provides a new route towards an electrochemical desalination system.

Phase Transitions and Water Splitting Applications of 2D Transition Metal Dichalcogenides and Metal Phosphorous Trichalcogenides

2D layered materials turn out to be the most attractive hotspot in materials for their unique physical and chemical properties. A special class of 2D layered material refers to materials exhibiting phase transition based on environment variables. Among these materials, transition metal dichalcogenides (TMDs) act as a promising alternative for their unique combination of atomic-scale thickness, direct bandgap, significant spin-orbit coupling and prominent electronic and mechanical properties, enabling them to be applied for fundamental studies as catalyst materials. Metal phosphorous trichalcogenides (MPTs), as another potential catalytic 2D phase transition material, have been employed for their unusual intercalation behavior and electrochemical properties, which act as a secondary electrode in lithium batteries. The preparation of 2D TMD and MPT materials has been extensively conducted by engineering their intrinsic structures at the atomic scale. In this study, advanced synthesis methods of preparing 2D TMD and MPT materials are tested, and their properties are investigated, with stress placed on their phase transition. The surge of this type of report is associated with water-splitting catalysis and other catalytic purposes. This study aims to be a guideline to explore the mentioned 2D TMD and MPT materials for their catalytic applications.