MoO2-CoO coupled with a macroporous carbon hybrid electrocatalyst for highly efficient oxygen evolution

Catalytic Activity and Electrochemical Stability of Ru1-xMxO2 (M = Zr, Nb, Ta): Computational and Experimental Study of the Oxygen Evolution Reaction

We use computations and experiments to determine the effect of substituting zirconium, niobium, and tantalum within rutile RuO2 on the structure, oxygen evolution reaction (OER) mechanism and activity, and electrochemical stability. Calculated electronic structures altered by Zr, Nb, and Ta show surface regions of electron density depletion and accumulation, along with anisotropic lattice parameter shifts dependent on the substitution site, substituent, and concentration. Consistent with theory, X-ray photoelectron spectroscopy experiments show shifts in binding energies of O-2s, O-2p, and Ru-4d peaks due to the substituents. Experimentally, the substituted materials showed the presence of two phases with a majority phase that contains the metal substituent within the rutile phase and a second, smaller-percentage RuO2 phase. Our experimental analysis of OER activity shows Zr, Nb, and Ta substituents at 12.5 atom % induce lower activity relative to RuO2, which agrees with computing the average of all sites; however, Zr and Ta substitution at specific sites yields higher theoretical OER activity than RuO2, with Zr substitution suggesting an alternative OER mechanism. Metal dissolution predictions show the involvement of cooperative interactions among multiple surface sites and the electrolyte. Zr substitution at specific sites increases activation barriers for Ru dissolution, however, with Zr surface dissolution rates comparable to those of Ru. Experimental OER stability analysis shows lower Ru dissolution from synthesized RuO2 and Zr-substituted RuO2 compared to commercial RuO2 and comparable amounts of Zr and Ru dissolved from Zr-substituted RuO2, aligned with our calculations.

Heterointerface engineering of cobalt molybdenum suboxide for overall water splitting

Highly active and earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are of great significance for sustainable hydrogen generation through alkaline water electrolysis. Here, with an aim to enhance the bifunctional electrocatalytic activity of cobalt molybdate towards overall water splitting, we demonstrate a simple method involving the modulation of the cobalt to molybdenum ratio and creation of phase-modulated heterointerfaces. Samples with varying Co/Mo molar ratios are obtained via a microwave-assisted synthesis method using appropriate starting precursors. The synthesis conditions are modified to create a heterointerface involving multiple phases of cobalt molybdenum suboxides (CoO/CoMoO3/Co2Mo3O8) supported on Ni foam (NF). Detailed electrochemical studies reveal that modulating the composition and hence the interface can tweak the bifunctional electrocatalytic activity of the material for HER and OER and thus improve the overall water splitting efficiency with very high durability over 500 h. To further evaluate the practical applicability of the studied catalyst in water splitting, an alkaline electrolyser is fabricated with the optimized cobalt molybdenum suboxide material (CMO-1.25) as a bifunctional electrocatalyst. A current density of 220 mA cm-2 @1.6 V and 670 mA cm-2 @1.8 V was obtained, and the device showed very good long-term durability.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Carbon-Based Composites as Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media

This review paper presents the most recent research progress on carbon-based composite electrocatalysts for the oxygen evolution reaction (OER), which are of interest for application in low temperature water electrolyzers for hydrogen production. The reviewed materials are primarily investigated as active and stable replacements aimed at lowering the cost of the metal electrocatalysts in liquid alkaline electrolyzers as well as potential electrocatalysts for an emerging technology like alkaline exchange membrane (AEM) electrolyzers. Low temperature electrolyzer technologies are first briefly introduced and the challenges thereof are presented. The non-carbon electrocatalysts are briefly overviewed, with an emphasis on the modes of action of different active phases. The main part of the review focuses on the role of carbon–metal compound active phase interfaces with an emphasis on the synergistic and additive effects. The procedures of carbon oxidative pretreatment and an overview of metal-free carbon catalysts for OER are presented. Then, the successful synthesis protocols of composite materials are presented with a discussion on the specific catalytic activity of carbon composites with metal hydroxides/oxyhydroxides/oxides, chalcogenides, nitrides and phosphides. Finally, a summary and outlook on carbon-based composites for low temperature water electrolysis are presented.

From NiMoO4 to γ-NiOOH: Detecting the Active Catalyst Phase by Time Resolved in Situ and Operando Raman Spectroscopy

Water electrolysis powered by renewable energies is a promising technology to produce sustainable fossil free fuels. The development and evaluation of effective catalysts are here imperative; however, due to the inclusion of elements with different redox properties and reactivity, these materials undergo dynamical changes and phase transformations during the reaction conditions. NiMoO4 is currently investigated among other metal oxides as a promising noble metal free catalyst for the oxygen evolution reaction. Here we show that at applied bias, NiMoO4·H2O transforms into γ-NiOOH. Time resolved operando Raman spectroscopy is utilized to follow the potential dependent phase transformation and is collaborated with elemental analysis of the electrolyte, confirming that molybdenum leaches out from the as-synthesized NiMoO4·H2O. Molybdenum leaching increases the surface coverage of exposed nickel sites, and this in combination with the formation of γ-NiOOH enlarges the amount of active sites of the catalyst, leading to high current densities. Additionally, we discovered different NiMoO4 nanostructures, nanoflowers, and nanorods, for which the relative ratio can be influenced by the heating ramp during the synthesis. With selective molybdenum etching we were able to assign the varying X-ray diffraction (XRD) pattern as well as Raman vibrations unambiguously to the two nanostructures, which were revealed to exhibit different stabilities in alkaline media by time-resolved in situ and operando Raman spectroscopy. We advocate that a similar approach can beneficially be applied to many other catalysts, unveiling their structural integrity, characterize the dynamic surface reformulation, and resolve any ambiguities in interpretations of the active catalyst phase.

Three dimensional Ni3S2 nanorod arrays as multifunctional electrodes for electrochemical energy storage and conversion applications

The increasing demand for energy and environmental protection has stimulated intensive interest in fundamental research and practical applications. Nickel dichalcogenides (Ni₃S₂, NiS, Ni₃Se₂, NiSe, etc.) are promising materials for high-performance electrochemical energy storage and conversion applications. Herein, 3D Ni₃S₂ nanorod arrays are fabricated on Ni foam by a facile solvothermal route. The optimized Ni₃S₂/Ni foam electrode displays an areal capacity of 1602 µA h cm^-2 at 5 mA cm^-2, excellent rate capability and cycling stability. Besides, 3D Ni₃S₂ nanorod arrays as electrode materials exhibit outstanding performances for the overall water splitting reaction. In particular, the 3D Ni₃S₂ nanorod array electrode is shown to be a high-performance water electrolyzer with a cell voltage of 1.63 V at a current density of 10 mA cm^-2 for overall water splitting. Therefore, the results demonstrate a promising multifunctional 3D electrode material for electrochemical energy storage and conversion applications.

Noble-Metal-Free Electrocatalysts for Oxygen Evolution

Oxygen evolution reaction (OER) plays a vital role in many energy conversion and storage processes including electrochemical water splitting for the production of hydrogen and carbon dioxide reduction to value-added chemicals. IrO₂ and RuO₂ , known as the state-of-the-art OER electrocatalysts, are severely limited by the high cost and low earth abundance of these noble metals. Developing noble-metal-free OER electrocatalysts with high performance has been in great demand. In this review, recent advances in the design and synthesis of noble-metal-free OER electrocatalysts including Ni, Co, Fe, Mn-based hydroxides/oxyhydroxides, oxides, chalcogenides, nitrides, phosphides, and metal-free compounds in alkaline, neutral as well as acidic electrolytes are summarized. Perspectives are also provided on the fabrication, evaluation of OER electrocatalysts and correlations between the structures of the electrocatalysts and their OER activities.

Engineering Cu2O/Cu@CoO hierarchical nanospheres: synergetic effect of fast charge transfer cores and active shells for enhanced oxygen evolution reaction

An OER catalyst that has a high conductivity Cu₂O/Cu core and a strong bonding interface with the active CoO shell was constructed.

One-dimensional MoO2–Co2Mo3O8@C nanorods: a novel and highly efficient oxygen evolution reaction catalyst derived from metal–organic framework composites

One dimensional MoO₂–Co₂Mo₃O₈@C nanorods were synthesized by using MoO₃@ZIF-67 composites as a precursor and the catalyst Co₂Mo₃O₈ shows excellent OER activity.

Colloidal synthesis of urchin-like Fe doped NiSe2 for efficient oxygen evolution

The search for highly efficient non-precious metal electrocatalysts toward the oxygen evolution reaction (OER) is extremely essential for renewable energy systems. Here, we report the colloidal synthesis of Fe doped NiSe₂, which functions as a high-performance electrocatalyst for the OER in alkaline solution. The NiFeSe catalysts are composed of urchin-like dendrites with a high number of active sites, which could provide fast transportation of electrons and electrolytes, and facile release of the evolved O₂ bubbles during the OER catalysis. Benefitting from this unique urchin-like structure and strong electron interaction between Fe, Ni, and Se, the Ni_1.12Fe_0.49Se₂ catalyst exhibits excellent electrocatalytic activity and high durability toward the OER in alkaline solution, with an overpotential of 227 mV at a current density of 10 mA cm^-2, which is, to the best of our knowledge, higher than most of the reported selenide-based electrocatalysts.

A free standing porous Co/Mo architecture as a robust bifunctional catalyst toward water splitting

A free standing porous Co/Mo architecture is fabricated by CAN etching and exhibits robust bifunctional catalysis towards OER and HER.

Porous MoO2 Nanosheets as Non-noble Bifunctional Electrocatalysts for Overall Water Splitting

A porous MoO2 nanosheet as an active and stable bifunctional electrocatalyst for overall water splitting, is presented. It needs a cell voltage of only about 1.53 V to achieve a current density of 10 mA cm(-2) and maintains its activity for at least 24 h in a two-electrode configuration.

Au-NiCo2O4 supported on three-dimensional hierarchical porous graphene-like material for highly effective oxygen evolution reaction

A three-dimensional hierarchical porous graphene-like (3D HPG) material was synthesized by a one-step ion-exchange/activation combination method using a cheap metal ion exchanged resin as carbon precursor. The 3D HPG material as support for Au-NiCo₂O₄ gives good activity and stability for oxygen evolution reaction (OER). The 3D HPG material is induced into NiCo₂O₄ as conductive support to increase the specific area and improve the poor conductivity of NiCo₂O₄. The activity of and stability of NiCo₂O₄ significantly are enhanced by a small amount of Au for OER. Au is a highly electronegative metal and acts as an electron adsorbate, which is believed to facilitate to generate and stabilize Co⁴⁺ and Ni³⁺ cations as the active centres for the OER.