Regulating the Electronic Structure of CoP Nanosheets by O Incorporation for High‐Efficiency Electrochemical Overall Water Splitting

The exploration of earth‐abundant and high‐efficiency bifunctional electrocatalysts for overall water splitting is of vital importance for the future of the hydrogen economy. Regulation of electronic structure through heteroatom doping represents one of the most powerful strategies to boost the electrocatalytic performance of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, a rational design of O‐incorporated CoP (denoted as O‐CoP) nanosheets, which synergistically integrate the favorable thermodynamics through modification of electronic structures with accelerated kinetics through nanostructuring, is reported. Experimental results and density functional theory simulations manifest that the appropriate O incorporation into CoP can dramatically modulate the electronic structure of CoP and alter the adsorption free energies of reaction intermediates, thus promoting the HER and OER activities. Specifically, the optimized O‐CoP nanosheets exhibit efficient bifunctional performance in alkaline electrolyte, requiring overpotentials of 98 and 310 mV to deliver a current density of 10 mA cm^?2 for HER and OER, respectively. When served as bifunctional electrocatalysts for overall water splitting, a low cell voltage of 1.60 V is needed for achieving a current density of 10 mA cm^?2. This proposed anion‐doping strategy will bring new inspiration to boost the electrocatalytic performance of transition metal–based electrocatalysts for energy conversion applications.     

Trimetallic Mn‐Fe‐Ni Oxide Nanoparticles Supported on Multi‐Walled Carbon Nanotubes as High‐Performance Bifunctional ORR/OER Electrocatalyst in Alkaline Media

Discovering precious metal‐free electrocatalysts exhibiting high activity and stability toward both the oxygen reduction (ORR) and the oxygen evolution (OER) reactions remains one of the main challenges for the development of reversible oxygen electrodes in rechargeable metal–air batteries and reversible electrolyzer/fuel cell systems. Herein, a highly active OER catalyst, Fe_0.3Ni_0.7O_X supported on oxygen‐functionalized multi‐walled carbon nanotubes, is substantially activated into a bifunctional ORR/OER catalyst by means of additional incorporation of MnO_X. The carbon nanotube‐supported trimetallic (Mn‐Ni‐Fe) oxide catalyst achieves remarkably low ORR and OER overpotentials with a low reversible ORR/OER overvoltage of only 0.73 V, as well as selective reduction of O₂ predominantly to OH^?. It is shown by means of rotating disk electrode and rotating ring disk electrode voltammetry that the combination of earth‐abundant transition metal oxides leads to strong synergistic interactions modulating catalytic activity. The applicability of the prepared catalyst for reversible ORR/OER electrocatalysis is evaluated by means of a four‐electrode configuration cell assembly comprising an integrated two‐layer bifunctional ORR/OER electrode system with the individual layers dedicated for the ORR and the OER to prevent deactivation of the ORR activity as commonly observed in single‐layer bifunctional ORR/OER electrodes after OER polarization.     

In Situ Induction of Strain in Iron Phosphide (FeP2) Catalyst for Enhanced Hydroxide Adsorption and Water Oxidation

Carbon‐based materials have been widely used in heterogeneous catalysis because of their advantages of high surface area, thermal stability, and chemical inertness. However, their role in the catalysis is not fully understood although most studies conclude that the coupling between the carbon support and catalyst could reduce the charge transfer resistance and improve the kinetics of catalytic reactions such as water splitting. In this study, a carbon‐modified FeP₂ electrocatalyst with a one‐step strategy is synthesized. The tensile strain is introduced in situ in the ab crystal plane of the FeP₂ catalyst. This leads to charge redistribution between H and O atoms in the OH bonds and enhances the adsorption of reaction intermediates. In the water oxidation process, this results in a decrease in the energy barrier for the rate‐determining step, specifically, the chemical step of *OH adsorption preceded by one‐electron transfer. Benefiting from the optimized adsorption energy, the strained catalysts exhibit excellent oxygen evolution reaction (OER) activity with a low overpotential in addition to their increased stability. This study provides a new strategy for the introducing of strains in functional materials and provides new insights into the influence of carbon modification on OER activity.     

Physically Adsorbed Metal Ions in Porous Supports as Electrocatalysts for Oxygen Evolution Reaction

Developing highly efficient and earth‐abundant electrocatalysts for the oxygen evolution reaction (OER) is significantly important for water‐splitting. Here, for the first time it is reported that the physically adsorbed metal ions (PAMI) in porous materials can be served as highly efficient OER electrocatalysts, which provides a universal PAMI method to develop electrocatalysts. This PAMI method can be applied to almost all porous supports, including graphene, carbon nanotubes, C₃N₄, CaCO₃, and porous organic polymers and all the systems exhibit excellent OER performance. In particular, the as‐synthesized Co_0.7Fe_0.3CB exhibits a small overpotential of 295 mV and 350 mV at the current density of 10 mA cm^?2 and 100 mA cm^?2, respectively, which exceeds commercial 40 wt% IrO₂/CB and most reported non‐noble metal‐based OER catalysts. Moreover, the mass activity of Co_0.7Fe_0.3CB reaches 643.4 A g^?1 at the overpotential of 320 mV, which is nearly 4.7 times higher than that of 40 wt% IrO₂/CB. In addition, the advanced ex situ and in situ synchrotron X‐ray characterizations are carried out to unravel the PAMI synthetic process. In short, this PAMI method will break the conversional understanding, i.e., the most OER catalysts are synthesized chemically, because the new PAMI method does not require any chemical synthesis, which therefore opens a new avenue for the development of OER electrocatalysts.     

Metal and Metal Oxide Electrocatalysts for Redox Flow Batteries

Redox flow batteries (RFBs) are one of the promising technologies for large‐scale energy storage applications. For practical implementation of RFBs, it is of great interest to improve their efficiency and reduce their cost. One of the key components of RFBs that can greatly influence the efficiency and final cost is the electrode. The chemical and structural nature of electrodes can modify the kinetics of redox reactions and the accessibility of the electroactive species to available active sites. The ideal electrocatalyst for RFBs must have good activity for the desirable redox reaction, provide a high surface area, and exhibit sufficient conductivity and durability over repeated use. One strategy is to coat the electrode with metal and metal oxide electrocatalysts. Metal electrocatalysts have the advantage of high conductivity, while metal oxide catalysts are usually less expensive and so more economically attractive. In order to gain a better understanding of the performance of the electrocatalysts in RFBs, a comprehensive review of the progress in the development of metal and metal oxide electrocatalysts for RFBs is provided and a critical comparison of the latest developments is presented. Finally, practical recommendations for advancement of electrocatalysts and effective transfer of knowledge in this field are provided.     

Laser Assisted Solution Synthesis of High Performance Graphene Supported Electrocatalysts

Simple, yet versatile, methods to functionalize graphene flakes with metal (oxide) nanoparticles are in demand, particularly for the development of advanced catalysts. Herein, based on light‐induced electrochemistry, a laser‐assisted, continuous, solution route for the simultaneous reduction and modification of graphene oxide with catalytic nanoparticles is reported. Electrochemical graphene oxide (EGO) is used as starting material and electron–hole pair source due to its low degree of oxidation, which imparts structural integrity and an ability to withstand photodegradation. Simply illuminating a solution stream containing EGO and metal salt (e.g., H₂PtCl₆ or RuCl₃) with a 248 nm wavelength laser produces reduced EGO (rEGO, oxygen content 4.0 at%) flakes, decorated with Pt (≈2.0 nm) or RuO₂ (≈2.8 nm) nanoparticles. The RuO₂–rEGO flakes exhibit superior catalytic activity for the oxygen evolution reaction, requiring a small overpotential of 225 mV to reach a current density of 10 mA cm^?2. The Pt–rEGO flakes (10.2 wt% of Pt) show enhanced mass activity for the hydrogen evolution reaction, and similar performance for oxygen reduction reaction compared to a commercial 20 wt% Pt/C catalyst. This simple production method is also used to deposit PtPd alloy and MnO_x nanoparticles on rEGO, demonstrating its versatility in synthesizing functional nanoparticle‐modified graphene materials.     

Molybdenum and Phosphorous Dual Doping in Cobalt Monolayer Interfacial Assembled Cobalt Nanowires for Efficient Overall Water Splitting

The necessity for better water splitting requires speedy development of efficient catalysts with high activity, long‐term stability, and cost effectiveness. In this work, a bifunctional catalyst originating from the interfacial assembly of a thin Mo,P‐codoped Co layer (≈50 nm) shelled Co nanowire (Co‐Mo‐P/CoNWs) network is fabricated via a facile approach. The catalyst exhibits low overpotentials of 0.08 and 0.27 V to reach a current response of 20 mA cm^?2 for the hydrogen evolution reaction and oxygen evolution reaction, respectively, together with long‐term stability in 1.0 m KOH medium. The outstanding performance is further demonstrated by a Co‐Mo‐P/CoNWs‐based electrolyzer, which enables a cell voltage of only 1.495 V to reach 10 mA cm^?2, superior to one derived from commercial (Pt/C + RuO₂/C) as well as to various reports recently published elsewhere. It is recognized that the formation of multiactive centers together with the increased active site number caused by Mo and P dual doping synergistically promote both hydrogen and oxygen evolution performance. Such a hybrid material opens a new approach for developing efficient and cost‐effective catalysts for water splitting application.     

Probing the Self‐Boosting Catalysis of LiCoO2 in Li–O2 Battery with Multiple In Situ/Operando Techniques

Designing high‐activity catalysts and revealing the in‐depth structure–property relationship is particularly important for Li–O₂ batteries. Herein, the self‐boosting catalysis of LiCoO₂ as an electrocatalyst for Li–O₂ batteries and the investigation of its self‐adjustment mechanism using in situ X‐ray absorption spectroscopy and other operando characterization techniques is reported. The intercalation/extraction of Li⁺ in LiCoO₂ not only induces the change in Co valence and modulates the electronic/crystal structure but also tunes the surface disorder degree, lattice strain, and local symmetry, which all affect the catalysis activity. In a discharge, highly ordered LiCoO₂ acts as a catalyst to boost oxygen reduction reaction. During charging, the initial extraction of Li⁺ from LiCoO₂ induces Li/oxygen vacancy and Co⁴⁺, which deforms CoO₆ octahedron as well as lowers the symmetry, and accordingly promotes oxygen evolution reaction. This article offers insights into tuning the activity of catalysts for Li–O₂ batteries with the intercalation/extraction of alkali metal ions in traditional cathodes.     

Earth‐Abundant Nanomaterials for Oxygen Reduction

Replacing the rare and precious platinum (Pt) electrocatalysts with earth‐abundant materials for promoting the oxygen reduction reaction (ORR) at the cathode of fuel cells is of great interest in developing high‐performance sustainable energy devices. However, the challenging issues associated with non‐Pt materials are still their low intrinsic catalytic activity, limited active sites, and the poor mass transport properties. Recent advances in material sciences and nanotechnology enable rational design of new earth‐abundant materials with optimized composition and fine nanostructure, providing new opportunities for enhancing ORR performance at the molecular level. This Review highlights recent breakthroughs in engineering nanocatalysts based on the earth‐abundant materials for boosting ORR.     

Pt monolayer electrocatalysts for O2 reduction: PdCo/C substrate-induced activity in alkaline media

We measured the activity of electrocatalysts, comprising Pt monolayers deposited on PdCo/C substrates with several Pd/Co atomic ratios, in the oxygen reduction reaction in alkaline solutions. The PdCo/C substrates have a core-shell structure wherein the Pd atoms are segregated at the particle’s surface. The electrochemical measurements were carried out using an ultrathin film rotating disk-ring electrode. Electrocatalytic activity for the O₂ reduction evaluated from the Tafel plots or mass activities was higher for Pt monolayers on PdCo/C compared to Pt/C for all atomic Pd/Co ratios we used. We ascribed the enhanced activity of these Pt monolayers to a lowering of the bond strength of oxygenated intermediates on Pt atoms facilitated by changes in the 5d-band reactivity of Pt. Density functional theory calculations also revealed a decline in the strength of PtOH adsorption due to electronic interaction between the Pt and Pd atoms. We demonstrated that very active O₂ reduction electrocatalysts can be devised containing only a monolayer Pt and a very small amount of Pd alloyed with Co in the substrate. Dedicated to Professor Oleg Petrii on the occasion of his 70th birthday on August 24, 2007.