Hierarchically Structured FeNiOx Hy Electrocatalyst Formed by In Situ Transformation of Metal Phosphate for Efficient Oxygen Evolution Reaction

Iodine-Mediated Redox Strategy for Sustainable Lithium Extraction From Spent LiFePO4 Cathodes

With the widespread application of lithium-ion batteries, the recycling of spent batteries, especially those involving LiFePO4 (LFP) cathodes for their low-cost and high safety, has become an urgent environmental and resource challenge. Traditional recycling methods (hydrometallurgy and pyrometallurgy) struggle to achieve green and efficient recycling. Herein, this study proposes an iodine-mediated electrochemical strategy to utilize a recyclable I3 -/I- redox system and efficiently extract Li+ from spent LFP through liquid-phase reactions on one side (achieving a 93% leaching rate and recovery as lithium carbonate), while simultaneously producing metallic zinc through electrodeposition, which can be directly used in Zn-air batteries or hydrogen production. Furthermore, the delithiated LFP is upcycled into an oxygen evolution reaction (OER) catalyst, achieving an overpotential of only 250 mV at 10 mA cm-2, superior to commercial RuO2 catalysts. Eventually, this system reduces energy consumption by 32% (9.2 MJ kg-1) compared to traditional hydrometallurgical processes, decreases greenhouse gas emissions by 35% compared to traditional pyrometallurgical processes, while achieving a net profit of ≈$0.44 per kg. This work establishes a novel, scalable recycling system, providing a robust sustainable solution for spent LFP cathodes recycling and clean energy storage.

A Chalcogenide-Derived NiFe2O4 as Highly Efficient and Stable Anode for Anion Exchange Membrane Water Electrolysis

Developing low-cost, highly active, and durable oxygen evolution reaction (OER) electrodes is one of the critical scientific issues for anion exchange membrane water electrolyzer (AEM-WE). Herein, we report a vacancy-rich and alkali-stable NiFe2O4-type electrode (named as NiFeOx-350-Ov), derived from the chemical-vapor deposited precursor NiFeSexSy-350, as an efficient and robust anode material. The obtained electrode affords current densities of 100 and 500 mA cm-2 at overpotentials of 245 and 270 mV, respectively, and displays excellent long-term durability sustaining 1.0 A cm-2 at least for 1000 h. When coupled with Ni4Mo/MoO2/NF as a hydrogen evolution reaction (HER) catalyst, the resulting platinum-group metal (PGM)-free single-cell AEM-WE exhibits a cell voltage of 1.71 V at the current density of 1000 mA cm-2 at 80 °C and long-term durability during a current-cycling test between 0.5 A cm-2 and 1.0 A cm-2 over 150 h at 60 °C. This work highlights a unique reconstruction strategy for preparing highly active and durable OER catalysts used in PGM-free AEM-WE.

Shining Light on Anion-Mixed Nanocatalysts for Efficient Water Electrolysis: Fundamentals, Progress, and Perspectives

This review introduces recent advances of various anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, (oxy)hydroxides, and borides) for efficient water electrolysis applications in detail. The challenges and future perspectives are proposed and analyzed for the anion-mixed water dissociation catalysts, including polyanion-mixed and metal-free catalyst, progressive synthesis strategies, advanced in situ characterizations, and atomic level structure-activity relationship. Hydrogen with high energy density and zero carbon emission is widely acknowledged as the most promising candidate toward world's carbon neutrality and future sustainable eco-society. Water-splitting is a constructive technology for unpolluted and high-purity H₂ production, and a series of non-precious electrocatalysts have been developed over the past decade. To further improve the catalytic activities, metal doping is always adopted to modulate the 3d-electronic configuration and electron-donating/accepting (e-DA) properties, while for anion doping, the electronegativity variations among different non-metal elements would also bring some potential in the modulations of e-DA and metal valence for tuning the performances. In this review, we summarize the recent developments of the many different anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, oxyhydroxides, and borides/borates) for efficient water electrolysis applications. First, we have introduced the general information of water-splitting and the description of anion-mixed electrocatalysts and highlighted their complementary functions of mixed anions. Furthermore, some latest advances of anion-mixed compounds are also categorized for hydrogen and oxygen evolution electrocatalysis. The rationales behind their enhanced electrochemical performances are discussed. Last but not least, the challenges and future perspectives are briefly proposed for the anion-mixed water dissociation catalysts.

Engineering single-atomic ruthenium catalytic sites on defective nickel-iron layered double hydroxide for overall water splitting

Rational design of single atom catalyst is critical for efficient sustainable energy conversion. However, the atomic-level control of active sites is essential for electrocatalytic materials in alkaline electrolyte. Moreover, well-defined surface structures lead to in-depth understanding of catalytic mechanisms. Herein, we report a single-atomic-site ruthenium stabilized on defective nickel-iron layered double hydroxide nanosheets (Ru₁/D-NiFe LDH). Under precise regulation of local coordination environments of catalytically active sites and the existence of the defects, Ru₁/D-NiFe LDH delivers an ultralow overpotential of 18 mV at 10 mA cm⁻² for hydrogen evolution reaction, surpassing the commercial Pt/C catalyst. Density functional theory calculations reveal that Ru₁/D-NiFe LDH optimizes the adsorption energies of intermediates for hydrogen evolution reaction and promotes the O–O coupling at a Ru–O active site for oxygen evolution reaction. The Ru₁/D-NiFe LDH as an ideal model reveals superior water splitting performance with potential for the development of promising water-alkali electrocatalysts.

Rational design of single atom catalyst is critical for efficient sustainable energy conversion. Single-atomic-site ruthenium stabilized on defective nickel-iron layered double hydroxide nanosheets achieve superior HER and OER performance in alkaline media.

Recent advances of noble-metal-free bifunctional oxygen reduction and evolution electrocatalysts

Bifunctional oxygen reduction and evolution constitute the core processes for sustainable energy storage. The advances on noble-metal-free bifunctional oxygen electrocatalysts are reviewed.

Integrating hydrogen production with anodic selective oxidation of sulfides over a CoFe layered double hydroxide electrode

Replacing the sluggish oxygen evolution reaction (OER) with oxidation reactions for the synthesis of complex pharmaceutical molecules coupled with enhanced hydrogen evolution reaction (HER) is highly attractive, but it is rarely explored. Here, we report an electrochemical protocol for selective oxidation of sulfides to sulfoxides over a CoFe layered double hydroxide (CoFe-LDH) anode in an aqueous-MeCN electrolyte, coupled with 2-fold promoted cathodic H2 productivity. This protocol displays high activity (85-96% yields), catalyst stability (10 cycles), and generality (12 examples) in selective sulfide oxidation. We demonstrate its applicability in the synthesis of four important pharmaceutical related sulfoxide compounds with scalability (up to 1.79 g). X-ray spectroscopy investigations reveal that the CoFe-LDH material evolved into amorphous CoFe-oxyhydroxide under catalytic conditions. This work may pave the way towards sustainable organic synthesis of valuable pharmaceuticals coupled with H2 production.

Urchin-Like Cobalt-Copper (Hydr)oxides as an Efficient Water Oxidation Electrocatalyst

The development of efficient and low-cost oxygen evolution reaction (OER) catalysts is essential for the generation of clean hydrogen energy from water splitting. Herein, a novel hierarchical urchin-like cobalt-copper (hydr)oxide in situ grown on copper foam (CoCuO_x H_y (S)/CF) was synthesized through the electrochemical transformation of cobalt-copper sulfides (Co₉ S₈ -Cu_1.81 S) via anodization process. This CoCuO_x H_y (S)/CF anode exhibited a low overpotential (η) of 274 mV at a current density of 100 mA cm^-2 with a robust durability over a period of 40 h when operated at 10 mA cm^-2 . Further investigations imply that the unique nanowires aggregated urchin-like structure of CoCuO_x H_y (S) derived from the in situ anion exchange process could facilitate the exposure of active sites and accelerate electron transfer. More importantly, the incorporation of copper resulted in an electronic delocalization around the cobalt species, which contributed to reach a high-valent catalytically active cobalt species and further improved the OER performance.

Na4Ni3P4O15-Ni(OH)2 core-shell nanoparticles as hybrid electrocatalysts for the oxygen evolution reaction in alkaline electrolytes

There is wide interest in developing efficient, robust and low-cost electrode materials for the electrolysis of water to produce clean hydrogen fuel. It is especially important to improve the performance and durability of electrocatalysts for the OER. Here we have shown that the transformation of nanoparticle (n-NNP) and crystalline (c-NNP) forms of mixed phosphate Na4Ni3(PO4)2P2O7 in highly alkaline solutions occurs along various routes and provokes the generation of 2D Ni(OH)2 nanosheets or stable core(phosphate)-shell(Ni(OH)2) particles, respectively. In both cases, in the carbon matrix (through chemical and electrochemical conversion of phosphate in situ during electrolysis in a 6 M KOH or NaOH solution) stable OER electrocatalysts with low overpotentials of 250-290 mV at a current density of 10 mA cm-2 were obtained. The best candidate for the OER process is core-shell particles, which maintain overpotentials of around 250 mV in 6 M KOH for more than 3 days. The activity enhancement can be attributed to the formation of abundant NiOOH nanoparticles on the shell surface due to improved lattice matching. This report discusses future prospects for the creation of core-shell particles to reduce the overpotential of durable electrocatalysts for the OER.

Oxygen vacancy-rich amorphous porous NiFe(OH)x derived from Ni(OH)x/Prussian blue as highly efficient oxygen evolution electrocatalysts

Oxygen vacancies or defects play a significant role in improving the intrinsic activities of bimetallic hydroxides towards the oxygen evolution reaction (OER); however, their rational design and preparation remain a great challenge. In this study, oxygen vacancy-rich amorphous porous nickel iron hydroxide nanolayers supported on carbon paper (NiFe(OH)_x/CP) are rationally prepared through a facile approach involving the sequential electrochemical deposition of a Prussian blue (PB) nanocrystal layer and Ni(OH)_x layer on carbon paper followed by an alkaline etching process, where PB nanocrystals act as an Fe source and template for the formation of an amorphous porous NiFe(OH)_x layer. NiFe(OH)_x/CP with an ultralow loading of 0.8 mg cm^-2 exhibits outstanding OER activities, showing a low overpotential of 303 mV at 100 mA cm^-2 and a small Tafel slope of 33.8 mV dec^-1 in an alkaline electrolyte, which are superior to the state-of-the-art IrO₂ catalysts, and among the best results compared to the reported bimetallic compounds. Moreover, NiFe(OH)_x/CP exhibits excellent long-term stability with negligible degradation after water splitting for 50 h. Its superior electrocatalytic OER performance benefits from the massive oxygen vacancies derived from the amorphous and distorted structures, the synergistic effect between Ni and Fe species with an optimized Ni/Fe ratio, and the efficient electron and mass transfer of carbon paper. This work paves a new avenue for the rational design and preparation of amorphous porous structures with abundant oxygen vacancies to improve the intrinsic activities for energy storage and conversion applications.

Engineering Active Fe Sites on Nickel-Iron Layered Double Hydroxide through Component Segregation for Oxygen Evolution Reaction

Nickel-iron layered double hydroxide (NiFe LDH) is a promising oxygen evolution reaction (OER) electrocatalyst under alkaline conditions. Much research has been performed to understand the structure-activity relationship of NiFe LDH under OER conditions. However, the specific role of the Fe species remains unclear and under debate. Herein, based on DFT calculations, it was discovered that the edge Fe sites show higher activity towards OER than either the edge Ni sites or lattice sites. Therefore, a facile acid-etching method was proposed to controllably induce the formation of edge Fe sites in NiFe LDH, and the obtained sample exhibited higher OER activity. X-ray absorption near edge structure and extended X-ray absorption fine structure analyses further revealed that the interaction of the edge Fe species with Ni is believed to contribute to the enhancement of the OER performance. This work provides a new understanding of the structure-activity relationship in NiFe LDH and offers a facile method for the design of efficient electrocatalysts in an alkaline environment.

NiFe2O4 hollow nanoparticles of small sizes on carbon nanotubes for oxygen evolution

CNT-supported Ni–Fe bimetallic oxide hollow nanoparticles with an ultra-small size based on Kirkendall effect are fabricated and this catalyst exhibits excellent OER performances and robust stability, superior to the benchmark IrO₂ catalyst.

Elemental selenium enables enhanced water oxidation electrocatalysis of NiFe layered double hydroxides

The oxygen evolution reaction (OER) is involved in various renewable energy systems, such as water-splitting, metal-air batteries and CO₂ electroreduction. Ni-Fe layered double hydroxides (LDHs) have been reported as promising OER electrocatalysts in alkaline electrolytes. Herein, we demonstrate that the introduction of elemental selenium (Se) with an optimized phase composition, i.e., monoclinic (m-) or trigonal (t-) Se, could effectively tailor the OER activity of NiFe-LDH. Compared to t-Se doped NiFe-LDH, the presence of hybrid m/t-Se could effectively tune the electronic states of Ni-O and Fe-O sites, promote the generation of OER-active γ-NiOOH, and inhibit Fe-migration during the OER process, thus enhancing the OER performance. The optimized Ni_0.8Fe_0.2-m/t-Se_0.02-LDH catalyst exhibits extraordinarily high OER activity, with an overpotential of 200 mV at 10 mA cm^-2, which is superior to those of IrO₂ and most of the reported Se-based OER catalysts. The Ni_0.8Fe_0.2-m/t-Se_0.02-LDH catalyst is further implemented as an anode for overall water splitting and demonstrates a low cell voltage of 1.50 V to achieve 10 mA cm^-2.

Free-Sustaining Three-Dimensional S235 Steel-Based Porous Electrocatalyst for Highly Efficient and Durable Oxygen Evolution

A novel oxygen evolution reaction (OER) catalyst (3 D S235-P steel) based on a steel S235 substrate was successfully prepared by facile one-step surface modification. The standard carbon-manganese steel was phosphorized superficially, which led to the formation of a unique 3 D interconnected nanoporous surface with a high specific area that facilitated the electrocatalytically initiated oxygen evolution reaction. The prepared 3 D S235-P steel exhibited enhanced electrocatalytic OER activities in the alkaline regime, as confirmed by a low overpotential (326 mV at a 10 mA cm^-2 ) and a small Tafel slope of 68.7 mV dec^-1 . Moreover, the catalyst was found to be stable under long-term usage conditions, functioning as an oxygen-evolving electrode at pH 13, as evidenced by the sufficient charge-to-oxygen conversion rate (faradaic efficiency: 82.11 and 88.34 % at 10 and 5 mA cm^-2 , respectively). In addition, it turned out that the chosen surface modification delivered steel S235 as an OER electrocatalyst that was stable under neutral pH conditions. Our investigation revealed that the high catalytic activities likely stemmed from the generated Fe/(Mn) hydroxide/oxohydroxides generated during the OER process. Phosphorization treatment therefore not only is an efficient way to optimize the electrocatalytic performance of standard carbon-manganese steel but also enables for the development of low-costing and abundant steels in the field of energy conversion.