Secondary battery inspired α-nickel hydroxide as an efficient Ni-based heterogeneous catalyst for sulfate radical activation

Redox-mediated decoupled seawater direct splitting for H2 production

Seawater direct electrolysis (SDE) using renewable energy provides a sustainable pathway to harness abundant oceanic hydrogen resources. However, the side-reaction of the chlorine electro-oxidation reaction (ClOR) severely decreased direct electrolysis efficiency of seawater and gradually corrodes the anode. In this study, a redox-mediated strategy is introduced to suppress the ClOR, and a decoupled seawater direct electrolysis (DSDE) system incorporating a separate O2 evolution reactor is established. Ferricyanide/ferrocyanide ([Fe(CN)6]3-/4-) serves as an electron-mediator between the cell and the reactor, thereby enabling a more dynamically favorable half-reaction to supplant the traditional oxygen evolution reaction (OER). This alteration involves a straightforward, single-electron-transfer anodic reaction without gas precipitation and effectively eliminates the generation of chlorine-containing byproducts. By operating at low voltages (~1.37 V at 10 mA cm-2 and ~1.57 V at 100 mA cm-2) and maintaining stability even in a Cl--saturated seawater electrolyte, this system has the potential of undergoing decoupled seawater electrolysis with zero chlorine emissions. Further improvements in the high-performance redox-mediators and catalysts can provide enhanced cost-effectiveness and sustainability of the DSDE system.

Surface Catalytic Repair for the Efficient Regeneration of Spent Layered Oxide Cathodes

Direct recycling is considered to be the next-generation recycling technology for spent lithium-ion batteries due to its potential economic benefits and environmental friendliness. For the spent layered oxide cathode materials, an irreversible phase transition to a rock-salt structure near the particle surface impedes the reintercalation of lithium ions, thereby hindering the lithium compensation process from fully restoring composition defects and repairing failed structures. We introduced a transition-metal hydroxide precursor, utilizing its surface catalytic activity produced during annealing to convert the rock-salt structure into a layered structure that provides fast migration pathways for lithium ions. The material repair and synthesis processes share the same heating program, enabling the spent cathode and added precursor to undergo a topological transformation to form the targeted layered oxide. This regenerated material exhibits a performance superior to that of commercial cathodes and maintains 88.4% of its initial capacity after 1000 cycles in a 1.3 Ah pouch cell. Techno-economic analysis highlights the environmental and economic advantages of surface catalytic repair over pyrometallurgical and hydrometallurgical methods, indicating its potential for practical application.

Controlling the amount of MoSe2 loaded SrTiO3 to activate peroxymonosulfate for efficient elimination of organic pollutants

It is critical to effectively eliminate recalcitrant organic pollutants from wastewater. In this paper, the MoSe2/SrTiO3 (MST) catalysts were synthesized through simply controlling the amount of MoSe2 in the hydrothermal method to activate peroxymonosulfate (PMS) for the degradation of pollutants. The results demonstrated that sulfamethoxazole and tetracycline were almost eliminated by PMS/MST-3 (MoSe2/SrTiO3 mass ratio 0.3: 1) activation system. The effect of inorganic anions (Cl -, H2PO4 -, HCO3 -) and metal ions (Cu2+, Ni2+, Zn2+) commonly found in actual water bodies on catalytic reaction was explored. Moreover, SO4• -, •OH and 1O2 were identified by EPR tests and scavenger experiments, where the SO4• - and •OH were the dominant reactive species. The XPS analysis indicated that the oxygen vacancies and charge transfer on the catalyst surface were the keys of PMS activation. The effect of active sites in SMX and TC on the catalytic degradation activity was explored by density functional theory, and it was obtained that the central nitrogen site of SMX was more vulnerable in the catalytic system, while the edge oxygen site of TC was more susceptible to attack.

Boosted micropollutant removal over urchin-like structured hydroxyapatite-incorporated nickel magnetite catalyst via peroxydisulfate activation

In this work, urchin-like structured hydroxyapatite-incorporated nickel magnetite (NiFe3O4/UHdA) microspheres were developed for the efficient removal of micropollutants (MPs) via peroxydisulfate (PDS) activation. The prepared NiFe3O4/UHdA degraded 99.0 % of sulfamethoxazole (SMX) after 15 min in 2 mM PDS, having a first-order kinetic rate constant of 0.210 min-1. In addition, NiFe3O4/UHdA outperformed its counterparts, i.e., Fe3O4/UHdA and Ni/UHdA, by giving rise to corresponding 3.6-fold and 8.6-fold enhancements in the SMX removal rate. The outstanding catalytic performance can be ascribed to (1) the urchin-like mesoporous structure with a large specific surface area and (2) the remarkable synergistic effect caused by the redox cycle of Ni3+/Ni2+ and Fe2+/Fe3+ that enhances multipath electron transfers on the surface of NiFe3O4/UHdA to produce more reactive oxygen species. Moreover, the effects of several reaction parameters, in this case the initial solution pH, PDS dosage, SMX concentration, catalyst loading, co-existing MPs and humic acid level on the catalytic performance of the NiFe3O4/UHdA + PDS system were systematically investigated and discussed in detail. The plausible catalytic mechanisms in the NiFe3O4/UHdA + PDS system were revealed via scavenging experiments and electron paramagnetic resonance analysis, which indicated a radical (•OH and SO4•-) as the major pathway and a nonradical (1O2) as the minor pathway for SMX degradation. Furthermore, NiFe3O4/UHdA exhibited fantastic magnetically separation and retained good catalytic activity with a low leached ion concentration during the performance of four cycles. Overall, the prepared NiFe3O4/UHdA with outstanding PDS activation could be a promising choice for the degradation of persistent organic pollutants from wastewater.

Different crystallographic Ni(OH)2 as highly efficient Fenton-like catalysts for sulfate radical activation

By a simple hydrothermal method, a phase boundary between α- and β-Ni(OH)₂ can be obtained. The Fenton-like performance of α@β-Ni(OH)₂ is 1.56 times higher than that of single β-Ni(OH). α@β-Ni(OH)₂ displays superior stability compared to α-Ni(OH)₂, β-Ni(OH)₂, and amorphous Ni(OH)₂, which makes significant contributions to developing advanced catalysts in diverse fields.

Layered double hydroxide based materials applied in persulfate based advanced oxidation processes: Property, mechanism, application and perspectives

Recently, persulfate-based advanced oxidation processes (persulfate-AOPs) are booming rapidly due to their promising potential in treating refractory contaminants. As a type of popular two-dimensional material, layered double hydroxides (LDHs) are widely used in energy conversion, medicine, environment remediation and other fields for the advantages of high specific surface area (SSA), good tunability, biocompatibility and facile fabrication. These excellent physicochemical characteristics may enable LDH-based materials to be promising catalysts in persulfate-AOPs. In this work, we make a summary of LDHs and their composites in persulfate-AOPs from different aspects. Firstly, we introduce different structure and important properties of LDH-based materials briefly. Secondly, various LDH-based materials are classified according to the type of foreign materials (metal or carbonaceous materials, mainly). Latterly, we discuss the mechanisms of persulfate activation (including radical pathway and nonradical pathway) by these catalysts in detail, which involve (i) bimetallic synergism for radical generation, (ii) the role of carbonaceous materials in radical generation, (iii) singlet oxygen (¹O₂) production and several special nonradical mechanisms. In addition, the catalytic performance of LDH-based catalysts for contaminants are also summarized. Finally, challenges and future prospects of LDH-based composites in environmental remediation are proposed. We expect this review could bring new insights for the development of LDH-based catalyst and exploration of reaction mechanism.

Heterogeneous activation of persulfate by Mg doped Ni(OH)2 for efficient degradation of phenol

Mg doped Ni(OH)₂ was synthesized and investigated as an efficient material to activate persulfate (PS) for phenol degradation. The property of the Ni(OH)₂ material was enhanced by Mg doping as the removal efficiency of phenol was increased from 74.82 % in Ni(OH)₂/PS system to 89.53 % in Mg-doped Ni(OH)₂/PS system within 20 min. Such a high removal efficiency revealed that doping Mg into Ni(OH)₂ brings about more defects (oxygen vacancies), which facilitated the formation of more active species in the degradation process. The removal efficiencies of phenol increased with the increase of the initial pH from 3 to 11. The influences of Cl^-, NO₃^- and HCO₃^- on the stability of the system were also studied and the results showed that removal rates of all systems in the presence of these different inorganic anions could reached about 90 % within 20 min. Based on the electron spin resonance (ESR) experiments, ¹O₂, O₂^·-, ·OH and SO₄^•- were identified as the active species in Mg-doped Ni(OH)₂/PS system for phenol degradation and a degradation mechanism was proposed for this system. In addition, the as-prepared material retained its activation performance even after 3 repeated cycles.

Cu-doped Ni-LDH with abundant oxygen vacancies for enhanced methyl 4-hydroxybenzoate degradation via peroxymonosulfate activation: key role of superoxide radicals

Oxygen vacancies (OVs) were introduced into Ni-based layered double hydroxides (LDHs) through Cu doping, and the catalytic performance of the resulting Ni_xCu-LDHs were investigated for peroxymonosulfate (PMS) activation and methyl 4-hydroxybenzoate (MeP) degradation. Compared with that of Ni-LDH, the catalytic performance of Ni_xCu-LDHs were significantly enhanced and increased with increasing OV content in the catalysts, indicating that Cu doping introduced OVs into Ni_xCu-LDHs and greatly improved their catalytic activity with PMS. Quenching experiments and EPR analyses confirmed that oxidation processes dominated by superoxide radicals (O₂^•-) and singlet oxygen (¹O₂), rather than sulfate radicals (SO₄^•-) or hydroxyl radicals (•OH) used by traditional LDH catalysts, were responsible for MeP degradation by Ni₁₅Cu-LDHs. In addition, quenching experiments with different systems showed the fate of reduced SO₄^•-and •OH, and demonstrated that O₂^•- and ¹O₂ concentrations grew with increasing OV content, confirming that the presence of OVs affected the process of PMS activation. Notably, O₂^•- mainly originated from adsorbed oxygen or dissolved oxygen (DO) by acquiring electrons from OVs in Ni₁₅Cu-LDHs, since OVs possess abundant localized electrons. Consequently, an OV-mediated oxidative mechanism was proposed for Ni₁₅Cu-LDHs/PMS. This study provides new clues for enhancing the catalytic performance of LDH catalysts by introducing OVs via metal doping in PMS-based AOPs systems.

π-π conjugation driving peroxymonosulfate activation for pollutant elimination over metal-free graphitized polyimide surface

A novel metal-free catalyst consisting of typical flower-like graphitized polyimide (g-PI) is first synthesized via an enhanced hydrothermal polymerization process, and it exhibits excellent performance for pollutant removal through peroxymonosulfate (PMS) activation over a wide pH range (3-11). The catalyst is especially effective for attacking the endocrine disruptor bisphenol A (BPA), which can be completely degraded in a short time. Based on the results of characterization, g-PI is consisted of abundant aromatic frameworks with π conjugates based on C-O-C linkages and N-hybrid rings, which play essential roles in the subsequent degradation of pollutants. In the g-PI/PMS/BPA system, BPA (rich in π bonds) is preferentially adsorbed to the catalyst surface through π-π interactions, accompanied by a decrease in its activation energy to produce surface-adsorbed BPA*. This species can be directly attacked and degraded by PMS without the need for the radical processes, which saves the energy required for the intermediate activation process of PMS. On the other hand, the electrons obtained from pollutants are rapidly transferred to the O center, driving PMS activation to generate free radicals. The synergetic interface process offers excellent potential for practical wastewater purification.

Transformation from a non-radical to a radical pathway via the amorphization of a Ni(OH)2 catalyst as a peroxymonosulfate activator for the ultrafast degradation of organic pollutants

The peroxymonosulfate (PMS) activation reaction using transition-metal-based catalysts has been proven to be a promising approach for the degradation of refractory organic contaminants; however, the ambiguous structure-property relationship between the intrinsic free-radical and non-radical mechanistic pathway selectivity and structural characteristics greatly hinders the development of active catalysts. Taking Ni(OH)2 as a model catalyst, this work reveals that the pathway selectivity during PMS activation can be controlled via the construction of crystalline and amorphous structures. Electron paramagnetic resonance and radical quenching experiments verified that amorphous Ni(OH)2 with disordered -OH, synthesized via a formamide-assisted precipitation method, dramatically promotes the generation of ˙OH and SO4˙- (the radical pathway), which highly improved the degradation efficiencies toward organic contaminants. However, crystalline Ni(OH)2 was found to activate PMS through via a non-radical pathway. Density functional theory calculations reveal that amorphous Ni(OH)2 possesses an electron-rich active surface, which favors the breaking of O-O bonds instead of O-H bonds in PMS molecules and triggers radical production. As confirmed via electrochemical measurements, the essence of PMS activation was uncovered; it was found that pathway selectivity was determined based on the electron-donating capabilities, which were highly dependent on the -OH group environments. Impressively, the catalytic mechanism of the same material can be successfully and precisely regulated from a non-radical to a radical pathway for PMS activation via a structural engineering method, which can simultaneously improve the catalytic performance for the effective elimination of emerging contaminants in aquatic environments.

Nickel-Nickel oxide nanocomposite as a magnetically separable persulfate activator for the nonradical oxidation of organic contaminants

The nanocomposite of metallic nickel and nickel oxide (denoted as Ni-NiO), synthesized by a simple sol-gel method, was found to activate peroxydisulfate (PDS), resulting in the effective oxidation of phenolic compounds and selected pharmaceuticals. A nonradical mechanism was proposed to explain the activation of PDS by Ni-NiO, in which organic contaminants are believed to be oxidized through an electron abstraction pathway mediated by the reactive complexes formed between PDS and the Ni-NiO surface. This mechanism was supported by multiple lines of evidence including radical scavenger experiments, the oxidation products, linear sweep voltammetry, and electron paramagnetic resonance spectroscopy. The Ni-NiO/PDS system exhibited a PDS utilization efficiency (expressed by the ratio of degraded organic contaminant to decomposed PDS) that was over 80%, and Ni-NiO showed a greater activity for PDS activation than a commercial nanoparticulate nickel oxide. This improved performance of Ni‒NiO was attributed to the disproportioned incorporation of the metallic Ni into the NiO matrix, creating more sites with oxygen vacancy. Also owing to the metallic Ni, Ni-NiO possessed magnetic properties and therefore could be easily separated and reused.

NiFe Layered Double Hydroxide (LDH) Nanosheet Catalysts with Fe as Electron Transfer Mediator for Enhanced Persulfate Activation

A highly efficient, durable, and cost-effective Fenton-like catalyst is desired to produce the sulfate radicals (^•SO₄^-) for energy and environmental applications. The M⁽ⁿ⁺¹⁾⁺/Mⁿ⁺ redox cycle in metal catalysts requires a high redox potential for ^•SO₄^- generation. NiFe layered double hydroxide (LDH) nanosheets with a suitable redox potential for persulfate (PDS) activation were prepared via incorporating Fe into the Ni based LDH. With the help of Fe, the charge-transfer kinetics for the reduction of Ni³⁺ to Ni²⁺ was improved and the formation of unwanted Ni component with higher oxidation state was suppressed. The incorporated Fe as the electron transfer mediator enhanced the process of Ni(OH)₂/NiOOH redox cycle. Therefore, NiFe LDH exhibited superior performance in PDS activation with exceptionally high activity for the phenolic compounds' degradation in neutral and basic conditions. This work is expected to inspire the rational design of LDHs based catalysts for PDS activation.

CuO-Co3O4@CeO2 as a heterogeneous catalyst for efficient degradation of 2,4-dichlorophenoxyacetic acid by peroxymonosulfate

CuO-Co₃O₄@CeO₂ nanoparticles used as a heterogeneous catalyst were prepared via a sol-gel method and characterized by various techniques. For comparison, a series of oxides was investigated for activating peroxymonosulfate (PMS) during the degradation of 2,4-dichlorophenoxyacetic acid (2,4-D). The results indicated that CuO-Co₃O₄@CeO₂ exhibited the highest catalytic performance among the catalysts. Complete degradation of 2,4-D (20 mg/L) was realized within 45 min at 1 mM PMS, CuO-Co₃O₄@CeO₂ loading of 0.07 g/L, and pH of 6. Recycling experiments confirmed that CuO-Co₃O₄@CeO₂ was very stable, and the 2,4-D degradation efficiencies ranged from 100% to 97.5%, decreasing by only 2.5% after the fifth run. The outstanding catalysis of CuO-Co₃O₄@CeO₂ resulted from the synergy of cerium, cobalt, and copper. Electron paramagnetic resonance and radical scavenger experiments confirmed the production of SO₄^{• -} and ^•OH radicals in the CuO-Co₃O₄@CeO₂/PMS system, which were responsible for efficient decomposition of 2,4-D. Furthermore, the combination of CuO-Co₃O₄@CeO₂ andPMS was applied to treat natural water containing 2,4-D, and a high 2,4-D removal rate was also achieved. Based on these results, it was deduced that CuO-Co₃O₄@CeO₂ can be utilized as a catalyst to activate PMS and destroy organic contaminants in aqueous solution.

Synergistically enhancing Fenton-like degradation of organics by in situ transformation from Fe3O4 microspheres to mesoporous Fe, N-dual doped carbon

Nanocarbon materials are emerging as alternative activators of peroxymonosulfate (PMS) for organics decomposition. However, the relatively low activity and complex syntheses hindered their practical application and innovation with respect to rational design of carbocatalysts is highly desired. Herein, an in situ replication and transformation strategy was employed to facilely convert porous Fe₃O₄ microspheres into novel Fe/N codoped large-pore mesoporous carbon spheres (M‑Fe/NC) as Fenton-like catalysts for PMS activation. Benefiting from the abundance of active sites induced by dual heteroatom doping, the enhanced active site exposure due to the unique mesoporous structure, and the high stability of carbon component, the derived M‑Fe/NC was superior to the pristine Fe₃O₄ for PMS activation to degrade various organics and was efficient over a wide pH range (2-9). Compared with the proposed mechanisms of previous reports, both radical (surface-bound SO₄^- and OH) and nonradical (¹O₂ and direct oxidation) pathways are involved in the M‑Fe/NC/PMS system. Furthermore, experimental observations in combination with DFT calculations reveal that graphitic N and FeN₄ sites serve as dual reaction centers in the catalysis. This research opened an avenue for development of novel multi-doped carbocatalysts used to activate PMS for sustainable remediation.