Valence States Of Co Research Articles

Synergistic interface engineering of ZnCo2O4/NiMnCo-layered double hydroxides for boosted reactivity in advanced supercapacitor electrodes

NiMn layered double hydroxide (LDH) exhibits high redox activity as a cathode material for hybrid supercapacitors, but it has the disadvantages of low conductivity and poor stability. In this work, the electrochemical properties of NiMnCo-LDH with different Co contents prepared by electrodeposition were tested. The results showed that the addition of an appropriate amount Co increased the adsorption of OH− on NiMn-LDH and improved the reactivity of the material. ZnCo-MOF grown on carbon cloth was transformed into flake ZnCo2O4 by calcination, and NiMnCo-LDH was electrodeposited on it to prepare ZnCo2O4/NiMnCo-LDH composite electrode. The core-shell structure increases the reaction area between the LDH material and the electrolyte. Physical characterization and density functional theory (DFT) calculations show that the structure and electronic distribution of ZnCo2O4/NiMnCo-LDH at the heterogeneous interface are reconstructed, which increases the valence state of Co and Mn elements, and finally exhibits enhanced electrochemical performance. Due to the existence of this synergistic effect, the specific capacity of the electrode is as high as 231.5 mAh g−1, with excellent stability demonstrated after 5000 cycles compared to the NiMnCo-LDH. The assembled hybrid supercapacitor exhibits a high energy density of 39.4 Wh kg−1, and has high cycle stability (90 % after 5000 cycles).

Highly sensitive 2D X-ray absorption spectroscopy via physics informed machine learning

Improving the spatial and spectral resolution of 2D X-ray near-edge absorption structure (XANES) has been a decade-long pursuit to probe local chemical reactions at the nanoscale. However, the poor signal-to-noise ratio in the measured images poses significant challenges in quantitative analysis, especially when the element of interest is at a low concentration. In this work, we developed a post-imaging processing method using deep neural network to reliably improve the signal-to-noise ratio in the XANES images. The proposed neural network model could be trained to adapt to new datasets by incorporating the physical features inherent in the latent space of the XANES images and self-supervised to detect new features in the images and achieve self-consistency. Two examples are presented in this work to illustrate the model’s robustness in determining the valence states of Ni and Co in the LiNixMnyCo1-x-yO2 systems with high confidence.

Performance improvement of oxygen electrode in reversible protonic ceramic electrochemical cell by co-doping of La and F

Reversible protonic ceramic electrochemical cell (R-PCEC) has demonstrated high potential as an energy storage and conversion device. Efforts have primarily concentrated on the development of oxygen electrodes being to increase their performance and reduce performance degradation. In this work, three perovskite materials, namely Ba0.9Co0.2Fe0.7Nb0.1O3-δ (BCFN), Ba0.8La0.1Co0.2Fe0.7Nb0.1O3-δ (BLCFN), and Ba0.8La0.1Co0.2Fe0.7Nb0.1F0.1O2.9-δ (BLCFNF) are investigated. Both doping with La and co-doping with La and F lead to a notable reduction in the average valence states of Fe, Co, and Nb elements, further resulting in an increase in oxygen vacancy concentration, thereby enhancing the oxygen migration ability of the material. The H2O partial pressure does not have a significant impact on the polarization resistance (Rp) of the oxygen electrode. Conversely, the rise in Rp is highly correlated with the decrease in oxygen partial pressure, suggesting that the primary factor influencing the reaction rate of the oxygen electrode process is the exchange of oxygen ions on the surface of the electrode. BLCFNF exhibits the best oxygen migration and surface exchange ability, resulting in the lowest Rp. When BLCFNF was used as the oxygen electrode in a R-PCEC with the humidity air (10%H2O) and humidified H2 (3%H2O) as oxidant and fuel, respectively, it exhibits the highest maximum power density of 472 mW cm−2, and a current density of −682.6 mA cm−2 (1.3 V) at 650 °C. In addition, BLCFNF also shows good durability in both discharge and electrolysis process. The results indicate that La and F co-doped BLCFNF material is a promising oxygen electrode material for R-PCECs.

Suppressing Structure Delamination for Enhanced Electrochemical Performance of Solid Oxide Cells.

Reversible solid oxide cells (rSOCs) have significant potential as efficient energy conversion and storage systems. Nevertheless, the practical application of their conventional air electrodes, such as La0.8Sr0.2MnO3-δ (LSM), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), and PrBa0.8Ca0.2Co2O5+δ (PBCC), remains unsatisfactory due to interface delamination during prolonged electrochemical operation. Using micro-focusing X-ray absorption spectroscopy (µ-XAS), a decrease (increase) in the co-valence state from the electrode surface to the electrode/electrolyte interface is observed, leading to the above delamination. Utilizing the one-pot method to incorporate an oxygen-vacancy-enriched CeO2 electrode into these air electrodes, the uniform distribution of the Co valence state is observed, alleviating the structural delamination. PBCC-CeO2 electrodes exhibited a degradation rate of 0.095mVh-1 at 650°C during a nearly 500-h test as compared with 0.907mVh-1 observed during the 135-h test for PBCC. Additionally, a remarkable increase in electrolysis current density from 636 to 934mAcm-2 under 1.3V and a maximum power density from 912 to 989mWcm-2 upon incorporating CeO2 into PBCC is also observed. BSCF-CeO2 and LSM-CeO2 also show enhanced electrochemical performance and prolonged stability as compared to BSCF and LSM. This work offers a strategy to mitigate the structural delamination of conventional electrodes to boost the performance of rSOCs.

Effect of fluoride (CoF2) based electrode material for high energy and power density asymmetric flexible supercapacitors

Herein, an asymmetric supercapacitor (ASC) device was assembled by employing the pristine CoF2 as active positive electrode material and activated carbon (AC) as negative electrode material. The CoF2 electrode exhibited an outstanding electrochemical behavior with enhanced specific capacitance (Cs) of 872 F/g at 1 A/g current density in three electrode configurations. The reversible Faradaic redox reactions in CoF2 instigate the formation of cobalt-oxyfluorides and generating the mixed valence state of Co (Co2+ and Co3+) which improves its electrical conductivity and electrochemical behaviors. Furthermore, the room temperature paramagnetic nature of CoF2 increases the gradient force of ions which could possibly increase the redox reactions at the electrolyte/electrode interface. The constructed asymmetric device structure of CoF2//AC revealed a quasi-rectangular shaped cyclic voltametric curve and symmetrical semi-triangular shaped galvanostatic charge-discharge profiles, respectively, which affirms the combined electric double layer capacitance and pseudocapacitive nature by its hybrid assembly. Hence, the constructed CoF2//AC ASC demonstrated a remarkable specific capacitance of 141 F/g, energy density of 44.06 Wh.Kg−1, and power density of 422.97 W.Kg−1 at 1 A/g with the high cell voltage of 1.5 V. The CoF2//AC ASC device retained c.a. 92 % of its initial capacitance and attained over 96 % of Coulombic efficiency after 2000 cycles of charge-discharge profiles at 1 A/g, which suggests the high electrochemical stability. This admirable electrochemical performance and the origin towards the magneto-capacitance of CoF2 could pave a new way to explore next generation high-performance supercapacitors.

Synthesis of nano-crystalline K2NiF4-type phases La1+xSr1-xCoO4 (x = 0.0, 0.2, and 0.5): Effect of mixed valence state of Co on structural, magnetic and photocatalytic properties

A lot of work has already been reported on bulk K2NiF4-type layered perovskite oxides but hardly is known about their nano-crystalline state as they are generally prepared at high temperatures. In this connection, we have synthesized nanocrystalline La1+xSr1-xCoO4 layered perovskites oxides via sol-gel Pechini method. Efforts were also done to synthesize phases with x > 0.5 but remained unsuccessful. The Rietveld refinement investigation has confirmed the successful formation of single-phase compounds that crystallize in the I4/mmm space group with tetragonal symmetry. The X-ray photoelectron spectroscopy (XPS) analysis revealed that Co in La1+xSr1-xCoO4 has mixed valence states of +2 and + 3. Dominant anti-ferromagnetic (AFM) super-exchange Co2+―O―Co2+ and Co3+―O―Co3+ interactions were observed in all the synthesized nanophases and the existence of weak ferromagnetic (FM) interactions due to the generation of double-exchange Co2+―O―Co3+ interaction because of mixed valent state of Co. The increase in Néel temperature (TN) values with La doping has been attributed by the increased prevalence of anti-ferromagnetic (AFM) super-exchange Co2+―O―Co2+ and Co3+―O―Co3+ interactions. The band gap energy values (Eg) for all the prepared nano samples vary from 1.57 to 1.64 eV, rendering them highly competent catalysts for degradation of dyes. Additionally, the photocatalytic performance of the prepared nano photocatalysts was evaluated using cationic (MB & RhB) and anionic (MO) dye contaminants as well as their ternary mixture. LaSrCoO4 photocatalyst has been found to efficiently and swiftly degrade all three dye pollutants separately as well as in mixtures, with outstanding recyclability and durability. It has been observed that the x = 0.0 photocatalyst showed best degradation efficacy for RhB dye, i.e., ∼99 % in 100 min, compared to ∼91 % and ∼84 % for the x = 0.2 and 0.5 photocatalysts in the same time interval, respectively. These results suggest that LaSrCoO4 nano powder can be considered a promising photocatalyst in the dyes degradation.

Construction of high performance fuel electrode in solid oxide electrolysis cells by tuning the surface oxygen defect to promote electrochemical reduction of CO2

B-site Co–Fe-based perovskite materials have shown potential as ceramic cathodes in solid oxide electrolysis cells (SOECs) for direct CO2 electrolysis. Nevertheless, their operational utilization is constrained by insufficient catalytic activity and durability in pure CO2. In this work, F− doping and A-site deficiency strategies are proposed and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF), F-doped La0.6Sr0.4Co0.2Fe0.8F0.1O2.9 (LSCFF) and A-site deficiency combined with F-doped (La0.6Sr0.4)0.95Co0.2Fe0.8F0.1O2.9 (LSCFF95) are prepared as cathode materials for CO2 electrolysis. After doping with F−, the valence states of both Co and Fe elements decrease, while generating more oxygen vacancies. With the further introduction of A-site deficiency, the valence states further decrease, resulting in more oxygen vacancies. The enhancement of oxygen vacancies leads to an improvement in the oxygen surface exchange coefficient (kchem) and the bulk diffusion coefficient (Dchem) of LSCFF and LSCFF95 materials. At 800 °C, kchem of LSCFF and LSCFF95 reach 24.21 × 10−4 and 33.54 × 10−4 cm s−1, respectively, which are 51% and 110% higher than the value of 16.03 × 10−4 cm s−1 for LSCF. Meanwhile, the Dchem values of LSCFF and LSCFF95 are measured to be 37.52 × 10−5 and 47.81 × 10−5 cm2 s−1, respectively. These values are 49% and 90% higher than the value of 25.22 × 10−5 cm2 s−1 for LSCF. Consequently, F− doping and A-site deficiency significantly increase the electrochemical performance. For example, F− doping decreases the polarization resistance (Rp) value of the SOEC with LSCF cathode from 0.19 Ω cm2 to 0.13 Ω cm2 at 800 °C. Furthermore, when the A-site deficiency is introduced, the Rp value is further reduced to 0.11 Ω cm2. Therefore, the SOEC with the LSCFF95 cathode exhibits excellent performance. Specifically, it has demonstrated an impressively high current density of 2.85 A cm−2 at 800 °C and 1.5 V, and has exhibited exceptional durability over prolonged operation.

Engineered oxidation states in NiCo2O4@CeO2 nanourchin architectures with abundant oxygen vacancies for enhanced oxygen evolution reaction performance

Sluggish oxygen evolution reaction (OER) as the half reaction of water electrolysis hindered the production of clean hydrogen, necessitates a cost-effective electrocatalyst. Spinel nickel cobaltite (NiCo2O4), characterized by its lower cost, flexibility in oxidation states of cations, and stable spinel structure, holds promise as an OER electrocatalyst. However, there is still room for improvement in terms of its intrinsic electrochemical activity, conductivity, and active surface area. Inspired by the flexible valence states of Ni and Co, in this study, CeO2 was introduced into NiCo2O4 for further electronic structure adjustment. The result was the synthesis of an urchin-like NiCo2O4@CeO2 (NCOC), and the elucidation of its formation mechanisms, expressed conclusively through reaction formulas. NCOC, characterized by an open architecture and a rough surface, ensures rapid mass transportation and charge diffusion. Within NCOC, numerous oxophilic Ce3+ sites with oxygen vacancies construct electron transport pathways, accelerating charge-transfer. This simultaneously induces an increase in the oxidation states and proportions of Ni3+ and Co3+ in NCOC, reinforcing the covalence of Ni–O and Co–O bonds, expediting the ad/desorption of intermediates during OER. The electronic structure variations of NCOC were further verified through density functional theory (DFT) analyses, revealing alterations in adsorption configurations of intermediates and a decrease in adsorption free energies for intermediates on NCOC during OER. In practical terms, NCOC demonstrated a low overpotential of 228 mV at a current density of 10 mA cm−2 under 1 M KOH, maintaining long-term electrochemical stability for 100 h. Overall, this study provides a universal approach for designing highly cost-efficient electrocatalysts for OER.

Switching from antiferromagnetism to ferromagnetism in nanocrystalline La1−xSr1+xCoO4 (x = 0.0, 0.2, 0.4 and 0.6) layered perovskite oxides: effects of mixed valence states of Co on their magnetic, optical and photocatalytic properties

Photocatalytic degradation of a mixture of dyes by La0.4Sr1.6CoO4 nanocatalyst.

Enhanced Degradation of Decabromodiphenyl Ether via Synergetic Assisted Mechanochemical Process with Lithium Cobalt Oxide and Iron

The removal of decabromodiphenyl ether (BDE 209), as a typical persistent organic pollutant (POP), is of worldwide concern. Mechanochemical (MC) processes are promising methods to degrade environmental pollutants, most of which use a single grinding reagent. The performance of MC processes with co-milling agents still needs to be further verified. In this study, an efficient MC treatment with combined utilization of lithium cobalt oxide (LiCoO2) and iron (Fe) as co-milling reagents for BDE 209 degradation was investigated. The synchronous action of LiCoO2 and Fe with a LiCoO2/Fe/Br molar ratio of 1.5:1.67:1 and a ball-to-powder ratio of 100:1 led to almost thorough-paced abatement and debromination of BDE 209 within 180 min using a ball milling rotation speed of 600 rpm. The reduction in particle sizes and the destruction of crystal structure in mixture powders with the increase in milling time induced the enhanced degradation of BDE 209, as characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The X-ray photoelectron spectroscopy (XPS) characterization showed that the valence state of Co was converted from Co(III) to Co(II), and Fe(0) was changed to Fe(III) when treated with an MC process. This indicated that the reductive debromination of BDE 209 by Fe and the following oxidative degradation of debrominated products by LiCoO2 were integrated in a concerted way. It proved the removal of BDE 209 via an MC treatment. The full breakage of C-Br and C-O bonds in BDE 209 was confirmed by Fourier transform-infrared spectrometry (FT-IR) spectra, and a possible abatement pathway was also proposed based on the identified intermediate products using gas chromatography–mass spectrometry (GC-MS). These obtained results indicated that a combination of LiCoO2 and Fe as co-milling reagents is promising in the MC treatment of toxic halogenated pollutants like BDE 209.

Optimized valence state of Co and Ni in high-entropy alloy for high active-stable OER

Optimized valence state of Co and Ni in high-entropy alloy for high active-stable OER

Constructing flexible composite electrodes of low crystalline cobalt (Oxy)hydroxides nanosheet grown on carbon fiber cloth via self-reconstruction for electrochemical nitrate-to-ammonia conversion

Electrochemical conversion of NO3−-to NH3 via the nitrate reduction reaction (NO3−RR) is a promising approach for ammonia synthesis and “green hydrogen” storage. Constructing self-supporting and flexible composite electrodes is critical for portable devices of electrochemical conversion of NO3−-to NH3. In this study, carbon fiber cloth is used for a three-dimensional flexible electronic collector, and low-crystalline CoOOH nanosheets (LC-CoOOH NSs) grown on carbon fiber cloth were synthesized by potential induced self-reconstruction of cobalt Prussian blue analogues. The as-prepared self-supporting composite electrode exhibited improved NH3 yield rate. Further investigation revealed that the higher valence state of Co and the lower crystallinity are beneficial for NO3−-to-NH3 conversion. This work shows an example of efficient flexible composite electrode for NO3−-to-NH3 conversion.

NiSe2-CoSe2 with a Hybrid Nanorods and Nanoparticles Structure for Efficient Oxygen Evolution Reaction

Hetero-structure induced high performance catalyst for oxygen evolution reaction (OER) in the water splitting reaction has received increased attention. Herein, we demonstrated a novel catalyst system of NiSe2-CoSe2 consisting of nanorods and nanoparticles for the efficient OER in the alkaline electrolyte. This catalyst system can be easily fabricated via a low-temperature selenization of the solvothermal synthesized NiCo(OH)x precursor and the unique morphology of hybrid nanorods and nanoparticles was found by the electron microscopy analysis. The high valence state of the metal species was indicated by X-ray photoelectron spectroscopy study and a strong electronic effect was found in the NiSe2-CoSe2 catalyst system compared to their counterparts. As a result, NiSe2-CoSe2 exhibited high catalytic performance with a low overpotential of 250 mV to reach 10 mA·cm-2 for OER in the alkaline solution. Furthermore, high catalytic stability and catalytic kinetics were also observed. The superior performance can be attributed to the high valence states of Ni and Co and their strong synergetic coupling effect between the nanorods and nanoparticles, which could accelerate the charge transfer and offer abundant electrocatalytic active sites. The current work offers an efficient hetero-structure catalyst system for OER, and the results are helpful for the catalysis understanding.

Exploration and Application of the Self-Optimization Phenomenon of a Trimetal-Based MOF Electrocatalyst in the Oxygen Evolution Reaction

Metal–organic frameworks (MOFs) have emerged as a promising class of catalysts due to their large specific surface area, high porosity, and dispersed metal sites. However, the inherent low electrical conductivity of organic ligands limits their application as electrocatalysts. Herein, we constructed a trimetal FeCoNi MOF/NF catalyst for oxygen evolution reaction (OER) on nickel foam substrates by electrodeposition and hydrothermal methods. The FeCoNi MOF/NF catalyst shows a unique self-optimization phenomenon, which originates from the increase in the pore structure on the topography, the increase in the valence state of Co and Ni ions, and the formation of crystalline/amorphous interfaces. The self-optimizing stabilized FeCoNi MOF/NF has excellent electrocatalytic performance, which achieves a low overpotential of 267 mV at the current density of 10 mA cm–2 in 1 M KOH with a small Tafel slope of 37.6 mV dec–1. The self-optimization transformation process proposed in this work provides a new insight for the understanding and application of the active center of MOFs during the OER catalytic process.

Dielectric and anti-reduction properties of BaTiO3-based ceramics for MLCC application

BaTiO3 ceramics doped with different contents of Co2O3(abbreviated as (1-x) BT-xCo2O3) and (1-x) BaTiO3-xBiCoO3(abbreviated as (1-x) BT-xBC) ceramics were prepared via conventional solid-state process and sintered in reducing atmosphere. The main crystal phase of both ceramic samples was characterized by X-ray diffraction (XRD), which was perovskite structure. Compared with (1-x) BT-xCo2O3, (1-x) BT-xBC ceramic samples were dense with uniform microstructure confirmed by scanning electron microscopy (SEM). The 0.95BT-0.05BC ceramics had good dielectric properties, with ε25°C = 2670, tanδ = 0.012, and resistivity was 1.71 × 1011 Ω cm at 25°C, meeting the X7S standard for multilayer ceramic capacitor (MLCC) applications. The mechanism of improved anti-reduction property of (1-x) BT-xBC ceramics was studied by X-ray photoelectron spectroscopy (XPS) and thermally stimulated depolarization current (TSDC) techniques. The XPS results showed that there were two valence states of Co in 0.95BT-0.05BC and 0.95BT-0.05Co2O3 ceramics, indicating that the introduction of Co can improve anti-reduction property of BT-based ceramics. The TSDC revealed the formation of defect dipoles [2CoTi’−Vo▪▪], which was considered to be the cause for the improvement in anti-reduction property.

Fe-doped Co3O4 anchored on hollow carbon nanocages for efficient electrocatalytic oxygen evolution

In this work, a Fe-doped Co3O4 OER electrocatalyst supported by an N-doped hollow nanocage carbon framework (Fe-Co3O4/NC) was successfully prepared by anion exchange and annealing in an air atmosphere strategy. XRD and HRTEM characterizations confirm that Fe the incorporation of Fe into the lattice of Co3O4. XPS characterization clarifies that the valence state of Co increases after the introduction of Fe, which originates from the electrons transfer from Co2+/Co3+ to Fe3+ and is induced by the valence electron configuration of cations. It simulates Co sites in-situ derived into CoOOH active species during the OER process, which is confirmed by the HRTEM and XPS characterization after the OER stability test. Electrochemical performance tests show that the Fe-Co3O4/NC electrocatalyst only exhibits 275 mV overpotential to achieve a current density of 10 mA/cm2 and stably maintains for 20 h at 100 mA/cm2. Together with 20% Pt/C electrocatalyst, the composed two-electrode system only needs 2.041 V applied potential to achieve 100 mA/cm2 for total water splitting in a self-made membrane electrode device, which has industrial application prospects.

Reducible Co3+–O Sites of Co–Ni–P–Ox on CeO2 Nanorods Boost Acidic Water Oxidation via Interfacial Charge Transfer-Promoted Surface Reconstruction

Developing efficient and durable earth-abundant electrocatalysts for the acidic oxygen evolution reaction (OER) is the bottleneck for water splitting using proton-exchange membrane electrolyzers. Herein, a heterostructured CeO2 nanorod-supported Co–Ni–P oxide (CeO2/Co-Ni–P–Ox) catalyst is prepared for acidic OER electrocatalysis and the valence states of Co is precisely tuned from 2 to 2.51 by introducing heterojunction interfaces and trace P atoms. The increased Co states favor the in situ transformation of surface Co2+–O sites into highly active reducible Co3+–O sites, which promotes the deprotonation of water molecules and accelerates the OER kinetics. Therefore, this catalyst exhibits extraordinarily low OER overpotentials of 166 and 262 mV at 5 and 10 mA cm–2, respectively, in 0.5 M H2SO4, which are among the best reported for precious-metal-free electrocatalysts so far. The stability of the catalyst is also greatly improved due to the increased vacancy formation energy of the Co site that restricts its dissolution in an acid.

Fast Degradation of Rhodamine B by In Situ H2O2 Fenton System with Co and N Co-Doped Carbon Nanotubes.

In this study, an E-fenton oxidation system based on Co-N co-doped carbon nanotubes (Co-N-CNTs) was designed. The Co-N-CNTs system showed fast degradation efficiency and reusability for the degradation of rhodamine B (RhB). The XRD and SEM results showed that the Co-N co-doped carbon nanotubes with diameters ranging from 40 to 400 nm were successfully prepared. The E-Fenton degradation performance of Co-N-CNTs was investigated via CV, LSV and AC impedance spectroscopy. The yield of H2O2 could reach 80 mg/L/h within 60 min, and the optimal voltage and preparation temperature for H2O2 yield in this system was -0.7 V (vs. SCE) and 800 °C. For the target pollutant of RhB, the fast removal of RhB was obtained via the Co-N-CNTS/E-Fenton system (about 91% RhB degradation occurred during 60 min), and the •OH played a major role in the RhB degradation. When the Fe2+ concentrations increased from 0.3 to 0.4 mM, the RhB degradation efficiency decreased from 91% to about 87%. The valence state of Co in the Co-N-C catalyst drove a Co2+/Co3+ cycle, which ensured the catalyst had good E-Fenton degradation efficiency. This work provides new insight into the mechanism of an E-Fenton system with carbon-based catalysts for the efficient degradation of RhB.

Doping of Cr to Regulate the Valence State of Cu and Co Contributes to Efficient Water Splitting.

Water electrolysis in alkaline media is the most promising technology for hydrogen production, but efficient electrocatalysts are required to reduce the overpotential in HER and OER processes. In this work, the multicomponent transition metal catalyst Cr-Cu/CoOx was loaded on copper foam by electrodeposition and annealing, and the catalyst exhibited excellent electrochemical activity. The HER overpotential is 21 mV and the OER overpotential is 252 mV at a current density of 10 mA cm-2. The overall water splitting voltage is 1.51 V, even better than the Pt/C//RuO2 two-electrode system (1.61 V). The excellent performance of this catalyst is mainly derived from the close synergistic interaction among Cu, Co, and Cr. The doping of Cr modulates the valence states of Cu and Co at the active sites and improves the adsorption of various reaction intermediates. Density functional theory (DFT) calculations show that the doping of Cr can optimize the adsorption of the reaction intermediate H*. Meanwhile, the high-valent Cr and Co promote hydrolysis through strong adsorption with OH-. The present work provides a reasonable strategy for designing low-cost transition metals as efficient catalysts for water electrolysis.

Electronegativity-Induced Valence State Augmentation of Ni and Co through Electronic Redistribution between Co-Ni3N/CeF3 Interfaces for Oxygen Evolution Reaction

The development of high-performance and inexpensive electrocatalysts for the oxygen evolution reaction is one of the most desired objectives in energy conversion reactions. Due to sluggish reaction kinetics, the commercialization of such processes could not be achieved yet. Choosing an appropriate coupling interface to boost the catalyst performance is essential for the creation of high-efficiency electrocatalysts. Herein, metal-nitride and metal-fluoride heterostructures are reported with an extremely low overpotential of 180 mV at a current density of 10 mA cm–2 for OER application. The Co–Ni3N/CeF3 catalyst shows strong interfacial interaction, causing significant electronic redistribution between Co–Ni3N and CeF3 phases. X-ray photoelectron spectroscopy studies reveal the charge transfer from the Co–Ni3N to the CeF3 phase, resulting in the augmentation in the valence states of Co and Ni and making them highly active sites for the adsorption of intermediates (O*, OH*, and HOO*). This phenomenon is possibly driven by the high polarity of CeF3 due to the presence of highly electronegative F atoms. The stability study of the catalyst was performed for 120 h at large current densities of 100 and 200 mA cm–2. The detailed analysis of the surface reconstruction of Co–Ni3N/CeF3 is also carried out after a long-term stability test. This work offers a fresh look at the possibilities of the design and fabrication of an efficient and low-cost catalyst.