Transition Metal Doping Research Articles

Unraveling Hydrogen Adsorption on Transition Metal-Doped [Mo3S13]2- Clusters: Insights from Density Functional Theory Calculations.

Molecular and dissociative hydrogen adsorption of transition metal (TM)-doped [Mo3S13]2- atomic clusters were investigated using density functional theory calculations. The introduced TM dopants form stable bonds with S atoms, preserving the geometric structure. The S-TM-S bridging bond emerges as the most stable configuration. The preferred adsorption sites were found to be influenced by various factors, such as the relative electronegativity, coordination number, and charge of the TM atom. Notably, the presence of these TM atoms remarkably improved the hydrogen adsorption activity. The dissociation of a single hydrogen molecule on TM[Mo3S13]2- clusters (TM = Sc, Cr, Mn, Fe, Co, and Ni) is thermodynamically and kinetically favorable compared to their bare counterparts. The extent of favorability monotonically depends on the TM impurity, with a maximum activation barrier energy ranging from 0.62 to 1.58 eV, lower than that of the bare cluster (1.69 eV). Findings provide insights for experimental research on hydrogen adsorption using TM-doped molybdenum sulfide nanoclusters, with potential applications in the field of hydrogen energy.

Entropy regulation in spinel oxide with narrowed band gap and lattice distortion toward high-temperature infrared radiation

Typical infrared (IR) radiation materials, such as spinel structure with transition metal doping, remain challenges on IR radiation performance and thermal stability. Inspired by manipulatable entropy with desired properties, (Mg1/5Co1/5Ni1/5Cu1/5Zn1/5)Al2O4 (5MAl2O4) with improved infrared radiation property and reduced thermal conductivity has been successfully synthesized. High-entropy strategy significantly narrows the band gap by impurity energy level formation, resulting in an improved 0.91 (0.78–2.5 μm). Simultaneously, 5MAl2O4 emerges site inversion (tetrahedral and octahedral sites) and lattice distortion which benefits the outstanding infrared emissivity of 0.94 (2.5–15 μm, measured at 800 °C). Furthermore, 5MAl2O4 reaches fairly low thermal conductivity of 2.1 W·m−1·K−1, 4–5 times decrease compared with MAl2O4 end members. The remarkable infrared emissivity and low thermal conductivity offer great potential in energy conservation and heat dissipation.

Origin of hydrogen adsorption free energy variation in electrochemical hydrogen evolution in transition-metal dichalcogenides

Two-dimensional transition-metal (TM)-doped transition metal dichalcogenides (TMDs) are among the most promising materials for water-splitting catalysts. Among various methods applied to promote the hydrogen evolution reaction (HER) in TMDs, doping with TM heteroatoms has attracted attention because this strategy allows optimization of hydrogen adsorption and H2 generation reactions. Herein, we conduct in-depth, systematic analyses of the trends in the change in the hydrogen adsorption free energy (ΔGH∗)—the most well-known descriptor for evaluating HER performance—for doped TMDs. The total of 150 different doped TMDs are used to conduct an atomic-level analysis of the origin of ΔGH∗ changes upon TM heteroatom doping. Moreover, we suggest two key factors that govern hydrogen adsorption over doped TMDs: 1) changes in the charge of chalcogen atoms, where hydrogen atoms are adsorbed onto early-TM-doped structures and 2) structural deformation energies accompanying the introduced dopants of the late-TM-doped structures. Furthermore, we propose a new perspective on how vacancies in TM-doped TMDs can result in the enhanced ΔGH∗ for the HER. We suggest that introducing electrostatic and structural controls in early- and late-TM-doped systems, respectively, are effective strategies for achieving thermoneutral ΔGH∗ in TMDs and promote the design of new TMD catalysts with superior water-splitting capabilities.

The regulation mechanism of transition metal doping on Hg0 adsorption and oxidation on Ce/TiO2(001) surface: A DFT study

The mercury oxidation performance of Ce/TiO2 catalyst can be further enhanced by transition metal modifications. This study employed density functional theory (DFT) calculations to investigate the adsorption and oxidation mechanisms of Hg0 on Ce/TiO2(001) and its transition metal modified surfaces. According to the calculation results, Ru-, Mo-, Nb-, and Mn-doping increased the affinity of the Ce/TiO2(001) surface towards Hg0 and HCl, thereby facilitating the efficient capture and oxidation of Hg0. The increased adsorption energy (Eads) of the intermediate HgCl on the modified surfaces could promote its conversion to the final product HgCl2. The modification of transition metals impeded the desorption of the final products HgCl2 and HgO, but it did not serve as the rate-determining step. The oxidation of Hg0 by lattice oxygen and HCl followed the Mars-Maessen and Langmuir-Hinshelwood mechanisms, respectively. HCl exhibited higher mercury oxidation ability than lattice oxygen. The reactivity of lattice oxygen could be further improved by doping transition metals, their promotion order was Ru > Nb > Mo > Mn. In a HCl atmosphere, Mn modification could significantly reduce the energy barrier for HCl activation and HgCl2 formation, providing the optimal enhancement for the mercury oxidation ability of Ce/TiO2 catalyst. The screening method of transition metal modified components based on surface adsorption reaction and oxidation energy barrier was proposed in this study, which provided theoretical guidance for the development of CeTi based catalysts with high mercury oxidation activity.

Cobalt concentration effect on the Structural, Magnetic and Optical properties of Zn1-xCoxSe Nanoparticles

Cobalt doped Zinc selenide (Zn1-xCoxSe where x = 0.00,0.03, 0.05, 0.07, 0.09, 0.1, 0.15) nanoparticles are synthesized using co-precipitation method. For studying structural properties of prepared samples, nanoparticles are characterized by XRD (X-ray diffraction),SEM (Scanning Electron Microscope) with EDX (Energy Dispersive X-ray Diffractometer), HRTEM (High resolution transmission electron microscope),Elemental composition confirmed by EDX (Energy Dispersive X-ray Diffractometer), Inductively coupled plasma (ICP) spectroscopy test and XPS (X-ray photoelectron microscope). XRD study confirms that prepared samples are crystalline in nature and size is appeared in nanometer range with cubic structure. Optical investigation is done by UV–visible spectra and PL spectra at room temperature which indicates change in energy band gap as doping of cobalt increases in host material due to quantum confinement which shows the altered optical properties due to doping of transition metal in host material. FTIR (Fourier transform infra-red) spectroscopy reports presence of different functional groups in nanoparticles and thermal stability is confirmed by TG/DTA(Thermogravimetric) analysis. Magnetic analysis by using ESR (electron spin resonance) and VSM (Vibrating Sample Magnetometer) confirms presence of ferromagnetic coupling in cobalt doped zinc selenide nanoparticles which make these nanoparticles suitable for using in different devices e.g. in magnetic storage devices, magnetic imaging, magneto-optics and sensors etc. Combined semiconducting and magnetic nature of these Co doped ZnSe nanoparticles make them useful in spintronics, magnetic storage devices etc applications.

CoMnOx Nanoflower-Based Smartphone Sensing Platform and Virtual Reality Display for Colorimetric Detection of Ziram and Cu2.

Transition metal doping is an ideal strategy to construct multifunctional and efficient nanozymes for biosensing. In this work, a metal-doped CoMnOx nanozyme was designed and synthesized by hydrothermal reaction and high-temperature calcination. Based on its oxidase activity, an "on-off-on" smartphone sensing platform was established to detect ziram and Cu2+. The obtained flower-shaped CoMnOx could exhibit oxidase-, catalase-, and laccase-like activities. The oxidase activity mechanism of CoMnOx was deeply explored. O2 molecules adsorbed on the surface of CoMnOx were activated to produce a large amount of O2·-, and then, O2·- could extract acidic hydrogen from TMB to produce blue oxTMB. Meanwhile, TMB was oxidized directly to the blue product oxTMB via the high redox ability of Co species. According to the excellent oxidase-like activity of CoMnOx, a versatile colorimetric detection platform for ziram and Cu2+ was successfully constructed. The linear detection ranges for ziram and Cu2+ were 5~280 μM and 80~360 μM, and the detection limits were 1.475 μM and 3.906 μM, respectively. In addition, a portable smartphone platform for ziram and Cu2+ sensing was established for instant analysis, showing great application promise in the detection of real samples including environmental soil and water.

Fe–Ni-doped metal-organic framework derived CoSe2 as an efficient and stable electrocatalyst for hydrogen evolution reaction

The development of efficient, stable and low-cost catalysts for hydrogen evolution reaction (HER) is one of the major difficulties in water electrolysis. It is also the key to realizing the large-scale production of hydrogen energy, a renewable and clean energy source. In recent years, metal-organic frameworks (MOFs) have attracted much attention as novel precursors for preparing water electrolysis catalysts due to their high porosity and large specific surface area. In this paper, we use ZIF-67 as a precursor and modify it by doping transition metal elements Fe and Ni. Afterwards, Fe–Ni–CoSe2@NC is synthesized by a thermal selenization process. The results show that the HER overpotential of the catalyst after modification is reduced by 50%, and the efficiency of water electrolysis is significantly improved. Moreover, the catalyst has good stability under alkaline condition and is more economical than the expensive Pt-based catalysts, which provides more possibilities for industrial applications.

A Computational Insight on the Effect of Doping of Transition Metals and Group V Elements on the Electronic Properties of Phosphorene

A Computational Insight on the Effect of Doping of Transition Metals and Group V Elements on the Electronic Properties of Phosphorene

Experimental and theoretical investigation on the high activity La0.8-xSr0.2KxMn1-yByO3-δ perovskite catalyst with combined A and B site modifications

In this work, a series of LaMnO3 (LM) perovskite catalysts modified at A and B sites were prepared to accelerate soot combustion. The La0.68Sr0.2K0.12Mn0.9Cu0.1O3-δ (LSK0.12MCu0.1) modified by Sr, K and Cu has good catalytic activity and cycle stability, the presence of NO can obviously promote the oxidation of soot. A variety of characterization results show that doping the A and B sites of perovskite will not destroy the original structure of perovskite, and the doped ions (Sr2+, K+, and Cu2+) can enter the perovskite catalyst well, which may be conducive to the formation of more oxygen defects and active oxygen species. According to the DFT calculation, adding Sr and K to the A sites can alter how atoms interact in a perovskite catalyst, increase the amount of unsaturated coordination bonds, and encourage the development of high-valence active ions (Mn4+) and oxygen defects. The doping of transition metal to the B sites results in the formation of the Cu-O-Mn link, which forms the Mn(Cu)O6 octahedron and stimulates the development of oxygen vacancies and the release of active oxygen. This study will provide an important reference for developing non-precious metal-based perovskite materials with high activity and low cost for catalytic combustion.

Enhanced co-production of high-quality syngas and highly-concentrated hydrogen via chemical looping steam methane reforming over Ni-substituted La0.6Ce0.4MnO3 oxygen carriers

Chemical looping steam methane reforming (CLSMR) is increasingly attracting more and more attention as a promising technology for co-production of syngas and hydrogen. However, low reactivity, poor selectivity and weak resistance to carbon deposition of oxygen carriers (OCs) probably discourage further commercial application of CLSMR. With this background, a novel strategy of incorporating Ce and Ni into LaMnO3 is advised to promote lattice oxygen migration rate and improve syngas selectivity. The redox activity and cycle stability of La0.6Ce0.4Mn1−xNixO3 for CLSMR are explored in a fixed-bed reactor coupling with various characterization methods. The characterization results show that La0.6Ce0.4Mn1−xNixO3 OCs still maintain the standard perovskite structures and well-ordered skeleton. The introduction of Ce3+ induces large amounts of oxygen vacancies on the surface of OCs and improve the syngas selectivity of methane reforming, leading to higher oxygen mobility and lower carbon deposition. Meanwhile, the experimental results also confirm that the doping of Ni into La0.6Ce0.4MnO3 can greatly improve the oxygen-donation ability and effectively provide more active sites for reaction with methane. While excessive Ni in perovskite will enhance the methane dissociation, which is the main reason of carbon deposits. The proper amount of Ni doping can well match the methane activation rate to migration rate of bulk lattice oxygen that is feasible to realize the balance between productivity and efficiency. In addition, the reduced metal elements along with great amounts of oxygen vacancies together contribute to more active sites for the following steam splitting which avails to produce high-purity H2. La0.6Ce0.4Mn0.7Ni0.3O3 oxygen carrier eventually achieve syngas yield of 2.8 mmol/g H2 and 1.4 mmol/g CO that is closed to the ideal value without obvious carbon deposition, and pure H2 yield of 3.8 mmol/g, exhibiting the best redox performance and good regenerability. Meanwhile, La0.6Ce0.4Mn0.7Ni0.3O3 can still maintain high redox activity and basic perovskite structure after 15 cyclic redox tests, illustrating a desirable stability and reliability. In summary, this study offers a valuable and effective strategy to tackle with the bottleneck of CLSMR, paving the way of doping transition metal as a pathway of modulating reaction performance and stability of OCs in chemical looping processes.

Optimizing Intermediate Adsorption over PdM (M=Fe, Co, Ni, Cu) Bimetallene for Boosted Nitrate Electroreduction to Ammonia

AbstractElectrochemical reduction of nitrate to ammonia (NO3RR) is a promising and eco‐friendly strategy for ammonia production. However, the sluggish kinetics of the eight‐electron transfer process and poor mechanistic understanding strongly impedes its application. To unveil the internal laws, herein, a library of Pd‐based bimetallene with various transition metal dopants (PdM (M=Fe, Co, Ni, Cu)) are screened to learn their structure–activity relationship towards NO3RR. The ultra‐thin structure of metallene greatly facilitates the exposure of active sites, and the transition metals dopants break the electronic balance and upshift its d‐band center, thus optimizing intermediates adsorption. The anisotropic electronic characteristics of these transition metals make the NO3RR activity in the order of PdCu>PdCo≈PdFe>PdNi>Pd, and a record‐high NH3 yield rate of 295 mg h−1 mgcat−1 along with Faradaic efficiency of 90.9 % is achieved in neutral electrolyte on PdCu bimetallene. Detailed studies further reveal that the moderate N‐species (*NO3 and *NO2) adsorption ability, enhanced *NO activation, and reduced HER activity facilitate the NH3 production. We believe our results will give a systematic guidance to the future design of NO3RR catalysts.

Optimizing Intermediate Adsorption over PdM (M=Fe, Co, Ni, Cu) Bimetallene for Boosted Nitrate Electroreduction to Ammonia.

Electrochemical reduction of nitrate to ammonia (NO3RR) is a promising and eco-friendly strategy for ammonia production. However, the sluggish kinetics of the eight-electron transfer process and poor mechanistic understanding strongly impedes its application. To unveil the internal laws, herein, a library of Pd-based bimetallene with various transition metal dopants (PdM (M=Fe, Co, Ni, Cu)) are screened to learn their structure-activity relationship towards NO3RR. The ultra-thin structure of metallene greatly facilitates the exposure of active sites, and the transition metals dopants break the electronic balance and upshift its d-band center, thus optimizing intermediates adsorption. The anisotropic electronic characteristics of these transition metals make the NO3RR activity in the order of PdCu>PdCo≈PdFe>PdNi>Pd, and a record-high NH3 yield rate of 295 mg h-1 mgcat -1 along with Faradaic efficiency of 90.9 % is achieved in neutral electrolyte on PdCu bimetallene. Detailed studies further reveal that the moderate N-species (*NO3 and *NO2) adsorption ability, enhanced *NO activation, and reduced HER activity facilitate the NH3 production. We believe our results will give a systematic guidance to the future design of NO3RR catalysts.

Green Synthesis of Metal-Doped ZnO Nanoparticles Using Bauhinia racemosa Lam. Extract and Evaluation of Their Photocatalysis and Biomedical Applications.

A fascinating problem in the fields of nanoscience and nanobiotechnology has recently emerged, and to tackle this, the production of metal oxide nanoparticles using plant extracts offers numerous benefits over traditional physicochemical methods. In the present investigation, ZnO nanoparticles were fabricated from Bauhinia racemosa Lam. (BR) leaves extract with various transition metal (TM) dopants (Ni, Mn, and Co). Plant leaves extract containing metal nitrate solutions were utilized as a precursor to synthesize the pristine and TM-doped ZnO nanoparticles. Structural, functional, optical, and surface properties of the fabricated samples were studied by using physicochemical and photoelectrochemical measurements. The organic pollutants tetracycline (TC), ampicillin (AMP), and amoxicillin (AMX) were used in the photocatalytic degradation assessment of the fabricated samples. Through X-ray diffraction (XRD) and transmission electron microscopy (TEM) investigation, the fabricated nanoparticles wurtzite crystal structure was verified. Moreover, Fourier transform infrared (FT-IR) analysis verified the existence of functional groups in the fabricated nanoparticles. The migration of electrons from the deep donor level and zinc interstitial to the Zn-defect and O-defect is related to the emission peaks seen at 468, 480, 534, and 450 nm in photoluminescence (PL) spectra. Co-ZnO nanoparticles demonstrated potent and excellent photocatalytic degradation performance for TC (91.09%), AMP (87.97%), and AMX (92.42%) antibiotics within 210, 180, and 150 min of visible light irradiation. Co-ZnO nanoparticles also demonstrated strong antimicrobial performance against Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae, Aspergillus flavus, Aspergillus niger, and Bacillus subtilis. Further investigation of in vitro cytotoxic potential against the A549 cell line (IC50 = 24 ± 0.5 μg/mL) utilizing MTT assay and the free radical scavenging performance of Co-ZnO nanoparticles estimated by DPPH assay utilizing l-ascorbic acid as a reference was also performed. Anti-inflammatory potential is also reviewed by comparing it with the standard drug Diclofenac, and the maximum activity was obtained for Ni-ZnO nanoparticles (IC50 = 72.4 μg/mL).

Building stabilized Cu0.17Mn0.03V2O5−□·2.16H2O cathode enables an outstanding room‐/low‐temperature aqueous Zn‐ion batteries

AbstractVanadium oxide cathode materials with stable crystal structure and fast Zn2+ storage capabilities are extremely important to achieving outstanding electrochemical performance in aqueous zinc‐ion batteries. In this work, a one‐step hydrothermal method was used to manipulate the bimetallic ion intercalation into the interlayer of vanadium oxide. The pre‐intercalated Cu ions act as pillars to pin the vanadium oxide (V‐O) layers, establishing stabilized two‐dimensional channels for fast Zn2+ diffusion. The occupation of Mn ions between V‐O interlayer further expands the layer spacing and increases the concentration of oxygen defects (Od), which boosts the Zn2+ diffusion kinetics. As a result, as‐prepared Cu0.17Mn0.03V2O5−□·2.16H2O cathode shows outstanding Zn‐storage capabilities under room‐ and low‐temperature environments (e.g., 440.3 mAh g−1 at room temperature and 294.3 mAh g−1 at −60°C). Importantly, it shows a long cycling life and high capacity retention of 93.4% over 2500 cycles at 2 A g−1 at −60°C. Furthermore, the reversible intercalation chemistry mechanisms during discharging/charging processes were revealed via operando X‐ray powder diffraction and ex situ Raman characterizations. The strategy of a couple of 3d transition metal doping provides a solution for the development of superior room‐/low‐temperature vanadium‐based cathode materials.

Tuning of physicochemical and electronic characteristics of Cu-doped NiCo2O4 ternary inverse spinel oxides for effective electrocatalytic hydrogen evolution reaction

The paper reports the development of Cu-doped NiCo2O4 ternary inverse spinel oxides through microwave-assisted hydrothermal method as effective catalysts for alkaline hydrogen evolution reaction (HER). The effective and controlled doping of Cu to NiCo2O4 lattice by the partial replacement of Ni2+/Ni3+ with Cu+/Cu2+ results in the fine-tuning of structural, morphological, and electronic characteristics, favourable for electrocatalytic HER. The study effectively correlates the influence of variation in the percentages of Cu-doping on the material properties and thereby the electrocatalytic characteristics. The experimental results indicate that Cu0.10Ni0.90Co2O4 electrode having optimum Cu-doping (10%) as the best electrocatalyst with the lowest onset potential for hydrogen evolution (242.7 mV) and reduced overpotential (72.5 mV) values by following a Volmer – Heyrovsky mechanism during HER. The improvement in HER performance after Cu-doping is due to the generation of greater number of active sites and reduction in electronic band gap. The present work advantageously utilises the scope of microwave-assisted transition metal doping strategy to fine tune the physicochemical and electronic characteristics, favourable for electrocatalytic HER, and this may pave a new way for tuning the electrochemical properties of metal oxides for various applications.

A Brief Review of Recent Theoretical Advances in Fe-Based Catalysts for CO2 Hydrogenation.

Catalytic hydrogenation presents a promising approach for converting CO2 into valuable chemicals and fuels, crucial for climate change mitigation. Iron-based catalysts have emerged as key contributors, particularly in driving the reverse water-gas shift and Fischer-Tropsch synthesis reactions. Recent research has focused on enhancing the efficiency and selectivity of these catalysts by incorporating alkali metal promoters or transition metal dopants, enabling precise adjustments to their composition and properties. This review synthesizes recent theoretical advancements in CO2 hydrogenation with iron-based catalysts, employing density functional theory and microkinetic modeling. By elucidating the underlying mechanisms involving metallic iron, iron oxides, and iron carbides, we address current challenges and provide insights for future sustainable CO2 hydrogenation developments.

Motional Narrowing Effects in the Excited State Spin Populations of Mn-Doped Hybrid Perovskites.

Spin-orbit coupling in the electronic states of solution-processed hybrid metal halide perovskites forms complex spin-textures in the band structures and allows for optical manipulation of the excited state spin-polarizations. Here, we report that motional narrowing acts on the photoexcited spin-polarization in CH3NH3PbBr3 thin films, which are doped at percentage-level with Mn2+ ions. Using ultrafast circularly polarized broadband transient absorption spectroscopy at cryogenic temperatures, we investigate the spin population dynamics in these doped hybrid perovskites and find that spin relaxation lifetimes are increased by a factor of 3 compared to those of undoped materials. Using quantitative analysis of the photoexcitation cooling processes, we reveal increased carrier scattering rates in the doped perovskites as the fundamental mechanism driving spin-polarization-maintaining motional narrowing. Our work reports transition-metal doping as a concept to extend spin lifetimes of hybrid perovskites.

Modeling the Zigzag Curves of Transition Metal Ions' Charge Transition Levels in Crystals.

Predicting the defect levels of transition metal (TM) dopants in the band gap of crystals is critical in determining the charge states of TM dopants and explaining their electronic and optical properties. By analyzing the calculated charge transition levels and the crystal-field strengths of all the 3d-TM ions in several insulators, we demonstrate that the variation trend of the 3d-TM dopants in a crystal is a scaling of the variation of 3d-electron binding energies (ionization potential) of the free TM ions corrected by adding the contribution of the 3d-orbital's crystal-field splitting. We therefore develop a model to predict the relative location of TM ions' defect levels in the band gap from the defect level and crystal-field splitting of a reference TM ion in the host of concern. The model is applied to predict the defect levels of the series of TM ions in β-Ga2O3 and ZnO, which have moderate to small band gaps, making some of the levels fall into the conduction or valence bands. These results show that the model may serve as a quick reference for related material design and optimization.

Size-selective trapping and photocatalytic degradation of PFOA in Fe-modified zeolite frameworks

Removal and destruction of perfluorooctanoic acid (PFOA) are challenging due to its extreme persistence and dilute concentrations. This study investigated dual-function adsorptive-photocatalytic zeolite materials to selectively adsorb and degrade PFOA via tuning pore structures and doping transition metals. It is found that the pore opening is critical in the size-selective trapping of PFOA, while the iron doped zeolites present excellent adsorption of PFOA (>80 mg g−1) combining hydrophobic and electrostatic interactions. The formation of PFOA-iron complexes has reduced bond dissociation energy of C−F, calculated from density functional theory, for favorable stepwise defluorination (over 60% defluorination in 4 hours) by superoxide radicals and ligand-to-metal charge transfer. This mechanistic investigation extends the potential of the concentrate-and-degrade concept to remove PFOA selectively and effectively from contaminated water.

Co–O bond covalency change in PrCoO3 by substituting Cu, Mn, and Nb

We investigated the effects of various transition metal dopants on the changes in B-site ion-oxygen (B–O) bonding characteristics and the subsequent effects on the electrochemical properties of PrCoO3 (PCMe, PrCo0.9Me0.1O3-δ, Me = Cu, Mn, Nb). The lattice constants of Mn, Cu, and Nb-doped PrCoO3 were increased compared with that of pure PrCoO3 (PCO), and the B–O–B bond angles were determined to be 163.709°, 167.323°, 165.149°, and 161.765° for PCO, PCCu, PCMn, and PCNb, respectively, by Rietveld refinement. The pre-edge peak of the O K-edge shifted to 0.7 eV of lower energy in PCCu as compared with PCO, and to 0.2 eV of lower energy in PCMn. By contrast, PCNb shifted to 0.3 eV of higher energy. Considering the B–O–B bond angle and pre-edge peak shift, the overlap between B 3d and O 2p orbitals can be expected to be the largest in PCCu, indicating the strongest covalency, whereas PCNb has the weakest covalency. The polarization resistance, as an indicator of the ORR activity, was 7.63, 6.67, 8.05, and 9.56 Ω cm2 in PCO, PCCu, PCMn and PCNb, respectively.