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Active and durable oxygen reduction reaction (ORR) catalysts are of utmost importance to realize the commercialization of hydrogen fuel cells and metal-air batteries. Al-substituted MnFe2O4-based ternary oxide and reduced graphene oxide (MAF-RGO) nanocomposite is synthesized using an in-situ co-precipitation followed by a hydrothermal process and verified for ORR electrocatalysis in the alkaline electrolyte (0.1 M KOH). MAF-RGO is first analyzed using physicochemical characterization tools including X-ray diffraction, Raman spectroscopy, sorption studies, electron microscopy, X-ray photoelectron spectroscopy, etc. Further, the characteristic ORR peak centered at 0.56 V vs. reversible hydrogen electrode (RHE) in cyclic voltammetry (CV) studies confirms the electrocatalytic performance of MAF-RGO. The ORR onset potential of 0.92 V vs. RHE is obtained in linear sweep voltammetry (LSV) measurement at 1600 rpm in O2-saturated electrolyte exhibiting an improved ORR performance as compared to the commercial electrocatalyst. The reduction kinetics is observed to follow the desirable near 4-e- mechanism. In addition, the electrocatalyst exhibits improved relative current stability of 86% and methanol poisoning resistance of 82%, which is better in comparison to the standard Pt/C. The observed electrochemical performance results from the synergism between the oxygen vacancy-rich Al-substituted metallic oxide active species and the functional group enriched predominantly mesoporous RGO sheets with excellent electrical conductivity. The introduction of metallic species enhanced the inter-planar spacing between graphitic sheets easing the maneuver of reactant species through the electrocatalyst and accessing more ORR-active sites. This study establishes the potency of mixed transition metal oxide/nanocarbon composites as durable high-performance ORR-active systems.

Aluminum (Al3+) doped manganese (Mn)/copper (Cu) ferrite impregnated rGO hybrids are verified for photo-electrocatalytic water-splitting and super-capacitor electrode applications. Energy band gaps of 2.17 eV (Mn) and 2.25 eV (Cu) confirm photocatalytic activity in the visible-band. High anodic photo-currents around 18–40 and 2–9 mA/cm2 (1 V vs. Ag/AgCl) are observed for Mn and Cu systems. Formation of oxygen vacancies due to aluminum doping assisted in efficient charge transfer behavior both at surface and lattice orders due to the structural adjustments. 2-D rGO surface disorders (ID/I2D) emphasized the retardation of the charge recombination at the metal-oxide and rGO junction. During visible-light irradiation current densities are increased by 0.7–9.2 mA/cm2 with a better redox behavior. The difference in transient currents are observed around 0.01–0.02 mA/cm2 at 0 V vs. Ag/AgCl [0.7 mA/cm2 at 0.5 V]. Intrinsic photo-current at zero voltage infers built-in potentials at the hybrid and electrolyte interface (Schottky- junction). Impedance study implicates better electron-transfer/ion diffusion characteristics during photon irradiation with a semi-infinite diffusion rate-limiting step. Specific-capacities (Three-electrode) are observed around 216 F g−1 (Mn), 142 F g−1 (Cu) with combined pseudo-capacitance and EDLC characteristics. Li+-intercalation results in ultrahigh stability up to 100,000 cycles (t ~ 69.4 h) at 0.8 V s−1 with a capacity retention of 127%. Two-Electrode performance shows 99.1% capacity retention after 5000 cycles with decrease in the impedance (1.38 Ω to 1.06 Ω), and a maximum areal energy density of 0.54 μWh cm−2 at power density of 50 μW cm−2.

Synergistic Mn-Al-Fe impregnated rGO hybrid (MAF-rGO) is developed and verified for arsenic remediation. Preliminary adsorption studies are observed through vibrational spectroscopy and morphological tools. Adsorption isotherms are fitted by Freundlich model multilayer sorption with a loading of 402 mg g−1 [As(III)], and 339 mg g−1 [As(V)]. Adsorption kinetics study in competing ion environment [SO42−, PO43−, NO3−, CO32−] confirms the PSO model-controlled chemisorption as the rate-limiting step. The irradiation of white light (> 420 nm) in adsorption kinetics study shows a two-fold increase in the arsenic loading parameters (5–10 min). Multiplet peaks in As3d spectra (XPS) confirm the transformation of arsenic species in the near-surface region of hybrid [i.e., As(III), As(V)]. The occurrence of redox-active, ligand-exchange reactions are confirmed by individual Mn2p, Fe2p, Al2p, O1s, and C1s spectra. The dedicated electrochemical study confirms arsenic decontamination occurred through both reduction and sorption (i.e., electrosorption). An energy bandgap (Eg) of 2.17 eV resulted in the photocatalytic activity, which enhanced the arsenic remediation. Photo-electrocatalytic synergism resulted in high photocurrent densities (0.2–0.9 mA cm−2) with charge separation [e−–h+], and built-in potential abilities (Schottky-junction; Water-splitting). Arsenic-treated hybrid (i.e., sludge) is observed with an ultra-high stable charge–discharge cyclic performance for 100,000 cycles (~77 %) with improved specific-capacity of 125 F g−1 over pristine hybrid (92 F g−1). The better electrochemical parameters of sludge are due to the efficient lithiation/de-lithiation process supported by adsorbed arsenic. This work proposes sustainable approach towards environmental remediation and energy storage applications.

The present study is about surface complexation behavior of arsenic species adsorbed onto ternary metal oxide adsorbents (Ni-Al-Fe, Co-Al-Fe, Mn-Al-Fe, and Cu-Al-Fe). The analysis is carried out by X-ray absorption spectroscopy (XAS) tool. XANES (μ(E) vs. E) spectra close to the absorption edge (i.e., As K-edge) of all samples are observed along with the As(III) and As(V) standards. The first derivative of XANES for Ni-As(V), and Cu-As(V) samples agree with that of As(V) standards, respectively. Whereas, As(III) adsorbed adsorbent systems (i.e., Ni, Co, Mn, and Cu) are observed with mixed oxidation state of arsenic. A total of 65-85 % is observed with initial oxidation state (As(III) or As(V)), and remaining 15-35 % is observed with modified oxidation state (As(V) or As(III)) that explains the occurrence of possible charge transfer. EXAFS analysis shows the As-O bond distances in the range of 1.7-1.8 A. The corresponding As-M bond distances are around 2.7, 3.2, and 3.6 A which confirms the formation various edge sharing (2E), and corner sharing (2C, 1V) surface complexes. Surface coverage is understood as an important parameter as bidentate attachments (2E, 2C) are evident in As(III), and As(V), but monodentate attachments (1V) are only observed in As(V).

Arsenic [As(III) and As(V)] adsorption on aluminum substituted cobalt ferrite (Co−Al−Fe) ternary metal oxide adsorbent is reported by means of qualitative and quantitative spectroscopy tools. IR and Raman active signals were observed around 810–920 cm–1 band indicate different As−OHcomplexed and As−Ouncomplexed stretching vibrations on to the adsorbent. The adsorption behavior of arsenic (III and V) onto these adsorbents is studied as a function of contact time, different concentrations, and pH conditions. The kinetics study on adsorption were performed to understand nature of adsorption which supports the Pseudo Second Order (PSO) model. The adsorption isotherms study indicates Freundlich type of adsorption. The maximum adsorption capacity of Co−Al−Fe adsorbent is observed around 130 and 76 mg g–1 for As(III) and As(V) systems, respectively. Detailed XPS study of As 3d, Fe 2p, Co 2p, and O 1s spectra has been reported in explaining the redox behavior and ligand exchange reactions in supporting arsenic ad...