Closed-loop chromium recovery from tannery wastewater: Mechanism of multiphase Cr(III) electrode position under low-concentration sulfate and chloride coordination.

Bioadsorption and membrane technology for reduction and recovery of chromium from tannery industry wastewater

In the context of the gradual depletion of global fossil fuel resources, it is increasingly necessary to explore new alternative energy. Hydrogen energy has attracted great interest from researchers because of its green and pollution-free characteristics. Moreover, the methanol oxidation reaction (MOR) can combine the hydrogen evolution reaction (HER), replacing the anode reaction (oxygen evolution reaction-OER) in overall water splitting and efficiently producing hydrogen. In this study, platinum-palladium nanoparticles on reduced graphene oxide (PtPd/rGO) were successfully synthesized as HER and MOR bifunctional electrocatalysts under alkaline conditions by the stepwise loading of Pt and Pd bimetallic nanoparticles on rGO using a simple liquid-phase reduction method. PtPd/rGO-2 with 0.99 wt% Pt and 2.86 wt% Pd in the HER has the lowest overpotential (87.16 mV at 100 mA cm-2), with the smallest Tafel slope (18.9 mV dec-1). The exceptional mass activity of PtPd/rGO-2 in the MOR reaches 10.75 A mg-1PtPd, which is 18.22 and 53.75 times greater than that of commercial Pt/C (Pt/C) and commercial Pd/C (Pd/C), respectively. PtPd/rGO-2 is 0.935 V lower in the coupling reaction of HER and MOR (MOR ∥ HER) compared to the overall water splitting (OER ∥ HER) without methanol (10 mA cm-2). This is probably because appropriate Pt and Pd loading exposes many more catalytic sites, and the synergistic interaction between Pt, Pd, and Pt-Pd enhances the catalytic performance. This strategy can be used for the synthesis of novel bifunctional electrocatalysts.

The hydrogen evolution reaction (HER) fundamentally limits aluminum electroreduction in aqueous electrolytes by dominating interfacial charge transfer. Here, we suppress HER by engineering deuterium bonds (D-bonds) through nuclear quantum effects, confining D between D₂O and DMF molecules. This quantum confinement weakens hydrogen delocalization and restructures the Al3+ solvation sheath, reducing water activity kinetically and thermodynamically. The regulated electrolyte enables uniform aluminum nucleation and dense plating layers, achieving 569h (0.05mA cm-2) and 379h (0.1mA cm-2) cycling stability in the 2D2O/1DMF electrolyte, which outperforms traditional sulfate electrolytes by 3.6 and 6.1 times, respectively. Our work uniquely leverages nuclear quantum confinement to engineer robust D-bonds, simultaneously suppressing HER and enabling atomic-level control over aluminum ion solvation structures for unprecedented Al redox reversibility in sulfate electrolytes. This exemplification pushes the electrolyte engineering from extensive component adjustment to quantum precision engineering, which provides an innovative solution for the high-activity water-based battery system.

Over the last decades, several studies have reported the effect of traces of iron species on the improvement of the oxygen evolution reaction (OER) kinetics for nickel-based anode materials in alkaline media. [1] However, the effect of iron impurities on the hydrogen evolution reaction (HER) is still a subject to debate. For instance, at low iron concentrations (0.03 – 0.5 ppm) a loss of the HER activity for Ni surface is observed due to the electrochemical deposition of less active Fe. [2,3] Other studies showed less deactivation behavior of the Ni cathode at 3 and 14 ppm iron ion concentration over time. [2,4] This improvement of the lifetime of the Ni electrode in the presence of iron cations is very likely related to the increased surface roughness by Fe electrodeposition and thereby the decrease in the formation of inactive NiH species. [2] Systematic investigations on Fe sputtered Ni surfaces with different coverages showed that ≥ 60% of Fe coverage stabilizes the Ni cathode by preventing hydrogen diffusion through the Ni lattice at constant current density of -100 mA cm-2. [5] Generally, the role of Fe impurities on the HER activity for nickel cathode surface in alkaline environments is still unclear to date.In this work, we comprehensively evaluated the influence of the iron ion concentration on the long-term HER performance for a polycrystalline Ni electrode by using rotating disc electrode (RDE) setup. The changes in HER activity as a function of the iron ion concentration was correlated with resulting surface roughness and layer thickness/coverage of the electrodeposited iron. Highly purified 0.1 M KOH was used as basic electrolyte solution and mixed with iron ion concentrations in the range of <1, 6 and 14 ppm. The initial HER activity on the Ni surface is unchanged irrespective of the iron ion concentration added into the electrolyte. However, at iron ion concentrations of <1 ppm and 6 ppm, the chronopotentiometric (CP) experiments at -10 mA cm-2 for 24 hours show a potential drop of ~70 mV and ~53 mV, respectively, resulting in a deactivation of the Ni surface by forming NiH species. Very interestingly, high iron ion concentrations (e.g. ~14 ppm) lead to an almost constant potential (~ 1 - 7 mV drop) during the CP experiment, by reducing the formation of inactive NiH. The electrochemically treated Ni electrodes were then characterized by X-ray fluorescence spectroscopy (XRF) and scanning electron microscopy in combination of energy dispersive X-ray spectroscopy (SEM-EDX). The SEM micrographs show iron particles electrodeposited uniformly on the polycrystalline Ni surface after the CP experiments with < 1, 6 and 14 ppm of iron ion concentrations in 0.1 M KOH. The analysis of the EDX maps reveals a coverage of 17, 54 and 58 % on the Ni surface at iron ion concentrations of < 1, 6 and 14 ppm, respectively. Obviously, iron ion concentrations of 6 and 14 pm show very similar coverages on the Ni surface, indicating their minor role on the HER activity. We suggest that during the HER the hydrogen bubbles suppress the formation of a dense and complete iron layer on the Ni electrode surface. The spatial distribution and thickness of the electrodeposited Fe particles were also evaluated after 1 h and 24 h of CP experiments with < 1, 6 and 14 ppm iron ion concentrations by using micro-XRF equipped with a polycapillary lens of 20 µm. Thicker iron layers after 24 hours are formed at high iron ion concentrations, preventing the hydrogen diffusion into Ni lattice to form inactive NiH species and to maintain the HER activity over time under alkaline conditions.In summary, we show that traces of iron impurities have a huge impact on the long-term durability of polycrystalline Ni electrodes for HER in alkaline environments. A critical iron ion concentration is required to suppress the formation of inactive NiH species during the HER at -10 mA cm-2 for 24 h and/or to electrodeposite enough iron for maintaining the HER kinetics on the polycrystalline Ni electrodes in alkaline environments.

Increasing demands for pollution-free energy resources have stimulated intense research on the design and fabrication of highly efficient, inexpensive, and stable non-noble earth-abundant metal catalysts with remarkable catalytic activity for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Morphology control of the catalysts is widely implemented as an effective strategy to change the surface atomic coordination and increase the catalytic behavior of the catalysts. In this study, we have designed a series of Mn-Co catalyts with different morphologies on the graphite paper substrate to enhance OER and HER activities in alkaline media. The prepared catalysts with different morphologies were successfully obtained by adjusting the amount of ammonium fluoride (NH4F) in the hydrothermal process. The electrochemical tests display that the cubic-like Mn-Co catalyst with pyramids on the faces at a concentration of 0.21 M NH4F exhibits the best activity toward both OER and HER. The cubic-like Mn-Co catalyst with pyramids on the faces showed overpotentials of 240 and 82 mV at a current density of 10 mA cm-2 for OER and HER, respectively. Also, the cubic-like Mn-Co catalyst with pyramids on the faces required overpotentials of 319 and 216 mV to reach the current density of 100 mA cm-2 for OER and HER, respectively. The current density of this catalyst at η = 0.32 V was 701.05 mA cm-2 for OER, and for HER, the current density of the catalyst was 422.89 mA cm-2 at η = 0.23 V. The Tafel slopes of the Mn-Co catalyst with cubic-like structures with pyramids on the faces were 78 and 121 mV dec-1 for OER and HER, respectively. A two-electrode overall water electrolysis system using this bifunctional Mn-Co catalyst exhibited low cell voltages of 1.60 in the alkaline electrolyte at the standard current density of 10 mA cm-2 with appropriate stability. These electrochemical merits exhibit the considerable potential of the cubic-like Mn-Co catalyst with pyramids on the faces for bifunctional OER and HER applications.

Removal and recovery of chromium and chromium speciation with MINTEQA2

Developing cost-effective, highly-active and robust electrocatalysts is of vital importance to supersede noble-metal ones for both hydrogen evolution reactions (HERs) and oxygen reduction reactions (ORRs). Herein, a unique vanadium-mediated space confined strategy is reported to construct a composite structure involving Co/Co9S8 nanoparticles anchored on Co-N-doped porous carbon (VCS@NC) as bifunctional electrocatalysts toward HER and ORR. Benefitting from the ultrafine nanostructure, abundant Co-Nx active sites, large specific surface area and defect-rich carbon framework, the resultant VCS@NC exhibits unexceptionable HER catalytic activity, needing extremely low HER overpotentials in pH-universal media (alkaline: 117 mV, acid: 178 mV, neutral: 210 mV) at a current density of 10 mA cm-2, paralleling at least 100 h catalytic durability. Notably, the VCS@NC catalyst delivers high-efficiency ORR performance in alkaline solution, accompanied with a quite high half wave potential of 0.901 V, far overmatching the commercial Pt/C catalyst. Our research opens up novel insight into engineering highly-efficient multifunctional non-precious metal electrocatalysts by a metal-mediated space-confined strategy in energy storage and conversion system.

Constructing catalysts with new and optimizational chemical components and structures, which can operate well for both the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER) at large current densities, is of primary importance in practical water splitting technology. Herein, the NiFe2O4 nanoparticles/NiFe layered double hydroxide (LDH) nanosheet heterostructure array on Ni foam was prepared via a simple one-step solvothermal approach. The as-prepared heterostructure array displays high catalytic activity toward the OER with a small overpotential of 213 mV at 100 mA cm-2 and can afford a current density of 500 mA cm-2 at an overpotential of 242 mV and 1000 mA cm-2 at 265 mV. Moreover, it also presents outstanding HER activity, only needing a small overpotential of 101 mV at 10 mA cm-2, and can drive large current densities of 500 and 750 mA cm-2 at individual overpotentials of 297 and 314 mV. A two-electrode electrolyzer using NiFe2O4 nanoparticles/NiFe LDH nanosheets as both the anode and the cathode implements active overall water splitting, demanding a low voltage of 1.535 V to drive 10 mA cm-2, and can deliver 500 mA cm-2 at 1.932 V. The NiFe2O4 nanoparticles/NiFe LDH nanosheet array electrodes also show excellent stability against OER, HER, and overall water splitting at large current densities. Significantly, the overall water splitting with NiFe2O4 nanoparticles/NiFe LDH nanosheets as both the anode and the cathode can be continuously driven by a battery of only 1.5 V. The intrinsic advantages and strong coupling effects of NiFe2O4 nanoparticles and NiFe LDH nanosheets make NiFe2O4 nanoparticles/NiFe LDH nanosheet heterostructure array abundant catalytically active sites, high electronic conductivity, and high catalytic reactivity, which remarkably contributed to the catalytic activities for OER, HER, and overall water splitting. Our work can inspire the optimal design of the NiFe bimetallic heterostructure electrocatalyst for application in practical water electrolysis.

Research on water splitting is paramount for developing low-carbon alternative energy sources. Nevertheless, creating an efficient, cost-effective, and bifunctional electrocatalyst that facilitates both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) remains an elusive goal. In this work, we report a novel hybrid nanostructured electrocatalyst by combining and pyrolyzing MXene, MIL-53(Fe), and ZIF-67. Comprehensive characterization of the synthesized nanocomposites was conducted using XRD, FESEM, TEM, EDX, and XPS. Notably, among the synthesized electrocatalysts, M3 demonstrated exceptional performance, achieving 10 mA cm-2 at 237 mV and 50 mA cm-2 at 292 mV for the OER, and 10 mA cm-2 at 307 mV and 50 mA cm-2 at 481 mV for the HER. The Tafel slope values were 64 mV dec-1 for the OER and 185 mV dec-1 for the HER at 10 mA cm-2. Moreover, M3 exhibited excellent stability, with negligible current density loss over 12 hours, and showed good mass activity of 57.5 and 54.6 A g-1 and TOFs of 1.56 and 2.97 s-1, for the OER and HER, respectively. This study highlights the efficacy of integrating MXene (Ti3C2T x ) with MIL-53(Fe) and ZIF-67, creating a potent bifunctional OER and HER electrocatalyst. The synergistic combination enhances electrical conductivity, active site availability, and structural stability, yielding superior performance. The findings of this investigation underscore the importance of strategic design and optimization of bifunctional electrocatalysts for energy conversion applications.

This study aims to establish easy-to-fabricate and novel structures for the synthesis of highly active and enduring electrocatalysts for the hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Gradient electrodeposition and four different time regimes were utilized to synthesize Ni-S 3D patterns with the optimization of electrodeposition time. Pulse electrodeposition was employed for the synthesis of Ni-Fe-S nanosheets at three different frequencies and duty cycles to optimize the pulse electrodeposition parameters. The sample synthesized at 13 min of gradient electrodeposition with a 1 Hz frequency and 0.7 duty cycle for pulse electrodeposition demonstrated the best electrocatalytic performance. The optimized electrode further showed remarkable performance for HER and UOR reactions, requiring only 54 mV and 1.25 V to deliver 10 mA cm-2 for HER and UOR, respectively. Moreover, the overall cell voltage of the two-electrode system in 1 M KOH and 0.5 M urea was measured at 1.313 V, delivering 10 mA cm-2. Constructing Ni-Fe-S nanosheets on 3D Ni-S significantly increased the electrochemical surface area from 51 to 278 for the Ni-S and Ni-Fe-S layers. Tafel slopes were measured as 138 and 182 mV dec-1 for the HER and UOR for the Ni-S coating layer and 97 mV dec-1 for the HER and 131 mV dec-1 for the UOR for the optimal Ni-Fe-S nanosheets on Ni-S. Minimal changes in the potential were observed at 100 mA cm-2 in 50 h regarding the HER and UOR, signifying exceptional electrocatalytic stability. This study provides economically viable, highly active, and long-lasting electrocatalysts suitable for HER and UOR applications.

Due to rising energy demand, renewable energy sources integrated processes have gained great interest in the recent years. Water electrolysis powered with renewable energy have potential to contribute to the development of sustainable hydrogen economy enormously. Despite the fact that “green” hydrogen is produced in the water electrolyzers with eco-friendly product O2, the excess energy input still hinders this technology to replace the fossil fuel-based technologies. Even with tremendous attempts and considerable contribution to the electrocatalyst development/optimization/improvement over last decades, there is a continued massive endeavor on the way of noble-metal based catalyst replacement for oxygen evolution reaction (OER) as well as for hydrogen evolution reaction (HER). From this point of view, transition metal-containing intermetallic compounds with well-defined electronic and crystal structures, are considered as promising electrocatalysts or precursors for them [1, 3]. Up to now, Mo-Ni and Mo-B systems have been explored for the hydrogen evolution reaction, and noticeable HER activities have been demonstrated for the binary compounds in these systems [4-6].The chemical behavior of ternary borides Mo2FeB2, Mo2CoB2 and Mo2NiB2 under hydrogen evolution (HER) reactions is investigated. Material synthesis and electrode manufacturing include the arc melting of elements in 2:1:2 atomic ratio, homogenization annealing at 1300-1400 °C and densification of grinded powder into cylindrically shaped electrodes via spark plasma sintering (SPS). The electrochemical measurements are carried out in 1M KOH and ambient conditions. Electrocatalytic activity and stability are monitored with cyclic voltammetry (CV) and 2-hour chronopotentiometry (CP) measurements performed at reducing current density of 10 mA cm-2, which are considered as conditions of standard benchmarking HER experiment. Furthermore, long-term stability of HER activity at elevated reducing current densities such as 50, 100, 200 mA cm-2 are essential to prove the preserved catalytic performance under harsh reaction conditions. For the comprehensive pre- and post-characterization of the electrode materials, bulk and surface-sensitive techniques are utilized.The pristine Mo2FeB2, Mo2CoB2 and Mo2NiB2 in HER region outperform their constituent TM components and indeed, their HER activity is close to that of the state-of-art Pt catalyst (Figure). During short-term CP experiment, Mo2NiB2 and Mo2FeB2 show continuing activation during the measurement, whereas Mo2CoB2 maintained its stability. Energy dispersive X-ray analysis (EDXS) of electrochemically treated areas indicate an enrichment in Fe, Co and Ni due to the noticeable leaching of Mo into electrolyte. Also, chemical analysis via ICP-OES reveals only Mo among all three constituent elements in the exploited electrolyte solution.As a conclusion, Mo2 TMB2 intermetallic compounds show outstanding HER activity and its stability over 2h of CP with respect to corresponding reference materials. Long-term chronopotentiometry in alkaline media were carried out and will be extensively presented. Figure. HER activity of Mo2 TMB2 (TM: Fe, Co, Ni) with respect to elemental references Fe, Co, Ni and state-of-art Pt. Inlets: Backscattered secondary electron images after 2h CP, including energy dispersive X-ray (EDXS) analysis results.

Developing earth-abundant highly efficient catalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is indispensable for the widespread implementation of electrochemical water splitting to store renewable energy. Herein, amorphous bimetallic selenide (Ni-Fe-Se) hollow nanospheres electrodeposited on nickel foam (Ni-Fe-Se/NF) are developed as a bifunctional catalyst for the HER and OER. The HER and OER bifunctional activity of Ni-Fe-Se/NF outperforms those of monometallic Ni-Se/NF and Fe-Se/NF owing to the synergy of Ni and Fe in Ni-Fe-Se/NF. Moreover, the amorphous hollow spherical morphology of Ni-Fe-Se/NF increases the active site density and facilitates the mass transfer of electrolytes and H2/O2 products. Ni-Fe-Se/NF drives a current density of 10 mA cm-2 with an overpotential of ∼85 mV for the HER and 100 mA cm-2 with an overpotential of ∼222 mV for the OER. As the HER and OER bifunctional catalyst, Ni-Fe-Se/NF can split alkaline water with total voltages of ∼1.52 V and ∼1.66 V at 10 mA cm-2 and 100 mA cm-2, respectively, and remain stable over 50 hours of operation in 1 M KOH.

Using abundant seawater can reduce reliance on freshwater resources for hydrogen production from electrocatalytic water splitting. However, seawater has detrimental effects on the stability and activity of the hydrogen evolution reaction (HER) electrocatalysts under different pH conditions. In this work, we report the synthesis of binary metallic core-sheath nitride@oxynitride electrocatalysts [Ni(ETM)]δ+-[O-N]δ-, where ETM is an early transition metal V or Cr. Using NiVN on a nickel foam (NF) substrate, we demonstrate an HER overpotential as low as 32 mV at -10 mA cm-2 in saline water (0.6 M NaCl). The results represent an advancement in saline water HER performance of earth-abundant electrocatalysts, especially under near-neutral pH range (i.e., pH 6-8). Doping ETMs in nickel oxynitrides accelerates the typically rate-determining H2O dissociation step for HER and suppresses chloride deactivation of the catalyst in neutral-pH saline water. Heterointerface synergism occurs through H2O adsorption and dissociation at interfacial oxide character, while adsorbed H* proceeds via Heyrovsky or Tafel step on the nitride character. This electrocatalyst showed stable performance under a constant current density of -50 mA cm-2 for 50 h followed by additional 50 h at -100 mA cm-2 in a neutral saline electrolyte (1 M PB + 0.6 M NaCl). Contrarily, under the same conditions, Pt/C@NF exhibited significantly low performance after a mere 4 h at -50 mA cm-2. The low Tafel slope of 25 mV dec-1 indicated that the reaction is Tafel limited, unlike commercial Pt/C, which is Heyrovsky limited. We close by discussing general principles concerning surface charge delocalization for the design of HER electrocatalysts in pH saline environments.

Removal of chromium(III) from tannery wastewater using activated carbon from sugar industrial waste

Chitosan, its nanoparticles and whiskers present an excellent capacity to complex chromium ions. However, this phenomenon is influenced by different parameters. In our search, we determined the appropriate range of pH to form chitosan–Cr(III), nanoparticles Cr(III) and whiskers–Cr(III) complex. We studied also the influence of chromium concentration and nature of chitosan-based materials on complexation process. Our main aim is approximate the optimal conditions to remove chromium(III) from tanning bath, recuperated from tannery wastewater of Marrakech in Morocco. However, the results of adsorption kinetic in tannery wastewater revealed that chitosan, its nanoparticles, whiskers and biocomposites are good sorbent of chromium as well, even if the adsorbed quantity is less compared to chromium solution. Although, according to ICP-OES analysis in this real effluent, nanoparticles are the best complexing ligand, after 24 h of contact nanoparticles can remove 70% of chromium from this tannery wastewater.