Electrochemical production of colloidal sulphur by oxidation of sulphide ion at lead coated-2- and -3-dimensional rotating cylinder anode surfaces

Trifunctional phosphorus-doped cobalt molybdate catalyst in self-driven coupling systems for synchronized sulfur recovery and hydrogen evolution

This study introduces a self-driven system that effectively achieves synchronized sulfur recovery and hydrogen production using a Zn-air battery. The system ingeniously integrates the sulfur oxidation reaction (SOR) and the hydrogen evolution reaction (HER) into a single, efficient process. Central to this system is the trifunctional phosphorus-doped cobalt molybdate catalyst (P-CoMoO4/NF), which exhibits superior performance in both HER (ηj = 100 = 0.13 V) and SOR (ηj = 100 = 0.30 V) with remarkable stability (∼360 h), reaching 0.64 V at 100 mA cm-2 for simultaneous sulfur ion degradation and hydrogen production. Through density functional theory simulations and extensive characterizations, it has been shown that phosphorus doping in the cobalt molybdate catalyst facilitates electron redistribution, enhancing the catalyst's conductivity, generating more oxygen vacancies, and promoting improved mass and electron transfer. This modification also lowers the energy barrier for adsorbing reaction intermediates, thus increasing the hydrogen production rate and sulfur oxide conversion in this self-powered system. In summary, this research marks a substantial advancement in the development of trifunctional catalysts and proposes an eco-friendly, cost-effective strategy for integrated reaction systems, paving the way for sustainable energy solutions.

Highly efficient electrochemical ammonia synthesis via nitrate reduction over metallic Cu phase coupling sulfion oxidation

Electrochemical nitrate reduction reaction (NO3RR) is a promising technology for ammonia production and denitrification of wastewater. Its application is seriously restricted by the development of the highly active and selective electrocatalyst and a rational electrolysis system. Here, we constructed an efficient electrochemical ammonia production process via nitrate reduction on the metallic Cu electrocatalyst when coupled with anodic sulfion oxidation reaction (SOR). The synthesized Cu catalyst delivers an excellent NH3 Faradaic efficiency of 96.0 % and a NH3 yield of 0.391 mmol h-1 cm-2 at -0.2 V vs. reversible hydrogen electrode, which mainly stem from the more favorable conversion of NO2 - to NH3 on Cu0. Importantly, the well-designed electrolysis system with cathodic NO3RR and anodic SOR achieves a dramatically reduced cell voltage of 0.8 V at 50 mA cm-2 in comparison with the one with anodic oxygen evolution reaction (OER) of 1.9 V. This work presents an effective strategy for the energy-saving ammonia production via constructing effective nitrate reduction catalyst and replacing the OER with SOR while removing the pollutants including nitrate and sulfion.

Colloidal sulphur production by electrochemical oxidation of sulphide in a swirling flow reactor

A swirling flow electrochemical reactor is suitable for the colloidal sulphur production by means of sulphide electrochemical oxidation.

Scalable Synthesis of Tungsten Disulfide Nanosheets for Alkali-Acid Electrocatalytic Sulfion Recycling and H2 Generation

WS₂ nanosheets hold great promise for a variety of applications yet faces a grand challenge in terms of large-scale synthesis. We report a reliable, scalable, and high-yield (>93 %) synthetic strategy to fabricate WS₂ nanosheets, which exhibit highly desirable electrocatalytic properties toward both the alkaline sulfion (S^2- ) oxidation reaction (SOR) and the acidic hydrogen evolution reaction (HER). The findings prompted us to develop a hybrid alkali-acid electrochemical cell with the WS₂ nanosheets as bifunctional electrode catalysts of alkaline anodic SOR and acidic cathodic HER. The proof-of-concept device holds promise for self-power or low-electricity electrolytic H₂ generation and environmentally friendly recycling of sulfion with enhanced electron utilization efficiency.

Electrochemical treatment of industrial sulfidic spent caustic streams for sulfide removal and caustic recovery

Alkaline spent caustic streams (SCS) produced in the petrochemical and chemical manufacturing industry, contain high concentrations of reactive sulfide (HS^-) and caustic soda (NaOH). Common treatment methods entail high operational costs while not recovering the possible resources that SCS contain. Here we studied the electrochemical treatment of SCS from a chemical manufacturing industry in an electrolysis cell, aiming at anodic HS^- removal and cathodic NaOH, devoid of sulfide, recovery. Using a synthetic SCS we first evaluated the HS^- oxidation product distribution over time, as well as the HS^- removal and the NaOH recovery, as a function of current density. In a second step, we investigated the operational aspects of such treatment for the industrial SCS, under 300 A m^-2 fixed current density. In an electrolysis cell receiving 205 ± 60 g S L^-1 d^-1 HS^- over 20 days of continuous operation, HS^- was removed with a 38.0 ± 7.7 % removal and ∼80 % coulombic efficiency, with a concomitant recovery of a ∼12 wt.% NaOH solution. The low cell voltage obtained (1.75 ± 0.12 V), resulted in low energy requirements of 3.7 ± 0.6 kW h kg^-1 S and 6.3 ± 0.4 kW h kg^-1 NaOH and suggests techno-economic viability of this process.

Anode materials for sulfide oxidation in alkaline wastewater: An activity and stability performance comparison

Electrochemical sulfide removal can be attractive as a zero-chemical-input approach for treatment of waste streams such as spent caustics coupled to caustic recovery. A key concern is possible decline in catalytic activity, due to passivation from deposited elemental sulfur (S⁰) on the anode surface and stability limitation, due to sulfide oxidation under highly alkaline conditions. In this study, six commercially available electrode materials (Ir Mixed Metal Oxide (MMO), Ru MMO, Pt/IrOx, Pt, PbOx and TiO₂/IrTaO₂ coated titanium-based electrodes) were tested to investigate the impact of the electrocatalyst on the process efficiency in terms of sulfide removal and final product of sulfide oxidation, as well as to determine the stability of the electrocatalyst under high sulfide concentrations (50 mM Na₂S) and high alkalinity (pH > 12). Short-term experiments showed that the catalyst type impacts the anode potential and the sulfide oxidation reaction products. Longer-term experiments under current densities up to 200 A m^-2 showed a high differentiation in stability performance among the catalysts. Ru MMO was the most active towards sulfide oxidation with a coulombic efficiency of 63.2 ± 0.5% at an average anode potential of 0.92 ± 0.17 V vs SHE. Ir MMO was the most stable, preserving 100% of its original catalyst loading during the tests. The results demonstrated that Ru MMO and Ir MMO were the most suitable electrode materials for sulfide oxidation under highly alkaline conditions, while the need for establishing a good trade-off between activity, stability and cost still persists.