Direct electrosynthesis and separation of ammonia and chlorine from waste streams via a stacked membrane-free electrolyzer

Conventional chemical production heavily relies on fossil fuels, leading to significant greenhouse gas emissions. Electrosynthesis, an emerging technology, offers a more sustainable alternative by utilizing renewable energy sources and ambient conditions to generate valuable chemicals. This approach is particularly attractive for producing ammonia (NH₃) and chlorine (Cl₂), which are globally produced at massive scales and are commonly found as pollutants in industrial wastewater. Traditional electrosynthesis often uses expensive electrolytes like tetrahydrofuran or toluene, generating secondary waste. This research proposes a novel approach that leverages wastewater itself as the electrolyte, thereby reducing costs and mitigating waste while simultaneously recovering valuable resources. Existing methods for simultaneous NH₃ and Cl₂ production often employ ion-selective membranes to prevent the rapid and undesirable reaction between these two products, but these membranes add significant cost and maintenance challenges. This study aims to overcome these limitations by developing a membrane-free electrolyzer capable of simultaneous electrosynthesis and separation of NH₃ and Cl₂ from waste streams, thus enhancing the efficiency and sustainability of chemical production.

Electrocatalytic nitrate reduction (NO₃RR) to ammonia has been demonstrated in previous studies, typically coupled with water oxidation. Similarly, chlorine gas (Cl₂) production is predominantly achieved through the chlor-alkali process, involving chlorine evolution reaction paired with hydrogen evolution. While these processes are established, simultaneously producing NH₃ and Cl₂ requires preventing their rapid reaction. Existing approaches use membranes to physically separate the anode and cathode compartments, but this increases costs and maintenance issues. The authors cite recent work exploring membrane modules with hydrophobic gas-diffusion layers for efficient gas extraction, creating a triphasic boundary that allows gas passage but blocks liquid water. This concept is leveraged to design a novel, membrane-free system.

The study utilizes a flow-type membrane-free electrolyzer featuring gas-extraction electrodes for simultaneous NH₃ and Cl₂ production and extraction. Metallic copper and ruthenium oxide are employed as electrocatalysts for NO₃RR and chlorine evolution reaction (CER), respectively, immobilized on a carbon-polytetrafluoroethylene (PTFE)-based gas diffusion layer. The electrode assembly's performance in terms of NH₃ and Cl₂ yield and transfer rates was evaluated separately, demonstrating a potential for efficient gas separation. A flow-type electrolyzer with separate ammonia and chlorine trap channels was constructed and tested with synthetic waste streams. The system's performance during simultaneous NH₃ and Cl₂ electrosynthesis and separation was evaluated, measuring product concentrations in the trap solutions and residual concentrations in the waste stream. Control experiments investigated the effects of various nitrogen and chloride concentrations and ratios, with and without separation, to understand product loss mechanisms. The process was then scaled up using a stacked electrolyzer system with three modules and a larger electrode area to treat actual reverse osmosis (RO) retentate from a desalination plant. The performance of the scaled system was evaluated and compared with the results from the synthetic waste streams. Detailed electrochemical analyses, including current-time curves and pH measurements, were conducted throughout the experiments. Techno-economic analysis was performed to evaluate the profitability of this technology and demonstrate the system’s flexibility in producing different ammonium salts using different acids for ammonia capture. Further experiments were carried out using a single-pass mode to assess the feasibility of application in various flow rates.

The membrane-free gas-extraction electrodes effectively balance NH₃ and Cl₂ production and separation. High NH₃ and Cl₂ separation efficiencies (90% and 99%, respectively) were achieved under optimized conditions. Simultaneous NH₃ and Cl₂ electrosynthesis resulted in minimal product loss, with final concentrations in trap solutions close to baseline values obtained during single-electrosynthesis. Control experiments confirmed that the separation process significantly reduced product loss caused by the undesired reaction between NH₃ and Cl₂. A stacked electrolyzer system with three modules efficiently processed real reverse osmosis retentate wastewater, achieving high concentrations of (NH₄)₂SO₄ (83.8 mM) and NaClO (243.4 mM) at an electrical cost of 7.1 kWh/kg of products. Residual concentrations of NH₃/NH₄⁺, NO₂⁻, and Cl₂/HClO/ClO⁻ in the treated effluent were all below regulatory limits for wastewater discharge. The techno-economic analysis demonstrated the economic viability of this method, particularly when scaled up to industrial levels. Experiments using different acids for ammonia capture showed comparable efficiency, highlighting the system’s adaptability. The single-pass mode demonstrated the efficacy of the system while highlighting the need for flow rate control to comply with discharge regulations.

The results demonstrate the feasibility of a highly efficient and cost-effective approach for ammonia and chlorine production from waste streams. The integrated electrosynthesis and separation process significantly reduces product loss compared to conventional methods. The successful application of the technology to real reverse osmosis retentate highlights its potential for industrial-scale implementation. The economic analysis shows profitability even with relatively high electricity costs. This approach is an important contribution towards a circular economy by transforming waste streams into valuable products, aligning with sustainability goals.

This study presents a novel membrane-free electrolyzer for efficient and sustainable ammonia and chlorine production from nitrate and chloride-containing waste streams. The integration of electrosynthesis and separation maximizes product yield and minimizes environmental impact. Successful scaling to a three-module system treating real wastewater shows strong potential for industrial applications. Future research should explore broader applications, optimize reactor designs, and integrate pre-concentration processes for dilute waste streams. This technology offers economic and environmental benefits by converting waste into valuable resources.

The current study focused on waste streams with relatively high nitrate and chloride concentrations. Further research is needed to optimize the process for wastewaters with lower concentrations. The techno-economic analysis is based on current market prices and electricity costs; changes in these factors could affect profitability. The long-term stability and durability of the gas-extraction electrodes under continuous operation require further investigation. The single-pass mode experiments, while demonstrating feasibility, highlighted the need for careful control of flow rates to maintain compliance with effluent regulations.