The global transition to a low-carbon economy necessitates large-scale green hydrogen production. Water electrolysis powered by renewable energy sources presents a promising solution, but current methods face challenges like slow power-to-hydrogen (P2H) conversion rates and high water consumption. Many existing P2H projects, particularly in water-stressed regions, highlight the critical need for alternative water sources and enhanced efficiency. Direct seawater electrolysis suffers from short electrolyzer lifespans and low hydrogen production rates due to issues such as cation precipitation, electrode corrosion, and chlorine byproduct formation. Pre-treatment of seawater using reverse osmosis (RO) improves the situation, but membrane fouling remains a problem. Forward osmosis (FO) offers a potential solution by mitigating membrane fouling, but its coupling with RO increases costs. Recent membrane-assisted seawater electrolysis approaches, while innovative, still yield low hydrogen production rates due to limitations in water flux and electrolyte conductivity. This research focuses on addressing these challenges by employing a novel FOWSAWE system. The hypothesis is that using KOH as both the draw solution in FO and the electrolyte in AWE will significantly improve hydrogen production rates. KOH facilitates high current densities in AWE and provides a suitable osmotic gradient for efficient water extraction from wastewater when paired with a compatible FO membrane. Utilizing wastewater effluent instead of seawater is proposed as a more sustainable and practical approach given its lower osmolarity and reduced chloride content, thus minimizing chlorine evolution and costly brine disposal. This study aims to investigate the optimal selection of FO membrane materials and electrolytes, the effects of wastewater impurities on hydrogen production, and the overall system stability and scalability.

The literature extensively discusses various approaches to green hydrogen production, highlighting the significant potential of water electrolysis powered by renewable energy. However, the intermittency of renewable energy sources necessitates efficient energy storage and utilization methods, making P2H systems crucial. Existing P2H projects globally are reviewed, indicating significant growth but also raising concerns about water resource management, particularly in water-stressed regions. Various strategies for using alternative water sources, such as seawater, are examined. Direct seawater electrolysis and seawater desalination using RO, are discussed. Studies that suggest using FO as a pretreatment step for RO are also reviewed, focusing on the trade-offs between fouling mitigation and increased costs. Finally, recent advances in membrane-assisted seawater electrolysis using hydrophobic membranes and FO coupled with water electrolysis are evaluated. These approaches, although promising, face limitations in hydrogen production rates due to low water flux and conductivity issues.

The research employed a FOWSAWE system integrating forward osmosis (FO) and alkaline water electrolysis (AWE). Three electrolytes (KOH, NaHCO3, and K4P2O7) were initially assessed for their suitability as draw solutions in FO. KOH was selected based on its superior current density in AWE, its effectiveness as a draw solution in FO, and its compatibility with the chosen TFC-FO membrane. The impact of KOH concentration on membrane integrity was also considered, selecting 1M KOH as the optimal concentration. Water flux and reverse salt flux (RSF) were measured over five cycles of 5 hours each to assess FO membrane performance with 1M KOH. Membrane integrity after these cycles was analyzed through SEM and FTIR, comparing it to a pristine membrane. Impurity rejection rates for NaAc, NH3/NH4+, and chloride were determined. Electrolysis performance was evaluated using a 5-cell alkaline stack with nickel alloy electrodes, comparing current density-voltage (J-V) curves generated from 1M KOH with and without various impurities. A water-hydrogen balance model was developed to dynamically balance water production from FO (QFO) and water consumption from AWE (QAWE), ensuring continuous operation. This model established a relationship between the specific normalized current and the concentration gradient to achieve equilibrium in the FOWSAWE system. The integration of FO and AWE modules was experimentally validated using deionized water and wastewater effluent. Continuous operation was tested over two cycles (5 hours each) and for a prolonged period of 168 hours. Hydrogen production rates, purity, and specific energy consumption (SEC) were compared with other membrane-based P2H systems, including seawater electrolysis using PTFE membranes and FOWS using CTA membranes. The impact of temperature on the FOWSAWE system was investigated by utilizing waste heat from AWE to elevate the KOH temperature. Wastewater samples from various regions in China were used to assess the system's adaptability to different wastewater conditions.

The FOWSAWE system demonstrated exceptionally high hydrogen production rates (448 Nm³/day/m²), significantly exceeding state-of-the-art methods by a factor of 10-14. The use of 1M KOH as both the draw solution and electrolyte was key to this success. The chosen TFC-FO membrane showed excellent KOH tolerance, maintaining its integrity and exhibiting stable water flux and low reverse salt flux even after prolonged use. Impurity rejection by the FO membrane was high for most components, and the presence of these impurities had a minimal impact on electrolysis efficiency. The water-hydrogen balance model accurately predicted the system's dynamic equilibrium, enabling stable and continuous hydrogen production. The system exhibited remarkable stability over multiple cycles (both short-term and 168-hour continuous operation) maintaining a stable cell voltage (1.79 ± 0.01 V) and high Faradaic efficiency (~99%). The SEC was exceptionally low (4.43 kWh Nm⁻³ at 23 °C and 3.96 kWh Nm⁻³ at 40 °C), comparable to commercial alkaline electrolysis using deionized water. The system showed adaptability to various wastewater conditions, maintaining consistent performance across different wastewater samples from China. The use of waste heat from AWE further improved the hydrogen production rate and reduced the SEC. The overall superior performance of the FOWSAWE system highlights its potential for large-scale, sustainable green hydrogen production.

The findings demonstrate the feasibility of high-rate, energy-efficient green hydrogen production directly from wastewater. The significant improvement in hydrogen production rate compared to existing membrane-based P2H systems is primarily attributed to the synergistic combination of a highly efficient FO process using KOH and a highly efficient AWE process. The use of wastewater as a feedstock enhances the sustainability and economic viability of the process by reducing freshwater consumption and avoiding costly brine disposal. The water-hydrogen balance model provides a robust framework for scaling up the system to meet varying demands, suggesting that this technology is highly scalable and can be adapted for various applications ranging from household to city-level hydrogen production. The low SEC achieved in this study is highly competitive, indicating its potential for commercial implementation. The results highlight the importance of considering both membrane properties and electrolyte selection for optimizing P2H systems. The combination of a hydrophilic TFC-FO membrane and KOH electrolyte creates a highly efficient system, surpassing the limitations of existing technologies. The adaptability of the system to various wastewater conditions further strengthens its applicability in diverse geographical locations.

This study presents a groundbreaking FOWSAWE system for ultra-fast and energy-efficient green hydrogen production directly from municipal wastewater. The system achieves record-high hydrogen production rates and low SEC, demonstrating its significant potential for large-scale, sustainable hydrogen production. The developed water-hydrogen balance model facilitates scalable design, making the system adaptable to different water sources and operational scales. Future research could focus on optimizing membrane fouling mitigation strategies for even longer-term operation and exploring the integration of this technology with various wastewater treatment systems to create truly circular economies.

While the FOWSAWE system shows remarkable promise, some limitations need to be acknowledged. The long-term study (168 hours) observed a slight decrease in water flux due to membrane fouling, although this fouling appeared to be reversible. The study primarily focused on specific wastewater compositions; further investigation is needed to evaluate its performance with a wider range of wastewater characteristics and potential variations in influent water quality. The scalability of the system beyond the laboratory scale remains to be demonstrated through pilot-scale testing and economic analysis to fully assess its commercial viability.