Synergistic Osmosis-Electrolysis Integration for Sustainable Hydrogen Production from Saline Water

Green hydrogen (H2) production by water electrolysis that utilizes renewable electricity has been recognized as a sustainable route towards decarbonizing the global energy system. Due to potable water scarcity in arid regions, using seawater or impaired water (henceforth, saline water) as feedstock is a promising way to generate clean hydrogen energy without stressing limited freshwater resources. Current research efforts have been focused on developing electrode materials and membrane technologies that improve the efficiency of direct saline water electrolysis. Yet, using saline water as the direct feed to the electrolyzer continues to face several challenges, with the major one being due to presence of high chloride concentrations, which leads to the undesired chlorine evolution reaction (CER) at the anode. Recent studies reported that this problem can be reduced by feeding seawater only to the cathode compartment, which requires the use of bipolar membranes (BPM) in an asymmetric-electrolyte electrolyzer. However, this approach is still problematic because saline water in the cathode compartment can lead to: i) corrosion of electrodes and other equipment, ii) precipitation of calcium and magnesium salts, which leads to electrode deactivation and fouling of membranes, and iii) accumulation of salts inside the electrolyzer as water is being converted to H2 and O2. Herein, we propose to overcome these challenges by studying and developing a pressure-retarded osmosis (PRO) system coupled with an asymmetric BPM water electrolyzer (BPMWE). In this combined process, an alkaline electrolyte is the anolyte and the PRO draw solution is the BPMWE catholyte. The draw solution is circulated between the PRO and BPMWE in a closed loop. It is diluted in the PRO unit as water is drawn into it from the saline source water through the PRO membranes. It is concentrated in the BPMWE as water is removed by conversion to H2 and O2. This approach allows saline waters to be used as the water source without the problems associated with feeding these waters directly to the electrolyzer. Also, no energy input would be required for desalination. Moreover, the PRO process will maintain high pressure of the BPMWE feed, enabling the production of compressed H2. The proposed research is based on five pillars: 1) Advancing the state of knowledge on design and manufacture of highly efficient and stable BPMWE tailored for this application, 2) Identifying optimum draw solutes and operating conditions that enhance water flux and solute rejection in the PRO unit, and accelerate water electrolysis kinetics in the BPMWE unit, 3) Development of robust PRO membranes tailored for this application, 4) Construction and performance evaluation of hybrid PRO-BPMWE system, and 5) Techno-economic and life cycle analysis of the proposed system. We will develop a fundamental understanding of how the hybrid system operates, and use it to optimize its application to these complex source waters.

Hamad Bin Khalifa University (HBKU)