Salinity gradient power-reverse electrodialysis and alkaline polymer electrolyte water electrolysis for hydrogen production

In this work, innovative use of Salinity Gradient Power (SGP) as renewable energy source for indirect production of hydrogen is addressed. A lab-scale reverse electrodialysis (RED) unit, fed with different NaCl solutions mimicking highly concentrated brine (5 M), Reverse Osmosis retentate (1 M), seawater (0.5 M) and brackish water (0.1 M), was coupled to an alkaline polymer electrolyte (APE) water electrolysis cell. SGP-RED unit, equipped with 27 cell-pairs, reached at best an Open Circuit Voltage (OCV) of 3.7 V and maximum gross power density of 3.2 W m⁻²MP (membrane pair) when feeding the low concentration compartment (LCC) with 0.1 M NaCl and the High Concentration Compartment (HCC) with 5 M NaCl. The single-cell APE water electrolysis unit, operated at 1.8 V, attained a current density of 120 mA cm⁻² under the following configuration: 10% w/w KOH electrolyte, highly conductive anion selective membrane composed of inert low-density polyethylene, finely milled anion selective particles and water-soluble poly (ethylene glycol-ran-propylene glycol), non-Platinum catalysts (NiCo₂O₄ and NiFe₂O₄) loading of 10 mg cm⁻² and 15%w/w polymer binder at both cathode and anode, and operational temperature of 65 °C. The integrated system resulted in a maximum hydrogen production rate of 44 cm³ h⁻¹ per cm² of electrode surface area.