Efficient caustic and hydrogen production using a pressurized flow-through cathode

Abstract

The emerging process of CO₂ capture and sequestration will likely require large volumes of caustic. The fossil fuel demand and carbon footprint of transporting liquid caustic is self-defeating, and hence, there is a need for energy-efficient, on-site caustic production for carbon capture projects. Caustic production is dominated by the well-established “chlor-alkali” process. This process requires highly concentrated (∼25% w/v) and pure (>99.5 wt%) NaCl feed brines, uses high-cost ion-exchange membranes and high operating temperatures (90 °C), and generates a highly-concentrated (>33% w/v) caustic stream that can be further concentrated using thermal evaporation. This highly concentrated caustic is then shipped to customers, where it is typically diluted to the required level. We have developed a flow-through membrane/cathode electrolysis process that produces a caustic solution (pH 10.22–12.26) at a specific energy consumption (SEC) of 1.71 kW h_e kg⁻¹ NaOH at room temperature using a 3.5% w/v NaCl solution as feed, while achieving pure H₂ generation without the use of ion exchange membranes. We demonstrate that the SEC is strongly dependent on the flow rate through the cathode, reaching a minimum at a high rate of 1200 L m⁻² h⁻¹. Electrochemical impedance spectroscopy, confocal microscopy, and finite element modeling show that the SEC is lowered through a combination of enhanced mass transport (of H⁺ and OH⁻ ions) to and from the cathode surface and H₂ gas stripping, both facilitated by the high flow rates. This technology offers the opportunity for the on-site production of dilute caustic streams (potentially from softened seawater) at a significantly reduced energy cost (compared to conventional chlor-alkali processes that consume >2.1 kW h_e kg⁻¹ NaOH).