Hydrogen Production through a Multi-Ion Exchange Membrane Based Electrolysis System

Abstract

Reverse electrodialysis (RED) is a membrane based electrochemical system used for capturing a portion nearly 2.6 TW of power released through the mixing of sea and river water at global estuaries[1]. This technology encompasses a series of alternating anion and cation exchange membranes (AEM and CEM) which actively separate channels with high and low concentration solutions (0.5 M and 0.011 M NaCl). Creating a concentration gradient across each ion selective membrane, generates a Donnan or Membrane potential. Typically, one cell pair (three membranes, and one high and low concentration chamber) produced an open circuit voltage of 120 mV. As more membranes are added to the system, the generated potential increases and drives faradic reactions at electrodes on either end of the membrane stack. To date, most RED work has focused on utilizing a large numbers of membranes to produce electrical power through a galvanic cell approach. Due to this focus, traditionally low overpotential reactions are preferred rather than the water splitting reaction (oxygen evolution and hydrogen evolution). Here we investigate the potential for using the RED electrochemical platform for hydrogen production through an electrolysis approach[2, 3]. The use of multiple monopolar membranes and multiple chambers, allows a sustained pH gradient to be maintained across the cell, minimizing the whole cell voltage necessary for hydrogen production. When the cell was operated with a pH gradient equal to 10 pH units, the steady state whole cell onset potential decreased by 700 mV when compared to no pH gradient. This represented a 33% decrease in the energy consumption for the water splitting reaction. Additionally, the steady state current density (11.7 mA/cm2) was achieved using an acidic cathode and basic anode, which was 12 x greater than no pH gradient. We also evaluate the potential tradeoffs between added ohmic resistances, decreased onset potential, and increased membrane potentials within this architecture. 1. Logan, B.E. and M. Elimelech. Nature, 2012. 488(7411): p. 313-319. 2. Hatzell, M.C., I. Ivanov, R.D. Cusick, X. Zhu, and B.E. Logan. Physical Chemistry Chemical Physics, 2014. 16(4): p. 1632-1638. 3. Hatzell, M.C., X. Zhu, and B.E. Logan. ACS Sustainable Chemistry & Engineering, 2014. 2(9): p. 2211-2216.