Zero-Gap Alkaline Water Electrolysis Using Ion-Solvating Polymer Electrolyte Membranes at Reduced KOH Concentrations

Abstract

Membranes based on poly(2,2 -(m-phenylene)-5,5-bibenzimidazole) (m-PBI) can dissolve large amounts of aqueous KOH to give electrolyte systems with ion conductivity in a practically useful range. The conductivity of the membrane strongly depends on the concentration of the aqueous KOH phase, reaching about 10 −1 S cm −1 or higher in 15-25 wt% KOH. Herein, m-PBI membranes are systematically characterized with respect to performance and short-term stability as electrolyte in a zero-gap alkaline water electrolyzer at different KOH concentrations. Using plain uncatalyzed nickel foam electrodes, the cell based on m-PBI outperforms the cell based on the commercially available state-of-the-art diaphragm and reaches a current density of 1500 mA cm −2 at 2.4 V in 20 wt% KOH at 80 • C. The cell performance remained stable during two days of operation, though post analysis of the membrane using size exclusion chromatography and spectroscopy reveal evidence of oxidative degradation of the base polymer at KOH concentrations of 15 wt% and higher. Converting surplus electrical energy into hydrogen by water electrolysis represents an attractive approach to balance the electric grid when an increasing fraction of the power input originates from fluctuating renewable sources.1 Characterized by their robustness and durability, alkaline electrolyzers have been available for a long time on a commercial basis for large scale hydrogen and oxygen production through electrochemical water splitting.2,3 The alkaline environment allows for a cell construction completely free from noble metals, where the oxygen 4-6 and hydrogen 7-9 evolution reactions readily occur on nickel and other transition metal-based materials. The electrodes are separated by an aqueous solution of potassium hydroxide confined in a porous diaphragm to reduce the intermixing of the product gasses. Between the diaphragm and the electrode is an electrolyte-filled gap to facilitate gas removal. This cell design gives an interelectrode distance in the range of a few millimeters and therefore a relatively large internal resistance, 3,10 even though the specific conductivity of the aqueous potassium hydroxide electrolyte used is remarkably high. Replacing the diaphragm with an ion-conducting membrane represents a new direction in the development of advanced alkaline electrolyzers as recently discussed by Pletcher and Li. 23 It allows for a zero-gap design with an inter-electrode distance of less than 100 μm, in which the membrane is sandwiched in between two porous electrodes. One approach in this connection is to use alkaline ion exchange membranes (AEM) based on quaternary ammoniumfunctionalized polymers, [24][25][26] but improving the stability of the polymer backbone 27 as well as of the anion exchange moieties 28 are apparent challenges.An alternative approach is to use a membrane based on an aqueous electrolyte dissolved in a polymer matrix. The ion-solvating polymer electrolytes are often soluble in water, 29 but as first demonstrated by Xing and Savadogo, 30 poly(2,2...

Blend membranes of polybenzimidazole and an anion exchange ionomer (FAA3) for alkaline water electrolysis: Improved alkaline stability and conductivity

“…In previous work, it was shown that the conductivity of KOH doped mPBI peaks at KOH concentrations of about 20-25%. [19,20,27,28]. After doping at 25wt% it reached about 0.1 S cm -1 [25], which was confirmed in the present work (Figure 6a).…”

“…This electrolyte system has been extensively evaluated for hydrogen [19] and direct alcohol [22][23][24] fuel cells. Recently, membranes of various polybenzimidazole chemistries [25][26][27][28][29] and polybenzimidazole-based polymer blend membranes [30] have been investigated as separator in alkaline water electrolysis. The reported data indicate reduced ohmic losses in the higher current density range, which clearly 4 demonstrate the feasibility of the concept.…”

“…However, an aging test at 88 °C indicated severe stability limitations of mPBI in KOH at higher KOH concentrations, as revealed by gradually decreasing molecular weight due to chain scission [31]. Furthermore, the degradation rate was increased during cell operation [28]. The assumed degradation mechanism is attack of hydroxide on the C2 position of imidazole, leading to ring opening and ultimately chain scission.…”

Ion-solvating membranes as a new approach towards high rate alkaline electrolyzers

“…The KOH/m-PBI system was first described by Xing and Savadogo 15 in 2000 for fuel cell applications, and we conducted the first demonstration for water electrolysis in 2013. 16 The membranes achieve the highest specific conductivity when equilibrated in bulk electrolyte concentrations in the range of 20-25 wt% KOH 17 and exhibit remarkable, though not complete stability, for up to six months in concentrated KOH. 18 They do, however, in contrast to AEMs, show negligible conductivity in KOH concentrations below 5 wt%.…”

Straight-Parallel Electrodes and Variable Gap for Hydrogen and Oxygen Evolution Reactions

“…When a concentration of 20-25 wt % aqueous KOH is analysed the best results are obtained. With 5,15, and 30 wt % the operation of the electrolyser is stable but its subsequent analysis evidenced oxidative degradation of m-PBI in 15 and 30 wt % KOH [16].…”