Alkaline water electrolysis

This chapter is based on results reported in publication:

J. Žitka, J. Peter, B. Galajdová, L. Pavlovec, Z. Pientka M. Paidar, J. Hnát, K. Bouzek: Anion exchange membranes and binders based on polystyrene-block-poly(ethylene-ran-butylene)block- polystyrene copolymer for alkaline water electrolysis, Desalination and Water Treatment, 142, 90-97 (2019)

doi:10.5004/dwt.2019.23411

A single-cell alkaline laboratory electrolysis was used to test the performance under alkaline water electrolysis conditions. Ni foam electrodes (INCO Advanced Technology Materials, Dalian Co., Ltd, Dalian, China) with pore size 580 mm and geometric area of 1.9 × 1.9 cm² were used as a catalytic layer support. As an anode, electrocatalyst served NiCo2O4 spinel prepared by a direct thermal decomposition of the Ni(NO3)2·H2O and Co(NO3)2·H2O (53). Catalytic ink was prepared by mixing 10 mL of chloroform (CHCl3), 5% solution of PSEBS-CM in CHCl3, and 0.192 g of catalyst powder. The ink was sprayed onto the surface of Ni foam using an airbrush.

Ni foam was heated at 50°C to accelerate chloroform evaporation and thus to prevent penetration

of the catalyst into the bulk of Ni support. The coated Ni foam was immersed into the solution of TMA in ethanol for 48 h to convert PSEBS-CM into PSEBS-CM-TMA. After removal from solution of TMA, the electrode was washed with demineralized water. The resulting anodes contained 8 mg cm^-2 of NiCo2O4 spinel electrocatalyst and 0.42, 0.88, or 2.67 mg cm^–2 of PSEBS-CM-TMA, respectively. For comparison, an anode containing 8 mg cm^–2 of NiCo2O4

and 2.67 mg cm^–2 of PTFE was used. The cathode contained catalyst layer composed of 0.3 mg Pt cm^–2 and 0.05 mg cm^–2 of PTFE binder. Operational temperature was 70 °C during load curve measurement or 50 °C in a long-term experiment. As a separator of the anode and cathode compartment, heterogeneous anion-selective membrane (54), (55) during load curve measurement or a PSEBS-CM-TMA membrane prepared within this work in a long-term experiment was used. The electrodes were attached directly to the membrane surface (zero-gap assembly). The rear sides of the electrodes were washed with tested electrolyte solution. The scheme of the electrolysis process with the role of the anion-selective membrane can be seen in Figure 24.

Figure 24 The scheme of the alkaline water electrolysis in zero-gap arrangement for a case of 10 wt.% KOH

PSEBS-CM-TMA was tested as a catalyst binder on the anode of an alkaline laboratory electrolyzer. Anodes with PSEBS-CM-TMA binder were compared with an anode with a standard polytetrafluoroethylene (PTFE) binder and the anode comprising only Ni foam. The influence of the amount of PSEBS-CM-TMA in the catalytic layer (anodic) was also tested.

Cathode catalytic layer was formed with Pt catalyst in PTFE binder and was the same in all experiments. Performance of electrolyzers with different anode compositions are shown in Figure 25. It is plotted as dependence of the current density obtained for particular anode composition at cell voltage 1.74 V depending on the concentration of the KOH liquid electrolyte.

The value of the cell voltage 1.74 V was chosen because it corresponds to the 85%

thermodynamic efficiency when compared with the thermoneutral voltage of water decomposition (1.48 V at 25°C). It can be seen that the anodes modified by catalyst bonded by PSEBS-CM-TMA binder greatly outperform not only those containing bare Ni foam but also the Ni foam/NiCo2O4 + PTFE binder. This is unambiguously due to the improved utilization of the NiCo2O4 catalyst allowing decrease in overpotential of the oxygen evolution reaction.

Figure 25 Current density achieved by laboratory alkaline water electrolysis cell with different anodes. Cell voltage 1.74 V, 70 °C, separator – heterogeneous anion-selective polymer membrane, cathode – 0.3 mg Pt cm^-2 + 0.05 mg PTFE cm^-2, liquid electrolyte - indicated on x axis. Anode – (a) 8 mg NiCo2O4 cm^-2 + 0.42 mg PSEBS-CM-TMA cm^-2, (b) 8 mg NiCo2O4 + 0.88 mg PSEBS-CM-TMA cm^-2, (c) 8 mg NiCo2O4 + 2.67 mg PSEBS-CM-TMA cm^-2, (d) 8 mg NiCo2O4 + 2.67 mg PTFE cm^-2, (e) bare Ni foam

The reason for better utilization of the catalyst when using the PSEBS-CM-TMA polymer binder instead of PTFE binder consists in the nature of both the materials. While the PTFE is the inert material which only can ensure the mechanical stability of the catalyst layer, the novel synthetized PSEBS-CM-TMA is able to ensure mechanical properties of the catalyst layer as well as the IC inside this layer. The later mentioned ability represents significant advantage to the inert PTFE binder. It significantly increases probability of the three-phase boundary occurrence in the catalytic layer. Three-phase boundary indicates part of the catalyst particle in contact with ionic conductor (electrolyte membrane), electron conductor (electrode), and reactant at the same time. The desired electrode reaction, i.e., oxygen evolution, can take place just on such places. The contact with reactant is easily achieved due to the presence of the liquid water in the system. The ionic contact represents more difficult point. The high concentration of the KOH in circulating water represents one possibility how to achieve a good ionic conductivity.

On the other hand, this is connected with the lower flexibility of the water electrolyzer and higher safety risk. The other possibility can be in utilization of the anion-selective material as a polymer binder. Such material should be able to ensure the ionic conductivity even in the environment of the diluted liquid electrolyte. Membrane PSEBS-CM-TMA fulfilled this role in present case.

The second difference in the nature of both used polymer binders consists in an affinity to the water. The PTFE represents the hydrophobic material which repeals the molecules of water. This may result in the accumulation of the evolved oxygen in the catalytic layer structure and subsequent disturbance of the water molecule supply. The PSEB-SCM-TMA is, in contrast, the hydrophilic material which will ensure better access of the water to the catalyst.

The best results were achieved using anode with the smallest used amount of PSEBS-CM- TMA in the catalytic layer. It is due to the fact, that a small amount of anion-selective polymer binder increases ionic conductivity of the catalyst particles, but it does not interfere with the transport of electrons and reactants. Figure 26 shows full load curves of the alkaline water electrolysis dependent on electrolyte concentration. Anode catalyst layer contained optimal concentration of PSEBS-CM-TMA binder (Figure 25). It can be seen that with increasing KOH concentration, the electrolyzer performance improves. This indicates that even an anion-selective

binder is not able to ensure a perfect ionic conduction between the electrodes and in the catalytic layer (though the improvement is great).

For comparison, Figure 26 shows the region of current densities achieved by commercial liquid alkaline electrolysis (56) which are usually operated with 25 - 30 wt.% aqueous KOH.

This comparison shows that the electrolysis with anion-selective binders outperform classical commercial electrolysis at least on a laboratory scale.

Figure 26 Load curves of the alkaline water electrolysis. Anode - 8 mg NiCo²O4 cm^-2 + 0.42 mg PSEBS-CM-TMA cm^-2, cathode - 8 mg NiCo2O4 cm^-2 + 0.42 mg PSEBS-CM-TMA cm^-2; 70 °C.

The values typical of industrial water electrolyzers are bordered by a dashed line. (a) 15 wt.%

KOH, (b) 10 wt.% KOH, (c) 5 wt.% KOH, (d) 1 wt.% KOH, (e) deionized water

Long-term stability of PSEBS-CM-TMA in an alkaline environment was tested in a laboratory electrolyzer where PSEBS-CM-TMA membrane was used as a separator of the anode and cathode compartments. Operation temperature was 50 °C, and the electrolyte was 10 wt.%

KOH.

Figure 27 shows time dependence of the cell voltage during the long-term alkaline water electrolysis experiment. In the first 80 h, the cell voltage increased from initial 1.76 to 1.84 V

followed by the decrease back to 1.76 V in next 80 h. This is most probably connected with the establishment of the equilibria in the system including phenomena such as activation of the catalysts, flooding the catalytic layers, establishing dynamic equilibrium between liquid electrolyte, and produced gases in the catalytic layers, equilibrating the temperature, etc.

Nevertheless, with respect to the average value and standard deviation of the cell voltage (average value showed in the Figure 27 as dashed line) reaching 1.78 ± 0.13 V, the maximum observed deflection is about 60 mV. It represents deviation of up to 4%, which can be considered with respect to its character as a stable system. The performance of the electrolyser can thus be considered as stable even after more than 800 h of operation. This shows a good stability of trimethyl benzyl ammonium groups on styrene block copolymers in alkaline environment as well as a good mechanical stability of PSEBS-CM-TMA membrane.

Figure 27 Voltage vs time. Current density 0.3 A cm^-1. Single-cell alkaline laboratory electrolyzer; PSEBS-CM-TMA membrane, anode and cathode: see captions to Fig. 26; 10 wt.%

aqueous KOH electrolyte, 50 °C.