Separating hydrogen and oxygen evolution in alkaline water electrolysis using nickel hydroxide

Low-cost alkaline water electrolysis has been considered a sustainable approach to producing hydrogen using renewable energy inputs, but preventing hydrogen/oxygen mixing and efficiently using the instable renewable energy are challenging. Here, using nickel hydroxide as a redox mediator, we decouple the hydrogen and oxygen production in alkaline water electrolysis, which overcomes the gas-mixing issue and may increase the use of renewable energy. In this architecture, the hydrogen production occurs at the cathode by water reduction, and the anodic Ni(OH)2 is simultaneously oxidized into NiOOH. The subsequent oxygen production involves a cathodic NiOOH reduction (NiOOH→Ni(OH)2) and an anodic OH− oxidization. Alternatively, the NiOOH formed during hydrogen production can be coupled with a zinc anode to form a NiOOH-Zn battery, and its discharge product (that is, Ni(OH)2) can be used to produce hydrogen again. This architecture brings a potential solution to facilitate renewables-to-hydrogen conversion.

According to the review article 1 , Co 3 O 4 -based anode for OER and metal Ni-based cathode for HER have been widely applied for alkaline water electrolysis. Therefore, we employed Co 3 O 4 and Ni-foam as OER and HER electrodes, respectively, to further investigate the separate steps. In this experiment, commercialized Co 3 O 4 powder was treated by ball milling for 4 hours, and then was used to fabricate OER electrode. The Co 3 O 4 -based OER electrode was obtained by mixing 80 wt % Co 3 O 4 powder, 10 wt % Ketjen Black (KB) as conductive agent, and 10 wt % polytetrafluoroethylene (PTFE) as binder. For a typical preparation, Co 3 O 4 , KB, and PTFE were dissolved in isopropanol to form a slurry with the weight ratio mentioned above, and then the slurry was rolled into a film. Finally, the film was pressed on stainless steel mesh to form OER electrode (2.5×4 cm 2 ). Commercialized Ni-foam was directly used as the HER electrode (2.5×4 cm 2 ). An alkaline water electrolytic cell was constructed with a Co 3 O 4 -electode for OER, a commercial Ni-foam electrode for HER and a Ni(OH) 2 electrode (See Supplementary  Movie 3 and 4). Water electrolysis of the cell was investigated by chronopotentiometry measurements with an applied current of 200 mA. Chronopotentiometry curve (cell voltage vs. time) of the electrolytic cell is shown in Supplementary Figure 13a. The chronopotentiometry curve of the cell using precious electrodes tested at the same condition is also shown in Supplementary Figure 13a for comparison. In addition, OER potential on Co 3 O 4 electrode and HER potential on Ni-foam electrode were investigated, in comparison with that on precious electrodes (see Supplementary Figure 13b). It can be observed from Supplementary Figure 13a that when using non-precious electrodes, the electrolysis process still includes two separate steps (Step 1 and 2) with different cell voltages. However, the cell voltages of the cell using non-precious electrodes (0.526V on step 1 and 1.611 V on step 2) are higher than that of the cell using precious electrodes (0.432V on step 1 and 1.553 V on step 2), which is owing to the lower catalytic ability of non-precious electrodes. As shown in Supplementary Figure 13b, the OER potential on Co 3 O 4 electrode (0.788 V vs. Hg/HgO) is higher than that on commercialized RuO 2 /IrO 2 coated Ti-mesh (0.691 V vs. Hg/HgO). The HER potential on Ni-foam electrode (-1.177 V vs. Hg/HgO) is lower than that on commercialized Pt coated Ti-mesh (-1.124V vs. Hg/HgO). The cycle of step 1 (H 2 production) and step 2 (O 2 production) in the cell using non-precious electrodes was also investigated with an applied current of 200 mA (Supplementary Figure 13c). The achieved cycle performance is similar to that achieved by the cell using precious electrodes. In addition, the video evidence was also given in Supplementary Movie 3 and 4 to confirm the separate H 2 /O 2 generation in the cell using non-precious electrodes.
Supplementary Figure 14|Driven voltage comparison at 200 mA between one-step system and two-step system. (a) one-step electrolysis using precious electrodes. (b) two-step electrolysis using precious electrodes. (c) one-step electrolysis using non-precious electrodes. (d) two-step electrolysis using non-precious electrodes.
In this experiment, chronopotentiometry measurement with an applied current of 200 mA was employed to investigate the one-step electrolysis process that is based on a commercialized RuO 2 /IrO 2 coated Ti-mesh anode (2.5×4 cm 2 ) for OER and a commercialized Pt coated Ti-mesh cathode (2.5×4 cm 2 ) for HER in an alkaline medium. The achieved chronopotentiometry curve of one-step electrolysis is shown in Supplementary Figure 14a, where it can be detected that the cell exhibits a voltage of 1.829 V with the applied current of 200 mA. The chronopotentiometry curve tested at 200 mA of two-step system using precious electrodes [RuO 2 /IrO 2 coated Ti-mesh electrode (2.5×4 cm 2 ) , Pt coated Ti-mesh electrode (2.5×4 cm 2 ) and Ni(OH) 2 electrode (2.5×4 cm 2 )] is shown Supplementary Figure 14b, where it can be observed that the two steps display a total cell voltage of 1.985 V (1.553 + 0.432 V). Therefore, the efficiency of the two-step cell using precious electrodes should be 92% (=1.829/1.985) compared to corresponding one-step system. In addition, the one-step system that is based on non-precious electrodes [Co 3 O 4 -based anode (2.5×4 cm 2 ) + Ni-foam cathode (2.5×4 cm 2 )] was investigated by chronopotentiometry measurement with an applied current of 200 mA (Supplementary Figure 14c). It can be observed from Supplementary Figure 14c that the one-step system using non-precious electrodes exhibits a cell voltage of 1.973 V. At the same test condition, the two-step system using non-precious electrodes displays a total cell voltage of 2.137 V (1.611 + 0.526 V) (Supplementary Figure 14d). The efficiency also is about 92% (=1.973/2.137) compared to corresponding one-step system. Self-discharge is a very common phenomenon for batteries. A very limited self-discharge is a quality index of any successfully commercialized battery. Ni(OH) 2 -based rechargeable batteries (such as nickel metal hydride (Ni-MH) batteries and nickel-cadmium (Ni-Cd) batteries) have been commercialized for a very long time, and still play an important role on current battery market. Therefore, it is undoubted that the self-discharge of nickel hydroxide electrode is very limited. We believe that a lot of readers have the experience that their full charged Ni-MH or Ni-Cd batteries still can work even after several weeks (or months) rest. To clarify this point, self-discharge performance of a NiOOH electrode (2.5×4 cm 2 ) was investigated with three-electrode method.
Firstly, the Ni(OH) 2 electrode was cycled with an applied current of 100 mA without any rest time between charge and discharge (Supplementary Figure 16a). In the experiment, the Ni(OH) 2 was charged for 2.25 hours, and then was directly discharged to 0 V (vs. Hg/HgO). As shown in Supplementary Figure 16a Figure 19, the active materials for the charge/discharge cycle are H 2 O, Ni(OH) 2 and Zn. According to these electrode reaction equations, it need 1 mol H 2 O (18g), 1 mol Ni(OH) 2 (92.7g) and 0.5 mol Zn (0.5× 65.38g) to store/deliver 1 mol electron (= 96500 C). Therefore, the energy density of the recharge cycle can be calculated by following equation

As shown in Supplementary
Herein, E is the energy density (Wh kg -1 ), Q is the quantity of electricity (96500C),V is the average discharge OER electrode) through the reaction of [2OH --2e -→ 1/2O 2 + H 2 O]. As mentioned above, the nickel hydroxide (Ni(OH) 2 /NiOOH) electrode is used as "a solid-state proton buffer" that can be moved between room-1 for H 2 production (step 1) and room-2 for O 2 production (step 2).