Comprehensive impedance investigation of low-cost anion exchange membrane electrolysis for large-scale hydrogen production

www.nature.com/scientificreports OPEN Comprehensive impedance investigation of low‑cost anion exchange membrane electrolysis for large‑scale hydrogen production Immanuel Vincent, Eun‑Chong Lee & Hyung‑Man Kim* Anion exchange membrane (AEM) electrolysis is a promising solution for large-scale hydrogen production from renewable energy resources. However, the performance of AEM electrolysis is still lower than what can be achieved with conventional technologies. The performance of AEM electrolysis is limited by integral components of the membrane electrode assembly and the reaction kinetics, which can be measured by ohmic and charge transfer resistances. We here investigate and then quantify the contributions of the ohmic and charge transfer resistances, and the ratedetermining steps, involved in AEM electrolysis by using electrochemical impedance spectroscopy analysis. The factors that have an effect on the performance, such as voltage, flow rate, temperature and concentration, were studied at 1.5 and 1.9 V. Increased voltage, flow rate, temperature and concentration of the electrolyte strongly enhanced the anodic activity. We observed that here the anodic reaction offered a greater contribution to the overpotential than the cathode did. Electricity production by renewable sources such as solar, wind and tidal hydraulics now offers the most promising solutions to our current energy demands, taking a clean environment into consideration1. However, the electricity produced directly from renewable sources, such as wind and solar, may be negatively impacted by fluctuations in relevant geographical factors, such as cloud cover and low winds2. This then leads to an interrupted supply of the renewable energy, hence renewable energy must be stored and then used on demand for specific applications2. Among the various energy storage technologies, storage in the form of hydrogen is considered most preferable, due to the ability to store large amounts of energy for short and long periods of time, which can be decoupled upon demand3. Low-temperature water electrolysis is one of the cutting edge technologies for the sustainable conversion of hydrogen from renewable energy, using water. This technology offers adequate energy storage and grid-balancing utility in power-to-gas o perations4. The advantages offered by low-temperature water electrolysis include its high efficiency, high product purity, stable output, the feasibility of large-scale production and the capability of incorporating renewable energy as power s ource5. Currently, the main commercially available water electrolysis technologies are proton exchange membrane (PEM) electrolysis and alkaline electrolysis. A PEM electrolysis performance of 3000 mA cm–2 at 1.8 V has been reported (2015)6. However, the acidic environment required in PEM electrolysis limits the choice of catalysts to the expensive noble metals, such as platinum, iridium and it oxides7. Furthermore, the Nafion-based PEM and titanium stack components directly increase the capital cost of the electrolysis process, hence hindering the wider application of this technology. On the other hand, we do have alkaline electrolysis that is a mature and less expensive technology, but it cannot be linked with the renewable energies (solar, wind, etc.) for power generation owing to its inability to maintain high-pressure hydrogen, because of the required use of a porous diaphragm and liquid e lectrolyte8. Anion exchange membrane electrolysis. Recently, researchers have developed an emerging third-gen- eration technology, anion exchange membrane (AEM) water electrolysis, which integrates the benefits of both Power System and Sustainable Energy Laboratory, Department of Nanoscience and Engineering, INJE University, 607 Eobang‑Dong, Gimhae‑si, Gyongsangnam‑do 621‑749, Republic of Korea. *email: Scientific Reports | (2021) 11:293 | https://doi.org/10.1038/s41598-020-80683-6 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1.  (a) Schematic of anion exchange membrane (AEM) electrolysis. AEM anion exchange membrane, AGDL anode gas diffusion layer, CGDL cathode gas diffusion layer. (b) Schematic diagram of AEM water electrolysis with EIS experimental setup. conventional PEM and alkaline electrolysis9–11. The AEM electrolysis technology adopts low-cost catalytic materials, as in alkaline electrolysis, and a solid polymer electrolyte architecture, as in PEM electrolysis t echnology12. A schematic of AEM electrolysis is shown in Fig. 1a. AEM electrolysis technology operates in an alkaline environment (pH ~ 10), making it possible the use modest non-noble-metal electrocatalysts, whilst accommodating a zero-gap architecture13. The membrane used in this type of electrolysis is a polymeric membrane, containing quaternary ammonium salts. It is relatively inexpensive and has low interaction with atmospheric CO214,15. Thus, it is expected that this electrolysis technology should offer better performances and at a lower overall c ost16. We have previously reported on performances achieved with AEM e lectrolysis17. In an earlier study of ours, we demonstrated a new membrane electrode assembly (MEA) combination; it comprised a polybenzimidazole (PBI) AEM, and Ni–Fe–Ox (for the OER, the oxygen evolution reaction) and Ni–Fe–Co (for the HER, the hydrogen evolution reaction). The best performance was obtained at 1000 mA cm−2, 1.9 V and 60 °C, an AEM electrolysis performance of 74% was r ecorded17. Although such AEM electrolysis performance is considered acceptable, it is still lower than that achieved with conventional PEM e lectrolysis12,13. To further advance in this field, it is mandatory to gather more information on the factors that may limit the performance of AEM electrolysis18. Accordingly, further consideration/evaluation of various factors are required, e.g., the OER and HER reaction mechanisms, and resistances offered by each of the integral parts/components of the M EA19,20. In addition, the following should also be considered: the rate-determining steps of AEM electrolysis, the feasible circumstances of mass transport limitations and degradation of catalytic layer and AEM involved in the electrochemical reaction. The performance of an AEM electrolyser is directly measured by means of polarization curves (I–V)21,22. Such curves offer a reflection of the macroscopic behavior of the whole AEM electrolyser, but reveal no precise information about the effect of the inner components and kinetics of the electrolysis reaction. In AEM electrolysers, the current expresses the rate of hydrogen production and the voltage is the driving force for the electrolysis reaction. When a voltage is applied between the anode and cathode, the electrons flow through an external circuit, balanced by O H– ion transfer through the AEM, thus the electrolysis takes place23. Upon the application of higher voltage, the electrons try to flow through the external circuit and the O H– ion (...truncated)