(Invited) Towards Entirely Platinum Group Metal-Free Water Electrolyzers: Innovative Electrocatalysts for Oxygen Evolution and Hydrogen Evolution Reactions

Making the production of "green" hydrogen (H₂) cost-effective requires the development of high-performance and affordable low-temperature water electrolyzers (LTWE).¹ Currently, the most mature technology for H₂ production using renewable electricity is the liquid alkaline electrolysis (AE). This technology suffers several major drawbacks such as gas crossover, relatively low current density, and the use of highly corrosive concentrated alkaline solutions (20-40% KOH). Proton exchange membrane (PEM) electrolysis, a valid alternative to AE technology, already commercialized on a large scale, enables operation on pure water thus eliminating corrosion, reducing gas crossover and allowing higher current density. However, the main drawback of PEM electrolyzers is the need of very expensive and rare platinum group metals (PGMs) such as Ir and Pt as catalysts for oxygen evolution reaction (OER) at the anode and hydrogen evolution reaction (HER) at the cathode, respectively.²

Recent advancements in performance and stability of anion exchange membranes (AEMs) have enabled a new type of alkaline membrane-based LTWE operating on pure water and with PGM-free catalysts.^3,4 If successful, this new AEM-LTWE technology will allow to overcome the drawbacks of AEs and PEM-LTWEs while benefiting from their respective advantages in a major breakthrough in the production of "green" H₂ at a low cost. In this scenario crucial is the development of high-performance PGM-free electrocatalysts for both OER and HER.

Due to the operation at high potentials, carbon-based catalysts and supports cannot be used at the anode. Therefore, the most common PGM-free anode catalysts are based on transition metal oxides, which suffer, however, from low surface area and electronic conductivity, limiting the electrocatalytic performance.^5,6 The catalysts with the most promising OER activity in alkaline environment are Ni-based alloys, oxides, and (oxy)hydroxides.⁷ The combination of Ni with other first-row transition metals such as Fe and Co was found to increase the OER catalytic activity.^8,9

In this work, we present a new method for synthesizing NiFe OER catalysts. The catalyst was synthesized via a sol-gel method, followed by a thermal treatment. The impact on the OER activity in alkaline liquid electrolyte of different synthesis parameters such as the Ni-to-Fe atomic ratio, the addition of a third transition metal (e.g., Co, Mn), the thermal treatment temperature and atmosphere were investigated. Then, the most promising electrocatalysts were tested in an AEM-LTWE operating with pure water and supporting electrolyte solution.

Bimetallic HER PGM-free catalysts were also developed by combining one a first-row transition metal, e.g. Ni, with a second-row transition metal, e.g. Mo. These HER catalysts were synthesized by either (i) using the sol-gel approach described above or (ii) via a metal organic framework (MOF) method similar to the one used in the synthesis of "atomically dispersed" M-N-C catalysts for oxygen reduction reaction.¹⁰

References

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2. H. A. Miller et al., Sustain. Energy Fuels, 4, 2114–2133 (2020).

3. J. Xiao et al., ACS Catal., 11, 264–270 (2021).

4. D. Li et al., Nat. Energy, 5, 378–385 (2020).

5. Q. Gao et al., Chem. Eng. J., 331, 185–193 (2018).

6. D. Xu et al., ACS Catal., 9, 7–15 (2019).

7. S. Fu et al., Nano Energy, 44, 319–326 (2018).

8. G. Zhang et al., Appl. Catal. B Environ., 286, 119902 (2021).

9. P. Chen and X. Hu, Adv. Energy Mater., 10, 1–6 (2020).

10. Y. He et al., Energy Environ. Sci., 12, 250–260 (2019).