Grafted Superstructure Renders Catalyst Stability in Alkaline Water Oxidation

Abstract

Large-scale alkaline water electrolysis, capable of producing green hydrogen (H2) from water at low temperatures (e.g., 20°C–80°C), is critical for renewable energy conversion and storage [1, 2]. Anion-exchange membrane water electrolysis (AEMWE) is particularly promising due to its low cost, scalable production, and ability to operate at large current densities. However, its practical implementation is still hindered by the poor operational stability of catalysts because of the dissolution of catalytic metal sites or phase segregation under industrial conditions (including large currents and high temperatures), especially for the first-row (3d) transition-metal (oxy)hydroxides. One critical limitation of metal (oxy)hydroxides arises from the inherently weak interactions between metal atoms and oxygen. Under operational conditions, these metal–oxygen bonds tend to break up, inducing localized structural distortions and rendering the dissolution of active metal sites into the electrolyte. Various strategies have been explored to enhance stability by structural design, including interlayer intercalation [3, 4] and incorporation of guest sites [5, 6]. However, the current optimization primarily centers on tuning the catalytic sites, the stability improvements often comes with a loss in catalytic activity [5, 7, 8]. Thus, the overall performance for alkaline water oxidation still falls short of industrial requirements. In a recent groundbreaking study published in Science, Yue et al. [9] reported a grated superstructure comprised of metal-organic framework (MOF) @ polyoxometalate (POM) derived from a self-assembly approach (Figure 1), which then electrochemically transforms into monolayer CoFe oxyhydroxides (CoFe-LDH) grafted onto POM, forming a triple layered superstructure. The mystery of enhanced stability lies in the electron transfer channel between metal atoms distributed in LDH, MOF, and POM. According to the authors, this superstructure affords mechanical bonding with newly formed CoFe-LDHs and establishes new electron-transfer from the nickel and tungsten in the POM to the catalytic iron and cobalt sites in LDH, and eventually to the electrode. These effects inhibit over-oxidation of metal to high-valence states and prevent dissolution. As for the boosted activity, the POM favors the formation of *OOH on CoFe-LDH, which is the rate-determining step for water oxidation. In addition, the hydrogen-bonded network is proven to be an important factor in determining the reaction pathway and activity. It is hypothesized, but not discussed in this study, that this grafted superstructure has somewhat altered the hydrogen-bonded network (Figure 1). This regulation increases the number of surface hydrogen bonds and improves water adsorption tendency, a step that often dictates the overall reaction kinetics. This effect could have been investigated by, for example, in situ Raman spectroscopy, for a clearer picture of the enhancement mechanism. Illustrative representation of the MOF@POM superstructure and the origin of enhanced stability and catalytic activity for alkaline water oxidation. The RDS indicates the rate-determining step during reaction. The catalyst electrodes exhibit outstanding performance. The obtained monolayer CoFe-LDH @ POM catalysts exhibit high activity and stability in alkaline water electrolysis, achieving a low overpotential of 178 mV at the current of 10 mA·cm−2, and stable operation for 1000 h at a higher current of 100 mA·cm−2. When assembled into a membrane electrolyzer, the system can operate stably for 3000 h under the working condition of 3 A·cm−2 and an applied voltage of 1.78 V—meeting the US Department of Energy’s (DOE’s) 2025 target for 2035 AEM technologies (1.8 V at 3 A·cm−2). At 2 A·cm−2, MOF@POM electrolyzer runs for 5140 h with a low decay rate of 0.02 mV·h−1. According to the technoeconomic analysis calculation, the MOF@POM shows the lowest annual operating expenditure and highest profitability among common catalysts including CoFe-LDHs and NiFe-LDHs. Because of the advantages in cost and high-performance of the obtained catalysts, this polyoxometalate-grating strategy in holds great potential in alkaline water electrolysis under industrial operation conditions. This work provides an innovative solution to address the dissolution and corrosion issues of crucial metal (oxy)hydroxide active sites by structural engineering instead of enhancing the density and/or activity of catalytic sites in most studies. It is noted that another recent paper reported that selectively docking PW12-polyoxometalate (PW12-POM) onto Fe sites of CoFe hydroxide catalysts anode can extend the operation stability over 1300 h at 1 A·cm−2 and 600 h at 2 A·cm−2 for seawater electrolysis [10]. Hence, we may infer that constructing superstructures by integrating metal (oxy)hydroxides with polyoxometalate or other similar substrates is an effective approach to the design of stable catalysts for a broader field of energy conversion and storage. Although the detailed mechanism for superstructures remains to be further investigated, this study represents a breakthrough in the design of nonplatinum group metals catalysts for industry-scale water electrolysis. Tao Zhang: writing – original draft; writing – review and editing. Hong Jin Fan: writing – review and editing. The authors declare no conflicts of interest. Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.

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