s-, p-, and f-block metals-mediated novel iron-based compounds as precatalysts for efficient alkaline OER

Background: Generating green hydrogen (H2) through alkaline water electrolysis (AWE) powered by renewable resources is crucial for decarbonizing our society. A significant challenge in AWE is kinetically sluggish anodic oxygen evolution reaction (OER). Currently, the anodes used in industrial AWE systems incorporate OER catalysts based on non-noble transition metals (TMs). These catalysts, however, only achieve moderate performance. As a result, substantial research has focused on developing a new generation of cost-effective and more efficient TM-based catalysts. Among these, iron (Fe)-based catalysts are particularly promising due to Fe's low cost, high abundance in the earth's crust, and its ability to exist in various redox states. Challenge: However, very few pure Fe-based compounds perform well for alkaline OER. During the reaction, these compounds primarily act as precatalysts, inevitably reconstructing into (oxy)hydroxides (FeOxHy) that exhibit low or non-conductivity and easily dissolve Fe atoms. Additionally, the reconstructed FeOxHy suffers from insufficient exposure of surface active sites and inadequate mass transport and bubble detachment capabilities. Moreover, although many Fe-based compounds are claimed to possess intrinsically "outstanding OER capabilities," their performances are often mis/overestimated due to unintentional interference from nickel (Ni) or cobalt (Co) (e.g., from the Ni foam substrate). Consequently, universally feasible strategies to optimize pure Fe-based compounds as efficient alkaline OER precatalysts for broader-scale applications have not yet been demonstrated. Solutions: Developing new Fe-based compounds that incorporate carefully chosen s-, p-, and f-block metals (spfMs) could effectively address the challenges mentioned above. spfMs typically exhibit two distinct behaviors during alkaline OER: ⅰ) dissolving into the aqueous electrolyte and ⅱ) remaining at the anode as new, insoluble, oxidized phases or as residual precatalysts. In the former scenario, the structural reconstruction of the targeted Fe-based precatalysts can facilitate the in-situ formation of FeOxHy, potentially increasing surface porosity, reducing crystallinity, and decreasing particle size. In the latter scenario, the remaining spfM oxides or precatalysts can enhance charge transfer, stabilize the active phase (sites), and optimize the binding and activation abilities toward reactants, intermediates, and products. As a result, the alkaline OER activity and stability can be significantly enhanced. Results: By incorporating the representative s-block metals lithium (Li) and calcium (Ca), p-block elements boron (B) and germanium (Ge), and f-block metal cerium (Ce), three unique novel bulk Fe-based compounds as precatalysts were developed, constituted by ⅰ) all leaching (LiFeBPO), ⅱ) both leaching and non-leaching (CaFe6Ge6), and ⅲ) all non-leaching (CeFe2) spfMs, respectively. We unequivocally determined that all of them underwent distinctive OER-driven reconstructions mediated by their respective SPFMs. Eventually, the precisely modified α-FeOOH in-situ evolved as the real active phase. Specifically, i) the drastic/complete loss of spfMs induced a deep/thorough reconstruction of LiFeBPO into ultrasmall low-crystalline α-FeOOH nanoparticles forming a 3D open porous skeleton. This structure increased the exposure and accessibility of active sites, leading to excellent bulk activity. Mass transport and bubble detachment were also improved. ⅱ) Regrading CaFe6Ge6, the partial loss of spfMs resulted in a porous heteroshell where α-FeOOH was coupled with CaCO3, promoting adsorption and transport of reactants, as well as activating more Fe sites for catalysis. The higher intrinsic activity of α-FeOOH was achieved, and the residual CaFe6Ge6 intermetallic core facilitated charge migration. ⅲ) All spfMs were preserved, mediating the surface conversion of CeFe2 into α-FeOOH nanodomains heterogeneously coupled with CeO2. The pseudo‐periodic confinement of insoluble CeO2 enhanced the performance of α-FeOOH by preventing active site blockage and dissolution while optimizing the electronic structure and intermediate adsorption energies. Meanwhile, the retained CeFe2 elevated the overall charge transfer. As a result, the reconstructed α-FeOOH from these three compounds exhibited boosted alkaline OER performances, surpassing those of directly synthesized FeOxHy and even comparable to or better than benchmark NiFeOxHy under identical operational conditions. Additionally, when deposited on a Ni foam substrate, they demonstrated impressive alkaline OER activity and stability, outperforming most documented TM-based counterparts. Conclusion & Perspective: This dissertation explicitly correlates the relationship between composition, reconstruction, active structure, and OER properties of the studied Fe-based compounds while avoiding interferences from Ni and Co. It illustrates that incorporating suitable spfMs is an effective strategy to optimize pure Fe-based compounds for efficient alkaline OER catalysis, enhancing the performance of anodes for practical AWE. Furthermore, exemplified by LiFeBPO, the practicality and versatility of spfMs-mediated Fe-based compounds were validated through the efficient and selective electrooxidation of various alcohols. This illustrates the broad perspective of this new concept.

Author dc:creator

• Yang, Hongyuan

Advisor dc:contributor.advisor

• Driess , Matthias

Licence dc:rights.uri

• https://creativecommons.org/licenses/by-nc/4.0/

Language dc:language.iso

en

Identifier URI

https://doi.org/10.14279/depositonce-22304

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/23490