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1. Morphology‐controlled synthesis of multi‐metal‐based spinel oxide nanocatalysts and their performance for oxygen reduction

   https://doi.org/10.1002/elt2.62  

   Li, Can; Pan, Jinfong; Chen, Xiaobo; Zhang, Lihua; Dennett, Anna; Bharathan, Prabhu; Lee, Douglas; Zhou, Guangwen; Fang, Jiye (August 2024, Electron)

   Abstract We present a one‐pot colloidal synthesis method for producing monodisperse multi‐metal (Co, Mn, and Fe) spinel nanocrystals (NCs), including nanocubes, nano‐octahedra, and concave nanocubes. This study explores the mechanism of morphology control, showcasing the pivotal roles of metal precursors and capping ligands in determining the exposed crystal planes on the NC surface. The cubic spinel NCs, terminated with exclusive {100}‐facets, demonstrate superior electrocatalytic activity for the oxygen reduction reaction (ORR) in alkaline media compared to their octahedral and concave cubic counterparts. Specifically, at 0.85 V, (CoMn)Fe₂O₄spinel oxide nanocubes achieve a high mass activity of 23.9 A/g and exhibit excellent stability, highlighting the promising ORR performance associated with {100}‐facets of multi‐metal spinel oxides over other low‐index and high‐index facets. Motivated by exploring the correlation between ORR performance and surface atom arrangement (active sites), surface element composition, as well as other factors, this study introduces a prospective approach for shape‐controlled synthesis of advanced spinel oxide NCs. It underscores the significance of catalyst shape control and suggests potential applications as nonprecious metal ORR electrocatalysts. 

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2. Octahedral spinel electrocatalysts for alkaline fuel cells

   https://doi.org/10.1073/pnas.1906570116  

   Yang, Yao; Xiong, Yin; Holtz, Megan E.; Feng, Xinran; Zeng, Rui; Chen, Gary; DiSalvo, Francis J.; Muller, David A.; Abruña, Héctor D. (December 2019, Proceedings of the National Academy of Sciences)

   Designing high-performance nonprecious electrocatalysts to replace Pt for the oxygen reduction reaction (ORR) has been a key challenge for advancing fuel cell technologies. Here, we report a systematic study of 15 different AB 2 O 4 /C spinel nanoparticles with well-controlled octahedral morphology. The 3 most active ORR electrocatalysts were MnCo 2 O 4 /C, CoMn 2 O 4 /C, and CoFe 2 O 4 /C. CoMn 2 O 4 /C exhibited a half-wave potential of 0.89 V in 1 M KOH, equal to the benchmark activity of Pt/C, which was ascribed to charge transfer between Co and Mn, as evidenced by X-ray absorption spectroscopy. Scanning transmission electron microscopy (STEM) provided atomic-scale, spatially resolved images, and high-energy-resolution electron-loss near-edge structure (ELNES) enabled fingerprinting the local chemical environment around the active sites. The most active MnCo 2 O 4 /C was shown to have a unique Co-Mn core–shell structure. ELNES spectra indicate that the Co in the core is predominantly Co 2.7+ while in the shell, it is mainly Co 2+ . Broader Mn ELNES spectra indicate less-ordered nearest oxygen neighbors. Co in the shell occupies mainly tetrahedral sites, which are likely candidates as the active sites for the ORR. Such microscopic-level investigation probes the heterogeneous electronic structure at the single-nanoparticle level, and may provide a more rational basis for the design of electrocatalysts for alkaline fuel cells. 

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3. Atomically dispersed Fe–N–C decorated with Pt-alloy core–shell nanoparticles for improved activity and durability towards oxygen reduction

   https://doi.org/10.1039/d0ee00832j  

   Ao, Xiang; Zhang, Wei; Zhao, Bote; Ding, Yong; Nam, Gyutae; Soule, Luke; Abdelhafiz, Ali; Wang, Chundong; Liu, Meilin (September 2020, Energy & Environmental Science)

   null (Ed.)

   One of the key challenges that hinders broad commercialization of proton exchange membrane fuel cells is the high cost and inadequate performance of the catalysts for the oxygen reduction reaction (ORR). Here we report a composite ORR catalyst consisting of ordered intermetallic Pt-alloy nanoparticles attached to an N-doped carbon substrate with atomically dispersed Fe–N–C sites, demonstrating substantially enhanced catalytic activity and durability, achieving a half-wave potential of 0.923 V ( vs.  RHE) and negligible activity loss after 5000 cycles of an accelerated durability test. The composite catalyst is prepared by deposition of Pt nanoparticles on an N-doped carbon substrate with atomically dispersed Fe–N–C sites derived from a metal–organic framework and subsequent thermal treatment. The latter results in the formation of core–shell structured Pt-alloy nanoparticles with ordered intermetallic Pt 3 M (M = Fe and Zn) as the core and Pt atoms on the shell surface, which is beneficial to both the ORR activity and stability. The presence of Fe in the porous Fe–N–C substrate not only provides more active sites for the ORR but also effectively enhances the durability of the composite catalyst. The observed enhancement in performance is attributed mainly to the unique structure of the composite catalyst, as confirmed by experimental measurements and computational analyses. Furthermore, a fuel cell constructed using the as-developed ORR catalyst demonstrates a peak power density of 1.31 W cm −2 . The strategy developed in this work is applicable to the development of composite catalysts for other electrocatalytic reactions. 

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4. Multi-walled carbon nanotube supported manganese selenide as a highly active bifunctional OER and ORR electrocatalyst

   https://doi.org/10.1039/d1ta09864k  

   Singh, Harish; Marley-Hines, McKenzie; Chakravarty, Shatadru; Nath, Manashi (March 2022, Journal of Materials Chemistry A)

   Transition metal selenides have attracted intensive interest as cost-effective electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) because of the continuous thrust in sustainable energy conversion. In this article a Mn-based bifunctional electrocatalyst, MnSe, has been identified which shows efficient OER and ORR activity in alkaline medium. The catalytic activity could be further enhanced by using multiwalled carbon nanotubes (MWCNTs) which increases the charge transfer and electronic conductivity of the catalyst composite. This MnSe@MWCNT catalyst composite exhibits a very low overpotential of 290 mV at 10 mA cm −2 , which outperforms state-of-the-art RuO 2 as well as other oxide based electrocatalysts. Furthermore, the composite's facile OER kinetics was evidenced by its small Tafel slope of 54.76 mV dec −1 and low charge transfer resistance, indicating quick transport of the reactant species at the electrode interface. The MnSe@MWCNT also exhibited efficient electrocatalytic activity for ORR with an E onset of 0.94 V, which is among the best reported to date for chalcogenide based ORR electrocatalysts. More importantly, this MnSe-based ORR electrocatalyst exhibits high degree of methanol tolerance, showing no degradation of catalyst performance in the presence of copious quantities of methanol, thereby out-performing the state-of-the-art Pt electrocatalyst. The catalyst composite also exhibited exceptional functional and compositional stability for OER and ORR after a prolonged period of continuous operation in alkaline medium. The surface Raman analysis after OER revealed the retention of manganese selenide surface with evidence of oxo coordination, confirming the formation of an (oxy)selenide as the active surface for OER. Such efficient bifunctional OER and ORR activity makes this MnSe based catalyst attractive for overall electrolysis in regenerative as well as direct methanol fuel cells. 

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5. Oxygen reduction reaction catalyzed by carbon composites with ruthenium-doped iron oxide nanoparticles

   https://doi.org/10.1039/d2ma00054g  

   Liu, Qiming; Zhou, Hong Bo; Nichols, Forrest; Kuo, Han-Lin; Mercado, Rene; Lu, Bingzhang; Zhu, Weiya; Liu, Yashu; Lu, Jennifer Q.; Bridges, Frank; et al (June 2022, Materials Advances)

   Carbon nanocomposites based on transition-metal oxides have been attracting extensive attention as cost-effective catalysts towards the oxygen reduction reaction (ORR). However, the activity remains subpar as compared to state-of-the-art platinum catalysts. One way to enhance the ORR performance is to dope a second metal into the nanocomposite to manipulate the electronic structure and hence the interactions with key reaction intermediates. Herein, dual metal (Ru and Fe) and nitrogen codoped carbon (RuFe-NC) nanocomposites were synthesized by controlled pyrolysis of a Fe–Ru–Fe trinuclear complex along with zeolitic imidazolate framework-8. The obtained porous nanocomposites consisted of Ru-doped Fe 2 O 3 nanoparticles embedded within a carbon scaffold, and exhibited an ORR activity in alkaline media rivaling that of commercial Pt/C, which was also markedly better than those of the monometallic counterparts and nanocomposites prepared with a simple mixture of the individual monometallic compound precursors. Structural characterization suggests that the use of the trinuclear complex facilitated the atomic dispersion of ruthenium within the iron oxide nanoparticles and charge transfer between the metal centers led to a high ORR activity. Results from this study suggest that rational design of heteronuclear complexes may be a unique strategy in the structural engineering of carbon-metal nanocomposites for high-performance electrocatalysis. 

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