O2 evolution electrocatalysis:Electronic, atomic, and nanoscale dynamics matter

In Nature, Mefford, Chueh, and colleagues describe how they investigated the oxygen evolution reaction (OER) in situ at sub-micrometer resolution. Nanoscale variations of current density, geometry, oxidation states show that currently emerging potentially paradigm-shifting picture redox-active, structurally dynamic catalyst materials might need to include nanoscale. The (O2) (OER), also denoted as water splitting or oxidation, proceeds photosynthetic organisms: cyanobacteria, algae, land plants. Because its fundamental importance for life on Earth, biological OER has been a topic intense research already decades revealing an astounding sophistication both, (1) atomic level catalytic site, protein-bound Mn4Ca-oxo cluster (2) nanoscale protein complexes lipid-bilayer membranes, latter representing nanostructures inter alia organize proton transport pH gradients.1Junge W. Oxygenic photosynthesis: history, status perspective.Quarterly Reviews Biophysics. 2019; 52https://doi.org/10.1017/S0033583518000112Crossref PubMed Scopus (40) Google Scholar,2Dau H. Limberg C. Reier T. Risch M. Roggan S. Strasser P. mechanism oxidation: from electrolysis via homogeneous catalysis.ChemCatChem. 2010; 2: 724-761https://doi.org/10.1002/cctc.201000126Crossref (1260) Scholar A well-known non-biological pendant is driven by electric potential difference between positive anode negative cathode, where O2 formed hydrogen (H2) cathode. For many decades, was subject dutifully had found way into textbooks, typically without sparking much excitement among students teachers. Also science, met with limited enthusiasm only—apart few dedicated electrochemists. situation changed within last 1–2 when meager scientific interest electrochemical replaced establishment almost hectic growth high-impact area. reason this pivotal role future technologies sustainable production non-fossil fuels. Noteworthily, full energy content fuel released (atmospheric) thereby producing, alia, H2O. Consequently, coupled formation essential any closed cycle technological usage, analogy respiratory dioxygen reduction. research, classical investigation metallic model electrodes became largely superseded comparative new materials, physical characterization methods reveal structural properties nanometer, atomic, electronic can be correlated macroscopic performance catalyst-coated electrodes. Such structure-function relations—an expression borrowed biochemical research—initially restricted correlating electrocatalytic as-synthesized material occasionally post-operation (or post-mortem) catalyst. Not only temporal corrosive processes remained unresolved. Reversible dynamics response application potentials escaped attention. Yet recently, experimental technology developed facilitate catalysts exposed electrolyte during well-defined potentials. techniques rapidly developing toolbox are often referred operando methods, today used synonymously situ. (We note passing spectroscopic do not differ spectroelectrochemistry applied processes.) Mostly investigations have addressing structures (geometrical arrangement nuclei) (orbital occupancies, states). Prominent examples use X-ray absorption spectroscopy (XAS),3Timoshenko J. Roldan Cuenya B. Situ/Operando Electrocatalyst Characterization Absorption Spectroscopy.Chem. Rev. 2021; 121: 882-961https://doi.org/10.1021/acs.chemrev.0c00396Crossref (111) hard soft regime, Raman spectroscopy.4Pasquini D’Amario L. Zaharieva I. Dau Operando tracks oxidation-state changes amorphous Co oxide electrocatalysis reaction.J. Chem. Phys. 2020; 152: 194202https://doi.org/10.1063/5.0006306Crossref (21) Both report potential-dependent time-dependent structure, which simultaneously measured parameters. Now connection added, “correlative microscopy.” smart combination particles recently facilitated success Tyler William their co-workers.5Mefford J.T. Akbashev A.R. Kang Bentley C.L. Gent W.E. Deng H.D. Alsem D.H. Yu Y.S. Salmon N.J. Shapiro D.A. et al.Correlative microscopy electrocatalysts.Nature. 593: 67-73https://doi.org/10.1038/s41586-021-03454-xCrossref (96) deposited substrate electrode hexagonal, micrometer diameter typical height 70 nm. At level, well-ordered cobalt oxyhydroxides parallel layers edge-sharing CoO6 octahedra, bridging oxygens completely partially protonated. hexagonal reflect structure oxyhydroxide plane. Layered hydroxide generally based transition metals described, perfectly defect-free intercalated ions (aside H+), MII(OH)2, MIIIOOH, MIVO2 (M = Co, Ni, Mn, Ru, Ir, others).6Cai Z.Y. Bu X.M. Wang Ho J.C. Yang J.H. X.Y. Recent advances layered double electrocatalysts Mater. Energy Sustain. 7: 5069-5089Crossref Often defect-rich layer fragments comprise corner-sharing MO6 octahedra well molecules counterions. nanoscale, morphology frequently encountered, marked contrast Mefford al. Their choice comparably large Co(OHx) crystalline order supports both correlative spatial resolution interpretation terms ideal materials. However, degree transferability conclusions other remains open question. After deposition flat planar electrode, al.5Mefford effect various near-catalytic using scanning cell (SECCM) simultaneous detection density particle height. these experiments, meniscus nanopipette scanned across electrode. resulting microscope images said determined tip 440 nm, but effective pipette mode appears being least one magnitude below diameter. separate experiment fully particles, 5Mefford spatially resolved state measuring LIII-edge spectra transmission (STXM) 50 nm 2 dimensions. Analysis three signals leads following conclusions:(1)Catalytic currents detectable margins implying basal planes oxides catalytically inactive, suggested before never visualized so impressively. Caveats possibility hydrated amorphized regions margins. Catalytic activity interior could SECCM method unlikely particles.(2)Conventional electrochemistry STXM suggest two redox transitions assigned follows: CoII(OH)2 ⇔ CoII/IIIO0.5(OH)1.5–(H2O)0.5intercalated CoIIIOOH, mixed-valent state, acquires molecules. swelling about 10% (ca. 7 nm) corresponding intermediate motivating proposed intercalation. Interestingly associated lateral expansion resolved, presently definitive assignment cannot made.(3)Transmission reveals heterogeneously distributed onset CoII→CoIII shifted higher than bulk. electrochemists discussed electrolytic taking place inert metal-oxide surfaces. Electrochemical started address occupancies states) given induced variation potential. begin shake assumption adsorption desorption metal surfaces related concepts describing reactions computationally. paradigm change dawning, materials.2Dau Scholar,7Risch Ringleb F. Kohlhoff Bogdanoff Chernev Water cobalt-based oxides: tracking catalysis.Energy Environ. Sci. 2015; 8: 661-674https://doi.org/10.1039/C4ee03004dCrossref Scholar, 8Nong H.N. Falling L.J. Bergmann A. Klingenhof Tran H.P. Spöri Mom R. Timoshenko Zichittella G. Knop-Gericke al.Key chemistry versus bias evolution.Nature. 587: 408-413https://doi.org/10.1038/s41586-020-2908-2Crossref (170) 9Anantharaj Noda Amorphous Catalysts Splitting: An Untold Story Harmony.Small. 16: e1905779https://doi.org/10.1002/smll.201905779Crossref (201) 10Pasquini González-Flores D. Mohammadi M.R. Guidoni Smith R.D.L. H/D Isotope effects factors controlling Co-based oxidation.J. Am. Soc. 141: 2938-2948https://doi.org/10.1021/jacs.8b10002Crossref (37) But paradigm-changing studies, features, nanometer were considered mechanistically irrelevant merely differences electrochemically active surface area modification. co-workers put focus. work unexpected explained interplay metal-ion chemistry, morphology. results definitive, still represent breakthrough possible kickstart effect: induce non-trivial now experiments. shown useful system will promote optimal because number reactive margin sites. development static optimized length scales ranging nanometers millimeters desirable. Optimization targets reactivity OER-active sites superior properties—water accessibility, electron transfer, transport, release relevant issues. Biological evolutionarily functionality billions years. Maybe we faster knowledge-based inorganic