Role of Fe decoration on the oxygen evolving state of Co3O4 nanocatalysts.

The production of green hydrogen through alkaline water electrolysis is the key technology for the future carbon-neutral industry. Nanocrystalline Co3O4 catalysts are highly promising electrocatalysts for the oxygen evolution reaction and their activity strongly benefits from Fe surface decoration. However, limited knowledge of decisive catalyst motifs at the atomic level during oxygen evolution prevents their knowledge-driven optimization. Here, we employ a variety of operando spectroscopic methods to unveil how Fe decoration increases the catalytic activity of Co3O4 nanocatalysts as well as steer the (near-surface) active state formation. Our study shows a link of the termination-dependent Fe decoration to the activity enhancement and a significantly stronger Co3O4 near-surface (structural) adaptation under the reaction conditions. The near-surface Fe- and Co-O species accumulate an oxidative charge and undergo a reversible bond contraction during the catalytic process. Moreover, our work demonstrates the importance of low coordination surface sites on the Co3O4 host to ensure an efficient Fe-induced activity enhancement, providing another puzzle piece to facilitate optimized catalyst design.

Deciphering the Structural and Chemical Transformations of Oxide Catalysts during Oxygen Evolution Reaction Using Quick X-ray Absorption Spectroscopy and Machine Learning.

Bimetallic transition-metal oxides, such as spinel-like CoxFe3-xO4 materials, are known as attractive catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. Nonetheless, unveiling the real active species and active states in these catalysts remains a challenge. The coexistence of metal ions in different chemical states and in different chemical environments, including disordered X-ray amorphous phases that all evolve under reaction conditions, hinders the application of common operando techniques. Here, we address this issue by relying on operando quick X-ray absorption fine structure spectroscopy, coupled with unsupervised and supervised machine learning methods. We use principal component analysis to understand the subtle changes in the X-ray absorption near-edge structure spectra and develop an artificial neural network to decipher the extended X-ray absorption fine structure spectra. This allows us to separately track the evolution of tetrahedrally and octahedrally coordinated species and to disentangle the chemical changes and several phase transitions taking place in CoxFe3-xO4 catalysts and on their active surface, related to the conversion of disordered oxides into spinel-like structures, transformation of spinels into active oxyhydroxides, and changes in the degree of spinel inversion in the course of the activation treatment and under OER conditions. By correlating the revealed structural changes with the distinct catalytic activity for a series of CoxFe3-xO4 samples, we elucidate the active species and OER mechanism.

Scalable Synthesis of Multi-Metal Electrocatalyst Powders and Electrodes and their Application for Oxygen Evolution and Water Splitting.

Multi-metal electrocatalysts provide nearly unlimited catalytic possibilities arising from synergistic element interactions. We propose a polymer/metal precursor spraying technique that can easily be adapted to produce a large variety of compositional different multi-metal catalyst materials. To demonstrate this, 11 catalysts were synthesized, characterized, and investigated for the oxygen evolution reaction (OER). Further investigation of the most active OER catalyst, namely CoNiFeMoCr, revealed a polycrystalline structure, and operando Raman measurements indicate that multiple active sites are participating in the reaction. Moreover, Ni foam-supported CoNiFeMoCr electrodes were developed and applied for water splitting in flow-through electrolysis cells with electrolyte gaps and in zero-gap membrane electrode assembly (MEA) configurations. The proposed alkaline MEA-type electrolyzers reached up to 3 A cm-2 , and 24 h measurements demonstrated no loss of current density of 1 A cm-2 .

Single-entity Electrochemistry Unveils Dynamic Transformation during Tandem Catalysis of Cu2 O and Co3 O4 for Converting NO3 - to NH3.

Electrochemically converting nitrate to ammonia is an essential and sustainable approach to restoring the globally perturbed nitrogen cycle. The rational design of catalysts for the nitrate reduction reaction (NO3 RR) based on a detailed understanding of the reaction mechanism is of high significance. We report a Cu2 O+Co3 O4 tandem catalyst which enhances the NH3 production rate by ≈2.7-fold compared to Co3 O4 and ≈7.5-fold compared with Cu2 O, respectively, however, most importantly, we precisely place single Cu2 O and Co3 O4 cube-shaped nanoparticles individually and together on carbon nanoelectrodes provide insight into the mechanism of the tandem catalysis. The structural and phase evolution of the individual Cu2 O+Co3 O4 nanocubes during NO3 RR is unveiled using identical location transmission electron microscopy. Combining single-entity electrochemistry with precise nano-placement sheds light on the dynamic transformation of single catalyst particles during tandem catalysis in a direct way.

Crystal Plane-Related Oxygen-Evolution Activity of Single Hexagonal Co3 O4 Spinel Particles.

The electrocatalytic activity for the oxygen evolution reaction in alkaline electrolyte of hexagonal spinel Co3 O4 nanoparticles derived using scanning electrochemical cell microscopy (SECCM) is correlated with scanning electron microscopy and atomic force microscopy images of the droplet landing sites. A unique way to deconvolute the intrinsic catalytic activity of individual crystal facets of the hexagonal Co3 O4 spinel particle is demonstrated in terms of the turnover frequency (TOF) of surface Co atoms. The top surface exposing 111 crystal planes displayed a thickness-dependent TOF with a TOF of about 100 s-1 at a potential of 1.8 V vs. RHE and a particle thickness of 100 nm. The edge of the particle exposing (110) planes, however, showed an average TOF of 270±68 s-1 at 1.8 V vs. RHE and no correlation with particle thickness. The higher atomic density of Co atoms on the edge surface (2.5 times of the top) renders the overall catalytic activity of the edge planes significantly higher than that of the top planes. The use of a free-diffusing Os complex in the alkaline electrolyte revealed the low electrical conductivity through individual particles, which explains the thickness-dependent TOF of the top planes and could be a reason for the low activity of the top (111) planes.

Single Particle Nanoelectrochemistry Reveals the Catalytic Oxygen Evolution Reaction Activity of Co3 O4 Nanocubes.

Co3 O4 nanocubes are evaluated concerning their intrinsic electrocatalytic activity towards the oxygen evolution reaction (OER) by means of single-entity electrochemistry. Scanning electrochemical cell microscopy (SECCM) provides data on the electrocatalytic OER activity from several individual measurement areas covering one Co3 O4 nanocube of a comparatively high number of individual particles with sufficient statistical reproducibility. Single-particle-on-nanoelectrode measurements of Co3 O4 nanocubes provide an accelerated stress test at highly alkaline conditions with current densities of up to 5.5 A cm-2 , and allows to derive TOF values of up to 2.8×104  s-1 at 1.92 V vs. RHE for surface Co atoms of a single cubic nanoparticle. Obtaining such high current densities combined with identical-location transmission electron microscopy allows monitoring the formation of an oxy(hydroxide) surface layer during electrocatalysis. Combining two independent single-entity electrochemistry techniques provides the basis for elucidating structure-activity relations of single electrocatalyst nanoparticles with well-defined surface structure.

Atmosphere-sensitive photoluminescence of Co x Fe3-x O4 metal oxide nanoparticles.

In this work the photoluminescence (PL) of Co x Fe3-x O4 spinel oxide nanoparticles under pulsed UV laser irradiation (λ exc = 270 nm) is investigated for varying Co/Fe ratios (x = 0.4⋯2.5). A broad emission in the green spectral range is observed, exhibiting two maxima at around 506 nm, which is dominant for Fe-rich nanoparticles (x = 0.4, 0.9), and at around 530 nm, that is more pronounced for Co-rich nanoparticles (x > 1.6). As examinations in different atmospheres show that the observed emission reacts sensitively to the presence of water, it is proposed that the emission is mainly caused by OH groups with terminal or bridging metal-O bonds on the Co x Fe3-x O4 surface. Raman spectroscopy supports that the emission maximum at 506 nm corresponds to terminal OH groups bound to metal cations on tetrahedral sites (i.e., Fe3+), while the maximum around 530 nm corresponds to terminal OH groups bound to metal cations on octahedral sites (i.e., Co3+). Photoinduced dehydroxylation shows that OH groups can be removed on Fe-rich nanoparticles more easily, leading to a conversion process and the formation of new OH groups with different bonds to the surface. As such behavior is not observed for Co x Fe3-x O4 with x > 1.6, we conclude that the OH groups are more stable against dehydroxylation on Co-rich nanoparticles. The higher OH stability is expected to lead to a higher catalytic activity of Co-rich cobalt ferrites in the electrochemical generation of oxygen.

Single-Entity Electrocatalysis of Individual "Picked-and-Dropped" Co3 O4 Nanoparticles on the Tip of a Carbon Nanoelectrode.

Nano-electrochemical tools to assess individual catalyst entities are critical to comprehend single-entity measurements. The intrinsic electrocatalytic activity of an individual well-defined Co3 O4 nanoparticle supported on a carbon-based nanoelectrode is determined by employing an efficient SEM-controlled robotic technique for picking and placing a single catalyst particle onto a modified carbon nanoelectrode surface. The stable nanoassembly is microscopically investigated and subsequently electrochemically characterized. The hexagonal-shaped Co3 O4 nanoparticles demonstrate size-dependent electrochemical activity and exhibit very high catalytic activity with a current density of up to 11.5 A cm-2 at 1.92 V (vs. RHE), and a turnover frequency of 532±100 s-1 at 1.92 V (vs. RHE) towards catalyzing the oxygen evolution reaction.

Intrinsic Activity of Oxygen Evolution Catalysts Probed at Single CoFe2O4 Nanoparticles.

Identifying the intrinsic electrocatalytic activity of nanomaterials is challenging, as their characterization usually requires additives and binders whose contributions are difficult to dissect. Herein, we use nano impact electrochemistry as an additive-free method to overcome this problem. Due to the efficient mass transport at individual catalyst nanoparticles, high current densities can be realized. High-resolution bright-field transmission electron microscopy and selected area diffraction studies of the catalyst particles before and after the experiments provide valuable insights in the transformation of the nanomaterials during harsh oxygen evolution reaction (OER) conditions. We demonstrate this for 4 nm sized CoFe2O4 spinel nanoparticles. It is revealed that these particles retain their size and crystal structure even after OER at current densities as high as several kA·m-2. The steady-state current scales with the particle size distribution and is limited by the diffusion of produced oxygen away from the particle. This versatilely applicable method provides new insights into intrinsic nanocatalyst activities, which is key to the efficient development of improved and precious metal-free catalysts for renewable energy technologies.