IN PRESS - Selected, just-accepted articles

  Colloidal Approaches to Zinc Oxide Nanocrystals

Joel van Embden, Silvia Gross, Kevin R. Kittilstved, Enrico Della Gaspera

Chemical Reviews

I.F. 60.62

DOI: https://doi.org/10.1021/acs.chemrev.2c00456

Zinc oxide is an extensively studied semiconductor with a wide band gap in the near-UV. Its many interesting properties have found use in optics, electronics, catalysis, sensing, as well as even biomedicine and microbiology. In the nanoscale regime the functional properties of ZnO can be precisely tuned by manipulating their size, shape, chemical composition (doping), and surface states. In this review, we focus on colloidal syntheses of ZnO nanocrystals (NCs) and provide a critical analysis of the synthetic methods currently available for preparing ZnO colloids. First, we outline key thermodynamic considerations for the nucleation and growth of colloidal nanoparticles, including an analysis of different reaction methodologies, and the role of dopant ions on nanoparticle formation. We then comprehensively review and discuss the literature on ZnO NC systems, including reactions in polar solvents that traditionally occur at low temperatures upon addition of a base, and high temperature reactions in organic, non-polar solvents. Here we also discuss the versatility of these methods in achieving morphological and compositional control in ZnO. A specific section is dedicated to doped NCs, highlighting both synthetic aspects and structure-property relationships. We then showcase some of the key applications of ZnO NCs, both as suspended colloids and as deposited coatings on supporting substrates. Finally, a critical analysis of the current state of the art for ZnO colloidal NCs is presented along with existing challenges and future directions for this field.

  High Open-Circuit Voltage Cs2AgBiBr6 Carbon-Based Perovskite Solar Cells via Green Processing of Ultrasonic Spray-Coated Carbon Electrodes from Waste Tire Sources

Fabian Schmitz, Nicolò Lago, Lucia Fagiolari, Julian Burkhart, Andrea Cester, Andrea Polo, Mirko Prato, Gaudenzio Meneghesso, Silvia Gross, Federico Bella, Francesco Lamberti, Teresa Gatti

ChemSusChem, e202201590

I.F. 9.14

Costs and toxicity concerns are at the center of a heated debate regarding the implementation of perovskite solar cells (PSCs) into commercial products. The first bottleneck could be over- come by eliminating the top metal electrode (generally gold) and the underlying hole transporting material and substituting both with one single thick layer of conductive carbon, as in the so-called carbon-based PSCs (C-PSCs). The second issue, related to the presence of lead, can be tackled by resorting to other perovskite structures based on less toxic metallic components. An interesting case is that of the double perovskite Cs2AgBiBr6, which at present still lacks the outstanding optoelectronic performances of the lead-based counterparts but is very stable to environmental factors. In this work, the processing of carbon electrodes onto Cs2AgBiBr6-based C-PSCs was reported, starting from an additive-free isopropanol ink of a carbon material obtained from the hydrothermal recycling of waste tires and employing a high-throughput ultrasonic spray coating method in normal environmental conditions. Through this highly sustainable approach that ensures a valuable step from an end- of-life to an end-of-waste status for used tires, devices were obtained delivering a record open circuit voltage of 1.293 V, which might in the future represent ultra-cheap solutions to power the indoor Internet of Things ecosystem.

  Selective Ion Sensing in Artificial Sweat Using Low-Cost Reduced Graphene Oxide Liquid-Gated Plastic Transistors

Rafael Furlan de Oliveira, Verónica Montes-García, Pietro Antonio Livio,  María Begoña González-García, Pablo Fanjul-Bolado, Stefano Casalini and Paolo Samorì

Small

IF = 13.281

DOI: 10.1002/smll.202201861

Health monitoring is experiencing a radical shift from clinic-based to point-of-care and wearable technologies, and a variety of nanomaterials and transducers have been employed for this purpose. 2D materials (2DMs) hold enormous potential for novel electronics, yet they struggle to meet the requirements of wearable technologies. Here, aiming to foster the development of 2DM-based wearable technologies, reduced graphene oxide (rGO)-based liquid-gated transistors (LGTs) for cation sensing in artificial sweat endowed with distin-guished performance and great potential for scalable manufacturing is reported. Laser micromachining is employed to produce flexible transistor test patterns employing rGO as the electronic transducer. Analyte selectivity is achieved by functionalizing the transistor channel with ion-selective membranes (ISMs) via a simple casting method. Real-time monitoring of K+ and Na+ in artificial sweat is carried out employing a gate voltage pulsed stimulus to take advantage of the fast responsivity of rGO. The sensors show excellent selectivity toward the target analyte, low working voltages (<0.5 V), fast (5–15 s), linear response at a wide range of concentrations (10 μm to 100 mm), and sensitivities of 1 μA/decade. The reported strategy is an important step forward toward the development of wearable sensors based on 2DMs for future health monitoring technologies.

  Characterization and Modeling of Reduced-Graphene Oxide Ambipolar Thin-Film Transistors

Nicolò Lago, Marco Buonomo, Rafael Cintra Hensel, Francesco Sedona, Mauro Sambi, Stefano Casalini, Andrea Cester

IEEE Transactions on Electronic Devices

IF = 2,917

DOI: 10.1109/TED.2022.3169451

The rise of graphene as an innovative electronicmaterial promoted the study and development of new 2-D materials. Among them, reduced graphene oxide (rGO) appears like an easy and cost-effective solution for the fabrication of thin-film transistors (TFTs). To understand the limits and possible application fields of rGO-based TFTs, a proper estimation of the device parameters is of extreme importance. In this work, liquid-gated ambipolar rGO-TFTs are characterized and a description of their working principle is given. Particular attention is paid toward the importance of the transistors’ OFF-state conductivity that was modeled as a resistance connected in parallel with the TFT. Thanks to this model, the main transistor parameters were extrapolated from rGO-TFTs with different levels of electrochemical reduction. The extracted parameters allowed understanding that rGO-TFTs have similar holes and electrons mobilities, and the more pronounced p-type behavior of the devices is due to a positive shift in the p-type and n-type threshold voltages.

  Real-time threshold voltage compensation on dual-gate electrolyte-gated organic field-effect transistors

Nicolò Lago, Marco Buonomo, Sara Ruiz-Molina, Andrea Pollesel, Rafael Cintra Hensel, Francesco Sedona, Mauro Sambi, Marta Mas-Torrent, Stefano Casalini, Andrea Cester

Organic Electronics

IF: 3.721

DOI: 10.1016/j.orgel.2022.106531

Electrolyte-Gated Organic Field-Effect Transistors (EGOFETs) offer many opportunities for the development of low-cost and low-power electronics suitable for applications like sensors and point-of-care tests; however, EGOFETs can be affected by the drift of their operative point that causes signals distortion and loss of information during sensing applications. Here, a blend of 2,8-Difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TES-ADT) and polystyrene (PS) is used as the active material for the fabrication of dual-gate EGOFETs. We exploited the dual-gate architecture to improve EGOFETs stability by implementing digital feedback that uses the back-gate electrode to compensate dynamically for the transistor threshold voltage allowing us to fix its operative point for prolonged tests (>10 hours) with different aqueous solutions (Milli-Q water, NaCl 0.1M and a physiological solution). The presented real-time threshold voltage compensation does not only allow to steady EGOFETs DC output current, but it also preserves EGOFETs sensing capability for the detection of signals with frequencies as low as 1 Hz.

  The impact of different conductive polymers on the performance of the sulfur positive electrode in Li-S batteries

Ahmed Shafique, Annick Vanhulsel, Vijay Shankar Rangasamy, Mohammadhosein Safari, ;Giulia Bragaggia, Silvia Gross,Peter Adriaensens, Marlies K. Van Bael, An Hardy, Sébastien Sallard

ACS Applied Energy Materials

In press, Just Accepted Manuscript

IF= 6.024

DOI:

Sulfur particles were coated with conductive polymer layers by dielectric barrier discharge (DBD) plasma technology under atmospheric conditions (low temperature and ambient pressure). The DBD-plasma process is a dry and sustainable (solvent-free, low energy consumption) technique compatible with upscaling. Different conductive coated sulfur materials were produced and labeled as poly(3,4-ethylene dioxythiophene-sulfur) (PEDOT-S), polyaniline-sulfur (PANI-S), polythiophene-sulfur (PTs-S), and polypyrrole-sulfur (PPy-S). The corresponding electrical conductivities were measured at 10-5, 10-6, 10-7, and 10-8 S/cm, respectively. The role of the conductive coating is to improve the electrochemical performance of Li-S cells by increasing the electronic conductivity of the sulfur particles and preventing the well-known polysulfide shuttle phenomenon. A wide variety of bulk and surface characterization methods including conductivity analysis, XRD (X-ray diffraction), SEM (scanning electron microscope), XPS (X-ray photoelectron spectroscopy), and solid-state 13C-NMR (nuclear magnetic resonance spectroscopy) were used to explain the chemical features using the different conductive polymer-coated sulfur materials. In coated sulfur samples, fragmentation in aromatic rings was observed, 88% for the PTs-S and 42% for the PEDOT-S while it is very limited for the PANI-S. Such a phenomenon has never been reported in the literature. The uncoated and coated sulfur powders were used as active material in positive electrodes of Li-S cells with high sulfur loading of ~ 4.5 mg/cm2 using lithium polyacrylate (LiPAA) as an (aqueous) binder. Long-term galvanostatic cycling at C/10 and multi-C rate tests showed the capacity fade and rate capability losses to be highly mitigated for the cells containing conductive polymer-coated sulfur in comparison to the reference Li-S cells with raw sulfur. Kinetic investigations by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analyses undoubtedly confirm improved electron and Li-ion transport within Li-S electrodes containing conductive polymer-coated sulfur. The electrochemical performance can be ranked as such PEDOT-S > PANI-S > PTs-S > PPy-S > raw sulfur.

  Atom-by-atom identification of catalytic active sites in operando conditions by quantitative noise detection

Marco Lunardon, Tomasz Kosmala, Christian Durante, Stefano Agnoli* and Gaetano Granozzi

Joule

IF: 41.428

DOI: 10.1016/j.joule.2022.02.010

Electrochemical STM (EC-STM) allows the direct observation of surface changes at the atomic scale in presence of an electrolyte at different electrochemical potentials. Recently, it has been demonstrated that the noise in the tunneling current of EC-STM allows identifying electrocatalytically active sites under reaction conditions. However, this method has never been applied to atom-by-atom investigations and could not provide a quantitative evaluation of the catalytic activity. Using the Hydrogen Evolution Reaction as case study, we demonstrate that the quantitative analysis of the noise in the tunneling current allows quantifying the local onset potential and provides information about the microscopic mechanism of electrochemical reactions on sub nanometric electrocatalytic sites, such as chemically heterogeneous flat interfaces, nanoparticles, and even single atom defects. This unique method allows surpassing the current limits not only of state-of-art EC-STM, but also of other operando and microscopy techniques.

  Suppressed charge carrier trap states and double photon absorption in substitutional Ta-doped TiO2-NT array

Xiaochen Huai, Gian Andrea Rizzi, Yanfeng Wang, Qige Qi, Gaetano Granozzi, Wangyang Fu and Zhengjun Zhang

Nano Today

IF: 20.722

DOI: doi.org/10.1016/j.nantod.2022.101407

Anatase-TiO2 nanotubes (A-TiO2 NTs) represent a great opportunity for the electron transport materials used in perovskite solar cells because of several intrinsic advantages, e.g. an improved light trapping effect, an inherent ion-blocking layer, a directed electron transmission channel without interfacial random scattering. Nevertheless, its severe double-photon absorption and charge carrier trap states badly jeopardize the stability and electron transport of the perovskite active layers (PALs) under visible light, representing a major obstacle for practical applications. In this paper, we introduce Ta to substitute Ti position in A-TiO2 NTs lattice through a simple fluorination process, and reveal its underneath mechanism on suppresing the abovementioned limiting factors of charge carrier trap states and double-photon absorption. Moreover, we use the effect of double-photon absorption of studied NTs to excite the photogenerated carriers under a modulated sinusoidal visible light with small amplitude, which can perturb the transport dynamics of photo-induced charge carriers and simulate the dynamic process of charge carriers at the interface between electron transport layer (ETL) and PALs in real time. These achievements highlight the unique potential of substitutional Ta doping for interfacing engineering of perovskite solar cells.

  The Effect of the 3D Nanoarchitecture and Ni-doping on the Hydrogen Evolution Reaction in MoS2/reduced GO Aerogel Hybrid Microspheres produced by a simple one-pot electrospraying procedure

Jiajia Ran, Leonardo Girardi, Zhanhua Wang, Stefano Agnoli*, Hesheng Xia and Gaetano Granozzi

Small

IF: 13.281

DOI: doi.org/10.1002/smll.202105694

Low-cost, efficient and durable electrocatalysts for green H2 production by water splitting are strategic for the transition towards renewable energy sources. Herein, we describe an easy and highly-scalable preparation of electrocatalysts made by MoS2 nanoparticles embedded in 3D partially reduced (pr) graphene oxide (GO) aerogel microspheres (MoS2/prGOAMs) with controlled morphology and composition. These materials exhibit remarkable electrocatalytic activity in the hydrogen evolution reaction (HER), thanks to the peculiar centre-diverging mesoporous structure, which allows easy access to the active sites and optimal mass transport, and to the efficient electron transfer provided by the intimate contact between the MoS2 and the highly-conductive and interconnected pr-GO sheets. To boost the HER activity, Ni atoms were introduced in the MoS2/prGOAMs hybrids either as small nanoparticles or single atoms, with the aim of facilitating water dissociation, which is the most critical HER step in alkaline medium. After an optimization procedure, Ni-promoted MoS2/prGOAMs reach a remarkable η10 (overpotential at 10 mA/cm2) of 160 mV in 1 M KOH and 174 mV in 0.5 M H2SO4. Moreover, after chronopotentiometry tests (15 h) at a current density of 10 mA/cm2, the η10 value improves to 147 mV in alkaline conditions, indicating an exceptional stability.

  Copper single-atoms embedded in 2D graphitic carbon nitride for the CO2 reduction

Claudio Cometto, Aldo Ugolotti, Elisa Grazietti, Alessandro Moretto, Gregorio Bottaro, Lidia Armelao, Cristiana Di Valentin, Laura Calvillo* and Gaetano Granozzi

npj 2D Materials and Applications

IF: 11.106

DOI: https://doi.org/10.1038/s41699-021-00243-y

We report the study of two-dimensional graphitic carbon nitride (GCN) functionalized with copper single atoms as a catalyst for the reduction of CO2 (CO2RR). The correct GCN structure, as well as the adsorption sites and the coordination of the Cu atoms, was carefully determined by combining experimental techniques, such as X-ray diffraction, transmission electron microscopy, X-ray absorption, and X-ray photoemission spectroscopy, with DFT theoretical calculations. The CO2RR products in KHCO3 and phosphate buffer solutions were determined by rotating ring disk electrode measurements and confirmed by 1H-NMR and gas chromatography. Formate was the only liquid product obtained in bicarbonate solution, whereas only hydrogen was obtained in phosphate solution. Finally, we demonstrated that GCN is a promising substrate able to stabilize metal atoms, since the characterization of the Cu-GCN system after the electrochemical work did not show the aggregation of the copper atoms.

  Operando visualization of the hydrogen evolution reaction with atomic scale precision at different metal-graphene interfaces

Tomasz Kosmala, Anu Baby, Marco Lunardon, Daniele Perilli, Hongsheng Liu, Christian Durante, Cristiana Di Valentin, Stefano Agnoli* and Gaetano Granozzi

Nature Catalysis

IF: 41.813

DOI: DOI.org/10.1038/s41929-021-00682-2

The development of catalysts for the hydrogen evolution reaction is pivotal for the hydrogen economy. Thin iron films covered with monolayer graphene exhibit outstanding catalytic activity, surpassing even that of platinum, as demonstrated by a method based on evaluating the noise in the tunnelling current of electrochemical scanning tunnelling microscopy. Using this approach, we mapped with atomic-scale precision the electrochemical activity of the graphene–iron interface, and determined that single iron atoms trapped within carbon vacancies and curved graphene areas on step edges are exceptionally active. Density functional theory calculations confirmed the sequence of activity obtained experimentally. This work exemplifies the potential of electrochemical scanning tunnelling microscopy as the only technique able to determine both the atomic structure and relative catalytic performance of atomically well-defined sites in electrochemical operando conditions and provides a detailed rationale for the design of novel catalysts based on cheap and abundant metals such as iron.

  H2S Dosimetry by CuO – Towards Stable Sensors by Unraveling the Underlying Solid-State Chemistry

S. Werner, C. Glaser, T. Kasper, T. N. Nguyên Lê,  S. Gross, and B. M. Smarsly

Chemistry: A European Journal

IF= 5.236

DOI:

The precise detection of the toxic gas H2S requires reliable sensitivity and specificity of sensors even at minute concentrations of as low as 10 ppm, the value corresponding to typical exposure limits. CuO can be used for H2S dosimetry, based on the formation of conductive CuS and the concomitant significant increase in conductance. In theory, by elevated temperature the reaction is reversed and CuO is formed, ideally enabling repeated, long-term and low-cost use of one sensor. Yet, the detection performance using pure CuO tends to drop upon repeated cycling. Utilizing defined CuO nanorods we thoroughly elucidated the associated detrimental chemical changes directly on the sensors, by Raman and electron microscopy analysis of each step during the sensing (CuO  CuS) and regeneration (CuS  CuO) cycles. We find the decrease in the sensing performance is mainly caused by the irreversible formation of CuSO4 even at 10 ppm H2S, as confirmed by ex-situ XRD experiments. The findings allowed us to develop strategies to reduce CuSO4 formation and thus to substantially maintain the sensing stability even for repeated regeneration cycles. We achieved CuO-based dosimeters possessing a response time of a few minutes only, even for 10 ppm H2S, and showing prolonged life-time.

  Universal Fabrication of Highly Efficient Plasmonic Thin-Films for Label-Free SERS Detection

Sara Gullace, Verónica Montes-García, Victor Martín, David Larios, Valentina Girelli Consolaro, Fernando Obelleiro, Giuseppe Calogero, Stefano Casalini, and Paolo Samorì

Journal: Small

IF=13.281

DOI: 0.1002/smll.202100755

The development of novel, highly efficient, reliable, and robust surface enhanced Raman scattering (SERS) substrates containing a large number of hot spots with programmed size, geometry, and density is extremely interesting since it allows the sensing of numerous (bio-)chemical species.
Herein, an extremely reliable, easy to fabricate, and label-free SERS sensing platform based on metal nanoparticles (NPs) thin-film is developed by the layer-by-layer growth mediated by polyelectrolytes. A systematic study of the effect of NP composition and size, as well as the number of deposition steps on the substrate’s performance, is accomplished by monitoring the SERS enhancement of 1-naphtalenethiol (532 nm excitation). Distinct evidence of the key role played by the interlayer (poly(diallyldimethylammonium chloride) (PDDA) or PDDA-functionalized graphene oxide (GO@PDDA)) on the overall SERS efficiency of the plasmonic platforms is provided, revealing in the latter the formation of more uniform hot spots by regulating the interparticle distances to 5 ± 1 nm. The SERS platform efficiency is demonstrated via its high analytical enhancement factor (≈106) and the detection of a prototypical
substance(tamoxifen), both in Milli-Q water and in a real matrix, viz. tap water, opening perspectives towards the use of plasmonic platforms for future high-performance sensing applications.

  Dielectric barrier discharge (DBD) plasma coating of sulfur for mitigation of capacity fade in lithium-sulfur batteries

A. Shafique, V. Shankar Rangasamy, A. Vanhulsel, M. Safari, S. Gross, P. Adriaensens, M. K. Van Bael, A. Hardy, S. Sallard

Article in press in ACS Applied Materials & Interfaces

I. F.: 8.758 (2019)

Sulfur particles with a conductive polymer coating of poly(3,4-ethylene dioxythiophene) “PEDOT” were prepared by dielectric barrier discharge (DBD) plasma technology under atmospheric conditions (low temperature, ambient pressure). We report a solvent free, low cost, low energy consumption, safe, and low risk process to make the material development and production compatible for sustainable technologies. Different coating protocols were developed to produce PEDOT-coated sulfur powders with electrical conductivity in the range of 10-8 - 10-5 S/cm. The raw sulfur powder (used as reference) and (low-, optimum-, high-) PEDOT-coated sulfur powders were used to assemble lithium-sulfur (Li-S) cells with high sulfur loading of ~ 4.5 mg/cm2. Long-term galvanostatic cycling at C/10 for 100 cycles showed that the capacity fade was mitigated by ~ 30% for the cells containing the optimum-PEDOT coated sulfur in comparison to the references Li-S cells with raw sulfur. Rate capability, cyclic voltammetry, and electrochemical impedance analyses confirmed the improved behavior of the PEDOT coated sulfur as an active material for lithium-sulfur batteries. The Li-S cells containing optimum-PEDOT coated sulfur showed the highest reproducibility of their electrochemical properties. A wide variety of bulk and surface characterization methods including conductivity analysis, X-ray diffraction (XRD), scanning electron microscope (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and nuclear magnetic resonance spectroscopy (NMR) were used to explain the chemical features and the superior behaviour of Li-S cells using the optimum-PEDOT coated sulfur material. Moreover, post-mortem (SEM and BET) analyses of uncoated and coated samples allow us to exclude any significant effect at electrode scale even after 70 cycles.

  Impact of inversion and non-stoichiometry on the transport properties of mixed zinc-cobalt ferrites

Mario F. Zscherp, Michele Bastianello, Silvia Nappini, Elena Magnano, Denis Badocco, Silvia Gross, Matthias T. Elm

Journal of Materials Chemistry C

IF=7.393

DOI: 10.1039/d1tc05871a

Metal spinel ferrites enable magnetic, electrical and (photo-)catalytic device applications. For example, tailoring the material's composition, the degree of inversion as well as the non-stoichiometry of the spinel enables controlling its electrical conductivity. The latter two, however, are rarely considered despite their vast impact on the structure–property relationship. Here, we elucidate their importance by carefully examining the temperature dependence (T = 600 °C to 50 °C) of the electrical conductivity of quaternary Zn(1−x)CoxFe2O4 ferrites under ambient and reducing atmospheric conditions. We show that the substitution of Co for Zn in bulk ZnFe2O4 results in a significant enhancement of the activation energy EA from 0.36 to 0.55 eV under an ambient atmosphere as mixed hopping between Co2+/Fe3+ sites dominates in Co containing ferrites, while the electrical conductivity in ternary ZnFe2O4 arises from electrons hopping between Fe2+/Fe3+ octahedral sites. More importantly, we demonstrate that hopping mainly occurs between Fe2+/Fe3+ octahedral sites (EA < 0.1 eV) under reducing conditions independent of the Co content as the release of oxygen increases the concentration of electrons. Our results highlight that controlling the non-stoichiometry is important for tuning of the electrical properties and essential for taking full advantage of quaternary ferrites in device applications..

  How do H2 oxidation molecular catalysts assemble onto carbon nanotube electrodes? A crosstalk between electrochemical and multi-physical characterization techniques

Ahmed Ghedjatti, Nathan Coutard, Laura Calvillo, Gaetano Granozzi, Bertrand Reuillard, Vincent Artero,Laure Guetaz, Sandrine Lyonnard,Hanako Okuno, Pascale Chenevier

Chem. Sci.

IF: 9.825

DOI: 10.1039/d1sc05168g

Molecular catalysts show powerful catalytic efficiency and unsurpassed selectivity in many reactions of interest. As their implementation in electrocatalytic devices requires their immobilization onto a conductive support, controlling the grafting chemistry and its impact on their distribution at the surface of this support within the catalytic layer is key to enhancing and stabilizing the current they produce. This study focuses on molecular bioinspired nickel catalysts for hydrogen oxidation, bound to carbon nanotubes, a conductive support with high specific area. We couple advanced analysis by transmission electron microscopy (TEM), for direct imaging of the catalyst layer on individual nanotubes, and small angle neutron scattering (SANS), for indirect observation of structural features in a relevant aqueous medium. Low-dose TEM imaging shows a homogeneous, mobile coverage of catalysts, likely as a monolayer coating the nanotubes, while SANS unveils a regular nanostructure in the catalyst distribution on the surface with agglomerates that could be imaged by TEM upon aging. Together, electrochemistry, TEM and SANS analyses allowed drawing an unprecedented and intriguing picture with molecular catalysts evenly distributed at the nanoscale in two different populations required for optimal catalytic performance.

  Electrocatalytic Site Activity Enhancement via Orbital Overlap in A2MnRuO7 (A = Dy, Ho, Er) Pyrochlore Nanostructures

V. Celorrio, D. Tiwari, L. Calvillo, A. Leach, H. Huang, G. Granozzi, J.A. Alonso, A. Aguadero, A.E. Russell, and D.J. Fermin

ACS Applied Energy

IF: 6.024

DOI: doi.org/10.1021/acsaem.0c02060

Oxygen electrocatalysis at transition metal oxides is one of the key challenges underpinning electrochemical energy conversion systems, involving a delicate interplay of the bulk electronic structure and surface coordination of the active sites. In this work, we investigate for the first time the structure−activity relationship of A2RuMnO7 (A = Dy3+, Ho3+, and Er3+) nanoparticles, demonstrating how orbital mixing of Ru, Mn, and O promotes high density of states at the appropriate energy range for oxygen electrocatalysis. The bulk structure and surface composition of these multicomponent pyrochlores are investigated by high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption spectroscopy, X-ray emission spectroscopy (XES), and X-ray photoemission spectroscopy (XPS). The materials exhibit high phase purity (cubic fcc with a space group Fd3̅m) in which variations in M−O bonds length are less than 1% upon replacing the A-site lanthanide. XES and XPS show that the mean oxidation state at the Mn-site as well as the nanoparticle surface composition was slightly affected by the lanthanide. The pyrochlore nanoparticles are significantly more active than the binary RuO2 and MnO2 toward the 4-electron oxygen reduction reaction in alkaline solutions. Interestingly, normalization of kinetic parameters by the number density of electroactive sites concludes that Dy2RuMnO7 shows twice higher activity than benchmark materials such as LaMnO3. Analysis of the electrochemical profiles supported by density functional theory calculations reveals that the origin of the enhanced catalytic activity is linked to the mixing of Ru and Mn d-orbitals and O p-orbitals at the conduction band which strongly overlap with the formal redox energy of O2 in solution. The activity enhancement strongly manifests in the case of Dy2RuMnO7 where the Ru/Mn ratio is closer to 1 in comparison with the Ho3+ and Er3+ analogs. These electronic effects are discussed in the context of the Gerischer formalism for electron transfer at the semiconductor/electrolyte junctions.