New insights into the oxygen reduction reaction mechanism on Pt(111): a detailed electrochemical study

Unifying the ORR and OER with surface oxygen and extracting their intrinsic activities on platinum

Common half-cell measurements deliver oxygen reduction activities linked to the chosen scan rate, limiting their value for fundamental and comparative studies on platinum. Here we show a deconvolution of the intrinsic kinetics from the effect of surface oxygen on platinum. We find an electronic effect of the surface oxygen, substantiate a Tafel slope of ~120 mV/decade, obtain an exchange current density of 13 ± 4 µA/cm2, and an activity of 7 mA/cm2 at 900 mV. Eventually, we broaden the scope of this analysis to the effects of surface rearrangement, alloying, and supported Pt nanoparticles, the latter providing insight into discrepancies between half-cell and fuel cell measurements. We find through computational methods that binding energies of intermediates would be weakened by the presence of highly coordinated oxygen atoms. Finally, we obtain a phenomenological rate equation for the oxygen reduction and evolution reaction, suggesting that both reactions follow a shared mechanism.

Effect of alkali-metal cation on oxygen adsorption at Pt single-crystal electrodes in non-aqueous electrolytes

The effect of Group 1 alkali-metal cations (Na+, K+, and Cs+) on the oxygen reduction and evolution reactions (ORR and OER) using dimethyl sulfoxide (DMSO)-based electrolytes was investigated. Cyclic voltammetry (CV) utilising different Pt-electrode surfaces (polycrystalline Pt, Pt(111) and Pt(100)) was undertaken to investigate the influence of surface structure upon the ORR and OER. For K+ and Cs+, negligible variation in the CV response (in contrast to Na+) was observed using Pt(111), Pt(100) and Pt(poly) electrodes, consistent with a weak surface-metal/superoxide complex interaction. Indeed, changes in the half-wave potentials (E1/2) and relative intensities of the redox peaks corresponding to superoxy (O2-) and peroxy (O22-) ion formation were consistent with a solution-mediated mechanism for larger cations, such as Cs+. Support for this finding was obtained via in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). During the ORR and in the presence of Cs+, O2- and weakly adsorbed caesium superoxide (CsO2) species were detected. Because DMSO was found to strongly interact with the surface at potentials associated with the ORR, CsO2 was readily displaced at more negative potentials via increased solvent adsorption at the surface. This finding highlights the important impact of the solvent during ORR/OER reactions.

Cations Determine the Mechanism and Selectivity of Alkaline Oxygen Reduction Reaction on Pt(111)

The proton-coupled electron transfer (PCET) mechanism of the oxygen reduction reaction (ORR) is a long-standing enigma in electrocatalysis. Despite decades of research, the factors determining the microscopic mechanism of ORR-PCET as a function of pH, electrolyte, and electrode potential remain unresolved, even on the prototypical Pt(111) surface. Herein, we integrate advanced experiments, simulations, and theory to uncover the mechanism of the cation effects on alkaline ORR on well-defined Pt(111). We unveil a dual-cation effect where cations simultaneously determine i) the active electrode surface by controlling the formation of Pt-O and Pt-OH overlayers and ii) the competition between inner- and outer-sphere PCET steps. The cation-dependent transition from Pt-O to Pt-OH determines the ORR mechanism, activity, and selectivity. These findings provide direct evidence that the electrolyte affects the ORR mechanism and performance, with important consequences for the practical design of electrochemical systems and computational catalyst screening studies. Our work highlights the importance of complementary insight from experiments and simulations to understand how different components of the electrochemical interface contribute to electrocatalytic processes.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

A density functional theory (DFT) study on reduced partition function ratios of oxygen species adsorbed on a Pt19 cluster and oxygen isotope effects

A density functional theory (DFT) computation on oxygen species adsorbed on platinum (Pt) catalyst surfaces has been carried out to elucidate oxygen isotope fractionation observed at the cathode of a polymer electrolyte membrane fuel cell (PEMFC). The Pt(111) catalyst surface was modelled by a Pt₁₉ cluster, and O, OH, OHH, OO, OOH, OHOH and HOHOH were assumed to be the oxygen species adsorbed on the Pt(111) surface. The oxygen isotope reduced partition function ratios (RPFRs) of the adsorbed species were calculated using the vibrational frequencies obtained by normal mode analyses performed on the optimized structures. Various oxygen isotope exchange equilibria among the adsorbed oxygen species and oxygen and water molecules in the gas phase were examined using their RPFRs. Experimental observation that the lighter ¹⁶O is enriched in water molecules exhausted from the cathode is explainable in a satisfactory manner by assuming oxygen isotope exchange equilibria of O₂ molecule with O, OH, OO and OOH adsorbed on the Pt(111) surface that appear in the first half of the conversion reaction from O₂ to H₂O and those of H₂O molecule with the adsorbed oxygen species, OHH, OHOH and HOHOH, formed in the latter half of the conversion reaction.

Detection of Superoxide Anion Oxygen Reduction Reaction Intermediate on Pt(111) by Infrared Reflection Absorption Spectroscopy in Neutral pH Conditions

In this work, in situ external infrared reflection absorption spectroscopy (IRRAS) is successfully employed for the detection of intermediate species in the oxygen reduction reaction (ORR) mechanism on a flat and well-defined Pt surface. Superoxide anion species (O2-) are detected on the Pt(111) surface in an O2-saturated solution with a NaF/HClO4 mixture with pH 5.5 by the observation of a O-O vibration band at ca. 1080 cm-1. The observation of O2- without the use of any other additional method of signal enhancement is possible because in these experimental conditions O2- is the main ORR-generated intermediate and its reactivity is limited in this pH. This leads to the accumulation of O2- near the Pt surface, facilitating its identification.

In Situ Quantification of the Local Electrocatalytic Activity via Electrochemical Scanning Tunneling Microscopy

Identification of catalytically active sites at solid/liquid interfaces under reaction conditions is an essential task to improve the catalyst design for sustainable energy devices. Electrochemical scanning tunneling microscopy (EC-STM) combines the control of the surface reactions with imaging on a nanoscale. When performing EC-STM under reaction conditions, the recorded analytical signal shows higher fluctuations (noise) at active sites compared to non-active sites (noise-EC-STM or n-EC-STM). In the past, this approach has been proven as a valid tool to identify the location of active sites. In this work, the authors show that this method can be extended to obtain quantitative information of the local activity. For the platinum(111) surface under oxygen reduction reaction conditions, a linear relationship between the STM noise level and a measure of reactivity, the turn-over frequency is found. Since it is known that the most active sites for this system are located at concave sites, the method has been applied to quantify the activity at steps. The obtained activity enhancement factors appeared to be in good agreement with the literature. Thus, n-EC-STM is a powerful method not only to in situ identify the location of active sites but also to determine and compare local reactivity.

Structure effects on electrocatalysts. Oxygen reduction on Te-modified Pt(111) surfaces: Site-blocking vs electronic effects

In this work, the oxygen reduction reaction (ORR) on tellurium-modified Pt(111) surfaces has been studied. Adsorption of Te adatoms on Pt(111) progressively shifts toward less positive values of both the ORR reaction onset and the half-wave potential in 0.1M HClO₄ for 0 < θ_Te < 0.25. However, at θ_Te > 0.25, the ORR activity increases relative to the one at θ_Te < 0.25, but remains lower than that on clean Pt(111). Results were analyzed in light of simulations of kinetic currents as a function of θ_Te, calculated by employing a simple mean field model including both site blocking and electronic effects. Inside this framework, experimental data are best explained by considering that oxygenated Te species inhibit the ORR by either negatively modifying adsorption energies of reaction intermediates or combined site-blocking and electronic effects. A redox ORR catalysis due to redox properties of Te adatoms is discarded. Contrarily, in 0.05M H₂SO₄, a positive catalytic effect has been found, interpreted in terms of a competitive adsorption-desorption mechanism involving the replacement of adsorbed sulfate by Te adatoms. On the other hand, despite the strong site-blocking effect on H_ads and OH_ads adsorption by Te adatoms, it appears that the reduced Te-Pt(111) adlayer does not inhibit the reaction, suggesting different active sites for H_ads and OH_ads adsorption and for the rate-determining step of the ORR mechanism.

Magnetic Field-Enhanced 4-Electron Pathway for Well-Aligned Co3 O4 /Electrospun Carbon Nanofibers in the Oxygen Reduction Reaction

The sluggish reaction kinetics of the oxygen reduction reaction (ORR) has been the limiting factor for fuel energy utilization, hence it is desirable to develop high-performance electrocatalysts for a 4-electron pathway ORR. A constant low-current (50 μA) electrodeposition technique is used to realize the formation of a uniform Co₃ O₄ film on well-aligned electrospun carbon nanofibers (ECNFs) with a time-dependent growth mechanism. This material also exhibits a new finding of mT magnetic field-induced enhancement of the electron exchange number of the ORR at a glassy carbon electrode modified with the Co₃ O₄ /ECNFs catalyst. The magnetic susceptibility of the unpaired electrons in Co₃ O₄ improves the kinetics and efficiency of electron transfer reactions in the ORR, which shows a 3.92-electron pathway in the presence of a 1.32 mT magnetic field. This research presents a potential revolution of traditional electrocatalysis by simply applying an external magnetic field on metal oxides as a replacement for noble metals to reduce the risk of fuel-cell degradation and maximize the energy output.

Mechanism and kinetics of the electrocatalytic reaction responsible for the high cost of hydrogen fuel cells

The sluggish oxygen reduction reaction (ORR) is a major impediment to the economic use of hydrogen fuel cells in transportation. In this work, we report the full ORR reaction mechanism for Pt(111) based on Quantum Mechanics (QM) based Reactive metadynamics (RμD) simulations including explicit water to obtain free energy reaction barriers at 298 K. The lowest energy pathway for 4 e^- water formation is: first, *OOH formation; second, *OOH reduction to H₂O and O*; third, O* hydrolysis using surface water to produce two *OH and finally *OH hydration to water. Water formation is the rate-determining step (RDS) for potentials above 0.87 Volt, the normal operating range. Considering the Eley-Rideal (ER) mechanism involving protons from the solvent, we predict the free energy reaction barrier at 298 K for water formation to be 0.25 eV for an external potential below U = 0.87 V and 0.41 eV at U = 1.23 V, in good agreement with experimental values of 0.22 eV and 0.44 eV, respectively. With the mechanism now fully understood, we can use this now validated methodology to examine the changes upon alloying and surface modifications to increase the rate by reducing the barrier for water formation.

Direct reduction of oxygen gas over dendritic carbons with hierarchical porosity: beyond the diffusion limitation

Direct activation of oxygen gas is achieved over dendritic carbons, resulting in an ultra-high current density without a diffusion plateau.

Combining theory and experiment in electrocatalysis: Insights into materials design

Electrocatalysis plays a central role in clean energy conversion, enabling a number of sustainable processes for future technologies. This review discusses design strategies for state-of-the-art heterogeneous electrocatalysts and associated materials for several different electrochemical transformations involving water, hydrogen, and oxygen, using theory as a means to rationalize catalyst performance. By examining the common principles that govern catalysis for different electrochemical reactions, we describe a systematic framework that clarifies trends in catalyzing these reactions, serving as a guide to new catalyst development while highlighting key gaps that need to be addressed. We conclude by extending this framework to emerging clean energy reactions such as hydrogen peroxide production, carbon dioxide reduction, and nitrogen reduction, where the development of improved catalysts could allow for the sustainable production of a broad range of fuels and chemicals.

Kinetics of Electrocatalytic Reactions from First-Principles: A Critical Comparison with the Ab Initio Thermodynamics Approach

Multielectron processes in electrochemistry require the stabilization of reaction intermediates (RI) at the electrode surface after every elementary reaction step. Accordingly, the bond strengths of these intermediates are important for assessing the catalytic performance of an electrode material. Current understanding of microscopic processes in modern electrocatalysis research is largely driven by theory, mostly based on ab initio thermodynamics considerations, where stable reaction intermediates at the electrode surface are identified, while the actual free energy barriers (or activation barriers) are ignored. This simple approach is popular in electrochemistry in that the researcher has a simple tool at hand in successfully searching for promising electrode materials. The ab initio TD approach allows for a rough but fast screening of the parameter space with low computational cost. However, ab initio thermodynamics is also frequently employed (often, even based on a single binding energy only) to comprehend on the activity and on the mechanism of an electrochemical reaction. The basic idea is that the activation barrier of an endergonic reaction step consists of a thermodynamic part and an additional kinetically determined barrier. Assuming that the activation barrier scales with thermodynamics (so-called Brønsted-Polanyi-Evans (BEP) relation) and the kinetic part of the barrier is small, ab initio thermodynamics may provide molecular insights into the electrochemical reaction kinetics. However, for many electrocatalytic reactions, these tacit assumptions are violated so that ab initio thermodynamics will lead to contradictions with both experimental data and ab initio kinetics. In this Account, we will discuss several electrochemical key reactions, including chlorine evolution (CER), oxygen evolution reaction (OER), and oxygen reduction (ORR), where ab initio kinetics data are available in order to critically compare the results with those derived from a simple ab initio thermodynamics treatment. We show that ab initio thermodynamics leads to erroneous conclusions about kinetic and mechanistic aspects for the CER over RuO₂(110), while the kinetics of the OER over RuO₂(110) and ORR over Pt(111) are reasonably well described. Microkinetics of an electrocatalyzed reaction is largely simplified by the quasi-equilibria of the RI preceding the rate-determining step (rds) with the reactants. Therefore, in ab initio kinetics the rate of an electrocatalyzed reaction is governed by the transition state (TS) with the highest free energy G_rds^#, defining also the rate-determining step (rds). Ab initio thermodynamics may be even more powerful, when using the highest free energy of an reaction intermediate G_max(RI) rather than the highest free energy difference between consecutive reaction intermediates, ΔG_loss, as a descriptor for the kinetics.

The oxygen reduction reaction (ORR) on reduced metals: evidence for a unique relationship between the coverage of adsorbed oxygen species and adsorption energy

The ORR current follows a volcano-like dependence on the coverage of oxygen species that adsorb upon exposure to dissolved oxygen and on their adsorption energy.

The bifurcation point of the oxygen reduction reaction on Au–Pd nanoalloys

The oxygen reduction reaction is of major importance in energy conversion and storage. Controlling electrocatalytic activity and its selectivity remains a challenge of modern electrochemistry. Here, first principles calculations and analysis of experimental data unravel the mechanism of this reaction on Au–Pd nanoalloys in acid media. A mechanistic model is proposed from comparison of the electrocatalysis of oxygen and hydrogen peroxide reduction on different Au–Pd ensembles. A H₂O production channel on contiguous Pd sites proceeding through intermediates different from H₂O₂ and OOH^σ adsorbate is identified as the bifurcation point for the two reaction pathway alternatives to yield either H₂O or H₂O₂. H₂O₂ is a leaving group, albeit reduction of H₂O₂ to H₂O can occur by electrocatalytic HO–OH dissociation that is affected by the presence of adsorbed OOH^σ. Similarities and differences between electrochemical and direct synthesis from H₂ + O₂ reaction on Au–Pd nanoalloys are discussed.

Transient study of the oxygen reduction reaction on reduced Pt and Pt alloys microelectrodes: evidence for the reduction of pre-adsorbed oxygen species linked to dissolved oxygen

Below 100 ms the reduction of oxygen on oxide-free Pt microdisc electrodes reveals the existence of oxygen species which adsorb on the electrode before the reduction is electrochemically driven.

Titanium oxynitride interlayer to influence oxygen reduction reaction activity and corrosion stability of Pt and Pt-Ni alloy

A key advancement target for oxygen reduction reaction catalysts is to simultaneously improve both the electrochemical activity and durability. To this end, the efficacy of a new highly conductive support that comprises of a 0.5 nm titanium oxynitride film coated by atomic layer deposition onto an array of carbon nanotubes has been investigated. Support effects for pure platinum and for a platinum (50 at %)/nickel alloy have been considered. Oxynitride induces a downshift in the d-band center for pure platinum and fundamentally changes the platinum particle size and spatial distribution. This results in major enhancements in activity and corrosion stability relative to an identically synthesized catalyst without the interlayer. Conversely, oxynitride has a minimal effect on the electronic structure and microstructure, and therefore, on the catalytic performance of platinum-nickel. Calculations based on density functional theory add insight with regard to compositional segregation that occurs at the alloy catalyst-support interface.

Metal-nitrogen doping of mesoporous carbon/graphene nanosheets by self-templating for oxygen reduction electrocatalysts

We demonstrate a general and efficient self-templating strategy towards transition metal-nitrogen containing mesoporous carbon/graphene nanosheets with a unique two-dimensional (2D) morphology and tunable mesoscale porosity. Owing to the well-defined 2D morphology, nanometer-scale thickness, high specific surface area, and the simultaneous doping of the metal-nitrogen compounds, the as-prepared catalysts exhibits excellent electrocatalytic activity and stability towards the oxygen reduction reaction (ORR) in both alkaline and acidic media. More importantly, such a self-templating approach towards two-dimensional porous carbon hybrids with diverse metal-nitrogen doping opens up new avenues to mesoporous heteroatom-doped carbon materials as electrochemical catalysts for oxygen reduction and hydrogen evolution, with promising applications in fuel cell and battery technologies.

Oxygen Reduction Reaction on Pt Surface in PEM Fuel Cell Cathode Based on the First-Principles Molecular Dynamics

In proton exchange membrane (PEM) fuel cell cathode, oxygen reduction reaction (ORR) behavior have important effects on fuel cell performance. In this paper, the dynamic oxygen adsorption model and then the redox reaction model for hydrogen and oxygen system were established on the surface of the electricity catalyst Pt. The reaction process of hydrogen and oxygen on the surface of Pt was simulated by first-principles molecular dynamics method, and the influence of temperature on oxygen reduction reaction characteristics was analyzed. The simulation results show that the oxygen atom adsorption on the Pt (111) surface and reaction with the first hydrogen atom are the control step of the oxygen reduction reaction; and the oxygen reduction reaction accelerates with increasing temperature, but the temperature does not affect the geometric structure of the products in the oxygen reduction reaction steps. The results of the simulation agree well with our previous first-principles calclation, and this will be helpful for understanding the mechanism of oxygen reduction reaction in PEM fuel cell cathode.

Specific adsorption of perchlorate anions on Pt{hkl} single crystal electrodes

Perchlorate anion adsorption inhibits the oxygen reduction reaction on Pt{hkl} electrodes in aqueous perchloric acid due to weak specific adsorption.

Some reflections on the understanding of the oxygen reduction reaction at Pt(111)

The oxygen reduction reaction (ORR) is a pivotal process in electrochemistry. Unfortunately, after decades of intensive research, a fundamental knowledge about its reaction mechanism is still lacking. In this paper, a global and critical view on the most important experimental and theoretical results regarding the ORR on Pt(111) and its vicinal surfaces, in both acidic and alkaline media, is taken. Phenomena such as the ORR surface structure sensitivity and the lack of a reduction current at high potentials are discussed in the light of the surface oxidation and disordering processes and the possible relevance of the hydrogen peroxide reduction and oxidation reactions in the ORR mechanism. The necessity to build precise and realistic reaction models, which are deducted from reliable experimental results that need to be carefully taken under strict working conditions is shown. Therefore, progress in the understanding of this important reaction on a molecular level, and the choice of the right approach for the design of the electrocatalysts for fuel-cell cathodes is only possible through a cooperative approach between theory and experiments.