The mechanistic interplay between the oxygen evolution reaction (OER) and material degradation during water electrolysis is not yet well understood even for the most studied OER electrocatalysts such ...as RuO2 and IrO2. It is still disputed whether the lattice oxygen mechanism (LOM) may be competitive with the conventional adsorbate evolving mechanism (AEM) of the OER in these materials and, if so, under what conditions. Herein, we employ density functional theory calculations to demonstrate that the LOM can give rise to higher OER activity than the AEM at the active sites involving structural defects, both intrinsic and extrinsic. Specifically, we show that, although the AEM is preferred for the perfect (110) and (211) surfaces, the formation of metal vacancies due to catalyst dissolution may lead to much lower OER overpotentials for the LOM. Also, by screening several metal impurities in RuO2, we reveal that dopants such as Ni and Co can promote the LOM over the AEM even for the perfectly structured surfaces. Overall, we demonstrate that defective IrO2 is less LOM active than RuO2 that should contribute to its superior stability under OER conditions.
Understanding the mechanistic interplay between the activity and stability of water splitting electrocatalysts is crucial for developing efficient and durable water electrolyzers. Ir-based materials ...are among the best catalysts for the oxygen evolution reaction (OER) in acidic media, but their degradation mechanisms are not completely understood. Here, through first-principles calculations we investigate iridium dissolution at the IrO2(110)/water interface. Simulations reveal that the surface-bound IrO2OH species formed upon iridium dissolution should be thermodynamically stable in a relatively wide potential window undergoing transformations into IrVI (as IrO3) at high anodic potentials and IrIII (as Ir(OH)3) at low anodic potentials. The identified high-valence surface-bound dissolution intermediates of Ir are determined to display greater OER activities than the pristine IrO2(110) surface in agreement with the experimentally observed high activity of an amorphous hydrated IrO x surface layer. Combined with recent experimental results, our simulations illuminate the mechanistic details of the degradation mechanism of IrO2 and how it couples to electrocatalytic OER.
Nitrate is a ubiquitous aqueous pollutant from agricultural and industrial activities. At the same time, conversion of nitrate to ammonia provides an attractive solution for the coupled environmental ...and energy challenge underlying the nitrogen cycle, by valorizing a pollutant to a carbon-free energy carrier and essential chemical feedstock. Mass transport limitations are a key obstacle to the efficient conversion of nitrate to ammonia from water streams, due to the dilute concentration of nitrate. Here, we develop bifunctional electrodes that couple a nitrate-selective redox-electrosorbent (polyaniline) with an electrocatalyst (cobalt oxide) for nitrate to ammonium conversion. We demonstrate the synergistic reactive separation of nitrate through solely electrochemical control. Electrochemically-reversible nitrate uptake greater than 70 mg/g can be achieved, with electronic structure calculations and spectroscopic measurements providing insight into the underlying role of hydrogen bonding for nitrate selectivity. Using agricultural tile drainage water containing dilute nitrate (0.27 mM), we demonstrate that the bifunctional electrode can achieve a 8-fold up-concentration of nitrate, a 24-fold enhancement of ammonium production rate (108.1 ug h
cm
), and a >10-fold enhancement in energy efficiency when compared to direct electrocatalysis in the dilute stream. Our study provides a generalized strategy for a fully electrified reaction-separation pathway for modular nitrate remediation and ammonia production.
Iridium-based materials are considered as state-of-the-art electrocatalysts for oxygen evolution reaction (OER), however, their stability and catalytic activity greatly depend on surface-state ...changes induced by electrochemical cycling. To better understand the behavior of the low-index Ir surfaces in an electrochemical environment, we perform a systematic thermodynamic analysis by means of the density functional theory (DFT) calculations. On the basis of computed surface energies of the Ir (111), (110) and (100) facets as a function of applied electrode potential and coverage of adsorbed water species we determine stability maps and predict equilibrium shapes of Ir nanoparticles. Our calculations also show that metastable oxide precursors formed at the initial stages of Ir surface oxidation are responsible for enhanced catalytic activity toward OER as compared to metal surfaces covered by oxygen adsorbates and thick-oxide films. Such enhancement occurs not only due to the modified thermodynamic stability of OER intermediates, but also because thin-oxide layers may display the more energetically favorable I2M (interaction of two M–O units) rather than WNA (water nucleophilic attack) OER mechanism.
Compared to the studies of new electrolyte and electrode chemistries aimed to push the energy and power density of battery systems, investigations of self-discharge reactions contributing to capacity ...fading are still very limited, especially at the molecular level. Herein, we present a computational study of oxidation–reduction reactions between vanadium ions in solution leading to battery self-discharge due to the crossover of vanadium species through the membrane in all-vanadium redox flow batteries (RFB). By utilizing Car–Parrinello molecular dynamics (CPMD) based metadynamics simulations in combination with the Marcus electron transfer theory, we examine the energetics of condensation reactions between aqueous vanadium ions to form dimers and their subsequent dissociation into vanadium species of different oxidation states after electron transfer has occurred. Our results suggest that multiple self-discharge reaction pathways could be possible under the vanadium RFB operation conditions. The study underscores the complexity of vanadium polymerization reactions in aqueous solutions with coupled electron and proton transfer processes that can lead to the formation of various mixed-valence vanadium polymeric structures.
A strategy to modulate the electrocatalytic activity of copper toward CO2 reduction involving adsorption of acrylamide, acrylic acid, and allylamine polymers is presented. Modification of ...electrodeposited copper foam with poly(acrylamide) leads to a significant enhancement in faradaic efficiency for ethylene from 13% (unmodified foam) to 26% at −0.96 V vs RHE, whereas methane yield is unaffected. Effects from crystalline phase distribution and copper oxide phases are ruled out as the source of enhancement through XPS and in situ XRD analysis. DFT calculations reveal that poly(acrylamide) adsorbs on the copper surface via the oxygen atom on the carbonyl groups and enhances ethylene formation by (i) charge donation to the copper surface that activates CO for dimerization, (ii) chemical stabilization of the CO dimer (a key intermediate for C2 products) by hydrogen-bond interactions with the −NH2 group, and (iii) facilitating the adsorption of CO molecules near the polymer, increasing local surface coverage. Poly(acrylamide) with copper acts as a multipoint binding catalytic system where the interplay between activation and stabilization of intermediates results in enhanced selectivity toward ethylene formation. Modification with poly(acrylic acid) which has a similar structure to poly(acrylamide) also shows some enhancement in activity but is unstable, whereas poly(allylamine) completely suppresses CO2 reduction in favor of the hydrogen evolution reaction.
Strain engineering is an effective strategy in modulating activity of electrocatalysts, but the effect of strain on electrochemical stability of catalysts remains poorly understood. In this work, we ...combine ab initio thermodynamics and molecular dynamics simulations to examine the role of compressive and tensile strain in the interplay between activity and stability of metal oxides considering RuO
2
${{}_{2}}$
and IrO
2
${{}_{2}}$
as exemplary catalysts. We reveal that although compressive strain leads to improved activity via the adsorbate‐evolving mechanism of the oxygen evolution reaction, even small strains should substantially destabilize these catalysts promoting dissolution of transition metals. In contrast, our results show that the metal oxides requiring tensile strain to promote their catalytic activity may also benefit from enhanced stability. Importantly, we also find that the detrimental effect of strain on electrochemical stability of atomically flat surfaces could be even greater than that of surface defects.
Effect of strain on oxygen evolution activity and stability of RuO2 and IrO2 is investigated. Compressive strain has different effects depending on oxygen evolution pathways. Activity and stability can be simultaneously enhanced if tensile strain is required to promote catalytic activity. The quantitative effect of lattice strain on electrochemical stability of atomically flat surfaces might be comparable to that of defective surfaces.
RuO2 is one of the most active electrocatalysts toward oxygen evolution reaction (OER), but it suffers from rapid dissolution in electrochemical environments. It is also established experimentally ...that corrosion of metal oxides can, in fact, promote catalytic activity for OER owing to the formation of a surface hydrous amorphous layer. However, the mechanistic interplay between corrosion and OER across metal-oxide catalysts and to what degree these two processes are correlated are still debated. Herein, we employ ab initio molecular dynamics-based blue moon ensemble approach in combination with OER thermodynamic analysis to reveal a clear mechanistic coupling between Ru dissolution and OER at the RuO2(110)/water interface. Specifically, we demonstrate that (i) dynamic transitions between metastable dissolution intermediates greatly affect catalytic activity toward OER, (ii) dissolution and OER processes share common intermediates with OER promoting Ru detachment from the surface, (iii) the lattice oxygen can be involved in the OER, and (iv) the coupling between different OER intermediates formed at the same Ru site of the metastable dissolution state can lower the theoretical overpotential of OER down to 0.2 eV. Collectively, our findings illustrate the critical role of highly reactive metastable dissolution intermediates in facilitating OER and underscore the need for the incorporation of interfacial reaction dynamics to resolve apparent conflicts between theoretically predicted and experimentally measured OER overpotentials.
Immune checkpoint blockade of signaling pathways such as PD‐1/PD‐L1 has recently opened up a new avenue for highly efficient immunotherapeutic strategies to treat cancer. Since tumor ...microenvironments are characterized by lower pH (5.5‐7.0), pH‐dependent protein‐ligand interactions can be exploited as efficient means to regulate drug affinity and specificity for a variety of malignancies. In this article, we investigate the mechanism and kinetics of pH‐dependent binding and unbinding processes for the PD‐1/PD‐L1 checkpoint pair employing classical molecular dynamics simulations. Two representative pH levels corresponding to circumneutral physiological conditions of blood (pH 7.4) and acidic tumor microenvironment (pH 5.5) are considered. Our calculations demonstrate that pH plays a key role in protein‐ligand interactions with small pH changes leading to several orders of magnitude increase in binding affinity. By identifying the binding pocket in the PD‐1/PD‐L1 complex, we show a pivotal role of the His68 protonation state of PD‐1in the complex stabilization at low pH. The results on the reaction rate constants are in qualitative agreement with available experimental data. The obtained molecular details are important for further engineering of binding/unbinding kinetics to formulate more efficient immune checkpoint blockade strategies.
Atomic-scale understanding of CO2 adsorption and reactivity on TiO2 is important for the development of new catalysts for CO2 conversion with improved efficiency and selectivity. Here, we employ ...Car–Parrinello molecular dynamics combined with metadynamics simulations to explore the interaction dynamics of CO2 and rutile TiO2(110) surface explicitly treating water solution at 300 K. We focus on understanding the competitive adsorption of CO2 and H2O, as well as the kinetics of CO and bicarbonate (HCO3 –) formation. Our results show that adsorption configurations and possible reaction pathways are greatly affected by proper description of the water environment. We find that in aqueous solution, CO2 preferentially adsorbs at the bridging oxygen atom Ob, while Ti5c sites are saturated by H2O molecules that are difficult to displace. Our calculations predict that further conversion reactions include spontaneous protonation of adsorbed CO2 and detachment of OH– to form a CO molecule that is significantly facilitated in the presence of a surface Ti3+ polaron. In addition, the mechanisms of HCO3 – formation in bulk water and near TiO2(110) surface are discussed. These results provide atomistic details on the mechanism and kinetics of CO2 interaction with TiO2(110) in a water environment.
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