Pseudo-Jahn-Teller Effect Breaks the pH Dependence in Two-Electron Oxygen Electroreduction

The hydrogenation of small molecules (like O2 and CO2) often exhibits strong activity dependence on pHs because of discrepant proton donor environments. However, some catalysts can show seldom dependence on two-electron oxygen electroreduction, a sustainable route of O2 hydrogenation to hydrogen peroxide (H2O2). In this work, a pH-resistant oxygen electroreduction system arising from the pseudo-Jahn-Teller effect is demonstrated. Thorough operando Raman spectra, local environment analyses and density function theory simulations, the lattice distortion of TiOxFy that introduces the pseudo-Jahn-Teller effect contributing to regulating local pHs at electrode-electrolyte interfaces and the absorption/desorption of key *OOH intermediate is revealed. Consequently, as comparison to 78.6% activity attenuation for common catalyst, the TiOxFy displays minor activity decay (3.2%) in the pH range of 1-13 with remarkable Faradaic efficiencies (93.4-96.4%) and H2O2 yield rates (595-614 mg cm-2 h-1) in the current densities of 100-1000 mA cm-2. Further techno-economics analyses display the H2O2 production cost dependent on pHs, giving the lowest H2O2 price of $0.37 kg-1. The present finding is expected to provide an additional dimension to pseudo-Jahn-Teller effect that leverages systems beyond traditional conception.

Anodic H2O2 Generation in Carbonate-Based Electrolytes-Mechanistic Insight from Scanning Electrochemical Microscopy

For the anodic H2O2 generation, it has been shown that the electrolyte composition can steer the reaction pathway toward increased H2O2 generation. Previous efforts made on composition optimization found that the impact of the molar fraction of carbonate species varies for different anodes, and therefore, controversies remain concerning the reaction pathways as well as the species involved in H2O2 formation. Considering that water oxidation results in the liberation of protons within the anode microenvironment, the corresponding acidification would cause an equilibrium shift between carbonate species, which in turn may modulate the reaction pathway. We determined the changes in the fraction of carbonate species in the vicinity of an anode by performing local pH measurements using a Au nanoelectrode positioned in close proximity to an operating anode by shear-force scanning electrochemical microscopy (SECM). It could be confirmed that the main anionic species at the interface is HCO3 -, at potentials where H2O2 is preferentially formed, regardless of the pH value in the bulk. The simultaneous use of a Au-Pt double barrel microelectrode in generator-collector SECM measurements demonstrates that the local HCO3 - concentration is collectively determined by the oxidation current, buffer capacity, and bulk pH of the electrolyte.

Efficient and durable seawater electrolysis with a V2O3-protected catalyst

The ocean, a vast hydrogen reservoir, holds potential for sustainable energy and water development. Developing high-performance electrocatalysts for hydrogen production under harsh seawater conditions is challenging. Here, we propose incorporating a protective V2O3 layer to modulate the microcatalytic environment and create in situ dual-active sites consisting of low-loaded Pt and Ni3N. This catalyst demonstrates an ultralow overpotential of 80 mV at 500 mA cm-2, a mass activity 30.86 times higher than Pt-C and maintains at least 500 hours in seawater. Moreover, the assembled anion exchange membrane water electrolyzers (AEMWE) demonstrate superior activity and durability even under demanding industrial conditions. In situ localized pH analysis elucidates the microcatalytic environmental regulation mechanism of the V2O3 layer. Its role as a Lewis acid layer enables the sequestration of excess OH- ions, mitigate Cl- corrosion, and alkaline earth salt precipitation. Our catalyst protection strategy by using V2O3 presents a promising and cost-effective approach for large-scale sustainable green hydrogen production.

One-Step Electrochemical Ethylene-to-Ethylene Glycol Conversion over a Multitasking Molecular Catalyst

Ethylene glycol is an essential commodity chemical with high demand, which is conventionally produced via thermocatalytic oxidation of ethylene with huge fossil fuel consumption and CO2 emission. The one-step electrochemical approach offers a sustainable route but suffers from reliance on noble metal catalysts, low activity, and mediocre selectivity. Herein, we report a one-step electrochemical oxidation of ethylene to ethylene glycol over an earth-abundant metal-based molecular catalyst, a cobalt phthalocyanine supported on a carbon nanotube (CoPc/CNT). The catalyst delivers ethylene glycol with 100% selectivity and 1.78 min-1 turnover frequency at room temperature and ambient pressure, more competitive than those obtained over palladium catalysts. Experimental data demonstrate that the catalyst orchestrates multiple tasks in sequence, involving electrochemical water activation to generate high-valence Co-oxo species, ethylene epoxidation to afford an ethylene oxide intermediate via oxygen transfer, and eventually ring-opening of ethylene oxide to ethylene glycol facilitated by in situ formed Lewis acid site. This work offers a great opportunity for commodity chemicals synthesis based on a one-step, earth-abundant metal-catalyzed, and renewable electricity-driven route.

Bifunctional Near-Neutral Electrolyte Enhances Oxygen Evolution Reaction

Performance of electrocatalytic reactions depends on not only the composition and structure of the active sites, but also their local environment, including the surrounding electrolyte. In this work, we demonstrate that BF2 (OH)2 - anion is the key fluoroborate species formed in the mixed KBi/KF (KBi=potassium borate) electrolyte to enhance the rate of the oxygen evolution reaction (OER) at near-neutral pH. Through a combination of electrokinetic and in situ spectroscopic studies, we show that the mixed KBi/KF electrolyte promotes the OER via two pathways: 1) stabilizing the interfacial pH during the proton-producing reaction with its high buffering capacity; and 2) activating the interfacial water via strong hydrogen bonds with F-containing species. With the KBi/KF electrolyte, electrodeposited Co(OH)2 is able to achieve 100 mA/cm2 at 1.74 V, which is among the highest reported activities with earth-abundant electrocatalysts at near neutral conditions. These findings highlight the potential of leveraging electrolyte-engineering for improving the electrochemical performance of the OER.

Tailoring a local acid-like microenvironment for efficient neutral hydrogen evolution

Electrochemical hydrogen evolution reaction in neutral media is listed as the most difficult challenges of energy catalysis due to the sluggish kinetics. Herein, the Ir-H_xWO₃ catalyst is readily synthesized and exhibits enhanced performance for neutral hydrogen evolution reaction. H_xWO₃ support is functioned as proton sponge to create a local acid-like microenvironment around Ir metal sites by spontaneous injection of protons to WO₃, as evidenced by spectroscopy and electrochemical analysis. Rationalize revitalized lattice-hydrogen species located in the interface are coupled with H_ad atoms on metallic Ir surfaces via thermodynamically favorable Volmer-Tafel steps, and thereby a fast kinetics. Elaborated Ir-H_xWO₃ demonstrates acid-like activity with a low overpotential of 20 mV at 10 mA cm^-2 and low Tafel slope of 28 mV dec^-1, which are even comparable to those in acidic environment. The concept exemplified in this work offer the possibilities for tailoring local reaction microenvironment to regulate catalytic activity and pathway.

Interfacial pH measurements during CO2 reduction on gold using a rotating ring-disk electrode

Insights into how to control the activity and selectivity of the electrochemical CO₂ reduction reaction are still limited because of insufficient knowledge of the reaction mechanism and kinetics, which is partially due to the lack of information on the interfacial pH, an important parameter for proton-coupled reactions like CO₂ reduction. Here, we used a reliable and sensitive pH sensor combined with the rotating ring-disk electrode technique, in which a functionalized Au ring electrode works as a real-time detector of the OH^- generated during the CO₂ reduction reaction at a gold disk electrode. Variations of the interfacial pH due to both electrochemical and homogeneous reactions are mapped and the correlation of the interfacial pH with these reactions is inferred. The interfacial pH near the disk electrode increases from 7 to 12 with increasing current density, with a sharp increase at around -0.5 V vs. RHE, which indicates a change of the dominant buffering species. Through scan rate-dependent voltammetry and chronopotentiometry experiments, the homogenous reactions are shown to reach equilibrium within the time scale of the pH measurements, so that the interfacial concentrations of different carbonaceous species can be calculated using equilibrium constants. Furthermore, pH measurements were also performed under different conditions to disentangle the relationship between the interfacial pH and other electrolyte effects. The buffer effect of alkali metal cations is confirmed, showing that weakly hydrated cations lead to less pronounced pH gradients. Finally, we probe to which extent increasing mass transport and the electrolyte buffer capacity can aid in suppressing the increase of the interfacial pH, showing that the buffer capacity is the dominant factor in suppressing interfacial pH variations.

Effect of Local pH Change in non-PGM Catalysts – A Potential Dependent Mechanistic Analysis of the Oxygen Reduction Reaction

Typically, the ORR show higher activity in alkaline electrolytes than in acidic medium on either platinum-group metal (PGM) or non-PGM catalysts, known from their outer sphere electron transfer (OSET) mechanism...

Quantum electrocatalysts: theoretical picture, electrochemical kinetic isotope effect analysis, and conjecture to understand microscopic mechanisms

The fundamental aspects of quantum electrocatalysts are discussed together with the newly developed electrochemical kinetic isotope effect (EC-KIE) approach.