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Fe–N/C catalysts with tunable mesoporous structures and carbon layer numbers were prepared and served as model catalysts for unraveling the important role of interlayer oxygen activation in boosting the oxygen reduction reaction activity.

Electrochemically robust oxide-supported dendritic Pt and Ir nanoparticles for PEM-URFC.

Symmetry breaking and valency change of Ni–N–C catalysts endow a CO2 electroreduction activity.

A peptide antibiotic, daptomycin, was repurposed to a multicopper catalyst presenting cooperative rate enhancement in O–O bond formation and activation reactions.

Photoelectrochemical (PEC) biomass oxidation by the substitution of an oxygen evolution reaction is considered a promising strategy for efficient hydrogen production.

Beyond great advances in initial activity, Fe-N-C catalysts face the next challenge of stability issue in acidic medium that must be overcome to replace Pt in fuel cell cathode. However, the complex phenomena in fuel cells and consequential difficulty in understanding deactivation mechanisms of Fe-N-C cathodes impede solutions for prolonged stability. Here, we show time-resolved changes in active site density (SD) and turnover frequency (TOF) of Fe-N-C along with concurrent decrease in oxygen reduction reaction (ORR) current in temperature/gas controllable gas-diffusion electrode (GDE) flow cell. In operando diagnosis of Fe leaching identifies a strong dependence of SD changes on operating parameters, and draws a lifetime-dependent stability diagram that reveals a shift in prime degradation mechanism during the operations. A proof-of-concept strategy with site-isolated Pt ions as a non-catalytic stabilizer, supported by theoretical calculation, demonstrates enhanced fuel cell stability with reduced Fe dissolution, offering new design principle for durable Fe-N-C catalysts.

Single-atom catalysts (SACs) featuring atomically dispersed metal cations covalently embedded in a carbon matrix show significant potential to achieve high catalytic performance in various electrocatalytic reactions. Although considerable advances have been achieved in their syntheses and electrochemical applications, further development and fundamental understanding are limited by a lack of strategies that can allow the quantitative analyses of their intrinsic catalytic characteristics, that is, active site density (SD) and turnover frequency (TOF). Here we show an in situ SD quantification method using a cyanide anion as a probe molecule. The decrease in cyanide concentration triggered by irreversible adsorption on metal-based active sites of a model Fe-N-C catalyst is precisely measured by spectrophotometry, and it is correlated to the relative decrease in electrocatalytic activity in the model reaction of oxygen reduction reaction. The linear correlation verifies the surface-sensitive and metal-specific adsorption of cyanide on Fe-N x sites, based on which the values of SD and TOF can be determined. Notably, this analytical strategy shows versatile applicability to a series of transition/noble metal SACs and Pt nanoparticles in a broad pH range (1-13). The SD and TOF quantification can afford an improved understanding of the structure-activity relationship for a broad range of electrocatalysts, in particular, the SACs, for which no general electrochemical method to determine the intrinsic catalytic characteristics is available.

Unlike planar photoelectrodes, bendable and malleable photoelectrodes extend their application to mechanical flexibility beyond conventional rigid structures, which have garnered new attention in the field of photoelectrochemical water splitting. A bendable metal (Hastelloy), which has both bendability and compatibility with various oxide layers, allows high-temperature processes for crystallization; therefore it is far superior as a substrate than a conventional flexible polymer. In this study, we fabricate bendable BiVO4 crystalline thin films on the metal substrates by employing template layers (SrRuO3/SrTiO3) to reduce the structural misfits between BiVO4 and the substrate. The crystallinities were verified through X-ray diffraction and transmission electron microscopy, and photocatalytic performances were examined. The crystallinity of BiVO4 was significantly improved by utilizing similar lattice constants and affinities between BiVO4 and the oxide template layers. We also formed a type II heterojunction by adding a WO3 layer which complements the charge separation and charge transfer as a photoanode. The photocurrent densities of tensile-bent BiVO4/WO3 thin films with a bending radius of 10 mm are comparable to those of pristine BiVO4/WO3 thin film in various aqueous electrolytes. Moreover, photostability tests showed that the tensile-bent crystalline photoanodes retained 90% of their initial photocurrent density after 24 h, which proved their exceptional durability. Our work demonstrates that the bendable photoelectrodes with crystallinity hold great potential in terms of device structure for solar-driven water splitting.

Electrocatalytic conversion of CO2 into value-added products offers a new paradigm for a sustainable carbon economy. For active CO2 electrolysis, the single-atom Ni catalyst has been proposed as promising from experiments, but an idealized Ni-N4 site shows an unfavorable energetics from theory, leading to many debates on the chemical nature responsible for high activity. To resolve this conundrum, here we investigated CO2 electrolysis of Ni sites with well-defined coordination, tetraphenylporphyrin (N4-TPP) and 21-oxatetraphenylporphyrin (N3O-TPP). Advanced spectroscopic and computational studies revealed that the broken ligand-field symmetry is the key for active CO2 electrolysis, which subordinates an increase in the Ni redox potential yielding NiI. Along with their importance in activity, ligand-field symmetry and strength are directly related to the stability of the Ni center. This suggests the next quest for an activity-stability map in the domain of ligand-field strength, toward a rational ligand-field engineering of single-atom Ni catalysts for efficient CO2 electrolysis.

Fuel cells, a sustainable technology that assures a cleaner earth, once experienced disillusion due to the issue of economic viability associated with the massive usage of Pt-based catalysts. To address this issue, research on cost-effective catalysts, especially on the cathode side, has been intensively carried out where the latest achievements have made the M–Nx/C material on its verge of application. Here, we present a comprehensive overview of the fundamental chemistry of oxygen reduction reaction (ORR) and the progress of the M–Nx/C catalysts. Efforts aimed at clarifying the nature of catalytic sites are summarized, highlighting the indispensable role of advanced characterization tools. A variety of successful attempts are exemplified to describe the synthetic methodologies, followed by close tracking of newly emerging single-atom or dual-atom sites. Future directions for this research area are believed to be the rational and/or controllable synthesis of catalysts with adequate active sites and the use of strategies to substantially alleviate catalyst degradation.

Carbon materials have been used as supporting substrates for electrocatalytically active species in many important reactions due to their high electrical conductivity and chemical inertness. To secure durable electrocatalysis, the fundamental understanding and prevention of corrosion are pivotal. The real-time monitoring of evolved CO2 enabled by differential electrochemical mass spectroscopy (DEMS) has provided potential-resolved information about carbon corrosion. However, this approach is typically limited to acidic media and it is unavailable in alkaline media because of the carbonation of the CO2 tracer. In this study, we investigate carbon corrosion using carbon monoxide (CO) as a corrosion marker. The electrochemical oxidation of selected model substrates (i.e., activated carbon, graphite, and Pt/C) is analyzed under dynamic and static potential excursions using DEMS. The results demonstrate that the CO tracer successfully monitored carbon corrosion in both acidic and alkaline media. The corrosion in alkaline media is qualitatively analogous to that in acidic media, but it is quantitatively alleviated, as confirmed by the decreased DEMS intensity and the decreased faradaic charge flowed.

The electrochemical synthesis of hydrogen peroxide (H2O2) can provide an attractive alternative to the current anthraquinone redox process, as it combines on-site chemical and electricity productions. A major challenge in the electrochemical H2O2 synthesis is the catalyst design which leads to a selective two-electron pathway in the oxygen reduction reaction (ORR) without dissociation of the O O bond. In the present work, we report that the partial oxidation of metal-nitrogen-carbon catalysts (Me-N-C, Me = iron, cobalt and manganese) can modify their ORR mechanisms from a four- to a two-electron pathway. Spectroscopic measurements reveal that ex situ H2O2 treatment introduces abundant oxygen functionalities on the Me-N-C surface without considerable changes to their bulk properties, such as crystallinity, degree of defects, surface area, and metal contents. Decreased H2O2 reduction kinetics on the oxidized catalysts confirm that the dissociation of the O O bond is strongly suppressed by the newly introduced oxygen functionalities. Among the three central metal candidates, the cobalt-nitrogen-carbon catalyst shows the highest H2O2 selectivity of >85%. This work provides a new simple guideline for designing Me-N-C catalysts for the efficient electrochemical synthesis of H2O2.

Corrosion of carbon support is one of the most crucial causes of the degradation of polymer electrolyte membrane fuel cells (PEMFCs) utilizing carbon-supported platinum nanoparticles (Pt/C) as a ca...

Ammonia has recently received considerable attention as an alternative energy carrier and a carbon-neutral fuel. In this future energy scenario, the ammonia oxidation reaction (AOR) is a pivotal pr...

The demand for non-noble bifunctional electrocatalysts for overall water splitting was increased for simplifying water-splitting systems and accelerating commercialization. Herein, oxygen vacancy-r...

A three-electrode system is typically utilized in many voltammetry studies to understand the behavior of an analyte at the electrode/electrolyte interface. A bulk Pt piece is usually used as a coun...

Converting carbon dioxide (CO2) into valuable products is one of the most important processes for a sustainable society. Especially, the electrochemical CO2 reduction reaction (CO2RR) offers an effective means, but its reaction mechanism is not yet fully understood. Here, we demonstrate that cation-coupled electron transfer (CCET) is a rate-determining step in the CO2RR to carbon monoxide. The first-principles-based multiscale simulation identifies a single cation that coordinates a CO2− intermediate adsorbed on Ag electrode. The CCET is experimentally verified by a collapse of the CO2RR polarization curves upon correcting Nernstianly for a bulk cation concentration. As further confirmation, a kinetic study shows that the CO2RR obeys first-order kinetics on a local cation concentration. Finally, this work unveils that the cation effect on CO2RR originates from the local colligative property, and further highlights the importance of ion-pairing tendency for electrochemical interface design to achieve high-performance CO2 electrolysis.

Electrocatalysis, whose reaction venue locates at the catalyst-electrolyte interface, is controlled by the electron transfer across the electric double layer, envisaging a mechanistic link between the electron transfer rate and the electric double layer structure. A fine example is in the CO

Electrocatalytic conversion of nitrogen oxides to value-added chemicals is a promising strategy for mitigating the human-caused unbalance of the global nitrogen-cycle, but controlling product selectivity remains a great challenge. Here we show iron–nitrogen-doped carbon as an efficient and durable electrocatalyst for selective nitric oxide reduction into hydroxylamine. Using in operando spectroscopic techniques, the catalytic site is identified as isolated ferrous moieties, at which the rate for hydroxylamine production increases in a super-Nernstian way upon pH decrease. Computational multiscale modelling attributes the origin of unconventional pH dependence to the redox active (non-innocent) property of NO. This makes the rate-limiting NO adsorbate state more sensitive to surface charge which varies with the pH-dependent overpotential. Guided by these fundamental insights, we achieve a Faradaic efficiency of 71% and an unprecedented production rate of 215 μmol cm−2 h−1 at a short-circuit mode in a flow-type fuel cell without significant catalytic deactivation over 50 h operation., Electrocatalytic conversion of nitrogen oxides to value-added chemicals is a promising strategy for mitigating the imbalance in the global nitrogen cycle. Here, the authors present iron–nitrogen-doped carbon as an efficient and durable electrocatalyst for selective nitric oxide reduction to hydroxylamine.

Converting carbon dioxide (CO2) into valuable products is one of the most important processes for a sustainable society. Especially, the electrochemical CO2 reduction reaction (CO2RR) offers an effective means, but its reaction mechanism is not yet fully understood. Here, we demonstrate that concerted cation–electron transfer (CCET) is a key catalytic step in the CO2RR to carbon monoxide. The first-principles-based multiscale simulation identifies a single cation that coordinates a CO2− intermediate adsorbed on Ag electrode. The CCET is experimentally verified by a collapse of the CO2RR polarization curves upon correcting for the thermodynamic activity of the cation. As further confirmation, a kinetic study shows that the CO2RR obeys first-order kinetics on the local cation concentration at the electric double layer (estimated by measuring the electrode surface charge). Finally, this work unveils the fundamental origin of different CO2RR activity depending on the species of alkali metal cation, and further highlights the importance of ion-pairing tendency of the cations for electrochemical interface design to achieve high-performance CO2 electrolysis.