Mixed Electron–Proton Conductors Enable Spatial Separation of Bond Activation and Charge Transfer in Electrocatalysis

Sodium tungsten bronze-supported Pt electrocatalysts for the high-performance hydrogen evolution reaction

NaxWO3 nanotube bundle was fabricated as a support for hosting Pt nanoparticle. Benefitting from the metal–support interaction, the optimal catalyst shows excellent activity with 46 mV overpotential at −100 mA cm−2, superior to the commercial Pt/C.

Recent Advances in Electrocatalysts for Alkaline Hydrogen Oxidation Reaction

With the rapid development of anion-exchange membrane technology and adequate supply of high-performance non-noble metal oxygen reduction reaction (ORR) catalysts in alkaline media, the commercialization of anion exchange membrane fuel cells (AEMFCs) become possible. However, the kinetics of the anodic hydrogen oxidation reaction (HOR) in AEMFCs is significantly decreased compared to the HOR in proton exchange membrane fuel cells (PEMFCs). Therefore, it is urgent to develop HOR catalysts with low price, high activity, and robust stability. However, comprehensive timely reviews on this specific subject do not exist enough yet and it is necessary to update reported major achievements and to point out future investigation directions. In this review, the current reaction mechanisms on HOR are summarized and deeply understood. The debates between the mechanisms are greatly harmonized. Recent advances in developing highly active and stable electrocatalysts for the HOR are reviewed. Moreover, the side reaction control is for the first time systematically introduced. Finally, the challenges and future opportunities in the field of HOR catalysis are outlined.

Electrolysis Can Be Used to Resolve Hydrogenation Pathways at Palladium Surfaces in a Membrane Reactor

For common hydrogenation chemistries that occur at high temperatures (where H₂ is adsorbed and activated at the same surface which the substrate must also adsorb for reaction), there is often little consensus on how the reactions (e.g., hydro(deoxy)genation) actually occur. We demonstrate here that an electrocatalytic palladium membrane reactor (ePMR) can be used to study hydrogenation reaction mechanisms at ambient temperatures, where the catalyst does not necessarily undergo structural reorganization. The ePMR uses electrolysis and a hydrogen-selective palladium membrane to deliver reactive hydrogen to a catalyst surface in an adjacent compartment for reaction with an organic substrate. This process forms the requisite metal-hydride surface for hydrogenation chemistry, but at ambient temperature and pressure, and without a H₂ source. We demonstrate the utility of this analytical tool by studying the hydrogenation of benzaldehyde at palladium nanocubes with dimensions of 13-24 nm. This experimental design enabled us to resolve that the alcohol product forms at the facial sites, whereas the hydrodeoxygenation step occurs at edge sites. These observations enabled us to develop the first site-specific definition of how a carbonyl species undergoes hydro(deoxy)genation.

Boosting thermo-photocatalytic CO2 conversion activity by using photosynthesis-inspired electron-proton-transfer mediators

Natural photosynthesis proceeded by sequential water splitting and CO2 reduction reactions is an efficient strategy for CO2 conversion. Here, mimicking photosynthesis to boost CO2-to-CO conversion is achieved by using plasmonic Bi as an electron-proton-transfer mediator. Electroreduction of H2O with a Bi electrode simultaneously produces O2 and hydrogen-stored Bi (Bi-Hx). The obtained Bi-Hx is subsequently used to generate electron-proton pairs under light irradiation to reduce CO2 to CO; meanwhile, Bi-Hx recovers to Bi, completing the catalytic cycle. This two-step strategy avoids O2 separation and enables a CO production efficiency of 283.8 μmol g-1 h-1 without sacrificial reagents and cocatalysts, which is 9 times that on pristine Bi in H2 gas. Theoretical/experimental studies confirm that such excellent activity is attributed to the formed Bi-Hx intermediate that improves charge separation and reduces reaction barriers in CO2 reduction.

Predicting the Energetics of Hydrogen Intercalation in Metal Oxides Using Acid-Base Properties

The ability to predict intercalation energetics from first principles is attractive for identifying candidate materials for energy storage, chemical sensing, and catalysis. In this work, we introduce a computational framework that can be used to predict the thermodynamics of hydrogen intercalation in tungsten trioxide (WO₃). Specifically, using density functional theory (DFT), we investigated intercalation energetics as a function of adsorption site and hydrogen stoichiometry. Site-specific acid-base properties determined using DFT were used to develop linear structure screening models that informed a kernel ridge energy prediction model. These regressions provided a series of hydrogen binding energy predictions across stoichiometries ranging from WO₃ to H_0.625WO₃, which were then converted to equilibrium potentials for hydrogen intercalation. Experimental validation using cyclic voltammetry measurements yielded good agreement with the predicted intercalation potentials. This methodology enables fast exploration of a large geometric configuration space and reveals an intuitive physical relationship between acidity, basicity, and the thermodynamics of hydrogen intercalation. Furthermore, the combination of theoretical and experimental results suggests H_0.500WO₃ as a maximum stable stoichiometry for the bronzes that arises from competition with hydrogen evolution rather than the inability of WO₃ to accommodate additional hydrogen. Our experimental results further indicate hydrogen insertion in WO₃ is highly irreversible for low H-stoichiometries, which we propose to be a consequence of the semiconductor-to-metal transition that occurs upon initial H-intercalation. Overall, the agreement between theory and experiment suggests that local acid-base characteristics govern hydrogen intercalation in tungsten trioxide, and this insight can aid the accelerated discovery of redox-active metal oxides for catalytic hydrogenations.

Electric Dipole Descriptor for Machine Learning Prediction of Catalyst Surface-Molecular Adsorbate Interactions

The challenge of evaluating catalyst surface-molecular adsorbate interactions holds the key for rational design of catalysts. Finding an experimentally measurable and theoretically computable descriptor for evaluating surface-adsorbate interactions is a significant step toward achieving this goal. Here we show that the electric dipole moment can serve as a convenient yet accurate descriptor for establishing structure-property relationships for molecular adsorbates on metal catalyst surfaces. By training a machine learning neural network with a large data set of first-principles calculations, we achieve quick and accurate predictions of molecular adsorption energy and transferred charge. The training model using NO/CO@Au(111) can be extended to study additional substrates such as Au(001) or Ag(111), thus exhibiting extraordinary transferability. These findings validate the effectiveness of the electric dipole descriptor, providing an efficient modality for future catalyst design.

Polyoxometalate-based electron transfer modulation for efficient electrocatalytic carbon dioxide reduction

The electrocatalytic carbon dioxide (CO2) reduction reaction (CO2RR) involves a variety of electron transfer pathways, resulting in poor reaction selectivity, limiting its use to meet future energy requirements. Polyoxometalates (POMs) can both store and release multiple electrons in the electrochemical process, and this is expected to be an ideal "electron switch" to match with catalytically active species, realize electron transfer modulation and promote the activity and selectivity of the electrocatalytic CO2RR. Herein, we report a series of new POM-based manganese-carbonyl (MnL) composite CO2 reduction electrocatalysts, whereby SiW12-MnL exhibits the most remarkable activity and selectivity for CO2RR to CO, resulting in an increase in the faradaic efficiency (FE) from 65% (MnL) to a record-value of 95% in aqueous electrolyte. A series of control electrochemical experiments, photoluminescence spectroscopy (PL), transient photovoltage (TPV) experiments, and density functional theory (DFT) calculations revealed that POMs act as electronic regulators to control the electron transfer process from POM to MnL units during the electrochemical reaction, enhancing the selectivity of the CO2RR to CO and depressing the competitive hydrogen evolution reaction (HER). This work demonstrates the significance of electron transfer modulation in the CO2RR and suggests a new idea for the design of efficient electrocatalysts towards CO2RR.