Overestimation of Photoelectrochemical Hydrogen Evolution Reactivity Induced by Noble Metal Impurities Dissolved from Counter/Reference Electrodes

Importance of broken geometric symmetry of single-atom Pt sites for efficient electrocatalysis

Platinum single-atom catalysts hold promise as a new frontier in heterogeneous electrocatalysis. However, the exact chemical nature of active Pt sites is highly elusive, arousing many hypotheses to compensate for the significant discrepancies between experiments and theories. Here, we identify the stabilization of low-coordinated Pt^II species on carbon-based Pt single-atom catalysts, which have rarely been found as reaction intermediates of homogeneous Pt^II catalysts but have often been proposed as catalytic sites for Pt single-atom catalysts from theory. Advanced online spectroscopic studies reveal multiple identities of Pt^II moieties on the single-atom catalysts beyond ideally four-coordinated Pt^II-N₄. Notably, decreasing Pt content to 0.15 wt.% enables the differentiation of low-coordinated Pt^II species from the four-coordinated ones, demonstrating their critical role in the chlorine evolution reaction. This study may afford general guidelines for achieving a high electrocatalytic performance of carbon-based single-atom catalysts based on other d⁸ metal ions.

Durable Nickel-Iron (Oxy)hydroxide Oxygen Evolution Electrocatalysts through Surface Functionalization with Tetraphenylporphyrin

NiFe-based oxides are one of the best-known active oxygen evolution electrocatalysts. Unfortunately, they rapidly lost performance in Fe-purified KOH during the reaction. Herein, tetraphenylporphyrin (TPP) was loaded on a catalyst/electrolyte interface to alleviate the destabilization of NiFe (oxy)hydroxide. We propose that the degradation occurs primarily due to the release of thermodynamically unstable Fe. TPP acts as a protective layer and suppresses the dissolution of hydrated metal at the catalyst/electrolyte interface. In the electric double layer, the nonpolar TPP layer on the NiFe surface also invigorates the redeposition of the active site, Fe, which leads to prolonging the lifetime of NiFe. The TPP-coated NiFe was demonstrated in anion exchange membrane water electrolysis, where hydrogen was generated at a rate of 126 L h^-1 for 115 h at a 1.41 mV h^-1 degradation rate. Consequently, TPP is a promising protective layer that could stabilize oxygen evolution electrocatalysts.

Synergistic effect of Cu and Fe small nanoparticles supported on porous N-doped graphitic framework for selective electrochemical CO2 reduction at low overpotential

Electrochemical CO₂ reduction is an appealing approach to diminish CO₂ emissions, while obtaining valuable chemicals and fuels from renewable electricity. However, efficient electrocatalysts exhibiting high selectivity and low operating potentials are still needed. Herein it is reported that Cu and Fe nanoparticles supported on porous N-doped graphitic carbon matrix are efficient and selective electrocatalysts for CO₂ reduction to CO at low overpotentials. XRD and Raman spectroscopy confirmed independent Cu and Fe metals as the main phases. HRSEM and HRTEM images show the coral-like morphology of the porous N-doped graphitic carbon matrix supporting Cu and Fe metal nanoparticles (about 10 wt%) homogeneously distributed with an average size of 1.5 nm and narrow size distribution. At the optimum Fe/Cu ratio of 2, this material present high activity for CO₂ reduction to CO at -0.3 V vs. RHE with a faradaic efficiency of 96%. Moreover, at -0.5 V vs. RHE this electrocatalyst produces 27.8 mmol of CO g_cat^-1 h^-1, the production rate being stable for 17 h. A synergy between Cu and Fe nanoparticles due to their close proximity in comparison with independent Cu or Fe electrocatalysts was observed.

On the importance of the electric double layer structure in aqueous electrocatalysis

To design electrochemical interfaces for efficient electric-chemical energy interconversion, it is critical to reveal the electric double layer (EDL) structure and relate it with electrochemical activity; nonetheless, this has been a long-standing challenge. Of particular, no molecular-level theories have fully explained the characteristic two peaks arising in the potential-dependence of the EDL capacitance, which is sensitively dependent on the EDL structure. We herein demonstrate that our first-principles-based molecular simulation reproduces the experimental capacitance peaks. The origin of two peaks emerging at anodic and cathodic potentials is unveiled to be an electrosorption of ions and a structural phase transition, respectively. We further find a cation complexation gradually modifies the EDL structure and the field strength, which linearly scales the carbon dioxide reduction activity. This study deciphers the complex structural response of the EDL and highlights its catalytic importance, which bridges the mechanistic gap between the EDL structure and electrocatalysis.

A Laser-Activated Membrane Introduction Mass Spectrometry Study of Proton Spillover Promoted Alkane Dehydrogenation

Operando high-throughput evaluation of heterogeneous catalysts by laser-activated membrane introduction mass spectrometry (LAMIMS) elucidates the Pt loading dependence of methylcyclohexane dehydrogenation on platinized γ-alumina beads. A CO₂ marking laser rapidly and sequentially heats catalyst beads positioned on a heat-dissipating carbon paper support that overlays a silicone membrane, separating the bead library reaction zone from a quadrupole mass analyzer. The toluene m/z peak varies logarithmically with Pt loading, suggesting that reactivity includes factors that are negatively correlated to Pt loading. These factors may include the Pt/γ-Al₂O₃ surface interfacial region as one component of a heterogeneous catalytically active surface area/mass. This work demonstrates LAMIMS as a broadly applicable high-throughput operando screening method for heterogeneous catalysts.

Template Engineering of CuBi2 O4 Single-Crystal Thin Film Photocathodes

To develop strategies for efficient photo-electrochemical water-splitting, it is important to understand the fundamental properties of oxide photoelectrodes by synthesizing and investigating their single-crystal thin films. However, it is challenging to synthesize high-quality single-crystal thin films from copper-based oxide photoelectrodes due to the occurrence of significant defects such as copper or oxygen vacancies and grains. Here, the CuBi₂ O₄ (CBO) single-crystal thin film photocathode is achieved using a NiO template layer grown on single-crystal SrTiO₃ (STO) (001) substrate via pulsed laser deposition. The NiO template layer plays a role as a buffer layer of large lattice mismatch between CBO and STO (001) substrate through domain-matching epitaxy, and forms a type-II band alignment with CBO, which prohibits the transfer of photogenerated electrons toward bottom electrode. The photocurrent densities of the CBO single-crystal thin film photocathode demonstrate -0.4 and -0.7 mA cm^-2 at even 0 V_RHE with no severe dark current under illumination in a 0.1 m potassium phosphate buffer solution without and with H₂ O₂ as an electron scavenger, respectively. The successful synthesis of high-quality CBO single-crystal thin film would be a cornerstone for the in-depth understanding of the fundamental properties of CBO toward efficient photo-electrochemical water-splitting.