We present a comprehensive experimental study of the formation and activity of dealloyed nanoporous Ni/Pt alloy nanoparticles for the cathodic oxygen reduction reaction. By addressing the kinetics of ...nucleation during solvothermal synthesis we developed a method to control the size and composition of Ni/Pt alloy nanoparticles over a broad range while maintaining an adequate size distribution. Electrochemical dealloying of these size-controlled nanoparticles was used to explore conditions in which hierarchical nanoporosity within nanoparticles can evolve. Our results show that in order to evolve fully formed porosity, particles must have a minimum diameter of ∼15 nm, a result consistent with the surface kinetic processes occurring during dealloying. Nanoporous nanoparticles possess ligaments and voids with diameters of approximately 2 nm, high surface area/mass ratios usually associated with much smaller particles, and a composition consistent with a Pt-skeleton covering a Ni/Pt alloy core. Electrochemical measurements show that the mass activity for the oxygen reduction reaction using carbon-supported nanoporous Ni/Pt nanoparticles is nearly four times that of commercial Pt/C catalyst and even exceeds that of comparable nonporous Pt-skeleton Ni/Pt alloy nanoparticles.
Recent advances in oxygen reduction reaction catalysis for proton exchange membrane fuel cells (PEMFCs) include i) the use of electrochemical dealloying to produce high surface area and sometimes ...nanoporous catalysts with a Pt‐enriched outer surface, and ii) the observation that oxygen reduction in nanoporous materials can be potentially enhanced by confinement effects, particularly if the chemical environment within the pores can bias the reaction toward completion. Here, these advances are combined by incorporating a hydrophobic, protic ionic liquid, MTBDbeti, into the pores of high surface‐area NiPt alloy nanoporous nanoparticles (np‐NiPt/C + MTBDbeti). The high O2 solubility of the MTBDbeti, in conjunction with the confined environment within the pores, biases reactant O2 toward the catalytic surface, consistent with an increased residence time and enhanced attempt frequencies, resulting in improved reaction kinetics. Half‐cell measurements show the np‐NiPt/C+MTBDbeti encapsulated catalyst to be nearly an order of magnitude more active than commercial Pt/C, a result that is directly translated into operational PEMFCs.
Nanoporous Ni/Pt nanoparticles exhibit mass activities nearly an order of magnitude higher than Pt/C when encapsulated with MTBDbeti ionic liquids, both in half‐cell measurements and fuel cell testing.
The sluggish kinetics of the alkaline hydrogen electrode have been attributed to the need to adsorb both H and OH optimally. In this work, single-crystal voltammetry and microkinetic modeling show ...that an OH-mediated mechanism is not viable on Pt(110). Only a direct Volmer step can explain observed kinetic trends with OH adsorption strength in KOH and LiOH electrolytes. Instead, OH behaves as a rapidly equilibrated spectator species that decreases available surface sites and slows hydrogen kinetics. These results identify kinetic barriers from interfacial water structure, not adsorption energies, as key to explaining changes in hydrogen kinetics with pH.
We have discovered a highly regioselective aminofluorination of cyclopropanes. Remarkably, four unique sets of conditionstwo photochemical, two purely chemicalgenerated the same aminofluorinated ...adducts in good to excellent yields. The multiple, diverse ways in which the reaction could be initiated provided valuable clues that led to the proposal of a “unifying” chain propagation mechanism beyond initiation, tied by a common intermediate. In all, the proposed mechanism herein is substantiated by product distribution studies, kinetic analyses, LFERs, Rehm–Weller estimations of ΔG ET, competition experiments, KIEs, fluorescence data, and DFT calculations. From a more physical standpoint, transient-absorption experiments have allowed direct spectroscopic observation of radical ion intermediates (previously only postulated or probed indirectly in photochemical fluorination systems) and, consequently, have provided kinetic support for chain propagation. Lastly, calculations suggest that solvent may play an important role in the cyclopropane ring-opening step.
The pH-dependent kinetics of the hydrogen oxidation and evolution reactions (HERs and HORs) remain a fundamental conundrum in modern electrochemistry. Recent efforts have focused on the impact of the ...interfacial water network on the reaction kinetics. In this work, we quantify the importance of interfacial water dynamics on the overall hydrogen reaction kinetics with kinetic isotope effect (KIE) voltammetry experiments on single-crystal Pt(111) and Pt(110). Our results find a surface-sensitive KIE for both the HER and the HOR that is measurable in base but not in acid. Remarkably, the HOR in KOD on Pt(111) yields a KIE of up to 3.4 at moderate overpotentials, greater than any expected secondary KIE values, yet the HOR in DClO4 yields no measurable KIE. These results provide direct evidence that solvent dynamics play a crucial role in the alkaline but not in the acidic hydrogen reactions, thus reinforcing the importance of “beyond adsorption” phenomena in modern electrocatalysis.
Perfluorosulfonic acid (PFSA)-based ionomers, most notably Nafion, provide the ionic connection between catalyst particles and solid polymer ion conducting membrane separators in polymer electrolyte ...membrane fuel cells (PEMFCs). While necessary, their presence adversely impacts the performance of the catalyst. PEMFC catalyst performance losses associated with Nafion ionomers are due to physical barriers to reactant gas transport and loss of active sites through specific adsorption of polymer-bound sulfonate groups on the catalyst surface. These impacts are particularly damaging for PEMFC high current density (HCD) performance. Here, we show that ionic liquids (IL) present at the catalyst interface can act to prevent the specific adsorption of sulfonate groups of Nafion on Pt surfaces, negating the detrimental impact of the ionomer on reaction rates, specifically for the oxygen reduction reaction (ORR). This result holds promise for further Pt loading reductions in PEMFCs.
Many receptor proteins of the GPCR family exist in multiple isoforms. A comprehensive analysis of different combinations of GPCR isoforms that produce diverse signalling patterns in cells has ...implications for drug development. See p.650
Many receptor proteins of the GPCR family exist in multiple isoforms. A comprehensive analysis of different combinations of GPCR isoforms that produce diverse signalling patterns in cells has ...implications for drug development.
CO2 electrochemical reduction to formate has emerged as one of the promising routes for CO2 conversion to useful chemicals and energy storage. Palladium has been shown to make formate with a high ...selectivity at minimal overpotential. However, production of CO as a minor product quickly deactivates the catalyst during extended electrolysis. Here, we present nanoporous Pd alloys (np-PdX) formed by electrochemical dealloying of Pd15X85 alloys (X = Co, Ni, Cu, and Ag) as active free-standing electrocatalysts with high formate selectivity and CO poisoning tolerance. Rate of deactivation under constant potential electrolysis, due to CO passivation, is strongly correlated to the identity of the transition metal alloying component. We purport that this composition dependent behavior is due to the induced electronic changes in the active Pd surface, affecting both the CO adsorption strength and the near surface hydrogen solubility, which can impact the adsorption strength of active/inactive intermediates and reaction selectivity. Free-standing np-PdCo and np-PdNi are found to exhibit high areal formate partial current densities, >20 mA cm–2, with high CO poisoning tolerance and minimal active area loss at cathodic potentials, demonstrating the utility of these materials for selective and stable CO2 electrolysis.
Although electrochemical hydrogen evolution and oxidation are arguably the best-understood reactions in electrocatalysis, the anomalous effect of pH on hydrogen reaction kinetics has defied simple ...explanation for decades. This longstanding puzzle exposes gaps in the fundamental understanding of electrocatalysis by showing that singular adsorption descriptors (e.g., the hydrogen binding energy) cannot describe kinetic effects across electrolytes. In this Perspective, we discuss the strengths and shortcomings of binding energies as HER/HOR activity descriptors across different electrolytes and catalyst surfaces, with a special emphasis on the bifunctional mechanism, and identify several “beyond adsorption” descriptors for chemical dynamics in the double layer, including the potential of zero (free/total) charge, the binding energy of coadsorbed spectator species, transition state barrier heights, and the solvation strength of electrolyte cations. Recent evidence for and against the importance of these phenomena is assessed in the context of hydrogen electrocatalysis to determine their feasibility to accurately predict catalyst behavior. Finally, we propose paths forward for improving the mechanistic understanding of how specific interactions between the surface and species in solution affect macroscopic rates, which include combining single-crystal voltammetry, electroanalytical chemistry, in-operando spectroscopy, atomic-scale DFT calculations, and molecular “double-layer dopants".
