Metal Chalcogenide Nanofilms: Platforms for Mechanistic Studies of Electrocatalysis

The systematic development of improved electrocatalysts requires strategies for preparing candidate materials as well-defined thin-film electrodes that are amenable to straightforward characterization of reaction mechanism and catalyst specific activity. While numerous thin film preparation methods are established for transition metals and metal alloys, few strategies exist for transition metal chalcogenides, despite growing recognition of their role as potent electrocatalysts. Herein we show that electrochemical atomic layer deposition (E-ALD) is a powerful tool for accessing well-defined metal chalcogenide electrocatalysts, by synthesizing, for the first time, crystalline conformal films of Co₉S₈, a promising earth-abundant oxygen reduction catalyst, with tunable nanoscale thickness. The as-prepared nanofilms display relatively high activity for the oxygen reduction reaction and provide a robust platform for detailed mechanistic investigations. Initial mechanistic studies reveal that oxygen reduction on Co₉S₈ nanofilms proceeds via rate-limiting one-electron transfer to O₂ with a specific activity of 20.6 μA cm^–2 at 600 mV vs RHE. This study opens the door to the systematic application of E-ALD to investigate chalcogenide electrocatalysts across the transition series

Donor-Dependent Kinetics of Interfacial Proton-Coupled Electron Transfer

The effect of the proton donor on the kinetics of interfacial concerted proton–electron transfer (CPET) to polycrystalline Au was probed indirectly by studying the rate of hydrogen evolution from trialkylammonium donors with different steric profiles, but the same p<i>K</i>_a. Detailed kinetic studies point to a mechanism for HER catalysis that involves rate-limiting CPET from the proton donor to the electrode surface, allowing this catalytic reaction to serve as a proxy for the rate of interfacial CPET. In acetonitrile electrolyte, triethylammonium (TEAH⁺) displays up to 20-fold faster CPET kinetics than diisopropylethylammonium (DIPEAH⁺) at all measured potentials. In aqueous electrolyte, this steric constraint is largely lifted, suggesting a key role for water in mediating interfacial CPET. In acetonitrile, TEAH⁺ also displays a much larger transfer coefficient (β = 0.7) than DIPEAH⁺ (β = 0.4), and TEAH⁺ displays a potential-dependent H/D kinetic isotope effect that is not observed for DIPEAH⁺. These results demonstrate that proton donor structure strongly impacts the free energy landscape for CPET to extended solid surfaces and highlight the crucial role of the proton donor in the kinetics of electrocatalytic energy conversion reactions

Suppressing Ion Transfer Enables Versatile Measurements of Electrochemical Surface Area for Intrinsic Activity Comparisons

Correlating the current/voltage response of an electrode to the intrinsic properties of the active material requires knowledge of the electrochemically active surface area (ECSA), a parameter that is often unknown and overlooked, particularly for highly nanostructured electrodes. Here we demonstrate the power of nonaqueous electrochemical double layer capacitance (DLC) to provide reasonable estimates of the ECSA across 17 diverse materials spanning metals, conductive oxides, and chalcogenides. Whereas data recorded in aqueous electrolytes generate a wide range of areal specific capacitance values (7–63 μF/real cm²), nearly all materials examined display an areal specific capacitance of 11 ± 5 μF/real cm² when measured in weakly coordinating KPF₆/MeCN electrolytes. By minimizing ion transfer reactions that convolute accurate DLC measurements, we establish a robust methodology for quantifying ECSA, enabling more accurate structure-function correlations

Minor Impact of Ligand Shell Steric Profile on Colloidal Nanocarbon Catalysis

Minor Impact of Ligand Shell Steric Profile on Colloidal Nanocarbon Catalysi

Separation of Polymeric Charge Enables Efficient Bipolar Membrane Operation

Efficient charge separation at the membrane-membrane junction is key to the operation of bipolar membranes (BPMs). Beyond co-ion crossover, there is a lack of understanding of the fundamental processes that control charge separation in BPMs. Herein, we employ polyelectrolytes to investigate the factors controlling charge separation in a cell devoid of co-ion crossover. We find that mobile polymeric charges at the bipolar interface can undergo charge-pairing and quenching in a process that we term bipolar pairing. This phenomenon attenuates membrane voltages and inhibits reverse bias water dissociation kinetics, leading to large overpotential penalties exceeding 7.5 V in some cases. However, we find that a sequential approach to constructing the catalyst and ionomeric binder layers of a BPM can more effectively minimize bipolar pairing than codepositing the two as a single mixed layer. These studies expose a hitherto unknown mechanism of efficiency loss in BPMs with implications for the design of high-efficiency bipolar interfaces

Dendritic Assembly of Gold Nanoparticles during Fuel-Forming Electrocatalysis

We observe the dendritic assembly of alkanethiol-capped gold nanoparticles on a glassy carbon support during electrochemical reduction of protons and CO₂. We find that the primary mechanism by which surfactant-ligated gold nanoparticles lose surface area is by taking a random walk along the support, colliding with their neighbors, and fusing to form dendrites, a type of fractal aggregate. A random walk model reproduces the fractal dimensionality of the dendrites observed experimentally. The rate at which the dendrites form is strongly dependent on the solubility of the surfactant in the electrochemical double layer under the conditions of electrolysis. Since alkanethiolate surfactants reductively desorb at potentials close to the onset of CO₂ reduction, they do not poison the catalytic activity of the gold nanoparticles. Although catalyst mobility is typically thought to be limited for room-temperature electrochemistry, our results demonstrate that nanoparticle mobility is significant under conditions at which they electrochemically catalyze gas evolution, even in the presence of a high surface area carbon and binder. A careful understanding of the electrolyte- and polarization-dependent nanoparticle aggregation kinetics informs strategies for maintaining catalyst dispersion during fuel-forming electrocatalysis

Controlled Chemical Doping of Semiconductor Nanocrystals Using Redox Buffers

Semiconductor nanocrystal solids are attractive materials for active layers in next-generation optoelectronic devices; however, their efficient implementation has been impeded by the lack of precise control over dopant concentrations. Herein we demonstrate a chemical strategy for the controlled doping of nanocrystal solids under equilibrium conditions. Exposing lead selenide nanocrystal thin films to solutions containing varying proportions of decamethylferrocene and decamethylferrocenium incrementally and reversibly increased the carrier concentration in the solid by 2 orders of magnitude from their native values. This application of redox buffers for controlled doping provides a new method for the precise control of the majority carrier concentration in porous semiconductor thin films

Mechanistic Studies of the Oxygen Evolution Reaction Mediated by a Nickel–Borate Thin Film Electrocatalyst

A critical determinant of solar-driven water splitting efficiency is the kinetic profile of the O₂ evolving catalyst (OEC). We now report the kinetic profiles of water splitting by a self-assembled nickel–borate (NiB_i) OEC. Mechanistic studies of anodized films of NiB_i exhibit the low Tafel slope of 2.3 × <i>RT</i>/2<i>F</i> (30 mV/decade at 25 °C). This Tafel slope together with an inverse third order rate dependence on H⁺ activity establishes NiB_i as an ideal catalyst to be used in the construction of photoelectrochemical devices for water splitting. In contrast, nonanodized NiB_i films display significantly poorer activity relative to their anodized congeners that we attribute to a more sluggish electron transfer from the catalyst resting state. Borate is shown to play two ostensibly antagonistic roles in OEC activity: as a promulgator of catalyst activity by enabling proton-coupled electron transfer (PCET) and as an inhibitor in its role as an adsorbate of active sites. By defining the nature of the PCET pre-equilibrium that occurs during turnover, trends in catalyst activity may be completely reversed at intermediate pH as compared to those at pH extremes. These results highlight the critical role of PCET pre-equilibria in catalyst self-assembly and turnover, and accordingly suggest a reassessment in how OEC activities of different catalysts are compared and rationalized

Mesostructure-Induced Selectivity in CO₂ Reduction Catalysis

Gold inverse opal (Au-IO) thin films are active for CO₂ reduction to CO with high efficiency at modest overpotentials and high selectivity relative to hydrogen evolution. The specific activity for hydrogen evolution diminishes by 10-fold with increasing porous film thickness, while CO evolution activity is largely unchanged. We demonstrate that the origin of hydrogen suppression in Au-IO films stems from the generation of diffusional gradients within the pores of the mesostructured electrode rather than changes in surface faceting or Au grain size. For electrodes with optimal mesoporosity, 99% selectivity for CO evolution can be obtained at overpotentials as low as 0.4 V. These results establish electrode mesostructuring as a complementary method for tuning selectivity in CO₂-to-fuels catalysis

Correlation between Electronic Descriptor and Proton-Coupled Electron Transfer Thermodynamics in Doped Graphite-Conjugated Catalysts

Graphite-conjugated catalysts (GCCs) provide a powerful framework for investigating correlations between electronic structure features and chemical reactivity of single-site heterogeneous catalysts. GCC-phenazine undergoes proton-coupled electron transfer (PCET) involving protonation of phenazine at its two nitrogen atoms with the addition of two electrons. Herein, this PCET reaction is investigated in the presence of defects, such as heteroatom dopants, in the graphitic surface. The proton-coupled redox potentials, EPCET, are computed using a constant potential periodic density functional theory (DFT) strategy. The electronic states directly involved in PCET for GCC-phenazine exhibit the same nitrogen orbital character as those for molecular phenazine. The energy εLUS of this phenazine-related lowest unoccupied electronic state in GCC-phenazine is identified as a descriptor for changes in PCET thermodynamics. Importantly, εLUS is obtained from only a single DFT calculation but can predict EPCET, which requires many such calculations. Similar electronic features may be useful descriptors for thermodynamic properties of other single-site catalysts