Kyoung-Shin Choi

Nanostructuring is established as a photoelectrode performance enhancing route for solar-driven catalytic processes. However, it is often unclear to what extent the increase in photoelectrode physical surface area directly results in a commensurate increase in photoelectrochemically active surface area. While there are established methods for estimating the physical and electrochemically active surface areas, it is challenging to quantify the operando photoelectrochemically active surface area of nanostructured or nanoparticulate photoelectrodes. Herein, we demonstrate that photoinduced absorption (PIA) spectroelectrochemistry can be employed to quantify the operando photoelectrochemically active area of photoanodes for the oxygen evolution reaction (OER) by exploiting the strong dependence of OER turnover frequency on physical surface hole density. We investigate BiVO4, α-Fe2O3, and TiO2 photoanodes with contrasting morphologies – planar, nanoparticulate, and nanostructured. Our study combines operando measurements of TOF and geometric surface hole density to quantify the photoelectrochemically active surface area of each catalytic system for OER. For the nanostructured photoelectrodes, the photoelectrochemically active surface areas determined from our PIA analyses are shown to be in agreement with their physical surface area, consistent with their good OER performance. This contrasts with the low photoelectrochemically active surface areas compared to their high physical surface area determined by the nanoparticulate photoelectrodes, resulting in poor performance. We conclude by demonstrating how the photoelectrochemically active surface areas of nanoparticulate photoelectrodes increase when driving more kinetically facile reactions or by adding co-catalysts.

The analytical chemistry classroom is naturally poised to demonstrate the use of orthogonal techniques to quantify different molecules such as polyphenols. Using food science as a lens and a molecule-centric, inquiry-based approach, this article describes a semester-long laboratory experience for students to adapt and use potentiometric titration, spectrophotometric, and chromatographic methods to analyze polyphenols (i.e., catechin and gallic acid) in instructor-provided samples. Students then are given the agency to select a sample of interest which (theoretically) contains the target analytes (or not) and justify the appropriate method for that sample. The implementation of this experience provides students with many opportunities to authentically engage in scientific practices, such as designing experiments, evaluating their results, and making evidence-based decisions about how to modify procedures and what samples to study. After completing this laboratory experience, students reported significant gains in learning laboratory techniques and moderate gains in designing experiments and analyzing data, while engaging in authentic analyses (including variability) in a supportive environment. Peer evaluations also indicated that students had positive experiences working in groups. Furthermore, the experience is incredibly flexible. Each standard method module (titration, spectrophotometry, chromatography) can be completed independently, or more orthogonal techniques (e.g., gas chromatography) could be added to expand the molecule-centric semester experience.

Molecules derived from lignocellulosic biomass are oxygenates with multiple oxygen-containing functional groups, such as hydroxyl and carbonyl groups. Therefore, the ability to selectively reduce a specific oxygenate group is essential for the reductive upgrading of such molecules. Previous studies on electrochemical biomass conversion have shown that alcohol hydrogenolysis, which involves cleavage of the σ(C-O_alcohol) bond, is extremely challenging for furfural and 5-hydroxymethylfurfural (HMF) derivatives, including furfuryl alcohol, 5-methylfurfuryl alcohol (MFA), and 2,5-bis(hydroxymethyl)furan (BHMF). In contrast, HMF itself undergoes alcohol hydrogenolysis relatively easily in acidic aqueous media. Considering that the only structural difference between HMF and BHMF or MFA is the presence of a carbonyl group, this observation raises the question of whether the carbonyl group in HMF facilitates alcohol hydrogenolysis. In this study, we designed systematic experiments to provide a coherent explanation of when and how a carbonyl group enables hydrogenolysis of a copresent alcohol group. Specifically, we show that alcohol hydrogenolysis in HMF proceeds via reduction of the carbonyl group to a ketyl radical, followed by a spin-center shift (SCS) through extended π conjugation. We also elucidate the effect of pH on the selectivity between carbonyl hydrogenation and alcohol hydrogenolysis, both of which share the ketyl radical intermediate. This mechanistic understanding enhances our ability to predict and control alcohol hydrogenolysis in the reductive upgrading of biomass-derived oxygenates.

The electrochemical reductive dimerization of small aromatic carbonyl compounds derived from lignocellulosic biomass is a crucial C-C coupling reaction for upgrading small molecules to long-chain hydrocarbons, particularly in the synthesis of drop-in sustainable aviation fuels. Although other electrochemical reduction reactions of these reactants (i.e., hydrogenation and hydrogenolysis) have undergone extensive mechanistic investigation, the understanding of dimerization remains relatively underdeveloped. Most importantly, there is a lack of understanding of the selectivity-determining step between dimerization and monomer reduction and critical factors that can affect this step. In this study, we provide a comprehensive mechanistic model to explain the competition between dimerization and monomer reduction of benzaldehyde under various conditions. Our model proposes that the selectivity between dimerization and monomer reduction depends on the competition between desorption of a ketyl radical from the electrode, necessary for dimerization, and further reduction of the ketyl radical to an alcohol on the electrode by proton-coupled electron transfer (PCET). Computationally comparing the adsorption/desorption energy and PCET activation barrier energy is challenging because conventional DFT calculations substantially underestimate the PCET kinetic barriers. In this study, we employed constrained DFT-based configuration interaction (CDFT-CI) to obtain a reliable comparison of these energies. Our mechanistic model was tested and supported by experimental results obtained with four electrodes (Cu, Pb, Bi, graphite), three pH conditions (acidic, neutral, basic), and three potentials. Our study offers a coherent mechanistic foundation that can explain how each of these conditions impacts the desorption and PCET processes and the selectivities for dimerization and alcohol production.

The oxygen evolution reaction (OER) is paired with various electrochemical and photoelectrochemical reduction reactions used for fuel and chemical production. As there is a strong interest in performing many of these reduction reactions in strongly acidic solutions to increase the reaction rate, efficiency, or selectivity, there is also a great interest in enabling efficient and stable OER in strongly acidic solutions. In this study, we report stable photoelectrochemical OER (POER) of a BiVO₄ photoanode in 0.1 M HNO₃ (pH 1). This was achieved by using Nb₂O₅ as a protection layer. While Nb₂O₅ was rarely used as a protection layer for photoelectrodes in the past, we show its excellent capability to suppress both the chemical and photoelectrochemical dissolution of BiVO₄ at pH 1. After stabilizing BiVO₄ with a Nb₂O₅ protection layer, we added Co²⁺ ions to the electrolyte as an OER catalyst to enhance the POER. We found that Co_(aq)²⁺ can serve as a homogeneous OER catalyst without being deposited as a CoO_x solid catalyst on Nb₂O₅. When we performed the POER using unprotected BiVO₄ with Co_(aq)²⁺ under the same condition, although POER was enhanced, the enhancement could not be sustained due to the chemical dissolution of BiVO₄. After the POER, we found that a Co³⁺-containing OER catalyst was deposited on the bare BiVO₄ surface. This result suggested that the use of Co²⁺ ions as a homogeneous catalyst was possible due to the inertness of the Nb₂O₅ surface toward the adsorption or deposition of Co ions. This study enabling stable POER of BiVO₄ in 0.1 M HNO₃ using the combination of a Nb₂O₅ protection layer and Co_(aq)²⁺ as a homogeneous OER catalyst provides promising possibilities for acidic POER and OER.

In photoelectrochemical cells, semiconductor electrodes are usually interfaced with protection layers to extend their stability. Ideally, the protection layer should not decrease photocurrent generation. Hence, the conduction band minimum (CBM) and valence band maximum (VBM) of the protection layer should appropriately align with those of the underlying semiconductor electrode to facilitate the desired interfacial charge transfer with minimal interfacial recombination. However, predicting interfacial band alignment can be challenging, as it may vary depending on the detailed interfacial atomic structure. Investigating the effect of the atomic structure at the semiconductor/protection layer junction on the band alignment is also challenging as it requires samples with varied interfaces without altering the semiconductor and protection layers. Here, we considered TiO₂, the most widely used material as a protection layer, interfaced with a BiVO₄ photoanode, and we fabricated two n-type BiVO₄(010)/TiO₂ photoanodes where a thin (∼4 nm) amorphous TiO₂ layer was deposited by atomic layer deposition (ALD). While the individual BiVO₄(010) and TiO₂ layers were identical in these two samples, we modified the interfacial atomic structure at the BiVO₄/TiO₂ junction by changing which precursor, Ti or O, was introduced first upon deposition of TiO₂. By experimentally and computationally investigating the differences in these two samples, we show that the band alignments between BiVO₄ and TiO₂ at the interface may not be straightforwardly predicted by the CBM and VBM of bulk BiVO₄ and TiO₂ and that interfacial atomic arrangements can have a marked impact on the electronic properties and photoelectrochemical performance of the BiVO₄(010)/TiO₂ photoanode.