AVS 72 Session SS-FrM: Photo- and Electrochemical Energy Conversion

Producing fuels and chemicals using electricity has drawn considerable interest in recent decades. However, research in electrocatalysis, which allows us to link electricity to chemical transformations, remains strongly focused on the electricity-driven transformation of small inorganic molecules such as CO₂, H₂O, N₂, as well as oxygen containing molecules derived from biomass. Yet, comprehensive industrial electrification will require electrocatalytic methods that can promote the reactions that make up the core of the chemicals and fuels industry: n–alkane transformations.

In this presentation, I will show that using electricity to promote n-alkane transformations opens entirely novel avenues of reactivity with the potential to address long-standing challenges in reactivity and selectivity plaguing catalytic alkane chemistry. Specifically, I will show how our group combined fundamental understanding of the interfacial processes occurring in electrocatalytic reactionswith in-situ product analysis using electrochemical mass spectrometry, to gain independent control over the elementary steps of complex alkane transformation reactions in real-time. Using the real-time modulation of the electrode potential applied to an electrocatalyst, we were able to independently control the adsorption of n-alkanes to electrocatalyst surfaces, initiate the transformation of adsorbates while they are bound to the catalyst, and selectively desorb desired products, while leaving others bound. These methods allowed us to demonstrate the room-temperature fragmentation of ethane and butane into shorter chain fragments, achieve the dehydrogenation of n-alkanes at room temperature, open pathways towards the oxidation of n-alkanes in fuel cells, and achieve other industrially relevant hydrocarbon transformations at mild conditions. I will discuss the fundamental science behind these pathways and show how they may chart a path towards more targeted transformation of chemical compounds than is possible with current chemical technologies.

Redox-active organic molecules on noble-metal surfaces provide model interfaces for studying molecular ordering, interfacial charge transfer, and surface-confined electrochemistry. In this study, the non-thiol adsorption and self-assembly of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), a strongly electron-accepting quinone, on Au(111) were investigated. Clean Au(111) substrates were immersed at room temperature in a saturated DDQ solution prepared in 0.1 M HClO₄ for selected immersion periods, allowing the time-dependent evolution of DDQ adsorption and surface assembly to be valuated. The resulting surfaces were characterized by Cyclic Voltammetry (CV) and ambient Scanning Tunneling Microscopy (STM), while X-ray Photoelectron Spectroscopy (XPS) is being used to evaluate surface composition. CVs showed reproducible DDQ-related redox features after immersion, indicating that electroactive DDQ species are retained at the Au interface after air exposure. The redox peak currents increase systematically with immersion time, consistent with progressive adsorption and increasing surface coverage. Scan-rate-dependent measurements support a surface-confined redox process, as the peak current scales linearly with scan rate rather than with the square root of scan rate. These results indicate that immersion time provides a direct way to tune the density of DDQ molecules on the Au(111) surface. Ambient STM was used to probe Au(111) after DDQ exposure. Compared with clean Au(111), DDQ-modified surfaces show clear changes in surface appearance, suggesting adsorbed molecular features and molecule-induced restructuring of the Au surface. The time-dependent STM observations indicate that DDQ adsorption evolves from early-stage surface modification toward more developed, linear assemblies at longer immersion times. Initial analysis suggests that the molecules tilt at elevated coverages, which allows the assemblies to be stabilized through possible pi-pi stacking and H-bonding. Because DDQ contains electron-withdrawing cyano groups and quinone carbonyl groups, adsorption may involve interactions between DDQ functional groups and Au surface atoms, including cyano-mediated binding or adatom-assisted adsorption. Ongoing XPS analysis will clarify the chemical composition of the DDQ-modified Au(111) surfaces and identify changes in the local chemical environment associated with adsorption. Complementary density functional theory calculations are planned to support plausible adsorption geometries and DDQ–Au interaction pathways.

Substrate selection is a decisive factor in the accurate electrochemical evaluation of active materials, as the support itself can obscure or distort the measured response. An ideal substrate combines electrical conductivity with chemical and electrochemical inertness under experimental conditions, ensuring that all measured signals are unambiguously ascribed to the material under investigation. In practice, however, true electrochemical neutrality is rarely achieved, since most conductive supports introduce parasitic contributions arising from double-layer capacitance, surface phase transitions, or residual catalytic activity.

In this context, we present copper-doped multi-walled carbon nanotube (Cu-CNT) films synthesized via spray pyrolysis using α-pinene as a renewable carbon precursor and ferrocene as catalyst, followed by microwave-assisted treatment to promote uniform copper incorporation at the nanotube surface. Films were deposited onto ITO substrates and characterized using scanning electrochemical cell microscopy (SECCM) coupled with cyclic voltammetry, enabling spatially resolved mapping of local electrochemical activity across thousands of discrete surface sites.

Systematic SECCM mapping of bare ITO first established its electrochemical profile as a reference baseline. CNT/ITO surfaces exhibited featureless voltammetric profiles with negligible background current, confirming that the nanotube overlayer effectively cover the ITO surface and yields a well-defined, electrochemically featureless support interface. Cu-CNT/ITO surfaces displayed a well-resolved anodic feature attributable to surface copper oxidation, clearly distinguishable from ITO contributions, alongside characteristic hydrogen and oxygen evolution onset potentials. Strikingly, statistical analysis of spatially resolved voltammograms revealed exceptional surface homogeneity across the Cu-CNT film, with markedly lower site-to-site variability in peak potential and current density than typically observed for heterogeneous metal–carbon interfaces.

These results demonstrate that spray pyrolysis combined with microwave treatment produces Cu-CNT surfaces with uniform electroactive site distribution and reproducible interfacial behavior. The electrochemical cleanliness of the CNT support, confirmed through rigorous substrate mapping, positions Cu-CNT films as model platforms for mechanistic studies of copper-mediated electrocatalysis.

Bimetallic Li alloys are very promising for solid-state battery applications, particularly as thin film interlayer in next-generation Li metal anodes. Exceptional lithiophilicity and Li-transport properties derive from the structure and reversible formation of Li-Ag alloys across a range of compositions. However, atomic-scale knowledge of the structure and Li-transport dynamics of Li-Ag alloy surfaces is limited.

We present an atomically detailed investigation of the formation and evolution of surface and near surface Li_xAg(111) alloys. Lithium is vapor deposited on a well-characterized Ag(111) single crystal substrate under UHV conditions. Lithium films, from sub-monolayer to multilayer thickness, spontaneously form surface alloys at room temperature. Surface alloys, in turn, support and stabilize lithium metallic islands, reflecting their lithiophilicity. The atomic-scale structures, energetics and evolution of Li-Ag surface alloys are mapped with scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), low energy electron diffraction (LEED). Unreported Li-Ag phases are revealed, including a Li_xAg(111) solid solution wetting layer (which supports Li islands) and local Li₁Ag₁(111)-(√3×1) and Li_xAg herringbone phases. Mechanisms of Li_xAg alloy formation, lithiophilicity, stability of Li/Li_xAg(111) interface, and the growth mode of Li islands are discussed. We further explore electric-field driven Li transport, using the STM tip for both introducing a localized field and subsequent characterization tool. Together, these results bridge atomic-scale surface science and device-level interface engineering, offering mechanistic insights for next-generation Li-metal batteries.

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Electrochemical reactivity hinges on the local environment at electrode-electrolyte interfaces, where interactions between the charged electrode, reactive species, and electrolyte ions combine to dictate charge transfer. Of significant interest is how interfacial ion assembly can be leveraged to control reactivity, and many recent studies have worked to identify how particular ions can influence the rate and selectivity of reactions such as hydrogen/oxygen evolution and CO₂ reduction. Electrostatically, ions with the opposite charge as the electrode (counterions) are expected to have a large presence at the interface, while ions with the same charge (co-ions) are expected to have a minor presence. However, many reaction systems utilize large polarizations and highly concentrated electrolytes, leading to correlated, charge-dense interfaces containing both counterions and co-ions. Therefore, it is important to consider the influence of both the counterion and the co-ion on reactivity and determine whether the combined effect of correlated ions differs from what would be expected from each ion individually. Here, we study the hydrogen evolution reaction in mixed salt electrolytes and show that counterions and co-ions act in tandem to significantly alter the interfacial water structure and influence the rate of reaction. We use a combination of interfacial techniques, including X-ray absorption spectroscopy and atomic force microscopy, to connect reactivity to changes in the structural and chemical makeup of the interface and again show that both ionic species together determine interfacial properties. Overall, our work provides fundamental insight into how correlated interfaces play a key role in modulating electrocatalytic reactions.

Self-assembly of nanocrystals by oriented attachment (OA) is a basis for growing larger crystals with defects organized along relict particle boundaries. A critical stage during OA events is the solvent separated state, in which the Angstrom-scale intervening water layers coating the particles provide the time in close proximity needed for interparticle torque acting across the gap to co-align particles.

This talk will feature new experimental and computational insights into how water structure on particle surfaces impacts particle interactions, using two illustrative systems.First, we examine the dispersion/aggregation of three morphologies of hematite nanocrystals in varied aqueous solutions using ex situ electron microscopy and in situ small-angle x-ray scattering. We demonstrate a unique tendency of (104) hematite nanoparticles to maintain a monodisperse state across a wide range of solution conditions not observed with (001)-and (116)-dominated particles, which instead form disordered aggregates. Density functional theory calculations reveal an inert, densely hydrogen-bonded first water layer on the (104) facet that favors interparticle dispersion.The finding clearly shows that facet-engineering is a route to controlling the hydration forces that influence the lifetime of the solvent-separated state.

Second, we demonstrate how macroscopic gibbsite mesocrystals emerge from nanoplates guided into staggered positions by directional sliding determined by intervening water layers. Electron microscopy and X-ray scattering reveal the monoclinic superlattice structure, based on nanoplate stacking with a uniform ≈50° stagger along the gibbsite [010] direction. In situ liquid-cell transmission electron microscopy captures preferential sliding along the gibbsite [010] direction, decelerating with increasing particle overlap. Molecular dynamics simulations reveal that this staggered arrangement corresponds to a global free-energy minimum, rather than full alignment. The simulations also confirm that with 1-2 intervening water layers, particle sliding along the [010] direction is energetically favored.The finding clearly shows the critical role of interfacial water for achieving long-range ordered assemblies.

Our collective insights highlight the central role that interfacial water plays in determining the residence time and energy landscape for interparticle configurational searching during aggregation, in turn determining the extent of order achieved and the corresponding properties of the aggregates themselves.

Zwitterionic materials are well known for their outstanding antifouling properties and have been widely explored in biomedical applications, drug delivery systems, and marine coatings. These properties are generally attributed to the strong surface hydration they maintain. However, probing this hydration in situ is challenging because it occurs at the solid–liquid interface. To address this, we have employed sum frequency generation (SFG) vibrational spectroscopy to study the interfacial hydration of several zwitterionic polymers, including poly(carboxybetaine), poly(sulfobetaine), and poly(trimethylamine N-oxide). As a second-order nonlinear optical technique, SFG provides intrinsic surface sensitivity at the sub-monolayer level due to its selection rules.

Our findings reveal that strongly hydrogen-bonded water molecules dominate the polymer–water interface, giving rise to the robust hydration layer characteristic of these materials. pH-dependent measurements further show that the surface hydration of poly(carboxybetaine) varies with pH, whereas poly(sulfobetaine) remains largely unaffected. These trends closely align with their respective antifouling performances, reinforcing the idea that surface hydration plays a central role in governing antifouling behavior.

Additional SFG studies demonstrate that protein adsorption does not significantly disrupt the hydration layer of zwitterionic polymers, in contrast to polyethylene oxide, whose surface hydration can be compromised by proteins. This highlights a key advantage of zwitterionic systems. Interestingly, mussels tend to avoid settling on zwitterionic surfaces; however, when physically constrained to remain in contact, they can still achieve strong adhesion. SFG results suggest that, unlike on conventional fouling surfaces where mussels first dehydrate the interface, they instead exploit interfacial water and adhesive proteins to bind to zwitterionic materials.

We also found that dissolved salts can weaken surface hydration through charge screening effects, potentially reducing antifouling performance. Nevertheless, when the spatial separation between charges within the zwitterionic structure is small, hydration remains strong even under high-salt conditions. Poly(trimethylamine N-oxide), with its closely spaced charges, is particularly effective at resisting salt-induced screening and maintaining a robust hydration layer.