Zeta potential measurements can provide valuable insights into colloidal stability and may reflect changes in the electrochemical double layer that influence electrocatalysis. In this study, we used zeta potential measurements to monitor and investigate the adsorption behavior of anionic sodium dodecyl sulfate (SDS), cationic dodecyl trimethyl ammonium bromide (DTAB), and cationic cetyl trimethyl ammonium bromide (CTAB) surfactants on SiO2 and TiO2 particles in DI water. The zeta potentials of SiO2 and TiO2 particles were measured as a function of particle concentration in 1.0 mM, 5.0 mM, and 10.0 mM surfactant solutions. Results indicate that there is no significant adsorption of DS- ions on SiO2 particles across all SDS concentrations investigated in this study. The zeta potential of TiO2 particles shifts from negative to positive as particle concentration increases at 1.0 mM SDS, indicating a decrease in DS- ions adsorption per TiO2 particle. Adsorption plateaus at 5.0 mM SDS for all TiO2 concentrations studied. In contrast, DTA+ and CTA+ions showed clear adsorption on SiO2 particles, but DTA+exhibited no apparent adsorption on TiO2 particles. Interestingly, CTA+ adsorption on TiO2 was only evident at higher surfactant concentrations. This study presents a systematic investigation of zeta potential behavior as a function of particle and surfactant concentration, highlighting adsorption saturation and electrostatic reversal. Unlike prior work that was limited to single particle concentration measurements, our approach reveals how surfactant adsorption per particle decreases with increasing surface area. Despite the structural similarity between DTAB and CTAB, this study captured the different adsorption behaviors of both surfactants on SiO2 and TiO2, emphasizing the role of the overall structure of the surfactant on surfactant adsorption behavior. These insights refine the interpretation of zeta potential data and advance understanding of surfactant adsorption on oxide particles. The insight provided in this study about surfactant adsorption on SiO2 and TiO2 particles could have important implications for the design of more effective colloidal and catalytic systems.

The Ensembles of Photosynthetic Nanoreactors (EPN) Energy Frontier Research Center is gaining new knowledge that will help bridge the gap in solar-to-hydrogen energy conversion efficiency between what is observed, i.e. <1%, and necessary, i.e. >10%, to substantially mitigate the effects of global climate change. A major focus is to couple correlative microscopic and spectroscopic measurements with numerical simulations. By doing so, we are overturning conventional wisdom in the understanding of the basic science and engineering that dictate several observations in the field of photocatalytic solar water splitting. Notably, charge separation in state-of-the-art Rh-doped SrTiO 3 and BiVO 4 nanoparticles is not driven by electric fields due to band bending, but instead by differences in mobility and/or lifetime of mobile electronic carriers. Moreover, we have observed that dopants in Rh,La-codoped SrTiO 3 nanoparticles sometimes reside in unexpected crystallographic locations. We have also observed extensive incorporation of Pt cocatalysts into the bulk of Rh-doped SrTiO 3 nanoparticles during Pt photodeposition, which coincides with the induction period for observation of H 2 . Also, using atomic layer deposition to deposit ultrathin permeable oxide coatings on Rh-doped SrTiO 3 nanoparticles, we have observed increased selectivity for photocatalytic H 2 evolution. Lastly, using thermodynamically rigorous detailed balance models, which support observations from experiments, we have shown that the solar-to-hydrogen energy conversion efficiency of an ensemble of optically thin light absorbers can exceed that of optically thick materials, providing new motivation for the study and advancement of photocatalytic, over photoelectrochemical, solar water splitting. Collectively, our discoveries support new approaches, and motivate additional research pathways, toward the development of technoeconomically promising artificial photosynthetic devices.

The performance of platinum electrodes towards the hydrogen evolution reaction (HER) during pH-neutral brine electrolysis was investigated in the presence of alkaline earth metal cations, with a focus on Mg2+ ions. This study aimed to understand the impact of these cations and/or their hydroxide deposits on electrode performance. Electrochemical measurements, including chronoamperometry and polarization curves, were conducted in pH-neutral electrolytes with varying Mg2+ concentrations (0 mM to 55 mM) in 1 M Na2SO4 solution. Contrary to concerns that Mg(OH)2 deposits might impair performance, Pt electrodes exhibited improved HER activity across all Mg2+ concentrations compared to Mg-free electrolytes. To distinguish the effects of hydroxide deposits from solvated cation interactions with the electrocatalyst, other divalent cations (Ca2+, Sr2+, Mn2+) with varying solubility product constants were also tested. All cations enhanced HER performance, including those unlikely to form deposits under the tested conditions. Our results point to cation acidity and Gibbs free energy of hydration as effective descriptors for predicting the role of divalent cations in HER kinetics. In situ image analysis revealed that Mg(OH)2 deposits are periodically removed by evolving hydrogen bubbles, preventing the blockage of active sites. In situ Raman spectroscopy further confirmed the formation of Mg(OH)2 deposits during HER.

Molecular catalysts, such as metalated porphyrins, are attractive cocatalysts for photocatalytic water splitting owing to their potential to simultaneously catalyze target reactions at their metal center, extend charge-separated-state lifetimes, and accumulate the requisite charge for product formation. However, porphyrin catalysts, like most molecular catalysts, are often limited by poor stability associated with demetalation, inactivation by undesired bonding (e.g., O2 coordination/redox/dimerization), and detachment from electrode supports or semiconducting photoabsorbers. In this study, nanoscopic titanium dioxide (TiO2) overlayers, deposited by atomic layer deposition (ALD), are demonstrated to encapsulate cobalt(III) meso-tetra(4-carboxyphenyl) porphyrin chloride (CoTCPP) molecular catalysts and thereby improve their adhesion to electrode surfaces over a wide range of electrode potentials spanning from -1.0 V vs RHE to +1.8 V vs RHE. Through analysis of Raman and ultraviolet-visible spectroscopy, it was confirmed that the metalloporphyrin structure was maintained when the surface-bound CoTCPP was encapsulated by 10 - 250 ALD cycles (≈2 - 18 nm thick) of TiO2. Additional characterization of CoTCPP catalysts before and after electrochemical measurements reveals that up to 97% of the encapsulated CoTCPP remains tethered to the electrode surface after chronoamperometry tests under hydrogen evolution reaction (HER) conditions, compared to <36% for unencapsulated CoTCPP. This study also shows that encapsulated CoTCPP molecules remain partially redox active for overlayers up to 8 nm, which can also attenuate undesired redox mediator back reactions like ferricyanide reduction.

Driven by the need to lower the cost of hydrogen (H 2 ) production via water electrolysis, extensive research is focused on enhancing the efficiency of proton exchange membrane (PEM) water electrolyzers, particularly at higher current densities. Currently, Nafion®, a perfluorinated sulfonic acid (PFSA)-based cation exchange membrane, is the dominant choice for commercial PEM electrolyzers due to its excellent stability and high proton (H + ) conductivity. However, limited membrane selectivity and harmful environmental impacts associated with the production of PFSAs motivate the search for alternative ion exchange membranes. As an alternative to PFAS-based membranes, proton-conducting oxide membranes, which are 2 to 4 orders of magnitude thinner than conventional Nafion, offer significantly reduced membrane resistance without compromising on safety. This talk presents a comprehensive approach to the design and electrochemical evaluation of submicron-thick SiO 2 membrane-electrode assemblies aimed at reducing ionic resistance and mitigating undesired H 2 crossover. We demonstrate the use of optimized microporous layers to enable the deposition of continuous, submicron SiO 2 membranes on porous electrode substrates via atomic layer deposition (ALD), achieving a three-order magnitude reduction in gas crossover. Electrochemical testing of submicron-thick SiO 2 membranes deposited on electrolyzer electrodes, including half-cell polarization curves, confirmed stable electrode performance and low membrane resistance, consistent with the H + conductivity of the ALD SiO₂. The performance of proof-of-concept membrane electrode assemblies based on this architecture was studied with regard to proton and hydrogen transport. These results highlight the potential of submicron oxide membranes for improving the efficiency and sustainability of water electrolysis.

Generating hydrogen from renewable resources would unlock a low-carbon energy carrier that could be used to reduce greenhouse gas emissions in sectors such as industry and transportation. Yet, the allocation...

Electrocatalysts encapsulated by nanoscopic overlayers can catalyze redox reactions at the outer surface of the overlayer or at the buried interface between the overlayer and the active catalyst, leading to complex behavior in the presence of two competing electrochemical reactions. This study investigated oxide encapsulated electrocatalysts (OECs) comprised of iridium (Ir) thin films coated with an ultrathin (2-10 nm thick) silicon oxide (SiOx) or titanium oxide (TiOx) overlayer. The performance of SiOx|Ir and TiOx|Ir thin film electrodes towards the oxygen evolution reaction (OER) and Fe(II)/Fe(III) redox reactions were evaluated. An improvement in selectivity towards the OER was observed for all OECs. Overlayer properties, namely ionic and electronic conductivity, were assessed using a combination of electroanalytical methods and molecular dynamics simulations. SiOx and TiO¬x overlayers were found to be permeable to H2O and O2 such that the OER can occur at the MOx|Ir (M = Ti, Si) buried interface, which was further supported with molecular dynamics simulations. In contrast, Fe(II)/Fe(III) redox reactions occur to the same degree irrespective of whether electrocatalysts are bare, have TiOx overlayers with thicknesses less than 4 nm, or have SiOx overlayers with thicknesses less than 2 nm. This observation is attributed to facile electronic transport between the buried interface and outer surface of the overlayer, as measured with through-plane conductivity and ionic permeability measurements of wetted overlayer materials. These findings reveal the influence of oxide overlayer properties on the activity and selectivity of OECs and suggest opportunities to tune these properties for a wide range of electrochemical reactions.

Multi-component photocatalytic particles can function as nanoscopic, light-driven reactors that absorb sunlight and use its energy to efficiently convert water into hydrogen fuel and oxygen. However, demonstrations of particle-based photocatalytic water splitting have been plagued by low solar-to-hydrogen conversion efficiencies. Gaining deeper insights into loss mechanisms and interactions between particles is essential for enabling rational design of high performance photocatalyst systems for which these losses are minimized. Achieving this goal becomes challenging, however, since photocatalysis studies are typically carried out on ensembles of millions to billions of particles, which often contain wide ranges of shapes, sizes, and/or compositions. Even for a population of identical photocatalyst particles, large variation in performance can be expected to result from different local microenvironments within the reactor set-up and proximity of a given particle to neighboring particles. To overcome these challenges, our team has been adapting and developing methodologies for evaluating the properties and performances of individual micro- and nano-sized photocatalyst particles. In this talk, I will present results from our studies of individual photocatalyst particles based on the use of local optical probes (optical tweezers) and electrochemical probes (scanning electrochemical microscopy) to measure the performances of individual photocatalyst particles and obtain a more quantitative understanding of the distribution of performances and structure-property relationships that underlie these differences.

Understanding proton transfer and water splitting reactions in nano-porous materials is critical for a wide range of emerging technologies, including hydrogen production through photoelectrochemical water splitting. However, elucidating mechanism and energetics of these processes remains a significant challenge for experimental probes. In this work, we combine large-scale molecular dynamics simulations with machine learning potential derived from first-principles calculations to investigate kinetics of proton transfer in nano-porous TiO2 as a representative photocatalyst material. We developed and applied a deep neural network potential to reconstruct the free energy surface of water dissociation and proton transport for a wide range of pore sizes to elucidate confinement effects. Although proton transfer mechanism is similar to that at a TiO2 interface with bulk water, confinement reduces the activation energy of this process, leading to more frequent proton transfer events. This enhanced proton transfer stems from the contraction of oxygen-oxygen distances dictated by the interplay between confinement and hydrophilic interactions. Our simulations also highlight the importance of the surface topology, where faster proton transport is found in the direction where a unique arrangement of surface oxygens enables the formation of an ordered water chain. Our study highlights the critical competition between kinetic and thermodynamic factors introduced by nano-confinement, suggesting potential strategies for optimization of photocatalytic systems for efficient water splitting reactions.

Encapsulating an electrocatalytic material with a semipermeable, nanoscopic oxide overlayer offers a promising approach to enhancing its stability, activity, and/or selectivity compared to an unencapsulated electrocatalyst. However, applying nanoscopic oxide encapsulation layers to high-surface-area electrodes such as nanoparticle-supported porous electrodes is a challenging task. This study demonstrates that the recently developed condensed layer deposition (CLD) method can be used for depositing nanoscopic (sub-10 nm thick) titanium dioxide (TiO2) overlayers onto high-surface-area platinized carbon foam electrodes. Characterization of the overlayers by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) showed that the films are amorphous, while X-ray photoelectron spectroscopy confirmed that they exhibit TiO2 stoichiometry. Electrodes were also characterized by hydrogen underpotential deposition (Hupd) and carbon monoxide (CO) stripping, demonstrating that the Pt electrocatalysts remain electrochemically active after encapsulation. Additionally, copper underpotential deposition (Cuupd) measurements revealed that TiO2 overlayers are effective at blocking Cu2+ from reaching the TiO2/Pt buried interface and were used to estimate that between 43 and 98% of Pt surface sites were encapsulated. Overall, this study shows that CLD is a promising approach for depositing nanoscopic protective overlayers on high-surface-area electrodes.

Photocatalysts are conceptually simple reaction units where nanoscale semiconductors integrated with catalysts drive a pair of redox reactions on illumination. However, the proximity of reaction sites performing cathodic and anodic reactions poses dire challenges to realize large light-to-fuel conversion efficiencies. In this study, a powerful, yet straightforward, equivalent-circuit detail-balance modeling framework is developed and applied to evaluate the performance of photocatalytic systems featuring multiple light absorbers. Specifically, low bandgap iridium-doped strontium titanate is modeled a Z-scheme photocatalyst to effect desired hydrogen evolution and iron-based redox shuttle oxidation reactions. Our model has unique capabilities to simulate competing redox reactions and address mass-transfer limitations. In a significant departure from state-of-the-art circuit models, our study develops tools to perform load-line analyses by incorporating a net electrochemical load curve that includes both desired and competing redox reactions. Consequently, reaction selectivity is predicted from equivalent circuit models for photocatalytic and photoelectrochemical systems. Our investigation into ensembles comprised of multiple, semi-transparent light absorbers reveals their potential to outperform a single, optically thick light absorber, particularly when operated under mass-transfer-limited conditions. However, this outcome hinges on minimizing mass-transfer rates of select redox species to prevent undesired reactions of hydrogen oxidation and/or redox shuttle reduction. Our findings demonstrate that reaction selectivity can be achieved by tuning asymmetry in redox species mass-transfer even with perfectly symmetric electrocatalytic charge-transfer coefficients. The influences of various kinetic, mass-transfer, and thermodynamic parameters are explored to offer crucial insights to inform the next-generation of photocatalysts, selective coatings, and reactor designs.

Photocatalytic water splitting using photocatalyst particles holds great promise for achieving efficient and cost effective production of renewable hydrogen (H2). However, demonstrations of photocatalytic water splitting have generally been plagued by low solar-to-hydrogen conversion efficiencies. Gaining deeper insights into loss mechanisms and interactions between different particles and components is essential for enabling rational design of ensembles of high performance photocatalysts for which these losses are minimized. Towards this end, our team has taken a two pronged approach that relies on the development and use of (i) model systems and (ii) single particle measurements. The first approach relies on the use and development of a planar analog photocatalyst for which all parts of a multi-component photocatalyst (e.g. light absorber, co-catalysts, selective coatings) can be integrated with excellent control over feature size, positioning, and density. This planar analog platform also allows for easier access of advanced analytical tools like scanning electrochemical microscopy, which can be used to directly probe spatial variation in performance and better understand interactions between components as a function of proximity and operating conditions. In this talk, I will describe how this approach can be especially useful for quantifying losses associated with undesired “back reactions” occurring at neighboring active sites, which can be a primary loss mechanism in photocatalyst systems. After describing efforts relating to planar analog photocatalysts, I will also present results from our studies of individual photocatalyst particles that rely on local optical and electrochemical probes to measure the relative performance of individual particles in the presence and absence of neighboring particles.

Photocatalytic water splitting is one promising route to reducing the cost of H2 production to industrially relevant levels. However, these particle-based systems are currently limited in their solar-to-hydrogen efficiency, requiring further development in photoabsorber and co-catalyst materials that can achieve both high activity and selectivity. One scientific challenge to advancing development of photocatalytic particles is that conventional analytical methods used to assess the performance of particle suspensions or sheets tend to measure the average or ensemble performance of a large population of particles having a distribution of sizes and compositions while often being exposed to both optical and chemical gradients within the reaction vessel. As a result, it is very difficult to uncover the structure-property-performance relationships that can guide rational design of photocatalysts. Local measurements, such as scanning electrochemical microscopy (SECM) and Raman microscopy, provide information at smaller length scales which can aid in understanding structure-property-performance relationships of individual photocatalyst particles as well as interactions between neighboring particles. Additionally, these tools can be utilized with model systems to understand how different phenomena are coupled, as well as with the introduction of selective coatings which are common in the field. As a part of broader efforts within the EFRC Ensembles of Photosynthetic Nanoreactors (EPN), our team is also advancing the use of these local techniques to correlate multiple measurements on identical particle, location or particle systems across of network of microscopists. Here, we explore the usage of Raman and SECM on model photocatalysts within the EPN correlative microscopy network. Extension of these observations to ensemble systems based on multiple particles will additionally be discussed.

Direct seawater electrolysis (DSWE) has been investigated as a candidate solution for hydrogen (H2) production using abundant seawater without the water purification requirements of conventional electrolyzers. Costly perfluorinated membranes employed in contemporary water electrolyzers (PEMWE) fail in the presence of trace contaminants due to cation exchange for H+ and scaling resulting from Mg2+/Ca2+ hydroxide precipitation caused by local pH increases. Herein, we disclose a membraneless electrolyzer and explore how symmetric and asymmetric electrolyte recirculation schemes impact pH gradients and performance of the system using in-house developed near-neutral pH electrocatalysts. Under the asymmetric recycle scheme, unbuffered seawater feed can be used whereby pH differentials caused by electrolysis are re-balanced by recirculating the liquid anode and cathode effluent streams to the cathode and anode inlet ports, respectively. Cell voltages of 2.61 and 3.50 V were required to attain 100 mA cm(-2) of current density under buffered (0.5 M phosphate buffer; pH 6.71) and unbuffered synthetic seawater operation, respectively, and 50 h of chronoamperometric stability at the same current density was achieved for the former. A mere 0.18 % of H-2 crossover from the catholyte to the anolyte at 250 mA cm(-2) of current density was recorded at prolonged operation.

Operating water electrolyzers at higher current densities is an attractive approach to improving the economics of hydrogen production from water electrolysis. Conventional proton exchange membrane (PEM) electrolyzers based on Nafion membranes already operate at relatively high current densities (1.5-2.5 A cm-2), but increasing the current density to 5 A cm-2 or higher would improve the economics even further. Currently, a major limitation of operating low-temperature PEM electrolyzers at such high current densities is the large ohmic drop across the Nafion membrane, which can dramatically lower electrolyzer efficiency. To reduce the membrane resistance and enable efficient operation at high current densities, our team has been exploring the use of nanoscopic proton-conducting silicon oxide (SiOx) membranes. Although the proton conductivity of these oxide membranes is lower than Nafion, we show that their total resistance can be made much lower than conventional Nafion-117 membranes by decreasing their thickness to the nanoscale. The use of sub-micron thick membranes is made possible by the high density of SiOx compared to Nafion, which makes SiOx membranes excellent hydrogen (H2) diffusion barriers for preventing H2 crossover. In this work, we show that the area specific membrane resistance of nanoscale SiOx membranes can be reduced to less than 20% that of Nafion-117 membranes while still maintaining desirable H2 blocking capabilities and avoiding problematic electronic leakage current.

Electronic and ionic conductivity of an oxide overlayer can dictate the active site location, which can increase OER selectivity over competing reactions.

Improved understanding of proton transfer in nanopores is critical for a wide range of emerging applications, yet experimentally probing mechanisms and energetics of this process remains a significant challenge. To help reveal details of this process, we developed and applied a machine learning potential derived from first-principles calculations to examine water reactivity and proton transfer in TiO2 slit-pores. We find that confinement of water within pores smaller than 0.5 nm imposes strong and complex effects on water reactivity and proton transfer. Although the proton transfer mechanism is similar to that at a TiO2 interface with bulk water, confinement reduces the activation energy of this process, leading to more frequent proton transfer events. This enhanced proton transfer stems from the contraction of oxygen-oxygen distances dictated by the interplay between confinement and hydrophilic interactions. Our simulations also highlight the importance of the surface topology, where faster proton transport is found in the direction where a unique arrangement of surface oxygens enables the formation of an ordered water chain. In a broader context, our study demonstrates that proton transfer in hydrophilic nanopores can be enhanced by controlling pore size, surface chemistry, and topology.

Electrooxidation of carbon monoxide (CO) and small oxygenate molecules is of great interest for direct alcohol fuel cell (DAFC) and electroorganic synthesis. Herein, we demonstrate that carbon-modified silicon oxide (SiOxCy) overlayers with nanoscopic thickness can greatly enhance the activity of Pt electrodes towards the oxidation of CO and five different oxygenate molecules. Trends in activity are reported with respect to the composition and structure of the SiOxCy overlayers, revealing that low-density, carbon-rich overlayers enhance peak current densities towards the oxidation of formic acid, methanol, ethanol, 1-propanol, and 1-butanol by 370%, 290%, 190%, 130%, and 30%, respectively, compared to bare Pt controls. Unlike conventional alloy electrocatalysts, the alcohol oxidation activity of SiOxCy|Pt electrodes did not strongly correlate with their ability to oxidize CO intermediates, suggesting that that C-H bond scission and/or oxidation of aldehyde or carboxylic acid intermediates, rather than CO intermediates, are the rate limiting steps during the oxidation of C1 and C2 alcohols. For larger alcohols like propanol and butanol, oxidation activity on encapsulated electrodes diminishes relative to bare Pt, which is attributed to mass transport limitations introduced by the overlayer. Overall, the structure-property-performance relationships uncovered in this study provide new insights into how overlayers can alter reaction mechanisms and can be used to guide design of encapsulated catalysts for alcohol oxidation and electroorganic synthesis reactions.

Direct seawater electrolysis is a promising approach to producing green hydrogen in water-scarce environments using renewable energy. However, the undesirable chlorine evolution reaction (CER) and hypochlorite evolution reaction (HCER) compete with the desired oxygen evolution reaction (OER) at the anode electrocatalyst. This issue is most pronounced in unbuffered pH neutral solutions due to local acidification resulting from the OER. To overcome this challenge, this study explores the use of silicon oxide (SiOx) and titanium oxide (TiOx) nanoscale overlayers coated on metallic ruthenium (Ru) and ruthenium oxide (RuOx) thin film electrodes to block chloride ions from reaching active sites during operation in unbuffered 0.6 M NaCl electrolyte. Using a combination of (electro)analytical techniques, encapsulated RuOx anodes are shown to effectively suppress Cl- transport to buried catalyst active sites while allowing for the desired OER to occur, leading to increases in OER faradaic efficiency at moderate overpotentials. Evidence for the ability of SiOx overlayers to block Cl- ions from reaching the active buried interface was obtained by monitoring the OH stretching mode of OH adsorbates using in situ Raman spectroscopy. This study also reports trade-offs between the activity, selectivity, and stability of bare and encapsulated Ru and RuOx electrocatalysts, finding that the magnitude of these trade-offs strongly depends on the nature of both the catalyst and overlayer material. The most promising anode electrocatalyst is RuOx encapsulated by 4 nm of SiOx, which gives the largest improvement in OER faradaic efficiency while demonstrating a relatively stable operating current and minimal increases in overpotential.