Rohini Bala Chandran

Our study focuses on an experimental demonstration of a novel method for CO 2 removal from ocean water, which has captured up to 40% of all anthropogenically released CO 2 emissions, that combines H 2 and redox salt looping to induce pH swings in electrochemical flow cells. Model-driven design optimization guides 3-D printed electrolyte flow channel designs to alleviate mass-transfer limitations. A ridged flow channel with 1 mm thick fins protruding at an angle of 30° from the base reduces ferri-/ferro-cyanide redox salt concentration boundary layer thickness by up to 35% while restricting the pressure drop to less than 415 Pa. A notable feature of this work is the experimental validation of the intrinsic operando cleaning capabilities enabled by the reversible looping process, eliminating what would otherwise be process down time. Over 4 acidification/basification cycles, 86% removal of fouling from electrode surfaces is demonstrated, while distinctly maintaining a constant electrochemical energy intensity. The lab-scale, proof-of-concept experimental demonstration shows promising results, which are interpreted to provide insights and guidelines toward further enhancing scalability and efficiency of oceanic carbon removal and utilization processes.

Our study focuses on an experimental demonstration of a novel method for CO₂ removal from ocean water, which has captured up to 40% of all anthropogenically released CO₂ emissions, that combines H₂ and redox salt looping to induce pH swings in electrochemical flow cells. Model-driven design optimization guides 3-D printed electrolyte flow channel designs to alleviate mass-transfer limitations. A ridged flow channel with 1 mm thick fins protruding at an angle of 30° from the base reduces ferri-/ferro-cyanide redox salt concentration boundary layer thickness by up to 35% while restricting the pressure drop to less than 415 Pa. A notable feature of this work is the experimental validation of the intrinsic <i>operando</i> cleaning capabilities enabled by the reversible looping process, eliminating what would otherwise be process down time. Over 4 acidification/basification cycles, 86% removal of fouling from electrode surfaces is demonstrated, while distinctly maintaining a constant electrochemical energy intensity. The lab-scale, proof-of-concept experimental demonstration shows promising results, which are interpreted to provide insights and guidelines toward further enhancing scalability and efficiency of oceanic carbon removal and utilization processes.

This study combines optical tweezers with integrated microelectrode sensors and fluorescence detectors to simultaneously measure the activity and photoluminescence of single photocatalyst particles suspended in aqueous solutions. Individual phosphorus-doped bismuth vanadate particles were studied under visible-light illumination, revealing a direct correlation between the rate of product generation and photoluminescence intensity. Additionally, a hybrid electrogenerated chemiluminescence (H-ECL) process was revealed that relies on the presence of both low-energy infrared light and a galvanic redox couple to emit photons. This phenomenon may be leveraged to estimate the loss of efficiency caused by self-parasitic back reactions. The demonstrated ability to study suspended photocatalysts on a single particle level highlights optical particle trapping with electrochemical sensors (OPTECS) as a powerful technique for elucidating single particle structure-property-performance relationships critical to the design of efficient photocatalysts.

Our study focuses on an experimental demonstration of a novel method for CO2 removal from ocean water that combines H2 and redox salt looping to induce pH swings in electrochemical flow cells. Model-driven design optimization guides 3-D printed electrolyte flow channel designs to alleviate mass-transfer limitations. A ridged flow channel with 1 mm thick fins protruding at an angle of 30◦ from the base reduces ferri-/ferro-cyanide redox salt diffusion boundary layer thickness by up to 41% while restricting pressure drop to less than 138 Pa. A notable feature of this work is the experimental validation of the intrinsic in-situ cleaning capabilities enabled by the reversible looping process, eliminating what would otherwise be process down time. Over 4 acidification/basification cycles, 86% removal of fouling from electrode surfaces is demonstrated, while distinctly maintaining a constant electrochemical energy intensity. The lab-scale, proof-of-concept experimental demonstration show promising results, which are interpreted to provide insights and guidelines towards further enhancing scalability and efficiency of oceanic carbon removal and utilization processes.

ABSTRACT In the current work, a hybrid and flipped (hybrid+flipped) course structure integrating open‐access pre‐recorded videos and in‐person class sessions was piloted with the objective of improving student experiences in a core undergraduate thermodynamics class in mechanical engineering. To minimize the barrier to instructor adoption, the course structure leveraged open‐access videos on the fundamentals of thermodynamics for the flipped portion of the class. The hybrid+flipped format allowed increased use of engaged‐learning tools during classroom sessions; methods known to improve learning outcomes (e.g., in‐class demonstrations, “think‐pair‐share” activities, and content and activities leveraging open‐access thermodynamics property data). Survey data from over 200 student participants from 2022 to 2023 document the student concerns at the start and exit of participation in the hybrid+flipped course sections. While the academic outcomes were unchanged compared with standard course delivery, student perceptions were dramatically affected. The survey data show significant positive shift in student concerns regarding online learning (44.2% were negative at the start of the course to 28.2% at the end of the class), and a large fraction of the students (45.6%) felt the hybrid+flipped format improved their learning outcomes. While the case study provides a valuable detailed example of how to successfully implement a hybrid+flipped class structure using open‐access tools, the results also show such efforts must correspondingly maintain high standards for organization and supplement the digital materials with community building and student and instructor engagement.

Two-step, Z-scheme reactors for photocatalytic water splitting have the potential to increase sunlight spectrum absorption while improving operational safety with an aqueous redox shuttle to transfer charge between two hydrogen- and oxygen-evolving catalysts. There is limited understanding of how the interactions between aqueous redox shuttle ions and the photocatalyst influences kinetics and reaction rates. To explore electrolyte-focused methods to enhance photocatalytic rates, this study presents a comprehensive investigation of the effects of electrolyte conditions --- pH (2 -- 2.5), redox shuttle concentration (0 - 6 mM FeCl3), nanoparticle size (30 nm and 100 nm diameter), and nanoparticle concentration (0.1 g/L and 0.5 g/L) on the oxygen evolution rates with TiO2 nanoparticles and Fe^3+ redox shuttles. Experimentally measured reaction rates with nanoparticle suspensions are uniquely connected with zeta potential measurements to probe electrostatic behavior at the solid-liquid interface and correlate observed enhancements with ionic behavior in the electric double layer. Additionally, thermodynamic analysis was performed to predict iron speciation and surface site composition as well as adsorption energies using Density Functional Theory simulations. Over the range of test conditions, reaction rate variations of nearly an order-of-magnitude (2 uM/min to 12 uM/min) were observed due to the interdependent effects of pH, particle diameter, and initial Fe^3+ concentration in solution. Experimentally measured zeta potentials informs theoretical ion loading density on surface sites and ion fluxes through the double layer, which prove to critically influence the photocatalytic reaction rates. An optimal pH of 2.3 was found to balance electric double layer thickness and proton site-blocking at low FeCl3 concentrations (&lt; 2 mM) for, counterintuitively, the larger diameter (100 nm) TiO2 particles. At a pH of 2.5, precipitation of Fe3+ as Fe2O3 was observed, indicating an irreversible redox reaction. Results motivate increased emphasis on quantifying electric double layer effects across solid-liquid interfaces to engineer advanced surface-electrolyte interactions to overcome limits in photocatalytic performance.

Carbon dioxide capture and removal technologies are critical to limit global warming to 2 °C. In this regard, ocean water CO2 capture can complement Direct Air Capture (DAC) technologies. In addition to the atmosphere, the oceans are huge sinks for carbon and have to-date captured about 25% of all anthropogenically released carbon. Previously, we have presented the performance of a new, reversible, electrochemical device that combines hydrogen and ferrocyanide redox salt looping to capture CO2 from ocean water via pH-shifting [1]. This process provides benefits including the potential to load shift the electricity demand of the electrochemical device and intermittent cleaning of the electrode surface. Cleaning is needed at ocean water fouls electrode surface when a locally high pH is present near the electrode surface. Results in a flow cell with custom manufactured flow plates were designed to reduce these limitations. This limitation was reduced through a computational design study to identify design/operational strategies to increase the operating current densities by lowering concentration/diffusion boundary layer thickness. Specifically, our experimental tests on flow cells demonstrate enhanced mass transport with up to 40% increase in the limiting current densities for a flow rate of 10 mL/min with the use of ridges as compared to the base case without ridges. Additionally, the energy intensities of carbon removal in alignment with the state of the art. However, due to ohmic resistance limitations, the current densities tested in the flow cell are inherently limited. Therefore, to probe a wider range of current densities a new H-cell architecture is created to enable viewing of the electrode during the pH shifting reactions. Different mechanisms to remove fouling seem to occur at different current densities. FTIR analysis is used to analyze the type of fouling observed at different current densities. Counterintuitively, low current densities may result in the least extreme local pH, they also result in the largest amount of fouling. To explain this phenomenon, boundary layer theory between concentration and momentum conditions is compared. This work seeks to identify the optimal current density to reduce the fouling constraint when operating a pH-shifting oceanic carbon removal electrochemical cell. Preliminary results indicate current densities above 5 mA/cm2 outperform low current densities, which aligns with the goal of achieving commercially viable (&gt;100 mA/cm^2) current densities for electrochemical pH-shifting methods of oceanic CO2 removal. [1] Rachel Silcox, Rohini Bala Chandran, Demand-side flexibility enables cost savings in a reversible pH-swing electrochemical process for oceanic CO2 removal, Cell Reports Physical Science, Volume 5, Issue 3, 2024, 101884, ISSN 2666-3864, https://doi.org/10.1016/j.xcrp.2024.101884.

Photocatalysis is a promising method for splitting water molecules into green oxygen and hydrogen gas via direct sunlight. However, gas evolution rates are limited due to the necessity of high-bandgap materials with low absorptance in the visible light range to overcome the energetic barriers of the redox reaction. The Z-scheme reactor design circumvents this limitation by separating the reaction into two distinct steps mediated by a charge-exchanging redox shuttle reaction. This design allows for more efficient use of the visible light spectrum via reduced bandgap catalysts, and individual optimization of the oxygen evolving (OER) and hydrogen evolving (HER) half-reactions. In this work, we demonstrate a scaled type-III Z-scheme reactor design which separates the OER and HER half-reactions into separate chambers between which the redox shuttle electrolyte is exchanged. While semiconductor nanoparticle sheets are the dominant design for large-scale photocatalytic demonstrations, semiconductor-coated ceramic photocatalysts show potential for improved flow design, optical absorption, reaction surface area, and mass transport. Additionally, optical concentrators are a relatively cost-effective method of enhancing reaction rates by increasing incident light intensity. Here, we experimentally compare large-scale reaction rates and feasibility of each photocatalyst design under varying illumination intensity via optical concentration, flow distribution design, redox shuttle concentration, electrolyte pH, and material composition. The photocatalytic materials tested include TiO2 and SrTiO3 nanoparticle sheets, as well as AlO3 foams coated in TiO2 and BiVO4. Results emphasize the importance of demonstrating photocatalytic water splitting at scale to determine the practical limitations and the potential for novel methods of reaction rate enhancement.

Production of nitrogen fertilizers has contaminated many sources of wastewater with nitrogen species (NO3-, NO2-, HN3, NH4+, N2) that harm aquatic and plant life, introducing a need for a method to treat this water for re-use [1]. Electrochemical reduction of these species is a low-energy method of combined wastewater treatment and recovery of nitrogen species as value-added products, such as ammonia. Mass transport impedes reaction rates of nitrate reduction in divided parallel-plate flow reactors as the reactant concentration at the electrode surface is insufficient. This limitation is especially exacerbated in low concentration electrolyte sources, such as polluted groundwater with predominantly nitrate contaminants at concentrations less than 10 mM [2]. Our work aims to compare the influences of pulsed electrolyte flow to specifically control the boundary layer thickness near electrodes to alleviate mass-transfer limitations; and pulsed electrolysis, which can impact selectivity. To explore the effects of pulsed flow, early experimental datasets are obtained in an analogous system, but with reversible ferri/ferrocyanide redox couple (Fe(CN)₆³⁻/Fe(CN)₆⁴⁻) instead of nitrate reduction, and paired with water reduction, to remove the influence of reaction kinetics and isolate the effect of mass transport. As compared to constant flow conditions, optimized pulsed electrolyte flow achieved up to a 25% reduction in reactant boundary layer, resulting in mass-transfer-limited current densities increasing from 25.0 to 29.3 mA/cm^2. At an average flow rate of 10 mL/min, the best performing pulsing conditions were a 50% duty cycle with electrolyte flow ranging from 0 to 20 mL/min over a 2-second pulse duration. Further improvements may be achieved by investigating the duty cycle or wave-form (triangular, saw-tooth etc.) of the pulses, coupling pulsed flow with pulsed potential, and monitoring local changes in pH within the cell. Additional experiments are being performed with nitrate salts to more directly probe and compare activity and selectivity performance under pulsed flow and electrolysis conditions. Moreover, the added energy requirements associated with pulsing potentials and flow conditions were quantified and evaluated against corresponding electrochemical performance enhancements. The advancements in this work demonstrate that pulsed flow is a valuable strategy to overcome mass transport limitations, and supports the feasibility of electrochemical nitrate reduction as a valid method for combined wastewater treatment and ammonia production. [1] L. Barrera and R. Bala Chandran, “Harnessing photoelectrochemistry for wastewater nitrate treatment coupled with resource recovery,” ACS Sustainable Chemistry &amp;amp; Engineering , vol. 9, no. 10, pp. 3688–3701, Jan. 2021. doi:10.1021/acssuschemeng.0c07935 [2] Y. Huang et al. , “Pulsed electroreduction of low-concentration nitrate to ammonia,” Nature Communications , vol. 14, no. 1, Nov. 2023. doi:10.1038/s41467-023-43179-1

In the global effort to achieve carbon neutrality, the sustainable production of clean energy, particularly green hydrogen due to its high gravimetric energy density and ultra-low carbon dioxide emission, has become essential. Photocatalytic water splitting is a proposed method for producing green hydrogen via semiconductor nanoparticles and sunlight at a potentially lower cost than electrolysis, but it suffers from low reaction rates and efficiencies. While there are many methods of attempting to increase reaction efficiency, the influence of temperature on photocatalytic reaction kinetics remains relatively understudied in previous literature, despite its potential to greatly boost or hinder hydrogen evolution. This work reports the rates of oxygen and hydrogen evolution half-reactions in the presence of anatase TiO 2 and SrTiO 3 nanoparticles for temperatures ranging from 25 to 70 ℃. To further interpret these results, we develop model-based predictions to especially quantify the interrelated effects of temperature on the charge-carrier generation and recombination rates in the photocatalyst, species mass transfer, reaction kinetics, and solubilities of gaseous species. Our current experimental results demonstrate a twofold reaction rate enhancement for Z-scheme photocatalysis with anatase TiO 2 towards reducing Fe(III) to Fe(II) together with O 2 evolution at 50 ℃. These results are co-interpreted with appropriate control measurements to further understand the effects of temperature. Overall, our study showcases the potential for substantially improving photocatalytic performance by controlling and optimizing the reaction temperature with minimal additional energy input, and motivates future reactor designs to harness both visible and thermal energy from incident sunlight to attain an optimal reaction temperature.

Electrified co-reduction of excess nitrates (NO3-) present in wastewater sources, and carbon dioxide (CO2) to synthesize urea is increasingly gaining attention towards electrocatalyst development to improve activity and selectivity. However, the influences of flow cell design and operational parameters including fluid flow including both the liquid and the gaseous phase, mass and charge transport remains relatively underexplored. For our application of urea production from liquid NO3- and gaseous CO2. Commercial-scale electrochemical cells require operation at current densities that exceed 100 mA cm-2. Achieving these current densities could be especially challenging for systems requiring a dedicated continuous feed of liquid and gaseous reactants at any electrode. Challenges stem in-part due to the increasing ohmic and mass transfer resistances when there is a separate electrolyte flow channel. While the state-of-the-art membrane electrode assembly (MEA) reactor designs have already demonstrated performance enhancements by lowering these resistances, this design doesn’t directly translate to applications requiring both gaseous and liquid phase reactants. To overcome this challenge, we propose both design and operational state changes, and evaluate enhancements in current densities gained through model predictions and experimental measurements. As a first-cut design modification to lower mass-transfer limitations, we introduced surface ridges with feature sizes on the order of 1 mm and custom manufactured flow plates to demonstrate up to a 40% enhancement to the mass-transfer rates, accompanied without much negative impacts to the ohmic resistance. We additionally test and report on the effectiveness of a new two-phase, co-feed, design architecture involving spatially distributed flow channels for the gas and the liquid phases in a crossflow configuration where the gas and liquid reactant meet at the GDE to drive the necessary reactions. Our design optimization targets to lower both the ohmic and mass transfer resistances, while balancing the losses in the contact surface between the gas and the liquid phases at the catalyst surface, and the aqueous phase pressure drop. To fast track traversing this design space, we use simplified computational fluid dynamic models paired with rapid prototyping and testing of downselected designs to assess limiting current densities in electrochemical flow cells. In addition to design-space modifications, we also assess the effects of operating conditions including gas and liquid flow rates, and the influences of pulsed flow of species on the achievable current densities. Preliminary modeling and experimental results with custom manufactured flow plates demonstrate the potential of new dual-phase co-feed designs to reduce ohmic and mass transport resistance, thereby improving electrochemical flow cell performance.

Salt hydrates are a promising thermochemical energy storage medium that stores heat through the reversible uptake (hydration) and release (dehydration) of water vapor. Our study deploys operando neutron imaging to investigate salt hydrate performance with high spatial resolution (42 μm pixels). For flow over a packed bed with diffusion-driven transport, measurements reveal the formation of a solid diffusion layer due to particle swelling for the pure SrBr 2 salt. In contrast, the SrBr 2 –vermiculite composite exhibits significantly less swelling and more than a 2-fold increase in the apparent water vapor diffusivity. For axial flow through a packed bed, neutron imaging confirms theoretically predicted transitions from a moving reaction front to a homogeneous profile with an increase in humid air flow rate. Our study establishes neutron imaging as a powerful technique to advance fundamental understanding of thermochemical systems and help guide composite material design.

Electrochemical reduction of CO 2 to methanol is a promising approach for renewable energy storage in chemical bonds. Immobilized cobalt phthalocyanine (CoPc) is capable of converting carbon dioxide to methanol via a cascade catalysis mechanism involving CO as the intermediate. However, weak binding of CO to CoPc leads to a low single-pass efficiency in CO 2 flow electrolyzers with gas diffusion electrodes. We show that by controlling the relative concentrations of CO 2 and CO near CoPc, we can enhance methanol production in flow electrolyzers. By adjusting the gas flow rate and the CO 2 partial pressure, we achieve methanol partial current densities of 20 mA/cm 2 at a large Faradaic efficiency of 46%. Our 3D multiphysics model predictions uniquely showcase that the factors that control methanol activity and selectivity change as a function of flow rate. While the ratio of local CO to CO 2 concentration shows strong positive correlations with methanol production for gas flow rates larger than 5 mL/min, we hypothesize methanol production is controlled by the local pH for the smaller flow rates (<2 mL/min). Overall, our integrated computational–experimental findings provide valuable insights into the effects of design and operating conditions of CO 2 flow electrolyzers with CoPc catalysts for producing drop-in methanol fuel.

Widespread air source heat pump (ASHP) adoption faces several challenges that thermochemical salt hydrate energy storage can mitigate. We quantify the space heating value of four salts— MgS O 4 , MgC l 2 , K 2 C O 3 , and SrB r 2 —coupled with ASHPs across 4,800 households in 12 US cities by embedding salt-hydrate-specific Ragone plots into a techno-economic model of coupled ASHP-thermal energy storage (TES) operations. In Detroit, salt hydrate TES can reduce household annual electricity costs by 5%–8%. Cost savings from TES vary widely between households, salts, and climates. We identify the most promising salt in this study, SrB r 2 , due to its high energy density and low humidification parasitic load. Breakeven capital costs of SrB r 2 -based TES can reach $17/kWh, making it the only salt studied to reach and exceed the US Department of Energy's $15/kWh TES cost target. Sensitivities highlight the importance of variable TES sizing and efficiency losses in the value of TES.

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. &lt;1%, and necessary, i.e. &gt;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.

Abstract Many packed bed thermochemical reactors are in an axial flow configuration. This configuration poses two challenges: increased pressure drops at large scales or large flow rates and a non-uniform chemical driving force, which can lower the utilization of the material. A radial flow design offers the potential to both decrease the pressure drop by decreasing the distance the feed goes through the packed bed and increase the chemical driving force across a larger reactor volume. In this study, we experimentally compare the performance of an axial and radial low packed bed reactor for thermochemical energy storage using salt hydrate materials. Towards studying thermochemical materials in packed bed reactor designs, we investigate low-temperature hydration reactions in a composite material comprising of SrBr2 impregnated in a porous vermiculite host material in both axial and radial flow configurations. Using the salt hydrate material with humid air flowing over the bed can mimic the behavior of high temperature thermochemical reactions in a packed bed. Our study found that although the pressure drop is lower for radial configuration, the radial distributor must be designed to provide uniform flow to the bed. This can improve efficiency in packed bed reactors for thermocatalysis applications.

Nanowire arrays present many unique advantages for solar-to-chemical energy conversion. One possible advantage is that photon recycling between neighboring nanowires has the potential to increase solar energy conversion efficiencies. Here, we explore three underlying mechanisms of optical and electronic coupling between neighboring nanowires─incident photon scattering, photon recycling, and charge-carrier transport from the photoexcited nanowire to the neighboring nanowire via the underlying substrate─using single nanowire-level microscopy and spectroscopy measurements. We present a comprehensive analysis of light absorption and emission of a single nanowire at open circuit, and subsequent re-absorption and re-emission by a neighboring nanowire. We developed a novel correlated single nanowire microspectroscopy and widefield imaging methodology to spatially resolve photon communication pathways between neighboring nanowires and selectively image re-emitted and reflected photons. We developed unique multiphysics models to couple wave optics and semiconductor photophysics to especially isolate contributions from photon recycling and electronic transport to photon emission from neighboring nanowires. By systematically varying the morphologies of the nanowires modeled, we identified pathways to maximize photon recycling between neighboring nanowires. We concluded that the measured photoluminescence is more strongly influenced by the diffusion of charge carriers as compared to photon recycling in materials with moderate-to-large charge-carrier mobilities (>10 cm 2 V –1 s –1 ), and that photon recycling dictates photoluminescence intensity only when the charge-carrier mobility is low (<1 cm 2 V –1 s –1 ). The experimental and simulation platforms developed herein for photon management strategies can be leveraged by the semiconductor photocatalysis community to enhance solar-to-chemical conversion efficiencies in semiconductor nanowire arrays.

Schematic of photocatalytic type 2 Z-scheme raceway design with hydrogen reactor cylinders floating on an oxygen reactor raceway pool. The raceway concept enables a scalable, low-cost, and low-carbon intensity method of hydrogen production.

Proton coupled electron transfer reactions that change the pH at electrode-electrolyte interfaces are at the heart of many electrochemical reactions. Specifically, an increase in pH accompanies H 2 evolution and CO 2 reduction with relevance to fuels production from water and CO 2 electrolysis or pH-swing processes for CO 2 removal. When this increase in pH occurs in carbon-containing and/or ocean water electrolytes, it leads to salt precipitation and fouling in porous electrodes. Magnesium hydroxide and calcium carbonates are suspected precipitates in ocean water solutions, while potassium carbonates are the only possible precipitates in many CO 2 reduction scenarios. This study develops and experimentally demonstrates a custom H-cell design that enables in-situ visualization of pH changes occuring during hydrogen evolution from a commercially available 0.5 mg/cm 2 Pt-C carbon paper gas-diffusion electrode. Bulk pH is measured by a pH probe and this is used to calculate a local pH, based on the Nernst diffusion layer thickness. Fouling extents are further characterized through optical visualization of the surface and the chemical composition is characterized by tracking characteristic absorption peaks in the infrared spectra. Experiments are conducted at several different current densities to explore the effect of local pH on the rate and extent of precipitate formation and dissolution. Results reveal that fouling due to the formation of hydroxide precipitates occurs on electrode surfaces, even when the bulk pH conditions are not large enough to cause precipitation. This underscores the necessity to track local pH conditions. Results also demonstrate the significant precipitation observed in ocean water solutions with relevance to ocean water electrolysis and oceanic CO 2 removal, as compared to solutions buffered with bicarbonate ions. Initial results also indicate how the rate of precipitate formation and removal from the porous surface is connected to the current density and pH conditions.

To meet increasing demands for carbon-reduction, renewable and low-emission fuel alternatives have been proposed as a replacement for carbon-intense natural gas and coal. Specifically, hydrogen and oxygen evolution have been demonstrated at the lab-scale through a room temperature water-splitting process using only water, sunlight, and suspended semiconductor nanoparticles as inputs. Recent advancements in improving the efficiency of this water-splitting process focus on methods to boost charge separation, chemical selectivity, and optical band-engineering. However, there is a growing need for demonstrations at scale that also consider nano-particle interactions as an ensemble. Additionally, Z-scheme reactor designs have motivated the study of oxidation and reduction half-reactions using charge-carrying ions as an intermediary. This work studies how photocatalytic rates of half-reactions driven by semiconductor nano-particle suspension are affected by the inevitable presence of aggregation and agglomeration. The effects of solution pH, Fe(III) concentration, input light intensity, reaction temperature, stirring and sonication, nanoparticle concentration and size, and material quality are experimentally characterized by the desired ion evolution reaction rates and external quantum yield. Additionally, the effects of these variables on the zeta potential, optical scattering, particle aggregation, and mass transport are also experimentally measured to probe ensemble behavior and nano-particle interactions. Results indicate the importance of balancing ion concentration and pH to reduce aggregation via enhanced zeta potential nanoparticle repulsion, while also avoiding competitive absorption from suspended intermediary ions and their respective pH-dependent hydrolysis states. Reaction modeling further motivates investigation into charge-transfer limitations and enhancements due to aggregation states that cannot be explained by optical effects alone.

Methanol is a desirable end-product from CO 2 reduction as it serves as a drop-in renewable fuel for the current fuel and transportation economy. Cobalt Phthalocyanine (CoPc) is an effective electrocatalyst to convert CO 2 into methanol by preventing C-C coupling. In prior work with gas-fed CO 2 electrolyzers, it has been shown that methanol production is suppressed when the relative concentration of CO 2 to CO increases, as reaction sites for CO to methanol are blocked [1]. Therefore, having knowledge of the local concentrations of CO 2 and CO can be significant to improve understanding and interpretation of experimental data. To probe these effects of mass-transfer in a gas-fed CO 2 flow cell, we develop a three-dimensional computational model of the gas flow adjacent to the gas diffusion electrode. Different flow rates (1-30 ml/min) and partial pressure of the feed CO 2 gas (0.02 - 1 atm) are modeled concurrently with inputs of experimentally measured partial current densities for methanol, carbon monoxide, and hydrogen formation. Model predictions determine space-time variations in the concentrations of all relevant reactant and product species. These are correlated with the relative concentrations of CO 2 and CO on the measured reaction rates and selectivity. Results highlight that methanol production only occurs at sufficiently low mass flow rates or partial pressures of CO 2 reactant gas, leading to low relative CO 2 to CO concentrations. The value of the companion model developed is that it identifies the maximum value of the relative CO 2 to CO concentration in the reactor that still yields methanol. Future predictive modeling studies can harness these local CO 2 :CO concentrations to relate to methanol productivity. Additionally, the modeling framework developed is applicable to probe and identify CO 2 electrolyzer design/operating conditions that can enhance the relative concentration of CO as compared to CO 2 . [1] Yao et al. (2024) Electrochemical CO 2 Reduction to Methanol by Cobalt Phthalocyanine: Quantifying CO 2 and CO Binding Strengths and Their Influence on Methanol Production. ACS Catalysis DOI:10.1021/acscatal.3c04957

Nanowire arrays present many unique advantages for solar-to-chemical energy conversion and are good model systems to investigate how the performance of one nanowire can influence others in an array. Spatially resolved photoluminescence is a powerful experimental characterization tool to quantify optical and electronic coupling between nanowires in an array. However, three underlying mechanisms of incident photon scattering, photon recycling, and charge-carrier diffusion dictate this coupling. In this study, we present a comprehensive analysis of light absorption and emission of a single nanowire at open circuit, and subsequent re-absorption and re-emission by a neighboring nanowire. We developed a novel correlated single nanowire micro-spectroscopy and widefield imaging methodology to spatially resolve photon communication pathways between neighboring nanowires and selectively image re-emitted and reflected photons. Unique multiphysics models have been developed to couple wave optics and semiconductor photophysics to especially isolate contributions from photon recycling and electronic transport to photon emission from neighboring nanowires. By systematically varying the morphologies of the nanowires modeled, we identify pathways to maximize photon recycling between neighboring nanowires. We conclude that the measured photoluminescence is more strongly influenced by the diffusion of charge-carriers as compared to photon recycling in materials with moderate-to-large charge-carrier mobilities (&gt; 10 cm2 V-1 s-1), and that photon recycling dictates photoluminescence intensity only when the charge-carrier mobility is low (&lt;1 cm2 V-1 s-1). The experimental and simulation platforms developed herein for photon management strategies can be leveraged by the semiconductor photocatalysis community to enhance solar-to-chemical conversion efficiencies in semiconductor nanowire arrays.

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.

The realization of an environmentally sustainable and widely-adopted hydrogen economy may require lowering hydrogen production costs of production pathways with ultra-low greenhouse gas emissions to $1/kg H 2 . The allocation of new or existing renewable electricity generation solely to hydrogen production remains contentious due to disputes regarding emissions accounting. Photoelectrochemical (PEC) hydrogen production technologies offer a unique solution, as hydrogen is produced directly from solar energy and water, without the need for electricity generation. However, cost projections for past photoelectrochemical designs have suggested that they are not cost competitive compared to conventional electrolysis systems manufactured at scale. Herein, we offer the first illustrative benchmark of cost and carbon intensity of hydrogen produced in a Type 2 Z-scheme photocatalytic reactor design that employs suspended semiconducting nanoparticles organized in two stacked baggies in a raceway design. To explore the near-term and future cost projection for hydrogen production via Type 2 photocatalytic Z-scheme raceways, the authors developed a bottom-up total installed capital cost model. This project cost model incorporates: 1) raceway reactor cost, derived from a Design for Manufacturing and Assembly (DFMA) process-based cost model, 2) mechanical balance of plant, including process equipment, piping, valves, and instrumentation, derived from equipment quotes, scaling, database values, and Aspen estimates; 3) electrical balance of plant, including wiring, derived from time and material cost correlations; 4) Site Preparation, focused on green field installation; and 5) Construction Overhead, including engineering, procurement, and construction (EPC) costs and project contingency. The capital cost model is fed into a Levelized Cost of Hydrogen (LCOH) discounted cash flow model that accounts for electricity and water consumption, in addition to other operating costs, including labor, maintenance, and raceway replacements. A hybrid life-cycle approach was used to develop an inventory of greenhouse gas emissions from all materials and energy flows associated with the facility life cycle. These flows are converted to emissions based on specific emission factors, or by using physical units-based input–output LCA models associated with the broader economic impacts of fuel and materials production, use, and end use. Estimates for levelized cost of hydrogen suggest that a 50 metric ton H 2 per day (MTD) raceway plant can approach $2.50/kg H 2 at an 8% solar-to-hydrogen efficiency with further cost improvements possible through increased performance and larger plant scales. Carbon intensity is estimated to be well under clean hydrogen targets for global warming potential of &lt;2-4 kg CO 2 e/kg H 2 produced. The results suggest a highly competitive and scalable technology, that justifies further experimental validation and prototyping in the field. Figure 1

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.

Widespread air source heat pump (ASHP) adoption faces several challenges that on-site thermal energy storage (TES), particularly thermochemical salt hydrate TES, can mitigate. No techno-economic analyses for salt-hydrate-based TES in residential applications exist. We quantify the residential space heating value of four salt hydrate TES materials - MgSO4, MgCl2, K2CO3, and SrBr2 - coupled with ASHPs across 4,800 representative households in 12 U.S. cities by embedding salt-hydrate-specific Ragone plots into a techno-economic model of coupled ASHP-TES operations. In Detroit, salt hydrate TES is projected to reduce household annual electricity costs by up to $\$$241 (8$\%$). Cost savings from TES can differ by over an order of magnitude between households and salt hydrates. We identify the most promising salt in this study, SrBr2, due to its high energy density and low humidification parasitic load. Break-even capital costs of SrBr2-based TES range from $\$$13/kWh to $\$$17/kWh, making it the only salt hydrate studied to reach and exceed the U.S. Department of Energy's $\$$15/kWh TES cost target. Sensitivities highlight the importance of variable TES sizing and efficiency losses in the value of TES.

Salt hydrates offer the dual potential of high energy densities and low material costs for building heating applications. Heat is released and consumed with uptake and release of water (vapor) respectively by the salt without dissolution. However, the adoption of salt hydrate reactors is hindered by the limited knowledge on how power and energy tradeoffs translate from the materials to the reactor scale. Our study addresses this knowledge gap with integrated modeling and measurements, focusing on a packed bed reactor with SrBr2 vermiculite composite particles. Results reveal a mass transport limited regime producing steady, low power with larger energy capacity utilization, and a kinetics limited regime offering high, but variable power and smaller energy capacity utilization. We use a modified Damkohler number to predict the specific power and performance limiting factors, with knowledge of the inlet state, materials-specific kinetics and thermodynamics, and reactor design/operating conditions. Comparison between the reactor-scale (100 g) with the material-scale (1 g) test using SrBr2 vermiculite composite highlights that the kinetics and specific power decreases nearly tenfold from 35 W/kg to 3.9 W/kg at full discharge, likely due to the larger temperature and morphological variations at the larger scale. These discrepancies emphasize the need for improved characterization of composite materials’ temperature-dependent kinetics, equilibrium vapor pressure, and structural/morphological changes associated with water vapor uptake and release. Our research enhances the understanding of salt hydrate performance from materials to reactors, applies the Ragone framework to contextualize and benchmark against other energy storage materials, and suggests strategies for performance improvement.

Granular flows with solid particles play an important role in energy and catalysis applications. To predict thermal performance, this study develops a comprehensive discrete particle tracking flow model fully coupled with conductive and radiative heat transfer. Leveraging LIGGGHTS, our model uniquely integrates particle-particle and particle-wall thermal radiation for grey radiative surfaces up to nearly 10 times of particle size. Models include: (a) plug flows of particles with a near-constant streamwise velocity and (b) gravity-driven, dense, moving beds of particles to determine wall-to-particle heat-transfer coefficients with isothermal channel walls. The plug flow model is used to interrogate the interdependent effects of flow-regime-dependent solid volume fractions (0.02 -- 0.48), particle size (0.4 -- 1 mm), operating wall temperatures (700 -- 1300 K), and thermophysical particles properties including emissivity (0.1 -- 1) and thermal conductivity (0.16, 33 W/m/K). The overall and conductive heat-transfer coefficients increase with solid volume fraction, while radiation contribution decreases. For a fixed solid volume fraction, heat-transfer coefficients exhibit linear dependence on particle emissivity and cubic dependence on wall temperature, driven by enhanced thermal radiation. For a fixed mass flow rate, the dependence of radiative heat-transfer coefficient on particle size changes with solid volume fraction. To generalize the applicability of our results, heat-transfer coefficients are transformed into a dimensionless Nusselt number, and establish its dependence on solid volume fraction. These results reveal the high sensitivity of the overall Nusselt number to even small changes in solid volume fraction in the dense flow regime, particularly for the high-conductivity alumina particles. In contrast, the radiative Nusselt number asymptotes for large solid volume fractions.

Granular flows with solid particles play an important role in energy and catalysis applications. To predict thermal performance, this study develops a comprehensive discrete particle tracking flow model fully coupled with conductive and radiative heat transfer. Leveraging LIGGGHTS, our model uniquely integrates particle-particle and particle-wall thermal radiation for grey radiative surfaces up to nearly 10 times of particle size. Models include: (a) plug flows of particles with a near-constant streamwise velocity and (b) gravity-driven, dense, moving beds of particles to determine wall-to-particle heat-transfer coefficients with isothermal channel walls. The plug flow model is used to interrogate the interdependent effects of flow-regime-dependent solid volume fractions (0.02 -- 0.48), particle size (0.4 -- 1 mm), operating wall temperatures (700 -- 1300 K), and thermophysical particles properties including emissivity (0.1 -- 1) and thermal conductivity (0.16, 33 W/m/K). The overall and conductive heat-transfer coefficients increase with solid volume fraction, while radiation contribution decreases. For a fixed solid volume fraction, heat-transfer coefficients exhibit linear dependence on particle emissivity and cubic dependence on wall temperature, driven by enhanced thermal radiation. For a fixed mass flow rate, the dependence of radiative heat-transfer coefficient on particle size changes with solid volume fraction. To generalize the applicability of our results, heat-transfer coefficients are transformed into a dimensionless Nusselt number, and establish its dependence on solid volume fraction. These results reveal the high sensitivity of the overall Nusselt number to even small changes in solid volume fraction in the dense flow regime, particularly for the high-conductivity alumina particles. In contrast, the radiative Nusselt number asymptotes for large solid volume fractions.

Oceans absorb 40% of all carbon dioxide (CO 2 ) emissions, making CO 2 removal from seawater as critical as from the air. A reversible pH-shifting seawater process uses hydrogen and redox salt to extract CO 2 , generating and consuming power during acidification and basification, respectively. An equivalent circuit model predicts performance dependent on key transport and thermodynamic parameters. The need to overcome mass-transport limitations, particularly liquid boundary-layer diffusion, is crucial for industrial current densities. Energy-intensity calculations indicate this process is competitive with prior oceanic CO 2 removal processes and even with direct air capture, when pumping energy cost is eliminated. Utilizing variable electricity pricing enables up to $65 (USD) savings per ton of CO 2 , which is 17% of oceanic and 52% of atmospheric CO 2 capture costs. At the present largest desalination plant, 100 kilotons of CO 2 can be captured per year, resulting in $6.5 million operational cost savings.

A computational framework integrating thermodynamics, kinetics, and mechanical stress calculations is developed to study supercritical CO2 induced corrosion in model Fe-based MA956 and Ni-based H214 alloys. Empirical models parametrized using experimental data show surface oxidation and sub-surface carburization for a wide range of thermodynamic conditions (800-1200 {\deg}C, 1-250 bar). CALPHAD simulations based on empirical models demonstrate higher carburization resistance in H214 compared to MA956 below 900 {\deg}C and through-thickness carburization in both alloys at higher temperatures. Finite element modeling reveals enhanced volumetric misfit induced stresses at oxide and carbide interfaces, and its critical dependence on the carbide chemistry and concentration.

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.

The radiative properties of sintered bauxite (ACCUCAST ID80) and AMS4003 particles were measured and compared to assess their performances as direct absorption and heat transfer media for particle-based concentrated solar power (CSP) plants. Reflectance measurements were performed over the spectral range 0.2–20 µm and used to calculate solar and thermal emission properties of the particles. In addition, reflectance was measured as a function of temperature up to 1000 °C for the spectral range 1–20 µm. The solar absorptance of AMS4003 was greater than that of ACCUCAST both before and after thermal cycling of the materials at 1000 °C, although thermal cycling was found to substantially decrease the solar absorptance of both materials. The thermal emittance of AMS4003 was also greater than that of ACCUCAST at all temperatures tested. Finally, the radiaitve properties measured in this study were used to estimate absorber efficiencies as a way to compare the particles in terms of their performance in a solar receiver. AMS4003 yielded greater efficiencies than ACCUCAST, suggesting its potential as a material for CSP.

A powerful detailed-balance model predicts optimal gains with many optically thin photo absorbers instead of one thick absorber. Selectivity and efficiency are controlled by redox species mass-transfer rates regardless of kinetic asymmetry.

Carbon dioxide (CO 2 ) capture and removal technologies are critical to limit global warming to 2 °C in the next few decades. In addition to the atmosphere, the oceans are large carbon sinks and have to-date captured about 25% of all anthropogenically released carbon. Therefore, ocean water CO 2 capture can complement Direct Air Capture (DAC) technologies. To harness this source of CO 2 , this talk proposes, models, and analyzes the performance implications of a reversible, aqueous electrochemical flow device, which uses hydrogen and redox salt looping to achieve pH shifting. The reversible nature of this device enables its use in both power production and consumption modes, enabling CO 2 capture to be powered exclusively by variable sources of renewable electricity. To model the performance of the proposed energy storage and CO 2 capture device, we developed an equivalent circuit model to predict the current-voltage performance of this device mediated by ferricyanide/ferrocyanide and H 2 . Performance predictions are modeled as a function of key transport and thermodynamic conditions, specifically, for different mass transport rates of gases and aqueous electrolytes and different extents of pH shift between acidification and basification steps. Estimations of device performance are developed at both laboratory-scale and industrially relevant current densities. Modeled electrochemical energy intensity values of the overall device operation are compared to current state-of-the-art electrochemical ocean CO 2 capture technologies to show the approach’s potential. Overall energy intensities are compared to current state-of-the-art DAC technologies. Our results show the energy intensity for CO 2 capture during the reversible process has the potential to be reduced compared to present day DAC systems, but is highly sensitive to parasitics, ohmic losses, and the extent of competing reactions. Additionally, projections of the device energy-time profile are demonstrated to show the diurnal energy storage potential, specifically with applications to solar energy. Energy storage capabilities in the proposed device provide additional benefits of powering this CO 2 capture device with renewable electricity while providing flexibility to manage grid-scale electricity demand. Overall, a reversible, pH-shifting based oceanic CO 2 capture device offers holistic benefits from designing CO 2 capture to operate exclusively from renewable electricity.

Nanoparticle-reinforced alloys offer the potential for high-strength, high-temperature alloys. While the classic Smith–Zener equation is widely used to predict grain size with pinning particles, it does not explicitly factor the influence of nucleation rate, which is a critical phenomenon that affects microstructure. A model is developed using the open-source phase-field modeling software , PRISMS-PF, to explore the impact of nucleation rate on alloy solidification for both random and clustered distributions of nanoparticles. Heterogeneous nucleation and grain boundary pinning are explicitly included, and a wide range of nanoparticle area fractions (0.01–0.1) and nucleation site densities (10^6–10^12nuclei/m^2), which affect nucleation rates, are modeled. Quantitative analyses inform a kinetically modified Smith–Zener relationship, which predicts grain size dependence on nucleation rate as d _z ~ 1/ J ^*0.15 . Simulations also reveal a strong preference of nanoparticle pinning grains, especially at triple points. Total pinning fraction increases rapidly with nucleation rate before saturating between 0.85 and 0.90 for both random and clustered 2D distributions. At low area fractions (< 0.05), particle clustering increases grain size between 15% and 45% compared to random distributions. A main advancement of this work is the quantification of how nucleation rate, in addition to nanoparticle size and concentration, affects grain size and therefore alloy strength.