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Waveguides with thicknesses similar to biofilms (10–100 µm) provide an opportunity to improve the bioenergy density of biofilm photobioreactors, avoiding the fundamental light- and mass-transport productivity limitations of planktonic photobioreactors. This report investigates the biofilm growth of a mutant of Synechococcus elongatus (PCC 7942) in evanescent light fields that can be scaled over large planar areas. In this study, areas of 7.2 cm2 are illuminated via frustrated total internal reflections on planar waveguides. The resulting photosynthetic biofilm growth showed resilience to surface intensities exceeding photosynthetic limits and a more uniform cell density distribution (1.0 ± 0.3 × 109 mL−1) than predicted from surface light distribution profiles. These results indicate potential for larger area biofilms using the uniform lighting conditions identified. The combination of evanescent illumination with biofilms indicates a modular reactor cell density on the order of 108 mL−1, representing a two orders of magnitude improvement over current facility architectures, with significant potential for further improvement through denser biofilms. [ABSTRACT FROM AUTHOR]

Abstract: This work explores how the degradation of the gas diffusion layer (GDL) under compression contributes to the formation of preferential pathways for water transport. Fluorescence microscopy is used to provide ex situ visualization of liquid water transport through the GDL placed beneath an optically transparent clamping plate. Transient image data obtained with a CCD camera indicates that areas of compression in the GDL coincide with preferential pathways for water transport and break-through. Preferential flow of water through the smaller pores resulting from GDL compression is contrary to the expected behaviour in a hydrophobic medium, and this suggests a loss of hydrophobicity. Scanning electron microscopy (SEM) is used to investigate the effect of compression on the morphology of the GDL. These SEM images show that compressing the GDL causes the breakup of fibers and, indeed, deterioration of the hydrophobic coating. [Copyright &y& Elsevier]

Abstract: The specific character of biological enzyme catalysts enables combined fuel and oxidant channels and simplified non-compartmentalized fuel cell assemblies. In this work, a microstructured enzymatic biofuel cell architecture is proposed, and species transport phenomena combined with consecutive chemical reactions are studied computationally in order to provide guidelines for optimization. This is the first computational study of this technology, and a 2D CFD model for species transport coupled with laminar fluid flow and Michaelis–Menten enzyme kinetics is established. It is shown that the system is reaction rate limited, indicating that enzyme specific turnover numbers are key parameters for biofuel cell performance. Separated and mixed enzyme patterns in different proportions are analyzed for various Peclet numbers. High fuel utilization is achieved in the diffusion dominated and mixed species transport regimes with separated enzymes arranged in relation to individual turnover rates. However, the Peclet number has to be above a certain threshold value to obtain satisfying current densities. The mixed transport regime is particularly attractive while current densities are maintained close to maximum levels. Optimum performance is achieved by mixed enzyme patterning tailored with respect to individual turnover rates, enabling high current densities combined with nearly complete fuel utilization. [Copyright &y& Elsevier]

Presented in this article is a computational analysis of the heat transfer due to an array of distributed heat sources on the bottom wall of a horizontal enclosure. The heat sources are modeled as flush-mounted sources with prescribed heat flux boundary conditions. Optimum heat transfer rates and the onset of thermal instability triggering various regimes are found to be governed by the length and spacing of the sources and the width-to-height aspect ratio of the enclosure. With respect to source spacing, we found that spacing equal to that of the source length provides effective convective heat transfer, and increasing the source spacing further does not result in significant improvements. The transition from a conduction-dominated regime to a convection-dominated regime is found to be characterized by a range of Rayleigh numbers, in contrast to the classical bottom wall heating problem. The range of Rayleigh numbers at which transition takes place decreases as the source length increases. At the transition region for very small source lengths, the Rayleigh-Bénard cell structure grows significantly to form fewer and larger cells, which accounts for higher heat transfer rates compared to configurations with longer heat sources where the cell structure remains the same throughout transition. Following the transition to a convection-dominated regime, bifurcations in the Rayleigh-Bénard cell structures as well as further regime changes are observed, reflecting the instabilities in the physical system. [ABSTRACT FROM AUTHOR]

Abstract: A novel fluorescence microscopy technique for visualizing the transport of liquid water in unsaturated hydrophobic fibrous media has been developed and is applied to the gas diffusion layer of a PEM fuel cell. In the experiments, fluorescein dye solution is pumped through the fibrous hydrophobic gas diffusion layer (GDL) and imaged with fluorescence microscopy. Transient image intensity data is correlated to the liquid surface height and is analyzed and presented in the form of three-dimensional reconstructions of the time-evolving gas/liquid interface inside the fibrous structure. The high spatial resolution of the visualization can resolve the dynamic transport of liquid water through distinct pathways, which helps to refine understanding regarding liquid water transport mechanism within these porous layers. The physical observations suggest that the water is not transported via a converging capillary tree as suggested in prior work and models. Rather, transport is dominated by fingering and channeling. Based on the physical insight obtained from the experiments, a new water transport scheme is proposed as the basis for developing improved models for water transport in hydrophobic gas diffusion layers. [Copyright &y& Elsevier]

We demonstrate surface plasmon resonance (SPR) sensing based on the polarization-dependent extraordinary optical transmission through a biaxial nanohole array. The biaxial array has two periodicities in a single array that can be individually probed by varying polarization. Here, the SPR polarization-spectral dependence is demonstrated for the detection of a self-assembled monolayer for four sets of biaxial array periodicities. By monitoring the polarization dependence of transmission through the nanohole arrays with biaxial periodicity, surface-sensitive refractive index induced intensity variations may be separated from other effects such as absorption, scattering, and intensity fluctuations, while using a single wavelength source. Biaxial sensing is useful for ongoing microfluidic integration of nanohole SPR, where the light source is transmitted through a microfluidic channel. [ABSTRACT FROM AUTHOR]

Abstract: We studied the interaction of a water droplet with a solid wall on a hydrophobic gas diffusion layer (GDL). Of particular interest is the stability of the droplet as a function of plate wetting properties and the potential for liquid entrapment in the GDL/land contact area. Such transport is of relevance to breakthrough dynamics and convective liquid droplet transport in polymer electrolyte membrane (PEM) fuel cell cathode gas channels. While a variety of complex coupled transport phenomena are present in the PEM fuel cell gas channel, we utilize a very simplified experimental model of the system where a droplet originally placed on a hydrophobic GDL is translated quasistatically across the GDL surface by a solid surface. Transport and entrapment are imaged using fluorescence microscopy. This work provides new insights into droplet behaviour at the GDL/land interface in a PEM fuel cell and suggests that hydrophobic land areas are preferable for mitigating the accumulation of liquid water under the land area of the gas flow channels. [Copyright &y& Elsevier]

The unique surface properties of grafted polydimethylsiloxane (PDMS) chains, particularly their omniphobicity and low friction, are influenced by molecular structure and tethering density. Despite molecularly smoothness and homogeneity, these surfaces exhibit significant variability in wettability and contact angle hysteresis (CAH). This work uncovers the molecular structure of grafted PDMS chains. Grafted PDMS chains synthesized using a difunctional chlorosilane initiator, which exhibits CAH <2° on silicon wafers, adopt a brush-to-mushroom conformation with a molecular weight ≈7,800 g mol -1 , a grafting density of 0.22 ± 0.4 chains nm -2 , and a thickness of ≈3 nm. Each PDMS chain terminates with a silanol group, and ≈96% of substrate silanols remain unreacted. The presence of these terminal silanols is confirmed with time-of-flight secondary ion mass spectroscopy, as is their removal when exchanged for trimethylsilyl groups, both on the substrate and terminating the PDMS chains. Quartz crystal microbalance with dissipation measurements show that this "capping" procedure exchanges ≈1.5 silanols nm -2 ; capping occurs at the substrate and PDMS chain end. The findings suggest that grafted, capped PDMS chains of this molecular weight are able to achieve excellent omniphobic properties even when the majority of surface silanols remain unreacted, which may aid in the design of future omniphobic materials., (© 2024 The Author(s). Small published by Wiley‐VCH GmbH.)

The electrochemical reduction of CO 2 in acidic media offers the advantage of high carbon utilization, but achieving high selectivity to C 2+ products at a low overpotential remains a challenge. We identified the chemical instability of oxide-derived Cu catalysts as a reason that advances in neutral/alkaline electrolysis do not translate to acidic conditions. In acid, Cu ions leach from Cu oxides, leading to the deactivation of the C 2+ -active sites of Cu nanoparticles. This prompted us to design acid-stable Cu cluster precatalysts that are reduced in situ to active Cu nanoparticles in strong acid. Operando Raman and X-ray spectroscopy indicated that the bonding between the Cu cluster precatalyst ligand and in situ formed Cu nanoparticles preserves a high density of undercoordinated Cu sites, resulting in a C 2 H 4 Faradaic efficiency of 62% at a low overpotential. The result is a 1.4-fold increase in energy efficiency compared with previous acidic CO 2 -to-C 2+ electrocatalytic systems.

To contribute meaningfully to carbon dioxide (CO 2 ) emissions reduction, CO 2 electrolyzer technology will need to scale immensely. Bench-scale electrolyzers are the norm, with active areas <5 cm 2 . However, cell areas on the order of 100s or 1000s of cm 2 will be required for industrial deployment. Here, we study the effects of increasing cell area, scaling over 2 orders of magnitude from a 5 cm 2 lab-scale cell to an 800 cm 2 pilot plant-scale cell. A direct scaling of the bench-scale cell architecture to the larger area results in a ∼20% drop in ethylene (C 2 H 4 ) selectivity and an increase in the parasitic hydrogen (H 2 ) evolution reaction (HER). We instrument an 800 cm 2 electrolyzer cell to serve as a diagnostic tool and determine that nonuniformities in electrode compression and flow-influenced local CO 2 availability are the key drivers of performance loss upon scaling. Machining of an initial 800 cm 2 cell results in a standard deviation in MEA compression that is 7-fold that of a similarly produced 5 cm 2 cell (0.009 mm). Using these findings, we redesign an 800 cm 2 cell for compression tolerance and increased CO 2 transport and achieve an H 2 FE in the revised 800 cm 2 cell similar to that of the 5 cm 2 case (16% at 200 mA cm -2 ). These results demonstrate that by ensuring uniform compression and fluid flow, the CO 2 electrolyzer area can be scaled over 100-fold and retain C 2 H 4 selectivity (within 10% of small-scale selectivity).

Reactive capture of carbon dioxide (CO 2 ) offers an electrified pathway to produce renewable carbon monoxide (CO), which can then be upgraded into long-chain hydrocarbons and fuels. Previous reactive capture systems relied on hydroxide- or amine-based capture solutions. However, selectivity for CO remains low (<50%) for hydroxide-based systems and conventional amines are prone to oxygen (O 2 ) degradation. Here, we develop a reactive capture strategy using potassium glycinate (K-GLY), an amino acid salt (AAS) capture solution applicable to O 2 -rich CO 2 -lean conditions. By employing a single-atom catalyst, engineering the capture solution, and elevating the operating temperature and pressure, we increase the availability of dissolved in-situ CO 2 and achieve CO production with 64% Faradaic efficiency (FE) at 50 mA cm -2 . We report a measured CO energy efficiency (EE) of 31% and an energy intensity of 40 GJ t CO -1 , exceeding the best hydroxide- and amine-based reactive capture reports. The feasibility of the full reactive capture process is demonstrated with both simulated flue gas and direct air input., (© 2024. The Author(s).)

Converting CO 2 to synthetic hydrocarbon fuels is of increasing interest. In light of progress in electrified CO 2 to ethylene, we explored routes to dimerize to 1-butene, an olefin that can serve as a building block to ethylene longer-chain alkanes. With goal of selective and active dimerization, we investigate a series of metal-organic frameworks having bimetallic catalytic sites. We find that the tunable pore structure enables optimization of selectivity and that periodic pore channels enhance activity. In a tandem system for the conversion of CO 2 to 1-C 4 H 8 , wherein the outlet cathodic gas from a CO 2 -to-C 2 H 4 electrolyzer is fed directly (via a dehumidification stage) into the C 2 H 4 dimerizer, we study the highest-performing MOF found herein: M' = Ru and M″ = Ni in the bimetallic two-dimensional M' 2 (OAc) 4 M″(CN) 4 MOF. We report a 1-C 4 H 8 production rate of 1.3 mol g cat -1 h -1 and a C 2 H 4 conversion of 97%. From these experimental data, we project an estimated cradle-to-gate carbon intensity of -2.1 kg-CO 2 e/kg-1-C 4 H 8 when CO 2 is supplied from direct air capture and when the required energy is supplied by electricity having the carbon intensity of wind.