Engineering topics
Chen, Yingying
Publications and source records attributed to Chen, Yingying.
Effect of isopropanol cosolvent on the rheology and spinnability of aqueous polyacrylic acid solutions
Abstract We investigate the effect of alcohol fraction (isopropanol, IPA) in a binary water‐alcohol solvent mixture on the shear and extensional rheological properties, as well as the role of viscoelasticity on fiber formation of poly(acrylic acid) (PAA) in electrospinning. Comparison of the scaling of both specific viscosities η sp and extensional relaxation times λ E of PAA in water–IPA mixtures, showed stronger scaling compared to salt‐free aqueous polyelectrolyte solutions, except for the η sp in the unentangled regime displaying a polyelectrolyte‐like scaling η sp ~ c 0.5 for all IPA%. Such deviation suggested IPA induces association/aggregation of PAA. However, the trends between η sp and λ E magnitudes as a function of IPA% differ for concentrations compared in the entangled regime. The η sp as well as their elastic moduli exhibit a maximum, whereas λ E increases monotonically with IPA%, suggesting a complex interplay of various interactions are dictating their structure in water‐IPA mixtures, affecting their shear and extensional response differently. Electrospinning experiments showed increasing IPA% reduces the onset of both beaded and uniform fibers. Analysis using dimensionless numbers indicated the enhancement of their elasticity by IPA, and the consequent stabilizing effect on their jets/filaments against break‐up during electrospinning, plays a role in the improvement of their fiber formation.
Enabling technologies for the continuous electrically driven conversion of CO 2 and water to multi-carbon products at high current densities
Here, we demonstrate greatly improved conversion of CO 2 using a gas diffusion electrode (GDE) with flowing electrolyte configuration for CO 2 gas delivery in combination with a high surface area nickel phosphide electrocatalyst. This configuration achieves 40–50% selectivity for total carbon products over H 2 formation (HER) at total current densities ranging from 50 to 300 mA cm -2 . We developed a soft-templating method using CTAB detergent micelles for synthesis of phase-pure Ni 2 P, achieving a 260-fold larger surface area (BET) and porous sponge-like morphology that produces stable currents. This catalyst produces mainly one C-product, methylglyoxal (MG, C 3 H 4 O 2 ) with 38–47% overall selectivity, the highest reported selectivity for a 12-electron reduction product. The versatile soft-templating method for electrocatalyst synthesis uses low-temperature (185 °C) that is permissive for incorporation of co-catalysts that are otherwise destroyed by the high temperatures used in traditional solid-state synthesis (SSS). The non-porous Ni 2 P-SSS catalyst produces mainly H 2 at these current densities. Achieving these high currents and C/H selectivity benefits from use of hydrophobic polymers as co-catalyst binders (cationic = Nafion, anionic = PFAEM and neutral = PTFE) to improve CO 2 conversion. PFAEM is the better ionomer for the CO 2 RR at high current density, postulated as due to suppressing CO 2 conversion to inactive bicarbonate. Precipitation of the carbon products as a polycarbonate polymer occurs at high currents.
An economic analysis of the role of materials, system engineering, and performance in electrochemical carbon dioxide conversion to formate
The development of technologies that utilize carbon dioxide is important to mitigating climate change. The electrochemical reduction of carbon dioxide is one technology that can utilize greenhouse gasses that would be otherwise be emitted to the atmosphere by producing chemicals and fuels from carbon dioxide and electricity. Significant progress has been made in the experimental performance of carbon dioxide reduction systems with novel catalyst designs, new materials, and systems engineering; however, no work has linked such changes in stack design and materials to capital costs for the stack itself. In this study, we present an analysis that accounts for and analyzes the impacts of alternative materials and system architectures on manufactured costs of carbon dioxide reduction stacks, thus providing a framework to understand exactly how these advances impact the at-scale capital costs of these systems. Specifically, we consider the impact that the addition of a catholyte buffer layer has on an electrolyzer reducing carbon dioxide to formate, finding that the cost of manufacturing this part only increases stack costs by about $30/m 2 at high manufacturing rates, while previous work finds that this part improves system performance. This work shows that the links between system performance, materials, and costs are nonlinear, and that achieving low-cost scalability requires optimization of not just performance parameters but also the use of low-cost and highly scalable materials. These results bridge experimental and techno-economic analysis of processes for carbon dioxide reduction, informing researchers by providing a quantifiable estimate of the impact of advances in electrochemical carbon dioxide reduction technology on manufactured stack capital costs.
The effect of catholyte and catalyst layer binders on CO 2 electroreduction selectivity
Electrochemical reduction of carbon dioxide (CO 2 ) is an attractive technology for converting CO 2 to value-added chemicals by using renewably generated electricity. This work combines both experimental and theoretical analyses to investigate the role of different catalysts (Ag/Vu and SnO 2 /Vu), binder materials, and catholyte compositions on the selectivity of a CO 2 electrolysis cell to the two-electron CO 2 reduction products CO and formate. As a complementary effort, a 2D multi-physics transport model was developed to elucidate the fundamental processes and species concentrations in the cathode components. It was shown that the selectivity of CO 2 reduction to CO can change significantly when different catholyte compositions and binder materials are used, whereas the selectivity of CO 2 to formate is relatively stable across the conditions studied. These insights can be used to inform decisions regarding the electrode development and system design in CO 2 electrolyzers and ultimately contribute to scaling energy-efficient electrochemical CO 2 -reduction systems.
Discovering and Demonstrating a Novel High-Performing 2D-Patterned Electrode for Proton-Exchange Membrane Water Electrolysis Devices
Proton-exchange membrane water electrolysis (PEMWE) produces hydrogen with high efficiency and purity but uses high-loading platinum-group metal (PGM) catalysts. Such concerns call for the development of novel electrode architectures to improve catalyst utilization and mass activity, thus promoting PEMWE cost competitiveness for large-scale implementation. In this study, we demonstrated, for the first time, a novel two-dimensional (2D)-patterned electrode with edge effects to address these challenges. The edge effect was induced by membrane properties, potential distribution, and counter electrode coverage and could be optimized by tuning the catalyst layer dimensions. To achieve identical PEMWE performance, the optimal pattern saved the 21% anode PGM catalyst compared with the conventional catalyst fully covered electrode. The PGM catalyst could be further reduced by 61% to boost mass activity with no significant performance loss. The results also indicated that the electrode uniformity in PEMWE cells might not be as critical as that in PEM fuel cells. Finally, the novel 2D-patterned electrode could effectively reduce PGM catalyst loading, accelerating affordable and large-scale production of hydrogen and other value-added chemicals via electrolysis.
Characterizing uncertainties in ground “truth” of precipitation over complex terrain through high-resolution numerical modeling
Ground observation of precipitation over complex terrain is subject to large uncertainties due to inadequate sampling. This study explores a method that combines limited gauge data and a high-resolution numerical simulation to quantify the precipitation uncertainties in central Himalaya. Specifically, the Coefficient of spatial Variability (CV) of precipitation and the minimum Number of Required Stations (NRS) to obtain areal-mean precipitation ground truth values within a 0.25-degree area are investigated using fine-scale meteorological simulation at 1.5 km grid spacing. Evaluation over a densely-gauged area demonstrates comparable CV and NRS values between station observations and simulations. The simulated CV and NRS values in a larger area show a strong and positive dependence on each other and an expected positive (negative) correlation with topographic complexity (temporal scale). The proposed method sheds lights on evaluating precipitation products and holds promise for informing the layout of rain gauge networks in complex terrain.
High-Performance Bipolar Membrane Development for Improved Water Dissociation
Not Available
Modeling Water Electrolysis in Bipolar Membranes
Bipolar membranes (BPMs) have proven useful in numerous electrochemical energy conversion and storage applications, including fuel cells and electrolyzers. However, water dissociation in bipolar membrane electrolysis cells (BPMECs) is a complicated phenomenon that occurs via several different pathways. In this work, we develop a model based on the Poisson-Nernst-Planck system that includes a multistep water-dissociation mechanism to observe the fundamental processes that contribute to BPMEC performance. The model, which is validated to in-house experimental data, demonstrates that the junction potential is the most significant contributor to the total electrolysis voltage. We investigated the effects of water-dissociation catalysts and found that the optimal catalyst $p{K}_{a}$ depends on how the catalyst is integrated into the BPM (although values near 7 are typically best, in accordance with conventional wisdom). We also simulated the water content across the BPM and found that dry-out is not a significant issue when the membrane is in contact with liquid water on both sides. The species conservation approach taken here leads to a physical understanding of the system without using any fitting parameters.
A Robust, Scalable Platform for the Electrochemical Conversion of CO 2 to Formate: Identifying Pathways to Higher Energy Efficiencies
This work demonstrated a robust, scalable cell architecture for electroreduction of CO 2 (CO 2 R). An up to 90% faradaic efficiency for the conversion of CO 2 R to formate at 500 mA/cm 2 was realized at a 25 cm2 gas diffusion electrode (GDE) with a carbon-supported SnO 2 electrocatalyst. Furthermore, a 1.27 mm thick catholyte was used between the bipolar membrane and cathode GDE, which could be further reduced to tens of micrometers upon refinement. The deconvolution of the potential drop from each individual component/process guides the pathways to higher energy efficiencies of CO 2 R at this platform. Significant changes in the agglomerate size and aspect ratio on the electrode before and after an 11 h test were revealed by nano-CT, suggesting reduced CO 2 accessibility from electrode degradation. The versatility of this CO 2 R testing platform enables the ability to assess materials, components, and interactions at scales more in line with future devices.