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Peng, Xiong (ORCID:0000000187375830)

Publications and source records attributed to Peng, Xiong (ORCID:0000000187375830).

5 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Modeling the Effect of Material Properties on Liquid-Alkaline Water Electrolysis

Liquid-alkaline water electrolyzers (LAWEs) use electricity to drive the conversion of water to H 2 and O 2 gas. These devices benefit from the use of low-cost nickel electrodes and metal-oxide separators, but suffer from lower current densities and higher cell voltages than proton-exchange-membrane water electrolyzers. Identifying the inefficiencies that result in this poor performance is key to mitigating losses and optimizing LAWEs. Here, we report an experimentally-validated 1-D continuum model of a LAWE that elucidates the gradients within the cell, simulates H 2 crossover, and projects the energy improvements made possible by modulating the properties of the electrodes and separator. The model captures the Nernstian polarization losses and the distribution of gas- and liquid-phases within the electrodes, enabling quantification of energy losses associated with kinetic, ohmic, and bubble-induced (mass-transport) resistances. Simulations demonstrate that LAWE can achieve energy intensities of 50 kWh kg −1 of H 2 at 1 A cm −2 using improved electrode and separator properties.

08 HYDROGEN↗
DOE OSTI2023

Tailoring the Mass Transport to Achieve High-Performing Unitized Regenerative Fuel Cells Considering Practical Operating Conditions

Unitized regenerative fuel cells (URFCs) convert electrical energy to chemical bonds in hydrogen during charge and convert chemical energy to output electricity during discharge, offering a promising solution to long-term energy storage. Recent studies indicate that the round-trip-voltaic efficiency (RTE) and longevity of URFCs are limited by complex mass transport during charging and discharging. Here, we first investigate how different porous transport layer (PTL) structures can impact URFC performance. The preferred PTL has a low tortuosity and high porosity, leading to a high RTE above 50% at 1 A cm −2 using Nafion 212. Moreover, thicker membranes, such as Solvay 90, are required to ensure mechanical stability and minimize H 2 crossover when operating under high differential pressure. Although this assembly inevitably leads to a higher ohmic loss, the RTE can be improved by further tailoring the electrode structures to facilitate mass transport by using supported catalyst, which still achieves over 50% RTEs at 1 A cm −2 . Optimization of porous structure to mitigate mass transport resistance with appropriate materials down selection considering practical application requirements can be a key design principle for achieving high-performing URFCs.

25 ENERGY STORAGE↗
DOE OSTI2022

Performance and Durability of Proton Exchange Membrane Vapor-Fed Unitized Regenerative Fuel Cells

With a growing demand for electricity, clean hydrogen production and usage can be an asset not only to mitigate emissions but for long-term energy storage as well. Hydrogen gas, a high-density energy carrier, can be made through electrolysis in charging mode and generate electricity via a fuel cell in discharging mode in a unitized regenerative fuel cell (URFC). While URFCs reduce cost by combining the charging and discharging modes into a singular device, switching between modes becomes burdensome, and water management is a major challenge. One way to mitigate these issues is to operate the entire system in the vapor phase. Vapor-phase operation simplifies the physics of the system but will introduce losses within the system, primarily ohmic and mass transport during the charging mode. In this study, we explore the performance of a Proton-Exchange-Membrane (PEM)-URFC under vapor-phase conditions and the impact of different PEMs, feed gases, and relative humidity on the performance and durability. By tailoring operating conditions and membrane, the vapor-URFC achieves a roundtrip efficiency of 42% and a lifetime of 50,000 accelerated stress test cycles for fully humidified feeds. Discussion of vapor-URFC for energy storage and extensions to look at various applications shows the promise of this technology.

25 ENERGY STORAGE↗
DOE OSTI2022

Influence of Supporting Electrolyte on Hydroxide Exchange Membrane Water Electrolysis Performance: Catholyte

Alkaline or hydroxide exchange membrane water electrolysis (HEMWE) is a promising technology for green hydrogen production using platinum group metal-free catalysts and stainless steel, an advantage of alkaline water electrolysis (AWE), and a gas-impermeable membrane, a parallel to proton exchange membrane electrolysis (PEMWE). However, the HEMWE requires supporting electrolytes and there is minimal understanding of their role on the respective reactions. Without SELs, HEMWE performance and durability are worse than PEMWE systems. Herein, consistently feeding potassium hydroxide anolyte, we systematically study the effects of catholyte SELs in HEMWEs including dry vs. wet operation, cation effects, anion effects, and cation/OH ratios on cell potential and stability. We report that (i) hydration of the cathode improves high current density operation by preventing dehydration of the hydroxide exchange membrane (HEM), (ii) there was no correlation between cation type and cell potential, (iii) cell potential and high frequency resistance did not correlate with SEL conductivity, (iv) cathodic carbonate SEL had a significant negative effect on cell performance, (v) increased cation/OH ratio also caused increased cell potentials. Overall, this study concludes that feeding water or potassium hydroxide solution is desirable to improve the AEMWE performance.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗