Inspired by the breathing process of mammalian alveoli, scientists at Stanford University have developed a breathing-mimicking, pouch-type, nanoporous polyethylene (PE) structure for two-way efficient gas transport from and to an electrocatalyst/electrolyte interface. Their research is published in the journal Joule.
Electrocatalytic oxygen evolution and reduction reactions play a central role in clean energy technologies. Despite recent efforts to achieve fast gas reactant delivery to the reaction interface, efficient gas product evolution from the catalyst/electrolyte interface remains challenging. Inspired by the mammalian breathing process, here we developed an efficient electrocatalytic system to enable ample gas-solid-liquid three-phase contact lines and bidirectional gas pathways for evolution and consumption.
During the oxygen evolution reaction, the newly formed O2 molecules quickly diffuse to the gas phase, waiving the bubble formation energy in the electrolyte. A record low overpotential of 190 mV at 10 mA·cm2 was achieved using Au/NiFeOxcatalysts.
During the oxygen reduction reaction, O2 gas can transport to the catalyst/electrolyte interface, overcoming low O2 solubility in water and leading to 25-fold higher current densities for Ag/Pt bilayer nanoparticle catalysts. This breathing-mimicking design demonstrates efficient three-phase catalysis with a minimal catalyst thickness.—Li et al.
For OER, O2 molecules are formed on the surface of the catalyst followed by fast O2 diffusion through the nanoporous PE membrane into the gas phase inside the pouch. For ORR, the O2 gas quickly diffuses through the PE membrane nanopores to the gas-liquid-solid three-phase contact reaction interface. Li et al.
The act of inhaling and exhaling is so automatic for most organisms that it could be mistaken as simple, but the mammalian breathing process is actually one of the most sophisticated systems for two-way gas exchange found in nature. With each breath, air moves through the tiny, passage-like bronchioles of the lungs until it reaches diminutive sacs called alveoli.
From there, the gas must pass into the bloodstream without simply diffusing, which would cause harmful bubbles to form. The unique structure of the alveoli—including a micron-thick membrane that repels water molecules on the inside while attracting them on the outer surface—prevents those bubbles from forming and makes the gas exchange highly efficient.
Scientists in senior author Yi Cui’s lab at the Department of Materials Science and Engineering at Stanford University drew inspiration from this process in order to develop better electrocatalysts: materials that increase the rate of a chemical reaction at an electrode.
Clean energy technologies have demonstrated the capability of fast gas reactant delivery to the reaction interface, but the reverse pathway—efficient gas product evolution from the catalyst/electrolyte interface—remains challenging.—Jun Li, first author
The team’s mechanism structurally mimics the alveolus and carries out two different processes to improve the reactions that drive sustainable technologies such as fuel cells and metal-air batteries.
The first process is analogous to exhalation. The mechanism splits water to produce hydrogen gas by oxidizing water molecules in the anode of a battery while reducing them in the cathode. Oxygen gas (along with the hydrogen gas) is rapidly produced and transported through a thin, alveolus-like membrane made from polyethylene—without the energy costs of forming bubbles.
The second process is more like inhalation and generates energy through a reaction that consumes oxygen. Oxygen gas is delivered to the catalyst at the electrode surface, so it can be used as reactant during electrochemical reactions.
Although it is still in the early phases of development, the design appears to be promising. The uncommonly thin nano-polyethylene membrane remains hydrophobic longer than conventional carbon-based gas diffusion layers, and this model is able to achieve higher current density rates and lower overpotential than conventional designs.
However, this lung-inspired design still has some room for improvement before it will be ready for commercial use. Since the nano-polyethylene membrane is a polymer-based film, it cannot tolerate temperatures higher than 100 degrees Celsius, which could limit its applications. The team believes this material may be replaced with similarly thin nanoporous hydrophobic membranes capable of withstanding greater heat. They are also interested in incorporating other electrocatalysts into the device design to fully explore their catalytic capabilities.
The breathing-mimicking structure could be coupled with many other state-of-the-art electrocatalysts, and further exploration of the gas-liquid-solid three-phase electrode offers exciting opportunities for catalysis.—Jun Li
This work was supported by Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division; the Stanford Nano Shared Facilities; and Stanford Nanofabrication Facility.
Li et al. (2018) “Breathing-Mimicking Electrocatalysis for Oxygen Evolution and Reduction” Joule doi: 10.1016/j.joule.2018.11.015