Dynamics Management of Intermediate Water Storage in an Air-Breathing Single-Cell Membrane Electrode Assembly

In air-breathing proton exchange membrane fuel cells (Air PEM FCs), a high rate of water evaporation from the cathode might influence the resistance of the membrane electrode assembly (MEA), which is highly dependent on the water content of the Nafion membrane. We propose a dead-end hydrogen anode as a means of intermediate storage of water/humidity for self-humidification of the membrane. Such an inflatable bag integrated with a single lightweight MEA FC has the potential in blimp applications for anode self-humidification. A dynamic numerical water balance model, validated by experimental measurements, is derived to predict the effect of MEA configuration, and the membrane's hydration state and water transfer rate at the anode on MEA resistance and performance. The experimental setup included humidity measurements, and polarization and electrochemical impedance spectroscopy tests to quantify the effect of membrane hydration on its resistance in a lightweight MEA (12 g) integrated with an inflatable dead-end hydrogen storage bag. Varying current densities (5, 10, and 15 mA/cm2) and cathode humidity levels (20, 50, and 80%) were examined and compared with the numerical results. The validated model predicts that the hydration state of the membrane and water transfer rate at the anode can be increased by using a thin membrane and thicker gas diffusion layer.

Ultralight, Safe, Economical, and Portable Oxygen Generators with Low Energy Consumption Prepared by Air-Breathing Electrochemical Extraction

Pure oxygen is vital in medical treatment, first aid, and chemical synthesis. Hypoxia can cause severe damage to the organ systems such as respiratory, digestive, and nervous systems and even directly cause death. Notably, the severe Coronavirus disease 2019 (COVID-19) pandemic has exacerbated the shortage of medical oxygen in the world. Hence, a safe, economical, and portable oxygen supply device is urgently needed. Here, we have successfully prepared a device with air-breathing electrochemical extraction of pure oxygen (ABEEPO) with light weight and high energy efficiency. By renovating the structure of the electrolytic cell, the components bipolar plate and end plate are replaced with a plastic membrane, and the component current collector is replaced with a highly conductive graphene composite membrane electrode. Due to the use of the plastic membrane and graphene composite membrane electrode, the weight of the electrolytic cell is reduced from 1319.4 to 1.6 g, and the flexibility of the electrolytic cell is successfully realized. Through optimizing anode catalysts, working area, and operating voltage, a high flow rate per mass (234 mL h^-1 g^-1) was achieved at a voltage of 1.2 V. The device exhibits high stability in 2 h. The new portable oxygen production device would be effective for hypoxia treatment.

The integument of the nonamphibious goby Gobionellus oceanicus: Its functional morphology and respiratory capacity

Facultative air-breathing fish can exchange respiratory gases using an air-breathing organ (ABO), such as the oral cavity of the integument, during environmental hypoxia. The goby Gobionellus oceanicus inhabits areas subject to environmental hypoxia; however, its ABO is unknown. To investigate the respiratory potential of G. oceanicus, the gill and integument surface area, diffusion capacity, and their diffusion barrier thickness were measured. Our results show that although gill surface area is smaller than observed in other facultative air-breathing fish, but it has all features necessary to perform aquatic gas exchange. Additionally the integument of the palate has a short diffusion barrier thickness and a large calculated O₂ -diffusion capacity suggesting that it functions as the ABO.

Acid-base and ion balance in fishes with bimodal respiration

The evolution of air breathing during the Devonian provided early fishes with bimodal respiration with a stable O2 supply from air. This was, however, probably associated with challenges and trade-offs in terms of acid-base balance and ionoregulation due to reduced gill:water interaction and changes in gill morphology associated with air breathing. While many aspects of acid-base and ionoregulation in air-breathing fishes are similar to water breathers, the specific cellular and molecular mechanisms involved remain largely unstudied. In general, reduced ionic permeability appears to be an important adaptation in the few bimodal fishes investigated but it is not known if this is a general characteristic. The kidney appears to play an important role in minimizing ion loss to the freshwater environment in the few species investigated, and while ion uptake across the gut is probably important, it has been largely unexplored. In general, air breathing in facultative air-breathing fishes is associated with an acid-base disturbance, resulting in an increased partial pressure of arterial CO2 and a reduction in extracellular pH (pHE ); however, several fishes appear to be capable of tightly regulating tissue intracellular pH (pHI ), despite a large sustained reduction in pHE , a trait termed preferential pHI regulation. Further studies are needed to determine whether preferential pHI regulation is a general trait among bimodal fishes and if this confers reduced sensitivity to acid-base disturbances, including those induced by hypercarbia, exhaustive exercise and hypoxia or anoxia. Additionally, elucidating the cellular and molecular mechanisms may yield insight into whether preferential pHI regulation is a trait ultimately associated with the early evolution of air breathing in vertebrates.

The importance of cutaneous gas exchange during aerial and aquatic respiration in galaxiids

The Canterbury mudfish Neochanna burrowsius was found to be a pseudo-aestivating galaxiid with a low metabolic rate and significant cutaneous oxygen uptake (c. 43%) in both air and water. Another galaxiid, inanga Galaxias maculatus, had a higher metabolic rate in both media but the proportion of oxygen uptake met by cutaneous respiration rose significantly from 38 to 63% when the fish were exposed to air. Besides its important role in oxygen uptake, the skin of both species also contributed significantly to excretion of carbon dioxide in air, indicating the critical role of the integument as a respiratory tissue. In air, G. maculatus may increase cutaneous gas exchange to meet metabolic demands owing to the reduced utility of the gills, but as emersed G. maculatus were only able to maintain metabolic rates at c. 67% of that measured in water, this strategy probably only permits short-term survival. By contrast, the low and unchanging metabolic rate in water and air in N. burrowsius is a feature that may facilitate tolerance of long periods of emersion in the desiccating environments they inhabit.

Short and long-term strategies to facilitate aerial exposure in a galaxiid

This study investigated two potential strategies to survive short and longer-term aerial exposure in a galaxiid. This scaleless fish possesses cutaneous pores that dilated in the short-term (15 min-3 h) but contracted over longer periods (15 h) out of water, suggesting that these organs are used to cope with shorter durations of air exposure. Pores on the abdominal surface showed the greatest variation while those on the operculum surface hardly changed. Conversely, thickening of the epithelial layer of secondary gill lamellae showed a slower increase but persisted in an approximately linear fashion over the duration of this study, indicating that this is a strategy that facilitates longer-term aerial exposure. Thus, this species has the capacity to accommodate both short and long-term exposure to air.