Electrochemical Oxygen Generator With 99.9% Oxygen Purity and High Energy Efficiency

Under the growing crisis of the coronavirus disease 2019 pandemic, the global medical system is facing the predicament of an acute shortage of medical‐grade oxygen (O2, ≥ 99.5% purity). Herein, an oxygen generation device is manufactured that relies on electrochemical technology. The performance of the electrochemical oxygen generator (EOG) is remarkably improved to a practically applicable level, achieving long‐term (>200 h), stable, and quick production (>1.5 L min−1) of high purity O2 (99.9%) at high energy efficiency (496 L kW−1 h−1), via simultaneous optimization for intrinsic electrochemical reaction mechanisms, electrocatalysts, and external cell structure. The EOG also presents powerful competitiveness in user experience, which finds expression in high portability (4.7 kg), nearly instant O2 production (<1 s), and a quiet working condition (<39 dB). The EOG shows great potential to substitute commercial pressure swing adsorption O2 generation devices, which may significantly impact the traditional oxygen production industry.

Apart from the physical air-separation technology, chemical water electrolysis can generate high-purity O 2 by the oxidation of water. However, this process suffers from high energy consumption and is accompanied by the production of explosive hydrogen as the by-product. [7] In 1964, Langer et al. first proposed an electrochemical strategy to generate O 2 by utilizing the electrolytic cell, where O 2 is selectively reduced from the air at the cathode by oxygen reduction reaction (ORR), and regenerated at the anode by oxygen evolution reaction (OER) via a four-electron transfer (4ET) pathway. [8] This electrochemical strategy was further developed by Goldstein et al. in 1972, [9] who experimentally verified a more efficient route to produce O 2 , by employing a two-electron transfer (2ET) pathway with the formation of HO 2 as the intermediate species. Afterward, the viability of the above 2ET-route was supported by a study in 1981, [10] which explicitly validated that the 2ET-electrochemical cell can greatly lower the energy consumption compared with those previous 4ET-ones. However, for those studies concerning electrochemical O 2 generation, [7][8][9][10][11] restricted by the primitive cell structure and catalyst with inadequate activity and selectivity, the electrochemical cells presented relatively low current density, poor energy efficiency, and weak stability, thus had not been further developed to an applicable device. Consequently, for decades, the electrochemical strategy has not received deserved attention followed by gradually faded out from the views. The O 2 production field has been invariably dominated by the physical air-separation strategy till now.
In the present work, we manufactured an oxygen generation device by means of electrochemical technology. The performance of the electrochemical oxygen generator (EOG) has been improved to a practically applicable level and finally achieved long-term, stable, and quick production of high purity O 2 under high energy efficiency via some targeted optimizations and elaborate designs. In comparison to previous studies, the optimized EOG cell presents notably surpassed performance in current density, electrode area, O 2 production rate, energy efficiency and O 2 purity, etc (Figure 1c and Table S1, Supporting Information). In particular, the energy efficiency of our EOG for the first time was improved to a comparable level (496 L kW −1 h −1 ) to commercial PSA O 2 generation devices (152-327 L kW −1 h −1 [5,6] ), confirming the practical availability of the electrochemical technology in the oxygen production field. For the actual user experience aspects, the EOG also shows strongly competitive power, reflecting in the generation of high O 2 purity reaching medical grade (99.9%), light and handy size to be easily carried and transported (4.7 kg), nearly instant O 2 generation for responding to sudden emergencies (<1 s), quiet working condition that is in favor of patients (<39 dB), and long stability for constantly providing O 2 (>200 h). As illustrated in Figure 1d, the EOG remedies the drawbacks of inconvenient transportation and storage for oxygen cylinders, and insufficient O 2 purity for PSA devices. It can directly and quickly supply high purity O 2 to patients. Our Figure 1. Schematic illustration of the comparison for three types of O 2 generation technologies a, b, d) and performance parameters for electrochemical cells for this work and other research c). a) CD technology produces O 2 for oxygen cylinder with a high purity over 99%, however, cannot satisfy the supply due to the transport and storage issues under urgent and massive demands. b) PSA technology can realize the in situ production of O 2 , but the purity of the produced O 2 is not up to the standard of medical-grade oxygen. c) the comparison of performance parameters for electrochemical cells for this work and other research. d), EOG technology combines the advantages of the high purity of CD and the in situ production of PSA, which can directly produce O 2 with the purity of 99.9% and then quickly supply to demanders. www.advancedsciencenews.com work not only designed a portable, low-cost, energy-saving, and in situ O 2 production device, but more importantly, developed the electrochemical strategy which was longtime neglected by the O 2 production field to a workable technology with industrial application significance.

Results and Discussion
The performance of the EOG cell is mainly affected by three polarization resistances, namely 1) electrochemical polarization: originates from the activation barriers of the cathodic ORR and anodic OER; 2) concentration polarization: brought by the rapid depletion of gas during the cathodic ORR under high current density, leading to retard the whole reaction rates; 3) ohmic polarization: derives from the ohmic losses due to the ionic conduction in the electrolyte. Aiming to lower the above three polarization resistances, we conducted specific improvements in oxygen production mechanism, electrocatalysts, and cell structure, which effectively enhanced the performance of the EOG cell.
The electrochemical oxygen production is realized by two electrocatalytic reactions: the extraction of O 2 from the air through the cathodic ORR, and the generation of O 2 with the anodic OER. At an alkaline condition, the ORR and OER reactions can occur via two pathways, namely the 4ET and the 2ET pathways. [12] The 4ET pathway adopts OHas the intermediate species, while the 2ET one applies HO 2 -. As shown in Figure 2a, the 2ET pathway only consumes half of the electrons compared with the 4ET pathway in both ORR and OER processes. The previous reports have confirmed that the 2ET route is more energy-efficient compared with the 4ET-route. [10][11] Therefore, we adopted the 2ET-route as the oxygen production mechanism to decrease the intrinsic energy consumption in electrode reactions. Moreover, it is of great necessity to employ 2ET ORR and OER electrocatalysts with high activity and selectivity, so as to reduce the electrochemical polarization resistance from the electrode reactions. For the cathodic ORR process, we developed a general, low-cost, and facile strategy to access the large-scale synthesis of the 2ET-ORR electrocatalyst with high selectivity and activity. Commercial carbon black was chosen as raw material due to its low cost, excellent conductivity, high specific surface area, and porous structure for the high mass activity. According to recent studies, the 2ET selectivity was demonstrated to be strongly correlated with the species and content of oxygen-containing functional groups on oxidized carbon-based materials. [13] In our study, carbon black was oxidized by treating with ozone under the specified reaction times, resulting in a series of oxidized carbon black (O-C) nanoparticles with different oxygen surface contents (see materials and methods in Supporting information). No morphological changes were observed for these O-C nanoparticles after ozone treatment ( Figure S1, Supporting Information). The specific surface oxygen contents of O-C catalysts were characterized to be 0.4%, 1.4%, 11.3%, 18.8%, and 25.1% by X-ray photoelectron spectroscopy (XPS) analysis ( Figure S2, Supporting Information), corresponding to 0, 4, 8, and 16 h reaction times of the ozone treatment, respectively. The ozone treatment introduced abundant oxygen-containing functional groups, including carboxyl (-COO -), carbonyl (CO), and hydroxyl (C-OH), on the surface of O-C catalysts, as deconvolved from carbon and oxygen 1s signals ( Figure S3, Supporting Information). Apparently, the longer ozone-treatment times gave rise to a higher oxygen content until 25.1% within 16 h (Figure 2b). The XPS analysis of C 1s and O 1s spectra in Figure 2c and S3 (Supporting information) revealed that the carboxyl group -COOis gradually enriched along with the ozone-treatment times. The above results are further reinforced by the Fourier-transform infrared (FTIR) spectra in Figure S4 (Supporting Information), showing a positively proportional relationship between the ozone-treatment times and the content of -COO -. XPS only detected C and O, and inductively coupled plasma optical emission spectrometry (ICP-OES) excluded other metal elements for the prepared catalysts, which verified the ozone treatment is a clean strategy without introducing impurities, consequently, getting rid of repeated purification steps. The ORR activity and selectivity for O-C catalysts with different oxygen contents were evaluated by a standard three-electrode rotating ring-disk electrode (RRDE) in a basic electrolyte (0.1 M KOH, pH ≈ 13), and the results are shown in Figure S5 (Supporting Information). With the increase of oxygen contents on O-C surfaces, the activity and selectivity towards the 2ET pathway for ORR both increase first and then start to decrease. Among them, O-C-18.8% exhibits the highest ORR and H 2 O 2 generation current densities ( Figure S6a, Supporting Information), as well as the best 2ET selectivity achieving 97.5% ( Figure S5b To compare the total energy consumption of whole electrochemical reactions regarding the 2ET and 4ET pathways, we will separately discuss ORR and OER processes by means of density functional theory (DFT) calculations. For the cathodic ORR electrocatalyst, the surface of O-C catalyst was simplified as a 2D graphene monolayer with several oxygen-containing www.advenergymat.de www.advancedsciencenews.com functional groups of -COOH, CO, and -OH that were observed in the characterization experiments (Figure 2c). The computational details can be seen in Supporting Information and all the configurations are displayed in Figures S14-S16 (Supporting Information). Figure 2f shows the relationship between the limiting potential (U L ) and the OH* binding energy (ΔG OH* ) for both 2ET-and 4ET-ORR pathways. For each type of O-C catalyst, whether it is on the left or right of the volcano map, the 2ET process always has a stronger thermodynamic driving force than the 4ET pathway due to the lower overpotential (η). In all the configurations, the carbon atoms adjacent to oxygen-containing functional groups are considered as active sites, and the 2ET-ORR activity varies by changing the local chemical environment of the oxygen-containing The plots based on the RHE scale with the equilibrium potentials for both 2ET-and 4ET-ORR are displayed as dashed red and blue lines, respectively. Note that the 4ET-ORR activity of Pt(111) indicated by a yellow star is from ref. [14]. Carbon, oxygen, and hydrogen atoms are shown by grey, red, and white colors, respectively. functional groups. Figure 2f indicates that the O-C catalyst with ΔG OH* > 1.3 eV does not significantly contribute to the 2ET-ORR activity due to the η larger than 0.45 V. However, the O-C configurations with appropriate ΔG OH* between 0.5 and 1.3 eV exhibit high 2ET-ORR activity due to the small η ranging from 0.06 to 0.41 V. Among these configurations, we find that the carbon site with two -COOH groups on zigzag carbon edges (Figure 2f) is most active, yielding a η of only 0.06 V. This result implies that the excellent activity of the prepared O-C catalysts toward 2ET-ORR might be ascribed to the high content ratio of -COOH in oxygen-containing functional groups (Figure 2b). It is also important to note that most of the computed η values on O-C following the 2ET-ORR mechanism are even lower than that (0.45 V) on the traditional Pt catalyst through the 4ET-ORR process. [14] The above calculated results provide a compact explanation for both the observed high activity and selectivity of O-C toward the 2ET-ORR. For the anodic OER process, FeNi-Layered Double Hydroxide (LDH) was used as the electrocatalyst, which shows superior performance to commercial IrO 2 and RuO 2 in both activity and stability (Figure S10, Supporting Information). As the reverse reaction of ORR, OER occurs via whether the 4ET or 2ET pathway is determined by the intermediate species OHor HO 2 generated during ORR. Previous theoretical studies [15] on FeNi-LDHs suggest that the 2ET-OER is easier to take place than the 4ET-OER (see Supporting Information for detailed discussion). The above theories demonstrated that ORR and OER both obtain lower overpotentials via the 2ET pathway than the 4ET pathway, indicating less energy consumption by adopting the 2ET pathway.
The cathodic ORR is essentially a three-phase interfacial reaction involving catalytic active sites, electrolytes, and gaseous reactants. Under a large current density, the activity of ORR is severely restricted by the insufficient mass transfer of the gaseous reactants toward ORR-catalyst surfaces. In view of this, we introduced a gas diffusion electrode (GDE) into the EOG cell to promote the ORR rate (Figure 3a,b and S11, Supporting Information). GDE consists of a current collector, a highly hydrophobic microporous gas diffusion layer (GDL), and a catalyst layer (CL). The current collector bears mechanical strength and conductivity, which supports the structure and electron transmission during the electrochemical reactions. The GDL contains numerous microporous flow channels which are in favor of gaseous reactants transport toward the CL, and meanwhile, preventing itself from being wetted by the electrolyte due to the high hydrophobicity. Such designs endow EOG cell with continuous, rapid, and stable gas-feeding for ORR, and accordingly, reduces its concentration polarization resistance.
In an actual electrochemical reaction system, the inner resistance of the electrolytic cell brought by the ohmic polarization will induce apparent potential drops, which may cause side reactions and significantly lower energy efficiency. Therefore, an imidazole-functionalized alkaline anion exchange membrane (AEM) was synthesized [16] and placed between cathodic GDE and anode. Such a compact sandwich structure makes the electrodes achieving "zero-gap", lowering the ohmic losses to the greatest extent, [17] and in particular, improving the stability of the cell system by lowering heat generation. Besides, the AEM merely allows anions to pass through but entirely blocks air, which plays an immensely critical role in obtaining O 2 with high purity. Figure 3a illustrates the exploded schematic of the EOG cell. Behind the cathodic GDE and anode sides, seal gaskets made of fluorinated ethylene propylene, serpentine flow plates made of stainless steel with nickel plating, copper conductive plates, and end plates made of epoxy resin were arranged in order, and all the elements were firmly secured by bolts. Figure 3c exhibits the working mechanism of the EOG cell: at the cathode, the upper vent is fed with air where O 2 is converted into an ionic state of HO 2 by ORR, and the O 2 -depleted air is expelled from the lower vent. At the anode, the intermediate species HO 2 migrates to here through AEM driven by potential difference, and is oxidized to generate pure O 2 . Finally, the generated O 2 is released along with the flowing anodic electrolyte. After passing through the water vapor filter, the produced O 2 can be directly collected, and the purity of O 2 was analyzed to 99.9% (Figures S12 and Table S3, Supporting Information). The whole electrochemical reactions take place in a mild reaction condition without producing any contaminations. The performance of the unit EOG cell (16 cm 2 working area) with the loading of Pt/C and a series of O-C catalysts were investigated. The corresponding current-voltage (I-V) curves and the Faradaic efficiency are shown in Figure 3d,e. Among O-C catalysts, O-C-18.8% presents the lowest cell voltage (0.87 V under 200 mA cm -2 ), as well as the optimal 2ET selectivity of ORR (over 90% Faradaic efficiency of 2ET pathway). Meanwhile, L-O-C-18.8% displays similarly excellent performance and selectivity with O-C-18.8%, confirming the reliability of the large-scale synthesis. No H 2 byproduct was detected from the cathode side under such high current density, indicating the exclusive selectivity for ORR. The numbers of transferred electrons over O-C-18.8% and L-O-C (18.8%) were calculated to be ≈2.1 (equation A in Supporting Information), which demonstrated a typical 2ET pathway for ORR. To validate the scalability of the electrochemical cell, the polar plates with a 100 cm 2 working area were used for performance evaluation in a one-unit modular cell. As illustrated in Figure 3d,f, the L-O-C (18.8%) in the amplified cell presents no degradation of performance compared with that in the small-scale cell (16 cm 2 working area). Moreover, the 2ET-ORR Faradic efficiency (FE, equation B in Supporting Information) of L-O-C achieves 95% while that of Pt/C stays nearly zero in contrast, reflecting an excellent 2ET selectivity of L-O-C (Figure 3g). In Figure 3g and S13, Supporting Information L-O-C (18.8%) exhibits 147 mL min −1 of O 2 production rate and 511 L kW −1 h −1 of energy efficiency (defined by the ratio of O 2 production rate to power) under the same cell current of 20 A. While for Pt/C, despite it shows a slightly lower cell voltage of ORR (Figure 3f,g), its O 2 production rate and energy efficiency are only approximate to a half level of the L-O-C (18.8%) (76 mL min −1 and 270 L kW −1 h −1 ). In conclusion, L-O-C-18.8% not only has a significantly cheaper cost of the raw material than Pt/C, but also greatly lowers the energy consumption through the 2ET pathway of ORR and OER.
Several polar plates with 100 cm 2 working areas are piled in series to assemble into a cell stack comprising 9-unit cells, which aims to reach a high O 2 production rate and minimize the volume and weight (Figure 4a). The bipolar plates are elaborately structured to guarantee the continuous and smooth operation of the cell stack under a high current density. As shown in the schematic (Figure 4a), the distribution area is added to uniformly allocate the fluid and air flows of the main channels toward the bipolar plate of each unit cell, creating a consistent working environment among each unit cell. The bipolar plate is configured with multiple sets of parallel serpentine flow channels, which ensures the uniform distribution of gas and liquid medium and the proportion of the reactive area. In addition, the multiple sets of serpentine flow fields can also minimize the number of turns and the length of flow channels, dramatically reducing the pressure loss, and ensuring adequate mass transfer. Hence, the cell stack can realize stable operation under a high current density to guarantee the production rate of O 2 . By controlling the applied voltage, the O 2 production rate of the as-assembled EOG cell stack can be regulated in the range from 0.35 to 2.66 L min −1 (Figure 4b). For the stability measurements, the EOG device exhibits a constant O 2 production rate ≈1.5 L min −1 and a steady current with <1% decay within 200 h (Figure 4c), exhibiting an excellent stability performance.
The main performance index of the EOG device designed in this study is compared with a commercial PSA device in energy efficiency, O 2 purity, noise level, weight, and startup time, and the results are shown in Table 1. The working conditions of the EOG and PSA devices are recorded in Figure 4d,e and Video S1 (Supporting Information). Results indicate that the energy efficiency of the EOG device attains a comparable level for various commercial PSA devices under the same O 2 generation rate (1.5 L min −1 ). The purity of O 2 generated by the EOG device is up to 99.9%, while that of PSA device is measured to be only 93.6%. In essence, EOG is a device based on the electrochemical technology, where nearly pure O 2 can be easily obtained via electrocatalytic reactions. That is distinguished from the PSA technology, where O 2 purity is limited due to the incomplete adsorption-desorption processes. Medically, less noise level of O 2 generation devices is of great importance Data for stationary PSA devices is from the ref.
[6a,b], respectively. The energy efficiency is varied depending on the model; c) Data is from the product information of the purchased PSA device in the present study. The detailed product information is in Supporting Information. to hospitals and patients. As shown in Table 1, the noise generated by the air compressor of the PSA device is up to 93.6 dB (equivalent to the start-up sound of an automobile engine), which may cause negative side effects for patients. In contrast, the noise level of the EOG device is only 38.5 dB (equivalent to the working sound of a laptop), which is more suitable for applying at home or wards. In terms of portability, the PSA device weighs about 15.6 kg, where the irreplaceable air compressor and adsorbing canister account for the weight in a large proportion. Relatively, the EOG device weighs as light as 4.7 kg with the generation of equivalent O 2 of the PSA device, realizing the portable in situ O 2 generation. Finally, the two devices will be compared regarding the start-up time. Generally, obtaining concentrated O 2 by PSA device costs ≈20 s from its startup due to multiple cycles of adsorption and desorption processes. It is hard to address the emergency need of patients in severe conditions for medical oxygen. However, the EOG device generates medical oxygen nearly instantly which can well satisfy the urgent needs in a variety of conditions. The items in Table 1 are plotted into a five-star chart to compare these two O 2 generation devices visually. As can be seen in Figure 4f, the EOG device shows apparently superior user experiences (O 2 purity, energy efficiency, startup time, weight, and noise level) to those of commercial PSA devices, which has great market potential in the O 2 production field.

Conclusions
In summary, our work designed an EOG and improved its performance to an applicable level. Such remarkable enhancement of the performance originates from the targeted optimizations for intrinsic oxygen production mechanism, electrocatalysts, and external cell structure. Our EOG is powerfully competitive compared with the commercial PSA devices, which is reflected in the comparable energy efficiency, higher O 2 purity, lighter device weight, lower noise level, and quicker start-up time. Under the severe pressure of COVID-19, our EOG is expected to be ideal medical-oxygen supply equipment for patients in a variety of emergency conditions, and fill the great shortage of O 2 generation devices in the worldwide market.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.