Flow Cell Device Research Articles

Mitigating Side Reactions in Electrocatalytic Ammonia Synthesis by Nitrate Reduction

Electrocatalytic nitrate reduction holds significant promise for generating green ammonia and mitigating pollution, reducing energy input, and exhibiting zero-carbon emissions. However, the intricate multiple electron and proton transfer processes complicate the pathways from nitrate to ammonia, hindering the development of electrocatalysts with high activity and selectivity for practical use. The primary side reactions, such as the conversion of nitrate to gaseous nitrogen species and the hydrogen reduction reaction, limit the Faradaic efficiency of nitrate-to-ammonia conversion. In this work, we have developed a controllable methodology for preparing highly active Cu-based nanocatalysts. These nanocatalysts exhibit remarkable activity and selectivity in the electrochemical nitrate reduction reaction within a flow-cell device, with a high ammonia yield and high Faradaic efficiency.

Efficient Neutral H2O2 Electrosynthesis from Favorable Reaction Microenvironments via Porous Carbon Carrier Engineering

AbstractThe efficient electrosynthesis of hydrogen peroxide (H2O2) via two‐electron oxygen reduction reaction (2e− ORR) in neutral media is undoubtedly a practical route, but the limited comprehension of electrocatalysts has hindered the system advancement. Herein, we present the design of model catalysts comprising mesoporous carbon spheres‐supported Pd nanoparticles for H2O2 electrosynthesis at near‐zero overpotential with approximately 95 % selectivity in a neutral electrolyte. Impressively, the optimized Pd/MCS‐8 electrocatalyst in a flow cell device achieves an exceptional H2O2 yield of 15.77 mol gcatalyst−1 h−1, generating a neutral H2O2 solution with an accumulated concentration of 6.43 wt %, a level sufficiently high for medical disinfection. Finite element simulation and experimental results suggest that mesoporous carbon carriers promote O2 enrichment and localized pH elevation, establishing a favorable microenvironment for 2e− ORR in neutral media. Density functional theory calculations reveal that the robust interaction between Pd nanoparticles and the carbon carriers optimized the adsorption of OOH* at the carbon edge, ensuring high active 2e− process. These findings offer new insights into carbon‐loaded electrocatalysts for efficient 2e− ORR in neutral media, emphasizing the role of carrier engineering in constructing favorable microenvironments and synergizing active sites.

Efficient Neutral H2O2 Electrosynthesis from Favorable Reaction Microenvironments via Porous Carbon Carrier Engineering.

The efficient electrosynthesis of hydrogen peroxide (H2O2) via two-electron oxygen reduction reaction (2e- ORR) in neutral media is undoubtedly a practical route, but the limited comprehension of electrocatalysts has hindered the system advancement. Herein, we present the design of model catalysts comprising mesoporous carbon spheres-supported Pd nanoparticles for H2O2 electrosynthesis at near-zero overpotential with approximately 95 % selectivity in a neutral electrolyte. Impressively, the optimized Pd/MCS-8 electrocatalyst in a flow cell device achieves an exceptional H2O2 yield of 15.77 mol gcatalyst -1 h-1, generating a neutral H2O2 solution with an accumulated concentration of 6.43 wt %, a level sufficiently high for medical disinfection. Finite element simulation and experimental results suggest that mesoporous carbon carriers promote O2 enrichment and localized pH elevation, establishing a favorable microenvironment for 2e- ORR in neutral media. Density functional theory calculations reveal that the robust interaction between Pd nanoparticles and the carbon carriers optimized the adsorption of OOH* at the carbon edge, ensuring high active 2e- process. These findings offer new insights into carbon-loaded electrocatalysts for efficient 2e- ORR in neutral media, emphasizing the role of carrier engineering in constructing favorable microenvironments and synergizing active sites.

The surface asymmetric site regulation of carbon nitride oxide enabling peroxymonosulfate activation for degrading tetracycline

Peroxymonosulfate (PMS) activation by the carbonaceous-based materials for degrading antibiotics is a recent emerging hotspot in water environment restoration. Herein, a triazine carbon material is prepared by urea thermal condensation follwed by water-mediated oxidation. The optimized triazine carbon material (CNOR15) can degrade around 100 % of tetracycline within 6 mins, thus pseudo-first-order kinetic constant (kobs) enhances approximately 49 folds compared to pristine carbon material. Experiments and characterizations demonstrate non-radical reactive species of 1O2 plays a crucial role in CNOR15/PMS system to eliminate tetracycline, in which performs at an asymmetric C = O site on the edge of triazine motief. Electron paramagnetic resonance and trapping tests comfirm the novel mechanism of electron conduction from sp/sp2-hybrdized triazine to C = O prompting PMS decomposition for 1O2 generation. Ex-situ argon sputtering XPS technology solidly unveils a underlysing linear corelation between catalysis activity and content of carbonyl-constructed assymetric sites. In addition, membranoid CNOR15/PVDF removes ca. 90 % tetracycline at a rate of 100 mL·min−1 in a continuous flow-cell device, exhibiting a promising application in the practical wastewater purification. Our work emphasized a significance of asymmetric site regulation of carbonaceous materials towards PMS activation for environmental remediation.

Enhancing H2O2 Electrosynthesis at Industrial-Relevant Current in Acidic Media on Diatomic Cobalt Sites.

Electrocatalytic synthesis of hydrogen peroxide (H2O2) in acidic media is an efficient and eco-friendly approach to produce inherently stable H2O2, but limited by the lack of selective and stable catalysts under industrial-relevant current densities. Herein, we report a diatomic cobalt catalyst for two-electron oxygen reduction to efficiently produce H2O2 at 50-400 mA cm-2 in acid. Electrode kinetics study shows a >95% selectivity for two-electron oxygen reduction on the diatomic cobalt sites. In a flow cell device, a record-high production rate of 11.72 mol gcat-1 h-1 and exceptional long-term stability (100 h) are realized under high current densities. In situ spectroscopic studies and theoretical calculations reveal that introducing a second metal into the coordination sphere of the cobalt site can optimize the binding strength of key H2O2 intermediates due to the downshifted d-band center of cobalt. We also demonstrate the feasibility of processing municipal plastic wastes through decentralized H2O2 production.

Polyacrylamide degradation in oil field wastewater by UV/H2O2 and UV/PDS: Rapid experimental measurement and model simulation

Ultraviolet-based advanced oxidation processes (UV-AOPs) are effective for degrading refractory pollutants in wastewaters. However, performing rapid efficiency evaluations represents a significant challenge for process selection and optimization in practical application. This study investigated polyacrylamide (PAM) degradation by UV/hydrogen peroxide (H2O2) and UV/potassium peroxydisulfate (PDS) in pure water and practical oil field wastewater. Based on a previously developed mini-fluidic photoreaction system that is easy to operate and requires low sample volumes, an experimental setup for rapid evaluation of UV-AOP efficiency was constructed that enabled on-line analyses of viscosity, UV absorbance, and pH via specially fabricated flow cell devices together with off-line analyses of PAM, chemical oxygen demand (COD), and acrylamide (AM). The PAM degradation rate constants (k′PAM) in practical oil field wastewater by UV/H2O2 and UV/PDS were 2.4 and 2.1 m2 einstein−1, respectively, which were much higher than those by sole UV process (0.8 m2 einstein−1). Combing with the analyses of viscosity, UV absorbance (220 nm), pH, COD, and AM concentration, the UV/H2O2 and UV/PDS demonstrated the effectiveness in PAM degradation in oil field wastewater. Additionally, the scavenging capacities of HO• and SO4•− of practical oil field wastewater were determined, and the k′PAM values were predicted by model simulation and verified experimentally. This study provides rapid evaluation methods for UV-AOP efficiencies, which is helpful for selecting and optimizing UV-AOPs for industrial wastewater treatments.

Efficient electrochemical reduction of CO2 to CO in a flow cell device by a pristine Cu5tz6-cluster-based metal-organic framework.

The electrochemical reduction of CO2 to CO is a powerful approach to achieving carbon neutrality. Herein, we report a five-nuclear copper cluster-based metal-azolate framework CuTz-1 as an electrocatalyst for the electrochemical CO2 reduction reaction. It achieved a faradaic efficiency (FE) of 62.7% for yielding CO with a partial current density of -35.1 mA cm-2 in flow cell device, which can be preserved for more than ten hours with negligible changes of the current density and FE(CO). Studies of electrocatalytic mechanism studies revealed that the distance of Cu-N was increased, and the coordination number of the Cu ion was reduced, while the oxidation state of Cu was decreased after the electrocatalysis. These findings offer valuable insights into structural changes that influence the performance of the catalyst during the process of the electrochemical reduction of CO2 process.

Highly coordinative molecular cobalt-phthalocyanine electrocatalyst on an oxidized single-walled carbon nanotube for efficient hydrogen peroxide production.

H2O2 production via the two-electron oxygen reduction reaction (2e- ORR) offers a potential alternative to the current anthraquinone method owing to its efficiency and environmental friendliness. However, it is necessary to determine the structures of electrocatalysts with cost-effectiveness and high efficiency for future industrialization demand. Herein, a supramolecular catalyst composed of cobalt-phthalocyanine on a near-monodispersed and oxidized single-walled carbon nanotube (CoPc/o-SWCNT) was synthesized via a solution self-assembly method for catalyzing the 2e- ORR for H2O2 electrosynthesis. Benefiting from the enhanced intermolecular interaction by introducing oxygen functional groups on o-SWCNTs, the oxidation states of single-atom Co sites were tuned via the formation of two extra Co-O bonds. Coupled with structural characterizations, density-functional theory (DFT) calculations reveal that the depressed d-band center of the Co site regulated by two axially-bridged O atoms gives rise to a suitable binding strength of oxygen intermediates (*OOH) to favor the 2e- ORR. Thus, the CoPc-6wt%/o-SWCNT-2 catalyst with optimized synthetic parameters delivers competitive 2e- ORR performance for H2O2 electrosynthesis in a neutral electrolyte (pH = 7), including enhanced H2O2 generation, satisfactory molar selectivity of ∼83-95% and long-period stability (75 h) in H-cell measurement. Moreover, it could also be boosted to show a high current of 45 mA cm-2, recorded turnover frequency of 25.3 ± 0.5 s-1 and maximum H2O2 production rate of 5.85 mol g-1 h-1 with a continuous H2O2 accumulation of 1.2 wt% in a flow-cell device, which outperformed most of the reported neutral-selective nonprecious metal single-atom catalysts.

Materials Properties vs. Device Performance: Batch Testing of MnOx-Loaded Carbon Nanofoam Papers for Low-Energy Faradaic Deionization

Electrochemical desalination is presently a popular topic in the scientific literature as a response to global challenges with production of potable water. Many of these reports focus on materials/electrode properties for ion capture yet fail to account for other factors that impact practical device performance. For example, gravimetric ion-storage capacity is commonly reported while areal and volumetric capacities may be more important for desalination cell performance. Furthermore, the dynamic fluid mechanics of flow-cell devices require deliberately engineered electrodes in contrast to their static-electrolyte analogs for energy-storage devices (batteries or supercapacitors). To break from the academic status quo, we have designed a computer-controlled batch-process system to investigate desalination performance of practical electrode materials and architectures. This system allows continuous desalination to high degrees of salt removal on a laboratory scale without the need to use large-footprint electrodes. We demonstrate the characteristics of materials-based vs. system-based desalination metrics using NRL-pioneered electrodes — MnOx-decorated carbon nanofoam paper (MnOx@CNFP)1 — relatives of which are effective for faradaic desalination via Na+ capture.2 Scalable and freestanding CNFP electrodes possess three-dimensionally interconnected pore structures with tunable porosities to facilitate ion transport,3 while the interior surfaces of the CNFP (>200 m2 g–1 in mesopores and macropores) are readily functionalized with nanoscale MnOx by self-limiting electroless deposition.1 We investigate the effect of MnOx@CNFP pore structure and electrode thickness on practical desalination performance, and demonstrate the importance of reporting throughput (L/m2/h) and energy consumption (Wh/L) to better validate new electrode materials and architectures in flow-cell desalination devices. E. Fischer, K. A. Pettigrew, D. R. Rolison, R. M. Stroud, and J. W. Long, Nano Lett., 2007, 7, 281–286.Hand and R. D. Cusick, Environ. Sci. Technol., 2017, 51, 20, 12027–12034.C. Lytle, J. M. Wallace, M. B. Sassin, A. J. Barrow, J. W. Long, J. L. Dysart, C. H Renninger, M. P. Saunders, N. L. Brandell, and D. R. Rolison, Energy Environ. Sci., 2011, 4, 1913–1925.

Defect Engineering of 2D Copper Tin Composite Nanosheets Realizing Promoted Electrosynthesis Performance of Hydrogen Peroxide.

The transformation of the two-electron oxygen reduction reaction (2e-ORR) to produce hydrogen peroxide (H2 O2 ) is a promising green synthesis approach that can replace the high-energy consumption anthraquinone process. However, designing and fabricating low-cost, non-precious metal electrocatalysts for 2e-ORR remains a challenge. In this study, a method of combining complexation precipitation and thermal treatment to synthesize 2D copper-tin composite nanosheets to serve as the 2e-ORR electrocatalysts is utilized, achieving a high H2 O2 selectivity of 92.8% in 0.1m KOH, and a bulk H2 O2 electrosynthesis yield of 1436 mmol·gcat -1 ·h-1 using a flow cell device. Remarkably, the H2 O2 selectivity of this catalyst decreases by only 0.5% after 10,000 cyclic voltammetry (CV) cycles. In addition, it demonstrates that the same catalyst can achieve 97% removal of the organic pollutant methyl blue in an aqueous system solution within 1h using the on-site degradation technology. A reasonable control of defect concentration on the 2D copper-tin composite nanosheets that can effectively improve the electrocatalytic performance is found. Density functional theory calculations confirm that the surface of the 2D copper-tin composite nanosheets is conducive to the adsorption of the key intermediate OOH* , highlighting its excellent electrocatalytic performance for ORR with high H2 O2 selectivity.

Tailoring ligand fields of metal-azolate frameworks for highly selective electroreduction of CO2 to hydrocarbons at industrial current density

The catalytic activity of a metal complex is often contingent upon factors such as its valence state, coordination configuration of the metal ion, and coordination ability of the ligand. Hence, revealing the influence of the ligand field of the active metal center on the selectivity of the product for electrochemical CO2 reduction is crucial. Herein, three isostructural metal-azolate frameworks (MAFs) (Cu-BTP, Cu-BTTri, and Cu-BTT) with the same cyclic tetracopper(II) cluster units, namely [Cu3(BTP)2] (Cu-BTP, H3BTP = 1,3,5-tris(1H-pyrazol-4-yl)benzene), [Cu3(BTTri)2] (Cu-BTTri, H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene), and [Cu3(BTT)2] (Cu-BTT, H3BTT = 1,3,5-tris(2H-tetrazol-5-yl)benzene) were synthesized using pyrazolate-, triazolate-, and tetrazolate-based ligands, respectively. The synthesized MAFs were subjected to analysis to evaluate their electrochemical capabilities for reducing CO2 under identical reaction conditions. Among them, the pyrazolate MAF, Cu-BTP, delivers a current density of 1.25 A cm2 in a flow cell device with the highest Faradiac efficiency for hydrocarbons (CH4, 60%; C2H4, 22%). Furthermore, the system shows no obvious degradation over 60 h of continuous operation. The order of selectivity of the three MAFs for hydrocarbon production is consistent with the corresponding pKa values of the azolate ligands. Theoretical calculations show that a stronger Lewis basicity of the organic ligand, resulting in a stronger ligand field strength, is conducive to strengthening the binding of metal centers with key intermediates, such as *CO and *CHO. This ultimately leads to the deep reduction of CO2 to hydrocarbons.

Nitrogen-doped carbon-encompassed Ni nanoparticles prepared from Ni (II) cation-exchanged metal organic framework for efficient electrochemical CO2 reduction

Carbon layer-encompassed nickel nanoparticles of core–shell structure (designated as Ni NPs@NC) show notable advantages toward electrochemical carbon dioxide reduction reaction (CO2RR). Core-shell structured Ni NPs@NC nanoparticles anchored on the carbon matrix have been conveniently built from a cationic metal–organic framework (CPM-72 herein) which incorporates the Ni2+ cations through the cation exchange before high-temperature pyrolysis. The designed Ni NPs@NC catalyst exhibited impressive CO2RR performance which could efficiently convert CO2 into CO (carbon monoxide). In the H-type cell, a maximal CO faradaic efficiency (FE) of 86.4% was achieved at −0.8 V (vs. RHE) with a high CO partial current density (jco) of −11.0 mA cm−2. In the flow cell device, the CO FE was further improved to 98.6% with the enhanced jco of −38.7 mA cm−2. Finally, Zn-CO2 battery test also delivered a peak power density of 0.39 mW cm−2 at 2.65 mA cm−2.

A 3D-printed flow-cell for on-grid purification of electron microscopy samples directly from lysate

While recent advances in cryo-EM, coupled with single particle analysis, have the potential to allow structure determination in a near-native state from vanishingly few individual particles, this vision has yet to be realised in practise. Requirements for particle numbers that currently far exceed the theoretical lower limits, challenges with the practicalities of achieving high concentrations for difficult-to-produce samples, and inadequate sample-dependent imaging conditions, all result in significant bottlenecks preventing routine structure determination using cryo-EM. Therefore, considerable efforts are being made to circumvent these bottlenecks by developing affinity purification of samples on-grid; at once obviating the need to produce large amounts of protein, as well as more directly controlling the variable, and sample-dependent, process of grid preparation.In this proof-of-concept study, we demonstrate a further practical step towards this paradigm, developing a 3D-printable flow-cell device to allow on-grid affinity purification from raw inputs such as whole cell lysates, using graphene oxide-based affinity grids. Our flow-cell device can be interfaced directly with routinely-used laboratory equipment such as liquid chromatographs, or peristaltic pumps, fitted with standard chromatographic (1/16") connectors, and can be used to allow binding of samples to affinity grids in a controlled environment prior to the extensive washing required to remove impurities. Furthermore, by designing a device which can be 3D printed and coupled to routinely used laboratory equipment, we hope to increase the accessibility of the techniques presented herein to researchers working towards single-particle macromolecular structures.

RESPONSE OF BLOOD PRESSURE TO RENAL DENERVATION IS NOT ASSOCIATED WITH GENETIC VARIANTS

Objective: Renal denervation (RDN) is an evidence-based therapeutic option in patients with hypertension. However, the blood pressure (BP) responses to RDN in individual patients is currently impossible to predict. We conducted an unbiased genomic screen to identify genetic variants that are associated with response to RDN. Design and method: We included 268 patients (34.7% female, mean age 62 ± 10 years, BMI 30.5 ± 5.1 kg/m2) with uncontrolled treatment resistant hypertension (baseline BP 166 ± 21 / 90 ± 15 mmHg) who underwent endovascular RDN using the Symplicity™ catheter (Medtronic Inc, USA). Response to 24 hour ambulatory BP was assessed after 6 months and divided into two groups, above and below the median response of 6.0 mmHg, taking pre-intervention 24-hour ambulatory BP and regression to the mean into account. Whole exome sequencing assessing 249,669 variants was conducted using Illumina NovaSeq technology read on a Nova Seq S4 Flow Cell device. Read quality was overall excellent with the eight poorest performing samples still covering 30 reads per base for 80% of the exome. Reads were analysed by a standardised variant calling pipeline, using principal component analysis followed by individual logistic regressions to model association of variants with phenotype where appropriate. Results: There has been no linkage found between individual gene variants and the response variable. These neutral findings were further confirmed after adjustment for sex and in a sensitivity analysis looking at tertiles of BP response. We also explored specific variants in AGT, ADD1, ADRB1, ADRB2 and SCNN1A that have been proposed as potential candidate genes for response of BP to RDN and again found no association (all p>0.13). Gene ontology analysis of variants across the two responder groups highlighted differences in biological processes such as cell adhesion and molecular function such as protein tyrosine kinase activity. Conclusions: Response to RDN is not robustly associated with the genetic make-up of patients with uncontrolled treatment resistant hypertension. Our data do not support the use of a genetic score to predict BP response. We found, however, different regulation of biological processes and cellular/molecular function that could provide insights into the mechanisms of RDN.

Metastable Hexagonal Phase SnO2 Nanoribbons with Active Edge Sites for Efficient Hydrogen Peroxide Electrosynthesis in Neutral Media.

Electrochemical two-electron oxygen reduction reaction (2 e- ORR) to produce hydrogen peroxide (H2 O2 ) is a promising alternative to the energetically intensive anthraquinone process. However, there remain challenges in designing 2 e- ORR catalysts that meet the application criteria. Here, we successfully adopt a microwave-assisted mechanochemical-thermal approach to synthesize hexagonal phase SnO2 (h-SnO2 ) nanoribbons with largely exposed edge structures. In 0.1 M Na2 SO4 electrolyte, the h-SnO2 catalysts achieve the excellent H2 O2 selectivity of 99.99 %. Moreover, when employed as the catalyst in flow cell devices, they exhibit a high yield of 3885.26 mmol g-1 h-1 . The enhanced catalytic performance is attributed to the special crystal structure and morphology, resulting in abundantly exposed edge active sites to convert O2 to H2 O2 , which is confirmed by density functional theory calculations.

Metastable Hexagonal Phase SnO2 Nanoribbons with Active Edge Sites for Efficient Hydrogen Peroxide Electrosynthesis in Neutral Media

AbstractElectrochemical two‐electron oxygen reduction reaction (2 e− ORR) to produce hydrogen peroxide (H2O2) is a promising alternative to the energetically intensive anthraquinone process. However, there remain challenges in designing 2 e− ORR catalysts that meet the application criteria. Here, we successfully adopt a microwave‐assisted mechanochemical‐thermal approach to synthesize hexagonal phase SnO2 (h‐SnO2) nanoribbons with largely exposed edge structures. In 0.1 M Na2SO4 electrolyte, the h‐SnO2 catalysts achieve the excellent H2O2 selectivity of 99.99 %. Moreover, when employed as the catalyst in flow cell devices, they exhibit a high yield of 3885.26 mmol g−1 h−1. The enhanced catalytic performance is attributed to the special crystal structure and morphology, resulting in abundantly exposed edge active sites to convert O2 to H2O2, which is confirmed by density functional theory calculations.

Stabilizing Copper by a Reconstruction-Resistant Atomic Cu-O-Si Interface for Electrochemical CO2 Reduction.

Copper (Cu), a promising catalyst for electrochemical CO2 reduction (CO2R) to multi-electron reduction products, suffers from an unavoidable and uncontrollable reconstruction process during the reaction, which not only may lead to catalyst deactivation but also brings great challenges to the exploration of the structure-performance relationship. Herein, we present an efficient strategy for stabilizing Cu with silica and synthesize reconstruction-resistant CuSiOx amorphous nanotube catalysts with abundant atomic Cu-O-Si interfacial sites. The strong interfacial interaction between Cu and silica makes the Cu-O-Si interfacial sites ultrastable in the CO2R reaction without any apparent reconstruction, thus exhibiting high CO2-to-CH4 selectivity (72.5%) and stability (FECH4 remains above 60% after 12 h of test). A remarkable CO2-to-CH4 conversion rate of 0.22 μmol cm-2 s-1 was also achieved in a flow cell device. This work provides a very promising route for the design of highly active and stable Cu-based CO2R catalysts.

Penta nitrogen coordinated cobalt single atom catalysts with oxygenated carbon black for electrochemical H2O2 production

The two-electrons (2e–) oxygen reduction reaction (ORR) offers a sustainable and decentralized alternative to the traditional synthetics for hydrogen peroxide (H2O2) production. Although various Co single atom catalysts (SACs) have been proposed as highly effective 2e– ORR catalysts, there is still room for improvement through fine-tuned coordination environment. Here, a Co-N5-O-C with the combination of highly coordinated Co-N5 moieties and nearby electro-withdrawing epoxides is first time developed to reach the optimal binding energy of *OOH intermediate, resulting in the ultrahigh mass activity of 87.5 A g−1 at 0.75 V vs. RHE. Moreover, a high H2O2 production rate of 11.3 mol g−1 h−1 at 200 mA cm−2 is also obtained by a flow cell device. Such an efficient in-situ generation of H2O2 further enables 100% degradation of the organic methylene blue pollutant within 15 min through the electro-Fenton process. These findings will provide a new direction for on-site H2O2 synthesis and wastewater treatment.

Rational design of heterogenized molecular phthalocyanine hybrid single-atom electrocatalyst towards two-electron oxygen reduction

Single-atom catalysts supported on solid substrates have inspired extensive interest, but the rational design of high-efficiency single-atom catalysts is still plagued by ambiguous structure determination of active sites and its local support effect. Here, we report hybrid single-atom catalysts by an axial coordination linkage of molecular cobalt phthalocyanine with carbon nanotubes for selective oxygen reduction reaction by screening from a series of metal phthalocyanines via preferential density-functional theory calculations. Different from conventional heterogeneous single-atom catalysts, the hybrid single-atom catalysts are proven to facilitate rational screening of target catalysts as well as understanding of its underlying oxygen reduction reaction mechanism due to its well-defined active site structure and clear coordination linkage in the hybrid single-atom catalysts. Consequently, the optimized Co hybrid single-atom catalysts exhibit improved 2e− oxygen reduction reaction performance compared to the corresponding homogeneous molecular catalyst in terms of activity and selectivity. When prepared as an air cathode in an air-breathing flow cell device, the optimized hybrid catalysts enable the oxygen reduction reaction at 300 mA cm−2 exhibiting a stable Faradaic efficiency exceeding 90% for 25 h.

Elucidating Spatial Distribution of Electrochemical Reaction in a Porous Electrode by Electrochemical Impedance Spectra for Flow Batteries

A porous electrode is an essential component in a flow battery, and its structure determines the battery’s performance. The coupling of the multi-temporal-spatial-scale processes (e.g., electrochemical reaction, mass transfer, charge transfer) makes the recognition of each process complicated. Herein, a symmetric flow cell device is developed, and the electrochemical impedance measurement (two- or three-electrode configuration) is realized to elucidate the electrochemical processes. First, the effect of flow rate and concentration on the impedance spectra is investigated to identify the electrochemical processes. Second, the distributed resistance is quantified to describe the spatial distribution of the electrochemical reaction. It is found that the electrochemical reaction occurs near the membrane side at a low polarization current, and the reaction zones spatially extend from the membrane side to the current collector with the increase of imposed polarization. Such an evolution of the spatial distribution stems from the trade-off between the mass transfer and the ion conduction in the porous electrode. This work provides an experimental method to nondestructively probe the electrochemical processes, and the result provides guidance for developing innovative electrode structures for flow batteries.