Two-electron Oxygen Reduction Research Articles

Insights into the effects of pulsed parameters on H2O2 synthesis by two-electron oxygen reduction under pulsed electrocatalysis

Pulsed electrocatalysis is emerging as a promising strategy for the improvement of electrocatalytic performance. Herein, the 2-electron oxygen reduction reaction (2eORR) for H2O2 synthesis was taken as a model reaction to explore the effects of pulsed parameters on electrocatalysis. Firstly, the key parameters of a square wave pulse were determined by theoretical analysis and electrochemical testing. Secondly, we found that the pulsed period of the square wave pulse should be neither too short nor too long due to the opposing effects of non-Faradaic and diffusion processes. The lower duty ratio was more favorable for 2eORR. Finally, the effects of the pulsed waveform were explored. We found that the enhanced effects on 2eORR under the rapid sweep potential of a triangular or sine wave were similar to those for a square wave with a short period. Under a sine wave pulse with a frequency of 0.2 Hz, the H2O2 production rate and Faradaic efficiency of 2eORR were increased by 1.4 times and 3.62 times, respectively, compared with the results obtained under constant potential, and the energy consumption was reduced by 75 %. The enhanced effects described above could be attributed to the diffusion layer becoming thinner while the pulsed potential was applied. The reaction rate was increased by the diffusion of reactants (i.e., O2, H+) to the electrode surface and the intrinsic selectivity of 2eORR was increased by the oxygen-rich environment. In addition, the ineffective electroreduction of H2O2 was also reduced by the diffusion of H2O2 toward the bulk solution. This work provides new insights into the determination of parameters, waveform effects and the mechanism of pulsed electrocatalysis.

Charge-Polarized Selenium Vacancy in Nickel Diselenide Enabling Efficient and Stable Electrocatalytic Conversion of Oxygen to Hydrogen Peroxide.

Vacancy engineering is deemed as one of the powerful protocols to tune the catalytic activity of electrocatalysts. Herein, Se-vacancy with charge polarization is created in the NiSe2 structure (NiSe2 -VSe ) via a sequential phase conversion strategy. By a combined analysis of the Rietveld method, transient photovoltage spectra (TPV), in situ Raman and density functional theory (DFT) calculation, it is unequivocally discovered that the presence of charge-polarized Se-vacancy is beneficial for stabilizing the structure, decreasing the electron transfer kinetics, as well as optimizing the free adsorption energy of reaction intermediate during two-electron oxygen reduction reaction (2e- ORR). Benefiting from these merits, the as-prepared NiSe2 -VSe delivered the highest selectivity of 96% toward H2 O2 in alkaline media, together with a selectivity higher than 90% over the wide potential range from 0.25 to 0.55V, ranking it in the top level among the previously reported transition metal-based electrocatalysts. Most notably, it also displayed admirable stability with only a slight selectivity decay after 5000 cycles of accelerated degradation test (ADT).

Iterative machine learning method for screening high-performance catalysts for H2O2 production

Electrochemical H2O2 production through a selective two-electron oxygen reduction reaction represents a promising alternative to the traditional anthraquinone oxidation process (AOP). However, it remains an unavoidable challenge to screen high-performance catalysts for the H2O2 production such as ambiguous activation mechanisms. Herein, we propose an iterative machine learning (iML) method that drastically reduces the required training set size. By introducing the feature of spatial coordinate information,we can rapidly screen out the optimal catalytic activity from hundreds of single-atom catalysts. It can be found that RhO2N2(A) is an ideal catalyst for H2O2production with an ultra-low overpotential of 0.013 V. The work sheds light on the path to accelerate the data-driven design and discovery of high-performance catalysts.

Recent advances in H2O2 electrosynthesis based on the application of gas diffusion electrodes: Challenges and opportunities

Over the past few years, the development of highly efficient alternative techniques for the in situ production of H 2 O 2 via two-electron oxygen reduction reaction (2 e – ORR) has been the focus of intense research in electrochemistry. This review presents an overview of the recent advances made in H 2 O 2 electrosynthesis based on the application of gas diffusion electrodes (GDE) in the last 3 years. In the first section, we will be looking at the findings in the development of novel modified GDEs with a view to enhancing the activity and selectivity for 2 e – ORR, and the novel methodologies that have been employed for the construction of GDEs. In the second section, we will delve into the advances in the H 2 O 2 electrogeneration in industrial scale and for the treatment of water and wastewater. Finally, we will be discussing about the challenges and opportunities related to the production of H 2 O 2 via 2 e – ORR.

Efficient sacrificial-agent-free solar H2O2 production over all-inorganic S-scheme composites

Solar H2O2 generated from O2 and H2O is an economical and green protocol. However, most photocatalysts only perform well in the presence of sacrificial donors or the photocatalysts are composed by organic materials, thus hindering their applicability. Herein, an inorganic matrix, i.e. ZnIn2S4@BiVO4 (ZIS@BVO), is constructed as a model catalyst for photocatalytic H2O2 production in non-sacrificial systems. Excellent performance (1.8 mmol g−1 h−1) under visible-light is realized, and an apparent quantum yield of 5.18% is realized at 420 nm. Thorough investigation helps us to rationalize its exceptional performance and structural characteristics. Of note, the electron spin resonance spectroscopy results, in combination with scavenger capture experiments, reveal the discovery of an overlooked pathway. Specifically, except for the well-known two-electron oxygen reduction reaction (ORR), the involvement of 1O2 intermediate is verified during the H2O2 generation. Beyond presenting high performance of the inorganic composites, this finding also discloses a different reaction path for H2O2 production.

Kapok fiber derived biochar as an efficient electro-catalyst for H2O2 in-situ generation in an electro-Fenton system for sulfamethoxazole degradation

Developing highly efficient, low-cost, and environmental-friendly catalysts to generate hydrogen peroxide (H2O2) through two-electron oxygen reduction reaction (2e−–ORR) is very important for the electro-Fenton systems. In this work, nitrogen (N) self-doped biochar were synthesized through the carbonization of natural Kapok fiber (KF), and they were used as 2e−–ORR catalysts to generate H2O2 in electro-Fenton systems. The KF-derived biochar that carbonized at 700 °C (named as KFBC-700) exhibited the optimal 2e−–ORR catalytic performance. The pyridinic-N and COOH played key roles on O2 adsorption and reduction. The maximum H2O2 accumulated concentration of the KFBC-700 electro-Fenton system was as high as 108.5 mg/L. Sulfamethoxazole (SMX) can be completely removed through both the Fe2+ catalyzed homogeneous and the FeOCl mediated heterogeneous electron-Fenton reactions within 60 min. Moreover, the electro-Fenton systems showed an excellent stability and reusability for SMX degradation. Additionally, SMX was possibly degraded through three pathways according to the detected degradation intermediates. This paper for the first time used KF-derived biochar as 2e−-ORR catalysts of the electro-Fenton system, and obtained a prominent H2O2 in-situ generation efficiency and a high efficient SMX removal.

Mesoscale Mass Transport Enhancement on Well-Defined Porous Carbon Platform for Electrochemical H2O2 Synthesis.

Two-electron oxygen reduction toward hydrogen peroxide (H2O2) offers a promising alternative for H2O2 production, but its commercial utilization is still hindered by the difficulty of transferring lab-observed catalyst performance to the practical reactor. Here we report the investigation of the porosity engineering effect on catalytic performance inconsistency through a material platform consisting of a series of hollow mesoporous carbon sphere (HMCS) samples. The performance comparison of HMCS samples in rotating ring-disk electrode and Zn-air battery together with the simulation of diffusion behavior reveals that, in low current density conditions, large surface area is preferred, but the mass transport governs the performance in high current density regions. On account of the favorable porous structure, HMCS-8 nm delivers the most excellent practical performance (166 mW cm-2) and performs well in the bifunctional Zn-air battery for the wastewater purification (70% RhB degraded after 2 min and 99% after 32 min).

Electrocatalytic selectivity to H2O2 enabled by two-electron pathway on Cu-deficient Au@Cu2-xS-CNTs electrocatalysts

Electrochemical hydrogen peroxide (H2O2) production by two-electron oxygen reduction reaction (ORR) is an efficient and promising method to alternate industrial anthraquinone process. Herein, the Cu-deficient Au@Cu2-xS-CNTs core–shell electrocatalysts were synthesized for two-electron ORR, and composition, surface and structural properties were investigated using physicochemical characterizations. The Cu-defective Au@Cu2-xS-CNTs catalysts exhibit higher catalytic ORR activity and H2O2 specific productivity than Au@Cu2O-CNTs, indicating that the introduction of Cu defects effectively improves the two-electron transfer of ORR. The composition-optimized Au@Cu1.75S-CNTs sample with the abundant Cu defects exhibits high H2O2 selectivity (∼94 %) and good stability. Theoretical calculations reveal that the introduction of Cu defects can significantly lower the reaction energy barrier of the determinant intermediate OOH* formation of two-electron ORR pathway. This endows the synthesized catalysts with improved resistance to H2O generation. Our work deepens the fundamental understanding for guiding the optimization of H2O2 electrosynthesis catalysts to enhance selectivity and efficiency of two-electron pathway.

Multiple active cobalt species embedded in microporous nitrogen-doped carbon network for the selective production of hydrogen peroxide

To obtain excellent electrocatalysts for improving H2O2 yield and selectivity, we formulated a novel method for embedding cobalt catalytic active sites in porous two-dimensional nitrogen-doped carbon network (CNy) network and employed them as excellent two-electron oxygen reduction reaction (2e−-ORR) electrocatalysts. The polymeric cobalt-based metal–organic framework (polyCo-MOF) and melamine–cyanuric acid–complex (MCA) hybrid (denoted as polyCo-MOF@MCA) was used as a precursor for preparing a series of electrocatalysts comprising multiple active sites such as metallic Co, CoOx, or Co-N, which are homogeneously embedded in the porous two-dimensional CNy network through pyrolysis at high temperatures (600 °C, 700 °C, and 800 °C) under N2 atmosphere. The obtained CoOx/Co@CNy,700 hybrid by pyrolyzing polyCo-MOF@MCA at 700 °C displayed remarkably high H2O2 production and large selectivity in an alkaline solution. The possible catalytic mechanism of CoOx/Co@CNy,700 toward 2e−-ORR was identified by determining the catalytic kinetics and control experiments. The cathode assembled with the CoOx/Co@CNy,700 hybrid showed the maximum H2O2 production of 405 mmol L−1gcat.−1h−1 with a high Faradaic efficiency of 88.9 % at 0.65 V. The present work demonstrated a novel strategy for identifying excellent electrocatalysts with homogeneously dispersed multiple active sites and high production and selectivity for H2O2 synthesis, extending the applications of porous organic frameworks to the field of clean energy.

N-doped graphene aerogel cathode with internal aeration for enhanced degradation of p-nitrophenol by electro-Fenton process.

A columnar N-doped graphene aerogel (NGA) was successfully fabricated by one-step hydrothermal synthesis using L-hydroxyproline as reductant, N-doping, and swelling agent, and it was used as the cathode with internal aeration mode for the electro-Fenton degradation of p-nitrophenol. Owing to the stable solid-liquid-gas three-phase interface, more active defects, and modulated nitrogen dopants, the NGA cathode exhibited enhanced electrocatalytic activity. H2O2 could be continuously electro-generated via a two-electron oxygen reduction, and the yield of H2O2 was 153.3mg·L-1·h-1 with the low electric energy consumption of 15.3 kWh kg-1. Simultaneously, the NGA cathode had better charge transfer capability with N-doping, which was conducive to the conversion of Fe3+/Fe2+. Under the optimal condition, nearly 100% removal of p-nitrophenol and 84% removal of TOC were obtained within 60 and 120min, respectively. The NGA cathode also presented good stability and versatile applicability in different water matrices. Therefore, the NGA is a cost-effective cathode material in electro-Fenton system with adequate activity and reuse stabilization.

In-situ efficient electrosynthesis of H2O2–NaClO based on the media pH and catalyst mutual selection mechanism

Electrochemical two-electron oxygen reduction (2e− ORR) as a promising H2O2 synthesis strategy has been extensively explored, while the study on the relation between active sites at the catalyst surface and pH of applied medium is still scarce. Here in, the as-prepared catalysts by anodizing graphite felt in different media exhibit excellent H2O2 production activity in acidic (151.73 mg L−1 h −1 cm−2), neutral (53.72 mg L−1 h −1 cm−2) and alkaline (188.85 mg L−1 h −1 cm−2) media, which is superior to the current advanced felt-supported carbon-based catalysts. According to characterizations, a mutual selection mechanism between the catalysts and media pH is proposed, in which it is the catalysts’ specific functional groups and structures matching with the pH-dependent electron transfer mechanism leading to the efficient H2O2 production. When anolyte simulates the seawater environment in H-type cell, the simultaneous production of NaClO and H2O2 and degradation of rhodamine B are realized, and H2O2 accumulative concentration in solution reaches a breakthrough of 6.58 g L−1. In general, this work provides new insights into the relation between the pH-dependent surface properties of catalysts and 2e− ORR reaction mechanism. In-situ H2O2–NaClO synthesis is also a promising route for chemicals synthesis and pollutant degradation.

Hydrogen Peroxide Assisted Electrooxidation of Benzene to Phenol over Bifunctional Ni-(O-C2 )4 Sites.

Direct electrocatalytic oxidation of benzene has been regarded as a promising approach for achieving high-value phenol product, but remaining a huge challenge. Here an oxygen-coordinated nickel single-atom catalyst (Ni-O-C) is reported with bifunctional electrocatalytic activities toward the two-electron oxygen reduction reaction (2e- ORR) to H2 O2 and H2 O2 -assisted benzene oxidation to phenol. The Ni-(O-C2 )4 sites in Ni-O-C ar proven to be the catalytic active centers for bifunctional 2e- ORR and H2 O2 -assisted benzene oxidation processes. As a result, Ni-O-C can afford a benzene conversion as high as 96.4 ± 3.6% with a phenol selectivity of 100% and a Faradaic efficiency (FE) of 80.2 ± 3.2% with the help of H2 O2 in 0.1m KOH electrolyte at 1.5V (vs RHE). A proof of concept experiment with Ni-O-C concurrently as cathode and anode in a single electrochemical cell demonstrates a benzene conversion of 33.4 ± 2.2% with a phenol selectivity of 100% and a FE of 44.8 ± 3.0% at 10mA cm-2 .

Polydopamine/graphite sheet electrode for highly efficient electrocatalytic hydrogen peroxide generation and bisphenol A removal

Electrocatalytic processes, are greatly limited by acid conditions during water/wastewater treatment, and lead to higher disposal costs. Herein, based on the principle of strong adhesion of invertebrate mussels, polydopamine with multiple functional groups was immobilized on graphite sheet electrodes for constructing the electrocatalytic system (GS-PDA), which achieved favorable stability in hydrogen peroxide (H2O2) yield and model pollutant Bisphenol A (BPA) removal at pH = 3–9. At natural pH (∼5.8), GS-PDA had a higher BPA removal efficiency with 113 % increase in degradation rate and an 1100 % increase in productivity of H2O2 than control group. O2 bubbling was the original contributor of oxidant and its two-electron reduction product (H2O2) had a significant contribution for BPA removal in GS-PDA. High pH (pH ≥ 5) increased the selectivity toward the four-electron oxygen reduction process (to H2O) in control group, while GS-PDA could inhibit this process, thus promoting the production of H2O2 from two-electron oxygen reduction process. Both hydroxyl radicals and singlet oxygen were effective in removing BPA and the former was the primary pathway. Furthermore, 18 dominant degradations were identified in GS-PDA by high resolution hybrid quadrupole time-of-flight mass spectrometer, and BPA degradation pathway was proposed. This work found a way to decrease the effects of pH on electrocatalytic purification of water and generation of H2O2, and provided a strategy for treating wastewater at neutral pH.

Efficient synthesis of H2O2via oxygen reduction over PANI driven by kinetics regulation of carbon dots

Electrocatalytic two-electron (2e−) oxygen reduction reaction (ORR) is a promising method to realize the sustainable production of H2O2. However, the 4e− pathway competes with 2e− ORR, presenting a challenge to the design of highly selective, low-cost catalysts. In addition to the design of active sites, the regulation on electrocatalytic kinetics is another strategy to get ideal 2e− ORR route. Here, cobalt doped carbon dots (CDs-Co) were used to regulate the electron transport kinetics on polyaniline (PANI) by constructing p-n heterojunction. The electron transfer kinetics and oxygen molecular activation process on PANI/CDs-Co were studied and analyzed using the transient photo-induced voltage (TPV) and pulse voltage-induced current (PVC) technologies, respectively. These results show that CDs-Co not only reduces the over-potential of oxygen molecular activation, but also reduces the transient electron concentration on PANI, which effectively improves the selectivity and activity of H2O2 production via ORR. The obtained PANI/CDs-Co-2 shows a H2O2 selectivity nearly 100.0% higher than that of PANI (76.3%). And it exhibits a H2O2 productivity of 3.5 mol g−1cat h−1 at 0 V vs. RHE tested by gas diffusion electrode device. This work provides new insights for the design of 2e− ORR catalysts and the study of electrocatalytic kinetics.

Multiatom activation of single-atom electrocatalysts via remote coordination for ultrahigh-rate two-electron oxygen reduction

Electrocatalytic oxygen reduction via a two-electron pathway (2e−-ORR) is a promising and eco-friendly route for producing hydrogen peroxide (H2O2). Single-atom catalysts (SACs) typically show excellent selectivity towards 2e−-ORR due to their unique electronic structures and geometrical configurations. The very low density of single-atom active centers, however, often leads to unsatisfactory H2O2 yield rate, significantly inhibiting their practical feasibility. Addressing this, we herein introduce fluorine as a secondary doping element into conventional SACs, which does not directly coordinate with the single-atom metal centers but synergize with them in a remote manner. This strategy effectively activates the surrounding carbon atoms and converts them into highly active sites for 2e−-ORR. Consequently, a record-high H2O2 yield rate up to 27 mol g−1 h−1 has been achieved on the Mo–F–C catalyst, with high Faradaic efficiency of 90%. Density functional theory calculations further confirm the very kinetically facile 2e−-ORR over these additional active sites and the superiority of Mo as the single-atom center to others. This strategy thus not only provides a high-performance electrocatalyst for 2e−-ORR but also should shed light on new strategies to significantly increase the active centers number of SACs.

Two-step electrochemical synthesis of “ant-nest” like Co-O, pyrrolic N co-modified graphite felt for high-efficient H2O2 production

Heteroatom modified catalyst applying for the two-electron oxygen reduction reaction has the potential application for efficient on-site production of H2O2. At present, state of the art catalyst synthesis is often energy-intensive and tends to destroy the original structure of heteroatoms. Based on the above considerations, Co-O, pyrrolic N structures co-modified graphite felt catalyst (Co-PDA-GF) is successfully synthesized by a simple two-step electrochemical method. Interestingly, the introduction of Co2+ during the dopamine (DA) polymerization process changes the surface morphology of GF from a “thin-film” like to an “ant-nest” like. The cumulative H2O2 concentration achieves 421 mg L−1 in 6 h test time, which is 16 times higher than that of R-GF, and the Methylene blue (MB) in-situ removal rate reaches 97.2% within 3 h. The enhanced H2O2 yield of Co-PDA-GF catalyst is attributed to the increased oxygen-containing functional groups (-COOH, -C-O-C) and defect structures, and most importantly, Co-O and pyrrolic N synergistic modulation effects on local electronic modulation. This work provides new insights on DA polymerization morphology control and designing of high-efficient 2e− ORR catalyst with a low-energy method.

Active Oxygen Functional Group Modification and the Combined Interface Engineering Strategy for Efficient Hydrogen Peroxide Electrosynthesis.

Cathodic catalytic activity and interfacial mass transfer are key factors for efficiently generating hydrogen peroxide (H2O2) via a two-electron oxygen reduction reaction (ORR). In this work, a carbonized carboxymethyl cellulose (CMC)-reduced graphene oxide (rGO) synthetic fabric cathode was designed and constructed to improve two-electron ORR activity and interfacial mass transfer. Carbonized CMC exhibits abundant active carboxyl groups and excellent two-electron ORR activity with an H2O2 selectivity of approximately 87%, higher than that of rGO and other commonly used carbonaceous catalysts. Carbonizing CMC and the agglomerates formed from it restrain the restacking of rGO sheets and thus create abundant meso/macroporous channels for the interfacial mass transfer of oxygen and H2O2. Thus, the as-constructed carbonized CMC-rGO synthetic fabric cathode exhibits exceptional H2O2 electrosynthesis performance with 11.94 mg·h-1·cm-2 yield and 82.32% current efficiency. The sufficient active sites and mass-transfer channels of the cathode also ensure its practical application performance at high current densities, which is further illustrated by the rapid organic pollutant degradation via the H2O2-based electro-Fenton process.

(Invited) Efficient and Selective Electrocatalytic and Photoelectrochemical Conversion of Energy and Chemicals

Due to the intermittent nature of most renewable energy sources (such as solar and wind), practical large scale renewable energy utilization demands both efficient energy conversion and large scale energy storage or alternative usage. Earth-abundant but highly active and selective electrocatalysts are needed to enable efficient and sustainable production of fuels and chemicals using electrocatalytic and photoelectrochemical (PEC) energy conversion. We developed earth-abundant electrocatalysts for highly efficient hydrogen evolution reaction (HER) and oxygen evolution (OER). We recently combined computations and experiments to develop metal compounds as selective catalysts for two-electron oxygen reduction reaction (2e- ORR) to produce hydrogen peroxide (H2O2) and the subsequent electro-Fenton process for upgrading biomass molecules. We have integrated these earth-abundant electrocatalysts with efficient semiconductor materials to demonstrate efficient photoelectrochemical hydrogen generation systems. We also developed high performance hybrid solar-charged storage devices that integrate photoelectrochemical solar cells and redox flow batteries (RFBs). In these integrated solar flow batteries (SFBs), solar energy is absorbed by semiconductor electrodes to directly charge up the redox couples without external electric bias. Such charged redox molecules can then be either discharged to generate the electricity when needed or used to produce fuels and chemicals.

Efficient hydrogen peroxide electrosynthesis using anthraquinone covalently bonded CNT on superhydrophobic air breathing cathode

Electrochemical H2O2 production through two-electron oxygen reduction (ORR) is an environmentally friendly process with high energy efficiency. The low catalysts’ activity, low oxygen transfer rate and less device demonstration restrict the ORR development. In this study, a superhydrophobic air breathing cathode (ABC) was constructed, which consisted of anthraquinone covalently linked CNT-NH2 (NCNT-AQ) and PTFE modified carbon felt. This ABC could keep stable liquid-gas-solid three phase interfaces and efficiently utilize O2 through natural air diffusion, and the H2O2 generation rate could arrive at 2.09 M gcatalyst−1 h−1 at current density of 50 mA cm−2, which was 4.45 times higher than traditional electrode. A flow cell was designed with this ABC, the concentration of produced H2O2 could reach 0.26 M (8.84 g L−1) within 7 h, while the energy consumption was only 44.88 kWh kg−1 and the cost was estimated as 4.20 $ kg−1. The application of this device was demonstrated by the treatment of organic polluted water and soil. This work presents a highly selective, durable, and low-cost H2O2 electrosynthesis cell, providing a promising approach for the in-situ production and utilization of H2O2.

Insights on Oxygen Reduction Reaction to H2O2: The role of functional groups and textural properties on the activity and selectivity of doped carbon electrocatalysts

Hydrogen peroxide (H2O2) is a key molecule in chemical industrial processes and alterative processes and catalysts are needed to achieve a green in-situ generation. The electrochemical two-electron oxygen reduction (2e-ORR) reactions are one of the most attractive routes along with the water oxidation or the direct synthesis. Here, we focused on the 2e-ORR, summarizing the recent advances in the finalization of catalysts for specifically catalyse the generation of H2O2 and technical measures to improve material screening, especially to obtain reliable activity and selectivity data. Firstly, a brief overview of metal-free catalysts is given and secondly a focus on how different strategies are adopted to improve activity and selectivity is reported. Such strategies include to tune the thermodynamic energy barriers by changing the material composition/functionalization and/or to facilitate mass transfer for reactants and products by acting on morphology. Finally, a brief overview of challenges in ink and electrode preparation for the correct screening of H2O2 electrocatalysts is reported.