Tailoring carbon-based catalysts for 2e- oxygen reduction to hydrogen peroxide production: Mechanisms, reactor optimization, performance enhancement, and application.

The two-electron oxygen reduction reaction (2e- ORR) is a pivotal pathway for the distributed production of hydrogen peroxide (H2O2). In nature, enzymes containing manganese (Mn) centers can convert reactive oxygen species into H2O2. However, Mn-based heterogeneous catalysts for 2e- ORR are scarcely reported. Herein, we developed a nature-inspired single-atom electrocatalyst comprising N, O co-coordinated Mn sites, utilizing carbon dots as the modulation platform (Mn CD/C). As-synthesized Mn CD/C exhibited exceptional 2e- ORR activity with an onset potential of 0.786 V and a maximum H2O2 selectivity of 95.8 %. Impressively, Mn CD/C continuously produced 0.1 M H2O2 solution at 200 mA/cm2 for 50 h in the flow cell, with negligible loss in activity and H2O2 faradaic efficiency, demonstrating practical application potential. The enhanced activity was attributed to the incorporation of Mn atomic sites into the carbon dots. Theoretical calculations revealed that the N, O co-coordinated structure, combined with abundant oxygen-containing functional groups on the carbon dots, optimized the binding strength of intermediate *OOH at the Mn sites to the apex of the catalytic activity volcano. This work illustrates that carbon dots can serve as a versatile platform for modulating the microenvironment of single-atom catalysts and for the rational design of nature-inspired catalysts.

Constructing alkaline microenvironment on catalyst surface for enhanced electrocatalytic synthesis of hydrogen peroxide in a neutral electrolyte

Electrochemical hydrogen peroxide (H2O2) production via two-electron oxygen reduction reaction (2e- ORR) has received increasing attention as it enables clean, sustainable, and on-site H2O2 production. Mimicking the active site structure of H2O2 production enzymes, such as nickel superoxide dismutase, is the most intuitive way to design efficient 2e- ORR electrocatalysts. However, Ni-based catalysts have thus far shown relatively low 2e- ORR activity. In this work, we present the design of high-performing, atomically dispersed Ni-based catalysts (Ni ADCs) for H2O2 production through understanding the formation chemistry of the Ni-based active sites. The use of a precoordinated precursor and pyrolysis within a confined nanospace were found to be essential for generating active Ni-N x sites in high density and increasing carbon yields, respectively. A series of model catalysts prepared from coordinating solvents having different vapor pressures gave rise to Ni ADCs with controlled ratios of Ni-N x sites and Ni nanoparticles, which revealed that the Ni-N x sites have greater 2e- ORR activity. Another set of Ni ADCs identified the important role of the degree of distortion from the square planar structure in H2O2 electrosynthesis activity. The optimized catalyst exhibited a record H2O2 electrosynthesis mass activity with excellent H2O2 selectivity.

Limited supply of precious metals impedes the scale up of electrolyzer production. To enable 100s of GWs of PEM electrolyzer capacity by 2030, iridium supply must grow by more than 10x.[1] Alternatively, catalysts made of cheaper, more abundant manganese could relieve these constraints, if engineered to have comparable activity and stability. The unique bifunctional OER/ORR activity of Mn oxide could also enable electrolyzers that both produce and consume fuel with a single catalyst. Yet rational design of Mn oxide catalysts requires a better understanding of the structural and chemical motifs that lead to high bifunctional performance.In this talk, we will discuss our investigation of the bifunctional OER & ORR activity of Mn oxide and its unusual pH dependence. Our model system, 𝛼-KxMnO2, is among the best reported Mn oxide catalysts for both the OER and the ORR, with ORR activity matching that of Pt.[2] While the OER activity of Mn oxide has been shown to increase at higher pH,[3] we will show that its ORR activity improves as well. We will discuss our attempts to understand this phenomenon with operando scanning transmission x-ray microscopy (STXM), which enables spectroscopic investigation of single catalyst particles during reaction with spatial resolution down to 25 nm. Complementary surface DFT calculations provide further evidence for identification of the active crystal facets and insights into the pH-dependency of the OER & ORR mechanisms. We will discuss the implications of this work on the design of bidirectional electrolyzers with low-cost, non-precious metal catalysts.[1] IRENA. Green Hydrogen Cost Reduction. 2020 [2] Meng, Y. et al. J. Am. Chem. Soc. 2014, 136 (32), 11452–11464.[3] Takashima, T. et al. J. Am. Chem. Soc. 2012, 134 (3), 1519–1527.

Regenerative polymer electrolyte membrane (PEM) fuel cell technology is promising for sustainable power generation in the future. The two reactions taking place on the oxygen electrode in a regenerative PEM fuel cell are oxygen reduction (ORR) in the fuel cell mode and oxygen evolution (OER) in the electrolyzer mode. State-of-the-art catalysts for these reactions are precious metal based – Pt for ORR and Ir (or Ru) for OER. These are extremely expensive and are active only for one of the two reactions, either ORR or OER, but not both. This necessitates the development of cheaper alternatives with good bifunctional characteristics as catalysts for both ORR and OER in regenerative PEM fuel cells. Bifunctional characteristics of nitrogen-doped carbon nanostructures (CNx) that have been shown to demonstrate high ORR activity in acidic medium [1], are explored in this work. ORR electrochemical activity measurements performed using a rotating disk electrode (RDE) show significant ORR activity of CNx catalysts which is lower than that of Pt/C but higher than that shown by Ir/C. Pt/C and Ir/C exhibit the lowest and highest OER activity, respectively, while OER over-potential of CNx is found to be similar to state-of-the-art Ir/C catalyst (1.62 V and 1.59 V vs. RHE). Analysis of bifunctional activity in terms of total ORR and OER over-potential, demonstrates much better bifunctional characteristics of CNx compared to Ir/C and Pt/C catalysts. Operando mass spectrometry experiments are performed to confirm the absence of carbon corrosion [2]. Use of poisoning probes is a common methodology to investigate the nature of active sites in catalytic materials. However, CNx catalysts cannot be poisoned using probe molecules like H2S, CO and CN- indicating the absence of a metal-centered active site [3]. Our recent work has identified, for the first time, that phosphate anions can be used as poisoning probes for CNx [4]. Poisoning phenomenon associated with phosphate anions reveals the presence of two possible active sites: (i) pyridinic N itself which is rendered inactive by protonation, and (ii) carbon next to pyridinic N (i.e. pyridinic N as an active site marker) where poisoning is caused by a site-blocking effect caused by H2PO4 - adsorption on carbon. As the H2PO4 - concentration increases, there is a decrease in pyridinic N content of CNx which correlates with the decrease in ORR activity. Based on the above observations, distribution of pyridinic N species in CNx is changed by changing the pyrolysis conditions during carbon growth. XPS performed on CNx samples pyrolyzed at different temperatures reveals an increase in pyridinic nitrogen content with increasing temperature, with C:N ratio remaining the same in all samples. Laser Raman Spectroscopy shows a slight increase in the degree of graphitization with increasing pyrolysis temperature. Electrochemical activity measurements of these samples show higher ORR and OER activity at higher pyrolysis temperatures. Specific ORR kinetic current (ik) and OER current at 1.63 V correlate very well with increasing pyridinic nitrogen content in the samples [2]. ORR activity and hence bifunctional characteristics are also found to improve by incorporation of chloride species in CNx samples. The insights gained in this study will help in the rational design of efficient bifunctional catalysts for PEM regenerative fuel cells.

Pyrimidine-assisted synthesis of S, N-codoped few-layered graphene for highly efficient hydrogen peroxide production in acid

Electrosynthesis of hydrogen peroxide (H2O2) via a two-electron oxygen reduction reaction (2e– ORR) provides a green and sustainable alternative to the current energy-intensive anthraquinone process. However, the development of inexpensive and efficient electrocatalysts under acidic and air-fed conditions remains a challenge. Herein, we constructed a three-dimensional nitrogen-doped carbon nanoweb (3D-N-CNW) electrocatalyst by using a naturally porous cross-linked skeleton of water hyacinth as a raw material. The 3D-N-CNW shows an ultrahigh specific surface area of 1464 m2 g–1 with interconnected hierarchical porous architecture, and abundant carbon defects, resulting in excellent 2e– ORR activity with high onset potential (0.64 V) and H2O2 selectivity (∼93%), which is the best-performing carbon-based catalyst reported in acidic media. Impressively, the flow cell utilizing 3D-N-CNW achieves an exceptionally high H2O2 yield of 4289 mg L–1 h–1 under air self-breathing conditions, enabling ultrafast degradation of representative organic pollutants. Theoretical calculations and control experiments reveal that the 3D cross-linked network facilitates mass transport and creates an enriched O2 microenvironment to promote the 2e– ORR to highly efficient H2O2 production. This work provides a low-cost, readily scalable, and convenient route for the construction of advanced carbon-based electrocatalysts from natural biomass materials; it can also be expected in other renewable areas.

Hydrogen peroxide (H2O2) is a versatile and environmentally friendly oxidant with broad applications in industry, energy, and environmental remediation. Electrocatalytic H2O2 production via the two-electron oxygen reduction reaction (2e− ORR) has emerged as a sustainable alternative to traditional anthraquinone processes. Covalent organic frameworks (COFs), as a class of crystalline porous materials, exhibit high structural tunability, large surface areas, and chemical stability, making them promising electrocatalysts for 2e− ORR. This review systematically summarizes recent advances in COF-based electrocatalysts for H2O2 production, including both metal-free and metal-containing systems. We discuss key strategies in COF design—such as dimensional modulation, linkage engineering, heteroatom doping, and post-synthetic modification—and highlight their effects on activity, selectivity, and stability. Fundamental insights into the 2e− ORR mechanism and evaluation metrics are also provided. Finally, we offer perspectives on current challenges and future directions, emphasizing the integration of machine learning, conductivity enhancement, and scalable synthesis to advance COFs toward practical H2O2 electrosynthesis.

Impact of catalyst loading of atomically dispersed transition metal catalysts on H2O2 electrosynthesis selectivity

Two-electron oxygen reduction reaction (2e− ORR) is a promising alternative to energy-intensive anthraquinone process for hydrogen peroxide (H2O2) production. Metal-free nanocarbon materials have garnered intensive attention as highly prospective electrocatalysts for H2O2 production, and an in-depth understanding of their porous structure and active sites have become a critical scientific challenge. The present research investigates a range of porous carbon catalysts, including non-porous, microporous, and mesoporous structures, to elucidate the impacts of porous structures on 2e− ORR activity. The results highlighted the superiority of mesoporous carbon over other porous materials, demonstrating remarkable H2O2 selectivity. Furthermore, integration of X-ray photoelectron spectroscopy (XPS) data analysis with electrochemical assessment results unravels the moderate surface oxygen content is the key to increase 2e− ORR activity. These results not only highlight the intricate interplay between pore structure and oxygen content in determining catalytic selectivity, but also enable the design of carbon catalysts for specific electrochemical reactions.

Electrosynthesis of hydrogen peroxide (H2O2) via the two-electron oxygen reduction reaction (2e- ORR) is promising for various practical applications, such as wastewater treatment. However, few electrocatalysts are active and selective for 2e- ORR yet are also resistant to catalyst leaching under realistic operating conditions. Here, a joint experimental and computational study reveals active and stable 2e- ORR catalysis in neutral media over layered PdSe2 with a unique pentagonal puckered ring structure type. Computations predict active and selective 2e- ORR on the basal plane and edge of PdSe2, but with distinct kinetic behaviors. Electrochemical measurements of hydrothermally synthesized PdSe2 nanoplates show a higher 2e- ORR activity than other Pd-Se compounds (Pd4Se and Pd17Se15). PdSe2 on a gas diffusion electrode can rapidly accumulate H2O2 in buffered neutral solution under a high current density. The electrochemical stability of PdSe2 is further confirmed by long device operational stability, elemental analysis of the catalyst and electrolyte, and synchrotron X-ray absorption spectroscopy. This work establishes a new efficient and stable 2e- ORR catalyst at practical current densities and opens catalyst designs utilizing the unique layered pentagonal structure motif.

CO2-derived edge-boron-doped hierarchical porous carbon catalysts for highly effective electrochemical H2O2 production

Electrosynthesis of H2O2 through a two-electron oxygen reduction reaction by carbon based catalysts: From mechanism, catalyst design to electrode fabrication

Pairing photocatalytic 1,2,3,4-tetrahydroisoquinoline semi-dehydrogenation reaction (THIQ-SDR) with two-electron oxygen reduction reaction (2e- ORR) is a green solar to chemical strategy by simultaneously utilizing the photo-excited electrons and holes. However, it is still short of high-efficiency photocatalyst to drive two reactions above. In the present work, crystalline pyrene-thiourea/urea covalent organic frameworks (COF-Py-S and -O) were synthesized and demonstrated as high-performance metal-free photocatalysts. In particular, COF-Py-S exhibited higher 2e- ORR to H2O2 yield rate of ~19 mmol g-1 h-1 under visible light and finally solid H2O2 products (Na2CO3 ⋅ 1.5H2O2) was collected after long-time irradiation. Under natural sunlight conditions, the H2O2 production rate over COF-Py-S further increased to ~51 mmol g-1 h-1, among the best COF photocatalysts. COF-Py-S also showed high THIQ-SDR conversion (~100 %) with 3,4-dihydroisoquinoline (DHIQ) selectivity of ~92 %. Theoretical calculations reveal stronger electron push-pull effect of COF-Py-S than COF-Py-O, enhancing its photoinduced charge carries, and a four-step 2e- ORR mechanism with thiourea as the active sites has been confirmed as well as the mechanism of THIQ-SDR to DHIQ, featuring lower energy barriers for O2 adsorption and *HOOH desorption over thiourea units. This work provides a paired photosynthesis strategy of H2O2 and DHIQ over high-efficiency pyrene-(thio)urea COF photocatalysts with electron push-pull effect.

The whole world has been in transition from a fossil fuel based economy to a clean energy economy. Especially in today's society, the demand for energy is increasing, the fossil fuel is exhausted and the environmental safety is becoming more and more important. [1-3]This inevitable process is being accelerated by recent active research on sustainable energy scavenging, conversion and storage. Batteries have long been accepted for their capacity to efficiently convert and store electrical energy[ 4] . To the best of our knowledge, there has been no report on the combination among La2O3, MnO2, CaO and carbon-based materials to form hybrid catalysts for improving the electro-catalytic performance of the materials in rechargeable metal-air batteries. It is expected that by combining the transition metal oxides with carbon-based material via a facile hydrothermal process would lead to a strong hybrid effect to enhance the catalytic performance. Based on this strategy, we have designed and synthesized the hybrid material via a facial method where the precursors of the metal-oxide (La, Mn and Co) and CNTs are justly mixed into a single reaction to produce the final catalysts. First, MnO2 nanotubes were prepared by a facile hydrothermal method. Then CaO nanoparticles and La2O3 nanoparticles were modified on the MnO2 nanotubes by a hydrothermal method combined with post-heat treatment. In a typical synthesis of La2O3/CaO/MnO2-CNTs hybrid nanomaterials, 0.25 g Ca(NO3)2▪4H2O and 0.25g La(NO3)2▪6H2O were dissolved in 15 mL of 1.3 mol L-1 ammonia solution. Afterwards, 0.125 g as-prepared MnO2 nanotubes and 0.125g CNTs were dispersed in the above solution by hydrothermal for 60 min. The resulting products. were separated by centrifugation, washed with deionized water, dried at 60 ℃ for 5 h, and then calcined in air at 400℃ for 1 h. As shown in Fig.1, the onset potential for was detected at 0.94 V for (MnO2/Co3O4)/CNTs,whereas it was 1.08 V and 0.78 V for Pt/C and CNTs, respectively. At 0.2 V, La2O3/CaO/MnO2-CNTs, Pt/C and CNTs afforded an ORR current density of 5.2mA cm-2 ,3.6mA cm-2and 2.2mA cm-2 .Apart from the ORR activity, excellent OER activity is particularly critical for bi-functional catalysts. As shown , the onset potential for was detected at 1.46V for La2O3/CaO/MnO2-CNTs, whereas it was 1.59V and 1.60V for CNTs and 20%Pt/C,the OER current density of La2O3/CaO/MnO2-CNTs,20%Pt/C and CNT at 1.67 V was 8 and 0.7 and 0.3mA cm- 2.A rotating disc electrode (RDE) half-cell setup was used to investigate the ORR and OER catalytic activity of the samples. The working electrode was fabricated by casting Nafion-impregnated catalyst ink onto a glassy carbon disk electrode (5.6 mm in diameter). 10 mg of the catalyst was ultrasonically dispersed into 1mL ethanol 8 and uL 5 wt% Nafion solution to form a catalyst ink. 5uL of the catalyst ink was deposited on the disk and dried at room temperature. The working electrode was allowed to achieve a catalyst loading of 0.1 mg cm-2. Electrochemical activity of the samples was studied using linear sweep voltammetry. The working electrode was immersed in a glass cell containing 0.1 M KOH aqueous electrolyte. A platinum foil and an Hg/HgO electrode were used as the counter and reference electrodes, respectively. Catalyst activity toward the ORR and OER was evaluated in oxygen-saturated electrolyte solution from 1.67 to 0.1 V vs. RHE. The potential of the reference electrode was normalized with respect to the potential of the reversible hydrogen electrode (RHE). The rotation rate is 1600 rpm and the scan rate is 5 mV s-1. A commercial Pt/C catalyst (30 wt% platinum on carbon) was tested using the same procedure. The battery had an open circuit voltage of 1.35 V. At a voltage of 600 mV, it showed a high current density of 375mA cm-2. The peak power density was 258mW cm-2. the battery discharge and discharge performance noticeably at lower current densities and through long cycle times. In summary, La2O3/CaO/MnO2-CNTs a new air electrode material have been synthesized via a two-step hydrothermal method. These hybrid nanomaterials display good bifunctional ORR/OER activity and cyclic stability in the discharge and charge process. Further studies are ongoing to improve the ZABs performance by manipulating the hybrid structure. References 1 M.Armand, J. M.Tarascon, Nature, 451, 652(2008). 2 A. S.Arico, P.Bruce, B.Scrosati, J. M.Tarascon, W. Van Schalkwijk, Nat. Mater., 4, 366(2005). 3 G.Girishkumar, B.McCloskey, A. C.Luntz, S.Swanson, W.Wilcke, J. Phys. Chem. Lett., 1, 2193(2010). 4 R.Cao, J.Lee, M. Liu, J.Cho, Adv. Energy Mater., 2, 816(2012) Figure 1