Toward the Decentralized Electrochemical Production of H2O2: A Focus on the Catalysis

Non-redox doping boosts oxygen evolution electrocatalysis on hematite

The oxygen evolution reaction (OER) is the major bottleneck to develop viable and cost-effective water electrolysis, a key process in the production of renewable fuels. Hematite, all iron α-Fe₂O₃, would be an ideal OER catalyst in alkaline media due to its abundance and easy processing. Despite its promising theoretical potential, it has demonstrated very poor OER activity under multiple experimental conditions, significantly worse than that of Co or Ni-based oxides. In the search for improving hematite performance, we have analysed the effect of doping with redox vs. non-redox active species (Ni or Zn). Our results indicate that Zn doping clearly outperforms Ni, commonly accepted as a preferred dopant. Zn-doped hematite exhibits catalytic performances close to the state-of-the-art for alkaline water splitting: reaching 10 mA cm⁻² at just 350 mV overpotential (η) at pH 13, thus twenty times that of hematite. Such a catalytic enhancement can be traced back to a dramatic change in the reaction pathway. Incorporation of Ni, as previously suggested, decreases the energetic barrier for the OER on the available centres. In contrast, Zn facilitates the appearance of a dominant and faster alternative via a two-site reaction, where the four electron oxidation reaction starts on Fe, but is completed on Zn after thermodynamically favoured proton coupled electron transfer between adjacent metal centres. This unique behaviour is prompted by the non-redox character of Zn centres, which maintain the same charge during OER. Our results open an alternative role for dopants on oxide surfaces and provide a powerful approach for catalytic optimisation of oxides, including but not limited to highly preferred all-iron oxides.

The distinct beneficial effect of Zn-doping on the OER alkaline activity of Fe-based catalysts points towards an alternative and faster two-site mechanism.

Selective Electrocatalytic Water Oxidation to Produce H2O2 Using a C,N Codoped TiO2 Electrode in an Acidic Electrolyte

Production of hydrogen peroxide (H₂O₂) via in situ electrochemical water oxidation possesses great potential applications in the energy and environment fields. In this work, for the first time, we reported a C,N codoped TiO₂ electrode for selective electrocatalytic water oxidation to produce H₂O₂ in an acidic electrolyte. An electrochemical anodic oxidation method combined with postcalcination in the presence of urea was applied to fabricate such a C,N codoped TiO₂ electrode, which was evidenced by detail structural characterizations. The calcination temperature and urea atmosphere were found to play key roles in its catalytic performances; the optimized 600N sample exhibited an onset potential of 2.66 V (vs Ag/AgCl) and a Tafel slope of 51 mV dec^-1 at pH 3. Under the optimal applied potential, the cumulative H₂O₂ concentration for this sample reached 0.29 μmol L^-1 cm^-2 h^-1. More importantly, a simple recalcination strategy was developed to recover the deactivation electrode. This study proposed an efficient C,N codoped TiO₂ electrode toward water oxidation to selectively produce H₂O₂ in the acidic electrolyte, which could be further used to in situ generate H₂O₂ for the energy- and environment-related fields with water as the precursor.

Room temperature and atmospheric pressure aqueous partial oxidation of ethane to oxygenates over AuPd catalysts

The aqueous partial oxidation of ethane over unsupported AuPd catalysts is investigated at 21 °C and 1 bar ethane.

A comparative perspective of electrochemical and photochemical approaches for catalytic H2O2 production

Recent advances in the design, preparation, and applications of different catalysts for electrochemical and photochemical H₂O₂ production are summarized, and some invigorating perspectives for future developments are also provided.

Theoretical insights into single-atom catalysts

Schematic diagram of theoretical models and applications of single atom catalysts. A review on the theoretical models, intrinsic properties, and the related application of SACs.

Dynamic structural changes of supported Pd, PdSn, and PdIn nanoparticles during continuous flow high pressure direct H2O2 synthesis

The structure of mono- and bimetallic supported Pd, PdSn, and PdIn NPs was monitored with a combination of techniques during continuous H₂O₂ synthesis with H₂O₂ production rates up to 580 mmol_H2O2 g_cat⁻¹ h⁻¹.

Analysis of Mass Flows and Membrane Cross-over in CO2 Reduction at High Current Densities in an MEA-Type Electrolyzer

Cell designs that integrate membrane-electrode assemblies (MEAs) with highly selective catalysts are a promising route to reduce ohmic losses and achieve high energy efficiency in CO₂ reduction at industrially relevant current densities. In this work, porous silver filtration membranes are demonstrated as simple and efficient gas-diffusion electrodes for CO₂ reduction to CO at high current densities in an MEA-type device. A partial current density for CO of up to ca. 200 mA cm^-2 was achieved at a cell voltage of ca. 3.3 V, in tandem with minimal H₂ production. However, the analysis of cathodic and anodic outlet streams revealed that CO₂ cross-over across the anion-exchange membranes, mostly in the form of CO₃^2- but partially as HCOO^- generated over the cathode, actually exceeds the amount of CO₂ converted to the target product, resulting in a poor utilization of the reactant and in the early onset of mass transfer limitations. In addition, CO₂ cross-over leads to a nonstoichiometric decrease of the outlet flow rate from the cathodic compartment. This effect can lead to a substantial overestimation of catalytic performance if the inlet flow rate of CO₂ is used as reference for calculating partial current densities and Faradaic efficiencies. The results of this work highlight the importance of carrying out a carbon balance, in addition to traditional measurements of activity and selectivity, to adequately assess the performance of CO₂ reduction devices at high current densities, and inform future efforts aimed at mitigating membrane cross-over in MEA-type electrolyzers for CO₂ reduction.

Highly efficient electro-generation of H2O2 by adjusting liquid-gas-solid three phase interfaces of porous carbonaceous cathode during oxygen reduction reaction

Equilibrium of three reactants (oxygen, proton and electron) in oxygen reduction reaction at large current flux is necessary for highly efficient electro-generation of H₂O₂. In this work, we investigated reactants equilibrium and H₂O₂ electrochemical production in liquid-gas-solid three phase interfaces on rolling cathodes with high electroactive area. Electrocatalytic reaction accelerated the electrolyte intrusion into hydrophobic porous catalyst layer for higher electroactive surface area, resulting in a 21% increase of H₂O₂ yield at 15 mA cm^-2. Air aerated cathode submerged in air/O₂ aeration solution was unable to produce H₂O₂ efficiently due to the lack of O₂ in three phase interfaces (TPIs), especially at current density > 2.5 mA cm^-2. For air breathing cathode, stable TPIs inside the active sites was created by addition of gas diffusion layer, to increase H₂O₂ production from 11 ± 2 to 172 ± 11 mg L^-1 h^-1 at 15 mA cm^-2. Pressurized air flow application enhanced both oxygen supply and H₂O₂ departure transfer to obtain a high H₂O₂ production of 461 ± 11 mg L^-1 h^-1 with CE of 89 ± 2% at 35 mA cm^-2, 45% higher than passive gas transfer systems. Our findings provided a new insight of carbonaceous air cathode performance in producing H₂O₂, providing important information for the practical application and amplification of cathodes in the future.

Hydrogenation of alkyl-anthraquinone over hydrophobically functionalized Pd/SBA-15 catalysts

Organosilane-functionalized mesoporous silica SBA-15 was prepared by the co-condensation method and then applied as a support of Pd catalysts for hydrogenation of 2-alkyl-anthraquinone (AQ, alkyl = ethyl, tert-butyl and amyl). The as-prepared Pd catalysts were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, N₂ adsorption–desorption, zeta potential, water contact angles measurement and transmission electron microscopy. By extending the pre-hydrolysis time of the silica source, the content of functional groups in the catalysts slightly increases. However, there is an initial increase in zeta potential and water contact angles up to a maximum at 2 h, followed by a decrease as the pre-hydrolysis time was further prolonged. The hydrophobicity created by organic functionalization has positive effects on AQ hydrogenation. The catalyst with the highest hydrophobicity exhibits the highest catalytic activity, with increments of 33.3%, 60.0% and 150.0% for hydrogenation of ethyl-, tert-butyl- and amyl-anthraquinone compared with the unfunctionalized one.

Enhanced AQ hydrogenation activity by hydrophobic functionalization of Pd/SBA-15 catalyst.

Direct electrosynthesis of pure aqueous H2O2 solutions up to 20% by weight using a solid electrolyte

Hydrogen peroxide (H2O2) synthesis generally requires substantial postreaction purification. Here, we report a direct electrosynthesis strategy that delivers separate hydrogen (H2) and oxygen (O2) streams to an anode and cathode separated by a porous solid electrolyte, wherein the electrochemically generated H+ and HO2– recombine to form pure aqueous H2O2 solutions. By optimizing a functionalized carbon black catalyst for two-electron oxygen reduction, we achieved >90% selectivity for pure H2O2 at current densities up to 200 milliamperes per square centimeter, which represents an H2O2 productivity of 3.4 millimoles per square centimeter per hour (3660 moles per kilogram of catalyst per hour). A wide range of concentrations of pure H2O2 solutions up to 20 weight % could be obtained by tuning the water flow rate through the solid electrolyte, and the catalyst retained activity and selectivity for 100 hours.

Highly selective oxygen reduction to hydrogen peroxide on transition metal single atom coordination

Shifting electrochemical oxygen reduction towards 2e- pathway to hydrogen peroxide (H2O2), instead of the traditional 4e- to water, becomes increasingly important as a green method for H2O2 generation. Here, through a flexible control of oxygen reduction pathways on different transition metal single atom coordination in carbon nanotube, we discovered Fe-C-O as an efficient H2O2 catalyst, with an unprecedented onset of 0.822 V versus reversible hydrogen electrode in 0.1 M KOH to deliver 0.1 mA cm-2 H2O2 current, and a high H2O2 selectivity of above 95% in both alkaline and neutral pH. A wide range tuning of 2e-/4e- ORR pathways was achieved via different metal centers or neighboring metalloid coordination. Density functional theory calculations indicate that the Fe-C-O motifs, in a sharp contrast to the well-known Fe-C-N for 4e-, are responsible for the H2O2 pathway. This iron single atom catalyst demonstrated an effective water disinfection as a representative application.

Electrosynthesis of Hydrogen Peroxide Synergistically Catalyzed by Atomic Co-Nx -C Sites and Oxygen Functional Groups in Noble-Metal-Free Electrocatalysts

Hydrogen peroxide (H₂ O₂ ) is a green oxidizer widely involved in a vast number of chemical reactions. Electrochemical reduction of oxygen to H₂ O₂ constitutes an environmentally friendly synthetic route. However, the oxygen reduction reaction (ORR) is kinetically sluggish and undesired water serves as the main product on most electrocatalysts. Therefore, electrocatalysts with high reactivity and selectivity are highly required for H₂ O₂ electrosynthesis. In this work, a synergistic strategy is proposed for the preparation of H₂ O₂ electrocatalysts with high ORR reactivity and high H₂ O₂ selectivity. A Co-N_x -C site and oxygen functional group comodified carbon-based electrocatalyst (named as Co-POC-O) is synthesized. The Co-POC-O electrocatalyst exhibits excellent catalytic performance for H₂ O₂ electrosynthesis in O₂ -saturated 0.10 m KOH with a high selectivity over 80% as well as very high reactivity with an ORR potential at 1 mA cm^-2 of 0.79 V versus the reversible hydrogen electrode (RHE). Further mechanism study identifies that the Co-N_x -C sites and oxygen functional groups contribute to the reactivity and selectivity for H₂ O₂ electrogeneration, respectively. This work affords not only an emerging strategy to design H₂ O₂ electrosynthesis catalysts with remarkable performance, but also the principles of rational combination of multiple active sites for green and sustainable synthesis of chemicals through electrochemical processes.

Activity-Selectivity Trends in the Electrochemical Production of Hydrogen Peroxide over Single-Site Metal-Nitrogen-Carbon Catalysts

Nitrogen-doped carbon materials featuring atomically dispersed metal cations (M-N-C) are an emerging family of materials with potential applications for electrocatalysis. The electrocatalytic activity of M-N-C materials toward four-electron oxygen reduction reaction (ORR) to H₂O is a mainstream line of research for replacing platinum-group-metal-based catalysts at the cathode of fuel cells. However, fundamental and practical aspects of their electrocatalytic activity toward two-electron ORR to H₂O₂, a future green "dream" process for chemical industry, remain poorly understood. Here we combined computational and experimental efforts to uncover the trends in electrochemical H₂O₂ production over a series of M-N-C materials (M = Mn, Fe, Co, Ni, and Cu) exclusively comprising atomically dispersed M-N_x sites from molecular first-principles to bench-scale electrolyzers operating at industrial current density. We investigated the effect of the nature of a 3d metal within a series of M-N-C catalysts on the electrocatalytic activity/selectivity for ORR (H₂O₂ and H₂O products) and H₂O₂ reduction reaction (H₂O₂RR). Co-N-C catalyst was uncovered with outstanding H₂O₂ productivity considering its high ORR activity, highest H₂O₂ selectivity, and lowest H₂O₂RR activity. The activity-selectivity trend over M-N-C materials was further analyzed by density functional theory, providing molecular-scale understandings of experimental volcano trends for four- and two-electron ORR. The predicted binding energy of HO* intermediate over Co-N-C catalyst is located near the top of the volcano accounting for favorable two-electron ORR. The industrial H₂O₂ productivity over Co-N-C catalyst was demonstrated in a microflow cell, exhibiting an unprecedented production rate of more than 4 mol peroxide g_catalyst^-1 h^-1 at a current density of 50 mA cm^-2.

Hydrogen peroxide generation from O2 electroreduction for environmental remediation: A state-of-the-art review

The electrochemical production of hydrogen peroxide (H2O2) by 2-electron oxygen reduction reaction (ORR) is an attractive alternative to the present complex anthraquinone process. The objective of this paper is to provide a state-of-the-arts review of the most important aspects of this process. First, recent advances in H2O2 production are reviewed and the advantages of H2O2 electrogeneration via 2-electron ORR are highlighted. Second, the selectivity of the ORR pathway towards H2O2 formation as well as the development process of H2O2 production are presented. The cathode characteristics are the decisive factors of H2O2 production. Thus the focus is shifted to the introduction of commonly used carbon cathodes and their modification methods, including the introduction of other active carbon materials, hetero-atoms doping (i.e., O, N, F, B, and P) and decoration with metal oxides. Cathode stability is evaluated due to its significance for long-term application. Effects of various operational parameters, such as electrode potential/current density, supporting electrolyte, electrolyte pH, temperature, dissolved oxygen, and current mode on H2O2 production are then discussed. Additionally, the environmental application of electrogenerated H2O2 on aqueous and gaseous contaminants removal, including dyes, pesticides, herbicides, phenolic compounds, drugs, VOCs, SO2, NO, and Hg0, are described. Finally, a brief conclusion about the recent progress achieved in H2O2 electrogeneration via 2-electron ORR and an outlook on future research challenges are proposed.

The Rise of Hydrogen Peroxide as the Main Product by Metal-Free Catalysis in Oxygen Reductions

One of the recent trends in electrocatalytic reactions involves the oxygen reduction reaction (ORR), where a new paradigm has been shaped to exploit this reaction for the synthesis of hydrogen peroxide (H₂ O₂ ). H₂ O₂ is a very versatile chemical of high commercial value, prepared currently through poorly sustainable processes. The emergence of metal-free carbon catalysts for the selective synthesis of H₂ O₂ is expected to revolutionize ORR research, beckoning at the development of new industrial schemes. The complexities of the mechanism and the factors dominating the selectivity of the process have been unveiled through a combination of theoretical and experimental studies.

Boosting the Characterization of Heterogeneous Catalysts for H2O2 Direct Synthesis by Infrared Spectroscopy

Infrared (IR) spectroscopy is among the most powerful spectroscopic techniques available for the morphological and physico-chemical characterization of catalytic systems, since it provides information on (i) the surface sites at an atomic level, (ii) the nature and structure of the surface or adsorbed species, as well as (iii) the strength of the chemical bonds and (iv) the reaction mechanism. In this review, an overview of the main contributions that have been determined, starting from IR absorption spectroscopy studies of catalytic systems for H2O2 direct synthesis, is given. Which kind of information can be extracted from IR data? IR spectroscopy detects the vibrational transitions induced in a material by interaction with an electromagnetic field in the IR range. To be IR active, a change in the dipole moment of the species must occur, according to well-defined selection rules. The discussion will be focused on the advancing research in the use of probe molecules to identify (and possibly, quantify) specific catalytic sites. The experiments that will be presented and discussed have been carried out mainly in the mid-IR frequency range, between approximately 700 and 4000 cm−1, in which most of the molecular vibrations absorb light. Some challenging possibilities of utilizing IR spectroscopy for future characterization have also been envisaged.

Changes in the oxidation state of Pt single-atom catalysts upon removal of chloride ligands and their effect for electrochemical reactions

The activity of Pt single-atom catalysts can be maximized by controlling the oxidation state of the single-atoms.

A COOH-terminated nitrogen-doped carbon aerogel as a bulk electrode for completely selective two-electron oxygen reduction to H2O2

A COOH-terminated nitrogen-doped carbon aerogel exhibited 100% selectivity to two-electron oxygen reduction, exceeding reported carbonaceous and noble metal catalysts. The optimal electrode with the synergistic effect of C-N/C-COOH resulted in a minimum ηO2/H2O2 and gave an evolution rate of 60 mg L-1 g-1 h-1 for H2O2 with satisfactory mechanical and electrochemical stability for practical applications.

Enhancing the Selectivity of H2O2 Electrogeneration by Steric Hindrance Effect

Selective hydrogen peroxide (H₂O₂) electrogeneration by oxygen reduction reaction (ORR) is an efficient and promising synthetic method for H₂O₂ production. Herein, we build a particular inorganic-organic interface to enhance the electrocatalytic selectivity of reduced graphene oxide (rGO) aerogels for H₂O₂ electrogeneration by modifying rGO aerogels with polyethyleneimine (PEI). The three-dimensional porous structure of aerogels and the steric hindrance effect of PEI on rGO endow PEI-functionalized rGO (rGO/PEI) aerogels with enhanced selectivity (90.7%), production rate (106.4 mmol g_cat^-1 h^-1), and durability for H₂O₂ electrogeneration by the two-electron pathway of ORR.