promising and environmentally friendly oxidant in advanced oxidation processes (AOPs). Besides, hydrogen peroxide received great attention in fuel cell as well, in which hydrogen still plays an important role. But the low volumetric energy density makes the storage of hydrogen a difficult issue. H 2 O 2 possesses a high energy density and oxidation potential [13] in full pH range (E 0 = 1.763 V at pH = 0, E 0 = 0.878 V at pH 14), [1] which enables it an ideal energy carrier alternative. [11] With a compounded increase of 6%, [14] the annual production of H 2 O 2 had reached 5.5 million tons in 2015. [1] At present, three main categories were conducted to produce H 2 O 2 : direct H 2 O 2 synthesis from H 2 and O 2 , [15] anthraquinone oxidation process (AO-process), [12] and oxygen electroreduction. [16][17] More than 95% of H 2 O 2 were synthesized by AO-process, involving hydrogenation and oxidation of the anthraquinone molecule over Ni or Pd catalysts in organic solvents. [14] However, AO-process is facing some challenges: 1) Low sustainability. The combustible and explosive substances used in AO-process, such as heavy aromatics, trioctyl phosphate, are not environmentally friendly, which brings a negative influence on the sustainability of the AO-process; 2) Transportation insecurity. The high concentration of H 2 O 2 has the potential to explode when flammable materials exist, which brings safety problems to transportation; 3) Exorbitant additional cost. Ahead of being transported, H 2 O 2 has to be concentrated up to 70 wt% with impurity separation [18] and requires acid promoter stabilizer, [14] causing the increase of extra cost. All these make AO-process not the best choice for low cost and distributed H 2 O 2 production.Comparatively, two-electron oxygen reduction reaction (ORR) provides an economic, efficient, and nonhazardous alternative process, achieving the in situ H 2 O 2 production under mild conditions. [1] Furthermore, ORR process could be coupled with renewable energy sources [18] and fuel cell, [19] recovering the energy released ( f 0 ∆G , 120 KJ mol −1 ). [20,21] For instance, solar energy can be directly used as an energy source to produce H 2 O 2 through photoelectric catalysis. [22] In brief, H 2 O 2 production via two-electron oxygen electro-reduction has emerged as a promising candidate to address the demand for distributed energy.Hydrogen peroxide (H 2 O 2 ) is one of the 100 most paramount chemicals in the world, which has been widely used in industrial synthesis, pulp and bleaching, semiconductor cleaning, medical sterilizing, environmental treatments, and energy storage. Among various H 2 O 2 production methods, anthraquinone process has intrinsic drawbacks such as energy-intensive and environmental pollution while 2-electron oxygen reduction reaction (ORR) provides an economic, efficient, and nonhazardous alternative process to realize the in situ production of H 2 O 2 instead. Recently, heteroatom-doped carbon electrocatalysts, especially the nitrogen-doped ones, receive special a...

Major challenges for effective implementation of the Electro-Fenton (EF) water treatment process are that conventional efficient cathodes are relatively expensive, and H 2 O 2 activation by Fe 2+ may cause secondary pollution. Herein, we propose a low-cost activated carbon/stainless steel mesh (ACSS) composite cathode, where the SS mesh distributes the current and the AC simultaneously supports H 2 O 2 electrogeneration, H 2 O 2 activation, and organic compounds (OCs) adsorption. The oxygen-containing groups on the AC function as oxygen reduction reaction (ORR) sites for H 2 O 2 electrogeneration; while the porous configuration supply sufficient reactive surface area for ORR. 8.9 mg/L H 2 O 2 was obtained with 1.5 g AC at 100 mA under neutral pH without external O 2 supply. The ACSS electrode is also effective for H 2 O 2 activation to generate ‧OH, especially under neutral pH. Adsorption shows limited influence on both H 2 O 2 electrogeneration and activation. The iron-free EF process enabled by the ACSS cathode is effective for reactive blue 19 (RB19) degradation. 61.5% RB19 was removed after 90 min and 74.3% TOC was removed after 720 min. Moreover, long-term stability test proved its relatively stable performance. Thus, the ACSS electrode configuration is promising for practical and cost-effective EF process for transformation of OCs in water.

Efficient H 2 O 2 electrogeneration from 2-electron oxygen reduction reaction (ORR) represents an important challenge for environmental remediation application. H 2 O 2 production is determined by 2-electron ORR as well as H 2 O 2 decomposition. In this work, a novel strategy based on the systematical investigation on H 2 O 2 decomposition pathways was reported, presenting a drastically improved bulk H 2 O 2 concentration. Results showed that bulk phase disproportion, cathodic reduction, and anodic oxidation all contributed to H 2 O 2 depletion. To decrease the extent of H 2 O 2 cathodic reduction, the pulsed current was applied and proved to be highly effective to lower the extent of H 2 O 2 electroreduction. A systematic study of various pulsed current parameters showed that H 2 O 2 concentration was significantly enhanced by 61.6% under pulsed current of "2s ON + 2s OFF" than constant current. A mechanism was proposed that under pulsed current, less H 2 O 2 molecules were electroreduced when they diffused from the porous cathode to the bulk electrolyte. Further results demonstrated that a proper pulse frequency was necessary to achieve a higher H 2 O 2 production. Finally, this strategy was applied to Electro-Fenton (EF) process with ibuprofen as model pollutant. 75.0% and 34.1% ibuprofen were removed under pulsed and constant current at 10 min, respectively. The result was in consistent with the higher H 2 O 2 and •OH production in EF under pulsed current. This work poses a potential approach to drastically enhance H 2 O 2 production for improved EF performance on organic pollutants degradation without making any changes to the system except for power mode.

Electrocatalytic oxygen evolution reaction (OER) has been recognized as the bottleneck of overall water splitting, which is a promising approach for sustainable production of H2. Transition metal (TM) hydroxides are the most conventional and classical non-noble metal-based electrocatalysts for OER, while TM basic salts [M2+(OH)2-x(Am−)x/m, A = CO32−, NO3−, F−, Cl−] consisting of OH− and another anion have drawn extensive research interest due to its higher catalytic activity in the past decade. In this review, we summarize the recent advances of TM basic salts and their application in OER and further overall water splitting. We categorize TM basic salt-based OER pre-catalysts into four types (CO32−, NO3−, F−, Cl−) according to the anion, which is a key factor for their outstanding performance towards OER. We highlight experimental and theoretical methods for understanding the structure evolution during OER and the effect of anion on catalytic performance. To develop bifunctional TM basic salts as catalyst for the practical electrolysis application, we also review the present strategies for enhancing its hydrogen evolution reaction activity and thereby improving its overall water splitting performance. Finally, we conclude this review with a summary and perspective about the remaining challenges and future opportunities of TM basic salts as catalysts for water electrolysis.