Integrating Nanoflower Morphology into Cr-N-C Catalysts for High-Rate H2O2 Electrosynthesis

As an environmentally benign chemical with robust oxidative reactivity, hydrogen peroxide (H2O2) has found broad applica- tions across wastewater decontamination,1 organic synthesis,2 medical sterilization,3 and industrial bleaching.4 Despite its versa- tility, large-scale H2O2 production remains highly dependent upon the anthraquinone route,5 which is inherently plagued by excessive energy consumption, hazardous organic solvent emis- sions, and cumbersome downstream purification procedures.6−9 Electrocatalytic two-electron oxygen reduction (2e− ORR) emerges as a compelling alternative for the mild, safe, and on- site synthesis of H2O2. Notably, acidic H2O2 production holds superior practical relevance.10,11 Unlike alkaline media, an acidic environment imparts decent stability to H2O2 while endowing it with stronger oxidative power.12,13 This unique combination translates into exceptional efficacy in target applications.

During the ORR process, the intense competition between the 4e− and 2e− pathways renders the catalytic site the decisive factor for selective H2O2 generation.14 Whether the O−O bond of the *OOH intermediate ruptures dictates the reaction coor- dinate, this requires balanced *OOH binding strength at the

Research Article

oxophilic, Cr-N4 can undergo in situ reconstruction to form O- Cr-N4 active centers, which markedly attenuates the binding strength toward *OOH intermediates.29 Moreover, electron- withdrawing O-functionalities on the carbon substrate synergis- tically modulate the electronic structure, steering the ORR pathway from the 4e− to the desired 2e− route. Compared with conventional Co-based counterparts, the Cr-N-C catalyst not only delivers comparable 2e− ORR performance in acidic media but also exhibits pronounced superiority in suppressing H2O2 decomposition and resisting metal leaching.

Beyond the widely studied electronic tailoring of active sites, mass transport is a pivotal factor that concurrently governs the apparent activity, selectivity, and durability of 2e− ORR catalysts.30−33 The sluggish outward diffusion of the product triggers undesired side reactions, which severely compromises the apparent H2O2 selectivity. More critically, the localized trapping of the produced H2O2 within obstructed pores induces oxidative degradation of the carbon-based catalysts under acidic conditions.34−37 Such deterioration in structural integrity fur- ther aggravates kinetic and transport polarizations, initiating a self-accelerating vicious cycle of performance decay.38,39 Rational design of catalyst mesoscopic geometry can theoretically boost reaction kinetics under practical operating condi- tions.17,18,40 However, dedicated research and structural engi- neering targeting mass transfer enhancement remain insufficient in current studies.41,42 This research bias leaves numerous outstanding catalysts, preliminarily screened by the rotating ring-disk electrode (RRDE), failing to deliver their promised per- formance in real device configurations. Metal−organic frame- works (MOFs) serve as excellent precursors for fabricating carbon-based single-atom catalysts, enabling uniform M-Nx sites coupled with inherent porosity. Three-dimensional open nano- flower morphologies exhibit distinctive merits, including a large specific surface area, short mass-transfer distance, and full active site exposure. Despite offering an effective platform to mitigate the above limitations, the precise construction of such well- defined architectures still faces substantial challenges.

In this study, a flower-like Cr−N−C single-atom catalyst (F- CrNC) is rationally designed using a MOF precursor, where precise morphological control is realized by simply adjusting the secondary ligand. The catalyst is constructed by the self- assembly of nanosheets into a three-dimensional (3D) open architecture. This unique structure renders active sites readily accessible and accelerates mass transport of O2 and H2O2. While supporting high intrinsic performance, such structural design appears to effectively limit secondary reactions of H2O2, potentially by minimizing its residence time on the cata- lyst surface. As a result, the as-prepared F-CrNC catalyst exhibits exceptional apparent activity and selectivity toward the 2e− ORR in acidic media. In H-cell and flow cell configura- tions, it delivers remarkable H2O2 production rates of 420 mmol gcat−1 h−1 (at 0.2 V vs RHE) and 7.45 mol gcat−1 h−1 (at 100 mA) both with Faradaic efficiencies (FE) exceeding 80%.