Operando Elucidation of Electrocatalytic and Redox Mechanisms on a 2D Metal Organic Framework Catalyst for Efficient Electrosynthesis of Hydrogen Peroxide in Neutral Media

Abstract

The practical electrosynthesis of hydrogen peroxide (H 2 O 2 ) is hindered by the lack of inexpensive and efficient catalysts for the two-electron oxygen reduction reaction (2e − ORR) in neutral electrolytes. Here, we show that Ni 3 HAB 2 (HAB = hexaaminobenzene), a two-dimensional metal organic framework (MOF), is a selective and active 2e − ORR catalyst in buffered neutral electrolytes with a linker-based redox feature that dynamically affects the ORR behaviors. Rotating ring-disk electrode measurements reveal that Ni 3 HAB 2 has high selectivity for 2e − ORR (>80% at 0.6 V vs RHE) but lower Faradaic efficiency due to this linker redox process. Operando X-ray absorption spectroscopy measurements reveal that under argon gas the charging of the organic linkers causes a dynamic Ni oxidation state, but in O 2 -saturated conditions, the electronic and physical structures of Ni 3 HAB 2 change little and oxygen-containing species strongly adsorb at potentials more cathodic than the reduction potential of the organic linker (E redox ∼ 0.3 V vs RHE). We hypothesize that a primary 2e − ORR mechanism occurs directly on the organic linkers (rather than the Ni) when E > E redox , but when E < E redox , H 2 O 2 production can also occur through Ni-mediated linker discharge. By operating the bulk electrosynthesis at a low overpotential (0.4 V vs RHE), up to 662 ppm of H 2 O 2 can be produced in a buffered neutral solution in an H-cell due to minimized strong adsorption of oxygenates. This work demonstrates the potential of conductive MOF catalysts for 2e − ORR and the importance of understanding catalytic active sites under electrochemical operation.

Metal-Compound-Based Electrocatalysts for Hydrogen Peroxide Electrosynthesis and the Electro-Fenton Process

“…An in situ ICP-MS technique using a stationary probe near a rotating disk electrode (SPRDE-ICPMS) has been implemented for real-time elucidation of the potential-dependent dissolutions of OER and 4e – ORR catalysts. In addition, electrochemical quartz crystal microbalance (EQCM) can also probe the dissolutions of electrocatalysts in real time by tracking their mass changes as a function of potential . These techniques will provide in-depth understanding and more guidance for developing more stable metal-compound-based 2e – ORR catalysts in the future.…”

“…Additionally, electrode preparation methods should be carefully described, as the catalyst microstructure can affect the diffusion of O 2 and H 2 O 2 into and out of the electrode, , which can further impact the ability for H 2 O 2 to accumulate and the Faradaic efficiency. Some factors to consider include the catalyst ink compositions (solvents, ionomers, catalyst concentrations), , the properties of the electrode substrates (hydrophobicity, porosity), , and the methods of depositing the catalyst onto the electrodes (drop-casting, spray-coating, − or direct growth − ). Because many factors at the device, electrode, and catalyst levels can impact the overall electrosynthesis performance, comparing the apparent H 2 O 2 electrosynthesis performances under different cell conditions can further obfuscate atomic-level insights into the structural design of metal-compound-based 2e – ORR catalysts.…”

“…In addition, the recently demonstrated computational approach of combining the bulk Pourbaix stability from the Materials Project and the oxygen adsorbate energetics (or surface Pourbaix diagrams in general) from the Catalysis Hub for OER catalyst discovery could also be used for screening metal compounds as 2e – ORR catalysts. To better understand the catalytic mechanisms and stability, in situ or operando techniques, such as X-ray absorption spectroscopy (XAS) and Raman spectroscopy, can be employed for probing the structural and electronic evolutions of the working catalysts. , Moreover, in situ or operando attenuated total reflectance infrared spectroscopy (ATR-IR) and ambient-pressure XPS (APXPS) can provide information about the catalyst/​electrolyte interface by capturing key ORR adsorbates, which can further complement the computational modeling and achieve atomic-level mechanistic insights into catalyst design. The use of flow cells that build up practical concentrations of H 2 O 2 (coupled with in-line detection of H 2 O 2 and/or catalyst leaching) while measuring XAS at metal K-edges using hard X-rays or at metal L-edges (or K-edges/L-edges of the catalytic inert sites) using soft X-rays could further inform catalyst stability as well as active surface speciation in the working environment.…”

“…Over the past decade, several classes of selective 2e – ORR catalysts, including noble metal alloys, − carbon nanomaterials, − single-atom catalysts, − and metal compounds, − have been studied for H 2 O 2 electrosynthesis under different pH conditions. , Among these reports, alkaline 2e – ORR catalysts have been most extensively studied, despite several limitations in alkaline H 2 O 2 electrosynthesis, including the instability of H 2 O 2 in alkaline solution and the less competitive anion-exchange membrane (AEM) technology . Moreover, carbon nanomaterials already perform quite well under alkaline conditions. , In contrast, the less studied acidic and neutral conditions are attractive for several reasons besides the chemical stability of H 2 O 2 .…”

“…On-site water disinfection and environmental remediation can also benefit from acidic H 2 O 2 electrosynthesis because the electro-Fenton process operates at the optimum pH of ∼3 to convert the produced H 2 O 2 into the more oxidizing hydroxyl radical ( • OH) for the removal of persistent bacteria and organic pollutants . For direct applications, the noncorrosive neutral solutions can avoid the need for neutralization. ,,, However, high-performance yet cost-effective 2e – ORR catalysts in acidic and neutral solutions are still being developed.…”