Delivering the Full Potential of Oxygen Evolving Electrocatalyst by Conditioning Electrolytes at Near-Neutral pH

A multimodal flow reactor for photocatalysis under atmospheric conditions

Photocatalysis is a promising concept for the direct conversion of solar energy into fuels and chemicals. The design, experimental protocol, and performance of a multimodal and versatile flow reactor for the characterization of powdered and immobilized photocatalysts are herein presented. Ultimately, this instrument enables rigorous evaluation of photocatalysis performance metrics. The apparatus quantifies transient gas-phase reaction products via online real-time gas analyzer mass spectrometry (RTGA-MS). For H2, the most challenging gas, the photocatalytic system's RTGA-MS gas detection sensitivity spans over three orders of magnitude and can detect down to tens of parts per million under atmospheric conditions. Using Pt nanoparticles supported on anatase TiO2 photocatalyst via wet impregnation, the instrument's capability for the characterization of photocatalytic H2 evolution is demonstrated, resulting in an apparent quantum yield (AQY) of 48.1% ± 0.9% at 320 nm, 45.7% ± 0.3% at 340 nm and 31% ± 1% at 360 nm. The photodeposition of Pt on anatase TiO2 was employed to demonstrate the instrument's capability to track the transient behavior of photocatalysts, resulting in an improved 55% ± 2% AQY for H2 evolution at 340 nm from aqueous methanol. This photocatalytic instrument enables systematic study of a wide variety of photocatalytic reactions such as water splitting and CO2 reduction to valuable C2+ fuels and chemicals.

Quantitative Description of Bubble Formation in Response to Electrolyte Engineering

The green hydrogen economy is expected to play a crucial role in carbon neutrality, but industrial-scale water electrolysis requires improvements in efficiency, operation costs, and capital costs before broad deployment. Electrolysis operates at a high current density and involves the substantial formation of gaseous products from the electrode surfaces to the electrolyte, which may lead to additional resistance and a resulting loss of efficiency. A detailed clarification of the bubble departure phenomena against the electrode surface and the surrounding electrolytes is needed to further control bubbles in a water electrolyzer. This study clarifies how electrolyte properties affect the measured bubble detachment sizes from the comparisons with analytical expressions and dynamic simulations. Bubble behavior in various electrolyte solutions and operating conditions was described using various physical parameters. A quantitative relationship was then established to connect electrolyte properties and bubble departure diameters, which can help regulate the bubble management through electrolyte engineering.

High Current Density Oxygen Evolution in Carbonate Buffered Solution Achieved by Active Site Densification and Electrolyte Engineering

High current density reaching 1 A cm^-2 for efficient oxygen evolution reaction (OER) was demonstrated by interactively optimizing electrolyte and electrode at non-extreme pH levels. Careful electrolyte assessment revealed that the state-of-the-art nickel-iron oxide electrocatalyst in alkaline solution maintained its high OER performance with a small Tafel slope in K-carbonate solution at pH 10.5 at 353 K. The OER performance was improved when Cu or Au was introduced into the FeO_x -modified nanostructured Ni electrode as the third element during the preparation of electrode by electrodeposition. The resultant OER achieved 1 A cm^-2 at 1.53 V vs. reversible hydrogen electrode (RHE) stably for 90 h, comparable to those in extreme alkaline conditions. Constant Tafel slopes, apparent activation energy, and the same signatures from operando X-ray absorption spectroscopy among these samples suggested that this improvement seems solely correlated with enhanced electrochemical surface area caused by adding the third element.

Oxygen Evolution Activity of Amorphous Cobalt Oxyhydroxides: Interconnecting Precatalyst Reconstruction, Long-Range Order, Buffer-Binding, Morphology, Mass Transport, and Operation Temperature

Nanocrystalline or amorphous cobalt oxyhydroxides (CoCat) are promising electrocatalysts for the oxygen evolution reaction (OER). While having the same short-range order, CoCat phases possess different electrocatalytic properties. This phenomenon is not conclusively understood, as multiple interdependent parameters affect the OER activity simultaneously. Herein, a layered cobalt borophosphate precatalyst, Co(H₂ O)₂ [B₂ P₂ O₈ (OH)₂ ]·H₂ O, is fully reconstructed into two different CoCat phases. In contrast to previous reports, this reconstruction is not initiated at the surface but at the electrode substrate to catalyst interface. Ex situ and in situ investigations of the two borophosphate derived CoCats, as well as the prominent CoP_i and CoB_i identify differences in the Tafel slope/range, buffer binding and content, long-range order, number of accessible edge sites, redox activity, and morphology. Considering and interconnecting these aspects together with proton mass-transport limitations, a comprehensive picture is provided explaining the different OER activities. The most decisive factors are the buffers used for reconstruction, the number of edge sites that are not inhibited by irreversibly bonded buffers, and the morphology. With this acquired knowledge, an optimized OER system is realized operating in near-neutral potassium borate medium at 1.62 ± 0.03 V_RHE yielding 250 mA cm^-2 at 65 °C for 1 month without degrading performance.

Electrolyte Engineering for Oxygen Evolution Reaction Over Non-Noble Metal Electrodes Achieving High Current Density in the Presence of Chloride Ion

Direct seawater electrolysis potentially simplifies the electrolysis process and leads to a decrease in the cost of green hydrogen production. However, impurities present in the seawater, especially chloride ions (Cl^- ), cause corrosion of the electrode material, and its oxidation competes with the anodic oxygen evolution reaction (OER). By carefully tuning electrode substrate and electrolyte solutions, the CoFeO_x H_y /Ti electrode with high double-layer capacitance actively and stably electro-catalyzed the OER in potassium borate solutions at pH 9.2 in the presence of 0.5 mol kg^-1 Cl^- . The electrode possesses an active site motif composed of either a Co- or Fe-domain and benefits from an enlarged surface area. Selective OER was demonstrated in Cl^- -containing electrolyte solutions at an elevated reaction temperature, stably achieving 500 mA cm^-2 at a mere potential of 1.67 V vs. reversible hydrogen electrode (RHE) at 353 K for multiple on-off and long-term testing processes with a faradaic efficiency of unity toward the OER.

Selective Electrooxidation of Biomass-Derived Alcohols to Aldehydes in a Neutral Medium: Promoted Water Dissociation over a Nickel-Oxide-Supported Ruthenium Single-Atom Catalyst

The biomass-derived alcohol oxidation reaction (BDAOR) holds great promise for sustainable production of chemicals. However, selective electrooxidation of alcohols to value-added aldehyde compounds is still challenging. Herein, we report the electrocatalytic BDAORs to selectively produce aldehydes using single-atom ruthenium on nickel oxide (Ru₁ -NiO) as a catalyst in the neutral medium. For electrooxidation of 5-hydroxymethylfurfural (HMF), Ru₁ -NiO exhibits a low potential of 1.283 V at 10 mA cm^-2 , and an optimal 2,5-diformylfuran (DFF) selectivity of 90 %. Experimental studies reveal that the neutral electrolyte plays a critical role in achieving a high aldehyde selectivity, and the single-atom Ru boosts HMF oxidation in the neutral medium by promoting water dissociation to afford OH*. Furthermore, Ru₁ -NiO can be extended to selective electrooxidation of a series of biomass-derived alcohols to corresponding aldehydes, which are conventionally difficult to obtain in the alkaline medium.