Importance of Site Diversity and Connectivity in Electrochemical CO Reduction on Cu.

Electrochemical CO2 reduction on Cu is a promising approach to produce value-added chemicals using renewable feedstocks, yet various Cu preparations have led to differences in activity and selectivity toward single and multicarbon products. Here, we find, surprisingly, that the effective catalytic activity toward ethylene improves when there is a larger fraction of less active sites acting as reservoirs of *CO on the surface of Cu nanoparticle electrocatalysts. In an adaptation of chemical transient kinetics to electrocatalysis, we measure the dynamic response of a gas diffusion electrode (GDE) cell when the feed gas is abruptly switched between Ar (inert) and CO. When switching from Ar to CO, CO reduction (COR) begins promptly, but when switching from CO to Ar, COR can be maintained for several seconds (delay time) despite the absence of the CO reactant in the gas phase. A three-site microkinetic model captures the observed dynamic behavior and shows that Cu catalysts exhibiting delay times have a less active *CO reservoir that exhibits fast diffusion to active sites. The observed delay times and the estimated *CO reservoir sizes are affected by catalyst preparation, applied potential, and microenvironment (electrolyte cation identity, electrolyte pH, and CO partial pressure). Notably, we estimate that the *CO reservoir surface coverage can be as high as 88 ± 7% on oxide-derived Cu (OD-Cu) at high overpotentials (-1.52 V vs SHE) and this increases in reservoir coverage coincide with increased turnover frequencies to ethylene. We also estimate that *CO can travel substantial distances (up to 10s of nm) prior to desorption or reaction. It appears that active C-C coupling sites by themselves do not control selectivity to C2+ products in electrochemical COR; the supply of CO to those sites is also a crucial factor. More generally, the overall activity of Cu electrocatalysts cannot be approximated from linear combinations of individual site activities. Future designs must consider the diversity of the catalyst network and account for intersite transportation pathways.

Sunlight-Driven Self-Cleaning Ultrafine Particulate Matter Filter with Antibacterial Activity.

Air pollution by particulate matter (PM) and airborne pathogens causes severe health problems in the human body. Presently, popular disposable air filters yield huge waste and have a fatal impact on the environment. Postuse cleaning of air filters also leads to secondary air and water pollution. Here, we report a sunlight-driven self-cleaning PM filter by coupling a full-solar-spectrum-active photocatalyst comprising up-conversion nanoparticles (UCNPs) decorated with semiconductor iron(III) oxide (UCNP@α-Fe2O3) shells stabilized upon graphene functionalized borosilicate fibrous membrane (rGO-BF). While rGO-BF ensures high PM adsorption, UCNP@α-Fe2O3 (NP) enables self-photodegradation of adsorbed PM under abundant sunlight and subsequent membrane regeneration, while preventing secondary air or water pollution. Rational surface chemistry and optimal microstructure enable our filters to remove >99% of PM2.5 under deplorable air-quality conditions. Moreover, our filter shows excellent antibacterial activity toward E. coli and S. aureus, demonstrating its potential for practical utilization in face masks, air filtering devices, and protective medical wear. This work successfully suggests an intriguing design platform for self-sustainable zero-waste air filter membranes.

Organometal Halide Perovskite-Based Photoelectrochemical Module Systems for Scalable Unassisted Solar Water Splitting.

Despite achievements in the remarkable photoelectrochemical (PEC) performance of photoelectrodes based on organometal halide perovskites (OHPs), the scaling up of small-scale OHP-based PEC systems to large-scale systems remains a great challenge for their practical application in solar water splitting. Significant resistive losses and intrinsic defects are major obstacles to the scaling up of OHP-based PEC systems, leading to the PEC performance degradation of large-scale OHP photoelectrodes. Herein, a scalable design of the OHP-based PEC systems by modularization of the optimized OHP photoelectrodes exhibiting a high solar-to-hydrogen conversion efficiency of 10.4% is suggested. As a proof-of-concept, the OHP-based PEC module achieves an optimal PEC performance by avoiding major obstacles in the scaling up of the OHP photoelectrodes. The constructed OHP module is composed of a total of 16 OHP photoelectrodes, and a photocurrent of 11.52 mA is achieved under natural sunlight without external bias. The successful operation of unassisted solar water splitting using the OHP module without external bias can provide insights into the design of scalable OHP-based PEC systems for future practical application and commercialization.

Atomic-Scale Homogeneous RuCu Alloy Nanoparticles for Highly Efficient Electrocatalytic Nitrogen Reduction.

Ruthenium (Ru) is the most widely used metal as an electrocatalyst for nitrogen (N2 ) reduction reaction (NRR) because of the relatively high N2 adsorption strength for successive reaction. Recently, it has been well reported that the homogeneous Ru-based metal alloys such as RuRh, RuPt, and RuCo significantly enhance the selectivity and formation rate of ammonia (NH3 ). However, the metal combinations for NRR have been limited to several miscible combinations of metals with Ru, although various immiscible combinations have immense potential to show high NRR performance. In this study, an immiscible combination of Ru and copper (Cu) is first utilized, and homogeneous alloy nanoparticles (RuCu NPs) are fabricated by the carbothermal shock method. The RuCu homogeneous NP alloys on cellulose/carbon nanotube sponge exhibit the highest selectivity and NH3 formation rate of ≈31% and -73 µmol h-1 cm-2 , respectively. These are the highest values of the selectivity and NH3 formation rates among existing Ru-based alloy metal combinations.

Generation of high-density nanoparticles in the carbothermal shock method.

The carbothermal shock (CTS) method has attracted considerable attention in recent years because it enables the generation of finely controlled polyelemental alloy nanoparticles (NPs). However, fabricating high surface coverage of NPs with minimized exposure of the carbon substrate is essential for various electrochemical applications and has been a critical limitation in CTS method. Here, we developed a methodology for creating NPs with high surface coverage on a carbon substrate by maximizing defect sites of cellulose during CTS. Cu NPs with high surface coverage of ~85%, various single NPs and polyelemental alloy NPs were densely fabricated with high uniformity and dispersity. The synthesized Cu NPs on cellulose/carbon paper substrate were used in electrocatalytic CO2 reduction reaction showing selectivity to ethylene of ~49% and high stability for over 30 hours of reaction. Our cellulose-derived CTS method enables the greater availability of polyelemental NPs for a wide range of catalytic and electrochemical applications.

Ultrafast-Selective Nanofiltration of an Hybrid Membrane Comprising Laminated Reduced Graphene Oxide/Graphene Oxide Nanoribbons.

In this study, reduced graphene oxide (rGO) and graphene oxide nanoribbons (GONRs) are used to fabricate a composite membrane that exhibits ultrafast water permeance (312.8 L m-2 h-1 bar-1) and precise molecular separation (molecular weight cutoff: 269 Da), which surpass the upper bound of previously reported polymer and graphene-based nanofiltration membranes. As two-dimensional GONR exhibits a width on the scale of nanometers, its nanochannels can be enlarged without hindering the stacking of rGO. Moreover, abundant oxygen-containing groups on the edge and surface of GONR enhance the electrostatic interactions between the filtered molecules and the membrane nanochannel. By the synergistic effect, rejection and water flux are considerably increased. Owing to the chemically stable nature of rGO, the composite membrane is highly stable in aqueous media (from acidic to alkaline) and is recyclable during repeated filtration tests.