This study explores the development of an effective cellulose acetate (CA)-based material for use as an anion exchange membrane (AEM) in fuel cell applications. The combination of CA and poly(epichlorohydrin) (PECH) cross-linked with 1,4-diazabicyclo[2.2.2]octane (DABCO) was investigated with factors varied, namely, CA/PECH blend ratios, DABCO cross-linking mole ratio (R D/P), and trimethylamine functionalization time. Increasing the CA content improved the tensile strength (TS), while a higher PECH content raised the ion exchange capacity, water uptake (WU), average degree of swelling (DSAve), and strain rate. Setting R D/P = 0.5 enhanced TS, elongation at break, WU, and DSAve, exhibited by improved stress-strain values and reduced hydrophysical properties in the 50:50 blend, which are attributed to successful cross-linking. The measured ionic conductivity was 32.7 mS cm-1 at 30 degrees C and peaked at 60 degrees C with a value of 51.1 mS cm-1. Thermogravimetric analysis revealed that the AEM remained thermally stable under typical operating conditions. An accelerated stability test of the AEM in 1 M KOH/D2O at 60 degrees C showed insignificant degradation, indicating superb chemical stability. Lastly, the fuel cell test at 30 degrees C measured a peak power density of 13.3 mW cm-2 at a current density of 24.7 mA cm-2. A high open circuit potential value of 1.02 V was recorded, suggesting that the voltage loss due to gas crossover is negligible. These findings indicate that the fabricated CA/PECH membranes have promising potential applications in AEM fuel cells.
Developing effective strategies to suppress the hydrogen evolution reaction (HER) under high potentials is pivotal for industrializing the electrocatalytic CO2 reduction reaction (CO2RR). However, progress remains constrained by an insufficient mechanistic understanding of the potential-dependent competition between CO2RR and HER. Motivated by previous experimental endeavors, we perform a systematic theoretical probing into the mechanism of HER and CO2RR under varying potentials over Cu as the prototype and find that potential-modulated interfacial K+ concentrations govern the HER/CO2RR selectivity transition. Specifically, the distinct K+ distribution state induced by different potentials can impose a distinguished effect on the free energy barriers of the rate-determining step (RDS) of HER and CO2RR, making HER or CO2RR the dominant reaction under different situations. The proposed innovative insights into reaction mechanisms can well unveil the experimentally observed potential-dependent HER/CO2RR antagonism and inspire us to propose a feasible strategy to inhibit HER over Cu-based catalysts, i.e., maintaining a CO2RR-favored cation concentration near the reaction interface under high potentials.
The carbonyl ligands that stabilize the Mn(I) center in the Mn(CO)5Br metal precursor are usually retained upon complexation with the tridentate ligand; however, their presence limits the hydrogenation performance of the Mn complex. Herein, we report the highly selective semihydrogenation of urea derivatives or carbamates, two of the most challenging carbonyl compounds, and polyurethanes to more active formamides using a catalyst system containing earth-abundant metal Mn under mild reaction conditions, which has been previously achieved using precious metal Ru or Ir catalysts. This catalytic activity stems from the fact that the Mn complex bears a noninnocent ligand, which facilitates the simultaneous transfer of both hydrogen atoms from a dihydrogen molecule, thereby avoiding the energetically demanding dihydrogen coordination step observed in systems with exclusively innocent ligands. Additionally, DFT calculations provide insights into the reason for the selective hydrogenation of urea or carbamates to more reactive formamides.
Molybdenum carbide (MoC) has attracted significant attention as a cocatalyst for photocatalytic hydrogen evolution. However, facile and scalable methods to synthesize hollow MoC materials with high specific surface areas remain limited. Herein, hollow-structured MoC/N-doped carbon (MCN) cocatalysts were innovatively synthesized via high-temperature calcination of MoO3-containing polydopamine (MoO3/PDA) precursors. These MCN materials were subsequently coupled with CdS through a solvothermal process to construct composite photocatalysts (MCN/CdS, denoted as MCS). Under simulated sunlight irradiation (AM 1.5G), the optimized MCS catalyst achieved an exceptional photocatalytic hydrogen evolution rate of 26.4 mmolg-1h-1, which is 10.8 times higher than that of the benchmark Pt/CdS catalyst. The MCS catalyst also demonstrated outstanding stability, maintaining efficient hydrogen production after 20 h of continuous illumination. The superior performance of MCS stems from two key advantages of the MCN cocatalyst: (i) its high specific surface area enhances CdS dispersion and provides abundant active sites for hydrogen evolution; (ii) the N-doped carbon matrix broadens light absorption and facilitates charge carrier transport. This work introduces an effective approach for the rapid and mild synthesis of ultrahigh surface area hollow nanospherical transition-metal carbons, providing valuable insights for advanced photocatalytic systems.
Partially premixed combustion has proven effective in enhancing flame stabilization and reducing emissions in hydrocarbon systems. However, the widespread adoption of ammonia (NH3) as a carbon-free fuel remains limited due to weak flame stabilization and high NO emissions. Therefore, this study investigates flame stability, NO emissions, flame appearance, and thermal structure in NH3/CH4/air flames under varying degrees of mixture inhomogeneity. Flames were stabilized using a conical nozzle fitted at the outlet of two concentric tubes: an NH3/air mixture was introduced through the inner tube, while a CH4/air mixture was flowed through the outer annular passage. The equivalence ratios and velocities of the inner (Phi(in), Vm(i)) and outer (Phi(out)= 0.9, 1.1, 1.3; Vm(o)) streams were systematically varied. The mixture inhomogeneity of NH3/air and CH4/air streams was controlled by adjusting the axial recession distance (L) between the two tube exits, which was expressed as L/D (0-10), where D is the outer tube's inner diameter. Flame stability was found to depend strongly on the interaction between L/D, Phi(out), and NH3 content. At lean Phi(out) = 0.9, increasing L/D (i.e., enhancing premixing) reduced flame stability, while at Phi(out) = 1.3, optimum stability occurred at L/D approximate to 5, with stability deteriorating at both lower and higher values. Partially premixed NH3/CH4 flames exhibited improved NO emission performance and combustion efficiency relative to fully premixed NH3/CH4/air and pure NH3/air flames. Co-firing NH3 in the inner stream with CH4 in the outer stream enabled the use of richer NH3 mixtures while limiting NH3 slip. Although NO emissions generally increased with L/D, enrichment of Phi(out) or reduction of the velocity ratio of the inner to the outer stream (V-R) mitigated this effect. Under lean Phi(out), increasing L/D or V R enhanced the mixing degree, shifting the overall mixture toward leaner local conditions and promoting NO formation via enhanced radical activity. Conversely, under rich Phi(out), these changes reduced flame temperature relative to those seen at lean Phi(out) and suppressed NO pathways by limiting radical production. These findings demonstrate that partially premixed NH3/CH4 combustion-when optimized through control of L/D, Phi(in), Phi(out), and V-R-can achieve stable, efficient, and ultralow NO operation, supporting its viability in future hydrogen-based energy systems.
