Effect of Dealloying Time and Post-Annealing on the Surface Morphology and Electrocatalytic Behavior of Nanoporous Copper Films for CO2 Reduction Reaction

Electrocatalytic reduction reaction of CO2 (CO2RR) is one of the promising routes to mitigate global warming via transforming greenhouse gas into valuable chemical feedstocks. By adding proper electrocatalysts, such as nanoporous copper (NPC) with an average ligament size of 37 ± 6 nm, hydrocarbons could be produced at a relatively low overpotential. As the dealloying time increased to 156 hrs, the NPC was transformed into CuO nanosheet structure, which yielded larger electrochemical surface area (ECSA) and current density than the as-prepared NPC films. However, the Faraday efficiency (FE) of the major conversion product, formic acid (HCOOH), decreased from 29 to 8% when the nanosheet structure was used as electrocatalyst. On the other hand, the surface morphology of the NPC films remained similar while the average ligament size increased from 37 to 63 nm after a post-annealing treatment at 500 °C for 4 hrs. Both the current density and ECSA of this post-annealed NPC film were nearly 3 times higher than those of as-prepared NPC film, and the FE toward HCOOH increased from 29 to 45%. X-ray photoelectron spectroscopy and Raman spectroscopy revealed that Cu2O were present on the nanoporous structure, which enhanced the selectivity and FE toward HCOOH in CO2RR.

NUMERICAL STUDY OF SCALE AND SHAPE PARAMETERS OF DROPLET-SIZE DISTRIBUTION IN AIR-ASSISTED ATOMIZER

The article is devoted to the development of approach to study the atomization in an air-assisted atomizer by the volume- of-fluid method. Received through the simulations in the axisymmetric swirl approximation, the ligament-size distributions were approximated by the translated Weibull distribution, which is determined by the scale, shape and location parameters. The dependences of the latter two parameters on the surface tension, viscosity, density and flow rate of the atomized liquid are a subject of this research. Since the problem hasn’t been well studied before, there is no convenient approach to determine the location parameter and as a result, we use two ways to find it. The first one is by changing the parameters of the translated Weibull distribution with aim to fit the ligament-size distribution as best as possible. The second one is by equating the location parameter to the minimal ligament size where the ligament-size distribution is zero. The choice of the approach not only changes values of the parameters, but also leads to appearance or disappearance of the dependence of the location parameter on the properties of the atomized liquid. Nevertheless, if the very weak decrease of the shape parameter on the surface tension is neglected, this parameter of the Weibull distribution doesn’t depend on the parameters of the liquid. Moreover, this conclusion is the same for both approach to determine the location parameter.

Interaction of void spacing and material size effect on inter-void flow localisation

The ductile fracture process in porous metals due to growth and coalescence of micron scale voids is not only affected by the imposed stress state but also by the distribution of the voids and the material size effect. The objective of this work is to understand the interaction of the inter-void spacing (or ligaments) and the resultant gradient induced material size effect on void coalescence for a range of imposed stress states. To this end, three dimensional finite element calculations of unit cell models with a discrete void embedded in a strain gradient enhanced material matrix are performed. The calculations are carried out for a range of initial inter-void ligament sizes and imposed stress states characterised by fixed values of the stress triaxiality and the Lode parameter. Our results show that in the absence of strain gradient effects on the material response, decreasing the inter-void ligament size results in an increase in the propensity for void coalescence. However, in a strain gradient enhanced material matrix, the strain gradients harden the material in the inter-void ligament and decrease the effect of inter-void ligament size on the propensity for void coalescence.

Annealing-Induced Changes in the Microstructure and Mechanical Response of a Cu Nanofoam Processed by Dealloying

Cu nanoporous foams are promising candidates for use as an anode material for advanced lithium ion batteries. In this study, Cu nanofoam was processed from pack-cemented bulk material via dealloying. In the as-processed Cu nanofoam, the average ligament size was ~105 nm. The hardness in this initial state was ~2 MPa, and numerous cracks were observed in the indentation pattern obtained after hardness testing, thus indicating the low mechanical strength of the material. Annealing for 6 h under an Ar atmosphere at 400 °C was shown to result in crystalline coarsening and a reduction in the probability of twin faulting in the ligaments. Simultaneously, the junctions of the ligaments became stronger and hence more difficult to crack. This study demonstrates that moderate heat treatment under Ar can improve the resistance against crack propagation in Cu nanofoam without a large change in the ligament size and the surface oxide content, which can thus influence the electrochemical performance of the material in battery applications.

Refinement of Nanoporous Copper with Poly(vinyl alcohol) During Dealloying Amorphous Mg65Cu25Y10 Precursors

Ultrafine nanoporous copper (UNP Cu) with a characteristic pore size of about 12 nm and a ligament size of about 14 nm was fabricated from amorphous Mg65Cu25Y10 precursor alloys after dealloying in a 0.1 M H2SO4 solution modified by poly(vinyly alcohol) polymers with a molecular weight of 105000 g/mol (PVA-124). The suppression of the surface diffusion from PVA-124 reduced the size of the nanopores and ligaments to 20 nm when the concentration of the added PVA-124 exceeded 0.1 g L−1. When the concentration of the added PVA-124 exceeded 2 g L−1, PVA-124 triggered the polymerization process. The resultant polymer surface layer on the fcc Cu ligaments was shown to reduce the rate of selective dissolution. It was also shown that extending the immersion time resulted in a suppression of coarsening. The introduction of PVA-124 polymer into acids resulted in a higher viscosity of the dealloying solutions, particularly when the concentration of PVA-124 was higher than 1.0 g L−1. This viscosity was shown not only to reduced rate of diffusion of Cu adatoms in PVA-124 solutions, but also forced the accumulation of Cu adatoms to form small scale UNP Cu.