Electrochemical Properties of Porous Metals Manufactured by Lost Carbonate Sintering

摘要

Porous metals have recently attracted much attention in both academia and industry. They have many potential applications ranging from light weight structure, thermal management and sound absorption, because of their unique structural, mechanical, thermal and acoustic properties. Open-cell porous metal can be an ideal material for electrochemical applications due to its high surface area, permeability and electric conductivity. The electro-active surface area of porous electrodes is the most important factor in electrochemical applications, such as fuel cells, because it largely determines the current density in the electrode. However, most research to date on the surface area of porous metals has been focused on the geometric and real surface areas. Very little research has been conducted to study the electro-active surface area. The mass transfer coefficient of a solid electrode is a very important factor in flow cell applications, such as flow battery and wastewater treatment, because high mass transfer coefficient means high reaction performance. However, very little research has been carried out on the mass transfer coefficient of open-cell porous metal manufactured by the space-holder method. The main objective of this study is to investigate the electrochemically relevant structural properties of porous Cu and Ni fabricated by the Lost Carbonate Sintering (LCS) process, which is a cost-effective process for manufacturing open-cell porous metals with controlled porosity, pore size and pore shape. In this project, the geometric, electro-active and real surface areas of porous Cu and Ni have been measured by quantitative stereology, cyclic voltammetry (peak current) and cyclic voltammetry (double layer capacitance) methods, respectively. The mass transfer coefficient of porous Ni has been measured by linear sweep voltammetry, using a purpose-built flow cell. The tortuosity of porous Cu has been measured by a diffusion method, using a purpose-built diaphragm cell. The geometric, electro-active and real surface areas of porous metal are due to the contributions from the primary porosity, the primary and secondary porosities, and the surfaces of metal particles, respectively. The geometric, electro-active and real surface areas of porous Cu and Ni samples with pore sizes 75-1500 µm and porosities 0.53-0.81 were in the ranges of 18-110 cm-1, 24-369 cm-1 and 700-1200 cm-1, respectively. Both the geometric and electro-active surface area increased with porosity and decreased with pore size. The real surface area decreased with porosity but the effect of pore size was not pronounced. The values of electro-active surface area of LCS porous metal can be similar to and often greater than those of the existing porous metals, e.g. incofoam Ni. The mass transfer coefficient of porous Ni with pore sizes 250-1500 µm and porosities 0.63-0.81 at different flow rates from 0.24 ml/s to 2.8 ml/s was in the range of 0.0035-0.0727 cm/s. Both the limiting current and mass transfer coefficient increased with flow rate and had the maximum values at a porosity of around 0.65-0.70. The maximum limiting current decreased and the maximum mass transfer coefficient increased with pore size. Compared with a smooth Ni plate at the same flow velocity, the mass transfer coefficient of the LCS porous Ni was increased by up to 9 times. The enhancement is due to the Ni particles providing a rough surface for the cell walls and the tortuous pore structure resulting in a high level of fluid turbulence. The tortuosity of the porous Cu samples, with pore sizes 250-1500 µm and porosities 0.56-0.84, was in the range of 1.33-1.78. It increased with pore size and decreased with porosity, agreeing with an empirical formula for porous media. This research has successfully applied cyclic voltammetry to the measurement of electro-active surface area of porous metals for the first time. It is the first systematic study of the structural properties of relevance to electrochemical applications of porous metals manufactured by LCS process.