title: ELECTROCHEMICAL PROPERTIES OF POROUS METALS MANUFACTURED BY LOST CARBONATE SINTERING PROCESS
Creators: name: Zhu, P
Publisher: name: University of Liverpool Repository
Year: 2019

description: Porous metals have attracted considerable attention in academia and industry due to their unique combinations of material and structural characteristics, including high surface area, good catalytic ability, high permeability, light weight, and good mechanical properties. They have been considered as excellent materials for applications in electrochemical detection and energy generation. This project measured the surface area and mass transfer coefficient of the porous metals manufactured by the Lost Carbonate Sintering (LCS) process and further studied the effects of structural parameters (porosity and pore size), manufacturing process (sintering temperature and metal particle size), chemical etching treatments and diffusion layer thickness. Additionally, a novel limiting current sensor with LCS porous Ni working electrode was developed for determination of ferricyanide concentration. The surface area of porous metals is normally measured by either the BET gas absorption method or the mercury intrusion method. However, both methods have their own limitations. BET is not applicable for pores too big, while the mercury intrusion method is not suitable for pores too small. Besides, the BET method measures the surface area at an extremely small length scale and the mercury intrusion method assumes all pores are cylindrical. Neither is appropriate for the measurement of electrochemical surface area. In this thesis, the geometric, electroactive and real surface areas of the LCS porous Cu and Ni were measured using quantitative stereology, cyclic voltammetry (CV) peak current and double layer capacitance methods, respectively. The cyclic voltammetry peak current method exactly determines the effective surface area in electrochemical reactions and the double layer capacitance method accurately measures the surface area where electrical double layer is formed. For the first time, two reactions with different diffusion layer thicknesses were employed to study the effect of diffusion layer thickness on the electroactive surface area of the LCS porous Cu. The electroactive surface area was increased by up to 2 times when the diffusion layer thickness was decreased from 50 µm to 1 µm. The effects of Cu particle size, sintering temperature and chemical treatment on the surface morphology and therefore the electroactive and real surface areas were also investigated. Cu particle size had a modest effect, with the medium particle sizes, 20 – 45 µm and 45 – 75 µm, showing the highest surface areas. Increasing sintering temperature from 850˚C to 950˚C or etching the samples by 5 M nitric acid for 5 minutes reduced the electroactive and real surface areas by 31% – 61% and 9% – 25%, respectively. For the surface area of the LCS porous Ni, the volumetric specific geometric, electroactive and real surface areas of the porous Ni samples, with pore sizes in the range of 250 – 1500 µm and porosities in the range of 0.55 – 0.85, are in the ranges of 20 – 100, 30 – 100 and 200 – 1000 cm-1, respectively. Their gravimetric specific geometric, electroactive and real surface areas are in the ranges of 5 – 65, 9 – 70 and 100 – 300 cm2/g, respectively. The electroactive surface area increases with increasing scan rate. The matrix material does not affect the geometric surface area and the real surface area is slightly affected by the metal particles used, while electroactive surface area is mainly affected by the diffusion layer thickness. The mass transfer coefficient of porous Ni was measured by the limiting current technique. For porous Ni samples with a porosity of 0.55 – 0.75 and a pore size of 250 – 1500 µm, the mass transfer coefficient, measured at an electrolyte flow velocity range of 1 – 12 cm/s, is in the range of 0.0007 – 0.014 cm/s, which is up to 7 times higher than that of a solid nickel plate electrode. The mass transfer coefficient increases with pore size but decreases with porosity. The porous nickel has Sherwood numbers considerably higher than the other nickel electrodes reported in the literature, due to its high real surface area and its tortuous porous structure, which promotes turbulent flow. A novel limiting current sensor containing a pumping system and a three-electrode electrochemical cell was developed for the determination of ferricyanide concentration. A porous Ni sample with a porosity of 0.7 and a pore size range of 425 – 710 µm was used as the working electrode. The limiting current sensor showed a limit of detection (LOD) range of 5.35×〖10〗^(-6) – 8.7×〖10〗^(-6) M and a sensitivity range of 7.47 – 20.24 mM/mA. The sensitivity increases with increasing fluid flow rate. A conventional three-electrode electrochemical sensor, with the same LCS porous Ni working electrode, was also used for measuring the concentration of ferricyanide. The three-electrode electrochemical sensor showed a LOD range of 0.21×〖10〗^(-4) – 0.58×〖10〗^(-4) M and a sensitivity of 0.33 – 1.50 mM/mA. Both LOD and sensitivity increase with increasing scan rate. The limiting current sensor showed a much lower LOD and much higher sensitivity than the three-electrode electrochemical sensor due to the turbulent fluid flow through the porous matrix in the limiting current sensor.