Ageing of nickel used as sensitive material for early detection of sudomotor dysfunction

The surface ageing of nickel electrodes was studied in the frame of the development of non-invasive biomedical devices, dedicated to the detection of sudomotor dysfunction manifested by an alteration of the ionic balance in human sweat. In this kind of technology, low voltage potentials with variable amplitudes are applied to nickel electrodes, placed on skin regions with a high density of sweat glands, and the electrical responses are measured. The trick is that nickel electrodes play alternately the role of anode and cathode, thus the analysis of the temporal evolution of the physico-chemical properties of nickel is of prime importance to ensure the good performance of the device. Electrochemical measurements coupled to surface chemical characterizations (X-ray photoelectron spectroscopy (XPS), Time of Flight-Secondary Ion Mass Spectrometry (ToF-SIMS)) were performed on pure Ni samples, immersed in buffered chloride solutions mimicking human sweat. The shapes of voltammograms, recorded in a restricted anodic potential range, show that the nickel surface was gradually passivated as a function of the number of scans. This was confirmed by XPS data, with the formation of a 1 nm thick duplex layer composed by nickel hydroxide (outermost layer) and nickel oxide (inner layer). In a negative extended potential range, though the electrochemical behavior of electrodes was not modified upon cycling the potential, XPS data show that the inner layer was thickening, indicating a surface degradation of the nickel electrode. Below pitting potentials, adsorbed chloride was only hardly detected by XPS, and the surface composition of the nickel samples was similar after treatments in chloride or chloride-free buffered solutions. In a larger potential range enabling to reach the breakdown potential, the highly chemically sensitive ToF-SIMS characterization pointed out that the surface concentration of adsorbed chloride was higher in pits than elsewhere on the surface sample.