Electrochemical energy conversion devices include state of the art energy storage systems such as lithium-ion batteries, electrolysers such as those used in the chlor-alkali industry, and fuel cells which could offer a clean alternative to the internal combustion engine for transport. The environment in electrochemical energy conversion devices is typically highly corrosive. Strongly oxidising species are normally present in the electrolytes and high positive potentials can occur at the electrodes. Corrosion of materials in electrochemical devices often leads to a rapid deterioration in performance, severely limiting the lifetime. At the same time, electrode materials are required to provide high conductivity, low contact resistance and in some cases electrocatalytic activity. In addition, material costs can make a significant difference to the commercial viability of these devices. This is particularly true for fuel cell systems, where material costs will need to be significantly reduced compared to the current state of the art if this technology is to make a significant impact. Coated materials may provide a practical solution, where a low cost substrate is coated with thin layer of a material which offers the required surface properties.

In this paper, the nature of electrochemical energy conversion devices will be reviewed, and the desired properties of materials, particularly the plate materials, will be identified. Methods for characterising material performance and performance targets will be considered in this context. The types of coatings that have been studied will be discussed and there performance will be evaluated. Coatings used in this context include conducting polymers, amorphous and diamond-like carbon, as well as ultra-thin noble metal coatings. Finally, the future directions and opportunities in this field will be considered.

NiO_x due to its high band gap (3.8 eV) and p-type nature has considerable potential for use as an electrode in a tandem dye-sensitized solar cells (DSSc)¹. However to-date the efficiency of sol gel deposited NiO_x semiconductors in DSSc has been found to be relatively poor², which limits the effectiveness of the tandem cells. One of the possible ways to improve this efficiency is to deposit the coating by the reactive sputtering method. This is because the uniformity, reproducibility, and mechanical durability of the sputtered deposited film is much higher compared to sol-gel or other wet chemical deposition techniques. In this study, NiO_x films were deposited using the unbalanced magnetron sputtering technique. The coating was deposited onto both silicon wafer and indium tin oxide glass (ITO) substrates. The influence of deposition parameters such as pressure, nickel target current and substrate bias voltage were correlated with coating properties, including surface roughness, thickness, crystallographic structure and surface energy. These evaluations were carried out using optical profilometry, spectroscopic ellipsometry, XRD and water contact angle measurements. The NiO_x coatings were sensitized with Ru-complex dye containing appropriate anchoring moieties (carboxylic group). The dye adsorption was investigated in transmission mode on the ITO using UV-Vis spectroscopy in the range 200 – 600 nm. Dye adsorption was enhanced on nickel oxide surfaces exhibiting higher surface energy values. For example for 70 nm thick NiO_x coatings, it was found that increasing surface roughness (Ra) from 4.0 to 4.4 nm, with corresponding increase in surface energy from 37.1 to 57.6 mN/mm, there was a 12 % increase in the level of light absorption.

¹E. L. Miller, R. E. Rocheleau, Journal of Electrochemical Society 144 (1997) 1995.

²J. He, H. Lindström, A. Hagfeldt, S. Lindquist, Journal of Physical Chemistry B 103 (1999) 8940.