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Solid oxide fuel cells (SOFCs) are currently the focus of intense investigation given their high chemical‐to‐electrical energy conversion efficiency and low carbon footprint. In this review, the development of thin film SOFCs, sometimes described as ‘micro‐SOFCs', is highlighted and analysed. Opportunities for reduced temperature operation and portable power generation arise from the decreased thickness of the solid electrolyte, as well at the metastable phases and nanoscale‐dependent effects that are a consequence of the reduced temperature of fabrication. Challenges such as enhanced cation diffusion along grain boundaries are; however, also observed, potentially impacting the long‐term stability of these devices. Recent progress achieved in understanding these and other challenges are reviewed and directions for future work identified.

The desire for ever smarter systems-on-a-chip and plug-free portable electronics with longer operating times between recharge has stimulated growing interest in micro-ionic systems. The use of thin film and photolithographic processing techniques, commonly at temperatures considerably below those utilized in conventional ceramics processing methods, leads to ionic or mixed ionic-electronic materials with nanosized dimensions. The implications for nanosized grains on the conductivity of thin film solid oxide electrolytes are examined. Grain boundary engineering, as a means of controlling and ultimately enhancing transport along and across grain boundaries, becomes essential given that such boundaries often dominate the transport properties of such nano-dimensioned materials. Heterogeneous doping by selective in-diffusion along grain boundaries was introduced as a potentially powerful means of achieving this. This is coupled with the modeling of space charge distributions at such boundaries, taking into account possible dopant segregation to the boundaries. The use of lithographic methods for generating geometrically well defined structures is used to illustrate how one can achieve a much improved understanding of electrode processes in SOFC structures. Indeed, the more idealized structures achievable by application of microelectronic processing provide a marvelous opportunity to uncover the science underlying the technology of micro- and ultimately macro-ionics.