This issue features articles on solid state electrochemistry, a subject dear to my heart. But before discussing the title subject, may I reminisce back to the start of the field? To my knowledge, the basis for understanding and interpreting the voltage generated by a solid state Galvanic cell arose from the early German electrochemists. By 1894, the Deutsche Elektrochemische Gesellschaft (later to become the Deutsche Bunsen_Gesellschaft) was formed, holding meetings, and publishing the Zeitschrift für Elektrochemie.
The earliest solid state electrochemistry paper that I recall was a discussion in the early 1900s by Walthur Nernst (the father of modern electrochemistry and 1920 Nobel Prize recipient) about the possibility of creating an in situ fuel cell by closing the opening to a coal mine with an oxygen_conducting solid electrolyte, providing some unspecified electrodes to each interface, and permitting a controlled fire inside to generate an internal reducing gas. Amazingly, he remarked that the in situ burning of coal in this manner, with the on_site conversion to electrical power, could replace the need to dig out and truck away the coal. In those days, German coal miners had battery_powered ‘Nernst glower’ helmet lights to illuminate their work. This ‘glower’ comprised a piece of stabilized_zirconia outfitted with electrodes that illuminated at high temperature. These two examples were realized before any mechanisms for solid state ionic conductivity were understood.
But in 1926, Frenkel introduced the interstitial point defect, and in 1930 Walter Schottky and Carl Wagner [Z. Phys. Chem. (1930) B11, 163] introduced vacancies (for ordered compounds). In 1943, Wagner correctly explained the conduction in Nernst’s device in terms of oxygen ion migration over vacant sites. In his classic paper on ‘parabolic scaling’ [Z. Phys. Chem. (1933) B21, 25], Wagner had already provided the thermodynamic basis for interpreting the emf of an open_circuit solid state Galvanic cell. Later, Koch and Wagner [Z. Phys. Chem. (1943) B38, 295] demonstrated quantitatively for the first time ‘the doping effect’, i.e. the creation of point and electronic defects in crystalline compounds to compensate for the substitution of aliovalent impurities (or solutes) into a lattice. The use of such solutes today is the common means to augment the ionic conductivity and to regulate the usually unwanted electronic conductivity contributions. Maybe we should call Wagner the ‘father of solid state electrochemistry’. As more recent authors sometimes claim excessive importance for their own work, we might remember that if you see a frog at the top of a flagpole, he didn’t get there without some help.
The early 1980s saw the rather surprising development of high temperature proton_conducting oxides. Actually, Wagner (with Stotz in 1966) had already speculated about charged hydrogen and water solutes in oxides, without appreciating the versatility of the perovskite lattice or foreseeing protonic conductors. Indeed, proton_conducting oxides sound like an oxymoron, for why would one expect such a reducing component to substitute into an oxide lattice? The dissolved proton is too small to occupy an interstitial lattice site, so it is complexed to form a lattice hydroxide ion. As the subject has advanced, largely through the efforts of Hiroyasu Iwahara’s research group at Nagoya University, it has emerged that the concentrations of solute protons can be significant and are thermodynamically favored in highly doped perovskite lattices, e.g. SrZr<_z7>1_xY<_z7>xO<_z7>3, BaCe<_z7>1_xGd<_z7>xO<_z7>3, SrCe<_z7>1_xYb<_z7>xO<_z7>3, with high oxygen vacancy concentrations. The competitive mixed conduction by protons, oxide ions, and positive holes, depending upon the electrolyte composition and the ambient conditions, make these studies interesting and complicated. Under some conditions, certain proton_doped oxides provide the dominant hydrogen ion conduction needed for sensors and fuel cells. For other applications requiring mixed ionic and electronic conduction, e.g. fuel conditioning, suitable electrolyte compositions have already been identified. As the engineering world continues to progress toward a hydrogen economy, proton conducting oxides should play an important role. But first, as was the case for stabilized zirconia around 1960, the fabricators of commercial ceramics will need to demonstrate that dense shock_resistant hardware can be produced in large sizes at low cost.