The perovskite-type materials, <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>Ln</mml:mi><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>NiO</mml:mi><mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo><mml:mi>δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>Ln2NiO4+δ , were prepared by a modified Pechini method and their electrochemical properties investigated using ac impedance and dc polarization methods from 300 to <mml:math display="inline"><mml:mrow><mml:mn>800</mml:mn><mml:mo>°</mml:mo><mml:mi mathvariant="normal">C</mml:mi></mml:mrow></mml:math>800°C under different oxygen partial pressures. Schouler methodology based on the frequency relaxation has been used to assign different electrochemical processes. A transfer process related to oxygen ions moving between the electrolyte and electrode interface was found in the mixed ionic and electronic conducting electrode/electrolyte system. The study shows that the <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>Nd</mml:mi><mml:mrow><mml:mn>1.95</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>NiO</mml:mi><mml:mrow><mml:mn>4</mml:mn><mml:mo>+</mml:mo><mml:mi>δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>Nd1.95NiO4+δ compound exhibits promising electrode properties due to its high chemical stability and low polarization resistance. The oxygen diffusion process is the rate-limiting step at low oxygen partial pressure, whereas the charge-transfer process becomes more important at higher oxygen partial pressure.

Nanocomposites of Ni(OH)₂ or NiO have successfully been used in electrodes in the last five years, but they have been falsely presented as pseudocapacitive electrodes for electrochemical capacitors and hybrid devices. Indeed, these nickel oxide or hydroxide electrodes are pure battery-type electrodes which store charges through faradaic processes as can be shown by cyclic voltammograms or constant current galvanostatic charge/discharge plots. Despite this misunderstanding, such electrodes can be of interest as positive electrodes in hybrid supercapacitors operating under KOH electrolyte, together with an activated carbon-negative electrode. This study indicates the requirements for the implementation of Ni(OH)₂-based electrodes in hybrid designs and the improvements that are necessary in order to increase the energy and power densities of such devices. Mass loading is the key parameter which must be above 10 mg·cm⁻² to correctly evaluate the performance of Ni(OH)₂ or NiO-based nanocomposite electrodes and provide gravimetric capacity values. With such loadings, rate capability, capacity, cycling ability, energy and power densities can be accurately evaluated. Among the 80 papers analyzed in this study, there are indications that such nanocomposite electrode can successfully improve the performance of standard Ni(OH)₂ (+)//6 M KOH//activated carbon (−) hybrid supercapacitor.

▪ Abstract This chapter on fuel cells covers the following topics: (a) fundamental electrochemical aspects and performance analysis; (b) technology research and development and demonstrations of fuel cell power sources for power generation, transportation, portable power, and space applications; (c) the role of fuel cells vs competing technologies, and (d) prospects for the applications and commercialization of fuel cell technologies in the twenty-first century. Although the fuel cell was invented in the nineteenth century, the twentieth century has been the period for technology development rather than widespread use. The fuel cell faces a great deal of competition in the proposed applications of power generation, transportation, and portable power. Significant work is still necessary, but intensified research and development activities could lead to the dawn of fuel cell commercialization and widespread use in the early part of the twenty-first century.