How do batteries work?

How do batteries really work? A convincing simple yet quantitative answer to this question has remained elusive. Textbooks and on-line sources have provided only descriptions but not explanations of basic electrochemistry. All calculations in electrochemistry are based on measured voltages, not atomic or molecular properties. Made-up explanations of batteries in terms of different “electron affinities” of different metals are widely believed but easily disproved, e.g. by concentration cells using the same metal for both electrodes.

A paper in the Journal of Chemical Education by Klaus Schmidt-Rohr (Chemistry) explains how batteries store and release energy, in quite simple terms but based on quantitative data. In the classical Zn/Cu galvanic cell, it is the difference in the lattice cohesive energies of Zn and Cu metals, without and with d-electron bonding, respectively, that is released as electrical energy. Zinc is also the high-energy material in a 1.5-V alkaline household battery. In the lead–acid car battery, intriguingly the energy is stored in split water (two protons and an oxide ion). Atom transfer into or out of bulk metals or molecules plays as big a role as electron transfer in driving the processes in batteries.

How Batteries Store and Release Energy: Explaining Basic Electrochemistry, Klaus Schmidt-Rohr, Journal of Chemical Education, 2018, 95 (10), pp 1801–1810.

Why Fire Is Hot

A recent paper by K. Schmidt-Rohr (Chemistry) answered the question why combustion reactions are always exothermic.  Every scientist should be able to explain what makes fire hot, but neither chemistry nor combustion textbooks have provided a valid answer. Schmidt-Rohr’s analysis shows that the reason lies in the double bond in O2, which is much weaker than other double bonds or pairs of single bonds in the biosphere, so that the formation of the stronger bonds in CO2 and H2O results in the release of heat. The bond energies in the fuel play only a minor role; e.g., the total bond energy of CH4 is nearly the same as that of CO2. A systematic analysis of bond energies gives the heat of combustion close to –418 kJ (i.e., –100 kcal) per mol O2, in good agreement (±3%) with data for >500 organic compounds; the heat of condensation of H2O (–44 kJ per mol H2O(l)) is also included in the analysis. For 268 molecules with ≥8 carbon atoms, the standard deviation is even smaller, 2.1%. For a fuel of composition CcHhOoNn, this gives DcH ≈ -418 kJ (c + 0.3 h – 0.5 o), which enables instant estimates of heats of combustion simply from the elemental composition, even for complex mixtures of unknown molecular composition, and explains principles of biofuels production. The analysis indicates that O2, rather than fuels like octane, H2, ethanol, or glucose, is the crucial “energy-rich” molecule. It also challenges common notions about a relation between the oxidation state and the energy content of biomolecules.

BlogCombustExotherm.0116

One then needs to explain why O2 is abundant in air despite its high enthalpy: All the O2 in the earth’s atmosphere has been produced by photosynthesis in cyanobacteria, algae, and higher plants, as a by-product of photosynthetic proton and electron production from H2O. The “price” of the production of O2, which is energetically so unfavorable, is paid by plants and algae (with “cheap” energy from the sun) in order to be able to live wherever H2O is present. So one can conclude that atmospheric O2 stores solar energy that sustains us with every breath we take.

Schmidt-Rohr K. Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O-2. J Chem Educ. 2015;92(12):2094-9.

Schmidt-Rohr to join Chemistry faculty

Klaus Schmidt-RohrThe Department of Chemistry is looking forward to welcoming Klaus Schmidt-Rohr to the faculty this July.

Prof. Schmidt-Rohr is a highly regarded spectroscopist, with a background in both physics and chemistry.  His research is focused on materials and his recent studies have revised our understanding of the structure of Nafion membranes (the proton selective membranes on which most hydrogen fuel cells now depend), the surfaces of nanodiamonds, the molecular bases of bone strength, and the molecular composition of biochar.  Schmidt-Rohr approaches materials primarily through solid state NMR, with a distinctive emphasis on skillful spectral editing.  He has also complimented these experiments with innovative analyses of small angle x-ray scattering data.

Prof. Schmidt-Rohr received his Ph.D. from the University of Mainz in Germany and continued at the Max-Planck Institute in Mainz as a staff scientist. Following postdoctoral work at UC Berkeley, as a fellow of the BASF AG and the German National Science Foundation, he took a faculty position in the Department of Polymer Science & Engineering at the University of Massachusetts at Amherst.  More recently, he has been a Professor of Chemistry at Iowa State University.

Prof. Schmidt-Rohr’s pioneering work has been recognized with prestigious awards, including the Rudolph-Kaiser Prize from the German Physical Society, a Beckman Young Investigator Award from the Arnold and Mabel Beckman Foundation, an Alfred P. Sloan Research Fellowship, the John H. Dillon Medal of the Polymer Division of the American Physical Society, and fellowship in the American Association for the Advancement of Science and in the American Physical Society.

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