(Invited) Change of Paradigms in Solid State Electrochemistry: Double-Cait Experiments with Two Clouds of Charge Carriers Representing Ideal Reversible Electrodes
Authors: Weitzel Karl-Michael 
Published: December 2024

Research and development in the field of solid state electrochemistry (SSE) is currently shaped by technical applications in particular regarding energy conversion and storage. There, the general goal is to safely transport as much charge as possible per unit time. For this purpose, the electrical circuit has to be closed. The classical circuit consist of a central sample (in general solid or liquid) connected to two electrodes (or series of electrodes) and an outer circuit in which the work is done. This implies the fact that there are at least two different interfaces between the sample and the electrodes. These are difficult to address selectively in classical approaches. The classical approach in solid state electrochemistry is built on a number of paradigms, which blur the view on and the access to new improved understanding and knowledge-based advance 1. Among these are:
Paradigm #1: Electrochemical experiments require two or more massive (solid or liquid) electrodes in contact with the sample.
Paradigm #2 Blocking of charge carrier transport is an unavoidable ingredient of SSE research.
The goal of this project is to bring about a change of paradigms in solid state electrochemistry. The pivotal concept is to replace the two classical massive electrodes by two charge carrier clouds, which represent ideal reversible electrodes. The charge carriers can e.g. be alkali ions, protons, electrons or oxygen anions. In one prototypical implementation, a beam of positive charge carriers, e.g. Li+ ions, is aimed at one side of a mixed ionic electronic conductor (MIEC) and an electron beam is aimed at the other side. The Li+ and electrons charge up the two sides positively and negatively respectively. This leads to well defined electrochemical surface potentials, +
surf and surf. Their corresponding gradients induce transport of cations and electrons inside the sample. This constitutes a double charge attachment induced transport experiment, Double-CAIT. The result of such macroscopic transport is quantified by means of secondary ion mass spectrometry (SIMS) and analyzed by means of the time dependent Nernst-Planck-Poisson (NPP) theory and time dependent Onsager theory. One pivotal characteristic is that any blocking of charge carrier transport can be completely avoided.
The experiment proposed here is the logical advancement of 10 years of CAIT experiments in the authors group, where so far still one side of the sample was connected to a metal electrode 2,3,4,5,6,7,8,9. In the future Double-CAIT experiment, there will be no metal electrode in contact with the sample.
As a result of these studies we expect to arrive at a new view on SSE free from the interference with any solid (or liquid) electrode. This includes equilibrium properties as well as transport properties but also their mutual relation. As an example of equilibrium properties we mention the advancement of a redox potential scale referenced to the vacuum state (sometimes not very clearly called “absolute redox scale”). The determination of transport coefficients will involve a truly time dependent approach.
1 K.-M. Weitzel, Charge attachment--induced transport -- Toward new paradigms in solid state electrochemistry, Curr. Opin. Electrochem., 2021, 26, 100672, DOI:10.1016/j.coelec.2020.100672.
2 M. Schäfer and K.-M. Weitzel, Bombardment induced ion transport. Part I, Physical Chemistry Chemical Physics : PCCP, 2011, 13, 20112–20122, DOI:10.1039/c1cp21215j.
3 P. V. Menezes, J. Martin, M. Schäfer, H. Staesche, B. Roling and K.-M. Weitzel, Bombardment induced ion transport--part II. Experimental potassium ion conductivities in borosilicate glass, Physical Chemistry Chemical Physics : PCCP, 2011, 13, 20123–20128, DOI:10.1039/c1cp21216h.
4 L. Rossrucker, P. V. Menezes, J. Zakel, M. Schäfer, B. Roling and K.-M. Weitzel, Bombardment Induced Potassium Ion Transport Through a Sodium Ion Conductor: Conductivities and Diffusion Profiles, Z. Phys. Chem., 2012, 226, 11083, DOI:10.1524/zpch.2012.0215.
5 K. M. Weitzel, Bombardment Induced Ion Transport through Ion Conducting Glasses, DF, 2016, 6, 107–143, DOI:10.4028/www.scientific.net/DF.6.107.
6 J. Martin, M. Gräf, T. Kramer, C. Jooss, M.-J. Choe, K. Thornton and K.-M. Weitzel, Charge attachment induced transport -- bulk and grain boundary diffusion of potassium in PrMnO$_3$, Phys. Chem. Chem. Phys., 2017, 19, 9762–9769, DOI:10.1039/C7CP00198C.
7 M. Schäfer and K.-M. Weitzel, Site energy distribution of ions in the potential energy landscape of amorphous solids, Materials Today Physics, 2018, 5, 12–19, DOI:10.1016/j.mtphys.2018.05.002.
8 M. Schäfer, D. Budina and K.-M. Weitzel, Site energy distribution of sodium ions in a sodium rubidium borate glass, Physical Chemistry Chemical Physics : PCCP, 2019, 21, 26251–26261, DOI:10.1039/c9cp05194e.
9 J. L. Wiemer, M. Schaefer and K.-M. Weitzel, Li+ Ion Site Energy Distribution in Lithium Aluminum Germanium Phosphate, J. Phys.
Chem. C, 2021, 125, 4977–4985, DOI:10.1021/acs.jpcc.0c11164.