Chemical Science Blog

Intercalation of multivalent ions, e.g., Al³⁺, has received increasing attention as an energy-boosting strategy for rechargeable batteries. The charge storage process of the wide-spread Li-ion batteries relies on Li⁺ intercalating electrodes with layered structures. Multivalent-ion batteries could accommodate more charges than Li-ion batteries because their ions carry more charge than Li⁺. This concept, however, has now been challenged by a research team led by BenoÎt Limoges and Véronique Balland of Université de Paris, France. Their mechanistic investigation, published in Chemical Science, revealed that Al³⁺ ions were unable to intercalate electrodes in aqueous electrolytes.

The authors selected TiO₂ arrays as the object of their study. These ~1 µm-high arrays were grown using the glancing angle deposition technique. By applying negative potentials to the TiO₂ arrays, cations such as Al³⁺ could diffuse through the inter-array slits and interact with TiO₂ (Figure 1).

Figure 1. Structure of the TiO₂ arrays. (left) Scanning electron micrographs and (right) a cartoon illustrating ion diffusion pathways.

Electrochemistry tests elucidated that the charge-storage process of TiO₂ in an Al³⁺-containing aqueous electrolyte correlated to proton intercalation. This conclusion was mainly based on the nearly identical cyclic voltammograms (Figure 2, top) and capacity vs. potential curves (Figure 2, bottom) of the TiO₂ arrays in both AlCl₃ and acetic acid aqueous electrolytes. Since acetic acid solution had no Al³⁺, the observed charge-storage activity could not be attributed to Al³⁺ intercalation. Instead, the authors argued that protons dissociated either from hydrated Al³⁺ cations, [Al(H₂O)₆]³⁺ or acetic acid must intercalate TiO₂ and result in the observed charge-storage capacities.

Figure 2. (top) Cyclic voltammograms and (bottom) capacity vs. potential curves of TiO₂ in (left) Al³⁺-containing and (right) acetic acid aqueous electrolytes. The electrolytes are 0.3 M KCl with different concentrations of AlCl₃ or acetic acid: 0 M (black), 25 mM (blue), 50 mM (purple), 100 mM (magenta), and 250 mM (red).

The authors believe that the misconception of Al³⁺ intercalation is due to the overlooking of Al³⁺ hydration, which is inevitable when Al³⁺ is present in aqueous electrolytes. Removing the water shell (a prerequisite for ion intercalation) is energy costly for Al³⁺ because of the strong binding force between water molecules and Al³⁺. Additionally, even if Al³⁺ ions intercalate TiO₂, their movement is strongly hindered by Coulombic interactions within the TiO₂ lattice. The immobilized intercalated ions would then block other ions from entering the TiO₂ lattice. Together, both factors prevent Al³⁺ from intercalating into TiO₂.

In summary, this work demonstrates that the charge-storage capacity of TiO₂ in Al³⁺-containing aqueous electrolytes is most probably due to proton intercalation. This conclusion also applies to other multivalent cations, including Zn²⁺ and Mn²⁺, as shown in this work.

To find out more, please read:

On the Unsuspected Role of Multivalent Metal Ions on the Charge Storage of A Metal Oxide Electrode in Mild Aqueous Electrolytes

Yee-Seul Kim, Kenneth D. Harris, BenoÎt Limoges, and Véronique Balland

Chem. Sci., 2019, doi: 10.1039/c9sc02397f

Tianyu Liu acknowledges Zachary L. Croft of Virginia Tech, the U.S., for his constructive comments on this post.

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the United States. He is passionate about the communication of scientific endeavors and cutting-edge research to both the general public and other scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

As we transition to an energy future composed primarily of intermittent, renewable technologies, finding ways to store the excess generated charges will be critical. While the general public typically thinks of batteries, another area of massive interest is storing the electrons in a chemical bond. This can be accomplished by creating electrocatalysts that can couple an electric current with chemicals like water and oxygen to generate stable new bonds in molecules for later use as fuels. These renewably generated fuels would then be available whenever needed, like at night. However, finding efficient electrocatalysts composed of earth-abundant materials has proven challenging. Many researchers have turned to nature for inspiration by designing molecules that mimic the active sites of enzymes.

Researchers in the United States used this approach, focusing on creating high-valent iron-oxo species, which others previously identified as the key catalytic intermediates in multiple enzymatic reactions. These types of species have traditionally been synthesized by reacting a reduced iron complex with an oxygen transfer reagent, but the researchers developed a system to generate highly reactive species using electricity as the reaction driving force and water as the oxygen source. The studied catalyst is a commercially available iron(III)-aquo complex with a tetraamido macrocyclic ligand (TAML) as the ancillary ligand (Figure 1B).

Figure 1. A. Cyclic voltammagram of (TAML)Fe in acetonitrile. B. Structure of (TAML)Fe and cyclic voltammagram showing increased current upon the addition of ethylbenzene.

When analyzed by cyclic voltammetry, an electrochemical technique where you cycle the voltage between set points and measure the current output, the (TAML)Fe shows two redox events at around 650 and 1250 mV that the researchers attributed to generating the Fe^IV-OH and Fe^V(O) species (Figure 1). Addition of ethylbenzene, which should react with the Fe^V species, increased the current at voltages of 1250 mV and higher, indicating (TAML)Fe turnover. However, isolating the Fe^v(O) species proved challenging as it reacts rapidly with the (TAML)Fe to form an Fe^IV dimer. This also limits the efficiency of the overall system by decreasing the amount of the most reactive species in solution.

Figure 2. A, B. Products generated by oxidation of various substrates screened with (TAML)Fe for electrocatalysis with isolated yield and calculated conversion in parenthesis. C. Substrates that did not react with the (TAML)Fe complex.

Both the Fe^IV dimer and Fe^V(O) species proved capable of oxidizing C-H bonds in ethylbenzene, but the Fe^V(O) is much more reactive and increases the oxidation rate at high electrochemical potentials. The researchers tested the scope of (TAML)Fe reactivity using a series of compounds with benzylic C-H bonds (Figure 2). They found that the (TAML)Fe performed well with electron-rich and electron-neutral derivatives, with an electron-deficient nitro-substituted derivative showing lower reactivity. Several substrates with non-benzylic C-H bonds showed high selectivity for oxidation at the benzylic C-H bond. (TAML)Fe also showed high electrocatalytic activity for oxidizing alcohols and converted substrates as simple as cyclohexanol and as complex as a steroid to ketones in high yields (up to 97%).

This study of an earth-abundant, stable, and commercially available electrocatalyst acts as a baseline for further studies with other similar metal complexes. Despite the efficiency limits attributed to dimerization, the high stability and selectivity of the (TAML)Fe could lead to its use with a broader range of substrates with varied functional groups.

To find out more please read:

Electrochemical C–H oxygenation and alcohol dehydrogenation involving Fe-oxo species using water as the oxygen source

Amit Das, Jordan E. Nutting and Shannon S. Stahl

Chem. Sci., 2019, 10, 7542-7548

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.