Product Leaves, Product Stays:
Chemical Capacitance as the Divide between Electrocatalysis and Battery Science

Electrochemistry uses a shared language across many research fields. Electrocatalysis & Battery intercalation, for example, both involve charge transfer across an interface. They use concepts such as overpotential, exchange current density, transfer coefficient, and reaction kinetics. Because of this shared vocabulary, it is tempting to imagine that the same electroanalytical intuition can be applied from one field to the other.

I used to think this way as well. As a graduate student trained mostly in electrocatalysis, I was once surprised when a friend told me that measuring or modeling charge-transfer kinetics in Li intercalation reactions was unusually difficult. At first, this felt counterintuitive to me. Charge transfer was one of the most basic concepts in electrochemistry. Why would it be so difficult to define or measure in a battery electrode during charge and discharge?

Recently, I began to understand what he meant.

In catalysis, the product leaves.
In batteries, the product stays.

In my view, this simple difference changes everything.

For an electrocatalytic reaction, or more broadly a heterogeneous catalysis, the active material ideally mediates the conversion of reactants into products without being changed. Of course, real catalyst surfaces are never perfectly passive. Surface reconstruction, electrolyte interaction, adsorbate coverage, and spontaneous thermochemical reactions with open-circuit potential shifts are representative phenomena of evolving surface state. Still, the basic expectation is that the catalyst is not meant to become the reservoir of the product itself.

Protons and electrons may combine to form hydrogen. An adsorbed intermediate may be converted into a product that desorbs into the electrolyte or gas phase. If reactants are continuously supplied and products are continuously removed, the catalyst can approach a steady state. The surface may be covered with intermediates, but the active material itself is not supposed to store the product as part of its bulk composition.

This makes it meaningful to ask a relatively direct kinetic question: "At a given overpotential, what is the reaction rate?"

Battery electrodes are different.

In an intercalation material, the reaction product does not simply leave the interface. A lithium ion and an electron are transferred into the active material and become stored inside the host lattice. The electrode is not a surface platform for a reaction. It is also the destination of the reaction. It is a 'reservoir that stores the product through changes in chemical potential.'

Every charge transferred across the interface changes the state of the material.

Here, the central issue appears. "A battery has its own capacity change."

Because of this Δcapacity, its kinetics are difficult to interpret or to measure.

This sounds almost kind of a paradox. Capacity is the reason a battery is useful in my opinion. It is the feature we want to pursue. However, capacity also means that the material remembers the reaction. When Li ion is inserted or extracted, the host composition changes. When the composition changes, the Li chemical potential changes (or more broadly chemical potential of an electrode changes). When this chemical potential changes, the equilibrium redox voltage changes. Therefore, during the very act of measuring charge transfer, the thermodynamic reference point of the reaction can swing.

The Role of Chemical Capacitance

This is where chemical capacitance becomes essential. However, chemical capacitance might be not familiar with heterogeneous catalysis guys. To me, however, it is becoming one of the clearest bridges between electrocatalysis and materials science. It tells us that an electrochemical signal is not only about electrons crossing an interface. It can also be a record of how a solid changes its internal thermodynamic state during its own catalysis procedure I believe.

In a conventional dielectric capacitor, charge is stored mainly through electrostatic separation. In a battery material, charge is also stored chemically. The host lattice accepts lithium, changes its composition, shifts its chemical potential, and therefore changes its equilibrium voltage. It is also why a battery electrode does not behave like a catalytic surface with a fixed thermodynamic identity.

That memory is encoded in composition, defect states, redox distributions, phase behavior, and local chemical environments. Every inserted or extracted Li ion changes the state from which the next charge-transfer event occurs. Therefore, the interface and the bulk cannot be cleanly separated in the way we often hope. The surface reaction modifies the solid, and the modified solid changes the meaning of the surface reaction.

This is the part I find scientifically beautiful.

Battery voltage is not just an electrical number displayed by an instrument. It is a compressed thermodynamic message from the material. It reflects where lithium prefers to reside, how transition-metal redox states respond, how strongly the host resists or accepts compositional change, and how defects reshape the available storage landscape. In that sense, voltage is a kind of language for the material.

A large chemical capacitance means that the material can store a large amount of charge with only a small change in its equilibrium voltage. A sharp feature in chemical capacitance may point to site filling, ordering, phase transformation, or a narrow redox-state distribution. A broad feature may suggest disorder, heterogeneous local environments, defect interactions, or distributed storage states. This is why I find chemical capacitance more informative than a simple capacity number.

A Coupled Event

This perspective also changes how I look back at my earlier experience in electrocatalysis. In catalyst studies, I was trained to care about activity, selectivity, overpotential, and mechanistic descriptors. Those concepts are still essential. But battery electrodes add another layer: the reacting material is also the storage medium. The reaction coordinate and the material state are coupled. The electron-transfer event cannot be fully understood without asking how the host lattice responds to the inserted or removed species.

This is why I no longer see battery intercalation simply as "charge transfer at an interface." I see it as a coupled event:

Charge Transfer + Chemical Storage + State Evolution

• The first term belongs to electrochemistry.
• The second belongs to thermodynamics.
• The third belongs deeply to materials science.

This comparison between electrocatalysis and batteries matters to me because the two fields share many of the same words while often pointing to different physical realities. Current, voltage, overpotential, resistance, and kinetics may appear in both fields, but their meanings can shift depending on whether the material remains steady or changes as part of the reaction.

For me, this has become a central theme: electrochemistry is not only about how electrons cross interfaces, but also about how materials respond after electrons cross. That response may appear as a voltage shift, a diffusion process, a phase boundary, a defect population, or a chemical capacitance. Each is a different way the material says:

This is the conceptual space I want to keep exploring. Not just better battery materials, and not just better kinetic models, but a clearer connection between what we measure and what the material is actually doing. This naturally leads to the next question: if the material remembers every charge-transfer event as a language of chemical potential, how can we measure the charge-transfer event itself?