Difference between OCV and OCP?

[eibar-flores]

In cell modelling, there is an explicit difference between the electrode OCP and the cell OCV. The OCP is understood to be inherent to the active material and measured versus Li+/Li in Li-ion cells. Instead, the OCV is understood to be the difference between individual electrodes OCPs. .Both are understood to be a function of the state of charge.

In the ontology, OCP and OCV are used as altLabels of the same quantity, OpenCircuitVoltage. However, from what I understand in the elucidations, OpenCircuitVoltage describes the OCV, while EquilibriumElectrodePotential describes better the OCP.

Should we consider using OCV as altLabel of OpenCircuitVoltage, and OCP as altLabel of EquilibriumElectrodePotential? Otherwise, is there an alternative mapping between model OCP/OCV to ontology concepts?

[jsimonclark]

I think the working understanding in the field is that Open-Circuit Potential refers to the half cell and Open-Circuit Voltage refers to the full cell. But there are some semantic issues with those terms and the root is the difference between Potential and Voltage.

The working understanding is that Potential is measured at a single point and Voltage is the difference between two potentials. So, to my understanding, as soon as you measure with respect to a reference electrode, it becomes a voltage and not a potential. Also note that the term "Open-Circuit" inherently means something that contains at least two terminals (i.e. you can't have an open-circuit with just one terminal).

Now, let's consider some authoritative sources.

IEC

• IEC 60050 defines Open-Circuit Voltage as "voltage of a cell or battery when the discharge current is zero", where Voltage is "equal to the negative of the electric potential difference between the two points" So, an open circuit voltage is the difference in electric potential measured between two electrodes when no current is flowing.
• IEC 60050 does not define or recognize the term Open-Circuit Potential, but it does recognize EquilibriumElectrodePotential, "electrode potential when the electrode reaction is in equilibrium".

IUPAC

• IUPAC Terminology of electrochemical methods of analysis deprecates terms containing "voltage" in favour of terms containing "potential".
• In the IUPAC Gold Book they say about voltage, "The use of this term is discouraged, and the term applied potential should be used instead, for non-periodic signals. However, it is retained here for sinusoidal and other periodic signals because no suitable substitute for it has been proposed."
• IUPAC defines applied potential as, "The difference of potential measured between identical metallic leads to two electrodes of a cell."
• Electric potential is defined as, "At a point, the work required to bring a charge from infinity to that point in the electric field divided by the charge."
• I struggle with this way of defining things because we don't really measure the electric potential itself - we are always measuring it with respect to some reference potential. And to say that an "applied potential" is the "difference of potential" does not really make sense to me. From the definition, we know that Electric Potential is a quantity at a single point, where as an applied potential requires two points.

In this case, I think the IEC terminology is more appropriate. So we go with that.

Now, what you are describing with modelling is that you need half cell open-circuit voltage profiles for both electrodes, against some common reference electrode. In the case of Li-ion battery cell modelling, it is usually both electrodes vs. Li+/Li. So, it seems to me that we should add a term for:

• HalfCellOpenCircuitVoltage or ReferenceOpenCircuitVoltage: the difference in the EquilibriumElectrodePotential values between a working electrode and a reference electrode.

This should align with the BPX activities, so lets also ask them to comment.