Nernst voltage loss in oxyhydrogen fuel cells

Jinzhe Lyu (Division for Experimental Physics, School of Nuclear Science & Engineering, National Research Tomsk Polytechnic University, Lenina Ave. 43, Tomsk, 634034 Russia,) 2891796456@qq.com *corresponding author czinchzhe1@tpu.ru ABSTRACT Normally, the Nernst voltage calculated from the concentration of the reaction gas in the flow channel is considered to be the ideal voltage (reversible voltage) of the oxyhydrogen fuel cell, but actually it will cause a concentration gradient when the reaction gas flows from the flow channel through the gas diffusion layer to the catalyst layer. The Nernst voltage loss in fuel cells in most of the current literature is thought to be due to the difference in concentration of reaction gas in the flow channel and concentration of reaction gas on the catalyst layer at the time when the high net current density is generated. Based on the Butler-Volmer equation in oxyhydrogen fuel cell, this paper demonstrates that the Nernst voltage loss is caused by the concentration difference of reaction gas in flow channel and on the catalytic layer at the time when equilibrium potential (Galvanic potential) of each electrode is generated.


Introduction
A fuel cell is a chemical device that direct converts the chemical energy contained in fuel into electric energy.It is the fourth power generation technology after hydroelectric, thermal, and atomic power generation.Since the fuel cell converts the Gibbs free energy of the chemical energy of the fuel into electric energy through an electrochemical reaction, it is not limited by the effect of the Carnot cycle, so the efficiency is high [1].The two parameters that are closely related to performance of the fuel cell are the output voltage and current of the battery.Under a certain temperature, pressure, and concentration of the substance, the maximum theoretical voltage that the fuel cell can output is determined by the Nernst equation, which is called reversible voltage.However, there are a series of irreversible processes in actual fuel cells.
These irreversible processes lead to a series of corresponding irreversible voltage losses in the fuel cell.There are currently three recognized irreversible voltage losses: 1) Activation overvoltage loss; 2) Ohmic loss; 3) Concentration loss.The activation overvoltage loss is the lost Galvani potential in order to generate a net current.The ohmic loss is due to the voltage loss caused by the ohmic resistance of the internal components of the fuel cell.Obviously, the ohmic loss and the current are linear.The literature considers that concentration loss leads to two different voltage losses: drop of Nernst voltage and increase of activation overvoltage (concentration overvoltage) and in most of literatures it is believed that both of these voltage losses caused by concentration loss are due to the concentration difference between the reaction gas in the flow channel and the reaction gas on the surface of catalytic layer [2,3].Based on the Butler-Volmer equation this paper aims to prove that the reduction in Nernst voltage and the increase in activation overvoltage are caused by different concentration effects.This result has important significance, especially in the case of a low exchange current density, because difference in Nernst voltage loss and activation overvoltage loss caused by different concentration differences is very large, therefore, it has a great influence on the prediction of the true output voltage of fuel cells.Using the wrong concentration difference to estimate the concentration loss will seriously affect the judgment of the fuel cell performance.

Galvani potential
When the external circuit of the fuel cell is disconnected, the reaction gas is continuously introduced into the flow channel.At this time, the electrode reaction starts at the catalytic layer.Take the anode reaction as an example: During the reaction, both the forward reaction and the reverse reaction need to overcome certain activation barriers (Fig. 1) [4,5], even though from the energy point of view the energy of the products is lower than the energy of the reactants (the reaction is spontaneous).However, due to the existence of activation barriers, the reaction rate is still limited.Current density is usually used instead of reaction rate as the basic performance parameter of the fuel cell.Forward current density is Where  -The anode current density of the forward electrode reaction, A • cm ;  -The rate of decay from the activated state to the product,  = , k -Boltzmann constant; h -Planck constant.Since the operating process of fuel cell is considered to be a process with constant temperature and pressure [6], the decay rate of each reaction in each electrode is the same.Replace  by f: Reverse current density is  As can be seen from the figure, the activation barrier of forward reaction has increased by  ∆ , the activation barrier of forward reaction has reduced by (1 −  )∆ .α is called transfer coefficient,  expresses how the change in the electrical potential across the reaction interface changes the sizes of the forward versus reverse activation barriers.The value of  is always between 0 and 1.For "symmetric" reactions, α = 0.5.For most electrochemical reactions,  ranges from about 0.2 to 0.5 [2,3,[6][7][8].
At this time current density is Where ∆ -Galvanic potential of Anodic, V; Similarly, the cathode exchange current density is: where ∆ -Galvanic potential of cathode, V; From the equations ( 5), ( 6), (7), and (8) for exchanging current density, the equilibrium potentials of the anode and cathode can be obtained as: In fact, ∆ and ∆ are the thermodynamic electrode potential formula [8].
The Nernst voltage of the oxyhydrogen fuel cell is: Where  -Standard reversible voltage for oxyhydrogen fuel cell.
Obviously, when the equilibrium potential is reached, the concentration of the reactants at anode and cathode is lower than the concentration of the reactant gas in the corresponding flow channel.The Nernst voltage loss caused by this concentration difference is: the cathode Nernst voltage loss is: From ( 13), ( 14) it can be seen that Nernst voltage loss is actually also a concentration loss.

Activation overvoltage loss
When the external circuit of the fuel cell is closed, the accumulated electrons are conducted from the anode to the cathode, and the hydrogen ions are transmitted from the anode side of the electrolyte to the cathode side.The electrode potential of the anode must be reduced, that is activation overvoltage.Activation overvoltage is a part of the voltage that must be sacrificed to start the operation of the fuel cell.As the activation overvoltage is generated, the activation barrier for the forward reaction decreases   , and the activation barrier for When the net current density is small, the concentration of the reactant and product on the catalyst layer does not change much compared with the concentration of the reactant and product at equilibrium potential, and the activation overvoltage is small, so for the anode, there are: * * ≈ 1, * * ≈ 1 When  = 0.5, the anode net current density can be written as follows: From equation ( 16) can get the relationship between overvoltage and net current density (Figure 4).It can be seen from this figure that the activation overvoltage increases rapidly at very small net current densities and at very large net current densities increases slowly and is almost constant.Obviously the higher the exchange current density, the smaller the activation overvoltage required to reach the same net current density.Should pay attention to the fact that the voltage at which the net current density is zero is not the ideal electromotive force but is the thermodynamically reversible voltage determined by the concentration of the reactants on catalytic layers of the anode and cathode at equilibrium potentials (not necessarily equal to 1.1 V but must be less than ideal reversible voltage).The difference between voltage at which the net current density is zero and the ideal electromotive force is the Nernst voltage loss, which is caused by the concentration difference between the reactants on the catalyst layer at the equilibrium potential and the reactants in the flow channel.

Concentration loss
The effect of change of the concentration of the reactants is neglected when the overvoltage is determined by equation ( 16).In fact, the net reaction rate will affect the surface concentration of reactants and products.For example, when the forward reaction rate increases drastically and the reverse reaction rate decreases sharply, the surface concentration of the reactants tends to be depleted.Obviously for a certain net current density, the lower the concentration of the reactants, the greater the activation overvoltage required to reach the same net current density, which leads to a sharp increase in the demand for additional activation overvoltages.This additional activation overvoltage is the concentration loss.At high current densities, the reactants in the catalytic layer are rapidly consumed, which leads to a sharp increase in the need for an activation overvoltage.The excess activation overvoltage is called concentration loss (concentration overvoltage) [12][13][14].
At high current densities, the second term in the Butler-Volmer equation can be discarded.
The net anode current density simplifies to: (17) can be expressed in the form of activation overvoltage: In this way concentration loss can be expressed as The sum of the Nernst voltage loss and the concentration overvoltage in anode can be obtained from (13) Since 0 <  < 1 and concentration of the reaction gas in the flow channel is constant, the smaller concentration of the reaction gas  * at equilibrium potential, the greater the deviation of the concentration loss calculated in literature and from equation (20), the greater the error.
As mentioned above, the typical voltage loss of a fuel cell is shown in Fig. 5.When the fuel cell supplies electronic power, the actual voltage will drop due to several non-reversible factors.

Fig. 1 .
Fig.1.Energy change of reaction process in anode electrode. -Gibbs free energy of anode reactants; ∆ -activation energy of positive reaction in anode;  -Gibbs free energy of anode products; ∆ -activation energy of reverse reaction in anode.

Where𝑗-Fig. 2 .
Fig.2.Diagram of the diffusion layer formed at the anode during the operation of the oxyhydrogen fuel cell.

Fig. 3 .
Fig.3.Energy change of reaction process during equilibrium potential of anode.
* -Concentration of reactant at equilibrium potential of anode, mol • cm ;  * -Concentration of product at equilibrium potential of anode, mol • cm ;  -Transfer coefficient of Anode electrode reaction;  -Anode exchange current density, A • cm .

𝑐 * -
Concentration of reactant at equilibrium potential of cathode, mol • cm ;  * -Concentration of product at equilibrium potential of cathode, mol • cm ;  -Transfer coefficient of cathode electrode reaction;  -Cathode exchange current density, A • cm .
)-( * ,  * ) of reactant in flow channel of anode, mol • cm ;  -Concentration of reactant in flow channel of cathode, mol • cm ; So, the anode Nernst voltage loss is:

the reverse reaction increases ( 1 −
) , the rate of forward reaction exceeds the rate of reverse reaction, and the fuel cell produces a net current density.Taking into account the influence of the net current generation on the concentrations of reactant and product, net current density is [2, 9-11]:  =  * * exp ( ) - * * exp [ * -Concentrations of Anode electrode reactants and product at any time when the circuit is closed, mol • cm ;  -Anode activation overvoltage.Similarly, the net current density of cathode is: and  * -Concentrations of cathode electrode reactants and product at any time when the circuit is closed, mol • cm ;  -Cathode activation overvoltage.

Fig. 4 .
Fig.4.Effect of activation overvoltage on fuel cell performance without considering the concentration loss.Reaction kinetics typically inflicts an exponential loss on a fuel cell's j-V curve as determined by the Butler-Volmer equation.The magnitude of this loss is influenced by the size of  .(curves calculated for various  values with  = 0.5,  = 2 and  = 298.15K.)

Fig. 5 .( 1 )
Fig.5.Ideal voltage and actual voltage diagram of oxyhydrogen fuel cell.(1) Open circuit voltage which is lower than reversible voltage due to Nernst voltage loss caused by difference in concentration of reaction gas in flow channel and reaction gas at the time of generation of equilibrium potential; (2) Activation overvoltage assuming that the concentration of the reaction gas at the time of generation of net current is equal to concentration of the reaction gas at the generation of equilibrium potential.It slowly increases in the high current density region because the decrease in the reactant concentration is not taken into consideration.At this time, the concentration term is large, and only a few increase of activation