The anode catalyst layer is a porous material transporting protons (H+) through and collecting electrons (e−). Here, we propose an ultrasound-based novel technique. Introducing ultrasound waves in a porous electron transport and thereby facilitates the efficiency of proton transport.
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Stimulation of porous anode catalyst layer by ultrasound
https://doi.org/10.21203/rs.3.rs-3229428/v1
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Proton Exchange Membrane Fuel Cell (PEMFC)
Catalyst Layer (CL)
Porous
Poroelasticity
Ultrasound
Charge density
A catalyst layer of the Proton Exchange Membrance Fuel Cell (PEMFC) is made of an inhomogeneous porous material, and its physicochemical and electrochemical properties have a strong effect on its performance 1. Hydrogen and oxidants are supplied to the flow channels, and they pass through gas diffusion layers (GDLs) and catalyst layers (CLs) of PEMFC. The catalyst surface absorbs hydrogen molecules and generates protons (H+) and electrons (e-), and the electron-conducting components and ionomers in the CL transport electrons and protons, respectively. The membrane in the cathodic CL allows protons to pass through it. The membrance blocks electrons and moves in the external circuit.
The CL is porous, and it is made of carbon-based platinum (or carbon support) and an ionomer (or Nafion polymer), which is a proton-conducting material. The carbon support provides an electron transport network between the platinum (Pt) and GDLs.
The application of ultrasound pulses (20 kHz) to an anode CL containing Nafion-bonded carbon polymers results in the CL absorbing a larger amount of Pt2. Furthermore, simplified experiments on glassy carbon (GC) have shown that ultrasound application can significantly increase Pt deposition on GC. In view of these findings, in the current study, I examined whether ultrasound application enhanced electron transport in CLs, by using poroelasticity theory.
Figure 1 shows how the charge density varied with the porosity and applied ultrasound frequency. With a decrease in the porosity and an increase in the frequency, the charge densities increased. Figure 2 shows that the cylindrical (i.e., tortuosity = one) water transport pathway (or a straight water transport pathway) involves higher charge densities than the twisted water transport pathway (i.e., tortuosity > 1).
From these results, we can design a new inhomogeneous porous CL microstructure’s carbon supports are connected in parallel, but Percolation theory starts the connection beyond the critical threshold in porous microstructures of CLs. More like parallelly connected microstructure is preferred. For example in percolation theory, the critical threshold establishes the first connection between one side and the other, but more links are required to realize stable microstructures. More experimental studies are required to devise ways for the ultrasound-based manufacture of optimized anode catalyst microstructures.
The authors of Reference 3 formulated poroelastic governing equations by including the charge density in the electrochemistry term, and numerically solved them for cancellous bone 4:
$${\rho _{11}}\ddot {u}+{\rho _{12}}\ddot {U} - N{u_{i,jj}} - \left( {A+N} \right){u_{j,ji}} - Q{U_{j,ji}}+b\left( {{{\dot {u}}_i} - {{\dot {U}}_i}} \right)+{q_s}{\Phi _{,i}}=0$$
1
$${\rho _{22}}{\ddot {U}_i}+{\rho _{12}}{\ddot {u}_i} - {\left( {R{U_{j,j}}+Q{u_{j,j}}} \right)_{,i}} - b\left( {{{\dot {u}}_i} - {{\dot {U}}_i}} \right)+{q_s}{\Phi _{,i}}=0$$
2
The values of the parameters in the above equations are provided in References 5,6 and the calculation steps are detailed in Reference 4. N, R, Q, and A are poroelastic parameters, u and U are displacements of fluid and solid constituents, and the variable \(\text{b}\) is defined by \({\phi ^2}\mu /K\), where \(\mu\) is the fluid viscosity, K is the permeability of bone fluid, ε is the permittivity, and the \(\phi\) is the porosity. G is the constant showing the linear relationship between the electric current and electrical potential due to the applied electrical potential. The mechanical properties of Pt and water were considered for solving the equations.
AUTHOR CONTRIBUTIONS
Young is the only author in this manuscript.
COMPETING INTERESTS
There is no competing interests in this manuscript.
DATA AVAIABILITY STATEMENT
All data anlaysied during this study are included in the published article4 except the mechanical properties of platinum are the bulk modulus is 230 GPa, density 4500 kg/m3, shear modulus 62 GPa. Also a supplement file is enclosed.
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- Neev, J. & Yeatts, F. Electrokinetic effects in fluid-saturated poroelastic media. Physical Review B 40, 9135 (1989).
- Yoon, Y. J. The effect of charge density on the velocity and attenuation of ultrasound waves in human cancellous bone. Journal of Biomechanics 79, 54–57 (2018).
- Williams, J. L. Ultrasonic wave propagation in cancellous and cortical bone: Prediction of some experimental results by Biot’s theory. The Journal of the Acoustical Society of America 91, 1106–1112 (1992).
- Yoon, Y. J., Chung, J.-P., Bae, C.-S. & Han, S.-Y. The speed of sound through trabecular bone predicted by Biot theory. Journal of biomechanics 45, 716–718 (2012).
No competing interests reported.