Reconciling temperature-dependent factors affecting mass transport losses in polymer electrolyte membrane electrolyzers

Abstract

In this work, we investigated the impact of temperature on two-phase transport in low temperature (LT)-polymer electrolyte membrane (PEM) electrolyzer anode flow channels via in operando neutron imaging and observed a decrease in mass transport overpotential with increasing temperature. We observed an increase in anode oxygen gas content with increasing temperature, which was counter-intu.itive to the trends in mass transport overpotential. We attributed this counterintuitive decrease in mass transport overpotential to the enhanced reactant distribution in the flow channels as a result of the temperature increase, determined via a one-dimensional analytical model. We further determined that gas accumulation and fluid property changes are competing, temperature-dependent contributors to mass transport overpotential; however, liquid water viscosity changes led to the dominate enhancement of reactant water distributions in the anode. We present this temperature-dependent mass transport overpotential as a great opportunity for further increasing the voltage efficiency of PEM electrolyzers.

Keywords: anode flow channels; hydrogen; mass transport; operating temperature; polymer electrolyte membrane electrolyzer; two-phase pressure drop.

Fig A.1.:

Sensitivity analysis of the effect of water diffusion across the membrane on the analytical model results. (a) Comparison of water flux due to electro-osmotic drag (red) and diffusion (blue). (b) Comparison of the analytical model results assuming the cathode side of the membrane is fully wet (pink-dashed) and fully dry (purple-dotted).

Figure 1:

(a) A schematic of the neutron imaging setup: incident beam originating from a nuclear reactor traverses in the through-plane direction (beam perpendicular to the membrane). The attenuated beam meets the scintillator, and the scintillator emits visible light based on the intensity of the attenuated beam. The inset schematic (black-solid box) shows the geometry of the flow field and the membrane-electrode-assembly (MEA). The yellow-dashed box indicates the imaged region. (b) Processed image of the electrolyzer. The position of the active area is indicated by the shaded red region, and channels are numbered as shown. Gas thicknesses were quantified for the regions dashed in red.

Figure 2:

Schematic outlining the parameters considered in the 1D analytical model to solve for theoretical gas thickness in the channels. This theoretical gas thickness assumes uniform reactant distribution and operating current density across the active area. Each term of the model is summarized in Table 1. The red-dashed box indicates the anode flow channel, which is the region of interest for the calculation.

Figure 3:

(a) Galvanostatic polarization curves and the ohmic resistances of the electrolyzer cell operated at 40 °C, 60 °C, and 80 °C. The decrease in ohmic resistance was a main contributor to the decrease in cell potential with increasing temperature in this experiment. We attributed a significant portion of this ohmic resistance decrease to the decreasing membrane resistance of the relatively thick membrane used (250 μm thick) as a function of increasing temperature. (b) Tafel plots for determining the kinetic overpotential. The Tafel region was considered from 10 mA/cm² to 100 mA/cm², indicated between dashed grey lines. This range was justified by the slight lift-off of the iR-corrected cell potential from the Tafel line when i > 0.1 A/cm² and by the demonstrated Arrhenius relation (inset of (b)). (c) The trend in mass transport overpotentials with increasing operating temperatures. We observed a decrease in mass transport overpotential with increasing temperature.

Figure 4:

(a) Voltage efficiency of the electrolyzer as a function of operating temperature. At i = 3.0 A/cm², we observed a 7 % increase in the voltage efficiency from T = 40 °C to T = 80 °C. (b) Breakdown of the overpotentials at i = 3.0 A/cm². The dashed-line indicates the reversible potential, the brown, orange, and green arrows indicate kinetic, ohmic, and mass transport overpotential, respectively. Mass transport overpotential contributed up to 28 % of the total overpotential, which highlights a major opportunity for improving the voltage efficiency of the electrolyzer.

Figure 5:

Processed images obtained from neutron radiographs during operating temperatures of 40 °C (a), 60 °C (b), and 80 °C (c) at an operating current density of 3.0 A/cm²; (d) Gas thicknesses in each channel shown in (a),(b), and (c). The error bars represent the 95 % confidence intervals. The mean exit gas thicknesses were 0.24 mm, 0.28 mm, and 0.35 mm for operating temperatures of 40 °C, 60 °C, and 80 °C, respectively. The error bars show 95 % confidence intervals based on the variation of thickness within each channel, and the value of the error bars was consistently ~ ±0.01 mm. We observed an increase in gas thickness with increasing temperature in the majority of channels.

Figure 6:

Mean exit gas thickness determined from in operando neutron radiography visualization at operating temperatures of (a) 40 °C, (b) 60 °C, and (c) 80 °C. The error bars represent the 95 % confidence intervals. The dashed lines indicate the theoretical exit gas thickness determined from the analytical model, assuming uniform liquid water distribution to each parallel flow channel and uniform current density operation. The root-mean-square deviations between the experimental and theoretical data are 0.066 mm, 0.034 mm, and 0.025 mm for 40 °C, 60 °C, and 80 °C, respectively. The theoretical and the experimental gas thicknesses at 80 °C exhibited the best agreement, whereas the theoretical and the experimental gas thicknesses at 40 °C exhibited the highest discrepancy, indicating a relatively uniform reactant delivery and local current density at higher temperature.

Figure 7:

(a) Two-phase pressure drop across a single flow channel calculated using the Lockhart-Martinelli correlation. The error bars represent the 95 % confidence intervals. We observe a decrease in two-phase pressure drop with increasing temperature, a trend that closely resembles that of the mass transport overpotential (Fig. 3c); (b) Sensitivity analysis of the two-phase pressure drop and temperature-dependent fluid properties. During the sensitivity analysis the fixed variables were based on T = 80 °C and i = 1.0 A/cm². We identified that the viscosity of water dominated the decrease in the two-phase pressure drop.