Optimal Surface Doping of Lead for Increased Electrochemical Insertion of Hydrogen into Palladium

Increasing the amount of hydrogen that is electrochemically inserted into materials is important for studying superconductivity and hydrogen embrittlement, and improving hydrogen storage capabilities. Surfaces can be engineered to accomplish this task with better insight into how the composition of a material's first few atomic layers affects the electrochemical insertion of hydrogen. To this end, different amounts of Pb were added to the 0.1 M LiOH electrolyte to be deposited onto Pd cathodes during galvanostatic experiments. The investigated amount of added Pb was between 1 μg cm−2 and 23 μg cm−2 with respect to the geometric area of the Pd cathode. The optimum surface doping level 2.9 μg cm−2 of Pb (∼1.4 mass equivalent monolayers) was found to achieve the highest quantity of inserted hydrogen at approximately −0.5 V vs RHE. Additionally, the hydrogen content increased from PdH0.75 to PdH0.86 with increasing Pb amounts up to 2.9 μg cm−2 at a constant current of −14.5 mA cm−2. For comparison, the same change in hydrogen content from pressurized gas loading experiments would require an increase in hydrogen fugacity from about 6 to 1420 atm. Preliminary analysis concerning the adsorbed hydrogen chemical potential suggests the Pb is affecting the balance between the Volmer, Heyrovsky, and Tafel reaction rates, which changes the hydrogen surface chemical potential, and ultimately controls the hydrogen insertion. Furthermore, the addition of Pb was found to decrease the rate of hydrogen insertion. This work provides a fundamental basis for the future design of metal surfaces yielding enhanced electrochemical hydrogen insertion in Pd and other hydrogen absorbing materials.