Endergonic Hydrogenation at Ambient Conditions Using an Electrochemical Membrane Reactor

Abstract

Here, we determine how the hydrogen loading (<i>x</i>) of an electrochemical palladium membrane reactor (ePMR) varies with electrochemical conditions (e.g., applied current density, electrolyte concentration). We detail how <i>x</i> influences the thermodynamic driving force of an ePMR. These studies are accomplished by measuring the fugacity (<i>P</i>) of hydrogen desorbing from the palladium-hydrogen membrane and subsequently relating <i>P</i> to pressure-composition isotherms to determine <i>x</i>. We find that <i>x</i> increases with both applied current density and electrolyte concentration, but plateaus at a loading of <i>x</i> ≅ 0.92 in 1.0 M H₂SO₄ at -200 mA·cm^-2. The validity of the fugacity measurements is supported experimentally and computationally by: (a) electrochemical hydrogen permeation studies; and (b) a palladium-hydrogen porous flow finite element analysis (FEA) model. Both (a) and (b) agree with the fugacity measurements on the following <i>x</i>-dependent properties of the palladium-hydrogen system during electrolysis: (i) the onset for spontaneous hydrogen desorption; (ii) the point of steady-state hydrogen loading; and (iii) the function describing hydrogen desorption between (i) and (ii). We proceed to detail how <i>x</i> defines the free energy of palladium-hydrogen alloy formation (Δ<i>G</i>(<i>x</i>)_PdH), which is a descriptor for the thermodynamic driving force of hydrogenation at the PdH_<i>x</i> surface of an ePMR. A maximum value Δ<i>G</i>_PdH of 11 kJ·mol^-1 is observed, suggesting that an ePMR is capable of driving endergonic hydrogenation reactions. We empirically demonstrate this capability by reducing carbon dioxide to formate (Δ<i>G</i>_{CO₂/HCO₂H} = 3.4 kJ·mol^-1) at ambient conditions and neutral pH.