Quantifying the Resistive Losses of the Catalytic Layers in Anion-Exchange Membrane Fuel Cells.

The existing gap in the ability to quantify the impacts of resistive losses on the performance of anion-exchange membrane fuel cells (AEMFCs) during the lifetime of their operation is a serious concern for the technology. In this paper, we analyzed the ohmic region of an operating AEMFC fed with pure oxygen followed by CO2 -free air at various operating currents, using a combination of electrochemical impedance spectroscopy (EIS) and a novel technique called impedance spectroscopy genetic programming (ISGP). Presented here for the first time in this work, we isolated and quantified the individual effective resistance (Reff ) values occurring in the AEMFC and their influence on performance as operating conditions change. We believe that this first work is vital to help distinguish the influence of the individual catalytic and mass-transfer processes in this technology thereby providing valuable data to the AEMFC community, with potentially wider applicability to other electrochemical devices where individual physical processes occur simultaneously and need to be sequestered for deeper understanding.

Optimization of Ni-Co-Fe-Based Catalysts for Oxygen Evolution Reaction by Surface and Relaxation Phenomena Analysis.

Trimetallic double hydroxide NiFeCo-OH is prepared by coprecipitation, from which three different catalysts are fabricated by different heat treatments, all at 350 °C maximum temperature. Among the prepared catalysts, the one prepared at a heating and cooling rate of 2 °C min-1 in N2 atmosphere (designated NiFeCo-N2 -2 °C) displays the best catalytic properties after stability testing, exhibiting a high current density (9.06 mA cm-2 at 320 mV), low Tafel slope (72.9 mV dec-1 ), good stability (over 20 h), high turnover frequency (0.304 s-1 ), and high mass activity (46.52 A g-1 at 320 mV). Stability tests reveal that the hydroxide phase is less suitable for long-term use than catalysts with an oxide phase. Two causes are identified for the loss of stability in the hydroxide phase: a) Modeling of the distribution function of relaxation times (DFRT) reveals the increase in resistance contributed by various relaxation processes; b) density functional theory (DFT) surface energy calculations reveal that the higher surface energy of the hydroxide-phase catalyst impairs the stability. These findings represent a new strategy to optimize catalysts for water splitting.

Tri-Functional Double Perovskite Oxide Catalysts for Fuel Cells and Electrolyzers.

Perovskite oxides are at the forefront of the race to develop catalysts/electrodes for fuel cells and electrolyzers. This work presents trifunctional properties of the double-perovskite oxide PrBa0.5 Sr0.5 Co1.5 Fe0.5 O5+δ and the PrBa0.5 Sr0.5 Co1.5 Fe0.5 O5+δ -Ag composite prepared by the glycine nitrate process. The electrocatalytic studies reveal that the Ag-based composite is an excellent catalyst for both oxygen evolution (OER) and hydrogen evolution reactions (HER) in alkaline solution. The electrochemical impedance spectroscopy analysis through distribution function of relaxation times (DFRT) suggests that the improved activity originates from the suppression of resistance contributed by various relaxation processes. The oxygen reduction reaction (ORR) kinetics in these oxide-based cathodes has been studied by performing symmetric-cell measurements at high temperatures using both oxygen-ion and proton-conducting cells. DC bias dependence of charge-transfer processes, oxygen-surface kinetics, polarization resistances, and activation energies are revealed by DFRT studies. Ag addition in PrBa0.5 Sr0.5 Co1.5 Fe0.5 O5+δ leads to enhanced kinetics of OER, HER, and ORR.