2 research outputs found
Steering Selectivity in the Four-Electron and Two-Electron Oxygen Reduction Reactions: On the Importance of the Volcano Slope
In the last decade, trends for competing electrocatalytic
processes
have been largely captured by volcano plots, which can be constructed
by the analysis of adsorption free energies as derived from electronic
structure theory in the density functional theory approximation. One
prototypical example refers to the four-electron and two-electron
oxygen reduction reactions (ORRs), resulting in the formation of water
and hydrogen peroxide, respectively. The conventional thermodynamic
volcano curve illustrates that the four-electron and two-electron
ORRs reveal the same slopes at the volcano legs. This finding is related
to two facts, namely, that only a single mechanistic description is
considered in the model, and electrocatalytic activity is assessed
by the concept of the limiting potential, a simple thermodynamic descriptor
evaluated at the equilibrium potential. In the present contribution,
the selectivity challenge of the four-electron and two-electron ORRs
is analyzed, thereby accounting for two major expansions. First, different
reaction mechanisms are included into the analysis, and second, Gmax(U), a potential-dependent
activity measure that factors overpotential and kinetic effects into
the evaluation of adsorption free energies, is applied for approximation
of electrocatalytic activity. It is illustrated that the slope of
the four-electron ORR is not constant at the volcano legs but rather
is prone to change as soon as another mechanistic pathway is energetically
preferred or another elementary step becomes the limiting one. Due
to the varying slope of the four-electron ORR volcano, a trade-off
between activity and selectivity for hydrogen peroxide formation is
observed. It is demonstrated that the two-electron ORR is energetically
preferred at the left and right volcano legs, thus opening a new strategy
for the selective formation of H2O2 by an environmentally
benign route
Temperature-Dependent Kinetic Studies of the Chlorine Evolution Reaction over RuO<sub>2</sub>(110) Model Electrodes
Ultrathin
single-crystalline RuO<sub>2</sub>(110) films supported
on Ru(0001) are employed as model electrodes to extract kinetic information
about the industrially important chlorine evolution reaction (CER)
in a 5M concentrated NaCl solution under well-defined electrochemical
conditions and variable temperatures. A combination of chronoamperometry
(CA) and online electrochemical mass spectrometry (OLEMS) experiments
provides insight into the selectivity issue: At pH = 0.9, the CER
dominates over oxygen evolution, whereas at pH = 3.5, oxygen evolution
and other parasitic side reactions contribute mostly to the total
current density. From temperature-dependent CA data for pH = 0.9,
we determine the apparent free activation energy of the CER over RuO<sub>2</sub>(110) to be 0.91 eV, which compares reasonably well with the
theoretical value of 0.79 eV derived from first-principles microkinetics.
The experimentally determined apparent free activation energy of 0.91
eV is considered as a benchmark for assessing future improved theoretical
modeling from first principles