Energy and climate change became one of the biggest preoccupations attracting the common interest of science, engineering and society. In this important field, electrochemistry provides a bridge for efficient conversion of chemical to electrical energy, i.e. in fuel cells. Oxygen reduction reaction (ORR) is a complex reaction and an effective and inexpensive catalyst that can compete with or exceed the efficiency of platinum remains an ongoing challenge. Nature provides an efficient solution to this problem, i.e., the O2 in our atmosphere is biogenic - the result of a catalytic process in the photosynthetic machinery of green plants. With increasing atmospheric oxygen levels higher life-forms with oxygen-metabolizing enzymes evolved. These naturally occurring oxygen activation catalysts are considered as viable substitutes for precious metals in ORR catalysts for fuel cells. The catalytically active sites, mostly composed of first-row transition metal atoms and clusters, share a common and important feature with classical inorganic catalysts, i.e., the presence of coordinatively unsaturated sites. New catalytic materials can be designed that are inspired by both natural and classical ORR catalysts by using organic molecules with specific molecular backbones (adequately separating the active sites) and selected functional groups (electron withdrawing or donating) to coordinate transition metal atoms or clusters directly on surfaces.
Two-dimensional metal-organic coordination networks (2D−MOCNs, Fig. 1), i.e., organic molecules and metal centers self-assembled on surfaces under well-controlled conditions, constitute a promising route to fabricate functional low-dimensional bio-inspired architectures. Here we show that rationally designed 2D-MOCN exhibit substantial electrocatalytic activity for the oxygen reduction reaction in alkaline media using trimesic acid (TMA) and 5,5´-bis(4-pyridyl)(2,2´-bipirimidine) (PBP) as organic linker. The presence of precisely defined and coordinatively unsaturated metal atoms (Fe or Mn) opens the possibility to explore the electrocatalytic properties of these nanostructures. Here, we explore the ability of the networks to catalyse the reduction of O2 in alkaline media. On Au(111) the reduction of O2 occurs via a 2e− pathway with the final product H2O2 (HO2− in basic media) according to the following mechanism:
O2 + H2O + 2e− → HO2− + OH− Eq.(1)
Figure 2(a) shows the polarization curve for the bare Au(111) (grey line) in O2 saturated 0.1M NaOH solution. The weak shoulder at -0.20V with a current density around 0.25mAcm-2 corresponds to the reduction of O2 to H2O2 (Eq.1). The polarization curves for TMA−Fe and PBP−Fe acquired under the same conditions are presented in Fig. 2(a) (red and green lines). TMA−Fe and PBP−Fe polarization curves differ significantly from bare Au(111). First, the shoulder at -0.2V shows an onset shifted ≈0.05V to lower overpotentials and has tripled current density (0.75mAcm-2) compared to bare Au(111). Second and much more striking is the presence of a second peak at about -0.80V with a large current density. The second shoulder can be assigned to the electroreduction of H2O2 to H2O (OH− in basic media) according to the reaction:
HO−2 + H2O + 2e− → 3 OH− Eq.(2)
For TMA−Fe and PBP−Fe networks we propose that the first wave at -0.2V in the polarization curve is related to reduction of O2 towards H2O2 mediated by both exposed Au surface areas and oxygen atoms present in the MOCNs. In fact, the polarization curve for pure TMA or PBP on Au(111) shows a small enhancement of the current density compared to bare Au(111). The presence of Fe in the TMA and PBP networks explains the second wave and a mechanism combining equations 1 and 2 is proposed.
The key role of the unsaturated metal centers in the electrocatalytic reduction of oxygen becomes evident when going from Fe- to Mn-based MOCNs. Figure 2(b) shows the polarization curve for TMA−Mn (blue solid line) in O2 saturated 0.1M NaOH solution. A distinctly different behavior is observed in comparison to bare Au(111) and TMA−Fe, PBP−Fe networks The first wave is shifted to a lower potential of -0.15V with an onset potential lowered by 0.1V (see inset in Fig. 2(b)) and presents a high current density (1mAcm-2), which is also better defined. Moreover, the wave at -0.8V found in both PBP−Fe and TMA−Fe ORR polarization curve is not present for the TMA−Mn network. A possible reason could be that the potential reduction of small amounts of the generated H2O2 is obscured by the onset of the hydrogen evolution reaction. We conclude that the TMA-Mn network reduces O2 directly to H2O trough a 4e− pathway according to:
O2 + 2H2O + 4e− → 4 OH− Eq.(3)
These results emphasize the importance of the complexation of the metal atoms and hint at the tremendous potential of this approach. On the one hand, the chemical activity of the metal centers is determined by both the nature of the metal ion and its coordination shell. Second, the ligation separates the unsaturated metal atoms preventing catalytic deactivation. Thus, the specific design of ligands allows tuning the catalytic activity of metal adatoms for a desired chemical conversion. This work provides a proof of concept that surface engineered metal-organic complexes and networks that display structural resemblance with enzyme active sites have a high potential for heterogeneous catalytic chemical conversions. The possibility to create novel and highly stable functional 2D coordination complexes at surfaces using specifically designed organic molecules and transition metal centers taking inspiration from nature opens up a new route for the design of a new class of nanocatalyst materials with promising applications in electrocatalysis.