Mixed Fe−Ni oxide electrocatalysts for the oxygen evolution reaction in alkaline electrolytes were synthesized using three different approaches: evaporation induced self-assembly, hard templating, and dip-coating. For each synthesis method, a peak in oxygen evolution activity was observed near 10 mol % Fe content, where the mixed metal oxide was substantially more active than the parent metal oxide electrocatalysts. X-ray diffraction (XRD) analysis showed the formation of a mixed NiO/NiFe 2 O 4 phase at low Fe concentrations, and formation of Fe 2 O 3 at compositions above 25 mol % Fe. Raman vibrational spectroscopy confirmed the formation of NiFe 2 O 4 , and did not detect Fe 2 O 3 in the electrocatalysts containing up to 20 mol % Fe. X-ray absorption near edge structure (XANES) showed the Fe in the mixed oxides to be predominantly in the +3 oxidation state. Extended X-ray absorption fine structure (EXAFS) showed changes in the Fe coordination shells under electrochemical oxygen evolution conditions. Temperature programmed reaction spectroscopy showed the mixed oxide surfaces also have superior oxidation activity for methanol oxidation, and that the reactivity of the mixed oxide surface is substantially different than that of the parent metal oxide surfaces. Overall, the NiFe 2 O 4 phase is implicated in having a significant role in improving the oxygen evolution activity of the mixed metal oxide systems.

The effects of varying alkaline electrolyte and electrolyte Fe levels on the performance and active-phase structure of NiOOH thin films for catalysis of the oxygen evolution reaction were studied. An electrolyte effect on catalytic performance was observed. Under purified conditions, current densities followed the trend Cs+ > K+ ≈ Na+ ≈ Li+ at current densities > 1 mA/cm2. Under Fe-saturated conditions, current densities followed the trend K+ ≈ Na+ > Cs+ > Li+ at all current densities. Voltammetry was coupled with Raman spectroscopy for studies in LiOH and CsOH. Raman spectra were fit to Gaussian functions and analyzed quantitatively based on mean peak positions. Both purified and Fe-saturated CsOH promoted slightly lower peak positions than purified and Fe-saturated LiOH, indicating that CsOH promoted a NiOOH active-phase structure with longer Ni–O bonds. Both Fe-saturated CsOH and LiOH promoted slightly lower Raman peak positions than purified CsOH and LiOH, but only for one of the two Raman peaks. These results indicate that Fe promoted an active-phase structure with slightly longer Ni–O bonds. This study shows that the catalytic performance and active-phase structure of NiOOH can be tuned by simply varying the alkaline electrolyte and electrolyte Fe levels.

Iron complexes of tetra-amido macrocyclic ligands are important members of the suite of oxidation catalysts known as TAML activators. TAML activators are known to be fast homogeneous water oxidation (WO) catalysts, producing oxygen in the presence of chemical oxidants, e.g., ceric ammonium nitrate. These homogeneous systems exhibited low turnover numbers (TONs). Here we demonstrate immobilization on glassy carbon and carbon paper in an ink composed of the prototype TAML activator, carbon black, and Nafion and the subsequent use of this composition in heterogeneous electrocatalytic WO. The immobilized TAML system is shown to readily produce O2 with much higher TONs than the homogeneous predecessors.