Challenges in determining the electrochemically active surface area of Ni-oxides in the oxygen evolution reaction

Effect of Fe on Calcined Ni(OH)2 Anode in Alkaline Water Electrolysis

Ni (hydr)oxide is a promising and inexpensive material for oxygen evolution reaction (OER) catalysts and is known to dramatically increase the activity when used with Fe. Herein, we basified a Ni(II) solution and coated layered Ni(OH)2 on Ni coins to prepare a template with high stability and activity. To evaluate the stability and catalytic activity during high-current-density operation, we analyzed the electrochemical and physicochemical properties before and after constant current (CC) operation. The electrode with a Ni(OH)2 surface exhibited higher initial activity than that with a NiO surface; however, after the OER operation at a high-current density, degradation occurred owing to structural destruction. The activity of the electrodes with a NiO surface improved after the CC operation because of the changes on the electrode-surface caused by the CC operation and the subsequent Fe incorporation from the Fe impurity in the electrolyte. After confirming the improvement in activity due to Fe, we prepared NiFe-oxide electrodes with improved catalytic activity and optimized the Ni precursor and Fe loading solution concentrations. The Ni-Fe oxide electrode prepared under the optimal concentrations exhibited an overpotential of 287 mV at a current density of 10 mA/cm2, and a tafel slope of 37 mV dec−1, indicating an improvement in the OER activity.

Anchoring Ce-modified Ni(OH)2 nanoparticles on Ni-MOF nanosheets to enhances the oxygen evolution performance

Constructing a heterostructure is an efficient strategy to enhance the catalytic activity toward the oxygen evolution reaction (OER). Herein, Ce-modified Ni(OH)₂ nanoparticles are anchored on Ni-MOF nanosheets by the electrodeposition strategy, forming a self-supporting electrode of Ce-m-Ni(OH)₂@Ni-MOF. The Raman spectrum proves that both Ce(OH)₃ and Ce doping exist in Ce-modified Ni(OH)₂ nanoparticles. The heterostructure possesses an open nanosheet structure, with a good interaction between Ni-MOF and Ce-m-Ni(OH)₂, which enables efficient mass/charge transfer and the synergetic effect between Ni and Ce, leading to a high-performance electrocatalyst. Specifically, Ce-m-Ni(OH)₂@Ni-MOF achieves current densities of 50 and 100 mA cm^-2 at low overpotentials of 219 and 272 mV, respectively, and retains high activity for at least 30 h.