RESUMO
Nickel iron hydroxides (NiFeOH) have been drawing enormous attention as effective catalysts for oxygen evolution reaction (OER), a key process in water splitting. Herein, we report that negatively charged iron(III) hydroxide colloidal particles, can significantly enhance the OER activity of NiFeOH in alkaline media. NiFeOH is grown on nickel foam in a supersaturated iron(III) salt solution, which also contains a high content of Fe(OH)3 colloidal nanoparticles, forming free-standing NiFeOH@Cx electrodes (with x being the Fe(OH)3 concentration). The interface between NiFeOH and Fe(OH)3 colloidal particles, as manifested by the unique volcano-like holes on the NiFeOH@Cx surface, is likely the OER active sites. In comparison to Fe(OH)3-free NiFeOH, NiFeOH@C1000 exhibits a 40-fold enhancement of the OER activity, confirming the significant effect of Fe(OH)3 colloidal nanoparticles in boosting the OER activity, likely as a result of enhanced charge transfer from Ni2+ to Fe3+ that facilitates the adsorption of key reaction intermediates. Furthermore, by coupling the free-standing NiFeOH@C1000 electrode with commercial Pt/C, full water splitting can occur and reach a current density of 10 mA cm-2 under a cell voltage of 1.51 V, which is lower than that (1.59 V) based on noble metal catalysts of RuO2 + Pt/C.
RESUMO
In recent years, low-cost, non-noble metal-based and stable catalysts have gained attention for the development of clean energy devices. Additionally, the synthesis of materials that can exhibit more than one electrocatalytic reaction is notable. In this work, stepwise electrodeposited nickel-iron hydroxide nanoarrays are investigated as anode electrocatalysts with enhanced performance towards the oxidation of water, urea, and hydrazine. The stepwise electrodeposited nickel-iron hydroxide (NiFe(OH)2-SD/NF) electrodes show excellent electrocatalytic activity and stability for the oxygen evolution reaction (OER) with a low potential of 1.45â¯V (vs RHE) at a current density of 10â¯mAâ¯cm-2. These electrodes further display excellent catalytic activity towards the urea oxidation reaction (UOR) and hydrazine oxidation reaction (HzOR) with potentials lower than 1.32â¯V (vs RHE) and 0.06â¯V (vs RHE), respectively. Owing to synergistic effects, a porous structure for mass transport leads to excellent electrocatalytic performance. This non-precious-metal nickel-iron hydroxide, prepared by a simple synthesis approach, is promising for hybrid water electrolysis applications and the development of environmentally friendly clean energy reactions.
RESUMO
Seawater is an abundant water resource on our planet and its direct electrolysis has the advantage that it would not compete with activities demanding fresh water. Oxygen selectivity is challenging when performing seawater electrolysis owing to competing chloride oxidation reactions. In this work we propose a design criterion based on thermodynamic and kinetic considerations that identifies alkaline conditions as preferable to obtain high selectivity for the oxygen evolution reaction. The criterion states that catalysts sustaining the desired operating current with an overpotential <480â mV in alkaline pH possess the best chance to achieve 100 % oxygen/hydrogen selectivity. NiFe layered double hydroxide is shown to satisfy this criterion at pHâ 13 in seawater-mimicking electrolyte. The catalyst was synthesized by a solvothermal method and the activity, surface redox chemistry, and stability were tested electrochemically in alkaline and near-neutral conditions (borate buffer at pHâ 9.2) and under both fresh seawater conditions. The Tafel slope at low current densities is not influenced by pH or presence of chloride. On the other hand, the addition of chloride ions has an influence in the temporal evolution of the nickel reduction peak and on both the activity and stability at high current densities at pHâ 9.2. Faradaic efficiency close to 100 % under the operating conditions predicted by our design criteria was proven using inâ situ electrochemical mass spectrometry.