Ni‐Based Electrocatalyst for Water Oxidation Developed In‐Situ in a HCO₃⁻/CO₂ System at Near‐Neutral pH

Abstract

that combine the anodic water oxidation products (protons and electrons) with a CO 2 reduction catalytic system at the cathode to generate liquid fuels such as formic acid or methanol ( Scheme 1 ). [16][17][18] Such a scheme represents a potentially attractive route for direct conversion of solar energy to produce carbon-based liquid fuels on a large scale for power generation or transportation applications and avoids the problem related to hydrogen storage and safety. [ 19 ] For the development of such systems, self-assembling and self-repairing water oxidation electrocatalysts that operate in a carbon dioxide enriched environment with good catalytic stability and effi ciency will be helpful. [ 18,20 ] A CO 2 saturated solution of a bicarbonate system (pH = 6.7-6.8) is an optimal environment to reduce and convert CO 2 into formic acid or desired products. [ 20,21 ] We report here that it allows for the in situ formation of an effi cient water oxidation electrocatalyst from easily available nickel(II) in a bicarbonate electrolyte. The nickel-bicarbonate type electrocatalysts self-assemble on a glassy carbon (GC) anode or ITO (indium tin oxide) surface via anodic electro-deposition from a Ni 2+ solution with (Ni/ HCO 3 /CO 2 ) or without (Ni/HCO 3 ) carbon dioxide saturation. The electrocatalytic system exhibits a remarkable activity for anodic oxygen evolution in a CO 2 enriched electrolyte. This new catalytic design also offers a direct solution for the extraction of protons and electrons from water for the generation of liquid fuels in combination with a suitable CO 2 reduction electrocatalytic module. [ 2,21 ] These nickel derived electrocatalysts (Ni/ HCO 3 /CO 2 and Ni/HCO 3 ) also show excellent performance in other neutral (phosphate) or near-neutral (borate) buffers and carbonate solution (pH > 10). Moreover, the Ni-derived electrocatalysts presented here do not require the proton abstracting phosphate or borate buffers for electrodeposition and for anodic water oxidation as recently shown to be essential for the generation and activity of Co-Pi and Ni-Bi based catalysts. [ 9,23,24 ] The Ni/HCO 3 /CO 2 type electrocatalyst is readily formed at the CO 2 /HCO 3 − phase boundary in the Pourbaix diagram on a freshly polished glassy carbon surface while scanning the potential between 0.97 and 0.85 V (vs. NHE) in a CO 2 saturated 0.2 M bicarbonate electrolyte near-neutral solution that contains 1.0 m M Ni 2+ . After the formation of the electrocatalyst fi lm on a glassy carbon anode, oxidative currents appear at about 1.05 V (vs. NHE) in the cyclic voltammetry (CV) curves ( Figure 1 ). This is ascribed to the formation of oxides of the nickel metal ions in carbon dioxide enriched bicarbonate electrolyte system. [22][23][24][25][26] The oxidative current wave is followed by a sharp catalytic current rise at >1.3 V (vs. NHE) that is accompanied by the generation of tiny oxygen bubbles at the anode. The oxygen evolution current density rises with further potential increase. The backward sweep generates the corresp...

Efficient electrochemical water oxidation in neutral and near-neutral systems with a nanoscale silver-oxide catalyst

“…[17, [20][21] The nanoscale AgOx-NP presented here displays very high catalytic stabilities in metal-ions free neutral or near-neutral bicarbonate, phosphate and borate buffer solutions as compared to recently reported CoOx and NiOx based heterogeneous electrocatalysts. Silver ions quickly precipitate in neutral pH solution.…”

“…[20][21] So, the continuous presence of Co or Ni ions in phosphate or borate aqueous buffers is necessary for the sustained activities of these catalysts. These results lead us to test the performance of AgOx-NP for anodic water oxidation in metal ions free phosphate (pH=7.1) and borate (pH=9.2) solutions.…”

“…[17, [20][21] Also, the presence of metal ions in the electrolyte system during water electrolysis may induce its reduction and metal deposition at the reduction site which can block and reduce the active catalyst performance. [15,19] Therefore, there is a need to develop low-cost heterogeneous electrocatalysts which remain stable and effective for long-term catalytic operation in metal-ions free neutral or near-neutral aqueous systems.…”

Atomically monodisperse nickel nanoclusters as highly active electrocatalysts for water oxidation

“…[3][4][5] The electrons and protons released from water oxidation reaction can also be combined with CO 2 to reduce it directly into liquid fuels like methanol or formic acid. 6,7 Therefore, this concept provides a route to renewable and alternative energy supplies to be obtained from abundant water and enormous sunlight available on earth. 6,8 But water oxidation is an energy intensive reaction and requires a minimum of 1.23 V (vs RHE) for its initiation.…”

“…6,7 Therefore, this concept provides a route to renewable and alternative energy supplies to be obtained from abundant water and enormous sunlight available on earth. 6,8 But water oxidation is an energy intensive reaction and requires a minimum of 1.23 V (vs RHE) for its initiation. Thus, the development of efficient water oxidation electrocatalysts (WOEc) is considered as a formidable challenge.…”