NiFe Alloy Integrated with Amorphous/Crystalline NiFe Oxide as an Electrocatalyst for Alkaline Hydrogen and Oxygen Evolution Reactions

Abstract

The rational design of efficient and low-cost electrocatalysts based on earth-abundant materials is imperative for large-scale production of hydrogen by water electrolysis. Here we present a strategy to prepare highly active catalyst materials through modifying the crystallinity of the surface/interface of strongly coupled transition metal−metal oxides. We have thermally activated the catalysts to construct amorphous/crystalline Ni−Fe oxide interfaced with a conductive Ni− Fe alloy and systematically investigated their electrocatalytic performance toward the hydrogen evolution and oxygen evolution reactions (HER and OER) in alkaline solution. It was found that the Ni−Fe/oxide material with a crystalline surface oxide phase showed remarkably superior HER activity in comparison with its amorphous or poorly crystalline counterpart. In contrast, interestingly, the amorphous/poorly crystalline oxide significantly facilitated the OER activity in comparison with the more crystalline counterpart. On one hand, the higher HER activity can be ascribed to a favorable platform for water dissociation and H−H bond formation, enabled by the unique crystalline metal/oxide structure. On the other hand, the enhanced OER catalysis on the amorphous Ni−Fe oxide surfaces can be attributed to the facile activation to form the active oxyhydroxides under OER conditions. Both are explained based on density functional theory calculations. These results thus shed light onto the role of crystallinity in the HER and OER catalysis on heterostructured Ni−Fe/oxide catalysts and provide guidance for the design of new catalysts for efficient water electrolysis.

Highly Active Nanostructured NiCoMo-Based Catalyst for Oxygen Evolution in Anion-Exchange Membrane Water Electrolysis

“…For all catalysts, double peaks were clearly observed at higher potentials, where the first peak is located at 460−480 cm −1 and the second peak is located at 540−560 cm −1 , corresponding to the E g Ni−O bending vibration and the A 1g Ni−O stretching vibration, respectively, which can indicate the presence of NiOOH. 16 At each potential, the peak intensity for Ni 0.8 Co 0.12 Mo 0.08 O (Figure 2g) was significantly higher than those for Ni 0.8 Co 0.2 O (Figure 2h) and NiO (lowest) (Figure 2i), and thus the superior OER activity of Ni 0.8 Co 0.12 Mo 0.08 O can be correlated with the increased formation of NiOOH species under anodizing conditions. In addition, the peak continuously shifted to lower wavenumbers (10−20 cm Given the outstanding catalytic performance of Ni 0.8 Co 0.12 Mo 0.08 O, a real AEM water electrolyzer using Ni 0.8 Co 0.12 Mo 0.08 O as the anode catalyst (commercial Pt/C catalyst as the cathode) was tested with an eye toward commercial use.…”

“…To investigate the surface chemical states of Ni 0.8 Co 0.12 Mo 0.08 O, XPS was performed. As shown in Figure k, three main Ni species were detected: metallic Ni (852.2 eV), Ni 2+ (855.1 eV), and Ni 3+ (855.9 eV). , Because the penetration depth sampled by XPS is several nanometers (tens of atomic layers) and the Ni 0.8 Co 0.12 Mo 0.08 O catalyst has experienced O 2 exposure, it is reasonable to assume that most of the Ni in the topmost layer has been oxidized and metallic Ni metal is present below this surface, as revealed in the HRTEM image. The Co also exhibited metallic and oxide features, as evidenced by peaks centered at 777.7 eV (Co 0 ), 779.8 eV (Co 3+ ), and 780.7 eV (Co 2+ ) .…”

“…Similarly, the Mo spectrum [each state has two spectral lines assigned to the Mo 3d 5/2 (low binding energy) and 3d 3/2 (high binding energy) spin-orbital components] indicated the coexistence of Mo metal and oxide species of Mo δ+ (0 < δ < 4), Mo 4+ , and Mo 6+ . Surface oxidation can also be revealed from the O 1s spectrum, showing characteristic peaks for metal–oxygen (M–O) bonds, hydroxyl (OH), and adsorbed H 2 O. , Compared with Ni 0.8 Co 0.2 O (Figure S1j–l) and NiO (Figure S2i,j), Ni 0.8 Co 0.12 Mo 0.08 O seems to have a lower degree of oxidation considering the larger peaks for metallic Ni and Co and the smaller peak for M–O, likely due to electronic interaction between Mo and Ni/Co–O, as elucidated later, which may be responsible for the superior electrical conductivity (Figure o), even higher than that of IrO x powder …”

“…The excellent OER catalytic activity of Ni 0.8 Co 0.12 Mo 0.08 O is thought to be partially attributable to the facile formation of NiOOH active sites, as revealed from the marked peak appearing at ca. 1.4 V [Ni­(OH) 2 + OH – → NiOOH + H 2 O + e – ]. , The CV, ECSA, and OER polarization curves at elevated temperatures were also measured in terms of the actual operating temperatures of AEMWE, as shown in Figures S3d–l, S4, and S5, respectively. Figure f shows the Arrhenius plot for j s at 1.6 V. At all tested temperatures, Ni 0.8 Co 0.12 Mo 0.08 O maintained the highest j s , followed by Ni 0.8 Co 0.2 O and NiO.…”

“…Figure 2a as well as most other transition-metal oxide-based OER electrocatalysts reported in the literature. 19 From the Tafel plots (Figure 2c), the Tafel slope was determined to be 48 mV dec 15,16 The CV, ECSA, and OER polarization curves at elevated temperatures were also measured in terms of the actual operating temperatures of AEMWE, as shown in Figures S3d−l, S4, and S5, respectively. Figure 2f shows the Arrhenius plot for j s at 1.6 V. At all tested temperatures, Ni 0.8 Co 0.12 Mo 0.08 O maintained the highest j s , followed by Ni 0.8 Co 0.2 O and NiO.…”