A NiCo₂O₄ Shell on a Hollow Ni Nanorod Array Core for Water Splitting with Enhanced Electrocatalytic Performance

Abstract

Nickel cobaltite (NiCo 2 O 4 ) nanostructures have been considered as promising bifunctional electrocatalyst materials for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline electrolyte. However, the activity of these materials toward overall water splitting is still unsatisfactory because a larger voltage is needed to drive the reactions. This study shows that more-efficient bifunctional catalysts based on NiCo 2 O 4 materials can be obtained by adopting an integrated composite electrode with robust hollow structures. This novel electrode is constructed by the growth of hollow Ni@NiCo 2 O 4 nanorod arrays on threedimensional (3D) Ni foam. The interlayer of Ni hollow nanorods is conformally covered by NiCo 2 O 4 nanosheets, providing large surface area and fast electron transport. Benefiting from these integrated characteristics, the hollow Ni@NiCo 2 O 4 electrode as an oxygen evolution electrocatalyst can deliver a low overpotential of 270 mV at 10 mA cm À2 and present excellent electrochemical durability. Meanwhile, the HER performance also impressive, with an overpotential of about 87 mV at 10 mA cm À2. As a result, the hollow Ni@NiCo 2 O 4 electrode as a bifunctional catalyst can achieve a current density of 10 mA cm À2 with a voltage of only 1.58 V, showing promise for high performance yet cheap electrocatalysts for the overall water splitting reactions.

Recent advances in transition metal-based electrocatalysts for alkaline hydrogen evolution

“…48 However, the electrocatalytic ability of such pristine TMOs in HER is still unsatisfactory. To break the bottleneck, several approaches have been developed, including doping, 52,134,135 defect (oxygen vacancy) engineering, 50,51 crystal-plane engineering, 49 compositing 48,124,126,136 and hierarchical structuring. 137 146 Figure 8f).…”

Sea coral-like NiCo2O4@(Ni, Co)OOH heterojunctions for enhancing overall water-splitting

“…22,23 For example, Wang et al reported a novel hollow core-shell Ni@NiCo 2 O 4 to improve the electronic conductivity of NiCo 2 O 4 owing to the hollow Ni directly attached to the conductive Ni foam. The hollow core-shell Ni@NiCo 2 O 4 electrode showed excellent catalytic activity with a current density of 10 mAcm −2 at a bias of 1.58 V. 24 These are not enough to reveal the catalytic nature and helpful to ARTICLE Journal Name 2 | J. Name., 2012, 00, [1][2][3] This journal is © The Royal Society of Chemistry 20xx…”

Single Step Grown NiFe Sponges as Efficient Water Splitting Electrocatalysts in Alkaline Medium

“…The NiFe5 sponge exhibits the lowest Tafel slope of 82 mV dec −1 among the various NiFe sponge and close to the value of the Pt/C catalyst (52 mV dec −1 ), indicating a faster HER catalytic kinetics. The Tafel slope and η 10 values of NiFe5 electrode are also on par with the several recently reported HER electrocatalysts (Table ) . Further, in order to evaluate the electron‐transfer kinetics of the electrodes, EIS measurements were performed.…”

“…Among the various alternatives, hydrogen (H 2 ) is considered as a promising and viable resource for future due to its high specific energy density and almost zero pollution on combustion, and recent studies showed its techno‐economical viability too . Among the various approaches of H 2 generation, electrochemical water splitting has attracted the attention of researchers and is considered as one of the most effective and eco‐friendly approaches than other conventional H 2 generation methods . The process of splitting water involves two‐half cell processes namely the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), which involve multi‐electron transfer reactions.…”

“…The process of splitting water involves two‐half cell processes namely the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), which involve multi‐electron transfer reactions. Considering the kinetics of these reactions, OER is more sluggish because of the involvement of four electron ‐ proton coupled reaction with multiple intermediates (eg., MOH, M−O, MOOH and O 2 ), where the HER is comparatively faster due to its two electron process with only one reaction intermediate . Metal based catalysts such as Ir/Ru oxides (for OER) and Pt and its alloys (for HER) have been regarded as the benchmarked ones .…”