Crafting Robust And Bifunctional Electrocatalysts For Electrochemical Water Splitting

Electrochemical water-splitting into hydrogen and oxygen is widely recognized as a green and effective approach to transform and store renewable energy. As such, it has garnered increasing interest as it enables a green energy future.

Water-splitting contains two half-reactions, that is, anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER). Unfortunately, these two half-reactions suffer from sluggish kinetics, and electrocatalysts are required to reduce overpotentials and achieve a desirable reaction rate.

Recently, precious Pt-, Ir-, and Ru-based catalysts have shown impressive performance for HER and OER [1]. However, their high cost and scarcity prevent them from viable commercial applications. In this context, efficient electrocatalysts based on earth-abundant elements, including transition metal (TM)-based materials [2] and heteroatoms-doped carbon materials [3], have been developed for HER or OER.

Nevertheless, despite significant advances in the synthesis of monofunctional electrocatalysts, effective methods to develop bifunctional electrocatalysts possessing both excellent HER and OER performances for overall water-splitting are comparatively few and limited in scope because of different reaction centers required by HER and OER, respectively.

Recently, a controlled incorporation of two different metal species into one single nanostructure to accelerate the activation of reactants has emerged as a powerful strategy for constructing superior electrocatalysts. [4] For example, bimetallic NiMo catalyst with moderate metal-hydrogen (M-H) strength displays a higher HER activity than single Ni and Mo, as Mo alone has too strong M-H binding strength while Ni alone has too weak M-H binding strength. [5] However, the NiMo catalysts carry low activity for OER with an overpotential of 368 mV at 10 mA cm^-2. [6]

On the other hand, incorporating Fe into Ni-based catalysts is found to be able to dramatically enhance the OER performance due to the stronger affinity of bimetallic NiFe to OH^– and the intermediates of OER. [7] Thus, it is plausible that the incorporation of Fe into bimetallic NiMo catalyst may improve the performance of OER, and the resulting new catalyst may serve as a bifunctional electrocatalyst for overall water splitting.

Because of the low conductivity and stability of TM materials, encapsulating TM nanoparticles (NPs) into graphene layers is crucial for developing robust TM-based electrocatalysts. Herein, we craft MoC₂-doped NiFe alloy NPs encapsulated within a-few-layer-thick N-doped graphene (denoted NG-NiFe@MoC₂, where the atomic ratio of Ni:Fe is 0.36:0.64, that is, Ni_0.36Fe_0.64) by one-step annealing of binary NiFe Prussian blue analogs (denoted NiFe-PBA) and Mo⁶⁺-grafted polyvinylpyrrolidone (PVP) hybrid precursors (denoted NiFe-PBA/PVP) at high temperature.

The unique hybrid precursors composed of PVP-encapsulating NiFe-PBA NPs and grafted Mo⁶⁺ enable the controlled and homogeneous carbonization reaction, meanwhile the outer PVP overlayers avoid the aggregation of the resulting NiFe alloy NPs and MoC₂ dopants. Consequently, the advantageous compositions (i.e., MoC₂ dopants and NiFe NPs, which synergistically play the key role in HER and OER) and unique architecture (i.e., a-few-layer-thick N-doped graphene-encapsulating shell) render the optimized NG-NiFe@MoC₂ nanohybrids with highly active and stable electrocatalysts for either HER or OER separately.

More importantly, they can also be exploited as both anodic and cathodic electrocatalysts for overall water splitting to achieve a current density of 10 mA cm^-2 at a voltage of 1.53 V with impressive durability of 10 h. As such, they emerge as promising bifunctional electrocatalysts to substitute precious metals for efficient overall water splitting.

These findings are described in the article entitled Crafting MoC₂-doped bimetallic alloy nanoparticles encapsulated within N-doped graphene as roust bifunctional electrocatalysts for overall water splitting, recently published in Nano Energy. This work was conducted by Qi Hu, Xiufang Liu, Bin Zhu, Liangdong Fan, Xiaoyan Chai, Qianling Zhang, Jianhong Liu, and Chuanxin He from Shenzhen University, PR China, and Zhiqun Lin from the Georgia Institute of Technology, USA.

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