Ir nanoclusters on ZIF-8-derived nitrogen-doped carbon frameworks to give a highly efficient hydrogen evolution reaction

Abstract

The precise change of the electronic structure of active metals using low-active supports is an effective way of developing high-performance electrocatalysts. The electronic interaction of the metal and support provides a flexible way of optimizing the catalytic performance. We have fabricated an efficient hydrogen evolution reaction (HER) electrocatalyst, in which Ir nanoclusters are uniformly loaded on a nitrogen-doped carbon framework (Ir@NC). The synthesis process entails immersing an annealed zeolitic imidazolate framework-8 (ZIF-8), prepared at 900 °C as a carbon source, into an IrCl3 solution, followed by a calcination-reduction treatment at 400 °C under a H2/Ar atmosphere. The three-dimensional porous structure of the nitrogen-doped carbon framework exposes more active metal sites, and the combined effect of the Ir clusters and the N-doped carbon support efficiently changes the electronic structure of Ir, optimizing the HER process. In acidic media, Ir@NC has a remarkable HER electrocatalytic activity, with an overpotential of only 23 mV at 10 mA cm−2, an ultra-low Tafel slope (25.8 mV dec−1) and good stability for over 24 h at 10 mA cm−2. The high activity of the electrocatalyst with a simple and scalable synthesis method makes it a highly promising candidate for the industrial production of hydrogen by splitting acidic water.

1. Introduction

Hydrogen energy with high energy density is a clean and sustainable energy resource which can be easily transported and stored, allowing for flexibility in energy distribution[1–3]. Additionally, hydrogen fuel cells have high energy conversion efficiency and produce only water as a byproduct, minimizing environmental impact[4–5]. However, obtaining green hydrogen via water electrolysis is largely hindered by its low energy efficiency. Recently, acidic electrolyzers, generally operate at lower voltages and have higher energy efficiency than alkaline counterparts, have become interesting alternatives[6–8]. Furthermore, acidic electrolyzers also exhibit faster reaction kinetics, enabling higher current densities and overall improved performance[9]. However, one major challenge in acidic catalytic systems is the stability of the catalysts. Most of the catalysts that can be utilized in alkaline conditions, especially the non-noble metal catalysts, are severely degraded in acidic electrolytes[10–12]. This instability can lead to decreased catalytic activity and shortened catalyst lifespan[13–14]. Addressing catalyst stability is crucial for the development and commercialization of efficient and durable electrocatalytic hydrogen production in acidic conditions[15].

Despite the low abundance and high cost, precious metals like Pt, Ir and Ru are still the main electrocatalysts that are extensively utilized in acidic electrolytes[16–17]. For instance, You et al. reported Ir nanoparticles anchored cucurbit [6] uril, which exhibited slightly worse HER performance (η10 = 54 mV) than Pt/C in 0.5 mol L−1 H2SO4[18]. Song et al. reported that a material with Ru dispersed on CoP nanoparticles exhibits superior HER catalytic activity, with a low overpotential of 49 mV to achieve 10 mA cm−2 in 0.5 mol L−1 H2SO4 solution, by lowering the energy barrier of proton-coupled electron transfer[19]. Drouet et al. reported a porous Ru nanomaterial, which needed an overpotential of 83 mV to deliver 10 mA cm−2 in 0.5 mol L−1 H2SO4 solution, owing to the porous structure of the material[20]. Although great progress has been made in this direction, methods for regulating the electronic structure while simultaneously increasing the utilization efficiency of precious metal atoms is still challenging[21].

Nanoscaling of material dimensions plays a critical role in enhancing the specific surface area of catalysts to provide more active sites[22]. The nano-catalysts often exhibit distinct and impressive properties compared to bulk materials. In particular, the metal nanoclusters with extremely high specific surface area and a lower surface metal-metal coordination number, improve the surface-t...