Recent progress on earth abundant hydrogen evolution reaction and oxygen evolution reaction bifunctional electrocatalyst for overall water splitting in alkaline media

With increasing global demand for energy, rapid depletion of fossil fuels and intensification of environmental concerns, exploring clean and sustainable energy carriers to replace fossil fuel is becoming critical. Among the various alternatives, hydrogen has been intensively regarded as a promising energy carrier to fulfill the increasing energy demand due to its large energy density per unit mass and eco-friendly production possibilities. However, hydrogen does not exist in molecular structure in nature, and it is essential to obtain efficient and sustainable H2 production technologies. Alkaline water electrolysis is an effective, clean and sustainable process to produce high-quality hydrogen. In this process, highly active electrocatalysts for the hydrogen evolution reaction (HER) are required to accelerate the sluggish kinetics and lower the overpotentials (η) for efficient hydrogen evolution. To date, a noble metal, platinum (Pt), is the state-of-art electrocatalyst for HER. However, exploration of alternative electrocatalysts with low cost and excellent electrocatalytic activity is of vital importance to realize large-scale hydrogen production through water electrolysis. Generally, an electrochemically active catalyst should have an optimal hydrogen adsorption free energy to allow efficient catalytic hydrogen adsorption/desorption. In alkaline solution, dissociation of water onto the electrocatalyst determines the overall HER efficiency. This thesis focuses on rational design and synthesis of different earth-abundant electrocatalysts for electrocatalytic HER in alkaline media. Through facile anion or cation doping strategies, electrocatalysts with abundant accessible active sites, enhanced electronic conductivity and accelerated HER kinetics have been systematically fabricated, characterized and evaluated. First, an efficient HER electrocatalyst in alkaline media was fabricated by incorporating sulfur atoms into a cobalt (hydro)oxide crystal structure. The resultant catalyst exhibits a remarkably enhanced HER activity with a low-overpotential of 119 mV at 10 mA/cm2 and an excellent durability. The results suggest that cobalt hydroxide benefits water adsorption and cleavage, while the negatively charged sulfur ligands facilitate hydrogen adsorption and desorption on the surface of electrocatalysts, leading to significantly promoted Volmer and Heyrovsky steps for HER in alkaline media. Second, exploring bifunctional electrocatalysts which can simultaneously accelerate the HER and oxygen evolution reaction (OER) activities plays a key role in alkaline water splitting. Here, sulfur atoms were incorporated into the mixed transition metal hydroxide with high OER performance to render excellent HER activity. The enhanced catalytic activity towards HER was confirmed by a synergistic effect between the retained metal hydroxide host and the incorporated sulfur atoms. In addition, the full water splitting electrolyzer equipped with fabricated bifunctional electrocatalysts as anode and cathode materials exhibited remarkable overall water splitting performance comparable to that with benchmark Pt and RuO2 electrocatalysts. The S/Se co-doped Co3O4 nanosheets on carbon cloth were fabricated by a facile room temperature chalcogen atom incorporation methodology and were applied as the electrocatalyst for HER in alkaline media. The sulfur and selenium atoms were homogeneously distributed on the surface by forming Co-S or Co-Se bonds which play a key role in the structural change in electrochemical activation. The obtained electrocatalysts demonstrated remarkably improved HER activity compared to that of the original Co3O4. Finally, molybdenum doped cobalt hydroxide was fabricated with significantly accelerated HER kinetics. The introduced Mo sites not only effectively facilitate water dissociation process and desorption of the OHads intermediates, but also simultaneously optimize the hydrogen adsorption free energy. Therefore, the in situ-generated Mo-doped amorphous cobalt hydroxide exhibited a remarkable HER performance in alkaline media with an overpotential of only -80 mV at a current density of 10 mA/cm2. This thesis innovatively explores strategies to improve the catalytic activity towards HER of metal (hydro)oxide in alkaline media. The surface foreign atom doping was demonstrated to manipulate the surface structure of catalysts, thus not only improving the water dissociation processes, but also facilitating the hydrogen adsorption/desorption on the catalysts. The demonstrated facile and effective strategies could be adopted for the fabrication of cost-effective and highly active catalysts for other important chemical reactions for energy conversion applications.

Nickel foam-supported Fe,Ni-Polyporphyrin microparticles: Efficient bifunctional catalysts for overall water splitting in alkaline media

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

The development of novel bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is urgently desired for improving the efficiency of overall water splitting but remains a great challenge. Herein, a metal–organic frameworks (MOFs)-derived hybrid nanostructure consisted of multiple transition metal phosphide (NiCoZnP) nanoclusters and hierarchical ultrathin N-doped carbon (NC) nanosheets is successfully synthesized as binder-free bifunctional electrocatalysts for overall water splitting. The ultrafine and monodispersed NiCoZnP nanoclusters with uniform sizes of 3 nm are homogeneously distributed on the ultrathin NC nanosheets, leading to self-supported NiCoZnP/NC nanosheets array on carbon cloth (CC). The NiCoZnP/NC exhibits large surface area and numerous active sites due to the small sizes of NiCoZnP and hierarchical nanostructures of NC. Furthermore, the NiCoZnP/NC nanosheets are directly grown on CC and avoids the usage of binder, which significantly reduces the electrical contact resistance and exhibits superhydrophilic performance. Therefore, the self-supported NiCoZnP/NC nanosheets array demonstrates high HER activity with low overpotential of 74 mV and OER activity with overpotential of 228 mV to reach current density of 10 mA cm−2 in 1.0 M KOH solution, and exhibits remarkable overall water splitting performances with low potential of 1.54 V to drive 10 mA cm−2 and good long-term stability.

In this work, several commonly used conductive substrates as electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) under alkaline conditions were studied, including nickel foam (Ni foam), copper foam (Cu foam), nickel mesh (Ni mesh) and stainless steel mesh (SS mesh). Ni foam and SS mesh are demonstrated as high-performance and stable electrocatalysts for HER and OER, respectively. For HER, Ni foam exhibited an overpotential of 0.217 V at a current density of 10 mA cm−2 with a Tafel slope of 130 mV dec−1, which were larger than that of the commercial Pt/C catalyst, but smaller than that of the other conductive substrates. Meanwhile, the SS mesh showed the best electrocatalytic performance for OER with an overpotential of 0.277 V at a current density of 10 mA cm−2 and a Tafel slope of 51 mV dec−1. Its electrocatalytic performance not only exceeded those of the other conductive substrates but also the commercial RuO2 catalyst. Moreover, both Ni foam and SS mesh exhibited high stability during HER and OER, respectively. Furthermore, in the two-electrode system with Ni foam used as the cathode and SS mesh used as the anode, they enable a current density of 10 mA cm−2 at a small cell voltage of 1.74 V. This value is comparable to or exceeding the values of previously reported electrocatalysts for overall water splitting. In addition, NiO on the surface of Ni foam may be the real active species for HER, NiO and FeOx on the surface of SS mesh may be the active species for OER. The abundant and commercial availability, long-term stability and low-cost property of nickel foam and stainless steel mesh enable their large-scale practical application in water splitting.

Constructing collaborative interface between Mo2N and NiS as efficient bifunctional electrocatalysts for overall water splitting

Two-dimensional double transition metal carbides as superior bifunctional electrocatalysts for overall water splitting

Lanthanum doped copper oxide nanoparticles enabled proficient bi-functional electrocatalyst for overall water splitting

SnO2@MoS2 heterostructures grown on nickel foam as highly efficient bifunctional electrocatalyst for overall water splitting in alkaline media

Bimetallic Ni-Hf tellurides as an advanced electrocatalyst for overall water splitting with layered g-C3N4 modification

Construction of cost-efficient and high-performance overall water splitting electrocatalysts for generating hydrogen and oxygen has recently received increasing research attention. Herein, hierarchical CoS2/MoS2 (CMS) nanoflakes were successfully decorated on graphene (denoted as CMSGr) as an efficient electrocatalyst for overall water splitting. Owing to the distinct hierarchical flakes morphology, and optimized interfacial microenvironment between CoS2 and MoS2, the fabricated CMSGr composites exhibit enhanced hydrogen and oxygen evolution reaction (HER/OER) activity. In particular, the optimized CMSGr-3 electrocatalyst presents the small overpotentials of 53 and 255 mV for HER and OER, and a low voltage of 1.55 V for overall water splitting in alkaline media under 10 mA cm−2. Theoretical calculations confirm that introducing CoS2 into MoS2 can greatly tune the electronic structure and optimize the adsorption energy during the water splitting. This work demonstrates that tuning the interfacial microenvironment of the binary metal sulfide composites is an effective route for developing high-performance water splitting electrocatalysts.

Connected design of Ni foam-supported porous Ni and FeOOH/porous Ni electrocatalysts for overall water splitting in alkaline media

Efficient and robust noble-metal-free bifunctional electrocatalysts for overall water splitting (OWS) is of great importance to realize the large-scale hydrogen production. Herein, we report the growth of undoped and Cr-doped NiCo2O4 (Cr-NiCo2O4) nanoneedles (NNs) on nickel foam (NF) as bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). We demonstrate that Cr-doping significantly improves activity for HER and OER by increasing the conductivity of NNs and allowing more active sites on NNs electrochemically accessible. When amorphous FeOOH is electrodeposited on the surface of Cr-NiCo2O4 NNs, the resulting FeOOH/Cr-NiCo2O4/NF exhibits itself as an excellent bifunctional catalyst for OWS. In the two-electrode cell where FeOOH/Cr-NiCo2O4/NF is used both as cathode and anode for OWS, a cell voltage of only 1.65 V is required to achieve an electrolysis current density of 100 mA·cm−2. In addition, the catalyst shows a very high stability for OWS, the two-electrode cell can operate at a consist current density of 20 mA·cm−2 for 10 h OWS with the cell voltage being stable at ca. 1.60 V. These results demonstrate that FeOOH/Cr-NiCo2O4/NF possesses an OWS performance superior to most of transition-metal based bifunctional electrocatalysts working in alkaline medium. The excellent bifunctional activity and stability of FeOOH/Cr-NiCo2O4/NF are attributed to the following reasons: (i) The NN structure provides a large specific surface area; (ii) the high conductivity of Cr-NiCo2O4 enables more active centers on the far-end part of NNs to be electrochemically reached; (iii) the deposition of FeOOH supplies additional active sites for OWS.

Zn(II)-MOFs nanosheets interaction with P-doped graphitic carbon nitride nanosheets for effective overall water splitting in alkaline medium

Developing efficient bifunctional electrocatalysts based on inexpensive and earth-abundant materials for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is essential for large-scale renewable energy storage and conversion processes but remains a major challenge. In this study, a bamboo-structured nitrogen-doped carbon nanotube coencapsulated with metallic cobalt and Mo2C nanoparticles (Co–Mo2C@NCNT) is designed and synthesized by a successive pyrolysis approach and demonstrated to be an efficient and stable bifunctional electrocatalyst for overall water splitting in alkaline medium. Attributing to favorable synergy interaction in composition and structure, the resultant Co–Mo2C@NCNT presents the superior performances toward HER, OER, and even overall water splitting in alkaline medium. To drive a current density of 10 mA cm–2, it needs only an overpotential of ∼186 and ∼377 mV for the electrocatalytic HER and OER, respectively, and a relatively low cell voltage (∼1.628 V) for overall...