Recent Progress of Transition Metal Compounds as Electrocatalysts for Electrocatalytic Water Splitting

Hydrogen has enormous commercial potential as a secondary energy source because of its high calorific value, clean combustion byproducts, and multiple production methods. Electrocatalytic water splitting is a viable alternative to the conventional methane steam reforming technique, as it operates under mild conditions, produces high-quality hydrogen, and has a sustainable production process that requires less energy. Electrocatalysts composed of precious metals like Pt, Au, Ru, and Ag are commonly used in the investigation of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Nevertheless, their limited availability and expensive cost restrict practical use. In contrast, electrocatalysts that do not contain precious metals are readily available, cost-effective, environmentally friendly, and possess electrocatalytic performance equal to that of noble metals. However, considerable research effort must be devoted to create cost-effective and high-performing catalysts. This article provides a comprehensive examination of the reaction mechanism involved in electrocatalytic water splitting in both acidic and basic environments. Additionally, recent breakthroughs in catalysts for both the hydrogen evolution and oxygen evolution reactions are also discussed. The structure-activity relationship of the catalyst was deep-going discussed, together with the prospects of current obstacles and potential for electrocatalytic water splitting, aiming at provide valuable perspectives for the advancement of economical and efficient electrocatalysts on an industrial scale.

2D MXene Nanomaterials as Electrocatalysts for Hydrogen Evolution Reaction (HER): A Review

MXenes, a novel family of 2D transition metal carbide, nitride and carbonitride materials, have been gaining tremendous interest in recent days as potential electrocatalysts for various electrochemical reactions, including hydrogen evolution reaction (HER). MXenes are characterized by their etchable metal layers, excellent structural stability, versatility for heteroatoms doping, excellent electronic conductivity, unique surface functional groups and admirable surface area, suitable for the role of electrocatalyst/support in electrochemical reactions, such as HER. In this review article, we summarized recent developments in MXene-based electrocatalysts synthesis and HER performance in terms of the theoretical and experimental point of view. We systematically evaluated the superiority of the MXene-based catalysts over traditional Pt/C catalysts in terms of HER kinetics, Tafel slope, overpotential and stability, both in acidic and alkaline electrolytic environments. We also pointed out the motives behind the electro catalytic enhancements, the effect of synthesis conditions, heteroatom doping, the effect of surface terminations on the electrocatalytic active sites of various MXenes families. At the end, various possible approaches were recommended for a deeper understanding of the active sites and catalytic improvement of MXenes catalysts for HER.

Green application of trimetallic nickel-cobalt-molybdenum nanocomposites on 3D graphene oxide as a powerful electrocatalyst for hydrogen evolution reaction

In-situ designing of multiple metals electrocatalysts with high active sites and performance is the main challenge for hydrogen evolution reaction (HER). So in this work, 3D-rGO was easily obtained from 2D-graphene by a simple one-step hydrothermal method to create the interspace sites and active surface area. The Ni-Co-Mo tri-metallic@3D-rGO was synthesized and fully characterized by different techniques, e.g., FT-IR, XRD, Raman, FE-SEM, TEM, EDS, mapping, ICP-OES, AFM, voltammetry, and electrochemical impedance spectroscopy. According to the FE-SEM and TEM images, the Ni-Co-Mo tri-metallic@3D-rGO has a crumpled-formed structure. The as-prepared nanocomposite has high HER performance with a low potential of -0.11 (vs. RHE) to deliver 10 mA cm^-2 and Tafel slope of 68 mV dec^-1 for Pt and -0.25 V (vs. RHE) to deliver 10 mA cm^-2 and Tafel slope of 110 mV dec^-1 for graphite counter electrode. Furthermore, the 3D structure illustrates high long-term durability in the HER process for 1000 continuous cycles and 12 h operation at -0.42 V (vs. RHE) for Pt and graphite counter electrode. It's noticeable HER performance has the synergetic effect between 3D-rGO and tri-metallic structure with high porosity and electrical conductivity, enhancing HER kinetic.

Highly Dispersed Pt Clusters on F-Doped Tin(IV) Oxide Aerogel Matrix: An Ultra-Robust Hybrid Catalyst for Enhanced Hydrogen Evolution

Dispersing the minuscule mass loading without hampering the high catalytic activity and long-term stability of a noble metal catalyst results in its ultimate efficacy for the electrochemical hydrogen evolution reaction (HER). Despite being the most efficient HER catalyst, the use of Pt is curtailed due to its scarcity and tendency to leach out in the harsh electrochemical reaction environment. In this study, we combined F-doped tin(IV) oxide (F-SnO₂) aerogel with Pt catalyst to prevent metallic corrosion and to achieve abundant Pt active sites (approximately 5 nm clusters) with large specific surface area (321 cm²·g^-1). With nanoscopic Pt loading inside the SnO₂ aerogel matrix, the as-synthesized hybrid F-SnO₂@Pt possesses a large specific surface area and high porosity and, thus, exhibits efficient experimental and intrinsic HER activity (a low overpotential of 42 mV at 10 mA·cm^-2 in 0.5 M sulfuric acid), a 22-times larger turnover frequency (11.2 H₂·s^-1) than that of Pt/C at 50 mV, and excellent robustness over 10,000 cyclic voltammetry cycles. The existing metal support interaction and strong intermolecular forces between Pt and F-SnO₂ account for the catalytic superiority and persistence against corrosion of F-SnO₂@Pt compared to commercially used Pt/C. Density functional theory analysis suggests that hybridization between the Pt and F-SnO₂ orbitals enhances intermediate hydrogen atom (H*) adsorption at their interface, which improves the reaction kinetics.

Electronic modulation of oxygen evolution on metal doped NiFe layered double hydroxides

The bottleneck of electrochemical water splitting is the sluggish kinetics of oxygen evolution reaction (OER). Layered double hydroxides (LDHs) have been proposed as active and affordable electrocatalysts in OER. It has been reported that the activity of LDHs can be effectively tuned by doping of other metals. Despite previous experimental synthesis and improved catalytic performance, the in-depth OER mechanism on metal doped LDHs remains ambiguous. In the present work, transition metals (Cr, Mn and Co) doped NiFe LDHs were designed to investigate the doping effect in OER by both experimental analysis and density functional theory calculations. Based on experimental results, the intrinsic OER activity is Cr-NiFe LDHs > Co-NiFe LDHs > Mn-NiFe LDHs > NiFe LDHs, while the enhanced catalytic performance upon doping can be attributed to the interface effect, which results in the tuning of the binding energies of the intermediate states in OER.

Hollow FeP/Fe3O4 Hybrid Nanoparticles on Carbon Nanotubes as Efficient Electrocatalysts for the Oxygen Evolution Reaction

We develop a method to prepare hollow FeP/Fe₃O₄ hybrid nanoparticles supported on carbon nanotubes (CNTs), which could be used as highly active and efficient electrocatalysts. The Fe@Fe₃O₄/CNT hybrids were first synthesized by annealing the CNTs adsorbed with Fe(NO₃)₃, followed by controlled phosphorization treatment. They exhibit an outstanding catalytic activity for oxygen evolution reaction (OER) with a low overpotential of 229 mV at a current density of 10 mA cm^-2, a high turnover frequency value of 0.35 s^-1 at an overpotential of 300 mV, and an ultralow Tafel slope of 27.6 mV dec^-1, which is much better than that of FeP/Fe₃O₄, FeP/CNTs, Fe₃O₄/CNTs, and the commercial RuO₂ electrocatalyst. More importantly, the Tafel slope is much lower than most non-noble metal-based OER electrocatalysts reported in the previous literature studies as far as we know. The excellent OER performance benefits from the electron transfer from Fe₃O₄ to FeP confirmed by X-ray photoelectron spectroscopy, together with good conductivity of CNTs. This hybrid structure supported on conductive CNTs may offer an efficient method to design earth-abundant and low-cost electrocatalysts for OER in practical applications.

Cu- and Fe-Codoped Ni Porous Networks as an Active Electrocatalyst for Hydrogen Evolution in Alkaline Medium

Highly active catalysts from the earth-abundant metals are essential to materialize the low-cost production of hydrogen through water splitting. Herein, nickel porous networks codoped with Cu and Fe prepared by thermal reduction of presynthesized Cu, Fe-codoped Ni(OH)₂ nanowires are reported. The sample consists of nanoparticles of ∼80 nm, which form highly porous network clusters of ∼1 μm with a pore size of 10-100 nm. Among the various doped compositions, the NiCu_0.05Fe_0.025 porous network exhibits the best catalytic activity with a low overpotential of 60 mV for a hydrogen evolution reaction (HER) in 1 M KOH solution and a specific activity of 0.1 mA cm^-2 at 117 mV overpotential calculated based on the electrochemical active surface area (ECSA). The density functional theory calculations reveal that codoping of Fe and Cu into the Ni lattice results in a shift of d-bands of nickel to lower energy levels and thus in the reduced hydrogen adsorption energy (ΔG_H = -0.131 eV), which is close to ΔG_H for Pt (-0.09 eV). When NiCu_0.05Fe_0.025(OH)₂ nanowires is used as an oxygen evolution reaction (OER) catalyst and is coupled with NiCu_0.05Fe_0.025 porous networks for overall water splitting, the NiCu_0.05Fe_0.025∥NiCu_0.05Fe_0.025(OH)₂ catalyst couple achieves a current density of 10 mA cm^-2 at 1.491 V, similar to that of the Pt/C∥RuO₂ couple and offers a negligible loss in the performance when operated at 20 mA cm^-2 for 30 h.

Oxygen Reduction Reaction on PtCo Nanocatalyst: (Bi)sulfate Anion Poisoning

Pt alloy electrocatalysts are susceptible to anion adsorption in the working environment of fuel cells. In this work, the unavoidable bisulfate and sulfate ((bi)sulfate) poisoning of the oxygen reduction reaction (ORR) on a common PtCo nanocatalyst was studied by the rotating disk electrode (RDE) technique, for the first time to the best of our knowledge. The specific activity decreases linearly with the logarithm of (bi)sulfate concentration under various high potentials. This demonstrates that the (bi)sulfate adsorption does not affect the free energy of ORR activation at a given potential. Moreover, it is speculated that these two conditions, the adsorption of one O₂ molecule onto two Pt sites and this adsorption as a rate-determining step of ORR reaction, are unlikely to exist simultaneously.

Applied potentials regulate recovery of residual hydrogen from acid-rich effluents: Influence of biocathodic buffer capacity over process performance

An absolute biological microbial electrolysis cell (MEC) was operated for a prolonged period under different applied potentials (Eapp, -0.2V to -1.0V) and hydrogen (H2) production was observed using acid-rich effluent. Among these potentials, an optimal voltage of -0.6 V influenced the biocathode by which maximum H2 production of 120 ± 9 ml was noticed. This finding was corroborated with dehydrogenase activity (1.8 ± 0.1 μg/ml) which is the key enzyme for H2 production. The in situ biocathode regulated buffer overpotentials which was remarkably observed by the change in peak heights of dissociation value (pKa) from the titration curve. Substrate degradation analysis gave an estimate of coulombic efficiency of about 72 ± 5% when operated at optimal voltage. Evidently, the electron transfer from solid carbon electrode to biocathode was analyzed by cyclic voltammetry and its derivatives showed the involvement of redox mediators. Despite, the MEC endures certain activation overpotentials which were estimated from the Tafel slope analysis.