Low-Loading of Pt Nanoparticles on 3D Carbon Foam Support for Highly Active and Stable Hydrogen Production

Hybrid Nanostructured Compounds of Mo2C on Vertical Graphene Nanoflakes for a Highly Efficient Hydrogen Evolution Reaction

Organizing a post-fossil fuel economy requires the development of sustainable energy carriers. Hydrogen is expected to play a significant role as an alternative fuel as it is among the most efficient energy carriers. Therefore, nowadays, the demand for hydrogen production is increasing. Green hydrogen produced by water splitting produces zero carbon emissions but requires the use of expensive catalysts. Therefore, the demand for efficient and economical catalysts is constantly growing. Transition-metal carbides, and especially Mo₂C, have attracted great attention from the scientific community since they are abundantly available and hold great promises for efficient performance toward the hydrogen evolution reaction (HER). This study presents a bottom-up approach for depositing Mo carbide nanostructures on vertical graphene nanowall templates via chemical vapor deposition, magnetron sputtering, and thermal annealing processes. Electrochemical results highlight the importance of adequate loading of graphene templates with the optimum amount of Mo carbides, controlled by both deposition and annealing time, to enrich the available active sites. The resulting compounds exhibit exceptional activities toward the HER in acidic media, requiring overpotentials of 82 mV at -10 mA/cm² and demonstrating a Tafel slope of 56 mV/dec. The high double-layer capacitance and low charge transfer resistance of these Mo₂C on GNW hybrid compounds are the main causes of the enhanced HER activity. This study is expected to pave the way for the design of hybrid nanostructures based on nanocatalyst deposition on three-dimensional graphene templates.

Controlled Synthesis of Carbon-Supported Pt-Based Electrocatalysts for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells are playing an increasing role in postpandemic economic recovery and climate action plans. However, their performance, cost, and durability are significantly related to Pt-based electrocatalysts, hampering their large-scale commercial application. Hence, considerable efforts have been devoted to improving the activity and durability of Pt-based electrocatalysts by controlled synthesis in recent years as an effective method for decreasing Pt use, and consequently, the cost. Therefore, this review article focuses on the synthesis processes of carbon-supported Pt-based electrocatalysts, which significantly affect the nanoparticle size, shape, and dispersion on supports and thus the activity and durability of the prepared electrocatalysts. The reviewed processes include (i) the functionalization of a commercial carbon support for enhanced catalyst–support interaction and additional catalytic effects, (ii) the methods for loading Pt-based electrocatalysts onto a carbon support that impact the manufacturing costs of electrocatalysts, (iii) the preparation of spherical and nonspherical Pt-based electrocatalysts (polyhedrons, nanocages, nanoframes, one- and two-dimensional nanostructures), and (iv) the postsynthesis treatments of supported electrocatalysts. The influences of the supports, key experimental parameters, and postsynthesis treatments on Pt-based electrocatalysts are scrutinized in detail. Future research directions are outlined, including (i) the full exploitation of the potential functionalization of commercial carbon supports, (ii) scaled-up one-pot synthesis of carbon-supported Pt-based electrocatalysts, and (iii) simplification of postsynthesis treatments. One-pot synthesis in aqueous instead of organic reaction systems and the minimal use of organic ligands are preferred to simplify the synthesis and postsynthesis treatment processes and to promote the mass production of commercial carbon-supported Pt-based electrocatalysts.

Graphical Abstract

This review focuses on the synthesis process of Pt-based electrocatalysts/C to develop aqueous one-pot synthesis at large-scale production for PEMFC stack application.

Low Platinum-Loaded Molybdenum Co-catalyst for the Hydrogen Evolution Reaction in Alkaline and Acidic Media

Developing an efficient catalytic system for electrolysis with reduced platinum (Pt) loading while maintaining performance comparable to bulk platinum metal is important to decrease costs and improve scalability of the hydrogen fuel economy. Here we report the performance of a novel sputter-deposited molybdenum (Mo) thin film with an extremely low co-loading of Pt, where Pt atoms were dispersed on Mo (Pt_d-Mo) as an electrocatalyst for the hydrogen evolution reaction (HER) in either alkaline or acidic media. The Pt_d-Mo electrocatalyst presents similar catalytic activity to bulk Pt in alkaline media, while the performance is only slightly decreased in acidic media. Differential electrochemical mass spectrometry (DEMS) results confirm that the Pt_d-Mo electrocatalyst produced hydrogen at a rate comparable with that of a pristine Pt sample at the same potential. A comparison with Pt-loaded degenerately doped p-type doped silicon (Pt_d-Si) suggests that Mo and Pt work synergistically to boost the performance of Pt_d-Mo catalysts. Cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) before and after 1000 cycles of continuous operation confirm the significant durability of the Pt_d-Mo performance. Overall, the Pt_d-Mo electrocatalyst, with comparable HER activity to bulk Pt despite an ultra-low Pt loading, could be a strong candidate for hydrogen production in either acidic or basic conditions.

Elucidating the Formation and Structural Evolution of Platinum Single-Site Catalysts for the Hydrogen Evolution Reaction

Platinum single-site catalysts (SSCs) are a promising technology for the production of hydrogen from clean energy sources. They have high activity and maximal platinum-atom utilization. However, the bonding environment of platinum during operation is poorly understood. In this work, we present a mechanistic study of platinum SSCs using operando, synchrotron-X-ray absorption spectroscopy. We synthesize an atomically dispersed platinum complex with aniline and chloride ligands onto graphene and characterize it with ex-situ electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectroscopy (XANES), and extended X-ray absorption fine structure spectroscopy (EXAFS). Then, by operando EXAFS and XANES, we show that as a negatively biased potential is applied, the Pt-N bonds break first followed by the Pt-Cl bonds. The platinum is reduced from platinum(II) to metallic platinum(0) by the onset of the hydrogen-evolution reaction at 0 V. Furthermore, we observe an increase in Pt-Pt bonding, indicating the formation of platinum agglomerates. Together, these results indicate that while aniline is used to prepare platinum SSCs, the single-site complexes are decomposed and platinum agglomerates at operating potentials. This work is an important contribution to the understanding of the evolution of bonding environment in SSCs and provides some molecular insights into how platinum agglomeration causes the deactivation of SSCs over time.

Microwave-Induced Structural Engineering and Pt Trapping in 6R-TaS2 for the Hydrogen Evolution Reaction

The nanoengineering of the structure of transition metal dichalcogenides (TMDs) is widely pursued to develop viable catalysts for the hydrogen evolution reaction (HER) alternative to the precious metallic ones. Metallic group-5 TMDs have been demonstrated to be effective catalysts for the HER in acidic media, making affordable real proton exchange membrane water electrolysers. Their key-plus relies on the fact that both their basal planes and edges are catalytically active for the HER. In this work, the 6R phase of TaS₂ is "rediscovered" and engineered. A liquid-phase microwave treatment is used to modify the structural properties of the 6R-TaS₂ nanoflakes produced by liquid-phase exfoliation. The fragmentation of the nanoflakes and their evolution from monocrystalline to partly polycrystalline structures improve the HER-activity, lowering the overpotential at cathodic current of 10 mA cm^-2 from 0.377 to 0.119 V. Furthermore, 6R-TaS₂ nanoflakes act as ideal support to firmly trap Pt species, which achieve a mass activity (MA) up 10 000 A g_Pt ^-1 at overpotential of 50 mV (20 000 A g_Pt ^-1 at overpotentials of 72 mV), representing a 20-fold increase of the MA of Pt measured for the Pt/C reference, and approaching the state-of-the-art of the Pt mass activity.

Platinum nanoparticle decorated vertically aligned graphene screen-printed electrodes: electrochemical characterisation and exploration towards the hydrogen evolution reaction

We present the fabrication of platinum (Pt⁰) nanoparticle (ca. 3 nm average diameter) decorated vertically aligned graphene (VG) screen-printed electrodes (Pt/VG-SPE) and explore their physicochemical characteristics and electrocatalytic activity towards the hydrogen evolution reaction (HER) in acidic media (0.5 M H₂SO₄). The Pt/VG-SPEs exhibit remarkable HER activity with an overpotential (recorded at -10 mA cm^-2) and Tafel value of 47 mV (vs. RHE) and 27 mV dec^-1. These values demonstrate the Pt/VG-SPEs as significantly more electrocatalytic than a bare/unmodified VG-SPE (789 mV (vs. RHE) and 97 mV dec^-1). The uniform coverage of Pt⁰ nanoparticles (ca. 3 nm) upon the VG-SPE support results in a low loading of Pt⁰ nanoparticles (ca. 4 μg cm^-2), yet yields comparable HER activity to optimal Pt based catalysts reported in the literature, with the advantages of being comparatively cheap, highly reproducible and tailorable platforms for HER catalysis. In order to test any potential dissolution of Pt⁰ from the Pt/VG-SPE surface, which is a key consideration for any HER catalyst, we additively manufactured (AM) a bespoke electrochemical flow cell that allowed for the electrolyte to be collected at regular intervals and analysed via inductively coupled plasma optical emission spectroscopy (ICP-OES). The AM electrochemical cell can be rapidly tailored to a plethora of geometries making it compatible with any size/shape of electrochemical platform. This work presents a novel and highly competitive HER platform and a novel AM technique for exploring the extent of Pt⁰ nanoparticle dissolution upon the electrode surface, making it an essential study for those seeking to test the stability/catalyst discharge of their given electrochemical platforms.

Fast and Facile Synthesis of Pt Nanoparticles Supported on Ketjen Black by Solution Plasma Sputtering as Bifunctional HER/ORR Catalysts

Hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are two core electrochemical processes involved in hydrogen fuel cell (HFC) technology. ORR is a cathodic reaction occurring in HFC, whereas HER can convert the H2O byproduct from HFCs into H2 gas via water splitting. Platinum (Pt)-based catalysts are the most effective catalysts for both reactions. In this work, we used a fast, facile, and chemical-free method, called solution plasma sputtering (SPS), to synthesize Pt nanoparticles supported on Ketjen Black (KB). The discharge time was varied (5, 10, and 20 min) to alter the Pt loading. Characterization results revealed that the plasma did not affect the morphology of KB, and the Pt loading on KB increased with increasing discharge time (5.5–17.9 wt%). Well-crystallized Pt nanoparticles, ~2–5 nm in diameter, were obtained. Electrochemical measurements revealed that Pt/KB exhibited bifunctional catalytic activity toward HER and ORR in 0.5 M H2SO4 solution. Both HER and ORR activities enhanced as the loading of Pt nanoparticles increased with a longer discharge time. Moreover, Pt/KB exhibited better HER and ORR stability than a commercial Pt-based catalyst, which was attributed to the stronger adhesion between Pt nanoparticles and KB support. Thus, SPS can be applied as an alternative synthesis method for preparing Pt/KB catalysts for HER and ORR.

Enhancement of the Electrochemical Properties of an Open-Pore Graphite Foam with Electrochemically Reduced Graphene Oxide and Alternating Current Dispersed Platinum Particles

This paper aimed to improve the electrochemical activity of a pitch-derived open-pore graphite foam (GF) by an electrochemical coating of reduced graphene oxide (RGO) and platinum particles without significantly affecting its 3D-structure. RGO was synthesized using cyclic voltammetry (CV) from a 3 g L−1 GO and 0.1 M LiClO4 solution. For the electrodeposition of Pt particles, an alternating current method based on electrochemical impedance spectroscopy (EIS) was used. A sinusoidal voltage from a fixed potential Ei was varied following a selected amplitude (ΔEac = ± 0.35 V) in a frequency range of 8 Hz ≤ fi ≤ 10Hz, where i = 500. This method proved its efficiency when compared to the traditional CV by obtaining more highly electroactive coatings in less synthesis time. For samples’ characterization, physical measures included permeability, pressure drop, and nitrogen adsorption isotherms. The electrochemical characterization was performed by CV. The surface morphology and chemical composition were examined using field emission electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDX), respectively. RGO improved the electron transfer rate constant of GF, and a more homogeneous coating distribution of reduced size Pt particles was obtained.

Self-terminating electrodeposition of Pt on WC electrocatalysts

Self-terminated electrochemical deposition is used to grow Pt nanoparticles on tungsten monocarbide (WC) from a pH 4 electrolyte of 3 mmol/L K₂PtCl₄-0.5 mol/L NaCl. An unconventional potentiodynamic deposition program is used where nucleation is promoted at large overpotentials followed by growth termination at still larger overpotentials to yield a high coverage of Pt nanoparticles. Following three deposition cycles between -0.8 V_SCE and -0.45 V_SCE, the surface is covered by a monolayer equivalent charge of Pt in the form of ≈3 × 10¹¹ particles/cm² that are ≈6.7 ± 1.1 nm in diameter. The number and size of nanoparticles increase monotonically for five deposition cycles. Area-normalized kinetics for hydrogen evolution (HER) and oxidation (HOR) on Pt-WC were determined in 0.5 mol/L H₂SO₄. For the lowest surface coverage of Pt nanoparticles on WC, ≈ 0.01, an exchange current density of ≈ 100 mA/cm² is achieved, comparable to the highest reported values for Pt nanoparticles and ultramicroelectrodes. The area normalized apparent exchange current density decreases with increasing Pt coverage as the relative contribution of point versus planar diffusion decreases. Self-terminated electrodeposition of Pt provides an attractive approach to achieving ultra-low loadings of well-dispersed Pt nanoparticles on a non-precious metal support like WC.