Synthesis of phosphorus-iridium nanocrystals and their superior electrocatalytic activity for oxygen evolution reaction

Iridium single-atoms anchored on a TiO2 support as an efficient catalyst for the hydrogen evolution reaction

Single-atom catalysts (SACs) play a vital role in the hydrogen evolution reaction (HER) owing to the highly desirable atom efficiency and the minimal amount of precious metals. Herein, we use TiO2 nanosheets to anchor stable atomically dispersed iridium (Ir) to be used as a catalyst (Ir@TiO2) for the HER. The atomic dispersion of Ir on the TiO2 substrate is confirmed by aberration-corrected scanning transmission electron microscopy and it is anchored by numerous surface functional groups on abundantly exposed basal planes in TiO2. In acidic media, the Ir@TiO2 catalyst (1.35 wt% Ir) shows a low overpotential (41 mV at 10 mA cm-2), a small Tafel slope of 42 mV dec-1, and a decent durability for 1000 cycles of the HER with the polarization curve having only a 1 mV shift, which are comparable with those of a commercial Pt/C catalyst with 20 wt% Pt. This work paves a way to design Ir atomically anchored catalysts with low cost and high activity.

Recent advances in iridium-based catalysts with different dimensions for the acidic oxygen evolution reaction

Proton exchange membrane (PEM) water electrolysis is considered a promising technology for green hydrogen production, and iridium (Ir)-based catalysts are the best materials for anodic oxygen evolution reactions (OER) owing to their high stability and anti-corrosion ability in a strong acid electrolyte. The properties of Ir-based nanocatalysts can be tuned by rational dimension engineering, which has received intensive attention recently for catalysis ability boosting. To achieve a comprehensive understanding of the structural and catalysis performance, herein, an overview of the recent progress was provided for Ir-based catalysts with different dimensions for the acidic OER. The promotional effect was first presented in terms of the nano-size effect, synergistic effect, and electronic effect based on the dimensional effect, then the latest progress of Ir-based catalysts classified into zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) catalysts was introduced in detail; and the practical application of some typical examples in the real PEM water electrolyzers (PEMWE) was also presented. Finally, the problems and challenges faced by current dimensionally engineered Ir-based catalysts in acidic electrolytes were discussed. It is concluded that the increased surface area and catalytic active sites can be realized by dimensional engineering strategies, while the controllable synthesis of different dimensional structured catalysts is still a great challenge, and the correlation between structure and performance, especially for the structural evolution during the electrochemical operation process, should be probed in depth. Hopefully, this effort could help understand the progress of dimensional engineering of Ir-based catalysts in OER catalysis and contribute to the design and preparation of novel efficient Ir-based catalysts.

Electrocatalysts for the Oxygen Evolution Reaction in Acidic Media

The well-established proton exchange membrane (PEM)-based water electrolysis, which operates under acidic conditions, possesses many advantages compared to alkaline water electrolysis, such as compact design, higher voltage efficiency, and higher gas purity. However, PEM-based water electrolysis is hampered by the low efficiency, instability, and high cost of anodic electrocatalysts for the oxygen evolution reaction (OER). In this review, the recently reported acidic OER electrocatalysts are comprehensively summarized, classified, and discussed. The related fundamental studies on OER mechanisms and the relationship between activity and stability are particularly highlighted in order to provide an atomistic-level understanding for OER catalysis. A stability test protocol is suggested to evaluate the intrinsic activity degradation. Some current challenges and unresolved questions, such as the usage of carbon-based materials and the differences between the electrocatalyst performances in acidic electrolytes and PEM-based electrolyzers are also discussed. Finally, suggestions for the most promising electrocatalysts and a perspective for future research are outlined. This review presents a fresh impetus and guideline to the rational design and synthesis of high-performance acidic OER electrocatalysts for PEM-based water electrolysis.

Anion Modulation of Pt-Group Metals and Electrocatalysis Applications

Pt-group metal (PGM) electrocatalysts with unique electronic structures and irreplaceable comprehensive properties play crucial roles in electrocatalysis. Anion engineering can create a series of PGM compounds (such as RuP₂ , IrP₂ , PtP₂ , RuB₂ , Ru₂ B₃ , RuS₂ , etc.) that provide a promising prospect for improving the electrocatalytic performance and use of Pt-group noble metals. This review seeks the electrochemical activity origin of anion-modulated PGM compounds, and systematically analyzes and summarizes their synthetic strategies and energy-relevant applications in electrocatalysis. Orientation towards the sustainable development of nonfossil resources has stimulated a blossoming interest in the design of advanced electrocatalysts for clean energy conversion. The anion-modulated strategy for Pt-group metals (PGMs) by means of anion engineering possesses high flexibility to regulate the electronic structure, providing a promising prospect for constructing electrocatalysts with superior activity and stability to satisfy a future green electrochemical energy conversion system. Based on the previous work of our group and others, this review summarizes the up-to-date progress on anion-modulated PGM compounds (such as RuP₂ , IrP₂ , PtP₂ , RuB₂ , Ru₂ B₃ , RuS₂ , etc.) in energy-related electrocatalysis from the origin of their activity and synthetic strategies to electrochemical applications including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), N₂ reduction reaction (NRR), and CO₂ reduction reaction (CO₂ RR). At the end, the key problems, countermeasures and future development orientations of anion-modulated PGM compounds toward electrocatalytic applications are proposed.

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.

Surface Engineering on Nickel-Ruthenium Nanoalloys Attached Defective Carbon Sites as Superior Bifunctional Electrocatalysts for Overall Water Splitting

Herein, we report a novel catalyst of nickel-ruthenium alloy nanoparticles (NPs) homogeneously enriched in the wall of multiwalled carbon nanotubes (denoted as NiRu@MWCNTs) via a facile plasma reduction method. The NiRu@MWCNTs exhibits remarkable electrocatalytic activity and stability for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The required overpotentials to drive a current density of 10 mA cm^-2 (η₁₀) over NiRu@MWCNTs are only 14 and 240 mV, corresponding to Tafel slopes of 32 and 55 mV dec^-1 for the HER and OER in alkaline medium, respectively. Furthermore, the NiRu@MWCNTs electrolyzer shows low η₁₀ of 330, 380, and 280 mV in acidic, neutral, and alkaline media, respectively. Density functional theory (DFT) calculations and experimental results reveal that the NiRu alloy NPs attached to the defective and nondefective carbon are the key active sites for the HER and OER, respectively, thus resulting in superior isolated synergistic bifunctional active sites for overall water splitting. Our work provides a promising strategy for efficient synthesis of robust catalysts with specific bifunctional active sites for overall water splitting in a wide pH range, along with deep insight into the catalytic mechanism.

An in situ grown lanthanum sulfide/molybdenum sulfide hybrid catalyst for electrochemical hydrogen evolution

An La₂S₃–MoS₂ catalyst with expanded interlayer spacing and engineered nano-interfaces was facilely synthesized, demonstrating enhanced catalytic activity for electrochemical hydrogen evolution.

Mass-producible 2D-WS2 bulk modified screen printed electrodes towards the hydrogen evolution reaction

A screen-printable ink that contained varying percentage mass incorporations of two dimensional tungsten disulphide (2D-WS2) was produced and utilized to fabricate bespoke printed electrodes (2D-WS2-SPEs). These WS2-SPEs were then rigorously tested towards the Hydrogen Evolution Reaction (HER) within an acidic media. The mass incorporation of 2D-WS2 into the 2D-WS2-SPEs was found to critically affect the observed HER catalysis with the larger mass incorporations resulting in more beneficial catalysis. The optimal (largest possible mass of 2D-WS2 incorporation) was the 2D-WS2-SPE40%, which displayed a HER onset potential, Tafel slope value and Turn over Frequency (ToF) of -214 mV (vs. RHE), 51.1 mV dec-1 and 2.20 , respectively. These values significantly exceeded the HER catalysis of a bare/unmodified SPE, which had a HER onset and Tafel slope value of -459 mV (vs. RHE) and 118 mV dec-1, respectively. Clearly, indicating a strong electrocatalytic response from the 2D-WS2-SPEs. An investigation of the signal stability of the 2D-WS2-SPEs was conducted by performing 1000 repeat cyclic voltammograms (CVs) using a 2D-WS2-SPE10% as a representative example. The 2D-WS2-SPE10% displayed remarkable stability with no variance in the HER onset potential of ca. -268 mV (vs. RHE) and a 44.4% increase in the achievable current over the duration of the 1000 CVs. The technique utilized to fabricate these 2D-WS2-SPEs can be implemented for a plethora of different materials in order to produce large numbers of uniform and highly reproducible electrodes with bespoke electrochemical signal outputs.

Recent Advances in the Development of Water Oxidation Electrocatalysts at Mild pH

Developing anodic oxygen evolution reaction (OER) electrocatalysts with high catalytic activities is of great importance for effective water splitting. Compared with the water-oxidation electrocatalysts that are commonly utilized in alkaline conditions, the ones operating efficiently under neutral or near neutral conditions are more environmentally friendly with less corrosion issues. This review starts with a brief introduction of OER, the importance of OER in mild-pH media, as well as the fundamentals and performance parameters of OER electrocatalysts. Then, recent progress of the rational design of electrocatalysts for OER in mild-pH conditions is discussed. The chemical structures or components, synthetic approaches, and catalytic performances of the OER catalysts will be reviewed. Some interesting insights into the catalytic mechanism are also included and discussed. It concludes with a brief outlook on the possible remaining challenges and future trends of neutral or near-neutral OER electrocatalysts. It hopefully provides the readers with a distinct perspective of the history, present, and future of OER electrocatalysts at mild conditions.