Palladium/cobalt nanowires with improved hydrogen sensing stability at ultra-low temperatures

Balancing Pd-H Interactions: Thiolate-Protected Palladium Nanoclusters for Robust and Rapid Hydrogen Gas Sensing

The transition toward hydrogen gas (H2) as an eco-friendly and renewable energy source necessitates advanced safety technologies, particularly robust sensors for H2 leak detection and concentration monitoring. Although palladium (Pd)-based materials are preferred for their strong H2 affinity, intense palladium-hydrogen (Pd-H) interactions lead to phase transitions to palladium hydride (PdHx), compromising sensors' durability and detection speeds after multiple uses. In response, this study introduces a high-performance H2 sensor designed from thiolate-protected Pd nanoclusters (Pd8SR16), which leverages the synergistic effect between the metal and protective ligands to form an intermediate palladium-hydrogen-sulfur (Pd-H-S) state during H2 adsorption. Striking a balance, it preserves Pd-H binding affinity while preventing excessive interaction, thus lowering the energy required for H2 desorption. The dynamic adsorption-dissociation-recombination-desorption process is efficiently and highly reversible with Pd8SR16, ensuring robust and rapid H2 sensing at parts per million (ppm). The Pd8SR16-based sensor demonstrates exceptional stability (50 cycles; 0.11% standard deviation in response), prompt response/recovery (t90 = 0.95 s/6 s), low limit of detection (LoD, 1 ppm), and ambient temperature operability, ranking it among the most sensitive Pd-based H2 sensors. Furthermore, a multifunctional prototype demonstrates the practicality of real-world gas sensing using ligand-protected metal nanoclusters.

Pd Decoration with Synergistic High Oxygen Mobility Boosts Hydrogen Sensing Performance at Low Working Temperature on WO3 Nanosheet

Pd-based materials have received remarkable attention and exhibit excellent H2 sensing performance due to their superior hydrogen storage and catalysis behavior. However, the synergistic effects originated from the decoration of Pd on a metal oxide support to boost the sensing performance are ambiguous, and the deep investigation of metal support interaction (MSI) on the H2 sensing mechanism is still unclear. Here, the model material of Pd nanoparticle-decorated WO3 nanosheet is synthesized, and individual fine structures can be achieved by treating it at different temperatures. Notably, the Pd-WO3-300 materials display superior H2 sensing performance at a low working temperature (110 °C), with a superior sensing response (Ra/Rg = 40.63 to 10 ppm), high sensing selectivity, and anti-interference ability. DFT calculations and detailed structural investigations confirm that the moderate MSI facilitates the generation of high mobility surface O2- (ad) species and a proper ratio of surface Pd0-Pd2+ species, which can significantly boost the desorption of intermediate PdHx species at low temperatures and contribute to enhanced sensing performance. Our work illustrates the effect of MSI on sensing performance and provides insight into the design of advanced sensing materials.

Pd-Co-Based Electrodes for Hydrogen Production by Water Splitting in Acidic Media

To realize the benefits of a hydrogen economy, hydrogen must be produced cleanly, efficiently and affordably from renewable resources and, preferentially, close to the end-users. The goal is a sustainable cycle of hydrogen production and use: in the first stage of the cycle, hydrogen is produced from renewable resources and then used to feed a fuel cell. This cycle produces no pollution and no greenhouse gases. In this context, the development of electrolyzers producing high-purity hydrogen with a high efficiency and low cost is of great importance. Electrode materials play a fundamental role in influencing electrolyzer performances; consequently, in recent years considerable efforts have been made to obtain highly efficient and inexpensive catalyst materials. To reach both goals, we have developed electrodes based on Pd-Co alloys to be potentially used in the PEMEL electrolyzer. In fact, the Pd-Co alloy is a valid alternative to Pt for hydrogen evolution. The alloys were electrodeposited using two different types of support: carbon paper, to fabricate a porous structure, and anodic alumina membrane, to obtain regular arrays of nanowires. The goal was to obtain electrodes with very large active surface areas and a small amount of material. The research demonstrates that the electrochemical method is an ideal technique to obtain materials with good performances for the hydrogen evolution reaction. The Pd-Co alloy composition can be controlled by adjusting electrodeposition parameters (bath composition, current density and deposition time). The main results concerning the fabrication process and the characterization are presented and the performance in acid conditions is discussed.

Characterization and Modeling of a Pt-In2O3 Resistive Sensor for Hydrogen Detection at Room Temperature

Sensitive H₂ sensors at low concentrations and room temperature are desired for the early warning and control of hydrogen leakage. In this paper, a resistive sensor based on Pt-doped In₂O₃ nanoparticles was fabricated using inkjet printing process. The H₂ sensing performance of the sensor was evaluated at low concentrations below 1% at room temperature. It exhibited a relative high response of 42.34% to 0.6% H₂. As the relative humidity of 0.5% H₂ decreased from 34% to 23%, the response decreased slightly from 34% to 23%. The sensing principle and the humidity effect were discussed. A dynamic current sensing model for dry H₂ detection was proposed based on Wolkenstein theory and experimentally verified to be able to predict the sensing behavior of the sensor. The H₂ concentration can be calculated within a short measurement time using the model without waiting for the saturation of the response, which significantly reduces the sensing and recovery time of the sensor. The sensor is expected to be a promising candidate for room-temperature H₂ detection, and the proposed model could be very helpful in promoting the application of the sensor for real-time H₂ leakage monitoring.

Porous Pd-Sn Alloy Nanotube-Based Chemiresistor for Highly Stable and Sensitive H2 Detection

While H₂ is indispensable as a green fuel source, it is highly flammable and explosive. Because it is difficult to detect due to its lack of odor and color, a solution for proper monitoring of H₂ leakage is essential to ensure safe handling. To this end, we have successfully fabricated hollow Pd-Sn alloy nanotubes (NTs) with a Brunauer-Emmett-Teller surface area of 223.0 m²/g through electrospinning and a subsequent etching method, which is the first demonstration of synthesizing Pd-based hollow alloy nanofibers with ultrafine grain sizes. We found that the alloying of Pd with Sn could effectively prevent degradation of the sensing performance upon the α-β phase transition during hydrogen detection. Besides, the highly porous structure with smaller nanograins offered more exposed active sites and higher gas accessibility to bulk materials. The resultant Pd-Sn NTs exhibited excellent sensitivity toward H₂ (0.00005-3%). Notably, the limit of detection of 0.0001% is an outstanding achievement on H₂ sensing among state-of-the-art H₂ sensors. Moreover, when exposed to a high concentration of H₂ (3%), Pd-Sn NTs showed excellent cycling stability with a standard deviation of 0.07% and a sensitivity of 9.27%. These obtained sensing results indicate that Pd-Sn NTs can be used as a highly sensitive and stable H₂ gas sensor at room temperature (25 °C).

Chemiresistive NH3 detection at sub-zero temperatures by polypyrrole- loaded Sn1-xSbxO2 nanocubes

Chemiresistive gas sensors operate mainly at high temperatures, primarily due to the need of energy for surface adsorption-desorption of analytes. As a result, the operating temperature of the chemiresistive sensors could be reduced only to room temperature. Hence, a plethora of sensing requirements at temperatures below ambient have remained outside the scope of chemiresistive materials. In this work, we have developed an antimony-doped SnO₂ nanocube-supported expanded polypyrrole network that could detect low ppm ammonia gas (≤20 ppm) at sub-zero temperatures with high response (∼4), selectivity, and short response and recovery times. The low temperature chemiresistive sensing has been explained in terms of the interplay of an extended conducting network of an in situ deposited polymer, effective transport properties of majority charge carriers and a loosely bound exciton-like electron-hole pair formation and breakage mechanism.

Palladium Nanosheet-Based Dual Gas Sensors for Sensitive Room-Temperature Hydrogen and Carbon Monoxide Detection

Palladium has long been explored for use in gas sensors because of its excellent catalytic properties and its unique property of forming hydrides in the presence of H₂. However, pure Pd-based sensors usually suffer from low response and a relatively high limit of detection. Palladium nanosheets (PdNS) are of particular interest for gas sensing applications due to their high surface area and excellent electrical conductivity. Here, we demonstrate the design and fabrication of low-cost PdNS-based dual gas sensors for room-temperature detection of H₂ and CO over a wide concentration range. We fabricated sensors using multiwalled carbon nanotube@PdNS (MWCNT@PdNS) composites and compared their performance against pure PdNS devices for hydrogen sensing based on electrical resistive response. Devices using PdNS alone had a response and response time of 0.4% and 50 s, respectively, to 1% H₂ in air. MWCNT@PdNS (1:5 mass ratio) showed enhanced performance at a lower hydrogen concentration with a limit of detection (LOD_H2) of 5 ppm. Nearly an order of magnitude increase in response was observed on increasing the amount of MWCNT to 50 mass % in the nanocomposite, but the response fell off at low H₂ concentration. Overall, these PdNS-based sensors were found to show good repeatability, stability, and performance under humid conditions. Their response was selective for H₂ versus CH₄, CO₂, and NH₃; the response to CO was comparable in magnitude but opposite in sign to the response to H₂. Upon simultaneous exposure to equal concentrations (10 ppm each) of H₂ and CO, the response to CO was dominant. The PdNS showed high sensitivity to CO, detecting as little as 1 ppm CO in air at room temperature. The sensitivity to CO could be used either in a stand-alone room-temperature CO detector, where H₂ is known not to be present, or in combination with CO and combustible gas detectors to distinguish H₂ from other combustible gases.

Anodic Alumina Membranes: From Electrochemical Growth to Use as Template for Fabrication of Nanostructured Electrodes

The great success of anodic alumina membranes is due to their morphological features coupled to both thermal and chemical stability. The electrochemical fabrication allows accurate control of the porous structure: in fact, the membrane morphological characteristics (pore length, pore diameter and cell density) can be controlled by adjusting the anodizing parameters (bath, temperature, voltage and time). This article deals with both the fabrication and use of anodic alumina membranes. In particular, we will show the specific role of the addition of aluminum ions to phosphoric acid-based anodizing solution in modifying the morphology of anodic alumina membranes. Anodic alumina membranes were obtained at −1 °C in aqueous solutions of 0.4 M H3PO4 added with different amounts of Al(OH)3. For sake of completeness, the formation of PAA in pure 0.4 M H3PO4 in otherwise identical conditions was also investigated. We found that the presence of Al(OH)3 in solution highly affects the morphology of the porous layer. In particular, at high Al(OH)3 concentration (close to saturation) more compact porous layers were formed with narrow pores separated by thick oxide. The increase in the electric charge from 20 to 160 C cm−2 also contributes to modifying the morphology of porous oxide. The obtained anodic alumina membranes were used as a template to fabricate a regular array of PdCo alloy nanowires that is a valid alternative to Pt for hydrogen evolution reaction. The PdCo alloy was obtained by electrodeposition and we found that the composition of the nanowires depends on the concentration of two metals in the deposition solution.

Recent Advances and Challenges of Nanomaterials-Based Hydrogen Sensors

Safety is a crucial issue in hydrogen energy applications due to the unique properties of hydrogen. Accordingly, a suitable hydrogen sensor for leakage detection must have at least high sensitivity and selectivity, rapid response/recovery, low power consumption and stable functionality, which requires further improvements on the available hydrogen sensors. In recent years, the mature development of nanomaterials engineering technologies, which facilitate the synthesis and modification of various materials, has opened up many possibilities for improving hydrogen sensing performance. Current research of hydrogen detection sensors based on both conservational and innovative materials are introduced in this review. This work mainly focuses on three material categories, i.e., transition metals, metal oxide semiconductors, and graphene and its derivatives. Different hydrogen sensing mechanisms, such as resistive, capacitive, optical and surface acoustic wave-based sensors, are also presented, and their sensing performances and influence based on different nanostructures and material combinations are compared and discussed, respectively. This review is concluded with a brief outlook and future development trends.

Sensitivity of PS/CoPd Janus particles to an external magnetic field

The dual nature of Janus particles confers fascinating properties such as a response to multiple stimuli. In this communication, we systematically study the sensitivity to a uniform external magnetic field of isolated Janus rod-shaped and spherical particles in water confined to two dimensions. The Janus asymmetry of the particles is given by magnetic [Co(0.28 nm)/Pd(0.90 nm)]8 multilayer films deposited onto monodisperse polystyrene (PS) nanorods and microspheres, respectively. It is shown that the particles dispersed in water respond to weak magnetic field applied in in-plane direction. Here we demonstrate that a precise control of the in-plane particle orientation can be obtained for magnetic field strengths higher than 0.1 mT for microspheres and 0.4 mT for nanorods.

Hydrogen Sensing at Room Temperature Using Flame-Synthesized Palladium-Decorated Crumpled Reduced Graphene Oxide Nanocomposites

We present a unique three-dimensional palladium (Pd)-decorated crumpled reduced graphene oxide ball (Pd-CGB) nanocomposite for hydrogen (H₂) detection in air at room temperature. Pd-CGB nanocomposites were synthesized using a rapid continuous flame aerosol technique. Graphene oxide reduction and metal decoration occurred simultaneously in a high-temperature reducing jet (HTRJ) process to produce Pd nanoparticles that were below 5 nm in average size and uniformly dispersed in the crumpled graphene structure. The sensors made from these nanocomposites were sensitive over a wide range of H₂ concentrations (0.0025-2%) with response value, response time, and recovery time of 14.8%, 73 s, and 126 s, respectively, at 2% H₂. Moreover, they were sensitive to H₂ in both dry and humid conditions. The sensors were stable and recoverable after 20 cycles at 2% H₂ with no degradation associated with volume expansion of Pd. Unlike two-step methods for fabricating Pd-decorated graphene sensors, the HTRJ process enables single-step formation of Pd- and other metal-decorated graphene nanocomposites with great potential for creating various gas sensors by simple drop-casting onto low-cost electrodes.

Effect of Hydrogen Electrosorption on Mechanical and Electronic Properties of Pd80Rh20 Alloy

The interaction of hydrogen with Pt-group metals and alloys is at the center of research in the fields of electrochemistry, electrocatalysis, hydrogen technologies and fuel cells developed under the Hydrogen Economy. In this work, the material under study was Pd₈₀Rh₂₀ alloy (50 μm foil) subjected to hydrogen electrosorption at potentials corresponding to formation of α, α-β and β phase in 0.1 M H₂SO₄ at 25 °C. The total amount of hydrogen adsorbed at the surface and absorbed in octahedral interstitial positions of fcc Pd₈₀Rh₂₀ alloy, was determined from the oxidation charges. The H/(Pd+Rh) was 0.002, 0.4 and 0.8 for α, α-β, and β Pd₈₀Rh₂₀H, respectively. Microindentation hardness testing and nanoindentation showed weakening of mechanical properties of the Pd₈₀Rh₂₀ alloy after hydrogen electrosorption due to internal stresses. Decrease of work function with increasing amount of hydrogen absorbed occurred due to the surface roughness changes and the presence of electropositive hydrogen atoms absorbed in the crystal lattice responsible for the dipole interaction. The detailed mechanism of hydrogen absorption/diffusion in the Pd₈₀Rh₂₀ alloy structure is discussed. The obtained results give a new insight into the relationship between the amount of absorbed hydrogen and mechanical and electronic properties of the Pd₈₀Rh₂₀ alloy at the micro- and nanoscale.