Efficient Water Reforming of Biomass to H2 via Well-Organized Redox-Neutral Cleavage of C-C, O-H and C-H Bonds

Hydrogen (H2) is a clean and environmentally friendly energy carrier. The depletion of fossil fuels makes renewable H2 production highly desirable. Water reforming of renewable biomass to hydrogen, with a relay of natural photosynthesis to biomass, would be an indirect pathway to realize the ideal but extremely challenging photocatalytic overall water splitting to hydrogen, with favorable thermodynamics. Since the seminal work of water reforming of biomass in 1980, great endeavors have been made. Nevertheless, hitherto, the entire kinetic pathway has been elusive, which seriously limits the reforming processes. Using a designed well-organized redox-neutral cleavage of C-C, O-H and C-H bonds enabled by photoelectrocatalysis, here, we show the efficient water reforming of biomass to hydrogen at room temperature, with a yield up to 93 %. The clear insights into the kinetic pathway with oxidation of carbon radicals to carbon cations as the indicated rate-determining step, would cast brightness for efficient and sustainable hydrogen production to accelerate the hydrogen economy.

Molecular Iridium Catalyzed Electrochemical Formic Acid Oxidation: Mechanistic Insights

Electrochemical formic acid oxidation reaction (FAOR) is a pivotal model for understanding organic fuel oxidation and advancing sustainable energy technologies. Here, we present mechanistic insights into a novel molecular-like iridium catalyst (Ir-N4-C) for FAOR. Our studies reveal that isolated sites facilitate a preferential dehydrogenation pathway, circumventing catalyst poisoning and exhibiting high inherent activity. In situ spectroscopic analyses elucidate that weakly adsorbed intermediates mediate the FAOR and are dynamically regulated by potential-dependent redox transitions. Theoretical and experimental investigations demonstrate a parallel mechanism involving two key intermediates with distinct pH and potential sensitivities. The rate-determining step is identified as the adsorption of formate via coupled or sequential proton-electron transfer, which aligns well with the observed kinetic properties, pH dependence, and hydrogen/deuterium isotope effects in experiments. These findings provide valuable insights into the reaction mechanism of FAOR, advancing our understanding at the molecular level and potentially guiding the design of efficient catalysts for fuel cells and electrolyzers.

PtRu mesoporous nanospheres as electrocatalysts with enhanced performance for oxidation of methanol

Composition and morphology are crucial factors in the design of Pt-based catalysts with high performance, particularly in direct methanol fuel cells (DMFCs). Herein, PtRu mesoporous nanospheres (PtRu MNs) with tunable compositions were synthesized via a facile method and then deposited on a carbon support to act as electrocatalyst materials for the methanol oxidation reaction (MOR). Superior catalytic activity, better catalytic stability, and good tolerance to CO were achieved by the optimum PtRu (2 : 1) MNs/C catalyst compared with Pt MNs/C. The mass activity on PtRu (2 : 1) MNs/C reached 111.77 mA mgPt -1, which was approximately 6.45-fold higher than that of Pt MNs/C (17.33 mA mgPt -1). Meanwhile, PtRu (2 : 1) MNs/C retained much more current density (84.7%) than Pt MNs/C (17.7%) after 500 cycles. The improved catalytic performance is due to several factors, including the formation of a mesoporous nanostructure with abundant active sites and the favorable effects of the Ru species. This work provides guidance toward designing and fabricating effective Pt-based electrocatalysts for DMFC applications.

Formic Acid Photo-Fuel Cells Consisting of TiO2 Photoanode and Pt Cathode

Formic acid (HCOOH) has attracted much attention as a promising power source for portable electronic devices because of its ease of storage and transportation. Here we report that a simple HCOOH photo-fuel cell (PFC) consisting of mesoporous anatase TiO2 photoanode and Pt cathode stably delivers a short-circuit photocurrent (Jsc) of 5.94 mA cm-2 and an open-circuit voltage of 0.94 V under UV-light irradiation (light intensity, I=200 mW cm-2). The incident photon-to-current conversion efficiency and Faradaic efficiency reach ~90 % and ~100 %, respectively. The excellent performances of this HCOOH PFC, designed based on the discovery that HCOOH provides a large photocurrent by current doubling even in the presence of O2, not only solves the problem of conventional HCOOH FCs, but also achieves the performances far exceeding those of PFCs using biomass-derived organics reported so far.

A Biphasic Strategy to Synergistically Accelerate Activation and CO Spillover in Formic Acid Oxidation Catalysis

Developing highly efficient and carbon monoxide (CO)-tolerant platinum (Pt) catalysts for the formic acid oxidation reaction (FAOR) is vital for direct formic acid fuel cells (DFAFCs), yet it is challenging due to the high energy barrier of direct intermediates (HCOO* and COOH*) as well as the CO poisoning issues associated with Pt alloy catalysts. Here we present a versatile biphasic strategy by creating a hexagonal/cubic crystalline-phase-synergistic PtPb/C (h/c-PtPb/C) catalyst to tackle the aforementioned issues. Detailed investigations reveal that h/c-PtPb/C can simultaneously facilitate the adsorption of direct intermediates while inhibiting CO adsorption, thereby significantly improving the activation and CO spillover. As a result, h/c-PtPb/C showcases an outstanding FAOR activity of 8.1 A mgPt-1, which is 64.5 times higher than that of commercial Pt/C and significantly surpasses monophasic PtPb. Moreover, the h/c-PtPb/C-based membrane electrode assembly exhibits an exceptional peak power density of 258.7 mW cm-2 for practical DFAFC applications.

Probing the adsorption configuration of methanol at a charged air/aqueous interface using nonlinear spectroscopy

Investigating the effects of electrolyte ions on the adsorption configuration of methanol at a charged interface is important for studying the interface structure of electrolyte solutions and the oxidation mechanism of methanol in fuel cells. This study uses sum frequency generation (SFG) and heterodyne-detected second harmonic generation (HD-SHG) to investigate the adsorption configuration of methanol at the air/aqueous interface of 0.1 M NaClO4 solution, 0.1 M HClO4 solution and pure water. The results elucidate that the ion effect in the electrolyte solution affects the interface's charged state and the methanol's adsorption conformation at the interface. The negatively charged surface of the 0.1 M NaClO4 solution and the positively charged surface of the 0.1 M HClO4 solution arise from the corresponding specific ionic effects of the electrolyte solution. The orientation angle of methyl with respect to the surface normal is 43.4° ± 0.1° at the 0.1 M NaClO4 solution surface and 21.5° ± 0.2° at the 0.1 M HClO4 solution surface. Examining these adsorption configurations in detail, we find that at the negatively charged surface the inclined orientation angle (43.4°) of methanol favors the hydroxymethyl production by breaking the C-H bond, while at the positively charged surface the upright orientation angle (21.5°) of methanol promotes the methoxy formation by breaking the O-H bond. These findings not only illuminate the intricate ion effects on small organic molecules but also contribute to a molecular-level comprehension of the oxidation mechanism of methanol at electrode interfaces.

Stabilizing Diluted Active Sites of Ultrasmall High-Entropy Intermetallics for Efficient Formic Acid Electrooxidation

The poisoning of undesired intermediates or impurities greatly hinders the catalytic performances of noble metal-based catalysts. Herein, high-entropy intermetallics i-(PtPdIrRu)2FeCu (HEI) are constructed to inhibit the strongly adsorbed carbon monoxide intermediates (CO*) during the formic acid oxidation reaction. As probed by multiple-scaled structural characterizations, HEI nanoparticles are featured with partially negative Pt oxidation states, diluted Pt/Pd/Ir/Ru atomic sites and ultrasmall average size less than 2 nm. Benefiting from the optimized structures, HEI nanoparticles deliver more than 10 times promotion in intrinsic activity than that of pure Pt, and well-enhanced mass activity/durability than that of ternary i-Pt2FeCu intermetallics counterpart. In situ infrared spectroscopy manifests that both bridge and top CO* are favored on pure Pt but limited on HEI. Further theoretical elaboration indicates that HEI displayed a much weaker binding of CO* on Pt sites and sluggish diffusion of CO* among different sites, in contrast to pure Pt that CO* bound more strongly and was easy to diffuse on larger Pt atomic ensembles. This work verifies that HEIs are promising catalysts via integrating the merits of intermetallics and high-entropy alloys.

Surface-Enriched Single-Bi-Atoms Tailoring of Pt Nanorings for Direct Methanol Fuel Cells with Ultralow-Pt-Loading

Single-atom decorating of Pt emerges as a highly effective strategy to boost catalytic properties, which can trigger the most Pt active sites while blocking the smallest number of Pt atoms. However, the rational design and creation of high-density single-atoms on Pt surface remain as a huge challenge. Herein, a customized synthesis of surface-enriched single-Bi-atoms tailored Pt nanorings (SE-Bi1/Pt NRs) toward methanol oxidation is reported, which is guided by the density functional theory (DFT) calculations suggesting that a relatively higher density of Bi species on Pt surface can ensure a CO-free pathway and accelerate the kinetics of *HCOOH formation. Decorating Pt NRs with dense single-Bi-atoms is achieved by starting from PtBi intermetallic nanoplates (NPs) with intrinsically isolated Bi atoms and subsequent etching and annealing treatments. The SE-Bi1/Pt NRs exhibit a mass activity of 23.77 A mg-1 Pt toward methanol oxidation in alkaline electrolyte, which is 2.2 and 12.8 times higher than those of Pt-Bi NRs and Pt/C, respectively. This excellent activity endows the SE-Bi1/Pt NRs with a high likelihood to be used as a practical anodic electrocatalyst for direct methanol fuel cells （DMFCs） with high power density of 85.3 mW cm-2 and ultralow Pt loading of 0.39 mg cm-2.

Heterogeneous Catalysis in Production and Utilization of Formic Acid for Renewable Energy

As the cleanest energy source, hydrogen has been followed with interest by researchers around the world. However, due to the internal low density of hydrogen, it cannot be stored and used efficiently which limits the hydrogen application on a huge scale. Chemical hydrogen storage is considered as a useful method for efficient handling and storage. Due to its excellent safety, formic acid stands out. It is worth noting that the matter and energy conversion is established based on formic acid, which is not referred to in the previous documentation. In this review, the latest development of research on heterogeneous catalysis via production and application of formic acid for energy application is reported. The matter and energy conversion based on formic acid are both discussed systematically. More importantly, with formic acid as the node, biomass energy shows potential to be in a dominant position in the energy conversion process. In addition, the catalytic mechanism is also mentioned. This review can provide the current state in this field and the new inspirations for developing superior catalytic systems.

Potential-Rate Correlations of Supported Palladium-Based Catalysts for Aqueous Formic Acid Dehydrogenation

Aqueous formic acid dehydrogenation (FAD) is a crucial process for hydrogen production, as hydrogen is a clean energy carrier. During this process, formic acid converts into hydrogen and carbon dioxide over a catalyst. Pd-based catalysts have exhibited significant potential in FAD due to their high activity and selectivity. In this study, we investigated aqueous thermal FAD in a mixture of formic acid and sodium formate using electrochemical open-circuit potential (OCP) measurement by loading the catalysts onto a conductive substrate as a working electrode. By varying the reaction conditions such as the concentration of reactants and modifying Pd with Ag, different FAD rates were obtained. Consequently, we revealed the correlation between the catalyst OCP and FAD rate; superior FAD rates reflected a more negative catalyst OCP. Furthermore, deactivation was observed across all catalysts during FAD, with a concurrent increase in catalyst OCP. Interestingly, we found that the logarithm of the FAD rate showed a linear correlation with the OCP of the catalyst during the decay phase, which we quantitatively explained based on the reaction mechanism. This study presents a new discovery that bridges thermal and electrocatalysis.

Enhanced formic acid electrolysis of Pd sites by improved OH adsorption assisted by MoP

MoP nanofiber-coupled Pd nanoparticles were demonstrated as efficient catalysts for formic acid-assisted water splitting in hydrogen generation. The theoretical calculations indicated that the OH on the surface of MoP through d-p bonding promoted the oxidation of CO at the Pd sites, and improved the ability to resist CO poisoning. As a result, enhanced catalytic performance was indicated.

Single-atom Mo-tailored high-entropy-alloy ultrathin nanosheets with intrinsic tensile strain enhance electrocatalysis

The precise structural integration of single-atom and high-entropy-alloy features for energy electrocatalysis is highly appealing for energy conversion, yet remains a grand challenge. Herein, we report a class of single-atom Mo-tailored PdPtNiCuZn high-entropy-alloy nanosheets with dilute Pt-Pt ensembles and intrinsic tensile strain (Mo1-PdPtNiCuZn) as efficient electrocatalysts for enhancing the methanol oxidation reaction catalysis. The as-made Mo1-PdPtNiCuZn delivers an extraordinary mass activity of 24.55 A mgPt-1 and 11.62 A mgPd+Pt-1, along with impressive long-term durability. The planted oxophilic Mo single atoms as promoters modify the electronic structure of isolated Pt sites in the high-entropy-alloy host, suppressing the formation of CO adsorbates and steering the reaction towards the formate pathway. Meanwhile, Mo promoters and tensile strain synergistically optimize the adsorption behaviour of intermediates to achieve a more energetically favourable pathway and minimize the methanol oxidation reaction barrier. This work advances the design of atomically precise catalytic sites by creating a new paradigm of single atom-tailored high-entropy alloys, opening an encouraging pathway to the design of CO-tolerance electrocatalysts.

Epitaxial Ultrathin Pt Atomic Layers on CrN Nanoparticle Catalysts

The construction of platinum (Pt) atomic layers is an effective strategy to improve the utilization efficiency of Pt atoms in electrocatalysis, thus is important for reducing the capital costs of a wide range of energy storage and conversion devices. However, the substrates used to grow Pt atomic layers are largely limited to noble metals and their alloys, which is not conducive to reducing catalyst costs. Herein, low-cost chromium nitride (CrN) is utilized as a support for the loading of epitaxial ultrathin Pt atomic layers via a simple thermal ammonolysis method. Owing to the strong anchoring and electronic regulation of Pt atomic layers by CrN, the obtained Pt atomic layers catalyst (containing electron-deficient Pt sites) exhibits excellent activity and endurance for the formic acid oxidation reaction, with a mass activity of 5.17 A mgPt -1 that is 13.6 times higher than that of commercial Pt/C catalyst. This novel strategy demonstrates that CrN can replace noble metals as a low-cost substrate for constructing Pt atomic layers catalysts.

A phosphate tolerant Pt-based oxygen reduction catalyst enabled by synergistic modulation of alloying and surface modification

Addressing phosphoric acid poisoning of platinum-based catalysts in high-temperature fuel cells still remains a strategic and synthetic problem. Here, we synthesized a Pt3Co@MoOx-NC catalyst with a Pt3Co active core and MoOx modification on the surface, which simultaneously exhibits high ORR activity and phosphate tolerance.

Exploiting Interfacial Effects between Collapsing Bubbles and Nanocarbon/TiN Substrates for the Green Synthesis of Self-Organized Noble Metal and Nanoalloy Nanoparticles

Noble metal nanoparticles and multi-materials thereof are processed on a substrate from aqueous solutions of the metallic ions, precluding any chemical additives/catalysts. The methods reported here take advantage of interactions between collapsing bubbles and the substrate that result in the generation of reducing radicals at the substrate surface and leading to the reduction of the metal ions on those sites, followed by nucleation and growth. Two selected substrates where these phenomena take place are nanocarbon and TiN. By either using ultrasonic radiation of the substrate in ionic solution or quenching the substrate in a solution from temperatures above the Leidenfrost temperature, a high density of nanoparticles of Au, Au/Pt, Au/Pd and Au/Pd/Pt are synthesized on the substrate surface. The sites where the reducing radicals are generated determine the self-assembly of the nanoparticles. The methods yield highly adherent surface films and nanoparticles; they are materials efficient and cost effective because only the surface is modified with costly materials. The formation mechanisms of these green multi-material NPs are described. Outstanding electrocatalytic performances in acidic solutions of methanol and formic acid are demonstrated.

Correlating the Experimentally Determined CO Adsorption Enthalpy with the Electrochemical CO Reduction Performance on Cu Surfaces

CO binding energy has been widely employed as a descriptor for effective catalysts in the electrochemical CO₂ and CO reduction reactions (CO₍₂₎ RR), however, it has yet to be determined experimentally at electrochemical interfaces due to the lack of suitable techniques. In this work, we developed a method to determine the standard adsorption enthalpy of CO on Cu surfaces with quantitative surface enhanced infrared absorption spectroscopy. On dendritic Cu at -0.75 V vs. SHE, the standard adsorption enthalpy, entropy and Gibbs free energy were determined to 1.5±0.5 kJ mol^-1 , ≈37.9±13.4 J/(mol K), and ≈-9.8±4.0 kJ mol^-1 , respectively. Comparison of the standard adsorption enthalpy of oxide-derived Cu and dendritic Cu, as well as their CORR activities, suggests the presence of stronger binding sites on OD Cu, which could favor multicarbon products. The method developed in this work will help establish the correlation between the CO binding energy and the CO₍₂₎ RR activity.

Leveraging Pd(100)/SnO2 interfaces for highly efficient electrochemical formic acid oxidation

The electrocatalytic formic acid oxidation (FAO) is the crucial anodic reaction of direct formic acid fuel cells (DFAFCs), but its activity remains to be largely improved in order to be practically viable. The rational development of enhanced catalysts requires thorough consideration of various contributing factors that are possibly integrated in composite systems. Here, we demonstrate that, Pd(100)/SnO₂ interfaces, provided being efficiently exploited, can significantly boost FAO activity by a factor of ∼10, compared with pure Pd(100) facets, with the mass activity reaching a record of 14.55 A mg_Pd^-1 at a 40 mV-lower peak potential. Unique Pd/SnO₂ nanocomposites with a myriad of Pd(100)/SnO₂ interfaces were obtained by a newly developed successive seeded growth strategy, wherein pre-formed SnO₂ nanospheres are used as seeds for two-round overgrowth of multitudinous Pd nanocubes. Using electron microscopic, electrochemical, spectroscopic and computational analyses, we found that the Pd(100)/SnO₂ interfaces induce lattice contraction and electron loss on Pd nanocubes, which optimize intermediate binding during FAO. Moreover, we showed that the good cubicity of the Pd nanocubes and the presence of SnO₂ nearby further promote the activity by facilitating the potential-determining step and the elimination of the poisoning CO intermediate, respectively. As such, the combined high intrinsic activity and number density of Pd(100)/SnO₂ interfaces enabled the superior activity of the Pd/SnO₂ nanocomposites. The composite material presented here holds promise for application in DFAFCs, but equally importantly, the insights regarding the structure-performance relationship would be beneficial for designing efficient metal/oxide composite catalysts for diverse electro- and photo-catalytic reactions.

Recent Advances in Anode Electrocatalysts for Direct Formic Acid Fuel Cell-II-Platinum-Based Catalysts

Platinum-based catalysts have a long history of application in formic acid oxidation (FAO). The single metal Pt is active in FAO but expensive, scarce, and rapidly deactivates. Understanding the mechanism of FAO over Pt important for the rational design of catalysts. Pt nanomaterials rapidly deactivate because of the CO poisoning of Pt active sites via the dehydration pathway. Alloying with another transition metal improves the performance of Pt-based catalysts through bifunctional, ensemble, and steric effects. Supporting Pt catalysts on a high-surface-area support material is another technique to improve their overall catalytic activity. This review summarizes recent findings on the mechanism of FAO over Pt and Pt-based alloy catalysts. It also summarizes and analyzes binary and ternary Pt-based catalysts to understand their catalytic activity and structure relationship.

Correlations between experiments and simulations for formic acid oxidation

Electrocatalytic conversion of formic acid oxidation to CO₂ and the related CO₂ reduction to formic acid represent a potential closed carbon-loop based on renewable energy. However, formic acid fuel cells are inhibited by the formation of site-blocking species during the formic acid oxidation reaction. Recent studies have elucidated how the binding of carbon and hydrogen on catalyst surfaces promote CO₂ reduction towards CO and formic acid. This has also given fundamental insights into the reverse reaction, i.e. the oxidation of formic acid. In this work, simulations on multiple materials have been combined with formic acid oxidation experiments on electrocatalysts to shed light on the reaction and the accompanying catalytic limitations. We correlate data on different catalysts to show that (i) formate, which is the proposed formic acid oxidation intermediate, has similar binding energetics on Pt, Pd and Ag, while Ag does not work as a catalyst, and (ii) *H adsorbed on the surface results in *CO formation and poisoning through a chemical disproportionation step. Using these results, the fundamental limitations can be revealed and progress our understanding of the mechanism of the formic acid oxidation reaction.

High-Entropy Intermetallic PtRhBiSnSb Nanoplates for Highly Efficient Alcohol Oxidation Electrocatalysis

The control of multimetallic ensembles at the atomic-level is challenging, especially for high-entropy alloys (HEAs) possessing five or more elements. Herein, the one-pot synthesis of hexagonal-close-packed (hcp) PtRhBiSnSb high-entropy intermetallic (HEI) nanoplates with intrinsically isolated Pt, Rh, Bi, Sn, and Sb atoms is reported, to boost the electrochemical oxidation of liquid fuels. Taking advantage of these combined five metals, the well-defined PtRhBiSnSb HEI nanoplates exhibit a remarkable mass activity of 19.529, 15.558, and 7.535 A mg^-1 _Pt+Rh toward the electrooxidation of methanol, ethanol, and glycerol in alkaline electrolytes, respectively, representing a state-of-the-art multifunctional electrocatalyst for alcohol oxidation reactions. In particular, the PtRhBiSnSb HEI achieves record-high methanol oxidation reaction (MOR) activity in an alkaline environment. Theoretical calculations demonstrate that the introduction of the fifth metal Rh enhances the electron-transfer efficiency in PtRhBiSnSb HEI nanoplates, which contributes to the improved oxidation capability. Meanwhile, robust electronic structures of the active sites are achieved due to the synergistic protections from Bi, Sn, and Sb sites. This work offers significant research advances in developing well-defined HEA with delicate control over compositions and properties.

Atomically Reconstructed Palladium Metallene by Intercalation-Induced Lattice Expansion and Amorphization for Highly Efficient Electrocatalysis

As an emerging class of materials with distinctive physicochemical properties, metallenes are deemed as efficient catalysts for energy-related electrocatalytic reactions. Engineering the lattice strain, electronic structure, crystallinity, and even surface porosity of metallene provides a great opportunity to further enhance its catalytic performance. Herein, we rationally developed a reconstruction strategy of Pd metallenes at atomic scale to generate a series of nonmetallic atom-intercalated Pd metallenes (M-Pdene, M = H, N, C) with lattice expansion and S-doped Pd metallene (S-Pdene) with an amorphous structure. Catalytic performance evaluation demonstrated that N-Pdene exhibited the highest mass activities of 7.96 A mg^-1, which was 10.6 and 8.5 time greater than those of commercial Pd/C and Pt/C, respectively, for methanol oxidation reaction (MOR). Density functional theory calculations suggested that the well-controlled lattice tensile strain as well as the strong p-d hybridization interaction between N and Pd resulted in enhanced OH adsorption and weakened CO adsorption for efficient MOR catalysis on N-Pdene. When tested as hydrogen evolution reaction (HER) catalysts, the amorphous S-Pdene delivered superior activity and durability relative to the crystalline counterparts because of the disordered Pd surface with a further elongated bond length and a downshifted d-band center. This work provides an effective strategy for atomic engineering of metallene nanomaterials with high performance as electrocatalysts.

Determining the role of Pd catalyst morphology and deposition criteria over large area plasmonic metasurfaces during light-enhanced electrochemical oxidation of formic acid

The use of metal composites based on plasmonic nanostructures partnered with catalytic counterparts has recently emerged as a promising approach in the field of plasmon-enhanced electrocatalysis. Here, we report on the role of the surface morphology, size, and anchored site of Pd catalysts coupled to plasmonic metasurfaces formed by periodic arrays of multimetallic Ni/Au nanopillars for formic acid electro-oxidation reaction (FAOR). We compare the activity of two kinds of metasurfaces differing in the positioning of the catalytic Pd nanoparticles. In the first case, the Pd nanoparticles have a polyhedron crystal morphology with exposed (200) facets and were deposited over the Ni/Au metasurfaces in a site-selective fashion by limiting their growth at the electromagnetic hot spots (Ni/Au-Pd@W). In contrast, the second case consists of spherical Pd nanoparticles grown in solution, which are homogeneously deposited onto the Ni/Au metasurface (Ni/Au-Pd@M). Ni/Au-Pd@W catalytic metasurfaces demonstrated higher light-enhanced FAOR activity (61%) in comparison to the Ni/Au-Pd@M sample (42%) for the direct dehydrogenation pathway. Moreover, the site-selective Pd deposition promotes the growth of nanoparticles favoring a more selective catalytic behavior and a lower degree of CO poisoning on Pd surface. The use of cyclic voltammetry, energy-resolved incident photon to current conversion efficiency, open circuit potential, and electrochemical impedance spectroscopy highlights the role of plasmonic near fields and hot holes in driving the catalytic enhancement under light conditions.

Observation and Analysis of Staircase Response of Single Palladium Nanoparticle Collision on Gold Ultramicroelectrodes

Collision (or impact) of single palladium nanoparticles (Pd NPs) on gold (Au), copper (Cu), nickel (Ni), and platinum (Pt) ultramicroelectrodes (UMEs) were investigated via electrocatalytic amplification method. Unlike the blip responses of previous Pd NP collision studies, the staircase current response was obtained with the Au UME. The current response, including collision frequency and peak magnitude, was analyzed depending on the material of the UME and the applied potential. Adsorption factors implying the interaction between the Pd NP and the UMEs are suggested based on the experimental results.

Au-Stabilized Nanoporous PdCuAu Alloys Exhibiting Outstanding Catalytic Activity and Durability for the Formic Acid Oxidation Reaction

Metallic Pd is widely recognized as an efficient electrocatalyst for the formic acid oxidation reaction (FAOR), which unfortunately suffers from poor durability owing to Pd dissolution and CO poisoning. The present work describes an effective method to enhance Pd durability by alloying with Cu and Au. Cu could provide surface OH at low potentials to remove poisonous CO for improved CO resistance. Au, the most inert metal, was added to reduce Pd and Cu dissolution. Moreover, alloying with Cu and Au could also modulate the electronic structure of Pd which is just profitable for the FAOR. The constructed PdCuAu with a nanoporous structure exhibits a specific activity of 14.9 mA cm^-2 and a Pd mass activity of 6012 A g^-1, which is ∼15 times and ∼14 times higher than those of commercial Pd/C. While these two electrocatalysts were used as fuel cell anodes, the maximum power density of the PdCuAu anode (Pd loading 10 μg cm^-2) is 93.2 mW cm^-2 and the value of the Pd/C anode (Pd loading 1.2 mg cm^-2) is 60.3 mW cm^-2. The power efficiency of Pd has been notably increased by 185 times in this home-made nanoporous PdCuAu ternary alloy electrocatalyst.

Recent Advances in Anode Electrocatalysts for Direct Formic Acid Fuel Cells - Part I - Fundamentals and Pd Based Catalysts

Direct formic acid fuel cells (DFAFCs) have gained immense importance as a source of clean energy for portable electronic devices. It outperforms other fuel cells in several key operational and safety parameters. However, slow kinetics of the formic acid oxidation at the anode remains the main obstacle in achieving a high power output in DFAFCs. Noble metal-based electrocatalysts are effective, but are expensive and prone to CO poisoning. Recently, a substantial volume of research work have been dedicated to develop inexpensive, high activity and long lasting electrocatalysts. Herein, recent advances in the development of anode electrocatalysts for DFAFCs are presented focusing on understanding the relationship between activity and structure. This review covers the literature related to the electrocatalysts based on noble metals, non-noble metals, metal-oxides, synthesis route, support material, and fuel cell performance. The future prospects and bottlenecks in the field are also discussed at the end.

Well-designed internal electric field from nano-ferroelectrics promotes formic acid oxidation on Pd

Pd-Based catalysts are considered the most efficient catalysts in direct formic acid fuel cells. However, the poisoning and dissolution of Pd in acidic systems limit its commercialization. Here, we propose an all-in-one solution for the anti-dissolution and anti-poisoning properties of palladium. A novel structured catalyst, Pd nanoparticles embedded in a carbon layer internally decorated with tourmaline nanoparticles (TNPs), is proposed for formic acid oxidation (FAO). The internal electric field strength of the catalysts is readily regulated by controlling the amount of TNPs. Remarkably, the prepared catalyst exhibits as high as 3.9 times mass activity (905 A g^-1) compared with the commercial Pd/C catalyst. The significant improvement in the electrocatalytic performance of the catalyst is mainly due to the polarized electric field of TNPs causing charge transfer from Pd to tourmaline, which weakens the O-H bond of HCOOH and the bond between Pd and CO_ad. Another advantage brought by the internal polarized electric field is that it facilitates water dissociation to produce OH_ad, thereby improving the anti-poisoning ability of the catalyst in acidic media. Moreover, the firmly anchored Pd nanoparticles can avoid dissolution and agglomeration during long-term use. 80.2% mass activity remained after the accelerated durability test.

Formic Acid Dehydrogenation Using Noble-Metal Nanoheterogeneous Catalysts: Towards Sustainable Hydrogen-Based Energy

The need for sustainable energy sources is now more urgent than ever, and hydrogen is significant in the future of energy. However, several obstacles remain in the way of widespread hydrogen use, most of which are related to transport and storage. Dilute formic acid (FA) is recognized asa a safe fuel for low-temperature fuel cells. This review examines FA as a potential hydrogen storage molecule that can be dehydrogenated to yield highly pure hydrogen (H2) and carbon dioxide (CO2) with very little carbon monoxide (CO) gas produced via nanoheterogeneous catalysts. It also present the use of Au and Pd as nanoheterogeneous catalysts for formic acid liquid phase decomposition, focusing on the influence of noble metals in monometallic, bimetallic, and trimetallic compositions on the catalytic dehydrogenation of FA under mild temperatures (20–50 °C). The review shows that FA production from CO2 without a base by direct catalytic carbon dioxide hydrogenation is far more sustainable than existing techniques. Finally, using FA as an energy carrier to selectively release hydrogen for fuel cell power generation appears to be a potential technique.

Solution-Phase Synthesis of PdH0.706 Nanocubes with Enhanced Stability and Activity toward Formic Acid Oxidation

Palladium is one of the few metals capable of forming hydrides, with the catalytic properties being dependent on the elemental composition and spatial distribution of H atoms in the lattice. Herein, we report a facile method for the complete transformation of Pd nanocubes into a stable phase made of PdH_0.706 by treating them with aqueous hydrazine at a concentration as low as 9.2 mM. Using formic acid oxidation (FAO) as a model reaction, we systematically investigated the structure-catalytic property relationship of the resultant nanocubes with different degrees of hydride formation. The current density at 0.4 V was enhanced by four times when the nanocubes were completely converted from Pd to PdH_0.706. On the basis of a set of slab models with PdH(100) overlayers on Pd(100), we conducted density functional theory calculations to demonstrate that the degree of hybrid formation could influence both the activity and selectivity toward FAO by modulating the relative stability of formate (HCOO) and carboxyl (COOH) intermediates. This work provides a viable strategy for augmenting the performance of Pd-based catalysts toward various reactions without altering the loading of this scarce metal.

Core-shell Pd-P@Pt-Ni nanoparticles with enhanced activity and durability as anode electrocatalyst for methanol oxidation reaction

Pd-P@Pt-Ni core-shell nanoparticles, which consisted of a Pd-P alloy as a core and Pt-Ni thin layer as a shell, were explored as electrocatalysts for methanol oxidation reaction. The crystallographic information and the electronic properties were fully investigated by X-ray diffraction and X-ray photoelectron spectroscopy. In the methanol electrooxidation reaction, the particles showed high catalytic activity and strong resistance to the poisoning carbonaceous species in comparison with those of commercial Pt/C and the as-prepared Pt/C catalysts. The excellent durability was demonstrated by electrochemically active surface area loss and chronoamperometric measurements. These results would be due to the enhanced catalytic properties of Pt by the double synergistic effects from the core part and the nickel species in the shell part.

How palladium inhibits CO poisoning during electrocatalytic formic acid oxidation and carbon dioxide reduction

Development of reversible and stable catalysts for the electrochemical reduction of CO₂ is of great interest. Here, we elucidate the atomistic details of how a palladium electrocatalyst inhibits CO poisoning during both formic acid oxidation to carbon dioxide and carbon dioxide reduction to formic acid. We compare results obtained with a platinum single-crystal electrode modified with and without a single monolayer of palladium. We combine (high-scan-rate) cyclic voltammetry with density functional theory to explain the absence of CO poisoning on the palladium-modified electrode. We show how the high formate coverage on the palladium-modified electrode protects the surface from poisoning during formic acid oxidation, and how the adsorption of CO precursor dictates the delayed poisoning during CO₂ reduction. The nature of the hydrogen adsorbed on the palladium-modified electrode is considerably different from platinum, supporting a model to explain the reversibility of this reaction. Our results help in designing catalysts for which CO poisoning needs to be avoided.

Tailor-designed nanowire-structured iron and nickel oxides on platinum catalyst for formic acid electro-oxidation

This investigation is concerned with designing efficient catalysts for direct formic acid fuel cells. A ternary catalyst containing iron (nano-FeOx) and nickel (nano-NiOx) nanowire oxides assembled sequentially onto a bare platinum (bare-Pt) substrate was recommended for the formic acid electro-oxidation reaction (FAOR). While nano-NiOx appeared as fibrillar nanowire bundles (ca. 82 nm and 4.2 μm average diameter and length, respectively), nano-FeOx was deposited as intersecting nanowires (ca. 74 nm and 400 nm average diameter and length, respectively). The electrocatalytic activity of the catalyst toward the FAOR depended on its composition and loading sequence. The FeOx/NiOx/Pt catalyst exhibited ca. 4.8 and 1.6 times increases in the catalytic activity and tolerance against CO poisoning, respectively, during the FAOR, relative to the bare-Pt catalyst. Interestingly, with a simple activation of the FeOx/NiOx/Pt catalyst at −0.5 V vs. Ag/AgCl/KCl (sat.) in 0.2 mol L⁻¹ NaOH, a favorable Fe²⁺/Fe³⁺ transformation succeeded in mitigating the permanent CO poisoning of the Pt-based catalysts. Interestingly, this activated a-FeOx/NiOx/Pt catalyst had an activity 7 times higher than that of bare-Pt with an ca. −122 mV shift in the onset potential of the FAOR. The presence of nano-FeOx and nano-NiOx enriched the catalyst surface with extra oxygen moieties that counteracted the CO poisoning of the Pt substrate and electronically facilitated the kinetics of the FAOR, as revealed from CO stripping and impedance spectra.

A FeOx/NiOx/Pt catalyst was recommended for formic acid electro-oxidation; the essential anodic reaction in direct formic acid fuel cells.

Quantum yield enhancement in the photocatalytic HCOOH decomposition to H2 under periodic illumination

Despite numerous studies on controlled periodic illumination to improve the quantum yield of photocatalytic reactions, debates still exist on the nature of such effect. In our system, we proposed that enhanced electron transfer is the promotion mechanism.

Phase-Controlled Synthesis of Pd-Sn Nanocrystal Catalysts of Defined Size and Shape

Bimetallic Pd-based nanoparticles (NPs) are of interest as electrocatalysts for formic acid electrooxidation (FAEO) because of their higher initial catalytic activity and CO tolerance when compared to Pt. Intermetallic NPs (i-NPs) with specific geometric and electronic structures generally exhibit superior catalytic activity, selectivity, and durability when compared to their disordered (random alloy) counterparts; however, the colloidal synthesis of i-NPs remains a challenge. Here, a one-pot method was demonstrated as a facile route to obtain monodisperse Pd-Sn NPs with phase control, including intermetallic hexagonal Pd3Sn2 (P63/mmc), intermetallic orthorhombic Pd2Sn (Pnma), and alloy cubic Pd3Sn (FCC, Fm3m) as size-controlled NPs with quasi-spherical shapes. Initial metal precursor ratios and reaction temperature were critical parameters to achieving phase control. Also, slight modifications of synthetic conditions resulted in either Pd2Sn nanorhombohedra or nanorods with tunable aspect ratios. A systematic evaluation of the Pd-Sn NPs for FAEO showed that most presented higher specific activities when compared to commercial Pd/C, in which Pd2Sn quasi-spheres and nanorhombohedra showed the highest catalytic activity for FAEO. These results highlight the benefits of phase-controlled Pd-based nanocatalysts with defined nanocrystal size and shape, with use of trioctylphospine (TOP) and oleic acid (OA) central to shape and size control.

Rich grain boundaries endow networked PdSn nanowires with superior catalytic properties for alcohol oxidation

Networked nanowire (NNW)-structured catalysts have attracted extensive attention due to their large surface area and structural stability, which mean that they have excellent catalytic activity and stability and can be used as anode reaction catalysts for use in direct alcohol fuel cells (DAFCs). Herein, a series of networked PdSn nanowires synthesized via a modified polyol strategy are used as efficient DAFCs anode reaction catalysts. The introduction of Sn plays an important role in the improvement of catalytic behavior, in which the existence of Sn promotes the oxidation of intermediates by providing abundant oxyphilic species. Moreover, the generated PdSn NNWs-3 with optimal content show rich grain boundaries and an even NNW structure, which provides more active sites to further improve catalytic performance, so it exhibits excellent activity toward alcohol oxidation. The mass activities of PdSn NNWs-3 toward the ethanol oxidation reaction (EOR) and the methanol oxidation reaction (MOR) are 8105.0 and 3099.5 mA mg_Pd^-1, which are 6.9 and 10.7 times higher than those of Pd/C, respectively. Compared with Pd/C, the PdSn NNWs also display enhanced stability towards the EOR and MOR. This work demonstrates that NNW nanocatalysts indeed exhibit excellent catalytic performance for alcohol oxidation reactions.

Machine Learning in Screening High Performance Electrocatalysts for CO2 Reduction

Converting CO₂ into carbon-based fuels is promising for relieving the greenhouse gas effect and the energy crisis. However, the selectivity and efficiency of current electrocatalysts for CO₂ reductions are still not satisfactory. In this paper, the development of machine learning methods in screening CO₂ reduction electrocatalysts over the recent years is reviewed. Through high-throughput calculation of some key descriptors such as adsorption energies, d-band center, and coordination number by well-constructed machine learning models, the catalytic activity, optimal composition, active sites, and CO₂ reduction reaction pathway over various possible materials can be predicted and understood. Machine learning is now realized as a fast and low-cost method to effectively explore high performance electrocatalysts for CO₂ reduction.

Rational Design of Single-Atom Site Electrocatalysts: From Theoretical Understandings to Practical Applications

Atomically dispersed metal-based electrocatalysts have attracted increasing attention due to their nearly 100% atomic utilization and excellent catalytic performance. However, current fundamental comprehension and summaries to reveal the underlying relationship between single-atom site electrocatalysts (SACs) and corresponding catalytic application are rarely reported. Herein, the fundamental understandings and intrinsic mechanisms underlying SACs and corresponding electrocatalytic applications are systemically summarized. Different preparation strategies are presented to reveal the synthetic strategies with engineering the well-defined SACs on the basis of theoretical principle (size effect, metal-support interactions, electronic structure effect, and coordination environment effect). Then, an overview of the electrocatalytic applications is presented, including oxygen reduction reaction, hydrogen evolution reaction, oxygen evolution reaction, oxidation of small organic molecules, carbon dioxide reduction reaction, and nitrogen reduction reaction. The underlying structure-performance relationship between SACs and electrocatalytic reactions is also discussed in depth to expound the enhancement mechanisms. Finally, a summary is provided and a perspective supplied to demonstrate the current challenges and opportunities for rational designing, synthesizing, and modulating the advanced SACs toward electrocatalytic reactions.

Bimetallic Pd-Co Nanoparticles Supported on Nitrogen-Doped Reduced Graphene Oxide as Efficient Electrocatalysts for Formic Acid Electrooxidation

In this work, bimetallic PdxCoy nanoparticles supported on nitrogen-doped reduced graphene oxide catalysts were synthesized and tested for formic acid oxidation as potentially efficient and durable electrocatalysts. Graphene oxide was nitrogen doped through hydrothermal chemical reduction with urea as a nitrogen source. The PdxCoy nanoparticles were deposited on the nitrogen-doped graphene oxide support using the impregnation-reduction method with sodium borohydride as a reducing agent and sodium citrate dihydrate as a stabilizing agent. The structural features, such as phases, composition, oxidation states, and particle sizes, of the nanoparticles were characterized using X-ray diffraction, transmission electron microscopy, scanning electron microscopy–energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The Pd nanoparticle sizes in Pd1Co1/N-rGO, Pd/N-rGO, and Pd1Co1/CNT were 3.5, 12.51, and 4.62 nm, respectively. The electrochemical performance of the catalysts was determined by CO stripping, cyclic voltammetry, and chronoamperometry. Pd1Co1/N-rGO showed the highest mass activity of 4833.12 mA–1 mg Pd, which was twice that of Pd1Co1/CNT. Moreover, Pd1Co1/N-rGO showed a steady-state current density of 700 mA–1 mg Pd after 5000 s in chronoamperometry carried out at +0.35 V. Apart from the well-known bifunctional effect of Co, nitrogen-doped graphene contributed to the performance enhancement of the Pd1Co1/N-rGO catalyst.

Highly Efficient Dehydrogenation of Formic Acid over Binary Palladium-Phosphorous Alloy Nanoclusters on N-Doped Carbon

Highly efficient dehydrogenation of formic acid (FA) at room temperature is a safe and suitable way to obtain hydrogen and promote the development of hydrogen storage application. Herein, the phosphorous-alloyed Pd nanoclusters loading on nitrogen-doped carbon (PdP/NC) were prepared and recognized as the highly active nanocatalysts for the dehydrogenation of FA. The PdP/NCs with controlled sizes and compositions were prepared by an easy self-limiting synthesis in an aqueous solution. The best PdP/NC exhibited a remarkable catalytic activity with a high turnover frequency of ∼3253.0 h^-1 than the compared nanocatalysts for the dehydrogenation of FA at room temperature. The catalytic kinetics and durability studies showed that both the alloyed P in Pd crystals and doped N in the carbon support could effectively tailor the electronic states of the Pd surface and further optimize the adsorption energy of FA. Based on the Sabatier principle, the proper adsorption energy accelerated the dehydrogenation reaction and correspondingly enhanced the activity and durability. The work proposed a high-efficiency nanocatalyst for safe hydrogen generation and may be extended to create other similar nanocatalysts with different compositions and nanostructures.