HMF Adsorption Research Articles

Synergistic enhancement of nickel-cobalt nano-catalysts for 5-hydroxymethylfurfural conversion via electronic structure engineering and solar intensification

The electrooxidation of 5-hydroxymethylfurfural (HMF) into vital chemical intermediates has received widespread attention. In order to achieve efficient conversion of HMF, the main focus has been on the design and development of delicate catalysts. In addition, the introduction of an external field in the reaction process can also improve the electrocatalytic efficiency. In this paper, nickel-cobalt composite oxide catalysts with controlled metal centers (Ni/Co) were synthesized by a simple “coating-calcination” method. The rational rearrangement of d-electron by Co-doping facilitated the improvement of HMF adsorption, which was further confirmed by Density Functional Theory calculations. The catalysts synthesized at the optimal Co-doping ratio could still achieve 100 % of HMF conversion, 95.65 % of 2,5-furandicarboxylic acid yield, and 92.97 % of Faraday efficiency after 25 electrochemical cycles. Crucially, we introduced solar illumination to further enhance the performance of electrooxidation HMF. The catalyst performance was further improved via local photothermal effect and additional photocurrent. Therefore, the yield of FDCA was increased by more than 27 % under 2.0 kW m−2 illuminations. An electrooxidation system constructed based on external field enhancement and catalyst design provides new insights into the efficient conversion of biomass.

Oxygen Defect Site Filling Strategy Induced Moderate Enrichment of Reactants for Efficient Electrocatalytic Biomass Upgrading.

The electrocatalytic oxidation of 5-hydroxymethylfurfural (HMF) provides a feasible approach for the efficient utilization of biomass. Defect regulation is an effective strategy in the field of biomass upgrading to enhance the adsorption capacity of reactants and thus increase the activity. However, how to select appropriate strategies to regulate the over-enrichment of reactants induced by excessive oxygen vacancy is still a huge challenge. In this work, the defect-filling strategy to design and construct an element-filled oxygen vacancy site layered double hydroxide (S─Ov─LDH) is adopted, which achieves a significant reduction in the electrolysis potential of biomass platform molecule HMF oxidation reaction and a significant increase in current density. Physical characterizations, electrochemical measurements, and theoretical calculations prove that the formation of metal─S bond in the second shell effectively regulates the electronic structure of the material, thus weakening the over-strong adsorption of HMF and OH- induced by excessive oxygen vacancy, promoting the formation of high-valence Co3+ during the reaction, and forming new adsorption sites. This work discusses the catalytic enhancement mechanism of defect filling in detail, fills the gap of defect filling in the field of biomass upgrading, and provides favorable guidance for the further development of defect regulation strategies.

Hydrogen-Bond-Assisted Electrocatalytic Semi-Oxidation of 5-Hydroxymethylfurfural into 2,5-Diformylfuran by Operando Dissociated N-Oxyl Mediator.

The conversion of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) is a promising approach for enhancing biomass utilization. Nevertheless, traditional methods using noble metal catalysts face challenges due to high costs and poor selectivity towards DFF. Herein, we developed a novel catalytic electrode integrating N-hydroxyphthalimide (NHPI) into a metal-organic framework on a hydrophilic carbon cloth. This design significantly enhances the selective adsorption of HMF due to stronger hydrogen-bond interaction between the electrode's hydrophilic surface and the C(sp3)-OH group in HMF compared to the C(sp2)=O in DFF. Additionally, the electro-driven dissociation of the NHPI-linker generates stabilized N-Oxyl radicals that promote selective semi-oxidation of HMF under neutral conditions. As a result, this approach achieves a high yield rate of 138.2 mol molcat -1 h-1 with a selectivity of 96.7 % for the HMF-to-DFF conversion. This work introduces a novel strategy for designing catalytic electrodes with stabilized N-Oxyl radicals, and offers a promising method for electrocatalytic DFF synthesis, leveraging hydrogen-bond interaction between electrode surface and HMF.

Selective Photocatalytic Oxidation of 5‐Hydroxymethylfurfural using Ce doped TiO2: A Crucial Role of Defective Sites

AbstractTransforming platform compounds with conventional catalysts, such as homogeneous and heterogeneous, into valuable chemicals encounters challenges associated with entailing high temperature/pressure and reusability during the reaction. To address these challenges, photocatalysis is one of the promising approaches, especially with visible‐light‐driven photocatalysts for selective transformation. A series of cerium‐doped TiO2 (CexTiO2; x=0.5, 1, 3 and 5 wt %) are prepared via a simple coprecipitation method for the cocatalyst‐free selective oxidation of 5‐hydroxymethylfurfural (HMF) to 2,5‐diformylfuran (DFF) under visible light illumination. Introducing Ce into the network to TiO2 generates impurity energy levels, reduces the band gap, and extends the spectral response range. This also causes slight distortion to the surface structure, thereby originating defective sites and various surface oxygen species, confirmed by X‐ray photoelectron spectroscopy and electron paramagnetic resonance studies. The HMF adsorption studies infer that HMF forms a surface complex with CeTiO2, facilitating the formation of ligand‐to‐metal charge transfer complex (LMCT) under visible light (~420 nm), which efficiently catalyzes the conversion of HMF (54.4 %) to DFF with a selectivity of >99 %. In‐situ DRIFTS and surface passivation studies provide insight into the interaction between HMF and surface acidic/basic sites along with hydroxyl groups of CeTiO2, which subsequently convert selectively to DFF.

Selective switching hydrogenation products of 5-hydroxymethylfurfural at high substrate concentrations by regulating Pd-MgO interactions

Controlling the selective activation of one or some functional groups as desired is still a challenge in hydrogenation, especially at high substrate concentrations. Herein, Pd-MgO interfaces over Al2O3 were finely regulated for efficiently selective hydrogenation of furan ring in 5-hydroxymethylfurfural (HMF) to produce 5-hydroxymethyltetrahydro-2-furaldehyde (5-HMTHFF) or total hydrogenation to 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF), also inhibiting side reaction. The Pd-MgO/Al2O3 with 12.0 wt% of MgO selectively hydrogenates the furan rings with a 5-HMTHFF yield of 82.4 % while that with 2.0 wt% of MgO totally hydrogenates HMF with a DHMTHF yield of 96.2 % at high substrate concentrations (400 mM). Characterizations and DFT calculations demonstrated that oligomeric MgO species suppress the agglomeration of Pd nanoparticles and strengthen HMF adsorption, consequently promoting total hydrogenation. Furthermore, Pdδ+-MgO1-x sites between the crystallized MgO and Pd metal favored tilted adsorption on the interface and weakened the activation of the CO bond, consequently selectively producing 5-HMTHFF.

Synergistic boosting the electrooxidation of biomass-based 5-hydroxymethylfurfural on cellulose-derived Co3O4/N-doped carbon catalysts

Catalysts play a pivotal role in the efficient conversion of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) through electrochemical oxidation. In this work, a cost-effective and highly efficient cobalt-based electrocatalyst for the synthesis of bio-based carboxylic acids was reported. By the employment of cellulose, which was dissolved in the alkaline-urea with cobalt ion, as the precursor, it derived a carbon-coated Co3O4 (Co3O4@NC) catalyst with a high specific surface area and rich porous structure. When utilized in the electrocatalytic conversion of HMF to FDCA, the catalyst exhibited exceptional yields and Faradaic efficiency which surpassed 95 %. In-situ Raman spectra unveiled that a dual-pathway process occurred on this catalyst, with part of Co3O4 serving as active sites for HMF adsorption, while other Co3O4 transformed into CoOOH during the reaction. This dual-pathway electrocatalysis facilitated the highly efficient conversion of HMF. Additionally, using bio-based alcohols/aldehydes as the feedstocks, eight carboxylic acids were successfully synthesized with yields ranging from 91.5 % to 99 %. This study presents a highly efficient electrocatalyst derived from biomass, enabling diverse bio-based carboxylic acid production with significant potential for sustainable chemical synthesis.

Regulating Adsorption Behavior of Reactants on NiO/CuO/Co3O4 Surface by Electronic Effect to Promote Electrosynthesis

AbstractDeveloping efficient electrooxidation 5‐hydroxymethylfurfural (HMF) catalysts with high selectivity and fast reaction kinetic is challenging. The HMF oxidation reaction (HMFOR) involves the adsorption of HMF and OH− on the catalyst, thus understanding the adsorption behavior between the catalyst surface and reactants is vital. In this work, by studying the relationship between HMFOR performance and the adsorption behavior of reactants on different transition metal oxides (TMOs), it is discovered that the catalytic performance of TMOs is related to the adsorption capacity of OH− and HMF simultaneously. Subsequently, TMOs with different HMF and OH− adsorption abilities are coupled to further optimize the catalytic performance of HMFOR. Experimental and theoretical calculation results indicate that the electronic interactions between different TMOs can regulate the substrate adsorption behavior and electron transfer ability of the catalysts, which is beneficial for HMFOR. Among them, due to the strong interaction between the three components optimizes the adsorption capacity for HMF and OH−, NiO/CuO/Co3O4 exhibits the best HMFOR performance with FDCA selectivity of 99.6 % and formation rate of 16.45 mmol gcat−1 h−1. This work provides a design principle for HMFOR catalysts by modulating the adsorption behavior of reaction molecules.

Introducing Fe into Cu-based catalyst to boost the electrocatalytic hydrogenation of 5-hydroxymethylfurfural.

Converting biomass-derived 5-hydroxymethylfurfural (HMF) into high-valued 2,5-bis (hydroxymethyl)furan (BHMF) via electrocatalytic hydrogenation (ECH) technology has been widely regarded as one of the most economical and eco-friendly routes. The high selectivity and activity depend on the reasonable regulation of the adsorption and activation of adsorbed hydrogen (H*) and HMF on the surface of the electrocatalyst. Herein, we report nanoflower-like CuFe-based electrocatalysts on copper foam (CF) substrates (CuFeOx/CF). BHMF was achieved on the optimal CuFeOx/CF with a selectivity of 93.3% and a yield of 90.1%. The H*, HMF and product were observed by in situ attuned total reflection Fourier transform infrared spectroscopy (ATR-FTIR). Moreover, in situ Raman spectra discloses the reconstruction of catalyst into CuFe-bimetal with low valence state. Density functional theory (DFT) calculations demonstrate that introducing Fe plays a role in regulating the electronic structure of Cu sites, which facilitate the generation of H* and adsorption of HMF, thus hampering the occurrence of dimerization. This study provides an innovative idea for the rational design of non-precious bimetallic electrocatalysts for ECH to produce high-valued chemicals.

Atomic-level rhodium doping into NiO for boosting the selective electrooxidation of HMF to FFCA in neutral electrolyte

The oxidation reaction of the biomass derivative 5-hydroxymethylfurfural (HMF) holds immense promise for the sustainable synthesis of high-value chemicals. Nevertheless, achieving the selective electrooxidation of HMF to generate high value-added formyl-2-furan carboxylic acid (FFCA) intermediate remains a serious challenge. Here, the V-NiO-Rh catalyst was constructed and demonstrates an impressive FFCA selectivity of 71 % for HMF electrooxidation reaction (HMFOR) in neutral electrolyte. The designed experiments elucidate that the HMFOR mechanism is the co-adsorption of OH* and HMF* on Ni active sites. Furthermore, the theoretical calculations prove that the modulation of Ni electronic structure by single atom Rh promotes the ability of HMF adsorption and the water electrolysis to release OH*. This paper establishes a theoretical foundation for the selective synthesis of high value-added intermediates products that are challenging to obtain in alkaline media.

Deciphering in-situ surface reconstruction in two-dimensional CdPS3 nanosheets for efficient biomass hydrogenation

Steering on the intrinsic active site of an electrode material is essential for efficient electrochemical biomass upgrading to valuable chemicals with high selectivity. Herein, we show that an in-situ surface reconstruction of a two-dimensional layered CdPS3 nanosheet electrocatalyst, triggered by electrolyte, facilitates efficient 5-hydroxymethylfurfural (HMF) hydrogenation to 2,5-bis(hydroxymethyl)furan (BHMF) under ambient condition. The in-situ Raman spectroscopy and comprehensive post-mortem catalyst characterizations evidence the construction of a surface-bounded CdS layer on CdPS3 to form CdPS3/CdS heterostructure. This electrocatalyst demonstrates promising catalytic activity, achieving a Faradaic efficiency for BHMF reaching 91.3 ± 2.3 % and a yield of 4.96 ± 0.16 mg/h at − 0.7 V versus reversible hydrogen electrode. Density functional theory calculations reveal that the in-situ generated CdPS3/CdS interface plays a pivotal role in optimizing the adsorption of HMF* and H* intermediate, thus facilitating the HMF hydrogenation process. Furthermore, the reconstructed CdPS3/CdS heterostructure cathode, when coupled with MnCo2O4.5 anode, enables simultaneous BHMF and formate synthesis from HMF and glycerol substrates with high efficiency.

Vacancy Mediated Electrooxidation of 5‐Hydroxymethyl Furfuryl Using Defect Engineered Layered Double Hydroxide Electrocatalysts

AbstractElectrochemical biomass oxidation coupled with hydrogen evolution offers a promising route to generate value‐added chemicals and clean energy. The complex adsorption behavior of 5‐hydroxymethyl furfural (HMF) and hydroxyl ions (OH−) on the electrocatalyst surface during HMF electrooxidation reaction (HMFOR) necessitates an in‐depth understanding of active sites available for adsorption. Herein, oxygen vacancy (VO) defects are introduced in NiFe layered double hydroxide (LDH) using Ce dopants to manipulate electronic structure. Synchrotron‐based HE‐XRD and XAS indicate negligible VO in La‐doped NiFe while Ce doping leads to VO defects due to flexible Ce redox (Ce3+↔ Ce4+). The VO‐rich Ce‐NiFe exhibits higher Faradic efficiency of ≈90% to produce 2,5‐furan dicarboxylic acid (FDCA), far greater than ≈60% for NiFe VO in Ce‐NiFe act as alternative active sites for OH− adsorption, hence reducing adsorption competition for the same metal sites. DFT calculation results corroborate experimental findings by showcasing that the presence of VO in Ce‐NiFe manipulates the adsorption energies and facilitates the chemical adsorption OH− in VO to improve HMFOR. In situ HE‐XRD derived pair distribution function coupled to RMC simulations confirm OH− trapping in VO and HMF adsorption on metal centers as evident by interlayer distance evolution. Taken together, this work showcases routes for dual‐site electrocatalyst design for improved biomass electrooxidation.

Ni-Cu-based phosphide heterojunction for 5-hydroxymethylfurfural electrooxidation-assisted hydrogen production at large current density

5-hydroxymethylfurfural oxidation reaction (HMFOR) offers a promising avenue to achieve energy-saving H2 production and produce value-added chemicals. However, the lack of HMFOR electrocatalysts with large current density and high selectivity impedes the whole productivity. Herein, a Ni3P-Cu3P heterojunction grown on Cu foam (Ni3P-Cu3P/CF) was successfully constructed, achieving large current density (300 mA cm−2 at 1.60 V vs. RHE) and high selectivity and Faradaic efficiency (>99 %) for HMFOR. The X-ray photoelectron spectroscopy and theoretical calculations reveal that the interface charge redistributes at the Ni3P-Cu3P heterointerface, resulting into the charge-deficiency Ni3P and charge-accumulation Cu3P. The charge-deficiency Ni3P induced by charge-attracting Cu3P favors to form more high-valence Ni species, which facilitates to optimize the adsorption of HMF and OH* species for improving current density and decreasing potential, while the charge-accumulation Cu3P enables to broaden the potential window by suppressing competitive oxygen evolution reaction, thus elevating the conversion rate and selectivity of products. Benefiting from the excellent performance of Ni3P-Cu3P/CF for HMFOR, when constructing a HMFOR-assisted H2 production system using Ni3P-Cu3P/CF and self-prepared MoNiNx/NF as anode and cathode, the energy consumption was substantially decreased to 3.8 kW•h/Nm3 relative to that of pure water splitting (4.66 kW•h/Nm3). Our work is instructive for achieving low energy consumption of H2 production and synthesis of valuable chemicals by constructing heterojunction.

Electron Structure Tuned Oxygen Vacancy-Rich AuPd/CeO2 for Enhancing 5-Hydroxymethylfurfural Oxidation.

The design of high activity catalyst for the efficiently conversion of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) gains great interest. The rationally tailoring of electronic structure directly affects the interaction between catalysts and organic substrates, especially molecular oxygen as the oxidant. This work, the bimetallic catalysts AuPd/CeO2 were prepared by the combining method of chemical reduction and photo-deposition, effectively concerting charge between Au and Pd and forming the electron-rich state of Au. The increasing of oxygen vacancy concentration of CeO2 by acidic treatment can facilitate the adsorption of HMF for catalysts and enhance the yield of FDCA (99.0 %). Moreover, a series of experiment results combining with density functional theory calculation illustrated that the oxidation performance of catalyst in HMF conversion was strongly related to the electronic state of interfacial Au-Pd-CeO2. Furthermore, the electron-rich state sites strengthen the adsorption and activation of molecular oxygen, greatly promoting the elimination of β-hydride for the selective oxidation of 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) to FDCA, accompanied with an outgoing FDCA formation rate of 13.21 mmol ⋅ g-1 ⋅ min-1 at 80 °C. The perception exhibited in this research could be benefit to understanding the effects of electronic state for interfacial sites and designing excellent catalysts for the oxidation of HMF.

Large enhancement of electrochemical biomass oxidation by optimizing the competitive adsorption of HMF and OH− on doped CoOx

By doping transition metals into CoOx, we can regulate the deintercalation capacity of OH−. The CoCuOx with enriched oxygen vacancies allowed the optimization of the competitive adsorption between HMF and OH−, and accelerated the kinetics of HMFOR.

Bimetallic nanozymes synergize to regulate the behavior of oxygen intermediates and substrate HMF adsorption.

We have constructed a bimetallic (CoNiP) nanozyme, leveraging the synergistic effect of cobalt and nickel, which efficiently catalyzes the oxidation of TMB from colorless to ox-TMB (blue). Density functional theory (DFT) calculations further highlight the pivotal role of this synergistic effect in improving the adsorption energy of oxygen intermediates, accelerating the catalytic process.

“Electrochemical reduction of HMF to synthesize added value compounds using CuAg electrodes”

In biorefining, Green electrochemistry offers an intelligent approach to produce valuable chemicals that are not derived from fossil fuels. In this study, we demonstrate the electrochemical reduction of 5-hydroxymethylfurfural (HMF), a biomass-derived compound, into 2,5-bis-hydroxymethylfuran (BHMF), which serves as a fundamental component in the polymer industry. This reaction was performed using Cu and Ag electrodes, but a better performance was obtained by combining the properties of the two metals in Ag decorated Cu foils. Additionally, by means of Impedance Spectroscopy IS we could uncover that the incorporation of Ag onto Cu electrodes not only increases the active area of the electrodes but also improves both the HMF adsorption on Cu and the catalytic properties of Ag. This synergistic effect leads to a more efficient reduction of HMF. Nonetheless, the diffusion of reactive species imposes a constraint on the increase in current. Finally, the observed inverted hysteresis in the cyclic voltammetry is attributed to variations in the amount of HMF adsorbed on the electrode surface during the forward and reverse directions of the cyclic voltammetry. Unlike other systems such as solar cells, where this effect is linked to the presence of a low frequency inductive behavior which, in this case, plays a negligible role.

Solid redox equilibrium via d-band centers regulation in the MoS2/Ni3S2 heterojunction for efficient selective oxidation of 5-hydroxymethylfurfural

The 5-hydroxymethylfurfural electrocatalytic oxidation reaction (HMFOR), in coupling with the hydrogen evolution reaction (HER), can replace the kinetically slow anodic oxygen evolution reaction (OER) to produce high value oxides and hydrogen. However, the commonly employed Ni-based electrocatalyst exhibits an unsatisfied performance due to the existence of OER. Here, we design and synthesize the Ni3S2/MoS2/NiMoO4 catalyst for efficient and stable HMFOR reaction. Through the in situ Raman spectroscopy, we demonstrate that the NiIII–OOH formed during the surface reconstruction process is the real reactive species for HMFOR. Moreover, the density functional theory results show that the upward shift of d-band centers in the reconstructed NiOOH/MoS2 is beneficial for the HMF adsorption and guarantees the smooth progression of redox equilibrium reactions. As a result, the prepared electrocatalyst shows a good HMFOR electrocatalytic performance in terms of selectivity (99.2%), conversion (96.7%), and Faraday efficiency (96.5%).

Anionic Regulation and Heteroatom Doping of Ni‐Based Electrocatalysts to Boost Biomass Valorization Coupled with Hydrogen Production

AbstractElectrocatalytic biomass valorization coupled with hydrogen production provides an efficient and economical way to achieve a zero‐carbon economy. Ni‐based electrocatalysts are promising candidates due to their intrinsic redox capabilities, but the rational design of active Ni site coordination is still a huge challenge. Herein, the combined strategies of surface reconstruction and heteroatom doping are adopted to modify Ni3S2 pre‐catalysts and the obtained bimetallic catalyst exhibits superior electrocatalytic performance toward 5‐hydroxymethylfurfural (HMF) oxidation to 2,5‐furanedicarboxylic acid (FDCA). Specifically, the oxysulfide‐coordinated amorphous NiOOH (NiOOH‐SOx) active phase is in situ constructed following the anionic regulation mechanism, which endows numerous defects and unsaturated sites for anodic HMF oxidation. Cu heteroatom doping further modulates the electronic structure of active sites with abundant Lewis acidic sites, offering advanced capability for HMF adsorption. Several operando characterization techniques (in situ Raman, infrared, and electrochemical impedance spectroscopies) are performed to disclose the reaction pathway and structure‐activity‐potential relationship. Theoretical results further demonstrate that Cu doping and oxyanionic regulation effectively modulate the local coordination environment of Ni sites and correspondingly tailor the intermediate adsorption behavior and then promote the reaction kinetics. Moreover, a two‐electrode system is assembled to pair HMF oxidation with cathode hydrogen production, demonstrating better energy conversion efficiency.

Impact of the Electrochemically Inert Furan Ring on the Oxidation of the Alcohol and Aldehyde Functional Group of 5‐Hydroxymethylfurfural (HMF)

AbstractThe electrochemical oxidation of bio‐based 5‐hydroxymethylfurfural (HMF) results in 2,5‐furandicarboxylic acid (FDCA), which is a renewable and environmentally friendly alternative to terephthalic acid. Using a gold electrode, we compare the electrochemical oxidation of the aldehyde and alcohol functionality in HMF to the isolated functionalities represented by ethanol and acetaldehyde. Thereby, we investigate the effect of the inert furan ring on the electrochemical reaction. The linear sweep voltammogram (LSV) of HMF in a weakly adsorbing electrolyte differs only marginally from the superposition of LSVs obtained in ethanol and acetaldehyde containing electrolytes. However, in the presence of strong adsorbates, only the kinetics of ethanol and acetaldehyde oxidation but not of HMF oxidation are hampered. We assign this to a stronger adsorption of HMF through the furan ring than through the alcohol and carbonyl functionality of ethanol and acetaldehyde. Hence, HMF is better equipped to compete for adsorption sites than aliphatic compounds.

Fluorine Induced In Situ Formation of High Valent Nickel Species for Ultra Low Potential Electrooxidation of 5-Hydroxymethylfurfural.

The Nickel-based catalysts have a good catalytic effect on the 5-hydroxymethylfurfural electrooxidation reaction (HMFOR), but limited by the conversion potential of Ni2+ /Ni3+ , 1.35V versus RHE, the HMF electrooxidation potential of nickel-based catalysts is generally greater than 1.35V versus RHE. Considering fluorine has the highest Pauling electronegativity and similar atomic radius of oxygen, the introduction of fluorine into the lattice of metal oxides might promote the adsorption of intermediate species, thus improving the catalytic performance. F is successfully doped into the lattice structure of NiCo2 O4 spinel oxide by the strategy of hydrothermal reaction and low-temperature fluorination. As is confirmed by in situ electrochemical impedance spectroscopy and Raman spectroscopy, the introduction of F weakens the interaction force of metal-oxygen covalent bonds of the asymmetric MT -O-MO backbone and improves the valence of Ni in tetrahedra structure, which makes it easier to be oxidized to higher valence active Ni3+ under the action of electric field and promotes the adsorption of OH- , while the decrease of Co valence enhances the adsorption of HMF with the catalyst. Combining the above reasons, F-NiCo2 O4 shows superb electrocatalytic performance with a potential of only 1.297V versus RHE at a current density of 20mA cm-2 , which is lower than the most catalyst.