Ultrahigh Hydrogen Evolution Performance of Under‐Water “Superaerophobic” MoS2 Nanostructured Electrodes

pH Overpotential for Unveiling the pH Gradient Effect of H ⁺ /OH ⁻ Transport in Electrode Reaction Kinetics

Anion exchange membrane (AEM) electrolysis remains an attractive alternative to conventional proton exchange membrane (PEM) electrolysis: AEM operates in base and therefore allows for the usage of non-Platinum Group Metal (PGM) catalysts. PEM operates in acid and only PGM materials can typically survive the high potentials and low pH while maintaining high activity. However, AEM is currently in the R&D stage at multiples levels of stack components, including ionomer-catalyst optimization.1 While the hydrogen evolution revolution (HER) is well-studied for metal surfaces, specifically, for mechanisms in acid, the specific mechanisms and contributing factors to HER in base and in the presence of an AEM ionomer requires further study. It is well-known that HER activity suffers a drop moving from acidic to alkaline media for PGMs (Pt, Pd, Ir, Rh).2, 3 Changes to hydrogen adsorption in particular would significantly alter the mechanism for HER, such as in the Volmer-Heyrovsky or Volmer-Tafel mechanisms:3-7 Volmer: H2O + e- -> Hads + OH- Heyrovsky: Hads + H2O + e- -> H2 + OH- Tafel: 2Hads -> H2 In base, hydrogen evolution may rely upon the Volmer step, a potentially, energetically uphill reaction requiring water to split to supply adsorbed H (Hads);3, 6, 7 moreover, the adsorbed H may have to compete with OH species for active sites in order to form H2.4, 8-10 Joint theoretical-experimental studies of both non-PGM catalysts and the model PGM, Pt, delving into the mechanistic considerations of HER for AEM electrolysis will be presented. We developed cost-competitive, high-performing Fe, Co-doped NiO catalysts for the oxygen evolution reaction.11 A similar theoretical screening of M-doped NiO catalysts for possible bifunctionality of these catalysts for combines both a mechanistic study of HER and electrochemical characterization of activity and site access via rotating disk electrode (RDE) half-cell tests. These initial water-splitting and deprotonation steps may be particularly influenced by the AEM ionomer and a model case of HER mechanisms in the presence of an ionomer on Pt will be explicated.(1) Pivovar, B. Current Status of Electrolyzer Technology and Needs for Successful Widespread Commercialization and Meeting Hydrogen Shot Targets. In Department of Energy (DOE) Hydrogen Shot Summit, August 31‒September 1, 2021, 2021.(2) Sheng, W.; Gasteiger, H. A.; Shao-Horn, Y. Hydrogen oxidation and evolution reaction kinetics on platinum: acid vs alkaline electrolytes. Journal of The Electrochemical Society 2010, 157 (11), B1529-B1536.(3) Zheng, J.; Sheng, W.; Zhuang, Z.; Xu, B.; Yan, Y. Universal dependence of hydrogen oxidation and evolution reaction activity of platinum-group metals on pH and hydrogen binding energy. Science advances 2016, 2 (3), e1501602.(4) Sheng, W.; Myint, M.; Chen, J. G.; Yan, Y. Correlating the hydrogen evolution reaction activity in alkaline electrolytes with the hydrogen binding energy on monometallic surfaces. Energy & Environmental Science 2013, 6 (5), 1509-1512.(5) Skúlason, E.; Karlberg, G. S.; Rossmeisl, J.; Bligaard, T.; Greeley, J.; Jónsson, H.; Nørskov, J. K. Density functional theory calculations for the hydrogen evolution reaction in an electrochemical double layer on the Pt (111) electrode. Physical Chemistry Chemical Physics 2007, 9 (25), 3241-3250.(6) Barber, J.; Conway, B. Structural specificity of the kinetics of the hydrogen evolution reaction on the low-index surfaces of Pt single-crystal electrodes in 0.5 M dm− 3 NaOH. J. Electroanal. Chem. 1999, 461 (1-2), 80-89.(7) Schouten, K.; van der Niet, M.; Koper, M. Impedance spectroscopy of H and OH adsorption on stepped single-crystal platinum electrodes in alkaline and acidic media. Physical Chemistry Chemical Physics 2010, 12 (46), 15217-15224.(8) Marković, N. The hydrogen electrode reaction and the electrooxidation of CO and H 2/CO mixtures on well‐characterized Pt and Pt‐bimetallic surfaces. Handbook of fuel cells 2010.(9) Sheng, W.; Zhuang, Z.; Gao, M.; Zheng, J.; Chen, J. G.; Yan, Y. Correlating hydrogen oxidation and evolution activity on platinum at different pH with measured hydrogen binding energy. Nature communications 2015, 6, 5848.(10) Alia, S. M.; Ha, M.-A.; Ngo, C.; Anderson, G. C.; Ghoshal, S.; Pylypenko, S. Platinum–Nickel Nanowires with Improved Hydrogen Evolution Performance in Anion Exchange Membrane-Based Electrolysis. ACS Cat. 2020, 10 (17), 9953-9966. DOI: 10.1021/acscatal.0c01568.(11) Ha, M.-A.; Alia, S. M.; Norman, A. G.; Miller, E. M. Fe-Doped Ni-Based Catalysts Surpass Ir-Baselines for Oxygen Evolution Due to Optimal Charge-Transfer Characteristics. ACS Cat. 2024, 17347-17359. DOI: 10.1021/acscatal.4c04489. Fig. 1 Schematic of mepiquat (representative of piperidium-based ionomers such as PiperION) functional group possibly enhancing or poisoning the hydrogen evolution reaction mechanism on (M=Co, Cu, Fe, Mn, Zn)-doped-NiO catalysts. Figure 1

Revealing the mechanism of bifunctional PtLa electrocatalyst for highly efficient methanol oxidation, hydrogen evolution, and coupling reaction

Introduction The recovery of Cu from wastewater is a key issue for environmental and economic aspects1. The wastewater treatment containing Cu has been carried out by various techniques such as chemical precipitation, coagulating sedimentation, and electrochemical methods. Although chemical precipitation method is the most commonly used technique due to relatively simple operation, the process needs to put additional chemicals and generates large amounts of sludge. On the other hand, electrodeposition is known as the “clean” method because it is able to achieve the recovery of Cu without requiring addition of chemicals and generating sludge. However, the current efficiency tends to decrease especially in a dilute solution, because hydrogen evolution and oxygen reduction reaction occur as competition reaction. Boron-doped diamond (BDD) electrode is a candidate for resolving the above problem because it has the excellent electrochemical properties2. Along these lines, we study on the recovery of Cu from dilute cupric sulfate solution as a model wastewater by electrodeposition method using BDD electrodes, and glassy carbon (GC) as a comparative carbon electrode. Experimental Two types of BDD films with different boron doping level (B/C = 0.1%, 1%) were deposited onto silicon wafer substrates by a microwave plasma-assisted chemical vapor deposition method. Electrochemical measurements were carried out using a conventional three-electrode system: BDD and GC as a working electrode, Pt plate as a counter electrode, and Ag/AgCl (saturated KCl) as a reference electrode. The surface of BDD electrodes was oxidized in 0.1 M H2SO4 at 2.0 V for 10 min before electrochemical measurements. The electrochemical behavior of Cu ions was evaluated by cyclic voltammetry (CV) in an aqueous solution of 0.5 mM CuSO4 and 0.1 M H2SO4 with a scan rate of 0.3 V s−1. Before CV measurements, the solution was deoxidized by bubbling with nitrogen for 10 min. Electrodeposition of Cu was carried out by chronoamperometry at various potentials from 0 to −0.8 V in 9-ml aqueous solution of 0.5 mM CuSO4 and 0.1 M H2SO4. During the electrodeposition, nitrogen kept bubbling in the solution. After electrodeposition, concentration of residual Cu ions was measured by an inductively coupled plasma atomic emission spectroscopy. Results and Discussion Figure 1 showed the time-dependent Cu recovery rates and current efficiencies at the various potentials on BDD and GC electrodes. When the electrodeposition was carried out at 0 V on all electrodes, Cu could hardly deposit on the electrodes. The Cu recovery rates on 0.1%BDD increased with increasing the applied negative potential, whereas the Cu recovery rates on GC decreased at −0.8 V, compared to the other potentials. On 1%BDD, Cu recovery rates hardly changed at any potential except 0 V. From CV measurements, reduction potentials corresponding to Cu2+ ions to metal Cu were −0.48 V, −0.28 V, and −0.20 V on 0.1%BDD, 1%BDD, and GC, respectively. Since the nucleation potential of Cu on 0.1%BDD is the most negative, Cu recovery rates at −0.2 V was lower than the other potentials. The nucleation overpotentials were different between 0.1%BDD and 1%BDD due to the difference of their conductive properties. As 0.1%BDD has a characteristic of a p-type semiconductor, a depletion layer occurs at the surface of the electrode with a band bending when the electrode contacts the solution. Therefore, cathodic reactions are inhibited due to increasing the depletion layer when applying the negative potential. On the other hand, 1%BDD has a characteristic of metal-like conductivity, so charge transfer from the electrode to Cu ions easily occurs. Also, the hydrogen evolution potential on 0.1%BDD was the most negative, followed in order by 1%BDD, and GC. Therefore, Cu recovery rates at −0.8 V on GC decreased due to the hydrogen evolution. The current efficiencies decreased with increasing the applied negative potentials on the all electrodes, because hydrogen evolution reaction easily occurs at more negative potential. Also, the current efficiencies on BDD were higher than GC at any potential. It is assumed that the current efficiency was improved using BDD electrodes due to inhibition of hydrogen evolution reaction, compared to GC electrode. Moreover, the current efficiency using 0.1%BDD was the higher than 1%BDD due to higher hydrogen overpotential, whereas 0.1%BDD also required high Cu deposition overpotential. Therefore, it was found that the recovery of Cu was achieved with high current efficiency using 1%BDD along with the lower power consumption. References (1) F. Fu and Q. Whang, J. Environ. Manage. 92, 407 (2011). (2) Y. Einaga, J. Appl. Electrochem. 40, 1807 (2010). Figure 1

In electrochemical hydrogen evolution reaction (HER), the produced hydrogen gas bubbles often adhered to the electrode surface and blocked the active catalytic site. While different catalysts are developed to improve the catalytic performance of HER, the design of a durable and universal approach for minimizing the force of nucleated hydrogen gas‐bubble adhesion to prevent blockage of electrocatalytic sites because of bubbles‐adhesion is unprecedented. Generally, buoyancy should outweigh the capillary force to remove nucleated bubbles, which means these forces ratio, Eötvös number Eo > 1. Herein, a chemically reactive multilayer coating on an electrode is reported to chemically modulate the adhesion force of nucleated gas‐bubble on the electrode. A dual modified coating on Ni‐foam provided a non‐adhesive superaerophobicity with nucleated bubble adhesion force of 4.6 ± 0.3 µN and displayed superior HER performance with lower overpotential (333 to 250 mV) at 100 mA cm−2 with respect to bare Ni‐foam. The chemically‐modulated low bubble‐adhesion facilitates the early removal of nucleated tiny hydrogen gas bubbles with a minimum size of 0.64 mm and Eo = 0.05 to keep catalytic sites available for superior electrochemical HER. Such a positive impact of the prepared coating is also noted for various other electrodes.

Hydrogen evolution reaction (HER) has been extensively studied using various kinds of electrodes because it is a fundamental electrochemical reaction and also because it is an important reaction for the production of H2. The HER can occur more or less at a cathode electrode in aqueous solution under a variety of conditions. When the overpotential of the cathode is high, i.e., when the rate of the HER is high, hydrogen bubbles are formed vigorously at electrode surface. This often causes a disturbance in electrochemical reactions, leading to difficulties in electrochemical measurements, controls, and analysis. Thus, the HER has been mostly studied in a low overpotential region where no bubbles are formed or in an intermediate region where bubbles do not disturb electrochemical measurements. On the other hand, the HER in a high overpotential region attracts recent interest from the viewpoint of application such as production of metal nanoparticles [1]. We have reported [2] that the HER on Pt and Au in 0.10 M (M = mol/dm3) H2SO4 solution is anomalously affected in a high overpotential region, e.g., more negative than -0.5 V vs. SHE, as shown in Figure 1a. The reduction current due to the HER became low (in absolute value) when salt such as Na2SO4 and K2SO4 was added to the solution. Interestingly, a potential oscillation, named HER oscillation, appeared under current controlled conditions (Figures 1b and 1c). We recently reported that the potential oscillation appeared during the HER not only on Pt and Au electrodes but also on Rh, Ag, Cu, Fe, Ni, W, Zn, Sn, and In electrodes in a variety of electrolytes such as H2SO4, HNO3, and HClO4 [3]. As discussed in the earlier paper [2], the decrease in the HER current by adding salts is caused by the presence of cations, Na+ or K+, which changes the transport velocity of H+ to the electrode surface. The transport is caused not only by diffusion and solution-stirring but also by electromigration, when no salt is added to the solution. When the salt is added to the solution, the cation reduces the electromigration transport of H+, resulting in the decrease in the HER current. On the other hand, the potential oscillation appears in the absence of the salts when the concentration of H2SO4 is as low as 0.03 M [3], and thus the appearance of the oscillation is not attributed to the presence of cations but to the hydrogen bubbles formed on the electrode surface. The behavior of hydrogen bubbles oscillate synchronously with the potential oscillation. The bubbles evolve more vigorously in the low potential region where the reduction of water occurs than in the high potential region where the reduction of H+ occurs. Therefore, we have proposed that the bubble evolution is involved in both the positive and negative feedback mechanisms, the combination of which causes an oscillatory instability. This proposed mechanism, however, cannot explain a negative differential resistance (NDR), which is essential generally for the appearance of electrochemical oscillations. In the present study, to clarify how the bubble evolution is related to an NDR, the bubble behaviour is carefully observed by using a high-speed camera. Furthermore, electrochemical impedance spectra are measured repeatedly during the HER on a Pt electrode because the Nyquist plots obtained by the impedance measurements can show the presence of the NDR. In this presentation, the major factor that induces the NDR will be discussed and the oscillatory instability, or the positive and negative feedback mechanisms, will be reconsidered. REFERENCES [1] T. Nishimura, T. Nakade, T. Morikawa, H. Inoue, Electrochimi. Acta, 129, (2014) 152. [2] Y. Mukouyama, M. Kikuchi, H. Okamoto, J. Electroanal. Chem., 617 (2008) 179. [3] Y. Mukouyama, R. Nakazato, T. Shiono, S. Nakanishi, H. Okamoto, J. Electroanal. Chem., 713 (2014) 39. FIGURE CAPTION Figure 1. Current (I) - potential (E) curves for a Pt electrode in 0.10 M H2SO4 solution with or without 0.05 M K2SO4, measured (a) under potential controlled conditions and (b) under current controlled conditions. (c) Time course of E measured at I = -16 mA. Figure 1

Magnetocatalysis: The Interplay between the Magnetic Field and Electrocatalysis

Anion-Modulated Platinum for High-Performance Multifunctional Electrocatalysis toward HER, HOR, and ORR.

Hydrogen is one of the promising sources of renewable energy to meet the impending energy crisis. Hydrogen can be produced using techniques such as steam reforming of methane, coal gasification, water electrolysis etc.[ 1 ] Among these techniques, water electrolysis produces purest H2 without any harmful by-products, where water is split in to H2 and O2 gas with the supply of a DC power.[ 2 ] Hydrogen Evolution Reaction (HER) is the cathodic reaction in water splitting, and attracts wide attention among the researchers. HER on various noble and transition metals has been studied well.[ 3 ] Pt is the best known catalyst for HER and located at the top of volcano curve while Au is found to have low activity for HER.[ 4 ] HER is oftentimes performed in alkaline media due to the higher stability of the electrode materials though the rates of the reaction are relatively slow to those in acidic media.[ 5 ] The mechanism of HER is proposed to be a combination of three elementary steps (Eqns. 1-3). The Volmer (V) step is the first step of the reduction of water molecule on the catalytic surface into an adsorbed hydrogen atom and negatively charged hydroxide anion (Eqn. 1). The adsorbed hydrogen atom can either be attacked by a water molecule to produce a hydrogen molecule and a hydroxide anion (the Heyrovsky step, Eqn. 2), or combine with another adsorbed hydrogen atom (formed by the Volmer step) to generate a hydrogen molecule that leaves the surface (the Tafel step, Eqn. 3).Though several studies have been published on HER studies, the investigations were limited to Tafel slope analysis in a stationery electrode or a rotating disc electrode(RDE) in a narrow potential range.[ 6 ] A major reason for this constrain is that in a normal RDE, many bubbles are produced at high over-potentials and block the electrode surface, since the electrode surface faces downward. Thus, it restricts the experimental range and the confidence in the model parameters.In this work, cathodic polarization of HER on Pt and Au disc electrode at different concentration and rotational speeds are performed in an inverted RDE (IRDE) experimental arrangement. In IRDE, the electrode surface is faced upward and the gas bubbles from the electrode are released rapidly. This allow us to perform experiments in a wide potential window and increase the confidence in the mechanism identified.In all the experiments NaOH is used as the electrolyte and sufficient NaClO4 was added so that the total concentration of anions/ cations was maintained at 1 M. MillQ water (Millipore) of high purity was used to prepare the solutions. A three-electrode setup with Pt mesh as a counter electrode and Ag/AgCl in 3MNaCl as a reference electrode was used. The experimental results are compared with model predicted results of VH, VT and VHT mechanisms and the best fit kinetic parameters are determined.HER studies were conducted at 10 and 100mM of NaOH on Pt and Au IRDE at 900 rpm and the cathodic polarization results are shown in Fig. 1. The polarization current density is more or less independent of concentration of alkali and rotational speed of the electrode (not shown).

Open AccessCCS ChemistryMINI REVIEWS28 Dec 2022Electrocatalytic CO2 Reduction over Bimetallic Bi-Based Catalysts: A Review Wei Chen, Yating Wang, Yuhang Li and Chunzhong Li Wei Chen Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 , Yating Wang Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 , Yuhang Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 and Chunzhong Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Engineering Research Center of Hierarchical Nanomaterials, Frontiers Science Center for Materiobiology and Dynamic Chemistry, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237 School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237 https://doi.org/10.31635/ccschem.022.202202357 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Electrocatalytic reduction of carbon dioxide (CO2) to valuable fuels is an up-and-coming approach. Owing to the low cost, environmental friendliness, and high selectivity to formate single product at low overpotentials, bismuth (Bi)-based catalysts have attracted extensive research attention. In this review, the reaction mechanisms of Bi-based catalysts are first introduced, and the bimetallic Bi-based catalysts synthesized by alloying, doping, and loading strategies are reviewed from the aspects of catalyst component, morphology, synthesis procedure, and performance optimization for electrocatalytic CO2 reduction. We provide an in-depth discussion of the existing challenges and an outlook for this highly promising kind of electrocatalysis. Download figure Download PowerPoint Introduction The energy crisis and environmental pollution have been ongoing significant issues and the focus of the international community. Over the past few decades, the strong dependence and overuse of fossil fuels has resulted in a rapid increase in the concentration of carbon dioxide (CO2) in the atmosphere.1,2 When the World Meteorological Organization released its latest Greenhouse Gas Bulletin, they pointed out that the current CO2 concentration in the atmosphere is 149% of preindustrial levels.3 The Bulletin additionally noted that greenhouse gases have increased by 47% through radiative forcing, with CO2 accounting for 80% of this increase from 1990 to 2020. The emission of an oversized quantity of CO2 and other harmful gases has resulted in global warming and environmental pollution, documented in numerous studies related to the energy and environment around the world. CO2 emission reduction is urgently needed. Electrocatalytic CO2 reduction reaction In order to curtail the global warming trend, scientists conduct in-depth discussions and research on the issue of reducing CO2 emissions. There are four primary strategies: (a) developing new clean energy technologies; (b) upgrading existing processes to eliminate and replace the low-efficiency sectors and equipment in traditional technologies;1 (c) afforestation and forestation; and (d) carbon capture, utilization and storage.4,5 Research on carbon capture, carbon storage, and carbon utilization has produced many advances and breakthroughs in CO2 storage and conversion. Among them, carbon capture and storage technology have certain limitations. First of all, the technology is expensive. Second, storage equipment may leak and cause a series of other hidden safety problems, such as local seawater acidification. In contrast, carbon capture and utilization technology, which reduce CO2 and convert it into usable chemical value-added fuels, possesses greater development prospects.6–8 Not only can this technology reduce high CO2 concentrations in the atmosphere, but it also produces renewable fuels to combat the energy crisis. In recent years, numerous catalytic conversion methods have been developed successively. The methods used for CO2 reduction chiefly include biological (enzyme) catalysis, photocatalysis, thermocatalysis, and electrocatalysis.9–12 At present, electrocatalysis as an emerging energy technology for CO2 emission reduction and production of value-added fuels receives a great deal of research attention.7 Compared with traditional industrial processes, electrocatalytic CO2 reduction reaction (CO2RR) can be carried out under milder environmental conditions, improving electrochemical stability and selectivity in CO2RR via selecting appropriate electrocatalysts to manipulate reaction-tailored products.13 In this regard, reports in the literature demonstrate the exploration of various electrocatalysts and electrode reaction mechanisms for CO2RR. CO2RR also utilizes renewable energy for catalytic reactions to achieve large-scale energy storage and production of high-energy products.14 Generally, CO2RR is in a position to produce a variety of reduction products through electrocatalysis. The corresponding reduction products are different as electron-transferred numbers change. Electrocatalytic CO2RR products are categorized as formic acid (HCOOH),12 carbon monoxide (CO), methane (CH4), methanol (CH3OH) as C1, and ethanoic acid (CH3COOH), ethanol (C2H5OH)15 as C2.16,17 The above reduction products are obtained by various electron-transfer mechanisms and half-reactions, as shown in Table 1.18,19 Table 1 | Electrochemical Potentials of Several CO2 Reduction Reactions CO2 Reduction Half-Reactions Electrode PotentialV (vs SHE) Electrode PotentialV (vs RHE) CO2 + 2H+ + 2e− → CO + H2O −0.52 −0.106 CO2 + 2H+ + 2e− → HCOOH −0.61 −0.250 CO2 + 4H+ + 4e− → HCHO + H2O −0.51 −0.070 CO2 + 6H+ + 6e− → CH3OH + H2O −0.38 0.016 CO2 + 8H+ + 8e− → CH4 + 2H2O −0.24 0.169 2CO2 + 12H+ + 12e− → C2H4 + 4H2O −0.34 0.064 2CO2 + 12H+ + 12e− → C2H5OH + 3H2O −0.33 0.084 During the electrocatalytic reduction process, not solely CO2RR but also other side reactions will occur, resulting in the complexity of the reaction. For instance, the hydrogen evolution reaction (HER) competes with CO2RR to generate H2, which decreases the performance of CO2RR20–22 in order to efficiently overcome the energy barrier of electron-transfer proton coupling, accelerate the catalytic reaction rate. and suppress the occurrence of side reaction processes. The development of ideal catalysts with remarkable CO2RR selectivity and activity is the focus of current research. Several parameters of electrocatalytic performance for CO2RR catalysts can primarily be evaluated, including Faradaic efficiency (FE), overpotential, current density, Tafel slope, stability, and so on.7,12,14 FE is the charge required as a percentage of the initial charge to cross the working electrode and facilitate the electrochemical reaction. In simple terms, FE is a significant indicator to measure the selectivity of electrocatalytic CO2RR products.23 Due to the complex reaction mechanism and sluggish kinetics, the actual reduction reaction working potential is more negative than the theoretical reduction potential. High reduction overpotentials lead to wasted energy and significant HER reactions. Therefore, overpotential is an important indicator in evaluating the electrocatalytic activity of CO2RR catalysts. Studies confirm that when current density goes above 300 mA cm−2, the production cost will be reduced as much as possible.24 The equation of Tafel (η = blgj + a) is able to directly reflect the rate of reaction dominated by kinetics. The smaller the Tafel slope of b in the equation, the faster the electrochemical reaction rate and the higher the catalytic activity, which is more favorable for the electrocatalytic reaction.25 Stability is an indicator of whether an excellent electrocatalyst possesses long-term stability and efficiency.26 The stability of electrocatalysts is usually effectively assessed with potentiostatic electrolysis or cyclic voltammetry. Bismuth-based electrocatalysts Recently, P-block electrocatalysts consisting of bismuth (Bi),27–30 tin (Sn),31–33 lead (Pb),34–36 and indium (In)37,38 have inarguably facilitated electrocatalytic CO2 reduction with remarkable selectivity for C1 products, especially formic acid or formate. The advantage of electrocatalytic generation of formate lies in the high selectivity and current density achieved by prohibiting competing side reactions. Compared with other value-added products from CO2 reduction, which are difficult to solely generate and low in yield, the FE of Bi-based CO2 toward formate can reach nearly 100%.39 In addition, formate is a liquid product with excellent chemical stability at room temperature for storage and transportation compared to gas-phase products such as CO. Experts evaluate various chemicals with economic viability in CO2RR and discovered that formate has considerable marketability.13 Moreover, converting CO2 toward formate is a 2-electron transfer process, leading to a low production cost of 1$US/0.59 kg, suggesting that CO2 electrolysis of formate is more cost-competitive than the C2 product production process.40 It may be difficult to attain widespread application of Pb and In metals on a marketable scale because of toxicity or low availability. However, Bi is a dramatic and promising electrocatalyst due to its low cost, nontoxicity, environmental safety, relatively single reduction product, and high formate generation activity. Bi-based catalysts can be traced back to the Bi electrocatalyst synthesized by Komatsu's team in 1995.41 In the following decades, research on Bi-based catalysts continued to deepen. Monometallic catalysts, including metallic Bi, are currently a hot topic in the field of electrocatalysis. Recently, with in-depth exploration and development of synthesis techniques, various nanostructured monometallic Bi catalysts have been designed, such as nanoparticles, nanowires, nanotubes, nanosheets, nanodendrites, and so on, in multidimensional aspects. However, monometallic Bi catalysts may be undesirable for the breakthrough of formate electrosynthesis, owing to the limited active sites on the catalyst surface. This means that monometallic Bi catalysts usually require high overpotential to achieve high formate FE and partial current density. Compared with the monometallic Bi catalysts that use advanced synthetic strategies or tedious structural optimization to enhance their performance, bimetallic Bi-based catalysts will further involve synergistic effects. The synergistic interaction between bimetals gives the bimetallic electrocatalyst a superior catalytic performance.42 The synergistic effects in bimetallic Bi-based catalysts can be broadly viewed as Bi acting as the active site and the other metals mainly playing three roles: (1) tailoring the electronic structures of Bi sites, (2) regulating the adsorption states of the key intermediates, and (3) generating interfacial active sites to further enhance performance. Through the electronic structure modulations by the second metal, the bimetallic Bi-based catalysts will boost the formation of the key intermediate OCHO*, thus improving the performance of CO2 electroreduction to formate. The activity, selectivity, and stability will be further improved via preparing bimetallic Bi-based materials through strategies such as alloying, surface doping, defect introduction, and nanoengineering. Here, in this review, the representative reaction pathways of Bi-based electrocatalysts are first introduced, and then the reaction mechanisms of bimetallic Bi-based heterogeneous CO2RR electrocatalysts are summarized with examples from the perspective of reaction pathways. Afterward, based on the Bi-based electrocatalysts in recent development, we divide bimetallic Bi-based catalysts into three categories: (1) alloyed Bi, (2) doped Bi, and (3) supported Bi. For each category, we describe in detail its performance-enhancing strategies and provide examples of catalysts, including descriptions of their preparation process, composition, morphology, catalytic activity, and product properties. Finally, we provide an in-depth analysis of the existing challenges and the current outlook for this field. Reaction Mechanisms of the Bi-Based Electrocatalysts Bi-based catalysts have high efficiency, selectivity, and stability to form formate via electrocatalytic CO2RR in aqueous solutions. They also possess the capacity to generate CO, according to some reports.43 The pathways of the formate and CO products are comparatively simple compared to that of other CO2RR products, and the essential difference is the intermediate products. An in-depth study of the electrocatalytic CO2RR process on the surface of Bi-based catalysts is required for a good understanding of the catalytic mechanism of Bi metals. In general, there are three types of steps involving the generation of products theoretically, consisting of: (1) reactant adsorption on the electrocatalytic surface, (2) transfer of electrons and protons to the reactant, and (3) the products desorption from the electrocatalyst surface.44 Research demonstrates that the first proton coupling determines the selectivity for a catalyst, which takes place at the C or O in CO2*− radical anion. Three reaction pathways for electrocatalytic CO2RR over Bi-based catalysts are displayed in Figure 1: (a) Generally, CO2 comes into contact with the catalyst via carbon or oxygen atoms. If the carbon atom binds to the catalyst electrode surface first, *COOH intermediate will be formed, which is the first intermediate for CO2 activation in this pathway. However, *COOH intermediate will have multiple pathways in the second proton coupling electron transfer (PCET) process, which is not conducive to promoting highly selective formate production. *COOH can lose H2O to form CO, or be reduced to form HCOOH. (b) Compared to pathway a, pathway b is different in that the oxygen is bound to the electrode surface, by which CO2* − hydrogenation forms the HCOO* intermediate.19 Based on the reaction mechanism of Bi-based catalysts, the second PCET process of the HCOO* intermediate can only generate HCOOH.(c) In the pathway c, CO2 forms the OCHO* intermediate during the first PCET when only one oxygen molecule is bound to the surface electrode. Formate is the only product obtained via a subsequent second PCET process. Theoretical analyses indicate that the formation energy barrier of OCHO* intermediate is lower than that of *COOH and HCOO* intermediates, leading to the importance of OCHO* intermediates in the Bi-based electroreduction process.45 Figure 1 | Possible electrochemical reaction pathways of CO2 over Bi-based catalysts. Download figure Download PowerPoint Theoretical calculations confirm that the CO2RR process of Bi-based catalysts follows the key intermediate of OCHO* from a PCET mechanism, facilitating highly selective formate production. The specific mechanism equations (1–5) for reducing CO2 pathway in solution to HCOOH are summarized as follows:46 CO 2 ( g ) → CO 2 * (1) CO 2 * + e − → CO 2 * − (2) CO 2 * − + e − + H + → OCHO * − (3) OCHO * − + e − + H + → HCOOH * (4) HCOOH * → HCOOH ( aq ) + * (5)where * denotes the catalytic surface or adsorption site, initially the CO2 molecules are dissolved in the solution and contact the electrode surface to form adsorbed CO2*. Afterwards, single electron is transferred to CO2* that forms the CO2* − radical anion. According to HCO3 − ↔ H+ + CO32−, electron transfer and proton coupling form OCHO* − intermediates. Ultimately, OCHO* − forms formic acid solution by PCET. Since the slow kinetics of the electrocatalytic CO2RR process, HER side reactions inevitably generate H2, and some reduction reactions are accompanied by CO generation, which negatively affects the highly selective production of formate from Bi-based materials. For the purpose of obtaining more formate, the production of H2 and CO is reduced as much as possible. The catalytic mechanism may be diametrically different in the same Bi-based alloy, depending on the content of two metal elements. Therefore, the reaction mechanism is manipulated via controlling different proportions of the two metals in bimetallic Bi-based electrocatalysts. For instance, Zhang et al.47 have developed CuO/Bi(OH)3 decorated on carbon nanotubes for CO2 electroreduction. CuBi#8 and CuBi#4 are bimetallic nanoparticles obtained via a two-step hydrolysis method and adjusted the Cu/Bi ratio. CuBi#8 and CuBi#4 exhibit CO and formate FE of 96% and 60% at −0.99 V versus reversible hydrogen electrode (RHE) (VRHE), respectively. It is found that with Bi content increasing, HER is well suppressed, and the products are mainly CO. By further increasing Bi content, FEHCOOH rapidly increases accompanied by the rapid decrease of FECO. FEHCOOH reaches the maximum of 96% at 12.5 mA cm−2. The conversion of intermediates from *COOH to OCHO* via increasing Bi content further illustrates the high selectivity of OCHO* to formate. Adjusting the optimal Cu/Bi ratio suppresses the production of CO and H2, leading to efficient production of formate. In addition, defects such as oxygen vacancies or doped atoms significantly improve the catalytic performance.12 Li et al.48 have prepared Sn atom-doped Bi2O3 nanosheet (NS) electrocatalysts by constant electrolysis. Three products can be detected during the electrolysis, including H2, CO, and HCOOH. The Sn-doped Bi2O3 NSs current density is significantly increased compared to the undoped Bi2O3 NSs. The 2.5% Sn-doped Bi2O3 NSs exhibit high selectivity for formate, obtaining a supreme FE of 93.4% at the potential of −0.97 V. The HER inhibition effect is significantly enhanced compared to the undoped Bi2O3 NSs. Moreover, the catalytic capacity is optimized by coping with significant HER and expanding its specific surface area. For the first time, metallic aerogel is a three-dimensional (3D) material that has attracted enormous attention due to its abundant specific surface area, contributing to the generation of more catalytic centers.49 In addition, adjusting the partial pH can also improve the selectivity of CO2RR, and proper control of pH into acidity facilitates the formation of formate.50 Advanced Bi-Based Electrocatalysts for CO2 Reduction In recent decades, various Bi-based CO2 reduction electrocatalysts have been exhaustively studied, mainly with formate as the end product. Especially, the preparation of bimetallic catalysts via different synthetic methods is the focus of most current studies. Bimetallic Bi-based catalysts can mainly be classified into three types: (1) alloyed Bi, (2) doped Bi, and (3) supported Bi. Detailed CO2RR performances of bimetallic Bi-based electrocatalysts are summarized in Table 2. Table 2 | Performance of Bimetallic Bi-Based Catalysts in Electrocatalytic CO2RR Catalyst Electrolyte Major Products FE (%) Potential at FEMax (V) Current Density (mA cm−2) Stability (h) References Bi5Sn60 0.1 M KHCO3 Formate 94.8 −1.0 (vs RHE) 34 20 52 BixSny/Cu 0.1 M KHCO3 Formate 90.4 −0.84 (vs RHE) 30 12 53 Bi-Sn aerogel 0.1 M KHCO3 Formate 93.9 −1.0 (vs RHE) 9.3 10 57 Cu-Bi 0.1 M KHCO3 Formate 90 −0.8 (vs RHE) >2 — 60 CuBi-100 0.5 M KHCO3 Formate 94.7 −1.0 (vs RHE) 12.8 8 61 CuBi 0.5 M KHCO3 Formate 94.4 −0.97 (vs RHE) 38.5 — 62 CuBi 0.5 M KHCO3 Formate 98.3 −1.07 (vs RHE) 56.6 — 62 Bi/Cu 0.5 M KHCO3 Formate 95 −0.9 (vs RHE) 59.7 12 64 CuBi75 0.5 M KHCO3 Formate 100 −0.77 (vs RHE) 33.65 24 66 Cu1-Bi/Bi2O3@C 0.5 M KHCO3 Formate 93.4 −0.94 (vs RHE) 10.1 10 67 Pd3Bi-IMA 0.1 M KHCO3 Formate >90 −0.35 (vs RHE) 3 8 68 a-NPSB 0.1 M KHCO3 Formate 88.4 −1.15 (vs RHE) 21.2 18 70 Bi–Pt complex 0.1 M TBAPF6/THF CO 82 −1.25 (vs NHE) 0.125 — 71 Mo-Bi BMC/CP 0.5 M [Bmim]BF4 CH3OH 71.2 −0.7 (vs SHE) 12.1 — 72 Sn-doped Bi2O3 NSs 0.5 M KHCO3 Formate 93.4 −0.97 (vs RHE) 24.3 8 48 Bi/Bi(Sn)Ox NWs 1 M KOH Formate ∼100 −0.7 (vs RHE) 301.4 20 77 Cu-Bi2Se3 0.5 M NaHCO3 Formate 65.31 −1.3 (vs RHE) 24.1 24 78 Ce–[email protected]x/C 0.5 M KHCO3 Formate 96 −1.7 (vs SHE) 15.2 10 79 BiIn5[email protected] 0.5 M KHCO3 Formate 97.5 −0.86 (vs RHE) 13.5 15 81 Bi-Sn/CF 0.5 M KHCO3 Formate 96 −1.1 (vs RHE) 45 100 82 Sn0.80Bi0.20@Bi-SnOx 0.5 M KHCO3 Formate 95.8 −0.88 (vs RHE) 74.6 — 83 Bi-SnO/Cu 0.1 M KHCO3 Formate 93 −1.7 (vs Ag/AgCl) — 30 84 [email protected] 0.5 M NaHCO3 Formate 95 (vs RHE) 15 12 45 NSs 0.5 M KHCO3 Formate −0.86 (vs RHE) 8 90 0.5 M KHCO3 Formate −0.8 (vs RHE) 0.5 M KHCO3 Formate −0.8 (vs RHE) 10 [email protected] 0.5 M KHCO3 Formate (vs RHE) 10 M Formate (vs RHE) 34 Bi The is one of the methods to enhance the electrocatalytic performance of Bi-based effects facilitate the catalytic reaction by a between two different metal elements. P-block catalysts have CO2 conversion that is highly selective for Among them, Bi-Sn bimetallic is the most is a and method for the preparation of The method a of two metallic elements. Bimetallic obtained via are or and possess such as high density and can improve the catalytic activity of the catalyst Li et have two Bi and on the an In this different electrode The at different of This structure is to the surface and more catalytic When the of metal Bi and the of metal Sn 60 the catalytic performance the as the of −1.0 with mA partial current density, the FE of formate it excellent formate yield, which superior to most electrocatalysts. The of bimetallic is to the OCHO* intermediate and suppress the HER process. Li et have also electrocatalysts on and discovered that the FE of formate enhanced by the Bi content in the BixSny/Cu electrode. is an emerging in electrocatalytic due to their structure and the of the catalyst, contributing to the generation of more catalytic Bimetallic prepared by the method it to compared with methods such as or solution et have prepared Bi-Sn bimetallic under with and abundant Bi and Sn the of via controlling the ratio. with with a electron as shown in Figure abundant of that favorable for and abundant active sites during electrocatalysis. The of the Bi-Sn aerogel well with of Sn and Bi suggesting that the obtained aerogel is a Bi-Sn The that the are and corresponding to the and of Bi and This that the Sn and Bi are and more reaction sites are during the catalytic process. The the of Sn and Bi in the The electrochemical performance displayed compared with Sn and Bi catalysts, Bi-Sn aerogel excellent performance for formate with FE as high as Density that electronic between Bi and Sn the energy barrier for formate, the electrocatalytic performance In can indicate the reaction pathway of Bi-Sn aerogel bimetallic catalyst in CO2RR. shown in Figure the at at V and more as the potential increased from to V. This is to the that in the formate intermediate HCOO* is in suggesting that the synergistic effects between Bi-Sn bimetals can the generation of Figure 2 | (a) of the synthesis of Bi-Sn (b) of (c) of (d) and the corresponding of the catalytic mechanism of the of Bi-Sn with various with from Download figure Download PowerPoint is a and catalyst in the field of electrocatalysis. The product selectivity of catalysts is C1 products such as formate and CH4 and value-added fuels such as The of the Bi and to form a Cu-Bi usually formate conversion and the HER The reaction intermediates with of the between bimetallic Cu-Bi facilitate the catalytic reaction. et have synthesized bimetallic Cu-Bi electrocatalysts with Owing to the difference between Bi and the of bimetallic defect sites with high density, leading to the formation of more catalytic Cu-Bi exhibit lower current for HER and CO evolution thus the higher selectivity of Cu-Bi for formate. At −0.8 the FE of the formate product In to materials obtained by have numerous and catalytic activity will be Through the of bimetallic catalysts, the of reaction sites of electrocatalysts are increased as much as and the electrochemical of product selectivity is For instance, et have synthesized bimetallic Cu-Bi electrocatalysts by at room temperature In the by the of CuBi-100 the electrochemical performance At the potential from −0.8 V to CuBi-100 excellent selectivity for formate and FE of more than The supreme at the potential of −1.0 electron that CuBi-100 on carbon and a The of CuBi-100 has

Due to the competitive relationship between nitrate reduction reaction (NO 3 − RR) and hydrogen evolution reaction (HER), the conventional approach to improve Faradaic efficiency is to select a catalyst without HER activity. Nevertheless, such a strategy not only limits the application of HER catalysts in NO 3 − RR, but also causes insufficient hydrogen source, thereby sacrificing ammonia yield rate. We believe that HER catalysts should not be excluded from hydrogenation reduction. Herein, taking traditional water electrolysis material Co 3 O 4 as model system, we reveal that the oxygen vacancies on crystal facet can greatly promote water dissociation and capture HER intermediate for NO 3 − RR, successfully shifting the reaction pathway from hydrogen evolution to nitrate hydrogenation. Beyond material development, we construct a hybrid reactor and achieve an ammonia recovery rate of 1216.8 g‐N m −2 d −1 in nuclear industry wastewater with ultra‐high nitrate concentration. This study breaks through the limitation of HER catalyst in NO 3 − RR, which provides a significant insight into the catalyst designing and hydrogenation mechanism.

Due to the competitive relationship between nitrate reduction reaction (NO3 -RR) and hydrogen evolution reaction (HER), the conventional approach to improve Faradaic efficiency is to select a catalyst without HER activity. Nevertheless, such a strategy not only limits the application of HER catalysts in NO3 -RR, but also causes insufficient hydrogen source, thereby sacrificing ammonia yield rate. We believe that HER catalysts should not be excluded from hydrogenation reduction. Herein, taking traditional water electrolysis material Co3O4 as model system, we reveal that the oxygen vacancies on crystal facet can greatly promote water dissociation and capture HER intermediate for NO3 -RR, successfully shifting the reaction pathway from hydrogen evolution to nitrate hydrogenation. Beyond material development, we construct a hybrid reactor and achieve an ammonia recovery rate of 1216.8g-Nm-2d-1 in nuclear industry wastewater with ultra-high nitrate concentration. This study breaks through the limitation of HER catalyst in NO3 -RR, which provides a significant insight into the catalyst designing and hydrogenation mechanism.

In the context of the gradual depletion of global fossil fuel resources, it is increasingly necessary to explore new alternative energy. Hydrogen energy has attracted great interest from researchers because of its green and pollution-free characteristics. Moreover, the methanol oxidation reaction (MOR) can combine the hydrogen evolution reaction (HER), replacing the anode reaction (oxygen evolution reaction-OER) in overall water splitting and efficiently producing hydrogen. In this study, platinum-palladium nanoparticles on reduced graphene oxide (PtPd/rGO) were successfully synthesized as HER and MOR bifunctional electrocatalysts under alkaline conditions by the stepwise loading of Pt and Pd bimetallic nanoparticles on rGO using a simple liquid-phase reduction method. PtPd/rGO-2 with 0.99 wt% Pt and 2.86 wt% Pd in the HER has the lowest overpotential (87.16 mV at 100 mA cm-2), with the smallest Tafel slope (18.9 mV dec-1). The exceptional mass activity of PtPd/rGO-2 in the MOR reaches 10.75 A mg-1PtPd, which is 18.22 and 53.75 times greater than that of commercial Pt/C (Pt/C) and commercial Pd/C (Pd/C), respectively. PtPd/rGO-2 is 0.935 V lower in the coupling reaction of HER and MOR (MOR ∥ HER) compared to the overall water splitting (OER ∥ HER) without methanol (10 mA cm-2). This is probably because appropriate Pt and Pd loading exposes many more catalytic sites, and the synergistic interaction between Pt, Pd, and Pt-Pd enhances the catalytic performance. This strategy can be used for the synthesis of novel bifunctional electrocatalysts.

Phase-mediated cobalt phosphide with unique core-shell architecture serving as efficient and bifunctional electrocatalyst for hydrogen evolution and oxygen reduction reaction

In hydrogen evolution reaction, inefficient mass transfer caused by bubble adhesion on electrode, bubble dispersion in electrolyte and slow H₂ diffusion, has greatly impeded the reaction process. Existing techniques can only resolve bubble adhesion or bubble dispersion problems. Strategy that simultaneously solve bubble adhesion, bubble dispersion and poor hydrogen diffusion problems is rarely reported. Recently, an article reported a new electrode with special wettability design, which can efficiently promote bubble transfer and dissolved H₂ diffusion. This design can simultaneously solve above mentioned three mass transfer issues and improve electrode efficiency. We summarize the remaining challenges of this work and outlook potential approaches to promote mass transfer in gas-evolution reactions.