Ultra-low Catalyst Loading Research Articles

Interfacial engineering via laser ablation for high-performing PEM water electrolysis

A rationalized interfacial design strategy was applied to tailor the porous transport layer (PTL)-catalyst layer (CL) contact and the PTL bulk-phase architecture. Particularly, at the PTL-CL interface, our results reveal that laser ablated sintered titanium power-based PTLs improve electrolyzer performance at both the H2NEW Consortium baseline catalyst loading of 0.4 mgIr·cm−2 as well as at the ultra-low catalyst loading of 0.055 mgIr·cm−2. Under ultra-low catalyst loadings, the laser ablated PTL demonstrates maximum reduction of 230 mV compared to the commercial PTL at 4 A·cm−2, and reduces by 68 mV at 3.2 A·cm−2 under H2NEW baseline loading. Laser ablation alters the titanium phase at the interface, so it forms more uniform structure like a microporous layer or a backing layer, leading to an increase in the surface area in contact with the catalyst layer while preventing the membrane from deforming into the PTL. Moreover, we reveal that bulk-phase architecture modification of the PTL by ablating patterned pores at the flow field-PTL interface improves mass transport without sacrificing contact at the CL-PTL interface. Overall, laser ablation of the PTL is an effective method to customize interfacial design to enhance proton exchange membrane electrolyzer performance.

Brønsted acid catalyzed mechanochemical domino multicomponent reactions by employing liquid assisted grindstone chemistry

Here, we have demonstrated a metal-free energy-efficient mechanochemical approach for expedient access to a diverse set of 2-amino-3-cyano-aryl/heteroaryl-4H-chromenes, tetrahydrospiro[chromene-3,4′-indoline], 2,2′-aryl/heteroarylmethylene-bis(3-hydroxy-5,5-dimethylcyclohex-2-enone) as well as tetrahydro-1H-xanthen-1-one by employing the reactivity of 5,5-dimethylcyclohexane-1,3-dione/cyclohexane-1,3-dione with TsOH⋅H2O as Brønsted acid catalyst under water-assisted grinding conditions at ambient temperature. The ability to accomplish multiple C–C, C=C, C–O, and C–N bonds from readily available starting materials via a domino multicomponent strategy in the absence of metal-catalyst as well as volatile organic solvents with an immediate reduction in the cost of the transformation without necessitates complex operational procedures, features the significant highlights of this approach. The excellent yield of the products, broad functional group tolerances, easy set-up, column-free, scalable synthesis with ultralow catalyst loading, short reaction time, waste-free, ligand-free, and toxic-free, are other notable advantages of this approach. The greenness and sustainability of the protocol were also established by demonstrating several green metrics parameters.

Analytical Model for the Hydrogen Transport Impedance at a PEMFC Anode

Impedance techniques are most useful for the characterization of kinetic processes in fuel cell electrodes. For 'proton exchange membrane fuel cells' (PEMFC) electrochemical impedance reflects cathode kinetics, where larger overvoltage occurs due to slower oxygen reduction reaction [1]. The anode kinetics is considered fast enough and therefore without a signal in the impedance. However this may not be the case under certain conditions, like high current densities, anodic flooding events, side hydrogen kinetics, ultra-low catalyst loading, among others.In order to study anode limitations and kinetics, independent from cathode kinetics, we have recently developed an impedance technique called ‘current modulation hydrogen flow-rate spectroscopy’ (CH2S), that relates the hydrogen inlet flux with fuel cell current. A general description of the technique and results are given in another communication of this congress (#I01A-1406). To help understanding the information provided by CH2S and the analysis of results, a theoretical analysis is desirable based on fundamental transport equations in the PEMFC anode.In this communication we have calculated the transfer function of CH2S for two cases of anode kinetics (Fig. 1a): Case 1 with pure hydrogen diffusive behavior in the gas diffusion layer (GDL), and Case 2 that includes the possibility of a hydrogen reversible chemical reaction in the GDL. Case 2 can be assimilated to a situation where hydrogen is stored in the electrode by the reversible formation of a hydrogen adsorbed species (X-H2). This possibility is of high interest for small portable PEMFC running without external hydrogen storage.Hence, the transfer function of CH2S is:H (jω)=nF QH2 / I = nFADH2 (dc H2 /dx)x=LGDL / I (Eq.1)Where I is the current perturbation, and Qi and cH2 are the perturbed flow rate and concentration, respectively. The electrode is considered as a continuous porous medium characterized by an effective diffusion coefficient for H2 (DH2 ) in the GDL, where transport is governed by the second Ficks´law. Hydrogen kinetics in the GDL is first order, with k1, k-1 rate constants for desorption and adsorption, respectively (Fig. 1a). Before the GDL, H2 transport is considered fast enough, without an impact on the transfer function.Transport equations for Cases 1 and 2 have been solved analytically by conventional methods using the appropriate boundary conditions. From the solutions, the CH2S transfer functions (Eq. 1) have been calculated and plotted in Fig. 1b) as Nyquist plot. For the simple diffusion, a skewed semicircle is obtained with high frequency cut at H' = 0 and low frequency at H' = 1. The null high frequency cut is due to limited hydrogen transport rate and experimental set-up response rate; the unit low frequency cut reflects the stoichiometric hydrogen consumption. The maximum of the semicircle is related with the diffusion constant and GDL thickness (ωmax=2DH2/LGDL 2). . For Case 2, the low frequency cut extends beyond unit (H'>1) , and a new signal can be distinguished due to hydrogen kinetics in the GDL.It is shown that the CH2S impedance technique can be used to study hydrogen transport and kinetics in the anode of a PEMFC.Acknowledgement: The work is partially financed by the ELHYPORT project (PID2019−110896RB-I00), Spanish Ministry of Science and Innovation.[1] A. Lasia, in 'Electrochemical Impedance Spectroscopy and its Applications', Springer Science+Business Media New York 2014 Figure 1

Advanced Porous-Transport-Layer Interface Design for PEM Electrolyzers

Proton-exchange-membrane (PEM) water electrolyzers are promising technologies for generating clean hydrogen for use in a variety of sectors. The anode catalyst layer/porous-transport layer (CL/PTL) interface plays a vital role in determining performance of proton-exchange-membrane (PEM) water electrolyzers. This interface impacts ohmic, kinetic, and mass-transport overpotentials experienced by the cell. PTLs with large surface pores cause undesired catalyst layer deformation into pores of the PTL, which exacerbates degradation, and leads to high ohmic and kinetic overpotentials due to insufficient interfacial contact area.1,2 On the other extreme, low surface porosity at the interface leads to accumulation of oxygen gas that inhibit water transfer, resulting in mass-transport overpotentials.3 Thus, ideal PTLs should have hierarchical structure with high bulk porosity and low tortuosity to mitigate mass-transport overpotentials, while low surface porosity is need to enhance interfacial contact area. In this talk, we explore controlling the interfacial pore structure of these layers through a subtractive method, namely, laser ablation. Laser ablation is a facile method for tailoring the interfacial pore structure of PTLs. Heat from the laser melts titanium, granting a unique morphology advantageous to electrolyzer performance. Laser-ablated PTLs provide improved contact with the CL, while maintaining the bulk pore structure elsewhere, thereby facilitating gas removal and water transport. We also examine how laser-modified PTLs can help enable ultra-low catalyst loadings by minimizing interfacial contact resistance with the thinner catalyst layers. Overall, our work reveals that laser ablation is a facile method to enhance PEM electrolyzer performance. Acknowledgements This work was funded under the H2NEW Consortium by the Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231. References T. Schuler, R. De Bruycker, T. J. Schmidt, and F. N. Büchi, J. Electrochem. Soc., 166, F270–F281 (2019)T. Schuler, T. J. Schmidt, and F. N. Büchi, J. Electrochem. Soc., 166, F555–F565 (2019).X. Peng et al., Adv. Sci., 8 (2021).

A novel electromagnetic mill promoted mechanochemical solid-state Suzuki–Miyaura cross-coupling reaction using ultra-low catalyst loading

The Nobel-prize-winning Suzuki–Miyaura cross-coupling (SMC) is a practical and attractive strategy for the construction of C–C bonds in both academic and industrial settings.

Catalytic selective oxidation of aromatic amines to azoxy derivatives with an ultralow loading of peroxoniobate salts

Tartaric acid-coordinated peroxoniobate salts demonstrate an exceptionally high TOF value (up to 4435 h−1) even at an ultralow catalyst loading for the oxidation of aromatic amines to azoxy compounds under green and very mild conditions.

Platinum Nanowire Based Electrodes with Boosted Catalyst Utilization for Efficient Hydrogen Production in PEM Electrolyzer Cells

Due to the limited triple phase boundary (TPB) sites caused by the intrinsically low electrochemical active surface area (ECSA) of catalysts and poor interfacial contact between a catalyst layer and a porous transport layer, high noble metal catalyst loadings are generally required to achieve high-efficiency hydrogen production in low-temperature proton exchange membrane electrolyzer cells (PEMECs). Our recent studies have demonstrated that direct deposition of platinum nanoparticles onto 25-µm-thick titanium liquid/gas diffusion layers (LGDLs) can significantly reduce the cathode catalyst waste.1-3 To further lower the cost of both catalysts and electrode fabrication, for the first time, we develop a green chemical synthesis approach at room temperature to fabricate ultrathin electrodes composed of platinum nanowires (PtNW) in-situ grown on thin titanium LGDLs.4 Our developed high-speed micro-scale visualization system reveals that ultrathin Pt nanowires display more uniform reaction sites and favorable bubble detachment compared to conventional Pt nanoparticles. Impressively, when integrated into a PEMEC, the PtNW electrode with a low catalyst loading of 0.2 mg cm−2 can achieve a low cell voltage of 1.643 V and high efficiency of 90.08% at 1000 mA cm−2, which significantly surpasses conventional electrodes with high loadings. Such performance enhancement is mainly attributed to the high ECSA of catalysts, favorable hydrogen bubble detachment, and good conductivity of the whole electrode. References Z.Y. Kang, G.Q. Yang, J.K. Mo, Y.F. Li, S.L.Yu, D.A. Cullen, S.T. Retterer, T.J. Toops, G. Bender, B.S. Pivovar, J.B. Green Jr, and F.Y. Zhang. "Novel thin/tunable gas diffusion electrodes with ultra-low catalyst loading for hydrogen evolution reactions in proton exchange membrane electrolyzer cells." Nano Energy. 47, 434-441 (2018).J.K. Mo, Z.Y. Kang, S.T. Retterer, D.A. Cullen, T.J. Toops, J.B. Green Jr, M.M. Mench, and F.Y. Zhang. “Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting.” Science Advances, 2, 1600690 (2016).Z.Y. Kang, J.K. Mo, G.Q. Yang, S.T. Retterer, D.A. Cullen, T.J. Toops, J.B. Green Jr, M.M. Mench, and F.Y. Zhang. “Investigation of thin/well-tunable liquid/gas diffusion layers exhibiting superior multifunctional performance in low-temperature electrolytic water splitting.” Energy & Environmental Science, 10, 166-175 (2017).Z.Q. Xie, S.L. Yu, G.Q. Yang, K. Li, L. Ding, W.T. Wang, D.A. Cullen, H.M. Meyer III, S.T. Retterer, Z.L. Wu, P.X. Gao and F.Y. Zhang. "Ultrathin platinum nanowire based electrodes for high-efficiency hydrogen generation in practical electrolyzer cells." Chemical Engineering Journal. 410, 128333 (2021).

Quantifying Lateral Current Spread While Measuring Performance Using a Segmented Polymer Electrolyte Water Electrolysis Cell

Conventional research and diagnostic techniques for fuel cells and electrolyzers, such as polarization curves and EIS measurements, predominantly probe macroscopic performance characteristics. The works of Schuler, et al. [1] have given a detailed understanding of transport mechanisms inside a polymer electrolyte water electrolyzer (PEWE) by drawing extensively on such ex-situ techniques. However, these techniques cannot be used to probe in-situ behaviors of a PEWE. A need for reliable experimental data that captures in-situ spatial and temporal current distribution is the motivation for the segmented cell used here. Current distribution measurement in PEWEs was performed by Bender et al. [2, 3] while Mench and coworkers pioneered the technique in similar flowing electrochemical systems [4, 5]; a major innovation was inclusion of a printed-circuit board (PCB) with an array of shunt resistors attached to each segment as shown in Figure 1a. The advantage of this design is that, although the PCB and flow-field are segmented and electronically isolated, the other components of the cell are not. However, this design is subject to error since current can spread laterally through the porous transport layer (PTL) as it passes from the catalyst layer to the segmented flow field. Since activity at the catalyst layer is of great interest, understanding current spread is a critical step to utilizing current distribution measurements.This work focuses on characterizing the lateral current spread through PTLs. Work by Philips, Ulsh, and Bender [3], showed that the experimental setup could be modified with the help of masks to characterize lateral current spread through the PTL. A set of experiments was thus designed to investigate current spread through the porous transport layer both “inward” and “outward” from a flow field segment. The goal of these experiments is to quantify current spread, allowing correction of its contribution to obfuscating measurement of current generated in the catalyst layer. High current was detected for the PCB segment corresponding to the unmasked area as expected, but substantial current was measured around the exposed area, as well (Figure 1b). If there was zero outward current spread, no current would be measured anywhere away from the single segment area generating current. Figure 1c shows average current density as a function of radial distance from a generating spot; Figure 1d plots normalized current density as a function of radial distance. This set provides calibration for how much current spread laterally outwards as detected at points located away from a current-generating area. Similarly, inward contributions were also mapped. These experiments were also carried out for two types of PTL – Ti-foil-based thin LGDLs [6] and Ti-based felt PTLs. Preliminary observations indicate that lateral current spread in the Ti-felt-based LGDLs is less than that of the Ti-based foil LGDLs. This work quantifies the lateral current spread seen inside a LGDL and also lays the groundwork to correct current spread recorded in a spatially-resolved current distribution measurement technique. It is part of DOE project #DE-EE0008426 "Developing novel electrodes with ultralow catalyst loading for high-efficiency hydrogen production in proton exchange membrane electrolyzer cells." References Schuler, T., T.J. Schmidt, and F.N. Büchi, Polymer Electrolyte Water Electrolysis: Correlating Performance and Porous Transport Layer Structure: Part II. Electrochemical Performance Analysis. Journal of The Electrochemical Society, 2019. 166(10): p. F555-F565. Reshetenko, T.V., et al., A segmented cell approach for studying the effects of serpentine flow field parameters on PEMFC current distribution. Electrochimica Acta, 2013. 88: p. 571-579. Phillips, A., et al., Utilizing a Segmented Fuel Cell to Study the Effects of Electrode Coating Irregularities on PEM Fuel Cell Initial Performance. Fuel Cells, 2017. 17(3): p. 288-298. Clement, J.T., D.S. Aaron, and M.M. Mench, In Situ Localized Current Distribution Measurements in All-Vanadium Redox Flow Batteries. Journal of The Electrochemical Society, 2015. 163(1): p. A5220-A5228. Ertugrul, T.Y., et al., In-situ current distribution and mass transport analysis via strip cell architecture for a vanadium redox flow battery. Journal of Power Sources, 2019. 437: p. 226920. Mo, J., et al., Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting. Science Advances, 2016. 2(11): p. e1600690. Figure 1

Spatially-Resolved Current Distribution Measurements in Polymer Electrolyte Water Electrolyzers

Operational cost, primarily power consumption, is one of the main cost obstacles for hydrogen produced by polymer electrolyte water electrolyzers (PEWE). Roughly 70% of the hydrogen production cost stems from electricity [2]. Thus, overall process efficiency has a direct influence on the hydrogen price. Dominating sources of electrochemical loss in PEWE cells are the kinetic, electric, and mass transport overpotentials. Ex-situ and modeling work have yielded insights into the physical processes occurring within an operating PEWE, but there is still need for in-situ measurements for verification [3]. Work carried out by Zhang et al. [4, 5] has successfully shown the bubble formation of water electrolysis inside a PEWE by means of an in-situ, high-speed micro-scale visualization system (HMVS). However the HMVS does not explicitly capture the local electrochemical behavior of the cell. Therefore, complex CFD models have been implemented to shed light on the inner workings of PEWE; such models need to be validated with experimental data. This work aims to provide a reliable source of experimental data by developing a technique that allows for in-situ spatial and temporal current distribution measurements, which can be used to validate existing modeling efforts.Current distribution measurements are described extensively in the works of Bender et al. [6, 7] and Mench et al. [1, 8]; a major innovation was the inclusion of a printed-circuit board (PCB) with an array of shunt resistors attached to each segment as shown in Figure 1a (4.5 mm x 4.5 mm segments in this work). The measured voltage drop across each shunt resistor is used to calculate the current in each segment. Passive data acquisition on the distributed current board is done simultaneously to electrochemical control of the electrolyzer, without interrupting its operation. This apparatus is capable of using flow fields of both 5cm2 (16 segments) and 25cm2 active area (100 segments). The distributed current is then interpolated to obtain smoothed contours as shown in Figure 1b. Preliminary current distribution experiments were carried out at 80⁰C with a water flow rate of 0.5 to 15 mL/min/cm2. Experiments were also carried out with two types of porous transport layers from Zhang and co-workers [4, 5]: thin titanium foil LGDLs with 80 nm pore size and Ir-coated titanium felt. Two types of flow field where also used – parallel and triple serpentine. Current distributions were found to respond to transport layer porosity and thickness, flow field design, flow rate, and current density. A non-uniform current distribution is suspected to be ultimately dominated by mass transport characteristics and these experiments support that. Additionally, conditions that yielded near-identical polarization performance were found to have disparate current distributions. This work demonstrates the benefit of a spatially-resolved current distribution measurement technique and the insights it offers which would otherwise be impossible to probe. This work is part of DOE project #DE-EE0008426 "Developing novel electrodes with ultralow catalyst loading for high-efficiency hydrogen production in proton exchange membrane electrolyzer cells." References Clement, J.T., D.S. Aaron, and M.M. Mench, In Situ Localized Current Distribution Measurements in All-Vanadium Redox Flow Batteries. Journal of The Electrochemical Society, 2015. 163(1): p. A5220-A5228.Grigoriev, S., V. Porembsky, and V. Fateev, Pure hydrogen production by PEM electrolysis for hydrogen energy. International Journal of Hydrogen Energy, 2006. 31(2): p. 171-175.Schuler, T., T.J. Schmidt, and F.N. Büchi, Polymer Electrolyte Water Electrolysis: Correlating Performance and Porous Transport Layer Structure: Part II. Electrochemical Performance Analysis. Journal of The Electrochemical Society, 2019. 166(10): p. F555-F565.Mo, J., et al., Discovery of true electrochemical reactions for ultrahigh catalyst mass activity in water splitting. Science Advances, 2016. 2(11): p. e1600690.Li, Y., et al., Wettability effects of thin titanium liquid/gas diffusion layers in proton exchange membrane electrolyzer cells. Electrochimica Acta, 2019. 298: p. 704-708.Bender, G., M.S. Wilson, and T.A. Zawodzinski, Further refinements in the segmented cell approach to diagnosing performance in polymer electrolyte fuel cells. Journal of Power Sources, 2003. 123(2): p. 163-171.Reshetenko, T.V., et al., A segmented cell approach for studying the effects of serpentine flow field parameters on PEMFC current distribution. Electrochimica Acta, 2013. 88: p. 571-579.Ertugrul, T.Y., et al., In-situ current distribution and mass transport analysis via strip cell architecture for a vanadium redox flow battery. Journal of Power Sources, 2019. 437: p. 226920. Figure 1

Graphite defect network constitutes a robust and polishable matrix: Ultralow catalyst loading and excellent electrocatalytic performance

An industrially attractive electrocatalyst needs to meet a series of criteria such as simple and scalable preparation, robust stability, as well as highly efficient catalytic activity. In this study, we demonstrate that a defect network present within high pure polycrystalline graphite can work as a wonderful matrix of electrocatalysts. The electrodes were prepared simply by drop-casting binder-free catalyst precursor solutions on the graphite surface. The precursor ions can infiltrate into the defect channels up to about 3.6 mm deep. Excellent electrocatalytic activity and robust stability were verified using hydrogen and oxygen evolution reactions with different electrocatalysts. The depth-dependent catalytic activities were measured and the depth-profile analyses were performed. The rigid defect network structure significantly improves structural integrity and robust stability and minimizes the electron transfer barrier. Such sustainable electrocatalysts fixed in a rigid defect network have the potential to provide all the features necessary for industrial applications.

Aqueous Suzuki–Miyaura Coupling with Ultralow Palladium Loading and Simple Product Separation

The diphosphine ligand N,N-bis(diphenylphosphanylmethyl)-aniline (bdppma) and PdCl2 afforded a Suzuki–Miyaura catalyst [(bdppma­)PdCl2] that was highly efficient at an ultralow catalyst loading (0.001 mol%) in 20:1 H2O–EtOH. This low catalyst loading in an aqueous solvent system permitted simple product separation by direct filtration without the need for chromatography. The ligand bdppma imparted surprisingly better reactivity than that achieved with other bidentate diphosphine ligands, but the catalytic system had a slightly narrower substrate scope than some similar Pd catalysts reported previously.

Building Electron/Proton Nanohighways for Full Utilization of Water Splitting Catalysts

AbstractLow electron/proton conductivities of electrochemical catalysts, especially earth‐abundant nonprecious metal catalysts, severely limit their ability to satisfy the triple‐phase boundary (TPB) theory, resulting in extremely low catalyst utilization and insufficient efficiency in energy devices. Here, an innovative electrode design strategy is proposed to build electron/proton transport nanohighways to ensure that the whole electrode meets the TPB, therefore significantly promoting enhance oxygen evolution reactions and catalyst utilizations. It is discovered that easily accessible/tunable mesoporous Au nanolayers (AuNLs) not only increase the electrode conductivity by more than 4000 times but also enable the proton transport through straight mesopores within the Debye length. The catalyst layer design with AuNLs and ultralow catalyst loading (≈0.1 mg cm−2) augments reaction sites from 1D to 2D, resulting in an 18‐fold improvement in mass activities. Furthermore, using microscale visualization and unique coplanar‐electrode electrolyzers, the relationship between the conductivity and the reaction site is revealed, allowing for the discovery of the conductivity‐determining and Debye‐length‐determining regions for water splitting. These findings and strategies provide a novel electrode design (catalyst layer + functional sublayer + ion exchange membrane) with a sufficient electron/proton transport path for high‐efficiency electrochemical energy conversion devices.

Semiconductor Quantum Dots Are Efficient and Recyclable Photocatalysts for Aqueous PET-RAFT Polymerization.

This Letter describes the use of CdSe quantum dots (QDs) as photocatalysts for photoinduced electron transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization of a series of aqueous acrylamides and acrylates. The high colloidal solubility and photostability of these QDs allowed polymerization to occur with high efficiency (>90% conversion in 2.5 h), low dispersity (PDI < 1.1), and ultralow catalyst loading (<0.5 ppm). The use of protein concentrators enabled the removal of the photocatalyst from the polymer and monomer with tolerable metal contamination (8.41 ug/g). These isolated QDs could be recycled for four separate polymerizations without a significant decrease in efficiency. By changing the pore size of the protein concentrators, the QDs and polymer could be separated from the remaining monomer, allowing for the synthesis of block copolymers using a single batch of QDs with minimal purification steps and demonstrating the fidelity of chain ends.

Organocatalytic Enantioselective Mukaiyama–Mannich Reaction of Isatin‐Derived Ketimines for the Synthesis of Oxindolyl‐β3, 3‐Amino Acid Esters

Mukaiyama-Mannich reactions of ester enolate equivalents with aldimines have been elegantly used for the asymmetric synthesis of β-amino acids; nevertheless, the corresponding asymmetric reaction employing ketimines are unexplored. Herein, the first organocatalytic enantioselective Mukaiyama-Mannich reaction employing isatin-derived ketimines with unsubstituted silyl ketene acetals is disclosed towards the scalable synthesis of 2-oxoindolinyl-β3, 3 -amino acid esters at room temperature with excellent enantioselectivities (ee >99.5 %). Ultra-low catalyst loadings (as low as 250 ppm) could be used for the quantitative product formation with high enantiopurity. The synthetic utility of this protocol has been showcased in the short formal synthesis of pharmaceutically demanded (+)-AG-041R, a potent gastrin/CCK-B receptor antagonist.

Microscopic insights on the degradation of a PEM water electrolyzer with ultra-low catalyst loading

The present work aims for a comprehensive understanding of the degradation of proton exchange membrane water electrolyzer (PEMWE) with a low catalyst loading of 0.3 mg cm−2 Pt and 0.08 mg cm−2 Ir after a long-term test of 4500 h at 1.8 A cm−2. For the first time, the mechanism of cathode degradation is proposed using established physical models from two aspects: (1) Pt dissolution from nanoparticles and (2) Pt dissolution due to rapid Pt oxide reduction at the start of operation. As Ir dissolves and migrates through the membrane, Pt-Ir precipitates are formed in the membrane. Furthermore, iridium dissolution and subsequent re-deposition on the cathode constitute a major portion (42%) of the anode catalyst loss. The overall picture of degradation mechanisms and the distribution of platinum and iridium across the membrane electrode assembly (MEA) are provided.

Investigation of Porous Transport Layer Parameters for Proton Exchange Membrane Water Electrolysis

Hydrogen production through proton exchange membrane water electrolysis (PEMWE) is regarded as a very promising technology that offers a highly efficient and robust path to store electrical energy in form of hydrogen and further enables the integration of renewable energy into the electrical grid 1-3. The total cell voltage of a single PEM water electrolyzer cell is composed of the open circuit voltage, activation overpotential, diffusion overpotential, and ohmic loss overpotential 4. The total voltage of the cell can be expressed as Eq(1) 5. V=VOCV+Vact+Vohm+Vdiff (1) Where VOCV is the Nernst potential which is also called the reversible voltage and can be calculated from the Nernst equation or Gibbs free energy, Vact is the activation overpotential due to the electrochemical reaction and it represents the overpotential to drive the electron transfer and electrochemical reaction kinetics as described by the Butler-Volmer model, Vohm is the ohmic overpotential caused by the resistances of the electronic/ionic components and interfacial contact resistances, and Vdiff is the diffusion overpotential related to mass transport. Porous transport layers (PTLs) are key components in PEMWE. They facilitate mass transport, thermal and electrical conduction, and are required to sustain a good contact with adjacent components 6-8. It is expected that various PTLs with different structures and wettability exhibit nonidentical performance in PEMWE. In this study, a general model is established to investigate the PTL parameters of four inherently different PTLs, including non-treated Toray paper, PTFE treated Toray paper, sintered titanium particles and Ti felt. SEM images highlighting the structures of these PTL materials are shown in Figure 1. The materials, which differed in thickness, porosity, material and composition were evaluated with identical MEAs over a range of operating conditions for model validation. The resulting data indicate that the hydrophobic additives in the Toray paper will decrease the PEMWE performance due to increased mass transport loss and ohmic loss. Ti felt with similar thickness and porosity as non-treated Toray paper showed slightly reduced performance due to its higher ohmic loss. Sintered Ti particle PTLs exhibit much higher ohmic resistance than Ti felt and nontreated Toray paper, which may be attributed to its significantly larger thickness and/or an increased contact resistance. The experimental data was employed to validate the MATLAB/Simulink model which enables the simulation of the contribution of each overpotential. The model was used to successfully describe the effects of PTFE loading on PEMWE performance, as shown in Figure 2. The hydrophobic treatment resulted, as expected, in detrimental performance effects. The diffusion loss of a 20% PTFE treated Toray paper is significantly higher than that of the nontreated PTLs (Figure 2). The modeling results also indicate that the change of the wettability of the PTLs by hydrophobic additives/agents, impacted not only the diffusion loss significantly, but also affected the ohmic loss and activation loss. This was specifically detrimental to PEMWE cell performance when operating at high current densities. The model that was validated in this study can be used to predict the effects of key PTL parameters and utilized to optimize PEMWE performance. Reference Bender, G.; Carmo, M.; Smolinka, T.; Gago, A.; Danilovic, N.; Mueller, M.; et al, Initial approaches in benchmarking and round robin testing for proton exchange membrane water electrolyzers. International Journal of Hydrogen Energy 2019, 44 (18), 9174-9187.Pivovar, B.; Rustagi, N.; Satyapal, S., Hydrogen at Scale (H2@ Scale): Key to a Clean, Economic, and Sustainable Energy System. The Electrochemical Society Interface 2018, 27 (1), 47-52.Kang, Z.; Yang, G.; Mo, J.; Li, Y.; Yu, S.; Cullen, D. A.; et al, Novel thin/tunable gas diffusion electrodes with ultra-low catalyst loading for hydrogen evolution reactions in proton exchange membrane electrolyzer cells. Nano Energy 2018, 47, 434-441.Rahim, A. A.; Tijani, A. S.; Kamarudin, S. K.; Hanapi, S., An overview of polymer electrolyte membrane electrolyzer for hydrogen production: Modeling and mass transport. Journal of Power Sources 2016, 309, 56-65.Han, B.; Steen III, S. M.; Mo, J.; Zhang, F.-Y., Electrochemical performance modeling of a proton exchange membrane electrolyzer cell for hydrogen energy. International Journal of Hydrogen Energy 2015, 40 (22), 7006-7016.Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D., A comprehensive review on PEM water electrolysis. International journal of hydrogen energy 2013, 38 (12), 4901-4934.Lettenmeier, P.; Kolb, S.; Sata, N.; Fallisch, A.; Zielke, L.; Thiele, S.; et al, Comprehensive investigation of novel pore-graded gas diffusion layers for high-performance and cost-effective proton exchange membrane electrolyzers. Energy & Environmental Science 2017, 10 (12), 2521-2533.Kang, Z., Development of Novel Thin Membrane Electrode Assemblies (MEAs) for High-Efficiency Energy Storage. 2018. Figure 1

(Invited) Developing Thin and Tunable Catalyst –Coated Liquid/Gas Diffusion Layers with Ultralow Catalyst Loadings

The thin liquid/gas diffusion layers (LGDLs) with well-tunable pore morphologies are developed by employing advanced manufacturing and show significant performance improvements in proton exchange membrane electrolyzer cells (PEMECs) for hydrogen/oxygen production and renewable energy storage. In addition, the LGDL thickness reduction from hundreds of µm for conventional LGDLs to tens of µm will decrease the weight and volume of the PEMEC stack. More importantly, by taking advantage of the novel material development coupled with a transparent PEMEC and a high-speed micro visualization system, the electrochemical reactions are revealed to occur only on the catalyst layer adjacent to good electrical conductors. Based on these findings, new fabrications of thinfilm catalyst depositions on the novel LGDLs exhibit much higher mass activity than conventional catalyst-coated membranes. This discovery presents an opportunity to enhance the multiphase interfacial effects, maximizing the use of the catalysts and significantly reducing the cost of these electrochemical devices. Furthermore, the well-tunable features of thin electrodes, including pore size, pore shape, pore distribution, and thus porosity and permeability, will be very valuable for developing models and validating simulations of electrochemical energy devices.

Highly Conductive Catalyst with Advanced Manufacturing for Efficient Oxygen Evolution Reaction

The electrochemical water electrolysis is a convenient technique to store intermittent, sustainable energies, such as solar and wind, into high-purity hydrogen. The state-of-the-art electrocatalysts for oxygen evolution reaction (OER) are Ir/Ru-based materials. However, their scarcity and high costs hamper the industrialization of water electrolysis. In addition, the low electrical conductivity (~ 1000 Ω/□) of those catalysts and the sluggish multistep proton-coupled electron transfer of OERs significantly reduce the reaction sites and performances of water splitting. To overcome those problems, this research will present a highly conductive multi-layer catalyst fabricated with advanced manufacturing methods. We employ spray printing, sputter coating, and inkjet printing to produce nafion layers, gold nanolayers (AuNL), and IrO2 catalyst layers, respectively. The mesoporous AuNLs provide the catalyst layer with not only excellent electrical conductivity (0.5 Ω/□) but also good proton permeability. Furthermore, the novel structure of the multi-layer catalyst in the electrolyser cell helps to directly investigate the influence of conductivity on OER kinetics by visualizing the generated oxygen bubbles, which enables the establishment of correlation between the conductivity and reaction sites. Most importantly, the faster OER kinetics can be achieved even with a much lower loading of IrO2 catalysts (as low as 0.1 mg/cm2) than the conventional IrO2 catalyst layer in pure water, acidic, and alkaline solutions. The novel multi-layer catalyst provides a new way to enhance the performance and reduce the usage of noble catalysts in energy conversion and storage devices. Keywords: Inkjet printing, mesopores, high conductivity, ultra-low catalyst loading, water electrolysis.

One‐Step Fabrication of Ultralow Pt Loading High Efficiency Proton Exchange Membrane for Water Electrolysis by Conventional E‐Beam Metal Deposition

AbstractProton exchange membrane water electrolysis (PEMWE) is known as an effective and environmental‐friendly method for hydrogen production. However, the process is hindered by the necessity of large loading of high‐priced noble metals in the conventional approaches. To resolve this problem, a highly efficient Nafion‐based catalyst and proton exchange system are developed, which only need ultralow platinum loading by direct e‐beam evaporation deposition. Scanning electron microscopy is performed before and after the deposition, which reveals a very smooth morphology. By assembling the PEMWE device with above‐mentioned cathode and conventional iridium black anode, the current density reaches 500 mA cm−2 with a platinum loading of only 0.00379 mg cm−2 at 1.64 V under 80 °C. A mass activity of ≈132 A mg−1 is achieved, which is one order of magnitude higher than previously reported results. Moreover, powered by a commercial silicon solar cell, it realizes a solar‐to‐hydrogen conversion efficiency up to 8.26%. This work opens up an avenue of fabricating PEMWE with an ultralow catalyst loading, which may have enormous potential for future industrial scale applications.

Novel thin/tunable gas diffusion electrodes with ultra-low catalyst loading for hydrogen evolution reactions in proton exchange membrane electrolyzer cells

Proton exchange membrane electrolyzer cells (PEMECs) have received great attention for hydrogen/oxygen production due to their high efficiencies even at low-temperature operation. Because of the high cost of noble platinum-group metal (PGM) catalysts (Ir, Ru, Pt, etc.) that are widely used in water splitting, a PEMEC with low catalyst loadings and high catalyst utilizations is strongly desired for its wide commercialization. In this study, the ultrafast and multiscale hydrogen evolution reaction (HER) phenomena in an operating PEMEC is in-situ observed for the first time. The visualization results reveal that the HER and hydrogen bubble nucleation mainly occur on catalyst layers at the rim of the pores of the thin/tunable liquid/gas diffusion layers (TT-LGDLs). This indicates that the catalyst material of the conventional catalyst-coated membrane (CCM) that is located in the middle area of the LGDL pore is underutilized/inactive. Based on this discovery, a novel thin and tunable gas diffusion electrode (GDE) with a Pt catalyst thickness of 15 nm and a total thickness of about 25 µm has been proposed and developed by taking advantage of advanced micro/nano manufacturing. The novel thin GDEs are comprehensively characterized both ex-situ and in-situ, and exhibit excellent PEMEC performance. More importantly, they achieve catalyst mass activity of up to 58 times higher than conventional CCM at 1.6 V under the operating conditions of 80 °C and 1 atm. This study demonstrates a promising concept for PEMEC electrode development, and provides a direction of future catalyst designs and fabrications for electrochemical devices.