A Permselective CeOx Coating To Improve the Stability of Oxygen Evolution Electrocatalysts.

Highly active NiFeOx electrocatalysts for the oxygen evolution reaction (OER) suffer gradual deactivation with time owing to the loss of Fe species from the active sites into solution during catalysis. The anodic deposition of a CeOx layer prevents the loss of such Fe species from the OER catalysts, achieving a highly stable performance. The CeOx layer does not affect the OER activity of the catalyst underneath but exhibits unique permselectivity, allowing the permeation of OH− and O2 through while preventing the diffusion of redox ions through the layer to function as a selective O2‐evolving electrode. The use of such a permselective protective layer provides a new strategy for improving the durability of electrocatalysts.

Water electrolysis is a key technology to convert and store sustainable energy into high-energy-dense hydrogen. However, slow kinetics of oxygen evolution reaction (OER) at the anode hinders overall reaction rate of water splitting and delays realization of hydrogen energy society. OER consists of four electron steps and an ideal catalyst has 1.23 V potential barrier for each step, but a real catalyst has a step which requires higher potential than 1.23 V, i.e., overpotential. Therefore, exploring OER catalyst with minimized OER overpotential is an important challenge in increasing water electrolysis efficiency. On the other side, the direct use of abundant seawater as a splitting reactant has advantage in terms of resources and cost. It can also help commercialization of water electrolysis. Since seawater contains chloride which causes undesirable side reactions and accelerated corrosion of the anode material, appropriate OER catalyst is necessary to avoid these problems.Conventional trial-and-error method in catalyst design usually requires many years of R&D and high cost. Computational screening can preemptively narrow down the candidate group of high-efficient catalyst to reduce time and economic cost. NiFe-based layered double hydroxide (NiFe-LDH) is a promising OER catalyst due to the comparable activity to commercial IrO2catalyst and can provide easily tunable metal composition during synthesis process. Its unique layered structure and reversible oxidation state change in redox condition also have arisen interest of many researchers. In this study, starting from the mechanism study of OER, chloride evolution reaction (ClER), and chloride-induced corrosion on this material, we performed DFT calculations to predict which transition metal dopants can enhance the OER activity of NiFe-LDH without accelerating selectivity and corrosion problems in seawater.NiFe-LDH is known to experience its transition to NiFeOOH phase under OER condition. [001] and [110] facets were set as edge and terrace sides of NiFeOOH, respectively, after their surface energies were investigated. Surface Pourbaix diagram showed that the [001] facet has clean termination without any adsorbate while [110] facet has dissociated-H2O covered termination under alkaline OER condition. OER energetic profile showed that the Fe site on the [110] surface is an OER active site with a theoretical overpotential of 280 mV, and seawater conditions do not significantly affect the OER activity itself. ClER mechanism study revealed that it occurs via *ClOH intermediates on [110] metal sites or *Cl intermediates on deprotonated [001] oxygen sites. Energy calculation of surface chlorination and metal dissolution steps showed that gradual chlorination accelerates metal dissolution and [110] facet is more vulnerable to chloride-induced corrosion than [001] facet.As the third metal candidate of NiFe-LDH, 3d to 5d transition metals excluding heavy metals were screened. Gibbs free energy correction terms such as vibrational entropy, zero-point energy and solvation parameter were calculated only for NiFeOOH case, and applied them to other candidates to simplify the screening process. NiFeOOH [001] surface could not be better than NiFeOOH [110] surface due to the inherent low activity of the [001] facet, but interestingly seven dopants could reduce the overpotential of NiFeOOH [110] surface by affecting the Fe active site or being active sites themselves. Since these dopants increased the oxidation ability of the active site, all of them also lowered chloride oxidation potential. However, fortunately, none of their ClER operating potential was lower than their OER operating condition, which means that there still exist potential windows where 100% OER selectivity can be achieved. In the last screening step, dissolution energies of fully chlorinated metal sites were calculated for the seven candidates, and two cases of them showed negative dissolution energy, which indicates very unstable doping state. As a result, five candidates passed through all three screening criteria: OER activity, OER vs. ClER selectivity, and durability against corrosion. Further, the experimental validation has performed on three promising candidates considering the raw material price, and it revealed that their activity is higher activity than NiFe-LDH. In addition, for the two abandoned cases in the third screening step (about metal dissolution), the experimental data showed that they were not seem to be actually doped into NiFe-LDH, indicating that the computational screening of seawater compatibility really worked. This study supports the utility of theoretical screening in electrochemical catalyst design and presents computational approach method to consider seawater compatibility.

Searching for high-activity, stable and low-cost catalysts toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are of significant importance to the development of renewable energy technologies. By using the computational screening method based on the density functional theory (DFT), we have systematically studied a wide range of transition metal (TM) atoms doped a defective BC3 monolayer (B atom vacancy VB and C atom vacancy VC), denoted as TM@VB and TM@VC (TM = Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir and Pt), as efficient single atom catalysts for OER and ORR. The calculated results show that all the considered TM atoms can tightly bind with the defective BC3 monolayers to prevent the atomically dispersed atoms from clustering. The interaction strength between intermediates (HO*, O* and HOO*) and catalyst govern the catalytic activities of OER and ORR, which has a direct correlation with the d-band center (εd) of the TM active site that can be tuned by adjusting TM atoms with various d electron numbers. For TM@VB catalysts, it was found that the best catalyst for OER is Co@VB with an overpotential ηOER of 0.43 V, followed by Rh@VB (ηOER = 0.49 V), while for ORR, Rh@VB exhibits the lowest overpotential ηORR of 0.40 V, followed by Pd@VB (ηORR = 0.45 V). For TM@VC catalysts, the best catalyst for OER is Ni@VC (ηOER = 0.47 V), followed by Pt@VC (ηOER = 0.53 V), and for ORR, Pd@VC exhibits the highest activity with ηORR of 0.45 V. The results suggest that the high activity of the newly predicted well dispersed Rh@VB SAC is comparable to that of noble metal oxide benchmark catalysts for both OER and ORR. Importantly, Rh@VB may remain stable against dissolution at pH = 0 condition. The high energy barrier prevents the isolated Rh atom from clustering and ab initio molecule dynamic simulation (AIMD) result suggests that Rh@VB can remain stable under 300 K, indicating its kinetic stability. Our findings highlight a novel family of efficient and stable SAC based on carbon material, which offer a useful guideline to screen the metal active site for catalyst designation.

Due to the relatively high energy barrier and the inertia involved in the 4e- process, oxygen evolution reaction (OER), greatly impedes the efficiency of water splitting devices.1 Ni-based sulfides, especially Ni3S2 based material has regarded as promising catalysts to replace noble OER catalysts due to its facile synthesis and good conductivity.2 Nevertheless, the intrinsic activity of Ni3S2 is still inferior to the Ru/Ir oxides catalysts. Heteroatom doping has been widely used to tune catalytic activity of Ni3S2. Similarly, nonmetal heteroatom can also engineer geometric and electronic structure and activate surface sites of catalysts.3 Moreover, as the radius and electronegativity of nonmetal atoms usually differ greatly from that of transition-metal atoms, nonmetal atoms are more likely to bring about changes in the local geometric and electronic structures of active sites of catalysts. This makes nonmetal heteroatoms more attractive in adjusting the electrocatalytic activity of materials.4 Herein, nonmetal doping would be a more promising way to regulate the electronic structure and OER activity of Ni3S2. Until now, there are few investigations on modulating the OER activity of Ni3S2 by doping nonmetal-atoms. [Figure] Figure 1. (a) The bond length of X-Ni1 (dX-Ni1) corelates with the electronegativity (χx) and atom radius (Rx) of X (the inset shows the top-view of Ni3S2(100) with labled Ni sites and two doping sites, Xout and Xin). (b)The Bader charge of Ni1, Ni2, Ni3 and Ni4 atoms. (c) The correlation between calculated overpotential (η) vs. difference between the adsorption energy of *OH and *O on different sites of all surfaces. (d) The improved active sites proportion (nsite) on X-Ni3S2(100) and the different between minimum η of each surface and η of Ni3S2(100). (e) OER polarization curves and (f) experimental overpotential (η) at current density of 10 mA cm-2, average turnover frequency and exchange current density(J0) of Ni3S2 NSs and C-Ni3S2 NSs.Inspired by the above considerations, we systematically investigated the OER activities of five kinds of nonmetal atoms (X, X = B, C, N, O, P) doped Ni3S2 (X-Ni3S2) electrocatalysts and screened out the most promising X-Ni3S2 OER catalysts. The geometric and electronic structure of X-Ni3S2, intermediates adsorption, potential determining step (PDS), and theoretical overpotentials of OER were studied using the density functional theory (DFT) calculations. With 2.5 at% X doping content, the X-Ni3S2 shows good electronic conductivity which can benefit the charge transfer between the surface and the intermediates during electrocatalysis. Among all X, C and N cause more prominent local structure disturbance on Ni3S2(100) surface (Fig.1a). This change of local coordination environment furhter disturbe the surface charge distribution, enableing the charge of adjacent Ni sites are intensely altered X (Fig.1b). Further, the OER free energy diagram denonting that the formation of OOH is the the PDS on pristine Ni3S2(100) and various X-Ni3S2(100). Among all X dopants, C is most effective dopant which can reduce the OER theorotical overpotential of adjacent Ni sites by 0.16 V and 0.23V when X doping on top most layer (Cout-)) and sublayer (Cin-) of Ni3S2(100) respectivelly (Fig.1c). Moreover, C doping can effectively active the surface Ni sites and enable the 62.5% of Ni sites on C-Ni3S2(100) have better OER activity than that of Ni sites on pristine Ni3S2 (Fig.1d).Furthermore, C doped Ni3S2 nanosheets (C-Ni3S2 NSs), and Ni3S2 nanosheets (Ni3S2 NSs) were synthesized through the same method3 to verify its application in OER. Our electrochemical results verify that C-Ni3S2 NSs catalyst has higher OER activity than pristine Ni3S2 NSs, exhibiting lower overpotential of 261 mV at current density of 10 mA cm-2 (Fig.1e) and lower Tafel slope of 96.18 mV dec-1 compared with Ni3S2 NSs (310 mV, 109.35 mV dec-1 respectively). Besides, C- Ni3S2 NSs has higher exchange current density (J0) and average turnover frequency than pristine Ni3S2 NSs(Fig.1f). Our DFT calculation and experimental results jointly highlight the promising C dopant in promoting the OER performance of Ni3S2. Our study offers guidance for screening and fabricating promising OER catalysts through nonmetal doping engineering, which can inspire more exploration of nonmetal doping of other electrocatalysts.References Reier, T.; Oezaslan, M.; Strasser, P., ACS Catal. 2012, 2 (8), 1765-1772.Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y., J. Phys. Chem. Lett. 2012, 3 (3), 399-404.Zheng, X.; Zhang, L.; Huang, J.; Peng, L.; Deng, M.; Li, L.; Li, J.; Chen, H.; Wei, Z., J. Phy. Chem. C 2020, 124 (44), 24223-24231.Zheng, X. Q.; Peng, L. S.; Li, L.; Yang, N.; Yang, Y. J.; Li, J.; Wang, J. C.; Wei, Z. D., Chem. Sci. 2018, 9 (7), 1822-1830. Figure 1

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

A bifunctional oxygen reduction reaction (ORR)/ oxygen evolution reaction (OER) catalyst is essential for rechargeable metal-air batteries and regenerative fuel cells. Platinum (Pt) and iridium oxide (IrO2) are the state-of-the-art ORR and OER catalysts, respectively. However the high price and scarcity of these platinum group metals (PGMs) has been an obstacle for wide spread application of these catalysts. Recently, in alkaline media, carbon based ORR and perovskite OER catalysts have demonstrated similar or even better catalytic activities compared to the counterpart PGM catalysts [1, 2]. Therefore, if we combine these two non-PGM catalysts, a non-PGM bifunctional ORR/OER catalyst can be obtained. A hindrance in this approach is the vulnerability of carbon-based ORR catalysts to oxidation in the OER potential range, i.e., potentials > 1.5 V vs. RHE. Thus development of robust ORR catalysts under practical OER conditions is a key to realize this kind of bifunctional catalysts. The carbon support used in our ORR catalysts was black pearl (BP) 2000 [1]. In preliminary tests, however, we found that BP 2000 undergoes oxidization at potentials around ca. 1.2 V vs. RHE and above (data not shown). In this work, we used reduced graphene oxide (rGO) as an alternative support to synthesize oxidation resistant ORR catalysts. The OER catalyst we chose was a perovskite (La1-xSrx)CoO3-δ (LSC). Pre-synthesized LSC was added into the initial solution of the rGO based ORR catalyst synthesis process, and after drying and heat-treatment, bifunctional (LSC + rGO) catalysts were obtained. In measuring the OER activity of the LSC catalyst, acetylene black (AB) carbon was added to the LSC (LSC + AB) to increase the electrical conductivity. Fig. 1 shows the comparison of ORR/OER activities between (LSC + AB) and (LSC + rGO). As expected, the ORR activity of (LSC + rGO) is greatly improved by ca. 200 mV in terms of E½ , in comparison to that of (LSC + AB). Interestingly even the OER activity of (LSC + rGO) becomes higher than that of (LSC + AB). Thanks to the enhancement of both ORR and OER activities with (LSC + rGO), highly active bifunctional catalysts are obtained. In this talk, material analysis results and diverse electrochemical performances of the (LSC + rGO) catalysts will be presented. Acknowledgements Support from the Directed Research of the Los Alamos National Laboratory’s Laboratory Directed Research & Development (LDRD-DR) is greatly acknowledged. References Chung et al., Nat. Commun. 4, 1922 (2013).Suntivich et al., Science, 334, 1383 (2011). Figure 1

The search for new materials can be laborious and expensive. Given the challenges that mankind faces today concerning the climate change crisis, the need to accelerate materials discovery for applications like water-splitting could be very relevant for a renewable economy. In this work, we introduce a computational framework to predict the activity of oxygen evolution reaction (OER) catalysts, in order to accelerate the discovery of materials that can facilitate water splitting. We use this framework to screen 6155 ternary-phase spinel oxides and have isolated 33 candidates which are predicted to have potentially high OER activity. We have also trained a machine learning model to predict the binding energies of the *O, *OH and *OOH intermediates calculated within this workflow to gain a deeper understanding of the relationship between electronic structure descriptors and OER activity. Out of the 33 candidates predicted to have high OER activity, we have synthesized three compounds and characterized them using linear sweep voltammetry to gauge their performance in OER. From these three catalyst materials, we have identified a new material, Co2.5Ga0.5O4, that is competitive with benchmark OER catalysts in the literature with a low overpotential of 220 mV at 10 mA cm-2 and a Tafel slope at 56.0 mV dec-1. Given the vast size of chemical space as well as the success of this technique to date, we believe that further application of this computational framework based on the high-throughput virtual screening of materials can lead to the discovery of additional novel, high-performing OER catalysts.

It is crucial but challenging to reduce the required noble‐metal loading without compromising the catalytic performance of oxygen evolution reaction (OER) catalysts. This study presents a highly active OER catalyst composed of IrO2 with Ir rich surface (IrO2@Ir) nanoparticles supported over nano TiN coated with TiOxNy (IrO2@Ir/TiN). The present approach demonstrates superior OER catalysts with high activity through small, uniformly dispersed IrO2@Ir nanoparticles, along with high durability owing to robust catalyst support and strong catalyst‐support interaction. The synthesized IrO2@Ir/TiN with an Ir loading of 40 wt % exhibits a mass‐normalized OER activity of 637 AgIr−1, which is 2.4 times that of the unsupported commercial benchmark IrO2 OER electrocatalyst. The fine nanoparticles and high activity enable significant (∼60 %) reduction in the Ir metal loading required to obtain equivalent OER performance. In addition, when evaluated through an accelerated stress test using potential cycling, the catalyst exhibits outstanding durability (79 % retention) compared to that of the commercial equivalent (66 % retention). The OER activity loss was attributed to the catalyst dissolution (30 % loss) and the catalyst particle growth (70 %), with no measurable loss due to the TiN support corrosion. The development of ultra‐fine IrO2@Ir nanoparticles and robust ceramic catalyst support significantly improved the Ir utilization and open a new perspective for supported OER catalyst.

To reduce damage to carbon containing components of fuel cell stacks during start-up and shutdown, extensive research has been carried out to produce reversal tolerant anodes (RTAs) using oxygen evolution reaction (OER) catalysts [1-4]. However, most of these results were obtained using resource intensive in-situ testing that suffers from long experimental times. To address this, a series of ex-situ experiments was devised to characterize the activity and durability of OER catalysts in a simulated polymer electrolyte fuel cell (PEMFC) environment. The dissolution/re-deposition mechanism of the OER catalysts were investigated using a combination of linear sweep voltammetry and potential stepping experiments within the normal operating range of PEMFCs (0 V to 1.2 V vs RHE). During normal fuel cell operation, IrO2 based catalysts form soluble Ir3+ species as an intermediate between metallic Ir and IrO2. Ir3+ ions can be washed out of the cell which diminishes the reversal tolerance of the anode. After our electrochemical testing, dissolved Ir3+ concentrations in the electrolyte were determined using the inductively coupled plasma mass spectrometry (ICP-MS) method. An in-line ICP-MS technique was previously used by Cherevko et al. to determine the potential resolved dissolution in real time [5]. To validate the ex-situ accelerated testing protocol, experimental reversal tolerance tests were carried out at four different temperatures (20, 40, 60 and 80 °C) and the results showed an increase in the concentration of Ir detected in the electrolyte solutions (Fig. 1). Additionally, different OER catalysts were tested and the results were correlated with in-situ reversal tolerance tests. The effect of OER catalyst structure and support interactions on catalyst stability was investigated by a combination of surface analytical and electrochemical techniques. The surface analysis revealed that there was dissolution and re-deposition occurring during accelerated degradation testing. SEM-EDX imaging of the catalyst layer after testing, showed relocation of the OER catalyst to the cracks of the gas diffusion layer supporting a microporous layer (GDL/MPL) substrate (Fig. 1). Taniguchi A, Akita T, Yasuda K, Miyazaki Y (2004) Analysis of electrocatalyst degradation in PEMFC caused by cell reversal during fuel starvation. J Power Sources 130:42–49. doi: 10.1016/j.jpowsour.2003.12.035Ralph TR, Hudson S, Wilkinson DP (2006) Electrocatalyst stability in PEMFCs and the role of fuel starvation and cell reversal tolerant anodes. ECS Trans 1:67–84. doi: 10.1149/1.2214545Mandal P, Litster S (2016) Investigation and mitigation of degradation in polymer electrolyte fuel cell due to cell reversal using oxygen evolution catalyst. Meet Abstr MA2016-01:1419–1419.Jung J, Park B, Kim J (2012) Durability test with fuel starvation using a Pt/CNF catalyst in PEMFC. Nanoscale Res Lett 7:34. doi: 10.1186/1556-276X-7-34Cherevko S, Geiger S, Kasian O, Kulyk N, Grote J-P, Savan A, Shrestha BR, Merzlikin S, Breitbach B, Ludwig A, Mayrhofer KJJ (2016) Oxygen and hydrogen evolution reactions on Ru, RuO2, Ir, and IrO2 thin film electrodes in acidic and alkaline electrolytes: A comparative study on activity and stability. Catalysis Today 262:170–180. doi: 10.1016/j.cattod.2015.08.014 Figure 1. Images: backscattered electron (BSE) detector images at 500x magnification of OER catalysts deposited on a GDL/MPL (a) before ex-situ testing and (b) after ex-situ testing. The bright dots were confirmed by EDX to contain Ir atoms. Bar graph: Electrolyte Ir concentration after accelerated degradation protocol. Performed in a flooded, N2 purged, 0.09 M H2SO4, three electrode cell for 30,000 cycles from 0.05 V to 1.2 V vs. RHE with a 1 s hold at each potential. OER catalysts were prepared by sonication in 2-propanol overnight and then 400 μg deposited dropwise onto the GDL/MPL substrate. Three trials were carried at each temperature and the error bars are the standard error of the trials. Figure 1

As a branch of catalysis, electrocatalytic energy conversion reactions have been extensively studied and widely applied in industry. Hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are the most common electrocatalytic reactions, which have been utilized in water splitting, fuel cells, and zinc-air batteries, and so on. The ideal benchmark catalysts for these reactions are noble metal based catalysts, like platinum (Pt), ruthenium (Ru) and iridium (Ir). However, their high cost and instable catalytic performance during long-term usage make it impractical to use them on a massive scale. To develop a commercially affordable electrocatalyst with a promising activity, a series of catalysts originated from the defective two-dimensional (2D) nanomaterials emerged, such as carbon, transition metal oxides and transition metal dichalcogenides. Their atomic thickness, large lateral size, high surface-to-volume atom ratio and large specific surface area render them promising for numerous applications, such as (electro)catalysis, electronics, sensors, energy storage and conversion, and so on. Meanwhile, the defects existed on these 2D materials can significantly tailor their intrinsic physical properties even in an extremely low concentration, and thus great efforts have been devoted into the artificially surface defect engineering. But there are three major challenges that need to be addressed in the application of defects for improving the performance of the 2D materials, including the control of defect type and concentration, defect stabilization and active site structure characterization. This thesis focuses on developing the high-performance nonprecious metal catalysts through defect engineering for OER. The studies include the preparation of active 2D nonprecious metal nanosheets for OER catalysis through different novel synthesis strategies, and the investigation of the effect of the various defects, such as oxygen vacancies (Vo) and selenium vacancies (Vse), on the OER activities of the corresponding materials. It aims to explore the efficient strategies for defect engineering on various 2D nanomaterials, and pave a route for developing new high-performance OER catalysts based on nonprecious metals.In the first part of the experimental chapters, a facile solution reduction method using sodium borohydride as reductant is developed to prepare the amorphous iron-cobalt oxide nanosheets (FexCoy-ONS) with a large specific surface area (up to 261.1 m2 g-1), ultrathin thickness (1.2 nm) and, importantly, abundant Vo. Particularly, the mass activity of Fe1Co1-ONS are clearly superior to those of commercial RuO2 and crystalline iron-cobalt oxide nanoparticles. The promising OER catalytic activity of Fe1Co1-ONS can be attributed to its specific structure. Its ultrathin ONS could facilitate the mass diffusion/transport of OH- ions and provide more active sites for OER catalysis, while the Vo could enhance the electronic conductivity and promote the adsorption of H2O onto the nearby Co3+ sites. But as these Vo are in-situ created during the preparation process, their density cannot be controlled.Therefore, the second part focuses on the controllable tuning of Vo density on the 2D iron-cobalt oxide (Fe1Co1Ox-origin) via hydrogen thermal treatment at different temperatures and hydrogen pressures. Notably, the hydrogen annealed iron-cobalt oxide at an optimized condition of 200 oC and 2.0 MPa exhibits a remarkably improved OER activity in 1.0 M KOH, 1.9 times that of Fe1Co1Ox-origin at an overpotential of 350 mV. The results reveal that the optimal Vo density on the 2D Fe1Co1Ox via hydrogenation can improve the electronic conductivity and promote the OH- adsorption onto nearby low-coordinated Co3+ sites, resulting in a significantly enhanced OER activity.To further stabilize the Vo during the highly oxidizing OER conditions, in the third part of the experimental chapters, the atomically distributed non-metallic elements, including sulfur (S), nitrogen (N), and phosphorus (P), were separately introduced onto the defective iron-cobalt oxide (FeCoOx-Vo) surface. S atoms can not only effectively stabilize the Vo, but also create some extra Vo on the nearby Co sites, modulating the electronic structure of the oxide material to exhibit promising OER activity. When paired with the commercial 20% Pt/C, an overall water splitting current density up to 245.0 mA cm-2 can be achieved at the cell voltage of 2.0 V in 1.0 M KOH, which can meet with the requirement of industrial water splitting.The developed defect engineering strategies (vacancy creation and heteroatom doping) are applied in other nonprecious metal catalysts to prove their universality in enhancing the electrocatalytic activities of the materials. In the fourth part, Vse are firstly created on CoSe2-x via Ar plasma, then single Pt atoms are loaded on CoSe2-x through photoreduction to construct the atomically coordinated Pt-Co-Se moieties. Owing to the filling of single Pt atoms, CoSe2-x-Pt shows much better OER performance than single Ni and even Ru atomic species filled CoSe2-x. Mechanism studies unravel that the single Pt can induce much higher electronic distribution asymmetry degree than both single Ni and Ru, and benefit the interaction between the Co sites and adsorbates (OH*, O*, and OOH*) during OER process, leading to a better OER activity.

Application of of polymer electrolyte fuel cells (PEFCs) in automotive sector shows great promise in arresting environmental degradation. They use hydrogen gas as a fuel that electrochemically reacts with air to produce electrical energy and water as a by-product. In a fuel cell electric vehicle (FCEV), these zero tail pipe emission systems offer high efficiency and power density for medium-heavy duty and long range transportation. However, PEFC technology is currently challenged by its high cost due to expensive platinum and limited durability when subjected to harsh and adverse operating conditions that can arise during the normal course of vehicle operation. A severe cause of PEFC degradation is “cell reversal”, resulting from partial and complete fuel starvation. Hydrogen starvation at the anode can arise from blokage in the hydrogen supply system by foreign impurities, water flooding or ice formation during winter [1,2]. Hydrogen starvation could be aggravated when FCEVs are operated under transient conditions such as start-up and rapid load change especially at the cells downstream in a cell stack. When the anode of a particular cell in the stack is starved of hydrogen, the anode requires an additional source of electrons and protons to complete the load circuit. At such a condition, the anode starts generating electrons and protons through water electrolysis reaction which is quickly followed by carbon corrosion reaction (in the presence of water). It causes the anode to consume itself to sustain the load demand, leading to severe degradation [3]. This process increases the anode potential while the cathode potential remains unchanged, leading to cell potential being reversed [1,3]. The existing concept of material-based solution involves adding a water electrolysis catalyst, i.e. an oxygen evolution reaction (OER) catalyst into the PEFC anode, which helps prolong the water electrolysis during hydrogen starvation and in the process preventing the anode potential from increasing further (i.e. cell potential plummeting down). These anodes are termed as Reversal Tolerant Anodes (RTA) [4]. The OER catalyst thus keeps the driving potential for carbon corrosion minimal and protects the cell by preventing self-consumption of the anode while fuel-starved under load demand. However, studies [2,4] indicate that the current strategy of adding expensive OER catalyst to enhance the durability of automotive fuel cells is only a temporary solution. The cell potential still plummets down to -2 V and lower, unless the load is terminated intentionally. Although the durability is increased to certain extent by slowing down the cell performance degradation caused by fuel starvation, the protection is not guaranteed for an indefinite time. The reversal tolerance increases with increasing loading of precious metal-based OER catalysts, however it further increases the PEFC cost. The question that we want to address is the cause of the ultimate failure or deactivation of the OER catalyst in the anode. The complexity of the electro-thermo-chemical phenomena occurring in a PEFC makes it difficult to pin-point the exact cause of degradation. In order to delineate the electrochemical phenomena under cell reversal condition, we need to subject each material component of the RTA to reversal condition and study its behavior. We propose to design well controlled experiment to explore the limitations of the materials used in PEFC electrodes and their vulnerability through insitu electrochemical diagnostics (Cyclic voltammetery, Chronopotentiometry, Electrochemical Impedance Spectroscopy, etc.) and also correlate the observations to the changes the material chemical-electronic and physical properties obtained through exsitu material characterization of the anode electrocatalyst layer. Custom anodes will be fabricated using (1) OER catalyst IrO2 with Nafion® ionomer binder, to study any intrinsic changes in IrO2 phisico-chemical properties and activity towards water electrolysis reaction with time. (2) IrO2 and different loadings of Carbon black (Vulcan, XC 72R) with Nafion ®ionomer binder, to find out if carbon corrosion alongside OER causes deactivation of the catalyst or if the deactivation is caused by electronic isolation of the OER aggregates in the anode. By gaining insight of how each of these materials influence the overall degradation mechanism when put together in an RTA we can eventually design better RTA by engineering new microstructure and material in an economical way. [1] A. Taniguchi, T. Akita, K. Yasuda, and Y. Miyazaki, J. Pow. Sources 130 (2004), 42-49. [2] T.P. Ralph, M. P. Hogarth, Platinum Met. Rev. 46 (2002) 117-135. [3] P. Mandal, B-K Hong, J-G Oh, S. Litster, ECS Trans. 69 (2015), 443-457. [4] T. R. Ralph, S. Hudson, D. P. Wilkinson, ECS Trans. 1 (2006), 67-84.

Current energy and environmental issues, alongside the depletion in fossil fuels, have increased the demand for the development and use of sustainable alternative energy resources. Due to their inexhaustibility, renewable energy resources such as solar and wind energies, are highly promising candidates for the construction of sustainable energy systems. To effectively use these intermittent sources in nature, it is essential that it is converted into chemical fuels that can be efficiently stored and transported. One such example of harnessing natural energy is the electrochemical water splitting of naturally abundant water to produce hydrogen, which is a promising carbon-free chemical fuel with a high energy density that, with some further work, can be stored and transported for use on a large scale. Electrochemical water splitting can be carried out using renewable energy generated electricity, making it a clean and cost-effective process. However, the anodic oxygen evolution reaction (OER) in the water splitting process has a large overpotential. Thus, there is a need to develop highly active and inexpensive electrocatalysts for OER to overcome this barrier. Iron (Fe) is an abundant element in the Earth’s crust that is inexpensive and has low toxicity, making it a potential candidate for widespread use in catalysts. Although simple Fe oxides, such as Fe2O3, have intrinsically low OER activity, the activity can be dramatically enhanced by combining the Fe species with other metals, resulting in optimization of the energy levels of the Fe 3d and O 2p bands and modification of the valence state of the Fe cation. Among the OER catalysts, perovskite-type metal oxides with the general formula ABO3, where the A- and B-sites are occupied by alkaline-earth or rare-earth metals and transition metals, respectively, have attracted great interest due to their prominent OER activities, facile synthesis and environmental friendliness. Although several active Fe-based perovskite-type OER catalysts have been previously reported,[1,2] most of these studies focused on the choice of elements for the A-site metal. While the effects of crystalline structures and elemental compositions of catalysts with Fe and A-site metals have not yet been systematically investigated, the knowledge is crucial to understanding the OER process catalyzed by Fe-based oxides and for the efficient design of promising electroactive materials. In this study, a variety of Fe-based oxides with the general formula AxFeyOz were synthesized, and their electrocatalytic OER activities in alkaline media were systematically investigated to elucidate the effects that the structure and composition of the materials have on the OER activity. Common alkaline-earth metals, Ca, Sr, and Ba, were selected as A-site metals, as shown in Figure 1A. Fe-based oxides were synthesized via a carboxylic acid-aided sol–gel method, which is a versatile synthetic tool to easily control the elemental compositions of the A- and B-site metals and has been previously employed to produce excellent electrocatalysts.[3] The structures of the fabricated oxides were characterized by X-ray diffraction, and the Brunauer–Emmet–Teller specific surface areas of the oxides were also measured. Subsequently, the OER activities of the oxides in alkaline medium were evaluated using a rotating-disk electrode and compared. Figure 1B shows the OER specific activities of the Fe-based oxides and reveals that a Ca-containing oxide was found to possess the highest OER specific activity among the synthesized oxides, exhibiting one of the best performances observed when compared to previously reported Fe-based oxides. These findings not only present an excellent electrocatalyst for OER in alkaline media, but also provide new guidelines for the design of metal oxide-type OER electrocatalysts.

Renewable energy-driven proton exchange membrane water electrolyzer (PEMWE) attracts widespread attention as a zero-emission and sustainable technology. Oxygen evolution reaction (OER) catalysts with sluggish OER kinetics and rapid deactivation are major obstacles to the widespread commercialization of PEMWE. To date, although various advanced electrocatalysts have been reported to enhance acidic OER performance, Ru/Ir-based nanomaterials remain the most promising catalysts for PEMWE applications. Therefore, there is an urgent need to develop efficient, stable, and cost-effective Ru/Ir catalysts. Since the structure-performance relationship is one of the most important tools for studying the reaction mechanism and constructing the optimal catalytic system. In this review, the recent research progress from the construction of unsaturated sites to gain a deeper understanding of the reaction and deactivation mechanism of catalysts is summarized. First, a general understanding of OER reaction mechanism, catalyst dissolution mechanism, and active site structure is provided. Then, advances in the design and synthesis of advanced acidic OER catalysts are reviewed in terms of the classification of unsaturated active site design, i.e., alloy, core-shell, single-atom, and framework structures. Finally, challenges and perspectives are presented for the future development of OER catalysts and renewable energy technologies for hydrogen production.

Templated synthesis of transition metal phosphide electrocatalysts for oxygen and hydrogen evolution reactions

Crystalline metal oxide catalysts operating under oxygen evolution reaction (OER) conditions invariably restructure, resulting in active sites with hydroxo/oxo species in an amorphous environment. An increase in the population of terminal hydroxo/oxo species (i.e., edge sites) facilitates proton-coupled electron-transfer (PCET) kinetics for oxygen generation and thus improves catalyst competency. While amorphous films benefit from a greater density of active sites, they suffer from diminished charge transport as compared to that of extended crystalline lattices. Managing this amorphous–crystalline dichotomy is essential when designing OER catalysts, which we highlight with the examination of electrodeposited PbOx materials, which historically are very poor OER catalysts. Along these lines, the presence of phosphate during PbOx electrodeposition truncates the growth of an extended lattice owing to its strong bonding to oxide surfaces to afford an amorphous catalyst film (A-PbOx) with significant charge-transfer resistance (138 ± 42 Ω) and poor OER kinetics (420 ± 105 mV dec–1 Tafel slope). Conversely, electrodeposition of Pb2+ in the presence of less coordinating electrolytes such as nitrate affords crystalline β-PbO2 with improved charge-transfer resistance (42.6 ± 1.1 Ω), though still poor OER kinetics (134 ± 36 mV dec–1 Tafel slope). By operating amorphous A-PbOx in less coordinating electrolytes, however, a new partially crystalline material can be generated (μc-PbOx) with further reduced charge-transfer resistance (33.0 ± 1.4 Ω) and improved OER kinetics (70 ± 15 mV dec–1 Tafel slope). The enhanced OER activity of μc-PbOx is the result of coupling the high edge-site population of an amorphous PbOx phase with crystalline-like charge transport properties. The ability to use an electrolyte to induce OER activity in an inactive amorphous form of PbOx highlights the benefits of optimizing the amorphous–crystalline phase compositions in the design of active OER catalysts.