Electrochemical property assessment of pr2cuo4 submicrofiber cathode for intermediate temperature solid oxide fuel cells

untitled Ting Zhao Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China e mail 295[.]

Ting Zhao Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China e-mail: 2959696606@qq.com Li-Ping Sun1 Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China e-mail: lipingsun98@yahoo.com Qiang Li Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China e-mail: lq1211@sina.com Li-Hua Huo Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China e-mail: lhhuo68@yahoo.com Hui Zhao1 Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China e-mail: zhaohui98@yahoo.com Jean-Marc Bassat CNRS, Universite de Bordeaux, ICMCB, 87 Avenue du Dr A Schweitzer, F-33608 Pessac-Cedex, France e-mail: bassat@icmcb-bordeaux.cnrs.fr Aline Rougier CNRS, Universite de Bordeaux, ICMCB, 87 Avenue du Dr A Schweitzer, F-33608 Pessac-Cedex, France e-mail: rougier@icmcb-bordeaux.cnrs.fr Electrochemical Property Assessment of Pr2CuO4 Submicrofiber Cathode for Intermediate-Temperature Solid Oxide Fuel Cells The Pr2CuO4 (PCO) submicrofiber precursors are prepared by electrospinning technique and the thermo-decomposition procedures are characterized by thermal gravity (TG), Xray diffraction (XRD), Fourier transform infrared spectoscopy (FT-IR), and scanning electron microscopy (SEM), respectively The fibrous PCO material was formed by sintering the precursors at 900  C for hrs The highly porous PCO submicrofiber cathode forms good contact with the Ce0.9Gd0.1O1.95 (CGO) electrolyte after heat-treated at 900  C for hrs The performance of PCO submicrofiber cathode is comparably studied with the powder counterpart at various temperatures The porous microstructure of the submicrofiber cathode effectively increases the three-phase boundary (TPB), which promotes the surface oxygen diffusion and/or adsorption process on the cathode The PCO submicrofiber cathode exhibits an area specific resistance (ASR) of 0.38 X cm2 at 700  C in air, which is 30% less than the PCO powder cathode The charge transfer process is the rate limiting step of the oxygen reduction reaction (ORR) on the submicrofiber cathode The maximum power densities of the electrolyte-support single cell PCO|CGO|NiOCGO reach 149 and 74.5 mW cm2 at 800 and 700  C, respectively The preliminary results indicate that the PCO submicrofiber can be considered as potential cathode for intermediate temperature solid fuel cells (IT-SOFCs) [DOI: 10.1115/1.4033526] S ebastien Fourcade CNRS, Universite de Bordeaux, ICMCB, 87 Avenue du Dr A Schweitzer, F-33608 Pessac-Cedex, France e-mail: fourcade@icmcb-bordeaux.cnrs.fr Jean-Claude Grenier CNRS, Universite de Bordeaux, ICMCB, 87 Avenue du Dr A Schweitzer, F-33608 Pessac-Cedex, France e-mail: grenier@icmcb-bordeaux.cnrs.fr Corresponding author Manuscript received February 15, 2016; final manuscript received April 26, 2016; published online May 17, 2016 Assoc Editor: Kevin Huang Journal of Electrochemical Energy Conversion and Storage C 2016 by ASME Copyright V FEBRUARY 2016, Vol 13 / 011006-1 Downloaded From: http://electrochemical.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jeecaj/935072/ on 01/27/2017 Terms of Use: http://www.asme.org/ab Introduction As a new power generation system, IT-SOFCs have been intensively studied in the last few decades, due to the high energy conversion efficiency, excellent fuel flexibility, and environment friendly properties One of the major problems encountered in developing IT-SOFC is the slow dynamic ORR on cathode, which accounts for most of the polarization losses in IT-SOFC [1,2] In order to overcome this problem, material composition modulation is considered as one of the good methods It includes the exploration of novel mixed ionic and electronic conducting (MIEC) materials and the preparation of composite cathodes [3–10] An alternative strategy concerns to tailor the electrode microstructure and enlarge the TPB length to promote the electrode reactions In the last decade, nanofiber materials have been intensively studied and showed potential applications in fabricating electronic and optoelectronic devices, biosensors, and electrochemistry devices [11–17] Very recent studies indicated that nanofiber oxides held great promise as cathode materials for intermediate-temperature SOFCs, due to their high porosity, large surface area, high gas permeability, and small interfibrous pore size [18–20] For example, a test cell with La1.6Sr0.4NiO4 nanofiber cathode on CGO electrolyte was constructed recently [21], which produced a polarization resistance (Rp) of 0.40 X cm2 at 700  C in air In another study, Nd1.93Sr0.07CuO4 nanofiber cathode was prepared by the electrospun method [22] A further reduced polarization resistance of 0.26 X cm2 was found at 700  C, almost two times smaller than the powder counterpart with the same composition It is postulated that the stacking of nanofiber forms a highly porous and wellconnected network structure, which provides continuous pathways for both electrons and oxygen ions throughout the bulk cathode At the same time, the well constructed microstructure of the fiber electrode ensures the rapid oxygen diffusion or adsorption processes on the cathode, which improves the electrochemical performance PCO has already been studied as potential cathode material in IT-SOFC [23–29] Compared to the other Ln2CuO4 materials, PCO was found to exhibit quite high electric conductivity [23], relatively high oxygen diffusion coefficient D*, and acceptable chemical stability toward ceria-based electrolyte [29] To further optimize the cathode performance, the PCO submicrofiber was prepared in this work by electrospun method and tested as SOFC cathode The electrochemical properties of the PCO submicrofiber cathode was evaluated and compared with the powder counterpart in detail Experimental 2.1 Sample Preparation The fibrous PCO was prepared by electrospinning technology In a typical procedure, polyvinylpyrrolidone (PVP) powder was dissolved in the mixture of ethanol and water under stirring to form a 10 wt.% PVP solution, to which stoichiometric amounts of Pr(NO3)36H2O and Cu(NO3)23H2O were added with continues stirring The obtained homogeneous hybrid sol was kept static for 10 hrs before being loaded into the electrospinning syringe During the electrospinning, the distance between the spinneret and the collector was fixed at 17 cm and the high-voltage supply was maintained at 10 kV A syringe pump was used to disperse the solution at a spinning rate of 0.08 mm/ The as-prepared hybrid fiber precursors sintered at 400  C for hrs and 900  C for hrs in air with a heating rate of  C/min to obtain PCO submicrofiber The fibrous PCO was thoroughly mixed with terpineol to form ink To fabricate the symmetrical cell PCO|CGO|PCO, the ink was manually brushed on both sides of the polished CGO pellet with an effective area of 0.62 cm2 Then, the cathode was first heated at 400  C for hrs to eliminate organic binders, followed by sintering at 900  C for hrs in air Fine platinum gauzes were pressed slightly on both sides of the cell as current collectors In order to measure the DC polarization experiments, three electrodes cell is made The PCO slurry was 011006-2 / Vol 13, FEBRUARY 2016 painted on one side of the CGO pellet as a working electrode (WE) Platinum paste was painted on the other symmetrical side of the CGO pellet to form the counter electrode A Pt wire was used as reference electrode pasted on the same side of the WE For the CGO electrolyte-supported single cell, NiO–CGO (NiO:CGO ¼ 60:40 wt.%) and PCO were coated on both sides of CGO electrolyte (0.4 mm in thickness) in the symmetrical positions with the reactive area of 0.25 cm2 and subjected to a final calcining step at 900  C for hrs 2.2 Characterizations The thermal decomposition behaviors of the hybrid fiber precursor were examined by a thermogravimetric analyzer (TGA) (Perkin-Elmer TG/DTA 6300) in the temperature range of 30–900  C under air flow The structure and phase purity of the PCO submicrofiber was determined by XRD on a Bruker D8-Advance diffractometer with Cu Ka radiation IR spectrum was recorded on a Bruker Equinox 55 FT-IR spectrometer with KBr pellet as background in the 4000–400 cm1 region Scanning electron microscopy (Hitachi, S-4700 FEG) was used to observe the morphology and microstructure of the submicrofiber The electrochemical impedance spectrum (EIS) analysis was carried out by using an electrochemical workstation (AUTOLAB, PGSTAT30) in the frequency range from MHz to 0.01 Hz, with the imposed AC voltage of 10 mV The measurements were performed at equilibrium potential (OCV) as a function of temperature (500–700  C) and oxygen partial pressure (in an N2/O2 mixed atmosphere) The DC polarization experiments were performed using the chronoamperometry method The cathode overpotential was obtained by the following equation: g ¼ DUWR  iRel (1) The performance of the CGO electrolyte-supported SOFC was evaluated from 600 to 800  C by the ProboStat device with humidified hydrogen (3% H2O) as fuel and static air as oxidant The current–voltage (I–V) characteristics were recorded using AUTOLAB (PGSTAT30) Results and Discussion 3.1 TG-DTG analysis TG-DTG result of the synthesized PVP/Pr(NO3)3/Cu(NO3)2 hybrid fiber is given in Fig In the TG curve, there are three discrete regions of weight loss occurred at about 95, 310, and 430  C, respectively In the corresponding DTG curve, three peaks could be observed, indicating the change of thermal decomposition mechanism with temperatures The weight loss below 95  C is probably due to the evaporation of moisture and trapped solvent such as water and ethanol In the temperature range of 310–430  C, there are continuous weight loss observed in the TG curve, due to the decomposition and burning out of PVP polymers, nitrate and other minor organic constituents introduced from the organic salt stocks When the temperature reaches 900  C, there is no further weight loss inspected, revealing the formation of pure PCO submicrofibers 3.2 XRD Analysis In order to verify the TG results, the samples that heat-treated at different temperatures were characterized by XRD Figure shows the XRD patterns of the as-prepared hybrid fibers and the samples that sintered at 450  C and 900  C for hrs, respectively The hybrid fibers exhibit a typical XRD pattern of polymers There is no diffraction peaks in the pattern, except a hump around 20 deg (Fig 2(a)) After heat-treated at 450  C for hrs, the hump absolutely disappears, meanwhile the diffraction patterns related to Pr6O11 and CuO phases appear in the XRD patterns (Fig 2(b)) When the sintering temperature reaches 900  C, all the observed diffraction peaks are consistent with the data of JCPDS standard card (49-1891), indicating the formation of PCO phase (Fig 2(c)) The Rietveld refinement result reveals that the PCO submicrofiber material crystallizes in Transactions of the ASME Downloaded From: http://electrochemical.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jeecaj/935072/ on 01/27/2017 Terms of Use: http://www.asme.org/ab tetragonal structure with the space group I4/mmm (Fig 3) The ˚ , c ¼ 12.225(0) A ˚ , which cell parameters are a ¼ b ¼ 3.960(6) A are in good agreement with the literature [29] This result proves that the pure phase PCO submicrofiber material is successfully prepared by the electrospinning method followed by a simple thermal treatment 3.3 FT-IR Spectra Analysis The thermal evolution process of the PCO submicrofiber is further studied by the FT-IR spectrum, and the results are presented in Fig Clearly the spectrum of pure PVP fiber and PVP/Pr(NO3)3/Cu(NO3)2 hybrid fibers are almost identical (Figs 4(a) and 4(b)) The PVP shows the characteristic vibration bands of the stretching vibration of hydroxyl group (tO–H), C–H bond (tC–H), carbonyl group (tC ¼ O), C–H bond (dC–H), and C–N bond or C–O bond (tC–N or tC–O) at around 3462, 2955, 1664, 1428, and 1283 cm1, respectively These bands are weakened or disappeared, and new band appears around 510 cm1 after sintered at 900  C for hrs (Fig 4(c)) This new band is due to the contribution of the stretching vibration of Cu–OII linkages in the basal planes [30] This result indicates that crystallized PCO is formed at 900  C, which is in quite agreement with the XRD conclusions 3.4 Morphology of the Fibers The morphology evolution of the electrospinning material was followed with SEM Figure 5(a) shows the SEM image of the PVP/Pr(NO3)3/Cu(NO3)2 hybrid material The fiber morphology is evidently observed The diameter of the fiber is about lm The surface of the fiber is smooth because of the amorphous nature of the polymer After sintering at 900  C for hrs (Fig 5(b)), the surface of the fiber becomes coarse and the diameter shrinks to 400–500 nm (in this case, we name it submicrofiber), due to the burning out of the PVP organic components and the crystallization of PCO material Figure 5(c) shows the surface image of the PCO submicrofiber cathode on CGO electrolyte It is observed that the fiber morphology remains unchanged after sintering at 900  C for hrs These submicrofibers are cross-linked together to form a three-dimensional porous framework (Fig 5(c) inlet) The cross section image shows that the submicrofiber cathode with thickness 24 lm forms good contact with the CGO electrolyte (Fig 5(d)) The porosity of the submicrofiber cathode (P) can be calculated by the following relation [19], P¼1 m ALq (2) Fig XRD patterns of various fiber samples: (a) PVP/ Pr(NO3)3/Cu(NO3)2 hybrid fiber; (b) calcination at 450  C for hrs; and (c) calcination at 900  C for hrs Fig Experimental (circles) and calculated (continuous line) XRD patterns (and their difference, dash line at the bottom) for Pr2CuO4 fiber Vertical bars indicate the positions of the Bragg peaks of the phases contained in the sample where m and A are the cathode mass and area (given the PCO loading mg cm2), L is the cathode thickness, and q is the theoretical density of PCO with 7.1 g cm3 The value of P is 71.3%, Fig TG-DTG curve of PVP/Pr (NO3)3/Cu (NO3)2 hybrid fiber Fig The FT-IR spectra of: (a) PVP; (b) Pr(NO3)3/Cu(NO3)2/ PVP hybrid fibers; and (c) submicro fibers after calcined at 900  C for hrs Journal of Electrochemical Energy Conversion and Storage FEBRUARY 2016, Vol 13 / 011006-3 Downloaded From: http://electrochemical.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jeecaj/935072/ on 01/27/2017 Terms of Use: http://www.asme.org/ab which indicating that a highly porous cathode was successfully prepared without adding any pore-formers It is speculated that this highly porous three-dimensional submicrofiber framework will supply a transport “highway” for both oxygen ions and electrons throughout the whole cathode As a result, the TPB sites are extended efficiently to the whole electrode and the ORR will be enhanced 3.5 Electrochemical Performance In order to understand the effect of material morphology on the performance of cathode, the Nyquist plot of both PCO submicrofiber electrode and the powder one are comparatively presented in Fig There is only one impressed arc that could be found in the spectrum of fiber cathode at 650  C For the powder cathode however, two consecutive depressed arcs are found Normally, the intercept value of the impedance arc with the real axis at high frequency corresponds to the resistance of the electrolyte and lead wires, and the value between the high-frequency intercept point of the x-axis and the low-frequency one is attributed to the total polarization resistance (Rp) of the test cell The ASR can be expressed as ASR ¼ 1/2 RpS, whereas S is the area of the cathode For PCO submicrofiber cathode, the ASR value is 0.38 X cm2 at 700  C in air, about 30% less than the PCO powder cathode [25] The Arrhenius plots of the polarization resistances for both the powder and the fiber cathode are presented in Fig inlet The activation energy (Ea) calculated from the Arrhenius plot is 1.27 eV for the submicrofiber cathode, which is comparable to the Ea value of PCO powder cathode (Ea ¼ 1.48 eV) Thus, it is supposed that the enhancement of the cathode performance is probably due to the fine microstructure of the PCO submicrofiber cathode, but not to the change of the oxygen reduction mechanism In order to further determine the mechanism of the ORR on the fiber cathode, the impedance spectra of the PCO submicrofiber cathode measured at 700  C under different oxygen partial pressures (pO2) is shown in Fig It is observed that the polarization resistance (Rp) decreases gradually with the increase of oxygen partial pressure, indicating that the oxygen concentration plays an important influence on the ORR process Generally, the relationship between Rp and the oxygen partial pressure can be described according to the following equation [31], Fig The Nyquist plot of PCO powder and submicrofiber cathodes that measured at 700  C in air; (inlet) Arrhenius plots of the polarization resistances of PCO powder and submicrofiber electrodes in air The electrolyte contribution has been subtracted from the impedance Rp ẳ Rop pO2 ịn (3) From the equation, the value of n decides the type of species involved in the electrode reactions [32,33] as follows: n¼1 n ¼ 1=2 n ¼ 1=4 n ¼ 3=8 n¼0 O2 gị $ O2;ads O2;ads $ 2Oads Oads ỵ 2e0 ỵ VO $ O O OTPB ỵ e0 $ O TPB O2 TPB ỵ VO $ O O Specifically, for metal oxide electrodes on solid electrolytes, n ¼ can represent the gaseous diffusions and adsorption of oxygen molecules; n ¼ 1/2 is the oxygen adsorption–desorption process, Fig SEM images of: (a) PVP/PCO hybrid fibers; (b) fibers calcined at 900  C; (c) the surface; and (d) the cross section image of the PCO submicrofiber cathode supported on CGO electrolyte after sintering at 900  C for hrs 011006-4 / Vol 13, FEBRUARY 2016 Transactions of the ASME Downloaded From: http://electrochemical.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jeecaj/935072/ on 01/27/2017 Terms of Use: http://www.asme.org/ab Fig Impedance diagrams of PCO submicrofiber cathode at 700  C under various oxygen partial pressures which includes oxygen diffusion at the interface of gas/cathode and surface diffusion of related intermediate oxygen species; n ¼ 1/4 can be attributed to the charge transfer process on the electrode, occurring at the interfaces of current collector/electrode; n ¼ 3/8 corresponds to the charge-transfer reaction at the TPB, and n ¼ relates to the oxygen ion transfer from the TPB to the electrolyte According to the results of Fig 8, all the n values are in the vicinity of 3/8 at different temperatures Therefore, in this study, the major rate-limiting step for the fiber cathode is the charge transfer reaction in the whole range of measurement oxygen partial pressures According to the literature, the ratedetermining step of the PCO powder cathode has been found to be the charge transfer process [25] In this sense, we proved that the modification of the PCO cathode microstructure has a positive impact on its performance, but has no effect on the oxygen reduction mechanism Cathodic overpotential is an important parameter to reflect the output characteristics of SOFCs In this work, the dc polarization curves were measured and the results are reported in Fig 9(a) As expected, under the same current density, the cathodic overpotential decreases with the increase of temperatures When the overpotential is 33 mV, the current density of PCO submicrofiber cathode reaches to 105 mA cm2 at 700  C in air Under low overpotentials (less than 20 mV), a linear expression from the Bulter–Volmer equation can be expected i ¼ i0 ZFg=RT (4) Fig Oxygen partial pressure dependence of ASR at 600–700  C Fig (a) Polarization curves of PCO submicrofiber cathode measured at different temperatures in air (b) Tafel curves of PCO powder and submicrofiber cathodes at 700  C where i and i0 are the current density and exchange-current density, respectively, g is the overpotential, F and R are their normal meanings The Rp value can be obtained from the inverse of the derivative of i against g, which is 0.35 X cm2 at 700  C in air, in good agreement with the results of the impedance measurement Figure 9(b) comparatively presents the log i–g characters of PCO submicrofiber and powder cathodes Clearly under the same overpotential, the submicrofiber cathode always exhibits a much higher current density, indicating that the submicrofiber electrode shows better performance than the powder electrode with the same composition The electrochemical properties of PCO submicrofiber cathode was further compared with the other fiber cathodes that reported in the literature and the results are listed in Table It is observed that the PCO fiber cathode exhibits improved cathode properties than the perovskite materials Pr0.6Sr0.4Fe0.8Co0.2O3 and La0.8Sr0.2MnO3, and even better than La2CuO4 and La1.6Sr0.4NiO4 The quite promising electrochemical properties of PCO fiber cathode may come both from the specific fiber morphology and the intrinsic physical properties of the material For example, PCO material has been found to exhibit the highest electrical conductivity and the oxygen trace diffusion coefficient (DT) among the Ln2CuO4 (Ln ¼ Pr, Nd, Sm) materials [23] To evaluate the possible applications of the fiber cathode in the cell, the electrolyte-supported cell NiO–CGO|CGO|PCO was constructed and the output performance was recorded as a function of current density using humidified H2 as fuel and ambient air as oxidant from 600  C to 800  C in Fig 10 The maximum power Journal of Electrochemical Energy Conversion and Storage FEBRUARY 2016, Vol 13 / 011006-5 Downloaded From: http://electrochemical.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/journals/jeecaj/935072/ on 01/27/2017 Terms of Use: http://www.asme.org/ab Table the electrochemical properties of fiber cathodes with different structures and compositions Composition PCO La2CuO4 La1.6Sr0.4NiO4-Ag La1.6Sr0.4NiO4 Pr0.6Sr0.4Fe0.8Co0.2O3 Nd1.93Sr0.07CuO4 La0.8Sr0.2MnO3 Morphology ASR at 700  C(X cm2) Ea (eV) Current density at 700  C (mA cm-2) Reference Submicrofiber Nanotube Nanofiber Nanofiber Nanotube Nanofiber Nanofiber 0.38 1.03 0.32 0.40 0.75 0.26 0.48 1.27 1.26 — 1.12 — 1.74 — 105 at g ¼ 33 mV 56 at g ¼ 130 mV — — — — — this work [34] [35] [21] [36] [22] [37] IT-SOFC ¼ L¼ m¼ MIEC ¼ ORR ¼ P¼ PVP ¼ pO2 ¼ R¼ Rel ¼ Rp ¼ SEM ¼ TGA ¼ TPB ¼ XRD ¼ DUWR ¼ Fig 10 I–V curves and corresponding power density curves of the electrolyte-supported cell NiO–CGO|CGO|PCO from 600 to 800  C density of the cell is 149 mW cm2 at 800  C, comparable to the traditional electrolyte-supported NiO–GDC|YSZ|LSM|LSM–YSZ cell [38], proving the potential application of the fibrous PCO material as cathode in SOFCs Conclusions The PCO submicrofiber was successfully synthesized by electrospinning method and used as prospective cathode for IT-SOFC The rate limiting step for the ORR on PCO submicrofiber electrode was the charge transfer process, the same as the powder electrode The lowest ASR obtained for the PCO submicrofiber cathode at 700  C was 0.38 X cm2 in air The three-dimensional porous morphology of the submicrofiber cathode promotes the ORR process, but has no effect on the ORR mechanism Acknowledgment The project was supported by National Natural Science Foundation of China (Grant Nos 51302069 and 51372073), Foundation of Heilongjiang Educational Department (Grant No 2013TD002), Scientific Research Foundation for Returned Scholars, Ministry of Human Resources and Social Security of People’s Republic of China (2014-240), and Nature Science foundation of Heilongjiang Province in China (E2016051) Nomenclature A¼ Ea ¼ EIS ¼ F¼ i¼ i¼ i0 ¼ cathode area activation energy electrochemical impedance spectrum Faraday constant current flowing through the test cell current density exchange-current density 011006-6 / Vol 13, FEBRUARY 2016 g¼ q¼ intermediate-temperature solid oxide fuel cells cathode thickness cathode mass mixed ionic and electronic conducting oxygen reduction reaction porosity of the submicrofiber cathode polyvinylpyrrolidone oxygen partial pressure universal gas constant resistance of the electrolyte obtained from the impedance spectrum polarization resistance scanning electron microscopy thermogravimetric analyzer triple-phase boundary X-ray diffraction applied voltage between working electrode and reference 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cathode The electrochemical properties of the PCO submicrofiber cathode was... dependence of ASR at 600–700  C Fig (a) Polarization curves of PCO submicrofiber cathode measured at different temperatures in air (b) Tafel curves of PCO powder and submicrofiber cathodes at