Patent Application: US-13039008-A

Abstract:
fuel cell structure . a plurality of microtubes are electrically inter - connected . each microtube includes an anode and a cathode layer separated by an electrolyte layer . the plurality of microtubes are arranged in at least two adjacent layers with microtubes in a first layer extending in a first direction and the microtubes in the second layer extending in a second direction , the first and second directions being non - parallel .

Description:
with reference first to fig1 , the novel fuel cell 10 disclosed herein includes an array of multiple microtubes 12 forming an interconnected , 3 - dimensional network sometimes referred to herein as a “ woodpile ” structure . each of the microtubes 12 includes an anode layer 14 , an electrolyte layer 16 and a cathode layer 18 . in this embodiment , air flows inside the microtubes 12 while fuel is supplied to the outer surfaces within the tube scaffold . those of ordinary skill in the art will appreciate that the layer arrangement can be altered so that fuel flows inside the tubes and air is supplied to the outer surfaces . one approach to manufacture the design disclosed herein is the use of a combined direct - ink - writing ( diw )- cvd process . this technology has been successfully employed to produce hollow woodpile photonic crystals ( cf . fig4 , adapted from [ gra1 ], and [ gra2 ]). the multi - tube sofc device disclosed herein can be prepared , as an example , according to the scheme depicted in fig5 . for the sake of clarity , only one tube is presented in this diagrammatical representation . either the inner or outer layer may be chosen as the cathode or anode i . e . the device can be designed to be cathode or anode supported . in a first step , solid tubes of polymer precursor are deposited by diw . this step determines the cross section of the miniaturized tubes and the piling of the rods , i . e ., the degree of interconnectivity . the mixed electronic - ionic conducting ( miec ) cathode material is deposited by cvd at low temperature . possible candidates are ( la , sr )( co , fe ) o 3 or ( ba , sr )( co , fe ) o 3 . an interconnected cathode network is formed automatically . optimum layer thickness ( 500 nm to some μm ) is fixed by considerations with respect to mechanical sustainability and electrode resistance . in a subsequent cvd ( or electrochemical vapor deposition ( evd ), see [ ped1 ] and references therein for details ) step , the electrolyte material , either doped ceria or ysz , is deposited . considering ohmic losses , this layer should be as thin as possible , ideally between 100 nm and 5 μm . the electrochemical vapor deposition ( evd ) process insures that a dense ysz film forms even over a porous cathode or anode layer . a cermet anode layer ( e . g . ni - ysz , ni - cgo ) with a thickness of about 1 to 2 μm is also deposited by cvd . one commonly begins by depositing nio - ysz or nio - cgo composites followed by a heat treatment in h 2 gas to reduce the nio phase to ni and thereby simultaneously to generate a percolating metallic phase and porosity . further improvements in porosity of the anode structures to achieve optimum fuel penetration may benefit from additional treatments ( thermal treatment , pre - treatment of the electrolyte surface ) prior to or post deposition in order to achieve a high degree of porosity . a calcination step ( t ≈ 475 ° c .) for template ( polymer ) removal and creation of a hollow tube is inserted between the cvd steps as appropriate depending on the temperatures needed for cvd . alternatively , the different layers can be applied by other deposition techniques such as sol - gel based dip coating or electrophoretic deposition . gas supply of air and fuel to the respective electrodes can be achieved using manifolds similar to the ones used in sodium / sulfur batteries [ mik1 ] or in agrichemical systems [ mcc1 ]. in order to attach the manifold , the bottom row of microtubes needs to be extended to one side of the substrate as shown in fig6 . gas tight sealing may be provided by glass seals which , given that they can be located at a much reduced temperature compared to the active part of the fuel cell , can be much more readily engineered than the high temperature seals required by the planar sofc design . all proposed layer thicknesses are feasible by cvd . as stated above , the deposition sequence might be switched to depositing the anode first and the cathode last . the maximum lateral dimensions of the diw - process can presently exceed 1 cm × 1 cm with rod diameters ranging from several hundreds of nm to several hundreds of μm [ gra1 ]. there appear to be no intrinsic features which , with further development , would limit the maximum lateral dimensions . the layers can either be closely packed , or the rods can form a scaffold - like structure as illustrated in the following . fig7 shows a diagrammatical cross - section through the 3d fuel cell scaffold , indicating the interconnected channels for the feed gases 1 and 2 . one gas flows inside the channel system formed by the hollow tubes ( gas 1 ), whereas the other gas ( gas 2 ) flows through the void space between the single tubes . in contrast to the conventional tubular design , a three - dimensional cell stack is created in one single manufacturing step . the tubes are intrinsically electrically connected in parallel . a series connection of single sofc scaffolds can be achieved in the following way . for this purpose , the planar substrates used for the diw process are selected to be good electronic conductors in order to serve as the interconnect between adjacent cells . the substrate can be prepared from a typical interconnect material e . g . la - chromite based —( la , sr ) cro 3 or for intermediate temperature operation , an oxidation resistant metal . by stacking diw sofc scaffold units ( each unit can contain from a minimum of a single layer to multiple layers ), one obtains the layer sequence depicted exemplarily in fig8 . thus , cathode and anode sides of the two cells are electrically connected ( series cell connection ). as an additional benefit , no further sealing is necessary in the stacked configuration , given that one gas is confined within the tubular channels by means of the manifold structure described above . the individual manifolds feeding each scaffold with one feed gas ( air or fuel ) can in turn be connected by a larger manifold as shown in fig9 . since the tubular structures are gas - tight , the entire stack can then be exposed to the second feed gas . it should further be noted that , in contrast to the siemens westinghouse tubular design , no complex interconnect design is required for stacking cells in the present invention . due to the small tube dimensions that can be achieved by diw - cvd , the surface - to - volume ratio of the cell stack is enhanced . the following calculations estimate this gain in active surface area . consider a conventional sofc tube as depicted in fig2 a . the conventional dimensions used for the siemens - westinghouse tubes according to [ www1 ] are summarized in table 1 . with the diw - technique , cell dimensions as low as 1 μm ( inner diameter ) are feasible . two possible dimensions for the woodpile stacks are included in table 1 . design ( a ) describes a very compact design with 1 cm tube length ( cf . presently feasible dimensions stated above ), an inner tube diameter of 5 μm and a wall thickness ( comprising electrodes and electrolyte ) of 5 μm . for design ( b ), the inner diameter is quadrupled and the wall thickness is doubled . using the dimensions included in table 1 , one tube in the siemens - westinghouse - sofc fills a total volume of π ·( 1 . 59 cm / 2 ) 2 · 50 cm = 99 . 3 cm 3 . the cathode presents a geometric surface area of 2 · π ·( 1 . 18 cm / 2 )· 50 cm = 185 cm 2 , the anode surface area equals 2 · π ·( 1 . 59 cm / 2 )· 50 cm = 250 cm 2 . assuming that the complete volume of such a tube is filled with piled multi - walled microtubes and assuming further that the piling of the single rods would lead to a maximum filling degree of 50 %, one would need approximately 1 . 4 · 10 5 tubes in the case of design a and 1 . 3 · 10 6 tubes for design b . the total available surface area for the cathode thus amounts to 99300 cm 2 ( design a ) and 44100 cm 2 ( design b ). for the anode , calculations yield for design a and b 198600 cm 2 and 66200 cm 2 , respectively . the available surface without increasing the volume of the device would thus increase considerably ( factors between 260 and 800 ). the proposed device can be further downscaled and thus be customized for specific applications , e . g . as an alternative for conventional batteries . in addition , the integration of mems and diw has been demonstrated and offers opportunities to integrate the novel cell concept with conventional platforms . the unique versatility of the diw process , which allows one to precisely control cross - section , dimensions , and piling , may be used to carefully optimize the three - dimensional cell stack to achieve best cell performance . the process can be adapted easily to alternative electrode or electrolyte materials . a very thin electrolyte thickness ( 100 nm to 5 μm ) compared to thicknesses & gt ; 10 μm used in conventional tubular designs [ sin1 ] can be prepared , thereby reducing ohmic losses of the cell . making use of the novel processing technique disclosed herein , the novel tubular design thus presents an attractive alternative to the conventional set - up with high potential for further optimization . it is recognized that modifications and variations will be apparent to those of ordinary skill in the art and it is intended that all such modifications and variations be included within the scope of the appended claims . 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