NOVELTY - A fuel cell comprises: an anode comprising an anode catalyst (1406); a cathode comprising a gas diffusion electrode (1418), and a cathode catalyst (1414) on gas diffusion electrode; a microfluidic channel (1408) contiguous with the anode; and a liquid comprising fuel in the channel, where the concentration of the fuel in the liquid is 0.05-0.5 M. The fuel is methanol. The concentration of the fuel in the liquid is (0.1-0.3, preferably 0.1-0.2) M. The liquid flows through the fuel cell at a stoichiometric fuel flow rate of (1-30, preferably 2-10, especially 3-7).    USE - As fuel cell and fuel cells stack (claimed), useful in portable and mobile fuel cell systems and electronic devices e.g. cellular phones, laptop computers, DVD players, televisions, personal data assistants, calculators, pagers, hand-held video games, remote controls, cassette players, CD players, radios, audio players, audio recorders, video recorders, cameras, navigation systems, and wristwatches; and in automotive and aviation systems, including systems used in aerospace vehicles.    ADVANTAGE - The fuel cell provides fuel cell that has advantages and electrochemical performance of flowing electrolyte fuel cell, but has smaller physical dimensions. The fuel cell suffers from first type of fuel loss, as fuel diffuses to cathode where it is either partially or completely oxidized or evaporates into air stream and exits system as either incomplete oxidation products, CO2 or fuel vapor; or second type of fuel loss that occurs from unspent fuel in stream that exits stack contains CO2 gas, where during separation fuel such as methanol evaporates into stream of CO2 gas bubbling through reservoir and venting. Because both of these fuel loss mechanisms are very strong functions of average concentration of fuel in fuel stream, the fuel cell minimizes these losses by building stack and system that operates at lowest feasible methanol concentration without imposing any other major system constraints or losses. The fuel cell provides high fuel efficiency cell for forming stack by using low concentration of fuel in the fuel stream of (0.05-0.5, preferably 0.1-0.2) M. Further, to obtain desirable power for a fuel cell stack using a low concentration of fuel in the fuel stream, the fuel stream is provided with high flow rate i.e. stoichiometric fuel flow rate of (1-30, preferably 2-10, especially 3-7). Using a combination of low concentration fuel in the fuel stream and a high fuel stream flow rate, fuel efficiency of ( greater than or equal to 70, preferably greater than or equal to 80, especially greater than or equal to 85)% can be achieved, and the fuel cell produces a current density of greater than or equal to 50 mA/cm2. Thus the fuel cell is a flowing electrolyte fuel cell or flowing fuel stream fuel cell including anode, cathode, and further microfluidic channel contiguous with anode containing the liquid electrolyte. The anode includes anode catalyst. The cathode includes gas diffusion electrode having cathode catalyst. Increased uniformity of fluid flow within flowing electrolyte fuel cell may be advantageous for a fuel cell that includes anode in convective contact with fuel; and can be facilitated by non-compressible spacer defining microfluidic channel for liquid electrolyte. In conventional flowing electrolyte fuel cells, diffusion medium is present between fuel stream and anode catalyst. It has been found that elimination of diffusion medium between anode catalyst layer and fuel stream provides unexpected improvement in performance of fuel cell. The fuel efficiency of the cell is increased when dissolved fuel does not have to pass through diffusion medium or other barrier to interact with anode catalyst. The rate of reduction of fuel at anode catalyst may be limited by mass transport of fuel through any material that is present between fuel stream and anode catalyst, since diffusion of solute in liquid is relatively slow process. When diffusion distance is minimized or eliminated, fuel molecules have higher probability of reacting at anode catalyst as fuel or fuel mixture flows past anode. Thus, for a given concentration of fuel in a mixture and a given flow rate of fuel in fuel cell, higher number of fuel molecules can react at anode to produce electrons. Increased uniformity of fluid flow within flowing electrolyte fuel cell may be facilitated by a non-compressible spacer defining microfluidic channel for liquid electrolyte. Microfluidic channel layer defines microfluidic channels for liquid electrolyte, and may be non-compressible. The use of non-compressible spacer defining microfluidic channel can help ensure that dimensions of microfluidic channels in cell constant during normal use conditions. In contrast, when compressible spacer is compressed between anode and cathode, cross-sectional area and volume of microfluidic channels can be diminished, leading to decrease in amount of fuel and/or oxidant that contacts electrode at given fluid pressure. In addition, if reduction in microfluidic channel dimensions is not uniform throughout the channels, then there can be variability in electrical potential across electrodes, leading to a decrease in performance of cell. Increased uniformity of fluid flow within flowing electrolyte fuel cell can facilitate uniform distribution of reagents and temperature throughout the cell. This allows for reducing physical dimensions of fuel cell without sacrificing electrochemical performance.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a fuel cell stack comprising: greater than or equal to 5 of the fuel cells, where fuel efficiency of fuel cell stack is greater than or equal to 70%.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-section and actual Z dimensions of the flowing fuel stream fuel cell.    Bipolar plate web (1402)    Anode support (1404)    Anode catalyst layer (1406)    Liquid channel/microfluidic channel (1408)    Photoetched kapton screen (1410)    Hydraulic barrier layer (1412)    Cathode catalyst layer (1414)    Microporous layer (1416)    Cathode gas diffusion layer/electrode (1418)    Gas channel (1420)