Usually, a nickel-iron battery is simply two electrodes -- of iron oxide and nickel(III)oxide -- suspended in potassium hydroxide solution. In a paper published in Nature Communications, the Stanford team (led by chemist Hongjie Dai) improved the nickel-iron battery's performance by growing nanocrystals of iron oxide on thin sheets of carbon, and nickel nanocrystals on carbon nanotubes. The result is a vastly increased surface area -- and a vastly increased charge rate.
Nickel-iron batteries were first invented and patented by Swedish inventor Waldemar Jungner in 1899, and his design was improved upon by Thomas Edison in 1901. Edison's batteries were huge and clunky, but that didn't matter since their main use was in many of the first electric cars. However, as gasoline engines became cheaper, and more reliable, the batteries fell out of favour -- much to Edison's regret. That's because they can't store as much as other battery types of the same size, and the nickel-iron design charges and discharges very slowly (often taking many hours at a time). By the 1970s nickel-iron batteries were so unpopular in the US that the one company left manufacturing them shut down production.
On the plus side, nickel and iron are both cheaper and less toxic than the chemicals found in acid-based batteries. The batteries also last a long time (as long as 20 years when regularly charged and discharged), because nickel and iron dissolve very poorly in potassium hydroxide. This has left them popular as a backup power source for sites disconnected from main electricity grids (such as construction sites), or with companies who need to store excess energy generated by wind and solar power.