countries, where many users still are not connected to the electric grid, it is clear that alternative energy technologies are needed that can contribute with renewable fuels in signifi cant amounts. In this challenging context, researchers have found inspiration in the process of natural photosynthesis when they attempt to convert the plentiful, but intermittent, solar irradiation incident on the Earth's surface into a storable fuel, using a range of methods commonly coined as artifi cial photosynthesis. [1][2][3][4][5][6][7][8][9] One emerging technology to achieve such solar-to-fuel conversion is the "artifi cial-leaf device", which essentially comprises an assembly of photovoltaics (PVs) that drives two electrocatalyst electrodes immersed in water. [10][11][12] This device produces molecular hydrogen and oxygen using only sunlight and water as the input, and during the past few years its performance has improved drastically, with the current record for the solar-to-hydrogen (STH) conversion efficiency being above 10%. [13][14][15][16] It is, however, clear that in order to become truly sustainable and useful, the artifi cial-leaf device should comprise solely, or predominantly, earth-abundant materials and be produced with scalable and low-cost methods. Moreover, from a processing and transport point of view, it is preferable if it can be lightweight and fl exible.Perovskite materials [ 14,[17][18][19][20][21] have recently emerged as an interesting option to incumbent silicon [ 15,20 ] and copperindium-gallium-selenide [ 16,20 ] for the active material in PVs, due to a high and quickly improving solar-to-electric (STE) conversion effi ciency and an opportunity for cost-effi cient, solutionbased fabrication. [ 17,18,21,23 ] The electrocatalyst electrode comprises a catalytically active material deposited on the surface of a current collector. Commonly employed materials for the current collector are bulk or porous metals, [ 15 ] whereas a popular choice for the catalyst is various types of metal-oxides. [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] From a system-cost and practicality perspective, it is desirable to develop a functional lightweight and low-cost current collector and to identify a catalyst that is bifunctional in that it can drive the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in the same water solution. [ 14,[39][40][41][42][43][44] Here, we present an artifi cial-leaf device that delivers an STH effi ciency of 6.2% and a Faradaic H 2 evolution effi ciency of 100%. We mention that our device is not fully integrated at this stage, Molecular hydrogen can be generated renewably by water splitting with an "artifi cial-leaf device", which essentially comprises two electrocatalyst electrodes immersed in water and powered by photovoltaics. Ideally, this device should operate effi ciently and be fabricated with cost-effi cient means using earth-abundant materials. Here, a lightweight electrocatalyst electrode, comprising large surface-area NiCo 2 O 4 nano...